Medical Parasitology and Basic Terminologies PDF

CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies SHEILA GRACE ALARILLA-MARTIN Parasitic infections represent more than 50% of those the chapters dealing with technical methods, the prob- listed in the WHO list of neglected tropical diseases. lems of technologis...

CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies SHEILA GRACE ALARILLA-MARTIN Parasitic infections represent more than 50% of those the chapters dealing with technical methods, the prob- listed in the WHO list of neglected tropical diseases. lems of technologists are discussed; physicians will be Research shows that helminths and protozoans are better able to utilize laboratory services if they under- among the common agents of infections that affect stand them. The manner in which parasitic organisms ASEAN countries that include these ten member states: are acquired and how they produce disease in humans Brunei Darussalam, Cambodia, Indonesia, Lao Peo- are perhaps of no direct importance to technologists. ple’s Democratic Republic (PDR), Malaysia, Myanmar, Yet a basic understanding of these matters should not the Philippines, Singapore, Thailand, and Vietnam. only make technologists’ work more interesting but en- These NTDs draw attentions among health clusters pri- able them to do it better and more efficiently. marily because of their public health impact especially Parasites, parasitism, and host relations in many marginalized areas. NTDs and poverty are in- Many terms have been devised to describe the terconnected ideas that should be given utmost atten- relationships that exist between different kinds tion by health workers and policy making bodies of the of plants and animals at the fundamental food- government. Extensive efforts were already made at the seeking or food-supplying level. community level however despite the several control Organism may at different times exhibit differ- measures that were instituted the problems still exist ent nutritional habits or at a given time obtain its and continually affecting one’s productivity. nutriment in more than one way. Studying the basic concepts of parasitology will en- Fundamentally, there are two ways in which an lighten us to the issues arising from existence of para- animal may obtain food at the expense of other sites and their effects to the general population. More animals. so, understanding the biology of the parasites and their Predation: It may attack another living animal, con- unique interaction with their host will provide clear suming part or all of its body for nourishment. evidence about their life cycle. Laboratory tests are rele- The attacker is the predator, and the victim is called vant for parasite diagnosis and require skills for proper the prey. In predator-prey relationship, the prey is processing and recognition. Misidentification of the usually killed because it is eaten by the predator. parasite may lead to serious consequence thereby af- Scavenging: An animal deriving its nutrition from fecting correct diagnosis. already dead animals, either devouring those The primary purpose of this book is to serve as a dead of natural causes or taking the leavings of a guide both to the clinical diagnosis and treatment and predator. Animals that subsist in this manner are to the laboratory diagnosis of the protozoan and hel- known as scavengers. minthic diseases of medical importance, and to a lesser Some animals are pure predators, others pure extent to the arthropods in relation to disease. While it scavengers, but many predators are not averse an is intended primarily for medical students and physi- occasional bit of scavenging. cians, it is hoped that this book will prove equally useful Some animals always seek their food by their to medical technologists and all others concerned with own efforts or in association with others of their laboratory identification of the animal parasites of hu- own species. mans. The success of the cooperative diagnostic efforts This is the most conspicuous and perhaps the of the physician and laboratory technologist depends most common way in which animals go about on a mutual appreciation of their several problems. In obtaining food. 1 2 Markell & Voge's Medical Parasitology  Symbiosis (“living together”): This refers to associa- Intermediate host tion of two species for food and shelter. Different Harbors the larval stages or asexual forms of forms of symbiosis may be distinguished on the the parasites. basis of whether or not the association is detri- First intermediate host (1st IH): harbors the mental to one of the two partners. early larval stage of the parasites. Commensalism: Latin for “eating at the same table,” Example: Snail is an intermediate of all the denotes an association that is beneficial to one trematodes. partner and at least not