Parasitism: Meaning and Types | Zoology

In this article we will discuss about the meaning and types of parasitism.

Meaning of Parasitism:

Parasitism means “eating from the table of another”. Parasitism is an obligatory relationship between two hetero-specific organisms in which the parasite, usually the smaller partner, either harms its host or in some sense lives at the expense of the host.

The organism which thrives at the expense of another organism in/on which it lives is called a parasite and the organism species which harbours the parasite is called its host.

The parasite is metabolically depen­dent on the host and the most commonly encountered dependency is nutrition. Unlike commensals, parasites derive their nutrients directly from their host. Parasitism is obli­gatory because the parasite cannot normally survive if it is detached from its host.

However, it is now possible to culture or maintain some parasitic species in vitro with the help of very complex chemical media and apparatuses.

The intimate relationship between a parasite and its host generally involves the exposure of the host to some antigenic substances of parasitic origin. Antibodies, on the other hand, are produced by the host in response to the parasites. Thus, unlike phoresis and commensalism, parasitism also involves immunologic response on the part of the host in addition to involving metabolic dependency on the part of the parasite.

Types of Parasitism:

In animal kingdom several types of parasitism are recognised. If a parasite lives on the surface of its host or superficially embedded in the host’s body surface, it is called an ectoparasite, e.g., ticks, mites etc.

Parasites that live within the body of their host in locations such as tissues, body cavity, alimentary canal, liver, and lung are known as endoparasites, e.g., Taenia solium living inside the intestine of human, Plasmodium spp in the human RBC.

Organisms that do not absolutely depend on the parasitic mode of life, but are capable of adapting to it if placed in such a relationship is known as facultative parasite. The amoeba, Naegleria fowleri, causing primary amoebic meningoencephalitis in human is a facultative parasite. Most parasites cannot complete their life cycle without spending at least part of the time in a parasitic relationship.

They are called obligatory parasite, e.g., Taenia solium. However, many such parasites have free- living stages outside any host, including some periods of time in the external environment within a protective eggshell or cyst. When a parasite enters or attaches to the body of a species or host, different from its normal one, it is called an accidental or incidental parasite.

For instance, some nematodes parasitic to insects may live for a short time in the intestine of birds or for a rodent flea to bite a dog or human. Accidental parasites usually do not survive in the wrong host, but in some cases they can be extremely pathogenic. For example, Toxocara canis, a common nematode of dog and cat, can cause visceral larva Migrans in humans.

Some parasites live their entire adult lives within or on their hosts, and may be called permanent parasites like Taenia solium. On the other hand, temporary, or intermediate parasites, such as mosquitoes or bedbugs, only feed on the host and then leave them.

Temporary parasites are often referred to as micro-predators in recognition of the fact that they usually “prey” on several different hosts or the same host at several discrete times.

Predation and parasitism are conceptually similar in that both the parasite and predator live at the expense of the host or prey. However, parasite normally does not kill its host, has only one host at each stage in its life cycle, and is symbiotic. The predator, on the other hand, kills its prey, has numerous prey, and is not symbiotic.

An erratic parasite is one that wanders into an organ in which it is not usually found. A periodic or sporadic parasite is one that visits its host intermittently to obtain some metabolic requirements. Pathogenic parasites are the causative agent of disease stage (chronic or acute) in the host. For instance, Plasmodium spp causes different malarias and Entamoeba histolytica causes amoebic dysentery in human.

Parasites that survive only on living organisms/tissues are known as biotrophs; e.g., Plasmodium spp and most other para­sites. Parasites that live on the host even after the death of the host are called necrotrophs; e.g., the blowfly Lucilia cuprina is parasitic to sheep but also help to destroy carcasses.

Parasitoids:

These are some insects, typically flies or wasps whose immature stages feed on their hosts’ bodies; usually other insects, and finally kill the hosts. Parasitoids resemble predators in this regard, but the former requires only a single host individual.

Many authors prefer to use the term protelean to describe all insects whose immature stages are parasitic but adults are free-living, e.g., the strepsipteran insect Corioxenos antestiae, whose larvae are parasitic to pentatomid bug.

Hosts:

On the basis of the role the host plays in the life cycle of a parasite, parasito­logists differentiate various types. A defi­nitive host is one in which the parasite reaches sexual maturity. For example, malarial parasite Plasmodium spp reach sexual maturity and undergo fertilization in the mosquito, which, by definition, is, there­fore, the definitive host.

Sexual repro­duction has not been clearly shown in some parasites, such as amoebas and trypanosomes, and in these cases the definitive host is arbitrarily considered to be the one most important to humans. An intermediate host is one that is required for the completion of the parasite’s life cycle and in which the parasite does not reach sexual maturity.

For example, molluscs and arthropods commonly serve as first and second intermediate hosts in which digenetic trematodes complete a part of their development.

A paratenic or transport or transfer host is one in which the parasite does not undergo any development but remains alive and infective to another host. Thus paratenic hosts may bridge an ecological gap between the intermediate and definitive hosts. The thorny-headed worms undergo development in insects that pick up the worms’ eggs from owl faeces.

Shrew eat the insects regularly where worms remain encysted in their mesenteries. Owls, when they catch these shrews, become heavily infected with the worms. In this case, the shrew acts as the transport host between the intermediate host (insect) and definitive host (owl).

Any animal that harbours an infection which can be transmitted to humans or other animals is called reservoir host. Examples include dogs with Leishmania spp that cause Leishmaniasis (Kala-azar) to human. Dogs, cats and armadillos with Trypanosoma cruzi that serve as reservoir host of Chagas’ disease to humans.

In this regard, it should be pointed out that a number of parasites can cause diseases in both human and other animals. The term zoonosis designates such diseases that are transmissible to human from other vertebrates like Chagas’ disease, Kala-azar etc. The disease, transmissible from invertebrates to humans, is also referred to as zoonotic.

Finally, many parasites may host other parasites, a condition known as hyper-parasitism. Thus a hyper parasite is an organism which parasitises another parasite. Examples are Plasmodium spp. in mosquitoes.

The protozoa Urosporidium spisuli is a parasite (hyper parasite) in the body cells of the nematode Sulcascaris sulcuta, which, in turn, is a parasite of the surf clam, Spisula solidissima and other marine molluses. Another protozoa Nosema dollfusi is a hyper parasite of the larval stage of a flat worm Bucephalus cululus, which, in turn, is a parasite of the American oyster Crassostrea virginica (Fig. 6.6).

Majority of the parasites are of different species than their hosts. However, exceptions do occur. In Trichosomoides, a nematode parasite of rats, the male lives its mature life within the uterus of the female worm, taking nourishment from her tissues. An even stranger relationship is evidenced in one species of anglerfish in which the male bites the skin of the female and sucks her blood and tissue fluids for nourishment.

Thus, many definitions are sometimes arbitrary and situations do exist that defy easy assignment to one of the precise categories. Many symbiotic associations cannot be classified with certainty as to the effects of the symbiont on the host. An apparent case of commensalism may have damaging effects on the host that human have not even thought about.

Conversely, a case of assumed parasitism may, on longer inspection, turn out to be commensalism. For instance, adult tape-worms are universally regarded as parasites, yet in some cases they have no known ill-effects on their host. So, definitions used here, are not exactly precise but are always useful and necessary in communication.