CONFIRM INFOZ

 The Adult Digenean Fluke

The basic body form of the adult trematode takes a number of different forms,some of which are describe below;

Amphistome

 

These have largefleshy bodies, witha prominent suckerat the posterior ofthe body(e.g.Gastrodiscoides hominis )

Distome

 

These are the mostcommon type, with the mouth surrounded by the oral sucker and a ventral sucker, present anywhere on the ventral surface except the extreme posterior (e.g. Fasciola hepatica )

Echinostome

Similar to the distomes, except that the oral sucker is surrounded by a prominent collar, equiped with spines (e.g.Echinostoma sp.)

Monostome

   In these there is either only one sucker present (usually only the oral sucker), or there are two suckers, but one very reduced, or in some cases no suckers)(e.g. Notocotylus attenuatus )

Schistosome 

 

 

    Elongate trematodes, with separate sexes, the male generally larger, holding the female within a groove formed by a folding of the male body (the gynaecophoric canal). Found within the circulatory system. (e.g.Schistosoma mansoni) There are other forms as well, for example the 'Holostome' type, where the body of the trematode is divided into two distinct regions, the anterior of which may hold an additional adhesive organ, (e.g. Diplostomum sp.), and the 'Gasterostome', where the gut is a very simple, sac like, structure, attached to a mouth situated near the centre of the body (reminiscent of the arrangement of some of the free living platyhelminthes).

The Basic Lifecycle of the Major Groups of the Digeneans

   Most digeneans are hermaphroditic (the major exception being the schistosomes, and one other group). In the majority of these parasites self fertilization may occur, but cross fertilization between different individuals is more generally the rule. The sperm enter the female system, either via the Laurers canal or more commonly through the common genital atrium, which opens into the uterus.

The Digenean Trematode Egg

  The formation of the digenean egg follows that described for the platyhelminthes as a group. Briefly, as the egg enters the öotype of the fluke it becomes surrounded  by a predetermined number of vitelline cells, the number of which will be specific for different parasites, which form the food reserve of the egg. These vitelline cells produce globules of a mixture of proteins and phenols, which are extruded to theouter surface of the developing egg.

  Here the phenols oxidise to form quinone, which then coalesces with the protein, reacting to form scleratin, a hard inert yellowish substance, making up the egg shell. As the eggs of different species may vary in thickness, their colours may vary from yellow, to a dark brown.The digenean egg is usually operculate, in common with other platyhelminthes.

  Exceptions to this may occur however, the most important being with the schistosomes. Here the eggs are non-operculate, and are ornamented with spines, the appearance of which are characteristic for different species of schistosome.

  The eggs hatch of operculate eggs involves the release of the opercular cap. This takes place under a variety of conditions, modified according to the particular species of trematode. For example some trematode lifecycles involve the ingestion of the egg before hatching (e.g. Dicrocoelium dendriticum, the lancet fluke), whilst others such as those of Fasciola hepatica, (the liver fluke), hatch in water.

   For the eggs that hatch in the external environment, a number of factors may be important,for example light, temperature and changes in osmotic pressure. Again the exact details of these environmental requirements will be optimised for the particular conditions which will maximise the chances of completion of the parasite lifecycle. In all cases the egg hatches to release the miracidium.

NEMATODE INFECTIONS

Enterobius vermicularis - Pinworm, Threadworm.

     E. vermicularis infection, a widespread nematode infection, particularly in temperate areas such as Western Europe and North America, ( being relatively rare in the tropics) and particularly in children. It has been estimated that the annual incidence of infection is over 200 million, this probably being a conservative figure. Samples of Caucasian children in the U.S.A. and Canada have shown prevalence of disease from 30% to 80%, with similar levels in Europe.

 

Ascaris lumbricoides - 

The Large Human Roundworm. Again the incidence rates for this parasite are very high with > 1500 million cases of infections annually, of which about 210 million cases are symptomatic.

  Trichuris trichiuria - The Large Human Roundworm.

The incidence rates for this parasite are also very high, with estimates of about
1300 million cases of infection annually, of which >133 million cases are symptomatic.

The Hookworms.

These are represented by two parasites, Necator americanus in the tropics and subtropics worldwide and the S. E. states of the U.S.A., and Ancylostoma duodenale, again with a worldwide distribution in the tropics and sub tropics as well as the Mediterranean region. There are > 1200 million cases of hookworm infection annually, of which about 100 million cases are symptomatic.