disadvantageous to the Blood–feeding arthropods: mosquitoes, other. tsetse flies, sandflies, triatomid bugs. Example: Entamoeba coli lives in the lumen of the intestine, subsists there on the bacterial flora of Vector is a blood feeding arthropod. The the gut, and does its host no appreciable harm. term mechanical vector is used to describe Mutualism: A specialized type of symbiosis that oc- a vector which assist in the transfer of a curs when associations are beneficial to both or- parasitic form between hosts but is not ganisms. essential in the life cycle of parasite. This Parasitism: Symbiotic relationship in which one ani- means that no parasitic development oc- mal, the parasite, lives at the expense of the other curs in the vector. A good example of a me- animal, the host. chanical vector is a fly that transfers amoe- Parasitism, like other forms of symbiosis, necessar- bic cysts from infected feces to food/drink ily involves an intimate relationship between the that is consumed by humans. Non-arthro- two species, and it is this close and prolonged pods mechanical vector is called transport contact that differentiates parasitism from the or paratenic host. In such a host, a parasite predatory activities of many nonparasites. remains viable but does not develop. Parasitism as a way of life may be the only possibil- ity for a given organism, or it may be but one Second intermediate host (2nd IH): harbors alternative. the infective larval stage of the parasite. Example: snail, fish, vegetable, crabs, ants. Paratenic host DEFINITION OF MEDICAL PARASITOLOGY Harbors the parasite in an underdeveloped or Medical Parasitology is a branch of biology that is fo- in arrested state of development. cused on the animal parasites of human and their med- It will only continue the life cycle of parasite ical significance and public health impact. It also covers only if this paratenic host carrying the parasite phenomena of dependence among living organisms. is consumed by a susceptible definitive host. Parasitology is the scientific study of a parasite, an Example: In Paragonimus westermani, fresh organism that depends, and a host, an organism that water prawn/crab acts as its second interme- provides shelter and nourishment. diate host while wild boar acts as its paratenic host. Both of these carries the infective meta- cercariae. HOST Reservoir host Host is known as the organism that provides physi- Alternative host to a parasite that is harbored cal protection and nourishment to the parasite. normally by humans. Different types of hosts: Host the continues the life cycle of the parasite Definitive or final host and acts as an additional source of infection. Harbors the adult or sexually mature stage of Must be considered in parasite control mea- parasite. sures. Example: Example: Pigs are known reservoir host of Bal- Human is definitive host for lymphatic fi- antidium coli. larial worms that cause elephantiasis and A parasitic infection in which the normal host is an trypanosomes that cause African trypano- animal but can produce disease in human if they be- somiasis. come infected accidentally is known as zoonosis. Ex- Anopheles mosquito is the definitive host amples of this include the following parasitic diseas- for Plasmodium spp. that causes malaria. es: leishmaniasis, South American trypanosomiasis,  CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies 3 African trypanosomiasis, schistosomiasis (S. japoni- Only groups of protozoans that contain nothing but cum), trichinellosis, echinococcosis. Lack of adapta- parasitic forms: Phylum Apicomplexa and Microsporidia. tion to human host may cause serious infection in Members of these phyla have no locomotor organ- the person infected. elles, although the structures are present in one form or another in all other phyla of protozoa, even in their parasitic representatives. PARASITE Most of the free-living turbellarian flatworms are pro- These are organisms that depend on the host for sur- vided with a ciliated epidermis in the adult stage. vival. Cilia are not found on the parasitic members of this group or on the related but strictly parasitic trema- Types of Parasites todes and cestodes. According to relationship between host and a A digestive tract, moderately complex in the turbel- parasite larians, is generally reduced in the trematodes and is Obligate parasite: Organism that cannot survive in absent in the cestodes. any other manner in the absence of a host. The reproductive system is highly developed in the Facultative parasite: Organism that may exist in a two latter groups; this seems a reflection of the dif- free-living state or as a commensal and that, if opportu- ficulties inherent in transfer of these organisms to nity presents itself, may become parasitic. new hosts. Temporary parasite: Parasites that are obligatory at Specialized