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What Is Parasite?

Updated on September 28, 2015

Parasitology primarily deals with the study of parasites and the relationships with their hosts. These parasites obtain shelter and nutrients from their host. The hosts on the other hand, do not enjoy any known form of benefit from the parasites except that the association between a parasite and host, leads to the formation of an ecological association termed parasitism. Also these organisms inflict harm on the host, thereby causing several forms of pathogenic effects on the host. They affect the health of man, his domestic animals and those in the wild which sometimes find their way into human food chain. The net effect is morbidity and in some cases,mortality of the hosts. Funds are wasted in attempts to cure, prevent and eradicate these parasitic infections. This no doubt, have adverse effect on the economy of any nation.

The host maybe:

1. Intermediate host: When it houses the immature stages of parasite. Development occurs here but these parasites do not reach sexually matured adult.

2. Paratenic (transport) host: When it harbours the immature stages but these stages do not undergo any known developmental stage. It is important to mention that these stages are ineffective and can be transmitted to the definitive host.

3. Definitive host: When it harbours the adult of sexually matured stage of a parasite. A parasite may be.

A. Ectoparasite: When it lives on the host

B. Endoparasite: When it lives inside the host

Most ectoparasites are arthropods such as lice while the bulk of the endoparasites are found among the protozoa. Platyhelminthes and the Nematodes.

Protozoa are microscopic acellular or unicellular, most primitive and simplest forms of organisms.

Basically, these protozoan parasites are conveniently grouped into:

1. Sarcomastigohora which is divided into sarcodina and mastigophora (flagellates)

2. Ciliates

3. Apicomplexa

4. Cnidospora

The platyhelminthes are commonly called flatworms. They dorsoventrally flattened and lack body cavity, hence called acoelomates. They are grouped into three classes as:

1. Turbellaria-Members of this class are free living flatworms

2. Trematoa- These are commonly referred to as flukes. The class is further divided into monogenea; those with single host and digenea; those with at least two hosts. The monogenea are mostly free living forms while digenea mostly exhibit parasitic life. Cestoda Cestodes are commonly called tapeworm and exhibit parasitic life.

Nematodes are commonly referred to as round worms. They have slender cylindrical body. They possess external protective cuticle. However, their body wall do not possess true segmentation. Also they lack true body cavity. Hence nematodes are called pseuodcoelomates. Nematodes exhibit sexual disnorphism and produce several larval forms. They can be divided into two broad group on the basis of presence of phasmids. Those that lack phasmids are referred to as aphasmidian. However, they possess highly developed amphids. Those with highly developed phasmid are called phasmidian. It is important to mention that these amphids and phasmids are cuticular depressions with sensory functions.

The aphasmidian is made up of only the following three families in the phylum as:

1. Family Trichuridae

2. Family Capillaridae

3. Family Trichinellidae

While the other families comprise the phasmidian nematodes. Parasites have developed various adaptive mechanisms which enable them to strive best in their immediate host environment and adapt favourably to their parasitic life, Nmorsi(1993) enumerated some of these mechanisms as:

1.The presence of adversive organs like hooks and suckers for attachment to the viscera of the host. These are common features of the platyhelminthes especially tapeworms.

2. Reduced or absence of sense organs

3. Various modifications in nutrition. This is typified by the absence of alimentary cana in cestodes. Instead, the tegument of the tapeworms is highly modified and it serves as an active metabolic absorptive surface for uptake of nutrients.

4. Increased reproductive capacity as manifested by production of numerous egg/ova in helminthes and cysts in some protozoa. These structure have strong ecological significance as they enable these parasites to adapt to new adverse environmental conditions like in the soil or water outside the body of the host. The net effect is the increase transmission potential of these parasites.

5. Presence of polyembryony; a phenomenon which involves the formation of several larval forms.This is common feature of the helminths. For instance, in digenean parasites, the larval forms are miracidium, sporocystredia, cerceria and metacerceria. In nematodes, there are about three stages of larvae sandwiched by moulting and growth. Also among the filarial worms, different forms of microfilarial stages are produced. An interesting fact here is the ability of these larval forms to occur in different intermediate hosts before reaching sexually matured adult in their respective definitive hosts. The utility of these numerous hosts by larva forms of any parasite makes prevention and control measures uneconomical, cumbersome and often extremely difficult. For instance, onchocerca volculus, a filarial worm and the causative agents of onchocerciasis develops in blackflies. Similium before infecting man. In this case, control must be directed towards this arthropod as well as man.

Some parasites are known to penetrate the host subcutaneously.The parasites can penetrate the skin before embarking on their somatic migrations in the body of their hosts. This is exemplified by hookworm and shistosoma.

Some parasites are transmitted by vectors which include mostly members of he phylum arthronoda andmolluscs. The parasites may undergo development to attain infective stages within the vector as in onchocerca volvulus that uses black fly and wuchereia bancrofti and plasmodium that use mosquitoes. In some cases, they do not undergo any known cyclical development as in Trypanosoma evansi are transmitted mechanically. Some parasites are mostly water borne and are transmitted only when the host takes in infected or contaminated water. This is exemplified by Guinea worm infection. Parasites like T.vaginallis is regarded as the cause of one of the major general disease globally. It is acquired through sexual intercourse. Therefore, the endemicity is related to the level of sexual abuse in any society


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