Insect Vectors & Vector-Borne Diseases PDF

BZYET-143 INSECT VECTORS Indira Gandhi AND VECTOR BORNE National Open University School of Sciences DISEASES VOL 2 MEDICALLY IMPORTANT DIPTERANS AND IVM BLOCK 3 MEDICALLY IMPORTANT INSECT VECTORS-II 7 BLOCK 4 MEDICALLY IMPORTANT INSECT VECTORS-II AND IVM 65 BZYET-143 INSECT VECTORS Indira Gandhi AND VECTOR BORNE National Open University School of Sciences DISEASES Block 3 MEDICALLY IMPORTANT INSECT VECTORS-II UNIT 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) 7 UNIT 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) 28 UNIT 12 Dipterans as Disease Vectors-III (Aedes Mosquitoes) 45 Course Design Committee Prof. M.S. Nathawat Dr. H.S. Pawar Former Director, School of Sciences Scientist D, NIMR IGNOU, Maidan Garhi, New Delhi-110068 Sector 8, Dwarka, New Delhi-110077 Prof. S. S. Hasan (Retd.) Dr. Ranjana Saxena School of Sciences, IGNOU Associate Professor in Zoology Maidan Garhi, New Delhi-110068 Dyal Singh College, Lodhi Road, New Delhi-110003 Dr. R.S. Sharma (Retd.) Prof. Neera Kapoor Head, Centre of Medical Entomology and Vector School of Sciences, IGNOU Control, National Centre for Disease Control, Maidan Garhi, New Delhi-110068 Ministry of Health and Family Welfare, 22, Sham Nath Marg, Delhi-110054 Dr. S.K. Sagar Associate Professor in Zoology Swami Shraddhanand College, Alipur Village University of Delhi, Delhi- 110036 Block Preparation Team Dr. H.S. Pawar School of Sciences Scientist D, NIMR, Sector 8, Dwarka New Delhi-110077 (Units 10 to 12) Prof. Neera Kapoor (Units 10 to 12) Course Coordinator : Prof. Neera Kapoor Course Editor : Prof. Sarita Kumar Professor in Zoology Acharya Narendra Dev College, Kalkaji New Delhi-110009 Production Mr. Hemant Kumar SO (P), MPDD, IGNOU Acknowledgement: Prof. Neera Kapoor and Mr. Ajit Kumar, Suggestions for figures and Cover Design. Mr. Vikas Kumar, JAT for word processing and CRC preparation. December, 2021 Indira Gandhi National Open University, 2021 ISBN: All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from Indira Gandhi National Open University. Further information on Indira Gandhi National Open University courses may be obtained from the University’s office at Maidan Garhi, New Delhi-110 068 or IGNOU website www.ignou.ac.in. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by the Registrar, MPDD, IGNOU. Printed at: BLOCK 3: MEDICALLY IMPORTANT INSECT VECTORS-II Vectors are living organisms that can transmit infectious diseases between humans or from animals to humans. Many of these vectors are blood sucking insects. Mosquitoes are the best known disease vectors. Others include certain species of ticks, flies, sandflies, fleas, bugs etc. In Volume 1 you have studied about some medically important insect vectors like ticks, fleas, bugs, lice. In Volume 2, that comprises Block 3 and Block 4 you will study about mosquitoes, flies and sandflies (dipterans) as major insect vectors and diseases borne by them. You will also study about IVM (Integrated Vector Management) as the best control option for these vectors vector-borne diseases are illnesses caused by pathogens and parasites in humans. Block 3 discusses the mosquito borne diseases. Mosquitoes are major insect vectors for transmission of many fatal diseases. Mosquitoes are vectors of malaria, filariasis, and some arboviral infections. Female mosquitoes may feed on a variety of mammals, birds, and reptiles, each species having its own preferences for a particular source of blood. Many species of mosquitoes bite people, but only some of them are vectors of disease. Although all mosquitoes lay their eggs either in water or on the moist surfaces at the water’s edge, each species has a particular ecological niche, and their breeding sites can be very specific. Mosquito disease vectors are divided into to two groups, the anophelines and the culicines, which can be readily distinguished by characters that can be seen by the naked eye. Block 3 comprises 3 units, Unit 10 discusses about Anopheles mosquitoes and diseases caused by them. All malaria is transmitted by Anopheles mosquitoes which fly up to 2-3 km from breeding site. Only older females (2 weeks or more) can transmit malaria. Transmits four species of malaria (ovale, malariae, vivax, and falciparum). Falciparum malaria is the most important as it is particularly dangerous to those without acquired immunity. In any one area of the world malaria is mainly transmitted by one or two Anopheles species. Control programmes are highly specific to the species of mosquito involved in transmission. If a National Malaria Programme is not being undertaken then specialist advice from an experienced entomologist will be needed to ascertain the likely effectiveness of a control programme. The term ‘arbovirus’ is used to describe any virus transmitted by an arthropod (insect, mite or tick). There are over 400 arboviruses but only a hundred of these have been shown to cause clinical symptoms in humans. Those which cause major diseases are well studied, at least in some areas. These include dengue and dengue haemorrhagic fever (DHF), yellow fever, and Japanese Encephalitis (JE). These diseases and the causative mosquito vectors are discussed in Units 11 and 12. Dengue, DHF yellow fever, Chickungunya and Zika are usually transmitted by culicine mosquitoes belonging to the Aedes genus. These are discussed in Unit 11. Aedes breeds in water held in man-made containers, such as water storage jars, pots, tin cans, and old car tyres. It is nearly always associated with human habitats and mainly bites humans. Dengue is endemic throughout the tropics but tends to occur in periodic epidemics. Dengue itself is fairly benign but it may be associated with dengue haemorrhagic fever (DHF) or dengue shock syndrome (DSS), which are severe, often fatal diseases. Yellow fever, in its sever form, is also often fatal. Effective vaccines are available against yellow fever and these should be used as the first preventive measure. Control of Aedes vectors is practicable in an urban environment but is usually impossible in a forested area, where the mosquitoes breed in dispersed natural water pools in leaf axils or tree holes. Unlike malaria or arboviral 5 infections that may be transmitted by a single bite of a single mosquito, filarial infections require repeated inoculations of the infective larvae, perhaps hundreds per year, before the worms are present in sufficient number to produce the symptoms of disease. Bancroftian filariasis (elephantiasis) is caused by the parasitic worm Wuchereria bancrofti. The disease is widespread in the tropics, especially in urban environments, and is a chronic disease that may take several years to manifest itself after infection. In urban environments the vector Culex quinquefasciatus is the most important vector. Culex is chararterised by the presence of distinct parivilli. Culex adults can mate only in darkness. It is stout mosquito with strong pair of wings. Brugian filariasis (elephantiasis) is caused by the parasitic worm Brugia spp. It is found in India and South Asia, predominantly in rural situations. It is a chronic disease that may take several years to manifest itself after infection. It is usually transmitted by night-biting mosquitoes Mansonia spp. You will discuss about these diseases in Unit 12. In Block 4, you will study about flies (houseflies and sandflies), diseases caused by them and their control measures. IVM is discussed in the last unit of this course. Objectives After studying this block you will be able to: identify the features, life cycle and behavior of Anopheles mosquitoes, explain the epidemiology of the diseases spread by the Anopheles species as vectors, describe the preventive and control measures of disease transmitted by the vector, Anopheles, explain the life cycle of Culex mosquito, explain the diseases transmitted by Culex species, recognize their symptoms and suggest the remedial measures, discuss the disease epidemiology and affecting factors, describe the various preventive and control measures of Culex mosquitoes, explain the life cycle of Aedes mosquito, know about the diseases transmitted by Aedes species, recognize their symptoms and discuss the remedial measures, discuss the disease epidemiology and affecting factors, and describe the various preventive and control measures of Aedes mosquitoes. You are advised to go through the Annexures given at the end of the Block and the Volume. They provide comprehensive overview of the Units. 6 Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) UNIT 10 DIPTERANS AS DISEASE VECTORSI (ANOPHELES VECTORS MOSQUITOES) Structure 10.1 Introduction Diagnosis Objectives Treatment 10.2 Biology 10.4 Prevention and Control Eggs Traditional Methods of Control Larvae Modern Approaches in Control Pupae 10.5 Summary Adults 10.6 Terminal Questions Duration of Life Cycle 10.7 Answers Behavior 10.3 Diseases Life Cycle of Malaria Parasite Symptoms 10.1 INTRODUCTION In the previous unit you have studied about the different types of vectors, mode of transmission of diseases and their relationships with the hosts. In the present unit you will learn the biology of the mosquito-borne diseases, their prevention and various control measures. As you are aware that mosquitoes are slender, long-legged insects that are easily recognised by long proboscis and presence of scales on most parts of the body. The mosquito larvae are, however, without legs. They have a distinct head bearing mouth brushes and antennae, bulbous thorax that is wider than the head and abdomen, and a slender abdomen with posterior anal papillae and either a pair of respiratory openings (subfamily Anophelinae) or an elongate respiratory siphon (subfamily Culicinae) borne at the end. 7 Block 3 Medically Important Insect Vectors-II There are approximately 3,500 species of mosquitoes grouped into 41 genera under family Culicidae and order Diptera. Meigen (1818) classified culicidae family according to presence of two wings. The subfamily Anophelinae comprises three genera, and Culicinae consists of 110 genera segregated into 11 tribes (Fig. 10.1). Fig. 10.1: Classification of Family Culicidae. Anophelines are distributed in the temperate and tropical regions of the world, O’nyong’nyong except the polar regions, extreme colder regions and higher altitudes. As fever. ONNV described by J. W. Meigen (1818), Anopheles mosquito has about 460 (O’nyong nyong species of which only 100 species are reported to be involved as vectors. Virus), a mosquito borne Alphavirus, is Among these, females of only 30-40 species are reported to transmit the transmitted through malarial pathogen, Plasmodium species. Some species of Anopheles can also the bite of Anopheles serve as the vectors of canine heartworm, Dirofilariaimmitis; the filariasis- funestus and causing worm, Wuchereria bancrofti and Brugiamalayi; and viruses such as Anopheles gambiae, Vectors in Africa. the alpha virus that causes O'nyong'nyong fever. An association of brain ONNV fever is tumour incidence and malaria suggests that Anopheles might transmit a virus characterised by or other agent that could cause a brain tumour. arthralgia, similar to chikungunya fever. Objectives Objectives After having read this unit you should be able to:  identify the features, life cycle and behavior of Anopheles mosquitoes,  explain the epidemiology of the diseases spread by the Anopheles species as vectors, and  describe the preventive and control measures of disease transmitted 8 by the vector, Anopheles. