A parasite usually parasitizes a host which is larger in body size than it. Further, a parasite does not ordinarily kill its host, at least not until the parasite has completed its reproductive cycle. However, a host may die due to some secondary infection or suffer from stunted growth, emaciation, or sterility.
The balance between Parasite and host is upset if the host produces antibodies or other substances which hamper normal development of the parasite. In general the parasite derives benefit from the relation while the host suffers harm. But this does not mean that all parasites are harmful. The former category constitutes the pathogenic parasites.
Classification of parasites:
Parasites exhibit a tremendous diversity in ways and adaptations to exploit their hosts. The parasites may be viral parasites (e.g., bacterial, plant animal viruses), microbial parasites (e g., bacteria, protozoa, etc.), phytoparasites (e.g., plant parasites) and zooparasites animal parasites such as platyhelminthes, nematodes, arthropods, etc.). They may parasitize micro-organisms, plants and animals. They may occur on the outside of the host (ectoparasites) or live within the body of the host (endoparasites).
The endoparasites usually live in the alimentary tract, body cavities, various organs or blood or other tissues of host. Ectoparasites may be parasitic only in the immature stages—the hairworm larvae, parasitic in aquatic insects; only the adults may be parasitic—fleas on birds and mammals; or both larvae and adults may be parasitic—the blood sucking lice and flies, biting lice, mites, and ticks that occur on birds, mammals, and sometimes reptiles, and the monogenctic trematodes on fish.
Similar relation occurs in endoparasites but in them usually all the stages of life are parasitic—entozoan amoebae, trichomonad flagellates, palinode cilia’s, sporozoans, pentastomids, nematodes, digenetic trematodes, acanthocephalans, custodies, and some copepods.
Further, endoparasites rely on various means of transport from one host to another so that their survival, range of distribution and life cycle are unaffected. For the end parasite; the interior of its host is its microhabitat and microenvironment, while the outside environment is the microenvironment.
The host acts as a bulfer between the parasite and the outside environment changes in the microenvironment directly influence the parasite alterations in the macroenvironment influence it indirectly. Thus, mutual adaptations and tolerance between the host and parasite are important factors facilitating successful transmission of endoparasites, and climatic fluctuations tend to alter the physiology of the parasite, host or vector.
Mutual adaptations are the parasite’s abilities to establish and maintain itself in a favorable location within the host and exist in a proper form for sufficient length of time enabling subsequent transport to another host and continuation of species.
Animals may also be parasitic to plants. Nematodes infest the roots of plants. Wasps or gnats form galls on plants such as oaks, hickories, willows, roses, goldenrods, and asters. Mites stimulate formation of witch’s brooms in hackberry. A variety of insects the larvae of which are leaf miners, wood borers, cambium feeders, and fruit eaters should be included here.
Lastly, parasites may be full-time (or permanent) parasites or part-time (or temporary) parasites. Mosquitoes and bugs that suck the blood of their hosts are temporary parasites. Some temporary parasites spend only a part of their life cycle as parasites.
For example, glochidium larva of Anodonta (fresh-water mussel) attaches itself to the body of the fish by means of its hooks and penetrates inside the fish integument to remain buried there for several weeks and finally emerges out as the young mussel to lead an independent existence.
Permanent parasites, however, spend their life completely on other organisms. The common examples of permanent parasites are—Plasmodium, Entamoeba histolytica, and other proto zoan pathogens, different platyhelminthes, nematodes, arthropods, etc.
While studying parasitism as a biotic factor influencing the life and activities of parasites and hosts we often encounter manifold adaptations, both offensive and defensive, being developed by the host, as well as by the parasite. An end parasite entering a host often meets with the antibodies or phagocytic cells produced by the host. The ectoparasites and endoparasites have following parasitic adaptations:
1. In parasitic animals, a reduction of organs of special sense of nervous system, and of locomotors organs occur.
2. Most ectoparasites develop some clinging organs such as hooks, suckers, etc., to get attached with the body of their hosts. They also develop special piercing and sucking organs to suck the blood of animals or sap of plants. Certain blood-sucking ectoparasites such as leeches, blood-sucking mosquitoes, etc., contain certain anticoagulant enzymes in their salivary secretions.
