MODULE-7.-Parasitic-Helminths.pdf

Page 1 of 37 Parasitology Chapter 7 PARASITIC HELMINTHS Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022...

Page 1 of 37 Parasitology Chapter 7 PARASITIC HELMINTHS Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 2 of 37 Chapter 6 PARASITIC HELMINTHS Introduction Helminth is a term literally means “worm”. They belong to two major groups of animal the Phylum Platyhelminthes (flatworms) and Nematoda (round worms). All helminths are multicellular eukaryotic invertebrates with round, tube-like or flattened bodies exhibiting bilateral symmetry. They are triploblastic (with endo-, meso- and ecto-dermal tissues) but the flatworms are acoelomate (do not have body cavities) while the roundworms are pseudocoelomate (with body cavities not enclosed by mesoderm). Helminths are some of the world's most common parasites. Helminths infect a range of hosts, including humans. Parasitic Helminths can infect every organ and organ system. Prevalent in the intestines, they are found in the liver, lungs, blood and occasionally the brain and other organs. Helminths are one of the leading causes of morbidity in the developing world with over two billion people are affected. This module introduced students in Clinical Parasitology of Parasitic Helminths. Students will learn more about the different classification and characteristics of Helminths. Pathogenicity, epidemiology and treatment some protozoal infection are also discussed in this chapter. Specific Objectives At the end of this chapter, the students should be able to: 1. Identify and describe different types of helminths belong to their respective classes 2. Learn about the causes, transmission and treatment of different protozoal infection 3. Gain knowledge on how to protect themselves and prevent the spread of different parasitic infections. Duration Chapter 6: Parasitic Helminths = 3 hours (1 hour discussion; 2 hours assessment) Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 3 of 37 Cestodes Cestodes (Greek kestos-girdle or ribbon) are multi-segmented, dorsoventrally flattened tape-like worms whose sizes vary from a few millimeters to several meters. The adult worms are found in the small intestine of humans. Classification of Cestodes The class Cestoidea belong to Phylum Platyhelminthes. The class Cestoidea includes two orders: Table 7.1. Classification of Medically Important Cestodes Order Family Genus Pseudophyllidea  Diphyllobothriidae  Diphyllobothrium  Spirometra Cyclophyllidea  Taeniidae  Taenia  Echinococcus  Hymenolepididae  Hymeno/epis  Dipylidiidae  Dipylidium Morphology In their life cycle, they exist in three morphological forms: 1. Adult worm 2. Egg 3. Larva Body Structure of Adult Worm a. Head or Scolex - It is the organ of attachment to the intestinal mucosa of the definitive host, human or animal.  In Cyclophyllidean cestodes, the scolex bear four cup like muscular suckers (or acetabula). In some species like T. solium and H. nana, scolex has a beak like apical protrusion called as rostellum, which may be armed with hooklets. (These species are called as armed tapeworms)  In Pseudophyllidean cestodes, the scolex doesn’t possess suckers but it bears a pair of longitudinal groove called as bothria by which it attaches to small intestine. Figure 7.1. Structure of the adult Cestode Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 4 of 37 b. Neck - It is the part, immediately behind the head and is the region of growth from where the segments of the body (proglottids) arises. c. Strobila - The trunk also called as strobila is composed of a chain of proglottids or segments surrounded by a body wall called as tegument  The proglotids near the neck, are the young immature segments, behind them are the mature segments, and at the hind end, are the gravid segments  Tapeworms are hermaphrodites (monoecious), the Proglottids bear the reproductive organs (both male and female)  Proglottids can be (1) immature, (2) mature and (3) gravid segments  Immature segments: Male and female reproductive organs are not differentiated  Mature segments: Contain male and female organs in the same segment, male organ appear first  Gravid segments or fertilized segments: Following fertilization, the uterus gets filled with eggs. Other organs are atrophied. Female Reproductive Organ The female reproductive part of Cestodes are present on the ventral side and consists of:  A bilobed ovary: Present in the middle and posteriorly  Oviduct: Arises from ovary, joins with spermatic duct and opens into the ootype  Ootype: It is the chamber where fertilization takes place. There may be self fertilization or cross fertilization between the segments  Vagina: A tube that connects genital pore to the ootype through which the sperm enters. At its inner end, it contains seminal receptacle (for storage of sperm) and spermatic duct  Uterus: Straight tube arises from the ootype where the eggs are stored after fertilization in the gravid females. Its end may be opened (in pseudophyllideans) or closed as blind sac (in cyclophyllideans)  Vitelline gland (vitellaria) and Mehlis’ gland are present near the ootype. They occur as single mass (in cyclophyllideans) or scattered mass (in pseudophyllideans). They release their secretion through their ducts into the ootype. Male Reproductive Organ Present on the dorsal side and consists of:  Testes: They exist as multiple follicles (except in Hymenolepis which are three in number). Sperms are released to vasa efferentia which join together to form vas deferens  Vas deferens: It is a convoluted tube, opens in the common genital pore. It bears a seminal vesicle and ends in the common genital pore as a swollen muscular and protrusible organ called as cirrus (equivalent of penis) surrounded by a cirrus sac. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 5 of 37 Figure 7.2. A to B. Schematic Diagram of the Reproductive Organ of (A) Pseudophyllidean cestodes, (B) Cyclophyllidean Cestodes Nervous System - It is rudimentary, consists of brain like structure (central ganglion, lateral and rostellar ganglia connected by central nerve ring) present in the scolex from which the longitudinal nerve trucks arise and pass through all the segments and joined by transverse nerves in each segment. Excretory System - It is also rudimentary and present in each segment. It consists of two lateral canals (dorsal and ventral) connected by transverse canals in each segment. The excretory canals are built up of flame cells (terminal cells) and canal cells. Circulatory System - Cestodes do not have circulatory system nor body cavity. Body Wall (or Tegument) - It is made up of three layers—outer microvillus like structure called as microthrix, middle basal plasma membrane and inner muscular layer (outer circular and inner longitudinal muscle coats). Eggs of Cestodes - Eggs are released into the uterus following fertilization and fill the gravid proglottids  Pseudophyllidean cestodes: Eggs are ovoid, operculated, surrounded by a single layer called as egg shell (or capsule), inside which the embryo is present containing hooklets (three pairs). Membrane lining the embryo is ciliated. The eggs when laid first in the feces, are not embryonated. Maturation takes place later in water.  