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) 10.2 BIOLOGY Like all mosquitoes, anophelines go through four stages in their life cycle: egg, larva, pupa, and adult. The first three stages are aquatic and last for 7-14 days, depending on the species and the ambient temperature. The adult stage is winged and terrestrial. The adult females can live up to a month (or more in captivity) but generally do not live more than 1-2 weeks in nature. 10.2.1 Eggs Adult females of Anopheles lay 50-200 eggs per oviposition. The eggs are laid after complete embryonic growth. Eggs are laid singly directly on water (Fig. 10.2) and are 5-8 mm in size. These are unique in having floats on the lateral sides which help the eggs to float on the water surface (Fig. 10.3) and are also an important tool in identifying the sibling species. For example, identification of bioforms of An. stephensi is based on number of ridges present on the float of eggs. The mysorensis bioform (Life form) has less than 14 egg ridges, while the bioforms with 15-16 ridges are classified as intermediate bioforms and if the ridge countis more than 16 in An stephensi, they are categorised as Type bioform. The freshly laid eggs of Anopheles are white in colour while they turn to dark black after coming into contact with light and external environment due to change in the chitinous structure. Eggs are not resistant to drying and hatch within 2-3 days, although hatching may take up to 2-3 weeks in colder climates. Fig. 10.2: Difference in the eggs of different genera of mosquitoes. 9 Block 3 Medically Important Insect Vectors-II Fig. 10.3: An enlarged egg of the common malaria mosquito, Anopheles (size 5- 8 mm) with floats on the lateral sides. 10.2.2 Larvae Mosquito larvae, commonly called wrigglers, have a well-developed head with mouth brushes used for feeding, a large thorax, and a slender 9-segmented abdomen (Fig. 10.4). They are apodous and thus, are devoid of legs. In contrast to other mosquitoes, Anopheles larvae lack a respiratory siphon on their abdominal end. Instead, they breathe through spiracles located on the 8th abdominal segment. The larvae depend upon the atmospheric air for breathing and thus they need to come to the surface frequently. Thus for effective breathing, they are positioned parallel to the surface of the water. The larvae feed on algae, bacteria and other microorganisms present in the surface micro-layer. They dive below the surface only when disturbed. Larvae swim either by jerky movements of the entire body or through propulsion with the mouth brushes. They develop through 4 stages, called instars, and finally they metamorphose into pupae. At the end of each instar, the larvae molt, shedding their exoskeleton, or skin, to allow for further growth. The larval development is temperature-dependent. Under optimum conditions, it is approximately 8-10 days. Fig. 10.4: Anopheles Larva. Note the position, parallel to the water surface. The Anopheles larvae occur in a wide range of habitats but most species prefer clean, unpolluted water. They have been found in fresh- or salt-water marshes, mangrove swamps, rice fields, grassy ditches, edges of streams and rivers, and even small, temporary rain pools. Many species prefer habitats with vegetation whereas others prefer habitats that have none. Some breed in open, sun-lit pools while others are found only in shaded breeding sites in forests. A few species breed in tree holes or the leaf axils of some plants (Fig. 10 10.5). Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) Fig. 10.5: Differences in the larvae of different genera of mosquitoes. 10.2.3 Pupae The pupa of Anopheles is comma-shaped when viewed from the side and is called tumbler (Fig. 10.6). The head and thorax are merged into a cephalothorax and the abdomen curves around underneath. Just like larvae, pupae breathe atmospheric air and thus, come to the water surface frequently. Fig. 10.6: Anopheles They breathe through a pair of respiratory trumpets present on their Pupa. cephalothorax. After two days, the dorsal surface of the cephalothorax of pupa splits and the adult mosquito emerges. The duration, however, can vary depending upon environmental conditions The duration of development from an egg to adult varies considerably among different species and is strongly influenced by ambient temperature. Anopheles can develop from the egg to adult in as little as 5 days but usually take 10-14 days in tropical conditions. 10.2.4 Adults Like all insects, body of adult anophelines is divided into 3 sections: head, thorax and abdomen. The head is specialized for acquiring sensory information and feeding. It contains eyes and a pair of long, many-segmented antennae. The antennae are important for detecting host odours as well as odours of breeding sites where females lay eggs. Males possess highly feathery plumose antennae while females have less hairy pylose antennae. Mouth parts are piercing and sucking type. These are characterized by an Fig. 10.7: Anopheles Adults. Bottom figures elongate, forward-projecting proboscis used for feeding, and two sensory show their typical resting palps. The thorax is 3-segmented and specialized for locomotion. It contains position. three pairs of legs and a pair of forewings. Hind wings are reduced to club- shaped structures called halteres which are used for balancing. The abdomen is slender and specialized for food digestion and egg development. 11 Block 3 Medically Important Insect Vectors-II Anopheles mosquitoes can be distinguished from other mosquitoes by the palps, which are as long as the proboscis, and by the presence of discrete blocks of black and white scales on the fore margin of their wings (Fig. 10.7). Such wings are known as dappled wings. Adult Anopheles can also be identified by their typical resting position at an angle of 45°rather than sitting parallel to the surface. Adult mosquitoes usually mate within 2-3 days after their emergence from the pupal stage. In most species, the males form large swarms, usually around dusk, and the females fly into the swarms to mate. Male adults live for about a week, feeding on nectar and other sources of sugar. Females also feed on sugar sources for energy but they also require a blood meal for the development of eggs. The abdomen of females expands considerably after a blood meal. The blood is digested over time serving as a source of protein for the production and maturation of eggs. The maturation of eggs depends on the temperature but usually takes 2-3 days in tropical conditions. Once the eggs are fully developed, the female lays them and resumes host seeking. The cycle repeats itself until the female dies. As discussed earlier, females can survive up to a month (or longer in captivity) but generally do not live longer than 1-2 weeks in nature. Their chances of survival depend not only on temperature and humidity, but also on their ability to successfully obtain a blood meal while avoiding host defenses. Fig. 10.8: Difference in the Adults of different genera of mosquitoes. 10.2.5 Duration of Life Cycle Life cycle of Anopheles mosquito comprises four major stages as discussed 12 above i.e. eggs, larvae, pupae and adults (Fig. 10.9). Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) Fig. 10.9: Life cycle of Anopheles mosquito. The length of the aquatic life cycle of Anopheles varies from 7-14 days under optimal conditions of water temperature 25-27°C, while adults can live for 1-2 months. Rise in temperature decreases the duration of life cycle. However, temperature higher than 36-40°C destroys a number of immature stages hampering the life cycle, Similarly, atmospheric temperature also affects longevity of the adult form. 10.2.6 Behavior Mosquito behavior like flying, feeding, biting preference, resting and oviposition (egg laying), impact life cycle of Anopheles. These studies are useful for planning any kind of control for specific type of Anopheles species. Flying Flight activity of malarial mosquito peaks a short period after dark. For remainder of the night, they exhibit limited flight for blood and at dusk the start searching for the resting sites (Carpenter et al. 1946). Flight range is usually regarded as less than one mile under normal conditions, but this species is capable of much longer flights as demonstrated by mark and recapture studies (Carpenter and LaCasse 1955).Mosquitoes can fly up to several kilometers! They can reach far off places by taking shelter in motor vehicles, ships and aircraft. A study on An. gambiae showed that the average speed of species was around 1 km/h. The maximal flight distance taken by sugar-fed females was 9 km, while blood-fed females could travel for10 km and starved females just below 3 km. In some Anopheles, the maximal flight distances were 10-12 km when sugar-fed, 4.5 km when blood-fed, and below 3.5 km when starved, with an average speed of 1.3 km/h. Flight performances consisted of 1-4 h intervals of continuous flights, but mainly of bouts shorter than an hour, randomly distributed during the long flight trials. 