Lymphatic filariasis - Elephantiasis

This disease is caused principally by two parasites, Wuchereria bancrofti with an annual rate of infection of about 106 million cases, and Brugia malayi with a yearly rate of infection of approximately 12.5 million. The total number of people infected with other types of lymphatic filarial worms is much smaller, at about 1.5 million cases. These lymphatic filarial worms, along with the related filarial parasite Onchocerca volvulus, are unusual among the nematodes in that they develop with,
and are transmitted by insect vector intermediate hosts.

 Onchocerca volvulus - River Blindness

   The incidence rates for this parasite are not as high as some of the previously described parasites, with an annual rate of infection of about 18 million, however, due to the extreme pathology associated with this parasite, often with all adult members of affected villages losing their sight, along with severe skin conditions, the infection is significant.

Dracunculus medinensis - Guinea Worm

The incidence rates for this parasite are much lower, with an estimated annual rate of infection of about 100 000. This is much lower than in the recent past when up to 50 million people were infected. This reduction in incidence illustrates how successful helminth control programs can be useful in reducing the disease caused by these organisms.


Other significant nematode infections include; Trichinella spiralis, Strongyloides stercoralis, and several more rare infections. Nematodes that usually infect other animals may still cause disease in man. These include Toxocara canis and several nematodes causing anisakiasis.

Digenean Trematode Infections 

Schistosomiasis - Bilharzia.

Schistosomiasis is caused by Schistosoma mansoni, S. haematobium, S. intercalated, S. japonicum, and S. mekongi. This disease is the most important human helminthiasis in terms of morbidity and mortality. The numbers of people infected are lower than those of many of the nematode infections, with an estimated annual incidence of infection of > 200 million cases. In terms of active

disease, however, the parasite is much more important, with an estimated annual
the mortality rate of about 1 million deaths directly due to infection with these parasites.

Opisthorchis Sinensis - The Chinese Liver Fluke

This is also a significant trematode infection, with an estimated annual incidence of infection of about 20 - 30 million cases, mostly in the Far East, in Japan, China, Taiwan, and South East Asia.

Paragonimus spp. - The Lung Fluke

This fluke causes pulmonary disease, the adult parasites living in the lungs of their definitive hosts (e.g., man). There are several different species of this parasite, the most well documented being P. westermani in the Far East. It may, however, be locally widespread, with up to 40 to 50% of the population infected.


There are several other digenean trematode infections. These include various
Echinostome infections as well as several other flukes. Also, there are
a number of these parasites that usually infect domesticated animals, but also
cause well known human infections as well. These include Fasciola hepatica and Dicrocoelium dendriticum.

Cestode (Tapeworm) Infections

Taenia saginata - The Beef Tapeworm 

This only causes very limited pathology in man, but the annual incidence of infection is high, at an estimated 50 million cases.


Taenia solium - The Pork Tapeworm


This has a similar estimated annual incidence of infection of about 50 million cases. However, in this case, the consequences may be more severe, due to the added risk of contracting infection with the larval metacestode, (cysticercosis).

This may have extreme consequences in terms of the pathology associated with infection, with an estimated annual mortality rate of about 50,000 deaths. For the cestodes, these annual incidence rates are based on the detection of infection with the adult parasite. This is achieved by examination of feces, urine or sputum for parasite eggs.

Diagnosis of infection with larval metacestode parasites, such as Echinococcus sp. is very difficult, due to the lack of noninvasive diagnostic techniques. It is in consequence very difficult to estimate annual rates of infection, even though these metacestodes may be very important pathogens.

SUMMARY

Parasitic helminths are grouped into nematodes, cestodes, and trematodes. The nematodes are the most diversified groups. Parasitic helminths infect a wide range of hosts, ranging between man, domestic animals, and wild animals. Morbidity is often high leading to the death of several thousand of people.

Types of Parasitic Helminths host

INTRODUCTION

Parasitic helminths may have either complicated or straightforward lifecycles. The terms used to describe the hosts harboring different stages in these lifecycles are however the same.

The degree of damage done to the hosts is however varied. For example, in the definitive host, the greatest harm is seen being the one the adult stage of the parasite is found. Sometimes, a host might assume dual functions, and therefore could be challenging to classify strictly into one type. Human host during infection by the malarial parasite is one of such. A human could be classified as the
definitive host is the one in which the greatest harm is seen. Also, intermediate host because of human harbors the asexual stages of the parasite (merozoites and trophozoites). A clear understanding of the relationship between host and parasite and function of the host in survival and transmission of the parasite is, therefore, necessary for a better classification.