3. Most endoparasites exhibit anaerobic respiration, high rate of reproduction, parthenogenesis, hermaphroditism, polyembrvony, intermediate hosts and a complicated life-cycle. Some endoparasites, such as tapeworm, have become so adapted to the host that they no longer require a digestive system.
They simply absorb their food directly through their body wall. Further, parasites that live within the bodies of plants and animals possess cuticles or develop cysts resistant to the digestive enzymatic action of the host.
4. Many parasites pass their entire existence in a single host; others require one, two even three intermediate host. It is of ecological significance that both primary and intermediate hosts of a parasite occur in the same habitat or community. Even then the hazards to successful passage from one host to another are so great and mortality so high that prodigious quantities of offspring are produced to ensure that at least a few individuals will complete the cycle.
5. Parasites are transfer from one host to another by active locomotion of the parasite itself; by ingestion, animal sucks the blood of or eats another; by ingestion as an animal takes eggs, spores, or encysted stages of the parasite along with its food or drinking water; as a result of bodily contact between hosts; or by transportation from host by way of vectors.
For example bacteria that cause tularemia in man are carried from rabbit to rabbit by ticks. Man contacts the disease when he handles infected rabbits but the incidence of infection is greatly reduced in the autum when cold weather forces the ticks (i.e., the vectors of bacteria) to leave the rabbits and go into hibernation.
Host specificity of parasites:
Certain parasites, such as copepodes, contain a wide range of hosts and they are present in various invertebrates and fishes. Most parasitic genera, however, are adapted to hosts of one phylum only.
For example, each order of birds possesses its own particular species of tapeworms. This is true even when several orders of birds live in the same habitats as do, for instance, grebes, loons, herons, ducks, waders, flamingoes and cormorants (Baer 1951). Species of flagellate protozoans that occur in termite alimentary canals are largely host-specific. Some species of gall wasps attack only one species of oak.
Where a single species parasitizes two or more host species, the shape and structure of the gall formed around the egg and larva on both hosts is essentially similar. When several insects are found on the same oak, each kind of parasite produces its own characteristic gall form. The parasite-specific gall formation is found to be related with parasite-specific enzyme (Kinsey, 1930).
Certain parasites remain restricted to special habitats within the host. The roundworm lives near the duodenum of the digestive tract and soil nematodes live in the rootlets of plants. Biting lice remain restricted to the head or body regions of birds.
Some zoo- parasitic nematode species are found throughout the body in connective tissue, but not in the gut; some occur only in the digestive tract and associated organs; certain species occur in the glandular crop of birds, but others only in the caecum; many species occur exclusively in the lungs or in the frontal sinuses.
Such fine restrictions of parasites to particular microenvironments—hosts or organs is a consequence of precise physiological and morphological adaptations that permit the parasite to survive and complete the life cycle only under very special conditions. Likewise, the life cycle of the parasite is often closely synchronized in time with that of its host (Foster, 1969).
Effects of parasite on the host: disease:
Parasites may not cause immediate mortality but they cause damage to body structures, should it become excessive, may cause death. Because of these parasitically caused anatomical damages, the finely adjusted balance of different vital processes of host’s body become disturbed and host is said to be diseased.
There are a number of causes of diseases. Parasites are one; physiological stress, nutritional deficiency and poisoning are others. Some of common parasitic agents of disease and consequent mortality of animals are following:
1. Viruses are the potent agents of several disastrous diseases of plants and animals including man For instance, some viruses are the agents of hoof and mouth disease in deer, spotted fever in rabbit, encephalitis and distemper in foxes.
2. Bacteria may produce localized inflammatory changes in tissue, enter the blood stream, or produce powerful poisons known as toxins. They cause a variety of disease, notably tularemia paratyphoid, and tuberculosis among birds and mammals, as well as other diseases in lower types of organisms.
3. Fungus spores of Aspergillus may are drawn into the lungs of ground-feeding birds, where they germinate and grow, causing aspergillosis disease. Fungus may also develop on the external surface of animals.