attachment organs in the form of suck- one or more stages of their life cycles but free living at ers and hooks have been developed by the parasitic others. flatworms. Intermittent parasite: Small organisms, such as mos- Body size may be greatly affected by the parasitic quitoes, which must periodically seek out other and state. larger forms on which to nourish themselves. These are The majority of free-living turbellarians are less parasites that visit their host during feeding time. than a half centimeter in length, and while some land planarians may reach a half meter, none According to habitat approaches the length of 10 m or more seen in Endoparasite: Parasites living within the host. some tapeworms. Ectoparasite: Parasites that are found on the surface Most free-living nematodes barely attain naked- of the body. eye visibility as adults, but Ascaris can reach The essence of the parasitic relationship, which sep- 35 cm and Dracunculus as much as 1 m. arates it from predation, is the protracted and inti- The parasitic mode of existence may result in pro- mate association between parasite and host. found biochemical changes. Streamlining Adaptations to Parasitism Loss of certain metabolic pathways common The parasitic relationship probably evolved early in to free-living organisms. the history of living organisms. The parasite, no longer able to synthesize cer- The possibility of the adaptation of a parasitic mode tain necessary cellular components, obtains of existence may depend on what is known as pread- them instead from its host. aptation, or evolutionary changes that make possible Profound differences between metabolic existence in an unfavorable environment. pathways in parasite and host characterize: Preadaptive changes is due to increased resistance to Kinetoplastida (Leishmania and Trypanosoma) the enzymatic activities of the host. species in humans. Physiologic adaptations to parasitism might involve Entamoeba histolytica the loss of enzymes or enzyme systems, which are Giardia duodenalis (formerly Giardia lamblia) then supplied by the host. Trichomonas vaginalis Profound morphologic adaptations modifications Helminth parasites are more striking in those groups that are wholly Specialized mechanisms for effecting entrance parasitic than in those that contain both free-living into the body or tissues. and parasitic species. E. histolytica (pathogenic intestinal amoeba) Organs not necessary to a parasitic existence are fre- elaborates a proteolytic enzyme that aids its quently lost. penetration of the intestinal mucosa. 4 Markell & Voge's Medical Parasitology  Schistosoma sp. (blood fluke) in its cercarial Immune system of the host may also be affected. stage has penetration glands which produce The most widespread type of injury is that brought enzyme capable of digesting the skin. about by interference with the vital processes of the Hymenolepis nana (dwarf tapeworm) during host through the action of secretions, excretions, or its embryonic stage prior to developing into a other products of the parasite. cysticercoid larva, penetrates an intestinal vil- Mechanisms of parasite infection lus with the help of the six hooklets it bears. Fasciolopsis buski (Giant intestinal fluke) when Once within the host’s body, the parasite is present toxic symptoms in large numbers but subject to those defense mechanisms mobi- the precise cause is unknown. It may produce lized in the immune response. severe local damage to the intestinal wall by Continuation of a parasitic relationship de- means of its powerful suckers. In high intensity pends on how successfully the immune re- infection when there is a significant number of sponse of the host is overcome. adult flukes, intestinal obstruction may likely Many different defense mechanisms have evolved. occur. Immune evasion may involve the following Entamoeba histolytica erodes the intestinal wall, factors: destroying the tissues locally by means of a pro- Location of the parasite in relatively pro- teolytic enzyme. tected sites. Plasmodium falciparum (pathogenic species of Changes in the parasite surface antigenic malarial parasite) invade and multiply in red structure. blood cells, which are destroyed in the process Active modification of the host immune re- and may also attach to the walls of smaller blood sponse by products of parasite metabolism. vessels in the brain, occluding them to produce Increased reproductive capacity localized ischemia. Most metazoan parasites exhibit such an The helminth parasites, by virtue of their size, increase, which in some cases involves lar- may damage the host in other ways impossible val stages as well as adults. for the smaller parasites. The chances that a particular egg will Ascaris lumbricoides (Giant intestinal round- successfully infect a new host are usually worm) may perforate the bowel