13 Block 3 Medically Important Insect Vectors-II Feeding The mosquito adults have host seeking ability through vision, olfactory sensation and thermal source detection. Typically, both male and female mosquitoes feed on nectar and plant juices, but females have mouthparts adapted for piercing the skin of animal hosts and sucking their blood as ectoparasites. As discussed earlier, females need blood proteins for the development and maturation of eggs. (That is why they suck blood) In a few species, the females can produce more eggs after a blood meal. Males are dependent only on the nectar and the sugary plant juices throughout their life. Biting Preference Most Anopheles mosquitoes are crepuscular (active at dusk or dawn) or nocturnal (active at night). The Anopheles sp. which prefers to feed on animals are referred to as zoophillic in nature whereas those who like to bite humans more are referred to as anthropophillic. This biting behavior is very important for an Anopheline species to become an efficient vector. Different Anopheles species also show preference for biting outdoors or indoors. Some feed indoors (endophagic), while others feed outdoors (exophagic). The biting time and host are highly significant to determine the control methods. Resting Resting is a significant behavior of mosquitoes. After emergence, adults rest on the surface of water for some time and on the wall of container. Similarly, after taking blood meal, mosquitoes require adequate rest as they become heavy and need to digest blood. Resting helps in the development of eggs which takes 2-3 days depending upon the temperature. After feeding on blood, mosquitoes may prefer to rest indoors (endophilic), or rest outdoors (exophilic), though this can differ regionally based on the local vector ecotype, vector chromosomal makeup, housing type and local microclimatic conditions. This behavior is very important for mosquito control. Indoor residual sprays (IRS) are effective against endophilic mosquitoes whereas in case of outdoor resting mosquitoes, the house sprays may not be so useful. Ovipositioning Behavior (Egg Laying) Anopheles mosquitoes breed in natural water collections, normally clean water (Fig. 10.10). The species, such as An. fluvitalis, like to breed in slow flowing streams while species like An. sundaicus prefer to breed in brackish water. The breeding of Anopheles increases dramatically in the rainy season when water collects in bottles, tins, tender coconut shells, buckets, tyres etc., that are thrown out in the open and provide ample breeding ground. Apart from these, ponds, water tanks, paddy fields etc., also act as breeding grounds. Construction sites provide ample breeding places for them– water used for curing on the concrete slabs, water collected in tanks, water collected in and around the construction site owing to blockage of water drains – all these help in breeding. It is very important to destroy these water collections or to keep 14 them properly covered to prevent breeding. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) Fountains Cement Tank Pits Stone Quarry Cement tanks Ponds Fig. 10.10: Some of the breeding places for Anopheles. SAQ 1 For each sentence choose the appropriate word given in the parentheses. i) Eggs are laid singly directly on the surface of (land/water) by adult females of Anopheles. ii) Mosquito larvae breathe through (gills/spiracles) located on the 8th abdominal segment. iii) The pupa of mosquito is (round/comma) shaped when viewed from the side. iv) Male mosquitoes live for about a week, feeding on the (blood/nectar of flower). v) Mosquitoes can develop from egg to adult in (5/10) days. vi) Anopheles gambiae can fly distances of (3.5/9 Km). vii) The Anopheles species with a preference to feed on animals is referred to as (Anthropophillic/Zoophillic). 15 Block 3 Medically Important Insect Vectors-II 10.3 DISEASES Malaria is an Anopheles-borne disease caused by the Plasmodium parasite (A protist). Symptoms are of fever, chills, and flu-like illness. If the patients are left untreated, they may develop severe complications and even die. About 3 billion population of world is under the threat of malaria. In 2019, approximately 229 million cases of malaria were reported worldwide with 409,000 deaths. Children, especially in the African Region, are more vulnerable to the disease. In 2020, India experienced 181831 malaria cases and 63 deaths. About 1,700 cases of malaria are diagnosed in the United States each year. The vast majority of cases in the United States are reported in travellers and immigrants returning from malaria-affected countries, such as sub-Saharan Africa and South Asia. Malaria has been a major public health problem in India. Intermittent fever in the diseases and high incidence during the rainy season, coinciding with agriculture, sowing and harvesting, was first recognized by Romans and Greeks who associated it with swampy areas. They postulated that intermittent fevers were due to the ‘bad odour’ coming from the marshy areas and thus gave the name 'malaria' ('mal'=bad + 'air'). Despite the causative organism known today, the name has stuck to this disease. Malaria is caused by different species of Plasmodium; Plasmodium vivax (P. vivax), Plasmodium falciparum (P. falciparum), Plasmodium malariae (P. malariae) and Plasmodium ovale (P. ovale).Among these, P. vivax and P. falciparum are commonly reported from India. Recently Plasmodium knowlesi, which causes malaria in primates has been reported in humans. Anopheles species is called disease vector as infective bite of the mosquito transmits the pathogen from infected to healthy person. Man develops disease after 10 to 14 days of being bitten by an infective mosquito. Inside the human host, the parasite undergoes a series of changes as a part of its complex life cycle. The parasite completes its life cycle in liver cells (pre-erythrocytic schizogony) and red blood cells (erythrocytic schizogony). Infection with P. falciparum is the deadliest form of malaria. 10.3.1 Life Cycle of Malaria Parasite The infection of malaria starts in human beings, when a female Anopheles bites a healthy human being to suck the blood and injects her saliva containing “sporozoites” of Plasmodium. A sporozoite travels in the bloodstream and enters the liver where it invades a liver cell. It matures into a “schizont” (mother cell) which produces 30000–40000 “merozoites” (daughter cells) within six days by multiple fission. The merozoites burst out of the liver cells and either re-infect liver cells or invade red blood cells (RBCs). In RBCs, the merozoite transforms into a trophozoite within two days and starts feeding on the haemoglobin breaking it down into haem pigment and globin protein. The trophozoite gradually develops into a schizont through a series of stages and 8–24 new merozoites burst out from the red blood cell. The ruptured cell releases toxic pigment (haem converts to haemozoin) which causes chills and fever. The released merozoites invade new red cells. Plasmodium can prevent 16 the destruction of an infected red blood cell in the spleen (the organ where old Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) and damaged red cells are destroyed) by sending adhesive proteins to the cell membrane of the RBC. The proteins stick the red blood cell to the walls of small blood vessels. This poses a threat to the human host since the clustered red blood cells might block the circulatory system. A merozoite, sometimes develops into a "gametocyte", the stage that enters the mosquito. The merozoites form two kinds of gametocytes: male gametocytes (microgametes) and female gametocytes (macrogametes). They get ingested by a mosquito, while sucking the blood. The gametocytes mature into gametes and fuse inside the mosquito's midgutto form "zygotes" which then elongate and develop into "ookinetes". The motile ookinetes penetrate the midgut wall and a cyst is formed around them forming "oocysts". The cysts undergo meiotic division and eventually release sporozoites. These migrate into the salivary glands from where they get injected into human blood. The development of Plasmodium inside a mosquito takes about two weeks and makes a mosquito capable to transmit the disease. Plasmodium cannot complete its life cycle at temperatures below 20 °C (Fig. 10.11). The life cycle of Plasmodium in liver cells is called exoerythrocytic cycle, while inside blood cells it is termed as erythrocytic cycle. On the other hand, the life cycle completed in mosquito is named as sporogonic cycle. Fig. 10.11: Life cycle of malaria parasite (courtesy CDC). 17 Block 3 Medically Important Insect Vectors-II 10.3.2 Symptoms After being bitten by an infected mosquito, symptoms usually begin in the human beings within 10–30 days. Malaria can be uncomplicated or severe. Symptoms of uncomplicated malaria might include; Chills, diarrhea, fever, headaches, muscle pain, nausea, sweating, vomiting and weakness. Populations, most at Some less noticeable manifestations of malaria are enlargement of the spleen risk are young (Splenomegaly) or liver (Hepatomegaly), increased breathing frequency, mild children, pregnant anemia and mild jaundice (yellowish coloration in the eye whites and skin). women, people living with HIV, people The disease, however, can turn into severe malaria, if there are serious organ affected with failures or abnormalities in the bloodstream or body metabolism. Symptoms of humanitarian emergencies and non severe malaria include; breathing difficulties, coma, confusion, death, focal immune travellers neurologic signs, seizures and severe anemia. Some less noticeable moving into endemic manifestations of severe malaria are abnormalities in blood coagulation, area. The poorest of presence of haemoglobin in the urine, high acidity of the blood, hypoglycemia the poor in vulnerable (low blood glucose), hypotension (low blood pressure) and kidney failure. communities, living in During pregnancy, malaria can lead to premature baby delivery or delivery of a remote rural areas with limited access to low-birth-weight baby. The infant can get the parasite from the mother and health facilities, suffer develop the disease. In small children, involvement of central nervous system the most. (cerebral malaria) can cause blindness, deafness, speech difficulty, paralysis and trouble with movements. 