Cyclophyllidean cestodes: Eggs are round to oval, covered by two layers—an outer egg shell (or capsule) filled with yolk material (thin, so might be lost) and an inner thick radially striated embryophore surrounding the embryo. Eggs are embryonated from the beginning, contains six hooklets but the lining membrane is not ciliated Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 6 of 37 Figure 7.3.A to C. Schematic Diagram of Cestodes Eggs. (A) Pseudophyllidean cestodes; (B) Cyclophyllidean cestodes; (C) Cyclophyllidean cestodes after the loss of egg shell Larva form of Cestodes Embryonated eggs undergo further development to form larva:  Pseudophyllidean cestodes: Larva is solid without any sac. They are: - Coracidium: First stage larva of Diphyllobothrium - Procercoid: Second stage larva of - Diphyllobothrium - Plerocercoid: Third stage larva of - Diphyllobothrium - Sparganum: Larval stage of Spirometra  Cyclophyllidean Cestodes: Larvae contain bladder like sacs. They are: - Cysticercus: Larval stage of Taenia - Hydatid cyst: larval stage of Echinococcus - Coenurus: Larval stage of Multiceps - Cysticercoid: Larval stage of Hymenolepis. Figure 7.4.A to E Larvae of Cyclophyllidean cestodes: (A) cysticercus bovis; (B) cysticercoid (C) cysticercus cellulosae; (D) coenurus; (E) hydatid cyst Life Cycle Cestodes complete their life cycle in two hosts (definitive host and intermediate host) except:  Hymenolepis (requires only one host—man)  Diphyllobothrium requires three hosts (one Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 7 of 37 Diphyllobothrium latum - It belongs to the Order Pseudophyllidea and Family Diphyllobothriidae - It is also known as fish tapeworm or human broad tapeworm (proglottids are broader than longer, latum means broader) - It is the largest cestode found in human intestine (mostly in jejunum and ileum) - Measures up to 10 meters or more with over 3,000 proglottids - Its life cycle was described by Rosen in 1917 - Few species other than D. latum can rarely infect humans like D. dendriticum, D. pacifi cum and D. nihonkaiense etc. Morphology A. Adult Worm Figure 7.5. A to B. Adult worm of Diphyllobothrium latum (A) scolex; (B) strobila  Head or scolex - It is spoon shaped, bears two longitudinal grooves called as bothria (one on ventral and other on dorsal surface) by which it attaches to the small intestine. There are no suckers and rostellum.  Neck - It is situated next to scolex and represents the growing end, from which the proglottids arise. It is unsegmented and longer than the head.  Strobila - There are more than 3,000 segments divided into immature, mature and gravid segments (in that order starting from neck) - The mature segment is broader (10–20 mm) than longer (2–4 mm) and contains the male and the female reproductive organs. Female organs consist of bilobed, coiled and rosette shaped uterus, vitelline gland scattered throughout the segment and a vagina. Genital pore is situated midventrally. Male organs consist of testes (follicles), vas deferens and cirrus. - Gravid segment: Uterus is filled with eggs which are discharged periodically through the uterine pole. Some terminal gravid segments become shrunken and empty due to constant discharge of eggs and break off from the body and passed in the feces. (This is known as pseudoapolysis). Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 8 of 37 B. Eggs - Fertilized eggs are oval, measuring 70 μm length and 50 μm width - Eggs are operculated at one end and bear a knob at the other end - When freshly passed in the feces, they are unembryonated, surrounded by egg shell - Embryonated egg contains a hexacanth oncosphere lined by a ciliated membrane. Figure 7.6. Egg of D. latum https://www.cdc.gov/dpdx/diphyllobothriasi s/index.html) C. Larva - There are three larval stages: 1. First stage larva (coracidium) 2. Second stage larva (procercoid) 3. Third stage larva (plerocercoid) Life Cycle  Infective form: Third stage plerocercoid larvae  Modes of transmission: Humans get infection by ingestion of undercooked fresh water fish containing third stage plerocercoid larva.  Host: Humans are the definitive host. Dogs, cats and foxes are the other rare definitive hosts. There are two intermediate hosts: 1. First intermediate hosts: Fresh water copepods mainly of the genera Cyclops and Diaptomus 2. Second intermediate hosts: Fresh water fishes (pike, salmon, perch and trout) Development in Definitive Host (Intestine) - The plerocercoid larvae undergo further development to form adult worms which attach to the small intestine by the help of bothria. Adult worms become sexually mature in 4 weeks, fertilization takes place and they begin to lay eggs. Millions of eggs are released everyday. Development in Fresh Water - Embryonation and formation of L1 larva (coracidium): Eggs are unembryonatedwhen freshly passed in the feces, but become embryonated after 8–12 days in fresh water at 16 20°C. This ciliated embryo is released through the operculum of the egg into fresh water, which is known as the first stage larva (coracidium). Development in first intermediate host (L1 to L2 transformation): - The coracidium swims in water and survives only for 12 hours within which it has to be ingested by small copepods (Cyclops and Diaptomus). It loses cilia and penetrates the intestine; enters the body cavity of copepods where it transforms within 2–3 weeks into 0.5 mm long, second stage procercoid larva (infective stage to the fresh water fish) Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 9 of 37 Development in second intermediate host (L2 to L3 transformation): - The fresh water fishes are infected by ingestion of copepods containing procercoid larva. The procercoid penetrates the intestine of fish; migrate to muscle, liver and fat of the fish where it transforms within 1–2 weeks into an elongated 10–20 mm × 2–3 mm size, L3 stage (plerocercoid larva). This stage is infective to man and the cycle is repeated. - Paratenic host: If small fishes are eaten by a big suitable fish, then the plerocercoid penetrates the intestine of the bigger fish and survives without further development. This type of host is called as paratenic host (A host in which the parasite survives without any development and is not essential for its life cycle). Figure 7.7. Life Cycle of Diphyllobothrium latum (Source: https://www.mcdinternational.org/trainings/malaria/english/DPDx5/HTML/Diphyllobothriasis) Pathogenesis and Clinical Feature  Most of D. latum infections are asymptomatic.  Minor manifestations may include abdominal discomfort, diarrhea, vomiting, weakness and weight loss or rarely acute abdominal pain and intestinal obstruction, cholangitis or cholecystitis (may be produced by migrating proglottids)  Vitamin B12 deficiency: The adult worm absorbs large quantities of vitamin B12 and interferes with ileal B12 absorption  Vitamin B12 defi ciency leads to development of megaloblastic anemia and some people may exhibit neurologic sequelae like paresthesia Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 10 of 37 Laboratory Diagnosis Diphyllobothrium latum  Stool examination—eggs and proglottids seen  Blood examination shows megaloblastic anemia Treatment of Diphyllobothrium latum  Praziquantel (5–10 mg/kg once) is highly effective (drug of choice)  Niclosamide is given alternatively  Parenteral vitamin B12 should be given if B12 deficiency is manifested. Prevention  Proper cooking of fish (10 minutes at 50°C)  Deep freezing (–10°C for 24 hours)—for the people who eat raw fish. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 11 of 37 Taenia Species  Taenia belongs to Order: Cyclophyllidea, family Taeniidae  Taenia species cause two types of manifestations in humans—intestinal taeniasis and cysticercosis.  Two important members are: 1. T. saginata (also called as beef tapeworm) causes intestinal taeniasis in man 2. T. solium (also called as pork tapeworm) causes both intestinal taeniasis and cysticercosis in man.  The adult worms of T. saginata and T. solium reside in the small intestine (jejunum and ileum) of humans  It exists in three forms—(1) adult worm, (2) egg and (3) larva Morphology A. Adult Worm  Head/scolex - The scolex bears four cup like muscular suckers (or acetabula) which helps in attachment - In T. solium, the scolex has a beak like apical protrusion called as rostellum. The rostellum is armed with two rows of hooklets (hence called as armed tapeworm). Figure 7.8. A to B. Carmine stained scolex of (A) T. saginata and (B) T. solium  Neck - Situated next to the head. It is the narrow growing region from which the proglottids arise. Neck is longer in T. saginata.  Strobila - Strobila is the trunk or body, consists of many segments (or proglottids). - The mature segment contains the male and the female reproductive organs - Female organs consist of ovary, branched and closed uterus, ootype, single mass of vitelline gland and laterally situated genital pore. Male organs consist of testes Figure 7.9. A to B. Carmine stained mature (follicles), vas deferens and cirrus. proglottids of (A) T. saginata and (B) T. solium Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 12 of 37 B. Eggs Following fertilization eggs are released into the uterus and fill the gravid proglottids. - Taenia eggs are round, 30–40 μm size, covered by two layers - An outer egg shell (or capsule) filled with yolk material (thin, so might be lost) and an inner embryophore (brown, thick walled and radially striated) surrounding the embryo - The embryo or oncosphere contains three pair of hooklets - Eggs of T. saginata and T. solium are indistinguishable from each other (except, - T. saginata eggs are acid fast), and are infective to cattle and pigs respectively - Some time, eggs of T. solium are infective to man (to cause cysticercosis). C. Larva - Cysticercus is the larval stage of Taenia. It contains a muscular organ with bladder like sac. It is called as:  Cysticercus cellulosae in T. solium  Cysticercus bovis in T. saginata - Larval stage of T. saginata and T. solium is infective to man (to cause intestinal taeniasis). Life Cycle of Taenia solium Life cycle of T. solium depends on the disease it causes When it causes intestinal taeniasis, the life cycle is exactly similar to that of T. saginata except: - The intermediate host is pig (hence called as pork tapeworm) - Men harboring the adult worm excrete the eggs in feces which can infect the same individual by autoinfection - In pigs, the development time is shorter (7–9 weeks). - But when it causes cysticercosis, the life cycle is diff erent and given as below:  Host: Man acts as both definitive and intermediate host.  Infective stage: Eggs of T. solium.  Mode of transmission: Firstly man acquire the infection by—(1) ingestion of contaminated food or water with eggs of T. solium and (2) autoinfection. Autoinfection: Eggs excreted from men reinfect the same individual. Autoinfection can be of two types: a. External autoinfection: Due to unhygienic personal habit, e.g., contaminated fi nger b. Internal autoinfection: Due to reverse peristaltic movements by which the gravid segments throw the eggs back into the stomach (equivalent to swallowing of the eggs)  Further life cycle in men is similar to that in pigs: - Oncosphere is released from the eggs, penetrates the intestine and enters into the portal circulation or mesenteric lymphatics and reaches to various organs like Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 13 of 37 subcutaneous tissue, muscle, eye and brain where it is transformed to the larval stage cysticercus cellulosae in 7–9 weeks and deposited as cyst. Full develop ment to mature cysts takes 2–3 months of time.  Cysticercus cellulosae: - A mature cysticercus cellulosae is 0.5–1.5 cm size, spherical (or slightly oval), yellowish white, separated from the host tissue by a thin collagenous capsule.  It contains two chambers: Outer one is a bladder like sac filled with 0.5 mL of vesicular fluid and the inner chamber contains the growing scolex with hooklets and a spiral canal  Racemose cysticerci: In some cases, when the parasites are lodged in spacious area, they grow and transform into larger lobulated cysticerci (> 20 cm), containing 60 mL of vesicular fluid Pathogenesis and Clinical features Intestinal Taeniasis Both T. saginata and T. solium can cause intestinal taeniasis. - Often, it is asymptomatic; patients become aware of the infection most commonly by noting the passage of proglottids in their feces - The proglottids are often motile, and patients may experience perianal discomfort (or ment to mature cysts takes 2–3 months of time. - pruritus) when proglottids are discharged - Mild abdominal pain or discomfort, nausea, loss of appetite, weakness, weight loss, headache and change in bowel habit (constipation or diarrhea) can occur - Occasionally obstruction by the migrating proglottids can result in appendicitis or cholangitis Cysticercosis - Clinical spectra of the disease depend upon the localization of the cyst. Though it is discovered from any site of the body but the common sites are central nervous system (CNS),subcutaneous tissue, skeletal muscle and eyes. Cysticercosis can be:  Subcutaneous cysticercosis: It is frequently asymptomatic but may manifest as palpable nodules  Muscular cysticercosis: Manifest as muscular pain, weakness or pseudo hypertrophy  Ocular cysticercosis: Can involve eye lids, conjunctiva and sclera. Common symptoms like proptosis, diplopia, loss of vision and slow growing nodule with focal inflammation  Neurocysticercosis: considered as the most common parasitic CNS infection of man and the most common cause of adult onset epilepsy throughout the world Neurocysticercosis can be classified based on the site of involvement: Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 14 of 37 1. Parenchymal: Involves brain parenchyma 2. Extraparenchymal sites are meninges, ventricles and spinal cord. Though brain parenchyma was thought to be the most common site but recent evidences have shown that subarachnoid space being the most common site, followed by brain parenchyma Laboratory Diagnosis for Intestinal Taeniasis  Stool examination—Detects eggs, proglottids  Taenia specific coproantigen detection in stool—ELISA  Antibody detection in serum—ELISA, CFT, Immunoblot  Molecular method—PCR Stool examination—Detects eggs, proglottids  Taenia specifi c coproantigen detection in stool—ELISA  Antibody detection in serum—ELISA, CFT, Immunoblot  Molecular method—PCR Laboratory Diagnosis for Cysticercosis  Radiodiagnosis— CT scan and MRI  Antibody detection in serum or CSF— ELISA, Western blot (EITB)  Antigen detection in serum or CSF- ELISA  Lymphocyte transformation test  Histopathology of muscles, eyes, subcutaneous tissues or brain biopsies  Del brutto diagnostic criteria Treatment for Intestinal Taeniasis  Praziquantel (drug of choice): Single dose of (10 mg/kg) is highly effective  Niclosamide (2 g) is also effective but is not widely available. Treatment for Cysticercosis 1. Antiparasitic agents:  For brain parenchymal lesions: - Albendazole (15 mg/kg per day for 8–28 days) or - Praziquantel (50–100 mg/kg daily in three divided doses for 15–30 days)  Longer courses are often needed in patients with multiple subarachnoid cysticerci 2. Symptomatic treatment of:  Seizures by antiepileptic drugs  High-dose glucocorticoids should be used to reduce the inflammatory reactions caused by dead cysticerci  Hydrocephalus: Attempts should be made to reduce intracranial pressure. In the case of obstructive hydrocephalus, cysticerci can be removed by endoscopic surgery or ventriculoperitoneal shunting. 