Definitive Host

The adult parasites are found in the definitive host. This is where the parasite's the sexual cycle usually takes place, with either cross or self-fertilization with hermaphroditic parasites, or sexual reproduction if the parasites have separate sexes, followed by the production of eggs, or more rarely with viviparous helminths, Larvae. The greatest harm is usually seen in this host.

Intermediate Host

  In many cases the larvae of the parasite are found in different hosts, these are called the Intermediate Hosts. Parasitic helminth larvae may have one, two or more intermediate hosts in their lifecycles, or they may have no intermediate hosts.

 Often asexual stages of reproduction occur in these intermediate hosts, (for example with Platyhelminth parasites). Note that when describing hosts of parasitic protozoa, these terms are slightly different owing to the sexual characteristics of many of these organisms.

With parasitic protozoa, the vertebrate host is generally referred to as the definitive host, while the invertebrate is the intermediate hosts. Some parasitic nematodes (e.g., Strongyloides stercoralis) are
Facultative parasites, having completely free-living lifecycles in addition to parasitic ones. The two terms definitive and intermediate host are the most important in Parasitology when referring to the type of host. A vector, however, should not be mistaken for an intermediate host. A vector actively transmits the infection to a host without necessarily harboring the asexual stage of the parasite, e.g., the vector of African trypanosomiasis Glossina spp.

  These groups of vectors pick up the parasite (the infective stage) from the reservoir hosts during the blood meal and transmit it to the susceptible definitive host. However, some vectors can still serve as intermediate hosts harboring the asexual or the larval forms of the parasite e.g.individual Anopheles mosquitoes that harbor the microfilariae of filarial worms.

Accidental Host

  Accidental hosts are those in which the parasite does not usually develop (due, for example to lack of exposure to infective forms of the parasite), but when occasionally chance infections occur, the parasite can complete its lifecycle.

Hosts, where the parasite can complete its lifecycle, are called Permissive hosts and include true definitive and intermediate hosts as well as many accidental hosts. Examples here include such parasites as Fasciola hepatica, where the normal definitive hosts are ruminants, but humans and other animals may also be infected, and viable adult parasites develop. Another example is a human infection
with the nematode Angiostrongylus cantonensis in the far East.

   In comparison another form of the accidental host is the Non-Permissive host where the parasite, although it may develop to some extent, reaches a dead-end effectively, the parasite not being able to complete its lifecycle and eventually dying within the host. These forms of infection often occur where the parasite has intermediate hosts which may be accidentally ingested by animals other than the true definitive host. For example, with various marine ascarids of the family Anisakidae such as Anisakis sp., which give rise to the condition of 'Anisakiasis.' on ingestion of raw infected fish.

Paratenic Host

  Paratenic hosts may also be included in parasitic helminth lifecycles. In these forms of infection, the parasites undergo an arrested development on contamination, larval forms accumulating in these hosts until they have a chance of infecting the definitive host (e.g., in the Pseudophyllidean tapeworms). These hosts are therefore not essential to the completion of the parasite's lifecycle. This is in contrast to the case with true intermediate hosts whose ingestion is necessary to the lifecycle, for
example, Echinococcus sp.

Reservoir Host

  These are accidental hosts and hosts of parasites which have zoonotic patterns of infection (i.e., usually infect a wide range of hosts), may act as Reservoir Hosts for the parasite. These are also a form of permissive hosts as fully viable infections develop, and a more accurate term would be definitive alternative hosts (though this is not in fact used). The term reservoir host is usually only used when describing the epidemiology of human infections. An example of parasites with zoonotic diseases is Schistosoma japonicum.

This parasite, as well as infected man, can also affect other mammals as definitive hosts, including rodents, cats, dogs, domesticated ruminants such as water buffalo and a wide range of other mammals. In Human African Trypanosomiasis (HAT), the reservoir host is cattle which serve as sources of active infection to man. The presence of these Zoonoses has implications for the control of the parasite in the field.

SUMMARY

  The host of a parasite is the organism that harbors the parasite. The intermediate host is the one in which part of the developmental stages in the life cycle of the parasite takes place, usually the larval or the asexual stage. A vector actively transmits an infective stage of parasite harbored by the reservoir host which serves as a source of active infection to the definitive host. The definitive host
harbors the adult or the sexual stage of the parasite and often experiences the most enormous harm during the life cycle of the parasite.