10.3.3 Diagnosis Malaria is usually diagnosed by examining a blood sample under a microscope. The test kits are also available that detect antigens of P. falciparum in the patient's blood. These immunologic tests, known as rapid diagnostic tests (RDTs), can detect two different kinds of malaria antigens, one of P. falciparum and the other found in all the four Plasmodium species. RDTs usually give results in about 20 minutes. It is a good alternative to microscopy in the absence of reliable microscopic diagnosis. However, RDT might not detect infections, if there are not enough malaria parasites in the patient’s blood. In that case, a negative RDT result can be followed by microscopy. If a patient with positive RDT result is not responding to treatment, the Plasmodium species should be diagnosed with microscopy for appropriate medication. Diagnosis of malaria can be challenging for many reasons. Many health workers in developing countries are insufficiently trained and supervised. The microscopes and reagents might be of poor quality and the supply of electricity might be unreliable. Storage of blood samples and delayed testing until a qualified person is available to perform the microscopy, results in incorrect diagnosis. Many malaria endemic communities do not have the proper diagnostic tools such as microscopes and RDT kits. 10.3.4 Treatment Malaria can lead to deaths in severe manifestations, though most deaths occur in rural areas. Quick progression from illness to death can be prevented 18 by fast and effective medication. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) When treating a malaria patient, the following should be taken into account: a) Age and weight of the person to give the correct amount of medication. b) Drug allergies or other medications taken by the patient. c) Place of Infection and Plasmodium species responsible for infection to decide the drug. Species, such as P. falciparum and P. vivax have become resistant (in some areas) to many antimalarial drugs. For example, chloroquine-resistant strain of P. falciparum has spread to most endemic areas. Listed below are some drugs that are usually recommended by national malaria control programmes. They might not be effective in many parts of the world due to drug-resistant strains. Artemisinin-containing combination treatments (for example, artemether- lumefantrine, Artesunate-amodiaquine) Atovaquone-proguanil Chloroquine Doxycycline Mefloquine Quinine Sulfadoxine-pyrimethamine. Primaquine, is used as an adjunct drug against certain Plasmodium species. It is active against the dormant liver forms, which is rare/nonexistent in P. falciparum. Primaquine is not recommended for people who are deficient in glucose-6-phosphate dehydrogenase enzyme or for pregnant women. Patients who have uncomplicated malaria can take treatment from a nearby hospital and rest at home. In emergency cases, rectal artesunate drug can be given as a first line treatment (if they cannot be treated orally). Patients with severe malaria can be hospitalized for many days. Treating all people simultaneously in a population, though can prevent major malaria epidemics, but unfortunately it can also increase drug resistance in the parasite and cause complications in those who are deficient in glucose-6-phosphate dehydrogenase. Humans living in areas where malaria is common can become partially immune. Travellers, young children, women having their first or second pregnancy and those who have weak immunity due to other diseases (such as AIDS) have little to no immunity against malaria. In malaria endemic areas, pregnant women are recommended to eat iron and folate supplements to prevent anemia, get a curative dose of an antimalarial drug at least twice during pregnancy (starting from the second trimester) and sleep under an insecticide-treated bed net (ITNs). 19 Block 3 Medically Important Insect Vectors-II SAQ 2 Read the following sentences and write True (T) or False (F). i) Malaria is a disease caused by Anopheles mosquito. ii) The vector for malaria is Culex species. iii) The infection of malaria starts when a female mosquito injects sporozoites of Plasmodium sp. present in her saliva into a human skin. iv) Plasmodium cannot complete its life cycle at temperature below 20°C. v) During pregnancy malaria can lead to premature baby delivery. vi) Most malarial deaths occur in urban areas. 10.4 PREVENTION AND CONTROL The best way to prevent malaria is to avoid mosquito bites. The mosquitoes that carry the pathogen usually bite during early hours of night. Anopheles mosquito can be controlled at the larval as well as adult stage. 10.4.1 Traditional Methods of Control Mosquito larvae can be eliminated before they reach adulthood. Rainfall forms water puddles where mosquitoes lay their eggs which hatch into aquatic larvae and they develop into adults in a few days. Draining or removal of small puddles can reduce the Anopheles population. Chemical insecticides can also be applied in the water but might harm the environment and other organisms living in that habitat. Other measures which can be taken against larvae are: Insect growth regulators (IGRs) Cover or spray the water surface with oil, such as kerosene, petroleum etc. which suffocates the aquatic larvae Use larvicides, such as temephos in domestic water Introduce larvivorous fishes, like Gambusia; naiads of dragonfly and mayfly; and cyclopoid copepods in water bodies which feed on the mosquito larvae Add toxins from the bacterium Bacillus thuringiensis var. israelensis (Bti), or Bacillus sphaericus in water Additional personal protection and other methods used against mosquito adults include: Use glass windows (a well-constructed house). Clear off bushes and shrubs where mosquitoes can breed. Fumigate the houses and neighbouring areas with insecticides, such as 20 pyethroids, malathion, etc. to kill the mosquito adults. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) Sleep under a mosquito net/insecticide treated mosquito net. Use mosquito repellent on exposed skin. Kill adults by spraying insecticides inside the human dwellings. Use mosquito mats, coils and anti-mosquito cream to repel the mosquitoes from biting. Cover exposed parts of the body with thick and white or light- colouredclothing. 10.4.2 Modern Approaches in Control (a) Bed-Nets Insecticide-Treated Nets (ITNs): These are simple mosquito nets that are usually made of polyester but sometimes cotton, polyethylene, or polypropylene is used. These are treated with an insecticide, generally pyrethroids which have low health risks to humans but are toxic to mosquitoes at low doses. Pyrethroids do not rapidly wear off, unless exposed to sunlight or washed. These nets require 're-dipping' to restore the insecticide every 6-12 months. Re-dipping involves soaking the nets in a bucket of insecticide solution and then hanging them up to dry. Simple mosquito nets: These are nets without any insecticide treatment. They do not kill but prevent mosquito bite and local transmission. LLINs: Long lasting insecticide nets (LLINs): These are used in malaria- endemic areas for mosquito control and prevention of bites. The insecticide is cleverly bound within the fibres that make up the netting and is 'slowly released' over a 4-5 years period. Hence, it is called ‘long lasting’. Insecticide-treated nets, therefore, provide two levels of protection. First, as a mechanical barrier against the bites of malaria-carrying mosquitoes and second, as a means of killing mosquitoes on contact with the insecticide. The mosquitoes are drawn to the nets by the odour of the person sleeping beneath it and are killed as soon as they land on the nets. The ‘knock down’ or killing of the mosquito protects the person sleeping under the net. Though, nets can develop small holes over time yet even with a few holes, they still remain 90 to 95% effective, and as and when the mosquito lands on the net it is knocked down and killed (Fig. 10.12). Fig. 10.12: Long lasting Insecticide treated Nets. 21 Block 3 Medically Important Insect Vectors-II These nets are safe for children as the quantity of insecticide, a child might ingest by licking their hands after touching the net, is small enough not to cause any harm. The small amount that is transferred to the tiny mosquito, however, is enough to kill it. Modified Nets: Some of Anophelines are developing resistance to LLINs, therefore, new combination nets are now tested for use. Piperonylbutoxide (PBO) (a synergist) + Variant LLINs is one such net targeting pyrethroid- resistant vector populations. (b) Indoor Residual Spray (IRS) Indoor residual spray refers to the deposition of insecticide on the wall of house. Most of the insecticides having residual effect are sprayed indoors. When mosquitoes after taking blood meal rest in the house, they pick up sufficient insecticide particles sprayed on the walls and other indoor surfaces of the house. As a result, the longevity of mosquito reduces so much that it does not survive. In areas where the vectors are strongly endophilic, i.e. they tend to rest indoors, IRS of human dwellings can give very effective control. Vectors that are exophilic, i.e. they tend to rest outdoor but tend to feed or rest indoors briefly, can also be effectively controlled by indoor residual spraying. In areas where vectors are strongly exophilic and/or exophagic, i.e. they rest and bite outdoors, other control methods, such as use of insecticide-treated mosquito nets or exterior space spraying (for emergency control), are practiced. Table 10.1 presents a few insecticides, their formulations and dosages used during spray. Different types of sprayers are used depending upon the area to be sprayed