3. Surgery: Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 15 of 37  Open craniotomy to remove cysticerci is rarely required nowadays  Surgery is indicated for ocular and spinal and ventricular lesions because anti parasitic drugs can provoke irreversible inflamatory damage. Prevention Intestinal taeniasis can be prevented by:  Adequate cooking of beef or pork viscera: - Exposure to temperatures as low as 56°C for 5 minutes - Refrigeration or salting for long periods or freezing at –10°C for 9 days - Eff ective fecal disposal to prevent infection to cattle and pigs. The prevention of cysticercosis involves:  Good personal hygiene to prevent autoinfection with eggs  Effective fecal disposal to prevent contamination of food and water with eggs  Treatment and prevention of human intestinal infections  Vaccines to prevent porcine cysticercosis.  Various antigens like T. solium oncosphere antigen, T. crassiceps and T. ovis recombinant antigens are attempted for vaccination of pigs. They are under development. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 16 of 37 Trematodes - Trematodes (also called as flukes) include the helminths that are unsegmented, flat (flatworms) and leaflike. Taxonomic Classification of Trematodes Systemic classification of medically important trematodes is proposed by Gibson and Bray (1994) Table 7.2. Trematodes belong to: Phylum: Platyhelminths Class: Trematoda or Digenea Table 7.2. Classification of Trematodes Order Superfamily Family Genus Species S. haematobiu m S. mansoni Schistosomatida Strigeida Schistosomatoidea Schistosoma S. japonicum e S. mekongi S. intercalatum Gastrodiscoide Paramphistomatoide G. hominis Zygocotylidae s Echinostomid a Watsonius W. watsoni a Fasciola F. hepatica Echinostomatoidea Fasciolidae Fasciolopsis F. buski O. felineus Opisthorchis Opisthorchiidae O. viverrini Clonorchis C. sinensis Opisthorchioidea H. Heterophyes Plagiorchiida heterophyes Heterophyidae M. Metagonimus yokogawai P. Plagiorchioidea Paragonimidae Paragonimus westermani Classification Based on the Habitat 1. Blood Trematodes (flukes)  Schistosoma haematobium: Resides in vesical venous plexus  Schistosoma mansoni, S. japonicum: Resides in rectal venous plexus and portal venous plexus. 2. Hepatic Trematodes (flukes) Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 17 of 37  Fasciola hepatica and Fasciola gigantica: Both reside in liver  Clonorchis species and Opisthorchis species: Both reside in bile duct. 3. Intestinal Trematodes (flukes)  Small intestine: Fasciolopsis buski, Heterophyes species, Metagonimus species, Watsonius species  Large intestine: Gastrodiscoides species 4. Lung Trematodes (flukes) Paragonimus westermani General Characteristics of Trematodes Morphology  Trematodes exist in three morphological forms— adult worm, egg and larva. The adult worms are unsegmented and flattened dorsoventrally but some have thick fleshy bodies (schistosomes).  Size: Range from less than 1 mm to ~60 mm  Suckers: They attach to host with two suckers—(1) oral sucker (anterior) which surrounds the mouth and (2) ventral sucker (acetabulum) on the ventral surface  Digestive system: It is incomplete,consists of anterior mouth, muscular pumping pharynx which continues as esophagus. Esophagus bifurcates in front of ventral sucker into a pair of blind intestinal pouches called caeca. The anus is absent  Most trematodes are hermaphrodites (monoecious) except the schistosomes, which are diecious (sexes are separate)  Male reproductive organs: Consist of number of testes present near the cecal end, vas efferens  arise from each testes join to form a common vas deferens which runs via a small seminal vesicle and opens at genital pore situated near the ventral sucker  Female reproductive organs: Consist of an ovary (present near the ventral sucker), vitelline glands surrounding ovary, oviduct, ootype and a uterus containing eggs that opens behind the ventral sucker  The excretory system is bilaterally symmetrical. It consists of flame cells and collecting tubules which lead to a median bladder opening at the posterior end of the body, usually on the dorsal aspect  The nervous system consists of paired ganglia at the anterior end. From this, nerves extend anteriorly and posteriorly  Oviparous: Trematodes are oviparous, i.e. they lay eggs.The eggs are operculated except those ofschistosomes  Larva: Trematodes have many laval forms such as miracidium, sporocyst, redia, cercaria, and metacercaria Life Cycle  Host: Trematodes complete their life cycle in three different hosts, one definitive host (man) and two intermediate hosts. The first intermediate host is fresh water snail or Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 18 of 37 mollusc and the second intermediate host is either aquatic plant or fish or crab. However, schistosomes don’t need a second intermediate host.  Mode of transmission: Man acquires infection by eating aquatic plants, fishes or crabs harboring infective form (metacercariae) or by the penetration of free living cercariae (schistosomes). Fasciola Hepatica - Fasciola hepatica, also known as the common liver fluke or sheep liver fluke. The disease is called fascioliasis. In addition to humans, it also infects sheep and other domestic animals. - The parasite lives in the liver and bile duct Morphology  Adult Worm Large in size (3 cm length by 1.2 cm breadth), flat, leafshaped, brown colored - Suckers: The anterior end has a conical projection (shoulder) containing oral sucker while the posterior end is rounded. The ventral sucker is situated away from the oral sucker - Intestine is bifurcated and incomplete and bears lateral branches - It is hermaphrodite with both male and female reproductive organs. Figure 13. Faciola hepatica  Egg Eggs are oval, bile stained, unembryonated and operculated. - Large size: Measures about 130–150 μm by 60–90 μm size - The eggs of F. hepatica are similar to that of Fasciolopsis buski and cannot be differentiated.  Larva Metacercaria larva is the infective form for man and other definitive hosts. Other larval forms are miracidia, rediae and sporocysts. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 19 of 37 Figure 7.14. A to B. Fasciola hepatica (A) adult worm; (B) egg Life Cycle of Fasciola hepatica  Host: Sheep is the principal definitive host. Goats cattle and humans are other definitive hosts. The amphibian snails (Genus: Lymnaea) are the first intermediate hosts and water plants serve as the second intermediate hosts.  