wall, cause intes- very small, and if more than one host tinal obstruction if present in large numbers, and species is involved, the chance of suc- ectopically may invade the appendix, bile duct, cessful completion of the cycle becomes or other organs. still smaller. Some parasites exert their effects by depriving the If a parasite is successful in infecting an host of essential substances. intermediate host, it is obviously advanta- Human Hookworms, like Ancylostoma duode- geous if the larval stage that develops there nale and Necator americanus, suck blood de- can multiply to produce many additional prive the host of more iron than is replaced organisms capable of infecting the defini- by diet and therefore leading to a case of mi- tive or a second intermediate host. crocytic hypochromic anemia. Such a modification is seen in the trema- The broad fish tapeworm Dibothriocephalus todes and many of the cestodes, where in latus (formerly Diphyllobothrium latum) selec- the intermediate host a single egg develops tively removes vitamin B12 from the alimen- into a larva, which in turn produces many tary tract, producing a megaloblastic anemia in larvae of a more advanced kind. some infected persons. PATHOLOGY: SIGNS AND SYMPTOMS Effects of the Host on the Parasite Effects of the Parasite on the Host Genetic constitution of the host influences the host- A parasite, by definition, is an organism that lives at parasite relationship. the expense of its host. Duffy null phenotype: Fy (a - b -): demonstrates Overt symptoms of infection with this parasite may resistance to Plasmodium vivax infection. depend on the number of worms present, the nutri- Sickle cell trait is associated with increased resis- tional status of the host, or both. tance to Plasmodium falciparum infection.  CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies 5 Diet or nutritional status of the host Infants born in such areas to a semi-immune A high-protein diet is unfavorable for the devel- parent are at birth, and for some time thereaf- opment of many intestinal protozoa. ter, partially protected by maternal antibodies A low-protein diet favors the appearance of acquired transplacentally. symptoms of amoebiasis. If infection with one such parasite takes A rich-carbohydrate diet favors the development place during the first few months of life, it of certain tapeworms. is likely not to be as severe as it would oth- Effects on the immune mechanisms of the host erwise have been, and repeated infections Every species of animal is naturally resistant to over the years keep the acquired immunity infection by many organisms that parasitize dif- at a high level and symptoms correspond- ferent species. ingly mild. In the case of certain strains of malaria, resistance If, on the other hand, such a person leaves may also be a racial phenomenon. the endemic area for a protracted period, the In some cases, it has been possible to adapt par- acquired immunity wanes, and on returning asites to hosts that they normally infect poorly to the endemic area that person may fare no or not at all. This does not necessarily involve better than someone becoming infected after changes in the host’s natural resistance but rather entering the endemic area for the first time. changes in the parasite. Acquired immunity can be demonstrated in Parasites and the Compromised Host many parasitic diseases, and it is generally found Natural defenses of a patient may be compromised and to be at a lower level than that produced by bac- poses him to be at risk of parasitic infection: teria and viruses. Issues arising to immunity to parasitic infec- tion: Risk factor Parasitic infection Occurs rarely following protozoal infec- Patient undergoing aggressive Toxoplasmosis tions. treatment for leukemia Probably never with helminth infections of Corticosteroids Acute amoebic colitis humans. State of malignancy: Primary gastric amoebiasis As yet, no useful vaccines have been devel- Reticulum cell carcinoma oped against protozoal or helminthic infec- Immunologically compromised Strongyloidiasis and tions. hosts trichinosis Although malaria is a likely candidate for a Acquired Immunodeficiency Toxoplasmosis vaccine, recent field trials of potential ma- Syndrome (AIDS) Cyclosporiasis laria vaccines have failed to meet expecta- Cryptosporidiosis tions. Cystoisosporiasis Primary infection with Leishmania seems to Strongyloidiasis confer a degree of immunity to reinfection. (disseminated form) While many protozoal and helminthic in- fections confer no long-lasting immunity to reinfection, they do seem to stimulate resistance while the parasites are still in the LIFE CYCLE: INFECTIVE AND DIAGNOSTIC body. STAGES, MODE OF TRANSMISSION This resistance to hyperinfection, known as Life Cycles of Protozoa and Helminths premunition, may be of great importance in Many parasitic organisms have but a single host, endemic areas in limiting the extent of infec- being transferred from one individual to another tion with plasmodia, hookworms, and other of the same species either through direct physical parasites. contact or by means of resistant or semi-resistant Acquired immunity may be very important in forms that are able to survive a period outside or modifying the severity of disease in endemic away from the host. areas, particularly diseases such as vector- Encysting protozoans: Infective stage is called the cyst. borne malaria and filariasis and snail-borne, Non-encysting protozoans: Infective stage is called schistosomiasis. the trophozoite. 