Classification of the parasitic organism

 

 

   Organisms in these associations may either be on the outer surface of the host organism, (in which case the prefix Ecto- is used) or inside the host organism, (in which case the prefix Endo- is used). These prefixes may be used with any of the animal associations listed above. For example, the flagellate protozoa in the termite guts are Endosymbionts, while the anemone can act as an Ectocommensal with the crab.

  Parasites may act as both ecto- and endoparasites. Parasites may also be classified according to the closeness of the relationship. For example, Facultative Parasites (such as many bacteria) are those where the parasitic lifestyle is only taken up opportunistically, whereas Obligate Parasites (such as all viruses, and most of the helminth parasites described below) are those in which the organism must parasitize another organism.


   These parasites may often cause diseases, in which case they are referred to as Pathogenic Parasites.
In a somewhat broader interpretation of the term parasitism, some organisms exhibit parasitic behavior only early in their life cycle, these being referred to as Brood Parasites. Examples of these include caterpillars of the Large Blue butterfly, which chemically mimic other caterpillars with mutualistic associations with ants but both fail to produce honeydew as compensation and consume ant grubs, and may, in fact, destroy the nest, (thereby acting as a pathogenic parasite for the ants).


In this case, the parasitic lifestyle probably evolved from the mutualistic lifestyle of the other, related butterflies, again illustrating how one form of association may change into another. Another well-known example of a brood parasite is a bird, the cuckoo.

    Some parasites establish themselves in hosts in which they do not ordinarily live. These are called the Incidental Parasites. A temporary parasite is free-living during part of its existence and seeks its host intermittently to obtain its nourishment whereas Permanent Parasite remains on or in its host’s body from early life until maturity, sometimes for its entire life. A parasite that has passed
through the alimentary tract without infecting the host are called Coprozoic or Spurious Parasite.

   Parasites often lack the necessary organs for assimilation of raw materials and depend upon the host for predigested food. An adequate supply of moisture is assured inside the host but during the free-living existence of the parasite,

Inadequate moisture may either prove fatal or prevent larval development. Temperature is likewise important. Each species has an optimal temperature range for its existence and development. Both high and low temperatures are detrimental and even lethal.

SUMMARY

  Organisms interact with each other at different degrees. While some are solely dependent on each other, however, some are opportunistic and can adopt various means for their survival. One association can evolve into the other. An example is seen in commensalism, which can develop into mutualism or parasitism.

   Some parasites reside within their hosts while others are outsides. Obligate parasites live in living tissues, whereas the facultative parasites are more successful in that they are capable of taking other sources of nutrition. Pathogenic parasites are of great medical concern because they cause diseases. The survival of a parasite in its habitat/microhabitat can be influenced by moisture and temperature.

Types Of Parasitic Association

Symbiosis

   Here both organisms are dependent on each other. Examples being the association of flagellate protozoa in the gut of termites, where termites are dependent on the protozoa for breaking down their foodstuff, and the protozoa are dependent on the termites as host organisms.

Another good example here which is often cited is the association between clownfish and anemones in tropical reefs; where the fish is dependent on anemone for protection and food while the anemone does not appear to gain anything from the association, except possibly cleaning. However, it has been observed that in some cases, in the absence of the fish partner the anemones tend to disappear from their reef home, indicating a true symbiotic rather than a mutualistic or commensal relationship.


Another well-known example is found with the lichens, symbiotic association composed of fungi and algae. These associations may become very close, and it is thought that the eukaryotes as a group evolved as a result of such an association. Intracellular organelles such as the mitochondria and chloroplasts appear to have their origin as intracellular symbiotes of early eukaryotes, (some extremely primitive eukaryotes, such as the intestinal parasite Giardia lamblia, lack these
organelles).

Other forms of symbiosis may be much less close, for example, an organism that uses another plant purely as a means of dispersal. For instance, bacterial or fungal spores on the legs of flies, or coelenterates and barnacles on the carapaces of marine crustaceans. This particular form of symbiosis is sometimes called Phoresis.

Mutualism

   Here the associates may or may not be dependent on each other for their existence, but both benefit when they are associated. An excellent example of this occurs with the association of sea anemones on the backs of crabs. Both gain from the association 10(the anemone providing some food for the crab, which in turn gives extra motility to the anemone), but both can survive on their own.