Mode of transmission: The sheep and other definitive hosts including man get infection by eating water plants and water cress containing metacercariae. Figure 7.15. Life of Faciola hepatica Source: https://www.cdc.gov/dpdx/fasciolopsiasis/index.html Development in Man or Sheep - In duodenum, the metacercariae excyst and penetrate through intestinal wall to reach peritoneal cavity. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 20 of 37 - The larvae invade liver tissue and migrate through the liver parenchyma into the hepatic ducts where they mature into adult worms in about 9 weeks of infection - Inside the hepatic duct the fluke starts laying unembryonated eggs which come back to the intestine and are excreted in the feces. Development in Water - The eggs further develop in water in 1–2 weeks to release miracidium at 22–26°C. Development in snails - The miracidium penetrates the suitable snail host. Inside the snail host, the miracidium multiplies and transforms into sporocysts, which further develop into two generations of rediae. Finally, the rediae give rise to cercariae. Development in aquatic plants - The cercariae escape from the snails and infect the water plants where they encyst to form metacercariae. Metacercariae when ingested by the defi nitive host, cause infection and the cycle is repeated. Pathogenesis of Fasciola hepatica Incubation period varies from days to few months.  Acute disease develops during metacercarial migration (1–2 weeks after infection) and includes fever, right upper quadrant pain, hepatomegaly and eosinophilia. Computed tomography (CT) scan of the liver may show migratory tracks. The adult worm can cause obstruction of the bile duct and dilatation of the biliary tract  In chronic phase, the liver parenchyma is inflamed with formation of multiple subcapsular abscesses (called as liver rot). Bile duct obstruction by adult worm and biliary cirrhosis are also reported but less commonly. However, liver malignancy is not associated. Laboratory Diagnosis of Fasciola hepatica  Stool microscopy—detects operculated eggs  Antibody detection—ELISA, CIEP, western blot techniques  Molecular tests—DNA probe, PCR  USG, CT scan, MRI—detect lesions in liver  zPeripheral blood eosinophilia Treatment of Fasciola hepatica  Triclabendazole (10 mg/kg once) is the drug of choice for fascioliasis  Bithionol and praziquantel are the other alternative drugs. Prevention of Fasciola hepatica Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 21 of 37 Fascioliasis can be prevented by:  Avoidance of consumption of raw water plants and cleaning them before use  Control of snails  Health education  Treatment of infected person. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 22 of 37 PARASITIC HELMINTHS Nematodes Nematodes are probably the most widespread animal group occurring in the world. Many of them are non-pathogenic and exist as free living forms in fresh or marine water and soil while few of the species can be pathogenic and exist as parasitic form in both animals and plants. Nematodes can be classified into: Oviparous (egg laying), Viviparous (give rise directly to larvae), Ovoviviparous (producing young by means of eggs which are hatched within the body of the parent). Nematodes can also be classified based on their habitat (Table 7.3.). Table 7.3. Classification of Nematodes based on Habitat Intestinal Human Somatic Human Animal nematodes infecting rarely to man nematodes nematodes Larva migrans Other Animal nematodes Small Intestine Filarial worm Visceral Larva Zoonotic filariasis migrans Dirofilaria Toxocara (Liver Ascaris lumbricoides Lymphatics Visceral Larva Intestine (Common round Wuchereria bancrofti migrans Capillaria worm) Brugia malayi Toxocara (Liver) philippinensis Ancylostoma Brugia timori Angiostrongylus Trichostrongylus spp. duodenale cantonensis (CNS) Strongyloides (Old world Angiostrongylus fuelleborni Hookworm) costaricensis Necator americanus (abdomen) Oesophagostomum (American or new Anisakis Ternidens spp. world Gnathostoma Conjunctiva Hookworm) Baylisascaris Thelazia spp. Skin Liver Loa loa (also eye) Capillaria hepatica Onchocerca (also eye) Mansonella streptocerca Mansonella ozzardi (Serous cavity) Mansonella perstans Large intestine Other Human Cutaneous larva Kidney Trichuris trichiura Somatic migrans Dioctophyma spp (Whip worm) nematodes Ancylostoma Respiratory Enterobius Trichinella spiralis braziliensis tract/lungs vermicularis Dracunculus Ancylostoma caninum Mammomonogammus (Thread or pin worm) medinensis Ancylostoma (Guinea worm) ceylanicum Capillaria aerophila Gnathostoma spp. Ascaris suum Uncinaria stenocephala Bunostomum spp. Abbreviations: CNS, central nervous system Nematodes I (Intestinal Nematodes) Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 23 of 37 General Characteristics of Nematodes Nematodes pass through six developmental stages (Fig.7.14) adult worm, egg stage and four larval stages (L1–L4). Each larval stage transforms to the next by shedding of the cuticle (called as molting). Adult Worm  Shape: Nematodes are elongated, cylindrical or filariform in shape with both the ends pointed. They are unsegmented without any appendages  Size: Variable, ranging from less than 5 mm (hookworm, Trichinella and Strongyloides) to as long as one meter (Dracunculus). Female worms are longer than male worms  Symmetry: Body is bilaterally symmetrical (one plane) while head is radially symmetrical (multiple plane)  Body wall: Made up of outer layer of tough acellular cuticle and inner layer of longitudinal muscle  Locomotion: Nematodes move by contraction of the longitudinal muscles  Alimentary canal: It is well developed and consists of mouth at the anterior end followed by a muscular and glandular esophagus, intestine and rectum that leads to subterminal anus at the posterior end. In some species (e.g hookworm) mouth bears the teeth (cutting plate). The esophagus (or pharynx) may bear posterior bulb (as in Enterobius). The intestine or midgut is lined by a single layer of columnar cells  Body cavity: They possess a body cavity or a pseudocele (space between body wall and alimentary canal) with high hydrostatic pressure which is filled with body fluid secreted by intestine and genital organs  Sexes: Nematodes are diecious (bisexual), i.e sexes are different  Male reproductive system: It consists of a long convoluted tube which can be differentiated into testes, vas deferens, seminal vesicle and ejaculatory duct. Some worms also bear accessory copulatory organs like a copulatory bursa with two spicules (rod like protrusible organ present at the posterior end) and gubernaculum (an elevation of cloaca that guides the spicule during copulation). The ejaculatory duct opens sub terminally at the posterior end into a common passage along with the rectum (known as cloaca).  Female reproductive system: It consists of two (common) or one convoluted tube. Each tube is differentiated into an ovary, oviduct, seminal receptacle, and uterus and then both the tubes joined to form a common vagina that opens outside through vulva (genital pore) either in the middle of the body or near the mouth.  