and chemical used. Nowadays, new type of pumps with control flow valve are being used with improved tip of spray nozzle to avoid choking and erosion (Fig. 10.13). Table 10.1: Insecticide formulations and dosages for IRS (https://nvbdcp.gov.in/). S. Name of Insecticide Preparation of Dosage per Residual Area to be covered No. suspension in sq. metre of effect in by 10 L of water active weeks suspension to get ingredient correct dosage 1. DDT 50% WP 1 kg/10 L 1g 10-12 500 sqm 2. Malathion 25% WP 2 kg/10 L 2g 6-8 250 sqm 3. Deltamethrin 2.5% WP 400 g/10 L 20 mg 10-12 500 sqm 4. Cyfluthrin 10% WP 125 g/10 L 25 mg 10-12 500 sqm 5. Lambda-cyhalothrin 125 g/10 L 25 mg 10-12 500 sqm 10% WP 6 Alpha-cypermethrin 5% 250g/10 L 25 mg 10-12 500 sqm WP 7. Bifenthrin 10% WP 125g/10 L 25 mg 10-12 500 sqm 22 Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) (c) Attractive Toxic Sugar Baits (ATSB) Attractive-toxic sugar baits (ATSBs) are considered a new vector control paradigm based on “attract and kill” approach. The concept is based on the fact that mosquitoes use plant sugars as an energy source; and the successful feeding helps in high survival rate and reproductive fitness of mosquito. The sugar feeding behavior of mosquito is tapped to formulate ATSBs by combining a concentrated sugar-based food source, an olfaction stimulant and a systemic insecticide.Vectors searching for natural sugar sources are diverted and attracted to baits. Vectors ingest a toxin orally as they feed on and are killed. (d) Biological Control The biological control focuses on the use of agents that act as pathogens or parasites impacting their survival or as predators and kill them. For example, larvivorous fishes, like Gambusia and Poecilia; naiads of dragonfly and mayfly; and cyclopoid copepods are used in water bodies. Different bacterial pathogens, such as Bacillus thuringiensis var. israelensis (Bti), or Bacillus sphaericus are also used to kill larvae. Other biological agents are Mermitid Nematod (Romanomermis culicivorax), Notonectid bug, Ambylospora (Protozoa), Coelomomyces (Fungus) and NPV (Nuclear Polyhedrosis Virus). Wolbachia, an intracellular bacterium, has been used to reduce mosquito fitness. Infected females can pass the bacteria on to their offspring in their Fig. 10.13: Improved IRS sprayer and eggs, and thus the maternally inherited endosymbiont infects the next Nozzle with control generation. The uninfected females when mate with Wolbachia-infected males flow valve. produce no progeny (Cytoplasmic incompatibility) (Fig. 10.14). Also, mosquitoes develop refractoriness to Plasmodium. However, if females are infected with Wolbachia, whether they mate with infected or uninfected males, there is no reduction in the number of offspring. Infected females thus have a strong selective advantage. Fig.10.14: Mosquito control with Wolbachia. 23 Block 3 Medically Important Insect Vectors-II Until recently it was thought that Wolbachia did not naturally infect An. gambiae, however, populations of An. coluzzi and An. gambiae, naturally infected with a strain of Wolbachia, have been discovered in Burkina Faso, West Africa. The exciting prospect of using a Wolbachia-based strategy to control malaria transmission by anopheline mosquitoes attracted the attention of vector biologists. However, the creation of stably transfected populations of these mosquitoes has proved technically difficult. But once established in Anophelines it can be of great productivity. Wolbachia trials are going on in India as a novel control approach. (e) Genetic Approaches Mosquitoes can be genetically modified to impair their ability to transmit the malaria parasite. Combined SIT and IIT: The sterile insect technique (SIT), incompatible insect technique (IIT) or a combination of the two could be used to suppress mosquito populations. Male mosquitoes are sterilised either by the application of irradiation (SIT) or infected with a bacterium, Wolbachia (IIT), or both. These are then released into the target population. These compete with the fertile males to mate with females and result in suppression of next generation (Fig. 10.15). Fig. 10.15: Combined Male Sterilization Technique using Radiation and Wolbachia. CRISPR/Cas – mediated Control: The recently developed CRISPR/Cas9- based genome editing tools for Anopheles mosquitoes provide new and promising opportunities for developing malaria control strategies through gene 24 deletion to achieve complete agonist inactivation. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) For example, gene editing can disrupt male-determining gene in the mosquitoes resulting in the production of males with feminized genitalia. Gene inactivation, obtained by CRISPR/Cas9 genome editing can reduce susceptibility to Plasmodium and mosquito fitness. Other measures: Apart from these, introduction of chromosomal translocations and aberrations, such as deletion, inversion, etc.; and use of cytoplasmic incompatibility for producing hybrid sterilty have been used as control measures in many Anopheles sp. (f) RS & GIS Remote sensing (RS) and Geographical Information system (GIS) are the tools initiated with satellite launching and advancement in Computer technology.These are now used to get information of large to very small areas as per the need and the geographical attributes which are confounding to malaria.. This helps to map the malaria transmission by mapping of breeding site and geo-spatial distribution of malaria using satellite information /image. GIS software combines ground collected data on mosquito-borne diseases and population epidemiology with the geospatial data through which mapping of risk areas and needs for control measures can be enumerated. Risk assessments/predictions of diseases can be done which helps to plan and formulate control strategies. Change in land use pattern and climates can be assessed that can impact disease transmission. Intensive control activities to reach the elimination target. Regular surveillance of vectors for changes in prevalence of vector species and their behavioral aspects. Regular monitoring of insecticide resistance. Identify the knowledge gap and generate data to fill it. Test new tools for their efficacy and their suitability in different ecosystems the vector species are occupying. SAQ 3 Match Column A and Column B. Column A Column B i) Long lasting Insecticide nets (LLINs) a) Parasitizes or kills larvae ii) Indoor Residual spray (IRS) b) CRISPR/Cas-9 iii) Attractants c) Mapping of breeding site iv) Biological Control d) Searching for natural sugar sources get diverted 25 Block 3 Medically Important Insect Vectors-II v) Genetic Approaches e) Deposition of insecticide on the wall of the house vi) Remote sensing of Geographical f) Insecticide is bound with Information System the fibres 10.5 SUMMARY Let us summarise what we have learnt so far: Human malaria is transmitted by female of the genus Anopheles. It has 460 species, of which 30-40 species are reported to transmit pathogen, Plasmodium species. Anophelines comprise four stages in their life cycle: egg, larva, pupa and adult. The first three stages are aquatic and last for 7-14 days, depending on the species and the ambient temperature. The adult female Anopheles mosquito acts as vector for malaria. It can live upto a month (or more in captivity) but generally does not live more than 1-2 weeks in nature. Malaria is a mosquito-borne disease caused by a Plasmodium parasite. Symptoms of the disease consists of fever, chills and flu-like illness. If left untreated, humans may develop severe complications and die. Anopheles mosquitoes feed during the night. Thus, use of simple bed nets or pyrethroid-treated bed nets prevent their bites. There are several traditional and modern approaches used in the control of mosquitoes. These include use of biological and microbial agents, chemical insecticides, net screens, gene editing, cytoplasmic incompatibility, SIT, sugar baits, remote sensing, etc. 10.6 TERMINAL QUESTIONS 1. Describe the life cycle of Anopheles mosquito. 2. Explain the epidemiology of malarial parasite. 3. Discuss the preventive and control measures of Anopheles mosquito. 10.7 ANSWERS Self Assessments Questions 1. i) Water, ii) Spiracles, iii) Comma, iv) Nectar, v) 5 days, vi) 9 Km, vii) Zoophillic. 2. i) F, ii) F, iii) T, iv) T, v) T, vi) F. 26 3. i) f, ii) e, iii) d, iv) a, v) b, vi) c. Unit 10 Dipterans as Disease Vectors-I (Anopheles Mosquitoes) Terminal Questions 1. Refer to Section 10.2. 2. Refer to Section 10.3. 3. Refer to Section 10.4. 27 Block 3 Medically Important Insect Vectors-II UNIT 11 DIPTERANS AS DISEASE VECTORSII (CULEX VECTORS MOSQUITOES) Structure 11.1 Introduction National Filaria Control Programme (NFCP) Objectives 11.2 Life Cycle of Culex Elimination of Lymphatic Filariasis (ELF) Adult 11.6 Japanese Encephalitis (JE) Eggs Pathogenicity Larvae Diagnosis Pupae Treatment 11.3 Behavior 11.7 Prevention and Control 11.4 Culex-borne Diseases Anti-Larval Measures 11.5 Filariasis Anti-Adult Measures Epidemiology Protection against Mosquito Causative Agent Bites Life Cycle of the Parasite – 11.8 Summary Wuchereria Bancrofti 11.9 Terminal Questions Pathogenicity 11.10 Answers Diagnosis Treatment 11.1 INTRODUCTION In the previous unit you have studied about the mosquito-borne diseases transmitted by Anopheles species. The present unit is focused on the diseases (pathogens) transmitted by another mosquito species, Culex. The distribution of Culex ranges from the tropics to cool temperate regions, but does not extend to the extreme northern latitudes where only Aedes and Ochlerotatus 28 mosquito species are found. Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) Culex sp. are frequently referred to as “nuisance mosquitoes”. In India, Culex quinquefasciatus is the most commonly found domestic species which inhabits closer to the human population. Other prevalent species include Cx. pipiens, Cx. fatigans, Cx. tritaeniorhynchus, Cx. tarsalis and Cx. gelidus. Diseases (pathogens) carried by one or more species of Culex mosquitoes vary in their dependence on the species of vector. Some are rarely and only incidentally transmitted by Culex species; but Culex and closely related genera of culicine mosquitoes, if established in a particular region, readily support perennial epidemics of certain major diseases. Arbovirus infections transmitted by various species of Culex include West Nile virus, Japanese Encephalitis (JE), St. Louis encephalitis, and, Western and Eastern equine encephalitis. It is believed that Culex species might have role in Zika virus transmission. Nematode infection, filariasis, is transmitted by Cx. quiquefasciatus in India, though in other countries it can spread by other mosquitoes and blood-sucking flies. Although the Culex mosquito is not a primary vector for prevalent mosquito-borne diseases such as malaria, dengue and Chikungunya, the illnesses transmitted by them can also pose serious health threats to human beings. Objectives After you have read this unit you should able to:  explain the life cycle of Culex mosquito,  explain the diseases transmitted by Culex species, recognize their symptoms and suggest the remedial measures,  discuss the disease epidemiology and affecting factors, and  describe the various preventive and control measures of Culex mosquitoes. 11.2 LIFE CYCLE OF CULEX Mosquitoes are holometabolous insects. Life cycle of all mosquitoes are alike and passes through four stages, i.e. egg, larva, pupa, and adult. The first three stages are aquatic and last for 7-10 days, depending on the species and the ambient temperature. Culex adults can mate only in the darkness. They form swarms after sunset which is a natural mating behavior. The sound produced by a female mosquito attracts the flying males and copulation occurs during flight. After mating, the female mosquito selects some stagnant water of low depth for laying eggs. 11.2.1 Adult (Fig. 11.1) Fig. 11.1: Culex mosquito. Culex was described by Linnaeus in 1758.It is a brown-coloured mosquito, the size of which ranges from 4–10 mm (0.2–0.4 in). Adults are usually unicolor, but some species of the subgenus Culex have wings without scales, markings on the legs and pale spots on the wings similar to Anopheles. 29 Block 3 Medically Important Insect Vectors-II Culex adults are characterized by the presence of distinct pulvilli and the absence of pre-spiracular setae and post-spiracular setae. The adults sit parallel to the surface unlike Anopheles which sits at an angle of 45°. They have long hairy antennae, short maxillary palps and long proboscis. The proboscis, thorax and wings are darker than the body. Tip of the abdomen is blunt and covered with broad scales. Males can be distinguished from the females in having feathery antennae and large palps. 11.2.2 Eggs (Fig. 11.2) Adult female lays 150-300 eggs per oviposition. Eggs are laid on the surface of fresh or stagnant water. Water sources include puddles, barrels, horse troughs, ditches, ornamental ponds, unmaintained swimming pools and marshy areas. Eggs are held together by adhesion forces and form a raft. It helps the eggs to float on the surface of water by trapping air bubbles. The eggs are brownish in colour and cigar-shaped, tapering at one end. Eggs are normally laid at night; Fig. 11.2: Egg raft of they hatch after 24-48 hours into free swimming larvae. the Culex mosquito 11.2.3 Larvae (Fig. 11.3) Culex larva is also called as wriggler because it exhibits a wriggling movement in water. The body is divided into head, thorax and abdomen. Head bears a pair of antennae, a pair of compound eyes, a pair of simple eyes and mouth parts bearing mandibles and maxillae. Feeding brushes are present around the mouth. Thorax is large, unsegmented and abdomen is 9-segmented. Abdomen is without palmate hairs unlike Anopheles. Eighth abdominal segment bears a long tubular respiratory siphon at the tip of which lies a spiracle for aerial respiration while four tracheal gills are located in the 9th segment for aquatic Fig. 11.3: Larva of respiration. Larva is bottom feeder but wriggles up frequently for taking Culex hanging at an atmospheric oxygen through respiratory siphon. At rest, larva hangs at an angle from the water angle to the surface of water with its head downwards. surface (Peter J. The larvae spend most of their time feeding on algae, bacteria, and other Bryant). microorganisms. They occur in a wide range of habitats but most species prefer polluted water or water with high biological waste and open drains. The species, Cx. tritaeniorhynchus, prefers to breed in rice fields, grassy ditches, the edges of streams and rivers, and small, temporary rain pools. 11.2.4 Pupae (Fig. 11.4) Culex pupa is comma-shaped and non-feeding stage. It swims actively and also called as tumbler. The head and thorax are merged into a cephalothorax and the abdomen is curved giving it the characteristic comma shape. The abdomen is long and 9-segmented. The abdominal 9th segment bears a pair of paddles for swimming and a pair of tracheal gills for aquatic respiration. Palmate hairs are present on all abdominal segments. A pair of large respiratory trumpets for aerial respiration is present on cephalothorax. Inside the pupal case, developing adult structures can be observed. Within 2-7 days as a pupa, the dorsal surface of the cephalothorax 30 splits and the adult mosquito emerges. Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) Fig. 11.4: Pupa of Culex (Source: Michelle Cutwa-Francis, University of Florida). Fig. 11.5: Life cycle of Culex mosquito. The duration from egg to adult varies considerably among species and is strongly influenced by ambient temperature and moisture conditions. Mosquitoes can develop from egg to adult in as little as 7 days but usually take 10-14 days in tropical conditions (Fig. 11.5). 11.3 BEHAVIOR Behavior of mosquito is highly significant to assess its pest status. It comprises the swarming, biting, flight and egg laying habits. Swarming: Among various species of mosquitoes, Culex mosquitoes form large swarms for mating in the evening that are visible from the very far off distances. This swarming behavior is a kind of mating behavior of Culex mosquito which is performed during the flight. Biting: Most of the Culex species are zoophilic in nature (like to bite animals). They enter the house at dusk and reach maximum density by midnight. These nocturnal insects are highly troublesome and irritating ones. Legs, particularly below the knee are the preferred biting sites. 31 Block 3 Medically Important Insect Vectors-II During day, adults rest in the darker regions of their habitat. Both males and females feed upon the plant juices, though females also bite vertebrates and suck the blood once every 2 to 3 days for the development of their eggs. Flight: The Culex has a strong pair of wings. They can fly up to eleven kilometers while sometimes the flight may be limited due to easy availability of host. They can also migrate to very long distance as the air drag may take them very far off places. Oviposition (egg laying): After three days of mating and blood feeding,adult females lay eggs for which they search safe water bodies containing high amounts of biological waste or nutrients (Fig. 11.6). They, then lay eggs in the form of a boat-shaped raft of 150-300 eggs. Culex are the only mosquitoes which lay eggs in rafts. A female can lay 5-6 egg rafts in her life time. Mostly, the eggs are laid in the night when the water surface is least disturbed. (a) Breeding sites of Culex tritaeniorhynchus, Culex vishnui and Culex pseudovishnui (b) Dirty stagnated water breeding site for Culex quinquefasciatus. 32 Fig. 11.6: Breeding Sites of Culex mosquitoes. Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) SAQ 1 Read the following question and tick (√) mark the correct option. i) Egg laying habit of mosquitoes is known as a) Ovitrap b) Oviposition c) Ova-positioning d) None of the above ii) At rest, Culex larvae a) remain horizontal to the surface of water. b) remain at the bottom of water container. c) hang at an angle with the surface of water. d) hang out side around the water container. 11.4 CULEX-BORNE DISEASES Culex is a vector of several diseases prevalent in human beings and vertebrates (Table 11.1). In context with the Indian perspective, Culex species transmit primarily two diseases, Filariasis and Japanese encephalitis. Let us study about the transmission and symptoms of Culex-borne diseases. Table 11.1: Diseases spread by Culex species. Vector Species Disease Pathogen Culex pipiens Filariasis Wuchereria bancrofti Cx. quinquefasciatus Cx. tarsalis, Cx. quinquefasciatus, West Nile Virus (WNV) Alphavirus Cx. stigmatosoma, Cx. thriambus, Cx. pipiens Cx. tarsalis, Cx. pipiens complex, St. Louis Encephalitis Flavivirus Cx. quinquefasciatus (SLE) Cx tritaeniorhynchus, Cx. vishnui Japanese Encephalitis Flavivirus and Cx. pseudovishnui (JE) 11.5 FILARIASIS Filariasis has been a major public health problem in India, only next to malaria. The disease was recorded in India as early as 6th century B.C. by the famous Indian physician, Susruta in his book Susruta Samhita. In 7th century A.D., 33 Block 3 Medically Important Insect Vectors-II Madhavakara described signs and symptoms of the disease in his treatise 'Madhava Nidhana' which holds good even today. In 1709, Clarke called elephantoid legs in Cochin as Malabar legs. 11.5.1 Epidemiology Filariasis is confined to tropical and sub-tropical regions and prevalent mainly in developing countries. It affects over 120 million people in 73 countries including India, West Indies, Southern China, Japan, South America, West and Central Africa, Pacific Islands and Korea. It is almost absent from Europe, North America and polar regions. Lymphatic filariasis is often associated with urbanization, industrialization, illiteracy, poverty and poor sanitation. Migration of people has not only favored the spread of filariasis in new areas but also led to the extension of filariasis into areas where filariasis was not so prevalent. Climate is an important factor in the epidemiology of filariasis. Regions which are damp and moist and have stagnant water round the year provide a good breeding ground for the mosquitoes. In India, it is chiefly distributed in the Southern area especially along the sea coast affecting more than hundred million people. Indigenous cases have been reported from about 256 districts in 21 states/Union Territories. These are Andhra Pradesh, Assam, Bihar, Chhattisgarh, Goa, Jharkhand, Karnataka, Gujarat, Kerala, Madhya Pradesh, Maharashtra, Orissa, Tamil Nadu, Telangana, Uttar Pradesh, West Bengal, Pondicherry, Andaman & Nicobar Islands, Daman & Diu, Dadra & Nagar Haveli and Lakshadweep. The North-Western States/UTs namely Jammu & Kashmir, Himachal Pradesh, Punjab, Haryana, Chandigarh, Rajasthan, Delhi and Uttaranchal and; North- Eastern States namely Sikkim, Arunachal Pradesh, Nagaland, Meghalaya, Mizoram, Manipur and Tripura are known to be free from indigenously acquired filarial infection. 