6 Markell & Voge's Medical Parasitology  Examples: In certain cases, like among the coccidians, a cer- Entamoeba gingivalis, a commensal organism that tain host may act both as an intermediate and inhabits the mouth, has no cyst stage or other definitive host. means of survival outside of the host, and it All parasites undergo the different developmen- probably is transferred by direct contact. tal stages before it reaches its mature state and Trichomonas spp. (T. tenax, T. hominis, T. vagina- reproduction. This life cycle can be classified into lis) likewise is unable to form cysts, but it prob- the following based on the number of intermedi- ably can survive for short periods outside the ate hosts required. body. Monoxenous: involves only one intermediate Egg laying nematodes: Infective stage is called the host. embryonated egg. Heteroxenous: involves more than one inter- Examples: mediate hosts. Ascaris lumbricoides and Trichuris trichiura are ex- Life Cycle refers to the development of a parasite in amples of soil-transmitted helminthes that infect its earliest form and its survival and development in host through ingestion of embryonated egg. the outside environment and within the host. The Enterobius vermicularis is a contact-transmitted life cycle of parasite may simple or complex. This parasite whose embryonated eggs can be either cycle divided in three groups. ingested or inhaled. Vector-transmitted parasites A vector may also be a host if development of the Parasite Definitive Intermediate parasite takes place within its body. Group cycle involved host host If the arthropod is simply an instrument of Those with Ascaris Man/ Pig None passive transfer, we refer to it as mechanical vec- definitive sp. tor. If a fly, feeding on fecal matter containing host but does cysts of E. histolytica, becomes contaminated not require with some of these cysts, which it then trans- intermediate fers to food, it is acting as a mechanical vector host. of the amoeba. Those with Echinococcus Dog and Sheep, Goat, When an Anopheline mosquito sucks blood definitive granulosus other Swine, from a malaria patient, the parasites must devel- host with one canidae Cattle, intermediate Horse, op in the mosquito before she is able to transmit host required. Herbivores the infection. In this instance the mosquito is Human both acting as a host and biologic vector. Those with Fasciola Water 1st IH: Snail Some protozoa and many helminths have complex definitive hepatica1 buffalo/ 2nd IH: life cycles, with not one but two, and sometimes host and with Man Aquatic more, hosts. two or more plants or When more than one host species is necessary to the intermediate vegetation development of the parasite, that host in which sex- host required. ual reproduction occurs is called the definitive host 1Nowadays, Fasciola hepatica may encyst in water and therefore (DH). transmission happens even in the absence of second intermediate The species in which larval (or asexual if both sexual host. (Tolan, 2011) and asexual forms occur) stages of the parasites de- velop are called intermediate hosts (IH); they are usu- ally designated first and second intermediate hosts CONTROL, PREVENTION, AND TREATMENT if there is more than one. Preventive and control measures may be taken against Disconcerting as it may be to those with a strictly every parasite that is infective to human. These are de- anthropocentric point of view, humans are but the signed to break the transmission cycle are therefore intermediate host of the malarial parasite Plasmo- crucial for successful parasite eradication. Therefore, dium, which undergoes sexual reproduction in mos- preventing the infections to happen is to intervene in quitoes of the genus Anopheles. [IH – human; DH the life cycle of the parasite and prevent the successful – female Anopheles mosquito). entry of the infective stage to the host.  CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies 7 How to break the chain of Infection Parasite cluster Mode of transmission Infective stage Representative parasite Preventive measure Soil-transmitted Ingestion Embryonated Ascaris lumbricoides Hand washing helminths (STH) egg Trichuris trichiura Proper disposal of excreta Provision of clean toilet Skin penetration Filariform Hookworm Personal Protective Equipment: larva (L3 gloves larva) Use of slippers Food and water- Ingestion cyst Balantidium coli Handwashing borne Proper disposal of excreta metacercaria Paragonimus westermani Proper cooking of crab Skin penetration cercaria Schistosoma spp. Avoid contact with water Provision of clean water and toilet Snail control Drinking contaminated cyst Giardia duodenalis Water purification system water Iodine treatment Vector-borne Bite of an infected sporozoite Plasmodium sp. Protective clothing vector Mosquito net Insect repellent Integrated vector control Contact transmitted Direct contact Embryonated Enterobius vermicularis Proper hygiene: - Inhalation egg Handwashing - Ingestion Clean clothing - Autoinfection Sexually transmitted Direct contact Trophozoite Trichomonas vaginalis Avoid unprotected sexual contact - Sexual intercourse or multiple sexual partners Use of condom (Beaver et al., 1984; Belizario and De Leon, 2015) Parasitic treatment options include the following: Popularity of the tropics and subtropics as vaca- 1. Antiparasitic medication tion areas. Representative Modifications of the environment. Parasite group anti-parasitic drugs Flooding of vast areas creating new habitats for Protozoan Metronidazole the snail hosts (i.e., schistosomiasis). Nematode Mebendazole Global warming as a reason for the eventual Platyhelminthes: Praziquantel spread of diseases now seen primarily in the trematodes and cestodes tropics to more temperature climates. Parasitic infections among immunosuppressed 2. Therapies option and immunocompromised. a. Change in diet Therapy for parasitic infections b. Vitamin supplements Differential toxicity: antiparasitic agent is more c. Fluid replacement toxic to the parasite than to the host. d. Blood transfusion Individual variation in host resistance. e. Bed rest Toxic side effects. Risk factors for transmission of parasitic infections Treatment of certain parasitic diseases is changing Modifications of host resistance resulting in the which necessitates every physician to keep abreast of appearance of numbers of organisms in unfamil- the advances in this field. iar pathogenic roles. The Medical Letter on Drugs and Therapeutics. Failure of chemical treatment to eliminate the The Tropical Diseases Bulletin: vector-transmitted parasitic infections like ma- A monthly abstracting journal published in laria and filariasis. England, lists the worldwide literature in trop- Increased mobility of large segments of the pop- ical medicine. ulation. 8 Markell & Voge's Medical Parasitology  Drug Information for the Health Care Profes- Mastigophora: flagellates: equipped with flagellum, sional: a whip-like structure. Published yearly, originally by the U. S. Phar- macopeia but, since January 2004, now main- Phylum Ciliophora tained by Thomson Healthcare, Inc., in which Ciliata: ciliates: equipped with cilia, hair-like struc- the USP-approved drugs are listed by disease. tures. Some ciliates are multinucleate, while others contain but two nuclei, a large macronucleus and EXCITING NEW AREAS OF RESEARCH a small micronucleus. Role of eosinophils in killing young schistosomes The only ciliate parasite of humans is Balantidium and microfilariae. coli, found in the intestinal tract. Although rare, it Ability of older schistosomes to induce immuno- is important, as it may produce severe intestinal suppression in the host. symptoms. Discovery of host-like antigens on the surface of some parasites. Phylum Apicomplexa Phenomenon of antigenic variation in trypano- Parasites that are not equipped with definite loco- somes. motor apparatus. Role of cytokines, tumor necrosis factor (TNF) or ca- Demonstrates asexual cycle called schizogony and a chectin. sexual cycle called sporogony. Cachectin, a major secretory product of activated Contains apical complex that is composed of the macrophages, in low doses is protective against following: micronemes, subpellicular tubules, polar experimental malaria in mice, stimulates the kill- rings, conoids, rhoptries. ing of schistosomules by eosinophils in vitro, but Example: Plasmodium spp.: causative agent of malaria paradoxically is thought to bring about the state Babesia microti: causative agent of Nantucket fever of cachexia seen in trypanosomiasis. Toxoplasma gondii: causative agent of Toxoplasmosis Side effects of administration of TNF to cancer patients are almost identical to the various signs Phylum Platyhelminthes and symptoms seen in severe falciparum malaria. The Platyhelminthes, or flatworms, are multicellular Importance of the “secretions and excretions” of animals characterized by a flat, bilaterally symmetri- protozoa and helminths as antigenic substances cal body. stimulating host resistance. Most flatworms are hermaphroditic, having both In Trypanosoma lewisi infections in rats, the meta- male and female reproductive organs in the same bolic products of the parasites are more effective individual except schistosomes that have separate in producing immunity than are the dead try- sexes. panosomes themselves. Adults may be less than 1 mm long or they may Immunologic tests on the ability of the serum of an reach a length of many meters. infected host to precipitate the secretions or excre- Most members of the phylum are parasites, living tions of eggs, larvae, or adults of a number of differ- on or in the body of their hosts. ent helminths. 