Nervous system: It is rudimentary and consists of circular nerve ring (brain) surrounding the esophagus and six longitudinal nerve trunks (one dorsal, one ventral and four lateral). The dorsal nerve is responsible for motor control, while the lateral nerves are for sensory and the ventral one combines both the functions. Some species possess sensory structure like sensory papilla and phasmid (chemoreception organs) over the cuticle  Excretory system: It is also rudimentary. Unlike cestodes, they don’t have flame cells. Various ways of waste disposal are:  Through anus,  Excretion of nitrogenous waste in the  form of ammonia through the body wall Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 24 of 37  In some species, H shaped canal along each side of body regulates nutrients and waste content  In few other species; An excretory gland is situated near esophagus Life Cycle Nematodes complete their life cycle in one host (man) except in filarial worms (need two hosts—definitive hostman and intermediate host—mosquito) and Dracunculus species (need two hosts—definitive host man and intermediate host cyclops). Enterobius vermicularis - also called as pin worm or threadworm. - It is described first by Leuckart, in 1865 - E. vermicularis is the only species. The second species E.gregorii is identical to E. vermicularis (except the basal portion of spicule) and now it is considered as the younger stage of E. vermicularis. - The adult worm remains attached to the large intestine (cecum, appendix and adjacent portion of colon) by their mouth end. Morphology  Adult worm - It is small, white and thread like (hence named as threadworm). - Cervical alae: The adult worm bears a wing like expansion of the cuticle near the anterior end - Double bulb esophagus: The posterior end of the esophagus is dilated to form globular bulb - Male worm is smaller (2–5mm long × 0.1– 0.2 mm wide) and the posterior one third is - tightly curved and bears a copulatory bursa with spicules at the posterior end. Males die soon after fertilization - Female worm is longer (8–13 mm long × 0.3–0.5 mm wide), and the posterior - One third is tapering, straight, thin and pointed (looks like a pin, hence called as pin worm).  Eggs - Shape: Oval or planoconvex (one side is plain and the other side is flat because it is compressed laterally) - Size: 50–60 μm long × 20–30 μm wide - Surrounded by: Double layered egg shell - Not bile stained, Colorless in saline mount - Embryonated when passed fresh; contains a tadpole larva inside - Floats in saturated salt solution. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 25 of 37 Life Cycle  Host: Humans are the only host.  Infective form: Embryonated eggs are infective to man.  Mode of transmission: Men (usually children) acquire infection by ingestion of embryonated eggs containing larva by: 1. Ingestion of eggs contaminated with fingers due to inadequate hand washing or nail biting habit 2. Autoinfection: Endogenous autoinfection by retrograde migration of the larva hatched from the eggs in the perianal skin Figure 7.22. Life Cycle of Enterobius vermicularis Laboratory Diagnosis of E. vermicularis Microscopy—wet mount of perianal swab collected by cellophane tape method or NIH swab method detects planoconvex eggs containing larvae Treatment of E. vermicularis  One of the following drugs can be given: - Mebendazole (100 mg once) - Albendazole (400 mg once) or - Pyrantel pamoate (11 mg/kg once; maximum, 1 g).  The same treatment should be repeated after 2 weeks  Treatment of household members is advocated to eliminate asymptomatic reservoirs of potential reinfection. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 26 of 37 Prevention  By improving personal hygiene such as proper washing of bed clothes and hand washing Small Intestinal Nematode Hookworm  Hookworm is one of the important causes of iron deficiency anemia in both tropics and temperate countries. It is so named because the anterior end of adult worm is bent.  duodenale was first detected by an Italian physician Dubini in 1843 and life cycle and pathogenesis was described by Arthur Loss in 1898  N. americanus was first described by Stites in 1902 in Texas, USA, hence called as American hookworm Classification of Hookworm Hookworm belongs to the family Ancylostomatidae which consists of two species infecting humans. Ancylostoma duodenale and Necator americanus. The word Ancylostoma is derived from hooked mouth (Ancylos—hooked, stoma—mouth) Human parasite: 1. N. duodenale or old world hook worm 2. N. americanus or new world (or American) hook worm Morphology Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 27 of 37 Figure 7.23. A to B. (A) Buccal capsule and copulatory bursa of Ancylostoma duodenale and Necator americanus; (B) buccal capsule of Ancylostoma spp  Adult worm (A. duodenale) - Size: Male worm is smaller (5–11 mm) than the female (9–13 mm) - Shape: Straight except the anterior end which is bent dorsally (in the same direction of body curvature), hence called as hookworm - Color: Adult worm is pink or grayish white but may look reddish due to ingested blood - Mouth: It is present at the anterior end, directed dorsally. It contains the buccal capsule which is lined by a hard substance bearing six teeth (four hook like teeth on ventral surface and two knob like teeth dorsally) - Glands: The digestive system is attached with five glands, one of them is the esophageal gland secreting a substance that prevents clotting - Presence of copulatory bursa in the caudal end of males differentiates it from the female worms of A. duodenale - Copulatory bursa: It is the umbrella like expansion of the posterior end of male worm bearing two spicules, consists of three lobes (one dorsal and two lateral). - All the three lobes again split in a tripartite fashion. - Dorsal lobe contains three dorsal rays; each lateral lobe contains five rays (two ventral and three lateral rays). So total numbers of rays are 13. - Adult worm of N. americanus is different from A. duodenale  Egg Hookworm eggs are: - Oval shaped, measures 60 μm long × 40 μm wide - Not bile stained, colorless - Surrounded by thin, hyaline, translucent egg shell - Ovum (embryo) is segmented (four blastomeres) - There is a clear space between the egg shell and the embryo - Floats on saturated salt solution Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 28 of 37 - Eggs of both A. duodenale and N. americanus are morphologically indistinguishable. Figure 7.24. A to C. Hookworm (A) egg with four blastomeres; (B) egg with many blastomeres; (C) rhabditiform larva  Larva - There are four stages of hookworm larva (L1 to L4) - First stage larva is called as rhabditiform larva - L3 stage larva is called as fi lariform larva and is the infective form to man - Filariform larva is longer (660–720 μm) than the rhabditiform larva (100– 150 μm), the esophageal bulb extends to about one third of the body length and the posterior end is more acutely tapered. - The rhabditiform larva of A.duodenale and N.americanus is morphologically similar but it differs in the morphology of their filariform larva Figure 7.25. A to B. Hookworm (Acylostoma duodenale) (A) fi lariform larva; (B) rhabditiform larva Table 7.4. Differences between Adult worm of Ancylostoma duodenale and Necator americanus Adult worm Ancylostoma duodenale Necator americanus Size Large and thick Smaller and more slender Bending of anterior end Bends in the same direction of Bends in the opposite direction of body curvature body curvature Buccal capsule Bears six teeth Four chitinous cutting plate Four hook like ventral teeth present, Two knob like dorsal Two ventral and two dorsal Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 29 of 37 Dorsomedian teeth are present Copulatory bursa Bifurcation is tripartite Bifurcation is bipartite Total number of rays 13 Total number of rays 14 Dorsal ray splits at the tip Dorsal ray splits from the base Two spicules present freely Both spicules fused at the tip Posterior end of female Bears a spine No spine present in females worm Vulva opens at Behind the middle of the body In front of middle of the body Pathogenicity More pathogenic because of Less pathogenic Larger size, armed with teeth Except: Ground itch and and more migratory dermatitis Blood loss 0.15–0.26 (more severe) mL/worm/day Blood loss 0.03 mL /worm/day Life Cycle  Host: Involves only one host (man).  