11.5.2 Causative Agent Three nematode parasites which cause lymphatic filariasis (LF) in human are Wuchereria bancrofti, Brugia malayi and Brugia timori. Of these, only Wuchereria bancrofti and Brugia malayi are found in India. In mainland India, Wuchereria bancrofti, transmitted by the ubiquitous vector, Culex quinquefasciatus, has been the predominant infection contributing to 99.4% of the problem in the country. The infection is prevalent in both urban and rural areas. The worm is not found in any other mammal and thus, is not a zoonotic disease. The worm is mainly found where Culex mosquitoes are found; in and around temporary pools or standing water. Brugia malayi infection is primarily spread by Mansonia (Mansonioides) annulifera and M. (M). uniformis. The disease has been reported earlier from some rural areas in seven states viz., Kerala, Orissa, Tamil Nadu, Andhra Pradesh, Madhya Pradesh, Assam and West Bengal. However, its prevalence is now reportedly restricted to rural areas of Kerala and the infection 34 disappeared in some pockets in other states. Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) 11.5.3 Life Cycle of the Parasite – Wuchereria Bancrofti Wuchereria bancrofti is a pseudocoelomate and cylindrical nematode. The body is covered with a thick cuticle. Man is the definitive host i.e. where the mature adult male and female parasites mate and produce microfilariae whereas the mosquito is the intermediate host (Fig. 11.7). The adult parasite worms, male and female, live in the lymph vessels and lymph nodes by making nest in the dilated lymphatics, particularly in the groin regions. The adult worms survive for about 5-8 years and sometimes for as long as 15 years. The discovery of Adults are elongated, hair-like, cylindrical worms with a curved body. Both the microfilariae (mf) in ends of worm are tapering with head end terminating in a slightly round the peripheral blood swelling. They are often creamy-white in colour, though sometimes they look was made first by transparent. Generally, males and females live coiled together and it is very Lewis in 1872 in difficult to separate them. Copulation between male and female adults takes Calcutta (Kolkata). place in the lymph glands of man. The females of Wuchereria bancrifti are ovo-viviparous. They liberate numerous, as many as 50,000, active embryos, called juveniles or microfilariae. Microfilariae are covered with a transparent sheath which is larger than the size of their body. The sheathed microfilariae begin to appear in the blood circulation in six months to one year after infection (prepatent period). They first enter the main lymphatic trunks from where they find their way into the circulating blood. They move with the blood stream and ultimately migrate to reside in the deeper blood vessel. The sexual cycle of the parasite takes place in the human host, where the adult worms ultimately die. The life cycle of the parasite is cyclo- developmental in the vector where the parasites do not multiply. The microfilariae need a lower temperature for their development. Thus, they need mosquito for their development. If they are not sucked by mosquitoes, they die in human body. The average life span of microfilariae in human body is approximately 70 days. You are aware that Culex is a nocturnal feeder, thus, to be sucked by them microfilariae exhibit nocturnal periodicity which coincides with the biting activity of the vector. During day time they reside in the large and deeper blood vessels of various organs, such as lungs, kidneys, heart and large arteries. However, during night, they appear in the peripheral blood vessels, especially between 10 pm to 4 am, to be sucked by Culex. Microfilariae, (when picked up by the mosquito during blood meal) undergo development in mosquitoes to form infective larvae which usually takes about 10 to 14 days. The ingested microfilariae first shed their sheaths and penetrate the stomach wall. They then migrate to the muscles of the thorax and develop there without multiplication. The slender and tiny microfilariae (124 μ - 290 μ long) transform into immobile and inactive sausage-shaped larva (L1). It is thick and has a spiky tail formed by the cuticle. The larva grows rapidly in length and breadth after their first moult to become L2 or pre-infective larva (225 μ - 330 μ long), which is recognized by the presence of one or two papillae at its caudal end and a short tail. The L2 stage metamorphose into L3 which is highly motile and infective. It is slender and thread like, measuring about 1500-2000 microns in length. 35 Block 3 Medically Important Insect Vectors-II When the infective mosquito feeds on another human host, the infective larvae are deposited at the site of mosquito bite. The larvae attracted by the warmth of human body invade the skin either through the puncture or on their own and enter the lymphatic system. In the human host, the infective larvae undergo two moults and develop into adult male and female worms. The adult worms survive for about 5–8 years or sometimes as long as 15 years or more. Fig. 11.7: Life cycle of Filarial worm,Wuchereria bancrofti 11.5.4 Pathogenicity The pathogenic effects in the human beings are caused by mainly the adults of Wuchereria bancrofti. The disease is commonly called Wuchereriasis, Bancroft’s Filariasis or Classical Filariasis. The disease is generally asymptomatic showing no external signs of infection. However, these cause the damage to the lymphatic system, kidneys and the immune system of the body. In case the infection is severe, the host shows various pathological symptoms. The microfilariae though circulate in the blood are not known to cause any pathogenic effects, except in severe cases. The disease caused by microfilariae is known as Occult Filariasis. Various pathogenic effects caused by filarial worm are: a) Inflammation of the lymphatic system – lymphangitis. Inflamed areas are extremely painful and appear as cord-like swellings. b) Inflammation of the lymph nodes – lymphadenitis. It is accompanied by 36 high fever for 3-5 days and increase in neutrophils Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) c) Accumulation of fluid in the serous membrane surrounding the testes – Hydrocoele. It is less common in India. d) Excretion of milky-white urine due to the presence of chyle (milky fluid of small intestine) – Chyluria. It contains fat particles, albumin and fibrinogen along with microfilariae and RBCs. e) Blockage of the lymph flows in the body due to their accumulation in the lymph nodes – Elephantiasis. The affected part of the body becomes edematous and enlarge enormously becoming solid like a tumour. The surface of the skin becomes rough, dry, thick and papillomatus. Wuchereria bancrofti can affect the legs, arms, vulva, breasts, and scrotum. 11.5.5 Diagnosis Individuals infected by filarial worms may be described as either “microfilaraemic” or “amicrofilaraemic”, depending on whether microfilariae can be found in their peripheral blood. Filariasis is diagnosed in microfilaraemic cases primarily through direct observation of microfilariae in the peripheral blood. Occult filariasis is diagnosed in amicrofilaraemic cases based on clinical observations and, in some cases, by finding a circulating antigen in the blood. Those who develop the chronic stages of elephantiasis are usually free from microfilariae (amicrofilaraemic), and often have adverse immunological reactions to the microfilariae, as well as the adult worms (Source NVBDCP). 11.5.6 Treatment Administration of single dose of anti-filarial drugs to the entire community (mass drug administration), yearly once for 5-6years. Diethylcarbamazine (DEC) and albendazole for mass drug administration. Other drugs used –Ivermectin, Benzimidazole, Levamisole, Piperazine, Mebendazole, Imidazole. 11.5.7 National Filaria Control Programme (NFCP) The National Filaria Control Programme (NFCP) was launched in the country in 1955 after pilot project conducted in Orissa from 1949 to 1954. The objective of NFCP are to delimit the problem, to undertake control measures in endemic areas and to train personnel to man the programme. The main control measures were mass administration of Diethylcarbamazine (DEC), antilarval measures in urban areas and indoor residual spray in rural areas. The NFCP strategy was based on use of recurrent anti-measures at weekly intervals. These included: (a) Environmental methods; such as source reduction by filling ditches, pits, low lying areas, de-weeding, desilting, etc. (b) Biological control of mosquito breeding through larvivorous fish. (c) Anti-parasitic measures through 'detection' and 'treatment' of microfilaria carriers and diseased person with DEC, by Filaria Clinics in towns covered under the programme (source NVBDCP). 