The association between parasite and host can be divided into spatial and temporal relationships. In spatial relationship the part of the host’s body is exploited by the parasite i.e. endow and ectoparasitism. Temporal relationship means that is the time that the parasite spends in or on its host.
Gut parasites are permanent lodgers, certain ectoparasites attach to host while they feed and mosquito and to tsetse fly are examples and are called occasional visitors. Parasitism is a close coexistence of two species; beneficial to one partner (the parasite) and at the expense of the other (the host).
A facultative parasite (e.g. Claviceps can live and grow without a host, and it can be easily cultivated in vitro. An obligate parasite (e.g. rickettsias, viruses, rust and smut fungi) cannot survive separately from its host, to which has become adapted by evolution.
Parasitism may be regarded as a special form of symbiosis in which the predator, or the parasite, is much smaller than the prey and remains closely associated with it. Parasitism is harmful to the prey organism and beneficial to the parasite. The concept of parasitism seems obvious, but individual instances are often surprisingly difficult to distinguish from predation and from other kinds of symbiosis
Parasites are of two types:
Parasites that feed on exterior surface of an organism are external or ectoparasites, e.g. lice, which live on the bodies of vertebrates.
Vertebrates are parasitized ‘ internally by endoparasities, members of many different phyla of animals and protists. Invetebrates also have many kinds of parasites that live within their bodies. Some parasites live in or, just beneath the skin, others in the gut, others in the tissue fluid between the cells (intercellular parasites), and so on. The most ‘endo’ of all parasites are those that penetrate individual cells. The malerial parasite and certain parasitic fungi are examples of such ‘intracellular parasites’.
Benefits Gained by Parasites:
The two main benefits gained by the parasites are shelter and food. For maximum shelter endoparasites are in the best situation they are ensured of a stable environment from which nourishment can be derived with the minimum of effort. In some cases this involves no more than soaking up the host’s body fluids.
Admittedly gut parasites have a special problem in that they must protect themselves from being digested by the host’s digestive enzymes, but this can be achieved by the production of appropriate inhibitors. These inactivate the enzymes in the immediate vicinity of the parasite’s body.
Harms Suffered by the Host:
In extreme cases, where the parasite feeds on the tissues, much harm, even death, may be suffered as a result of sheer damage to the cells. Such is the case with parasitic fungi such as Phytophthora infestans, the cause of potato blight, and Pythium debaryanum which causes dumping off in seedlings.
Poisonous substances that stimulate the formation of antitoxin in the animal body. Several fungi produce toxins. Toxins produced by Aspergillus niger are termed aflatoxins. These aflatoxins belong to the group mycotoxins. They are natural carcinogens causing liver cancers, and they are 100 times as active as previously known liver carcinogens.
They are also produced by Aspergillus flavus, A. parasiticus and A. oryzae as well as Penicillium islandicum. They are coumarin difuran derivatives. In higher plants toxins are less common. Aflatoxins are produced by various fungi while growing on mustrard and groundnut cakes. This substance if eaten by animals cause disease in animals. From Claviceps fungus is extracted a substance called LSD (diethylamide tartarate of dilysergic acid of ergot or lysergic acid diethylamide) which is hallucinogenic in man i.e., causes distortion of perception.