Infective stage: Third stage filariform (L3) larva.  Mode of transmission: Through penetration of skin by the third stage larva (by walking bare foot in dampen soil). Though rare, but other routes of transmission of the larva has been reported through oral, in utero and transmammary routes. Migratory Phase - Following penetration, the L3 larvae enter into the small subcutaneous venules and through venous circulation, reach to the right side of heart and finally to the lungs. Here, they enter into the alveolar space and migrate up to bronchi, trachea and finally by swallowing of sputum, they enter GIT. Intestinal Phase 1. Develop into adults - The L3 larvae undergo third molt (either in the migratory phase or on reaching esophagus) to form L4 larvae that reach the small intestine where they undergo the final molt to develop into adult worms. 2. Laying eggs - The adultworms attach to the intestinal mucosa by their teeth in buccal capsule. In about 5 months following infection, the adult worms mature and then following fertilization, the female worms start laying the eggs, which are excreted in the feces. A gravid female of A. duodenale can lay 10,000–25,000 eggs/day where as that of N. americanus can lay 5000–10,000 eggs/ day. The female worm survives for about 1 year (A. duodenale) and 3–5 years (N. americanus). Development in Soil - Embryonation takes place in moist, sandy and warm soil. The first stage (rhabditiform) larvae hatch out from eggs which then molt twice and finally the infective stage, i.e. L3 larvae are developed within 5–8 days and they remain viable in soil for several weeks Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 30 of 37 Figure 7.26. Life Cycle of Hookworm (Source: https://www.cdc.gov/parasites/hookworm/biology.html) Pathogenicity  Hookworm has ability to suck blood from the intestinal vessels by:  Attaching and making cuts in the intestinal wall by buccal capsule and teeth followed by sucking the blood through contraction of their muscular esophagus  Secreting hydrolytic enzymes  Releasing anticoagulants like factor VIIa/ tissue factor inhibitor  Ingestion of extravasated blood  It can also penetrate the skin which is facilitated by proteolytic enzymes (like aspartyl proteases) and hyaluronidase secreted by hookworm  It is postulated that, hookworm infection can protect the individual from asthma and malaria but predispose to human immunodeficiency virus (HIV), tuberculosis and other intestinal helminthic infections Laboratory Diagnosis of Acylostoma duodenale  Stool microscopy—detects non bile stained oval segmented eggs  Stool culture - Harada Mori fi lter paper tube method - Petridish (slant culture) technique - Baermann funnel technique - Charcoal culture method - Agar Plate technique (more sensitive)  Other findings - Hypochromic microcytic anemia - Eosinophilia Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 31 of 37 - Hypoalbuminemia Treatment of Acylostoma duodenale  Antiparasitic - Antiparasitic drugs like albendazole (400 mg once), mebendazole (500 mg once), and pyrantel pamoate (11 mg/kg for 3 days) can be given - However, due to the widespread use of the drugs, their effi cacy is decreased compared to past. Resistance to albendazole and mebendazole has also been reported  Symptomatic Treatment - Mild iron-defi ciency anemia can often be treated with oral iron alone. - Severe hookworm disease with protein loss and malabsorption warrants nutritional support and oral or parenteral iron replacement Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 32 of 37 Nematode II (Somatic Nematodes) Filarial Nematode General Characteristics  Habitat: Filarial worms reside in the lymphatic system, skin, subcutaneous tissue and rarely body cavity  Adult worm: The adult worms are slender, round measuring 2–10 cm in length (except the female Onchocerca 35–50 cm). Some adult filarial worms can survive for many years in humans causing a number of chronic obstructive and inflammatory conditions including elephantiasis and hydrocele  Microfilariae: The female worm produces large number of L1 larvae called as microfilariae which are highly motile thread like larvae. They are usually non pathogenic but sometimes, hypersensitivity reactions can occur against the microfilarial antigen resulting in tropical pulmonary eosinophilia (TPE). Lymphatic Filarial Nematode Wuchereria bancrofti Morphology Figure 7.27. A to B. Wuchereria bancrofti (A) microfilaria (schematic diagram); (B) microscopic view of adult worm male (left) and female (right) 1. Adult worm Adult worms are located in the lymphatic vessels and lymphnodes. - They are long, slender, creamy white thread like filariform shaped with tapering ends - Adult males (4 cm × 0.1 mm) are smaller than females (6–10 cm × 0.2–0.3 mm) - Male worms can be differentiated from female worms by their small size, corkscrew like tail and presence of two spicules (helps in copulation) at posterior end Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 33 of 37 - Both adult male and female remain coiled together - Females are viviparous and they directly discharge larvae without any eggs. 2. Larva - Like other nematodes, there are four larval stages. The first stage larva is called as microfilaria. The third stage larva is called as filariform larva; which is the infective form to humans. 3. Microfilaria - Microfilariae are the diagnostic forms, found in the blood vessels (Fig. 14.2). - It measures 240–300 μm × 7.5–10 μm covered by a long hyaline sheath (360 μm) within which it moves - The head end is blunt while the tail end is pointed - In unstained film, microfilariae are transparent and colorless. But when stained with Giemsa or other Romanowsky stains they look pink with a column of violet nuclei - The nuclei are present throughout the body except near the head and the tail end. - Nuclei are also absent in few places which represent various primordial organs like nerve ring, excretory pore, anal pore and genital cells Life Cycle  Host: W. bancrofti completes its life cycle in two hosts. 1. Definitive host: Man 2. Intermediate host: Mosquito named Culex quinquefasciatus is the principle vector worldwide. Rarely Anopheles (rural Africa) or Aedes (Pacific Island) can serve as a vector.  Infective form: Third stage filariform larvae are the infective form found in the proboscis of the mosquito.  