37 Block 3 Medically Important Insect Vectors-II 11.5.8 Elimination of Lymphatic Filariasis (ELF) In 1997, The World Health Assembly adopted resolution, (WHA 50.29), for the Elimination of Lymphatic Filariasis as a global public health problem by 2020.In 2002, National Health Policy set a goal for ELF in India by 2015, which was extended to 2017. In 2004, ELF covered 202 endemic districts in 20 States/UTs. Subsequently it was scaled up to cover all the 256 endemic districts (21 States/UTs) targeting a population of about 650 million. The twin pillars of LF elimination strategy include: Transmission control: To prevent occurrence of new infections by annual MDA with DEC and Albendazole for 5 years or more to population at risk except children below 2 yrs, pregnant women and seriously ill persons. Disability prevention and morbidity management for those who already have the disease: Home based management –limb hygiene for lymphoedema and Hospital based management – surgical correction for hydrocele. JE virus is transmitted 11.6 JAPANESE ENCEPHALITIS (JE) between mosquitoes, in particular, Culex Japanese Encephalitis is a viral disease (Fig. 11.8) transmitted by infective tritaeniorhynchus and bites of female Culex tritaeniorhynchus, Culex vishnui and Culex animals such as pigs. pseudovishnui. However, some other mosquito species also play a role in Humans are incidental or ‘dead- transmission under specific conditions. It is primarily zoonotic in its natural end’ hosts, because cycle and man is an accidental host. they usually do not Natural hosts of JE virus include water birds of Ardeidae family (mainly pond develop high enough herons and cattle egrets). Pigs play an important role in the natural cycle and concentrations of the virus in their serve as an amplifier host since they allow manifold virus multiplication without bloodstreams to infect suffering from disease and maintain prolonged viraemia.Due to prolonged feeding mosquitoes. viraemia, mosquitoes get opportunity to pick up infection from pigs easily and transmit to human beings.Man is a dead end in transmission cycle due to low and short-lived viraemia. Mosquitoes do not get infection from JE patient. 38 Fig. 11.8: JE cycle in human as well as zoonotic. Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) JE virus is neurotropic arbovirus and primarily affects central nervous system. The infection leads to classical symptoms similar to any other virus causing encephalitis. Infections of JE virus are widespread in India. First evidence of the presence of JE virus dates back to 1955 when first case was confirmed. In India, annual incidence of JE ranges between 1714 and 6594 causing fatalities between 367 and 1665 (NVBDCP). 11.6.1 Pathogenicity JE virus infection may result in febrile illness of variable severity. In children, gastrointestinal pain and vomiting may be the dominant initial symptoms. Severe disease is characterized by rapid onset of high fever, stupor, disorientation, coma, tremors, spastic paralysis, hypertonia, loss of coordination, focal neurological deficit etc. Prodromal stage (the period after incubation and before the characteristic symptoms of infection occur) may be abrupt (1-6 hours), acute (6-24 hours) or more commonly subacute (2-5 days).The disease usually lasts for a week but may prolong due to complications. Amongst patients who survive, some are fully recovered through steady improvement, however some suffer with permanent intellectual, behavioral or neurological sequelae such as paralysis, recurrent seizures or the inability to speak. 11.6.2 Diagnosis Clinically it is difficult to differentiate between JE and other viral encephalitis as JE virus infection presents classical symptoms similar to any other virus causing encephalitis. A laboratory test is required to confirm JE infection and to rule out other causes of encephalitis. Testing for JE-specific IgM antibody in a single sample of cerebrospinal fluid (CSF) or serum by Enzyme Linked Immuno-Sorbant Assay (ELISA) has been recommended by WHO. Other laboratory tests available for detection of JE virus include; Haemagglutination Inhibition Test (HI);Compliment Fixation Test (CF);Antigen Detection - RPHA, Indirect Florescence Antibody test, Immuno-peroxidase etc.; Genome Detection - RTPCR, etc. 11.6.3 Treatment JE vaccine is produced in limited quantities at the Central Research Institute, Kasauli (HP). Three doses of the vaccine provide immunity lasting for a few years. The vaccine is procured directly by the state health authorities. Vaccination is not recommended as an outbreak control measure as it takes at least one month after second dose to develop antibodies at protective levels and the outbreaks are usually short-lived. Ironically, there is no specific treatment of JE. Clinical management is supportive and in the acute phase. The treatment is directed at maintaining fluid and electrolyte balance in the body, and control of convulsions, if present. Maintenance of airway is crucial for the patient to recover. The state 39 Block 3 Medically Important Insect Vectors-II governments have been advised to make anticipatory preparations in the endemic districts for timely availability of medicines, equipment and accessories as well as sufficient number of trained medical, nursing and paramedical personnel. The Government of India supports training programmes (source NVBDCP). SAQ 2 Read the following question and tick mark (√) the correct answer. i) Filaria is caused by a a) Bacterium b) Nematode c) Protozoan d) Virus ii) Japanese Encephalitis is a a) Muscular disease b) Bacterial diseases c) Aedes-borne disease d) Neurotropic disease iii) What is NFCP? a. National Fertilizer Control Programme b. National Filarial Control Programme c. National Food Corporation of India d. National Fund for Control of Population iv) Filariasis is caused by a. Entamoeba histolytica b. Plasmodium vivax c. Wuchereria bancrofti d. Ascaris lumbricoides 11.7 PREVENTION AND CONTROL Culex-borne diseases can be controlled or prevented by keeping the vector population under control. The use of excessive insecticides to control mosquito population has not only caused development of resistance in the mosquitoes but has also increased the environmental pollution to a great 40 extent. Integrated vector management approach is, thus recommended for the Unit 11 Dipterans as Disease Vectors-II (Culex Mosquitoes) control of mosquito population which involves use of more than one method with a view to obtain maximum benefit. The various methods to control the Culex vector are as below. 11.7.1 Anti-Larval Measures This method deals with reducing mosquito population by removing the breeding ground and killing larva. A few measures are as follows: (a) Stagnant water bodies act as breeding ground for mosquito. By not allowing water to remain accumulated for long, breeding of mosquitoes can be prevented. (b) Application of kerosene, fuel oil, burnout diesel, mosquito larvicidal oil etc., in stagnant water once a week, prevents aerial respiration by mosquito larva without which they cannot survive. (c) In larger water bodies, like ponds, application of Paris green is recommended to prevent mosquito breeding. Paris green contains copper aceto arsenite which causes stomach poisoning in the larva. (d) Introduction of larvicidal fish such as Gambusia and Lebistes (Guppy) in water bodies acting as mosquito breeding ground is recommended as a measure to control mosquito population. These fishes feed upon mosquito larva voraciously and kill them. (e) Introduction of naiads(nymphs) of dragonfly and mayfly; and cyclopoid copepods in water bodies can also control mosquito larvae by feeding upon them. 11.7.2 Anti-Adult Measures This measure deals with killing of adult mosquitoes. It includes: (a) Use of mosquitocidal chemicals: Spraying of DDT@ 1+2 gm/Sq meter applied 1 to 3 times in a year or spraying of lindane, malathion etc. in a lesser dose can kill adult mosquitoes. (b) Application of space sprays and fumigants: Spraying or fumigation of the houses and neighbouring areas with insecticides, such as pyethroids, malathion, etc. can kill the mosquito adults. (c) Genetic control: Mosquitoes can also be controlled by genetic methods, such as sterile male technique, chromosomal translocation, sex distortion and gene replacement methods. However, these methods have limitations and are expensive. 11.7.3 Protection Against Mosquito Bites Infection occurs due to biting of parasite carrying mosquitoes. Protection against mosquito bite is the most effective measure to keep the disease under control. It can be achieved through: (a) Use of mosquito net with not less than 150 holes/sq inch. (b) Screening of building with copper and bronze gauze. (c) Use of mosquito coils, mosquito mats and repellent cream to keep the mosquito away. 41 Block 3 Medically Important Insect Vectors-II SAQ 3 Fill in the blanks : i) Culex-borne diseases can be controlled by killing …….……….. in their breeding grounds and ……………….. using space ………………. ii) Stagnant water bodies act as …………….., ……..…………. for mosquito. iii) Mosquitoes can be controlled genetically by …………..male technique and ………………… replacement. iv) Mosquito nets used against Culex mosquitoes should not have ………… holes per square inch. 11.8 SUMMARY Let us summarise what we have learnt so far: Culex adults are usually unicolour mosquitoes. Some species of the subgenus Culex have wings without scales, markings on the legs and pale spots on the wings similar to Anopheles. The Culex species is different from that of Anopheles and Aedes in appearance and habits. It is known to contribute to the spread of diseases; West Nile Virus, filariasis and encephalitis. Culex is a common mosquito found in natural as well as manmade environments. Culex breeds in wide variety of habitats; natural ponds, dirty water, sewage water, rice fields wells, etc. filled with water containing high biological content. Culex mosquito life cycle has four major stages, i.e. Eggs, larvae, Pupae & Adults.The length of life cycle can vary from 7-14 days in optimal conditions of water temperature at 25-27°C. Adult females lay 150-300 eggs per oviposition. Eggs are held together in the form of a raft which traps air bubbles to keep eggs floating on surface of water. At rest, larva lies at an angle to the surface of water. Abdomen lacks palmate hairs. It has long tubular respiratory siphon at the tip of which lies a spiracle for aerial respiration while four tracheal gills are located in the 9th segment for aquatic respiration.. The pupa is comma-shaped. The head and thorax are merged into a cephalothorax with the abdomen curving around underneath. In India, Culex species transmits mainly two diseases; Filariasis and Japanese Encephalitis.

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