3 classes are recognized: Class Turbellaria (Roberts and Janovy, 2013) Mostly existing as free-living forms inhabiting IMPORTANT GROUPS OF ANIMAL terrestrial, freshwater and marine environments. PARASITES Ciliated epithelium envelops the body of the Protozoan adult worm. Exist as motile trophozoite stage and the nonmotile With or without suckers. infective cyst. Direct development without metamorphosis. Locomotor apparatus serves as one of the bases of These hermaphrodites are commensals or para- classification. sites of invertebrates especially of echinoderms and molluscs. Phylum Sarcomastigophora Class Trematoda: “flukes” Sarcodina: amoebae: equipped with pseudopods Leaf-shaped or elongate, slender organisms. (“false feet”).  CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies 9 Possess attachment organs in the form of cup- The life cycle requires an arthropod intermediate shaped muscular depressions called suckers. host: beetle, cockroach. An incomplete digestive tract is present. While thorny-headed worms are widely distributed Of the three orders of the Trematoda, the order among wild and domestic animals, only three gen- Digenea contains all the species that are parasitic era have been reported in human beings: Monilifor- in humans. mis, Macracanthorhynchus, Bulbosoma. Members of this order have complex life histo- ries, with at least one intermediate molluscan Phylum Arthropoda host. Arthropods are segmented and bilaterally symmetri- Included in the digenetic trematodes of humans cal animals with a body enclosed in a stiff, chitinous are forms that inhabit: small intestine, liver, covering or exoskeleton and bearing paired, jointed lungs, pancreas and blood vessels. appendages. Class Cestoda: “tapeworm” The digestive system is well developed. Elongate, ribbon-like, segmented body that bears Sexes are separate. a specialized anterior attachment organ called The phylum is subdivided into a number of classes, the scolex. many of which are of medical importance. A digestive tract is absent. Adult cestodes or tapeworms inhabit the small Class Crustacea intestine. Primarily aquatic forms, which breathe by means of Cestode larvae require an intermediate host for gills. development. However, Hymenolepis nana or the Crabs, shrimps, crayfish, and copepods. dwarf tapeworm, may or may not require an in- Serve as intermediate hosts of human parasites. termediate host. Autoinfection is also observed Freshwater crab: Paragonimus westermani: oriental in its life cycle. lung fluke. Humans may be host to either adult or larval Copepods: Dibothriocephalus latus (formerly stages, depending on the species of cestode. Diphyllobothrium latum): broad fish tapeworm. Class Chilopoda Phylum Nemathelminthes Centipedes are characterized by the possession of The nematodes, or roundworms, are elongate, cylin- one pair of legs on each body segment. drical worms, frequently attenuated at both ends. First pair of appendages is modified as poison claws. They possess a stiff cuticle, which may be smooth or may be extended to form a variety of structures, Class Arachnida particularly at the anterior and posterior ends. Spiderlike animals, possess a body divided into two The sexes are separate, the male frequently being parts, the cephalothorax and the abdomen. considerably smaller than the female and posses Adults have four pairs of legs. copulatory structures like spicule or bursa. Scorpions, the spiders, and the ticks and mites. A complete digestive tract is present. Scorpions and spiders produce venom, which in While most nematodes are free living, a large number some species may be extremely toxic. of species parasitize humans, animals, and plants. Ticks and mites may act as intermediate host for cer- Intermediate hosts are necessary for the larval devel- tain parasitic infection.Ixodes tick: Babesia sp. opment of some forms. Parasites of humans include intestinal and tissue- Class Insecta inhabiting species. From a medical or economic point of view, the most important of the arthropods. Phylum Acanthocephala Three pairs of legs and a body divided into three dis- The thorny-headed worms are all endoparasitic or- tinct parts: head, thorax, and abdomen. ganisms, the anterior