Mode of transmission: L3 filariform larvae get deposited in skin by the insect bite. Residents living in the endemic areas are exposed to about 50–300 L3 larvae every year. Human Cycle  Develop into adults: Larvae penetrate the skin, enter into lymphatic vessels and migrate to the local lymph nodes where they molt twice to develop into adult worms in few months (4–6 weeks for B. malayi)  Adults lay L1 larvae (microfilariae): Adult worms reside in the afferent lymphatics or cortical sinuses of the lymph nodes where they mate and start laying the first stage larvae (microfilariae). Male worms die after mating where as the female worms live for 5–10 years. A gravid female can discharge 50,000 microfilariae/day  Prepatent period: It is the time period between the infection (entry of L3 larvae) and diagnosis (detection of microfilariae in blood). This is variable ranging from 80days to 150 days. Mosquito Cycle Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 34 of 37  Transmission: When the mosquito bites an infected man, the microfilariae are ingested. Culex bites in night where as Aedes bites in daytime  Exsheathing: Microfilariae come out of the sheath within 1–2 hours of ingestion  Migration to thoracic muscle: L1 larvae penetrate the stomach wall and migrate to thoracic muscle in 6–12 hours where they become sausage shaped (short and thick)  Develop to infective L3 larvae: L1 larvae molt twice to develop L2 (long and thick form) followed by L3 (long and thin form). The highly active L3 larvae migrate to the labella (distal part of proboscis) of the mosquito and serve as the infective stage to man  Extrinsic incubation period: Under optimum conditions, the mosquito cycle takes around 10–14 days. Figure 7.28. Life Cycle W. bancrofti (Source: https://www.cdc.gov/dpdx/lymphaticfilariasis/index.html) Clinical Features Incubation period is about 8–16 months. Clinical manifestations can be categorized into: 1. Lymphatic filariasis – Also known as elephantiasis - Chronic filariasis can also cause Hydrocele (most common manifestation): Accumulation of fluid in the cavity of tunica vaginalis of testes 2. Tropical pulmonary eosinophilia (TPE)/ (Occult filariasis) - Also called as Weingarten’s syndrome: It is a distinct syndrome that develops in some infected individuals of endemic places. - include nocturnal paroxysmal cough and wheezing (due to nocturnal periodicity of microfilariae), weight loss, low-grade fever Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 35 of 37 3. Immune complexes mediated manifestations Figure 7.29 A to C. Clinical features of filariasis (real images) (A) lymphoedema; (B) elephantiasis; (C) hydrocele of scrotum Laboratory Diagnosis Wuchereria bancrofti  Demonstration of microfilariae  Antigen detection—ELISA, ICT  Antibody detection—IHA, IFA, ELISA  Imaging methods—USG, X-ray  Molecular methods—PCR, PCR-RFLP  Xenodiagnosis  Other methods—eosinophil count, cellular assay, etc Treatment of Wuchereria bancrofti  Diethylcarbamazine (DEC) - It is the drug of choice for the treatment of filariasis - It is given 6 mg/kg daily for 12 days - Can kill both adult worms and microfilariae.  Albendazole: 400 mg twice daily for 21 days) has also demonstrated effi cacy against adult worms and microfi lariae  Ivermectin: 400/kg single dose can kill the microfilariae but has no effect on adult worms. High rate of recurrence occurs, hence not used in India (used only in Africa)  Doxycycline: It is given to target the intracellular Wolbachia. It also shows significant microfilaricidal activity as DEC. Brugia malayi - Microfilariae were described first by Lichten stein in blood films from natives in Indonesia - Brug had described it as a new species (1927) - Rao and Maplestone (India) were the first to describe the adult worm (1940). - B. malayi occurs primarily in eastern India, Indonesia, Malaysia and Philippines Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 36 of 37 Morphology Table 7.30. A to B. Thick blood smears stained with Giemsa showing microfilaria of (A) Wuchereria bancrofti; (B) Brugia species Table 7.5. Microfilariae of Wuchereria bancrofti and Brugia malayi Microfilariae Wuchereria bancrofti Brugia malayi Appearance Graceful and sweeping curves Crinkled with secondary curves Size 240–300 μm long Smaller and average 220 μm long Cephalic space Length to width ratio is 1:1 Longer (length to width ratio is 2:1) Excretory pore Not prominent Prominent Nuclei column Large, coarse and well Darkly stained, large, coarse, separated overlapping and extended till the tail tip Tail  Pointed tail tip  Pointed tail tip  No nuclei in the tail region  Four to five nuclei are present in the tail region  Two widely spaced nuclei at the tail tip—terminal and sub terminal Life Cycle The life cycle is similar to W. bancrofti except:  Vector: Mansonia (M. annulifera and M. uniformis) is the main vector for the nocturnal strains, Anopheles and Aedes also can transmit the infection. The sub-periodic strains are transmitted by Coquillettidia and Mansonia  Reservoir: Humans are the main reservoir; except for the sub-periodic strains of B. malayi where monkeys, cats and dogs are the animal reservoirs.  Shorter pre-patent period: 3–4 months.  Shorter life cycle in mosquito (external incubation period) Brugia timori  B. timori was first detected by David and Edeson on 1965.  Its distribution is limited to the Timor islands of southeastern Indonesia  Morphologically microfilariae are similar to that of B.malayi except: - Longer: Measures 265–325 μm (average 310 μm long) - Cephalic space—length to width ratio is 3:1 - 5–8 nuclei are present in the tail region (with two nuclei in tail tip) Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022  Page 37 of 37 - Sheath doesn’t stain with Giemsa stain - Transmitted by Anopheles barbirostris Loa loa - Loa loa (also called as African eye worm) was first reported in West Indies in 1770. - Later in 1895, Argyll-Robertson described the adult worm from the subcutaneous swelling of the eye of a woman residing in Calabar from West Africa. Hence, this condition is named as Calabar swelling. Loa loa is restricted to the rain forests of West and Central Africa  Morphology - Adult worms (females, 50–70 mm long and 0.5 mm wide; males, 30–35 mm long and 0.3 mm wide) live in subcutaneous tissues. Microfilariae circulate in the blood with a diurnal periodicity that peaks between 12:00 noon and 2:00 p.m. They are sheathed, measure 250–300 μm long and bear a column of nuclei extending till the tail tip  Life Cycle - Life cycle is similar to that of W. bancrofti except the vector is female Chrysops species (deerflies, mango flies, red flies or tabanid flies)  Mode of Transmission - Infective (L3) larvae are transmitted by the bite of female Chrysops species during the blood meals in the daytime - Larvae transform into adult worms over 6–12 months and migrate in subcutaneous tissues and eyes. Microfilariae released from gravid female worms migrate to the blood and exhibit a diurnal periodicity - Microfilariae are ingested by the deerflies during the blood meal, loose sheath, penetrate the gut wall, then migrate to fat body and molt twice to become the infective - L3 larvae in about 10–12 days of time. Self-Learning Module in Genetics (BIO 4) Estigoy and Sazon.., 2022