   Another less well-known example is found between certain species of ants and the caterpillars of some of the Lycaenidae butterflies (notably the 'Blues'), where the caterpillar is protected by the ants within their nests, in return for which the caterpillar secretes a honeydew which the ants collect. In this case, from the ant, it benefits from the association but does not appear to need it, (i.e., the association is facultative, or opportunistic).
However, from the caterpillar, this association is required for its survival (i.e., the association is obligatory). This illustrates that these definitions may become blurred, and, over time, one form of association may evolve into another.

Commensalism

  Neither organism is dependent on the other for its existence, but in this case, only one of the partners benefits from the association, the other being unaffected. An
example of this, found in humans, are the non-pathogenic obligate commensal protozoa such as the amoebae Entamoeba gingivalis, commonly found in the
mouth, feeding of bacteria, dead epithelial cells and food particles. Purely commensal relationships tend to be rather rare, as on a closer inspection element of mutualism or parasitism may become apparent.

Parasitism

Here one of the associates lives either partly or wholly at the expense of the other associate, the other partner (the host organism) not gaining anything from the association. This association may give rise to extreme pathology in the host, or the parasitism may generally be not very pathogenic.

Dependency is carried out by many organisms, the main groups including viruses, bacteria, protozoa (these usually being endoparasitic), and various metazoan groups (multicellulareukaryotic animals), these were mostly groups of helminths (often endoparasitic), and arthropods (usually ectoparasitic), as well as some higher organisms, such as ectoparasitic lampreys and hagfish. Generally, however, for partly historical reasons, the term parasitology only refers to the study of infection with the eukaryotic protozoan, and invertebrate metazoan parasites, not bacteria, viruses or the higher chordate parasites, even though these are parasites in the real sense.

Introduction to Parasitology

Parasitology to a lay man is the study of parasites. A parasite is an organism that lives in or on a host causing harm to the host. Also, parasites are metabolically dependent on their hosts. Good knowledge on the biology of a parasite, transmission cycles, and human behaviors that promote transmission is necessary to eliminate or reduce infection.

The course, therefore, introduces students to the biology of common parasites in the helminth category in tropical countries and some other countries of the world. To facilitate learning, diagrams on their morphologies and life cycles are included in the guide. For coherence, epidemiology and control are treated simultaneously.

The contents of this course, therefore, will serve as the basis for studying parasites.
Information on the diagnosis of parasites will equip those willing to practice clinical parasitology with practical knowledge.

The Course

This course Guide tells you briefly what to expect from reading this material which bothers on the study of parasite “Parasitology.”
The association ‘Parasitism’ is a harmful association in favor of the parasite which likely evolved from a non-harmful association called commensalism. The location
and the behavior of parasites has enabled us to place them into different groups.
The parasite can not survive without a host organism. Therefore the study of the host in parasite transmission is equally important.
Parasites can be grouped into those with zoological, veterinary, and medical importance. Those invading livestock (veterinary parasites) and humans (human parasites) are touched in this Course Guide. Generally, parasitic helminths are
grouped into trematodes, cestodes, and nematodes. The digeneans are the only groups with medical importance in the class Trematoda, while nematodes form the majority of the helminthic infection. The entire course focuses on the following; lifecycle,
morphology, diagnosis, epidemiology, and control of the parasite.

Life cycle:

This involves the routes of transmission and parasite development from the immature to the adult stage. Some helminths have direct lifecycles involving no intermediate host. Examples in this category are Ascaris lumbricoides, hookworms, and Enterobius vermicularis. On the other hand, some show long lifecycles, which
could either be homogenous or heterogeneous. Homogenous indirect life cycle utilizes a single intermediate host. e.g., Schistosoma spp that make use of specific
snail intermediate hosts for intramolluscan development. In heterogenous long lifecycle, two or more intermediate hosts are utilized in the transmission cycle. e.g.
Diphyllobothrium latum that makes use of cyclops and fish as the first and second intermediate hosts respectively.

Morphology:

This is gross anatomy. Each helminth group has generalized features. The morphologies of the adults are often different from the immature stages. The adult trematodes are often leaf-like with two suckers (the oral and ventral suckers).
The reproductive system is often hermaphroditic. The cestode body is divided into head, neck, and strobila. The integuments are metabolic in function. The distinctive
morphological features of either the adult or the larval form are often used in the diagnosis of helminth infection.

Diagnosis:

The methods used in the detection of the parasite in the host is dependent on various factors. The site of infection of the parasite, morphological indifferences in some closely related species, the cost-effectiveness of different diagnostic methods and
introduction of new technologies and proficiency of laboratory technologists to manage these new technologies. 