end of which is modified into Several orders of insects are worthy of special mention. a hook-bearing, retractable proboscis that serves in attachment. Order Anoplura A digestive tract is absent. Sucking lice Sexes are separate, and males are usually smaller Wingless, dorsoventrally compressed insects than females. Human lice 10 Markell & Voge's Medical Parasitology  Order Hemiptera Derived from their body shape, which is elongate, True bugs, wingless bedbugs. and in some species tonguelike. Two pairs of wings, first pair has thickened membra- Other species have a ringed or annulated body. nous bases. Lack external appendages and possess two pairs of Cone-nosed bugs, or reduviids: vectors of Trypano- hooks near the mouth. soma cruzi that causes American trypanosomiasis or Adults live in the respiratory tract of vertebrates. Chaga’s disease. Encysted larval stages may occur in the lungs and other internal organs of humans, and they are found Order Coleoptera principally in tropical areas. Beetles Two pairs of wings, but the anterior pair is thickened Phylum Microsporidia throughout. Formerly classified with the Sporozoa Beetles are intermediate hosts of tapeworms: Minute intracellular parasites of many kinds of ver- Grain beetle - Hymenolepis nana tebrates and invertebrates Flour beetle – Hymenolepis diminuta Differ significantly in structure from the Apicom- plexa Order Hymenoptera Rarely cause disease in immunocompetent persons, Ants, bees, wasps but may do so with greater frequency in immuno- Medically important source of venom suppressed persons Ants serve as intermediate hosts for flukes: Fasciola Fungi-related organisms (Metenier and Vivares,2004) lanceolata, Eurytrema pancreaticum. Most human infections are caused by the following genera: Enterocytozoon and Encephalitozoon Order Siphonaptera Fleas Wingless and laterally compressed EPIDEMIOLOGY Fleas act as intermediate hosts of tapeworm: Dypi- Epidemiology is the body of knowledge that concerns lidium caninum. with occurrence and distribution of disease in hu- man population and communities. It may include Order Diptera the study to the manifestation of any time. Even One pair of true wings. though treatment, prevention, and control measure Mosquitoes, flies, and gnats. are available, parasitic infection still occurs and thus Some larval flies are parasitic in humans and ani- it is important to study and monitor their trends. mals. Prevalence is a statistical study referring to the num- Acts as vectors of parasites. ber of cases of a disease that are present in a particu- lar population at a given time. Mosquito : Plasmodium sp. : Malaria Incidence refers to the number of new cases that de- : Wuchereria bancrofti : Lymphatic filariasis velop in a given period of time Tse-tse fly : Trypanosoma brucei : Sleeping sickness Distribution of disease (Belizario and De Leon, 2015). gambiense : Trypanosoma brucei 1. Endemic When a disease in human population rhodesiense maintains a relatively steady, moderate Sand fly : Leishmania donovani : Kala-azar level Deer fly : Loa loa : Calabar swelling 2. Epidemic If there is a sharp rise in the incidence or an outbreak of considerable intensity Black fly : Onchocerca volvulus : River blindness occurs Gnats : Mansonella sp. : Commensal filarial 3. Hyperendemic If the prevalence of a disease in a worms community is high 4. Sporadic If it appears only occasionally in one or at Phylum Pentastomida most few members of the community All are endoparasitic forms. 5. Pandemics The disease covers extensive area of the Known as tongue worms, or linguatulids. world  CHAPTER 1 Introduction to Medical Parasitology and Basic Terminologies 11 Prevalence of parasitic infections Heterophydiasis: 1.6% prevalence in 2004 Estimates of the prevalence of parasitic diseases but increased to 10% in 2005 (Philippines) are at best extremely rough, as reporting of mor- (Montejo and Yumang, 2008). bidity is essentially nonexistent in many of the areas in which these diseases occur. WORLD DISTRIBUTION OF PARASITIC The following estimates are based on those of DISEASES the World Health Organization (WHO) through Figs. 1.1 through 1.12 show in rough outline the world 2002, the Centers for Disease Control and Pre- distribution of many of the important parasitic diseas- vention (CDC) through 2004, and others: es. Those with a restricted distribution are omitted, as Amoebiasis: approximately 1% of world popula- are those that occur essentially worldwide. tion infected; annual deaths, to 100,000; 2nd leading cause of death due to parasitic disease. NEWS UPDATE Giardiasis: worldwide, G. duodenalis (formerly G. lamblia) approximately >300 million cases 60 TO 80% OF POPULATION AT HIGH RISK FROM are reported annually; Giardiasis is 20% to PARASITES 40% of infection is highest among children Cebu City – As many as 80% of the Philippines’