4. Protozoan parasites are especially important in the alimentary tract and in the blood. A sporozoan species of Eimeria damages the walls of the intestine in upland game birds and causes coccidiosis disease; Toxoplasma becomes encysted in the brain of rodents; Leucocytozon is a common blood parasite of waterfowl and game birds.
5. Worm parasites, such as tapeworms, nematodes, and acanthocephalans may wander through the hosts body doing mechanical injury as well as destroying and consuming tissues. The host may respond by forming a fibrous capsule or cyst around an embedded parasite.
6. External parasites such as ticks, fleas, lice, mites, and flies do not commonly produce serious mortality by themselves, but they are often vectors, transmitting protozoa, bacteria and viruses from one animal to another. Heavy infestations of external parasites however lower the vitality of an animal and cause diseases of feathers.
7. Nutritional deficiency in vitamins or minerals, or improper balance among carbohydrates, proteins, and fats may produce malformations, lack of vigour, even death.
8. Food poisoning, botulism, occurs when certain food become contaminated with the toxins released by the bacterium Clostridium botulinum.
9. Physiological stress (Selye, 1955) is a term that has come to be applied to changes produced in the body non-specifically by many different agencies which may accompany any disease. Effects of stress include loss of appetite and vigour, ache and pains, and loss of weight.
Internally, physiological stress causes following abnormalities, such as, acute involution of the lymphatic organs, diminution of the blood eosinophils, enlargement and increased secretor activity of the adrenal cortex, and a variety of changes in the chemical constitution of the blood and tissues.
10. Accidents, aging, starvation, and so on, must also be included as important causes of mortality.
Organisms that produce disease generally fall into one or two categories. They are either present in the body at all times but not normally virulent, or they are normally absent but are virulent the moment the host is infected by them. Further, a single attack, even a mild one of some diseases often confers a partial or complete immunity from further attacks of same disease.
Like parages, disease can reduce populations, exterminate them locally, or strict the distribution of host. For example, smallpox, intention-unintentionally introduced nearly exterminated some tribes 5 North American Indians (viz., Human populations).
Social parasitism describes the exploitation of one species by another, for various advantages. It is a kind of parasitism in which the parasite foists the rearing of its young into the host. Social parasitism in various stages of development is found among some higher vertebrates and insects.
For example, there occurs a egg parasitism in two species of birds —old world cuckoos and the brown-headed cowbirds of North America, both of which do not build nests of their own, rather they deposit their eggs in two nests of other species, abandoning eggs and young to the care of foster parents. Similarly, there occurs a brood parasitism in between two Indian birds—Indian koel and crow.
The Indian koel (Endynamys scolopacea scolopacea), like many of its cuckoo cousins (family Cuculidae), neither incubates its eggs nor feed its young. But these functions are secretly forced upon the house crow (Corvus splendens) and jungle crow (Corvus macrorhynchus (Davis, De and Pal, 1977).
One species of ant waylays foraging workers of another species and snatches away the food they are transporting or the robber species may deliberately rob another nest of food. Some species of ants make slaves of the workers of other species.
Such kind of dependency of one species on another also occurs in other social insects such as termites, waps and bees. Social insects are apparently the only animals other than man to have succeeded in domesticating other species and of cultivating plants particularly fungi for food.
Some Diptera and Hymenoptera deposit their eggs in the immature stages of other insects the larvae on hatching feed on the host until they are fully grown. In such type of parasitism in which host generally dies of the larval depredations before the larva emerges and the parasitoid larva generally lives in spite of the hosts death, is called parasitodism.
It stands in between parasitism and predation. The parasitoid may in turn be infested with hyperparasicoids. In the Chicago, Samia cecropia, a saturniid moth, suffers the destruction of nearly 23% of its cocoon by an ichneumonid parasitoid, Spilocryptus extrematis, which deposits an average of 33 eggs on the inside of each cocoon or on the surface of larva.
The host larva dies in a few hours after the parasitoid hatches and the ichneumonid larva moves about freely, feeding on the cuticle by burrowing into the tissue to drink the body fluids. S. extrematis is infested by another hyperparasitoid, aeno- plex smithii which causes about 13% destruction of S. extrematis cocoons. A chalcidid, Dibrachys boucheanus, fed both upon S. extrematis and as a tertiary parasitoid, upoh A. smithii.