Parasites that inhabit the intestine will best be diagnosed through stool examination .e.g. Ascaris lumbricoides. In infection due to Paragonimus spp whose adult flukes reside in the lungs, diagnosis is made through sputum examination. Usually, the determination of parasitic helminths is made by looking out for the striking feature(s) of the eggs laid by the adult parasites. However, when the
eggs of two or more closely related species are morphologically indistinguishable, such as those of the two Hookworm species (Necator americanus and Ancylostoma duodenal) and the hexacanth eggs of Taenia spp., the adult parasites could be used.


The conventional method diagnosis (the use of a microscope) can sometimes show limitation, especially in the detection of parasites with very low intensity. Therefore,
more sensitive methods like Enzyme-Linked Immunosorbent Assay (ELISA) and parasite gene amplification using Polymerase Chain Reaction (PCR) are some of the
modern technologies currently available. Nevertheless, these methods are underused in Nigeria due to the cost and lack of technological know-how.

Epidemiology and control

– Epidemiology deals with the study of the cause of diseases and factors favoring transmission. In epidemiology, a good understanding of the transmission cycle is necessary. This will enable one to target control at stage(s) which will break the transmission cycle of the parasite. 

The killing of the snail intermediate hosts of Schistosomiasis will lead to a significant reduction in
disease occurrence. In this case, the intramolluscan development, which gives rise to the infective stage ‘cercaria’ will not occur. Epidemiology also offers knowledge of
the behaviors that predispose people to infection.

THE BASIC LIFE CYCLE OF THE MAJOR GROUPS OF NEMATODES

The life cycles of the parasitic species vary considerably, as would be expected from such a large and diverse group. There are however a number of common features. Firstly, the parasite undergoes a series of moults through larval stages (designated L1 to the adult L5 form). Secondly, in most (but not all) nematodes it is the L3 larvae that is the infective form, important exceptions to this being the
Ascarids, such as Ascaris lumbricoides and the pinworms, where it is either the L1 larvae, or eggs containing L1 or L2 larvae that are infective.

    Thirdly the L3 form onwards in all species undergoes a migration within the body of the definitive host as it matures into the adult parasite, usually via the bloodstream or lymphatic system to the heart, lungs, trachea, and then to the intestine. Finally, in most cases  the parasite leaves the definitive host as thin walled eggs in the faeces, important exceptions being the viviparous filarial worms (where L1 larvae infect intermediate hosts, usually in the blood meals of biting arthropods), Strongyloides stercoralis, (where the L1 larvae are found in the faeces), and the viviparous Trichinela spiralis, where the larvae do not leave the body as such but develop to the L3 stage which then encysts in the muscles, infection being by ingestion of undercooked contaminated meat.

  Infection of the definitive host may be by a variety of routes, such as the oral route, where eggs are accidentally ingested, also many filarial worms are infective via the bite of flies, as previously decribed, and the L3 larvae of many nematodes such as the hookworms and other related nematodes are directly invasive. In terms of complexity, the purest life cycles are those of the pinworms, where adults living in the colon mate and lay eggs which pass out in the faeces, infection being either by the oral route with berries, or perianal, where eggs hatch around the anus, and L1 larvae migrate back through the anus.


  The most diverse is probably that of S. stercoralis, where there are a number of alternative lifecycles which it may undergo, either as a completely free-living soil nematode, or as the standard infective L3 larvae with tissue migration to the intestine, or even occasionally full completion of the life cycle within the intestine, and finally immunocompromised hosts a life-threatening disseminated infection can occur, with parasites found throughout the body

Nematodes are roundworms with pseudocolour (lacking a mesothelium lining). The alimentary canal is complete having mouth and anus. The intestine is a tubular structure composed of a single layer of columnar cells possessing prominent microvilli on their luminal surface. The excretory system of some nematodes consists of an excretory gland and a pore with a complex tubular excretory system, which is usually H-shaped.

   Nematodes are generally bisexual. Males are generally smaller than females. A copulation between a female and a male nematode is necessary for fertilization except in the genus Strongyloides, in which parthenogenetic development occurs (i.e., the event of an unfertilized egg into a new individual). The parasite undergoes a series of moults through larval stages
with L3 larva mostly being the infective stage. Infection of the definitive host maybe by accidental ingestion of eggs, bite of flies and skin penetration by the infective L3 larval form as in the case of hookworms.