Types of parasitism

There are several types of parasitism. Parasitism itself is defined as an association between two specifically distinct organisms in which one lives on or within the other in order to obtain sustenance.

Symbiosis is the permanent association of two specifically distinct organisms so dependent upon each other that life apart is impossible under natural conditions. The relation between many termites and their intestinal protozoa is symbiotic. The termites eat wood, but they cannot digest it; the protozoa can digest wood, turning it into glucose, but they have no way of obtaining it; working together, the termites ingest wood particles, the protozoa break the cellulose down to glucose, and the termites then digest the glucose. Lichens furnish another example of symbiosis. They are composed of certain species of algae and fungi living together.

Many insects, ticks and mites have symbiotic bacteria and rickettsiae. The symbiotic organisms are found either in special cells, the mycetocytes, in modified parts of the Malpighian tubules, or in special organs, the mycetomes. It is significant that, among blood-sucking arthropods, symbiosis occurs in those which live on blood thruout their life cycles (ticks, lice, bedbugs, kissing bugs, tsetse flies, hippoboscid flies) but not in those in which only the adults suck blood while the larvae are free-living (fleas, mosquitoes, phlebotomines, tabanids and stable flies). Blood lacks some metabolites which the arthropods are unable to synthesize themselves and for which they depend on their symbiotes. These metabolites appear to include vitamins of the B group and probably other substances as well (Buchner, 1953; Koch, 1956; Weyer, 1960).

Mutualism is an association of two organisms by which both are benefited. It differs from symbiosis in that it is not obligatory for both partners. One example often cited is that of a sea anemone living upon the back of a crab. The anemone is benefited by being moved to new hunting grounds and by obtaining morsels of food torn off by the crab, while the crab is protected by the bulk and stinging tentacles of the anemone. Another marine example is that of the scorpion fish of Indo-Malaya. It lives on the bottom of the sea, where it lies in wait for passing fish. It is covered with a crust of hydroids which camouflage it so that it can seize its unwary prey more easily. The hydroids presumably benefit by being moved to new sources of food and by being provided with a dwelling-place. However, since they can live other places beside the scorpion fish’s back, their relation is mutualistic.

Another example of mutualism, and one closer to us, is the relationship between ruminants and the cellulose-digesting bacteria and other microorganisms in their rumens. The latter are furnished a favorable home by their hosts and aid them by breaking down cellulose to usable compounds. The rumen-dwelling bacteria which produce B group vitamins and thus make an outside source of them unnecessary for ruminant nutrition probably belong here too, altho they verge on the symbiotic. The bacteria which produce these vitamins in the large intestine of swine are more nearly mutualistic, since the pigs cannot absorb the vitamins thru the colon wall but must re-ingest their feces to obtain them. The same is true of rabbits, and is undoubtedly responsible for their coprophagy.

The bizarre protozoa which swarm in the rumen and reticulum are almost certainly mutualistic. Their host can get along without them, but they may benefit it by providing a better type of protein than it ingests. In addition, they are an important source of volatile fatty acids, and they smooth out the carbohydrate fermentation process.

Commensalism is an association between host and parasite in which one partner is benefited and the other is neither benefited nor harmed. Many intestinal bacteria such as Escherichia coli are normally commensals, as are many intestinal protozoa such as Entamoeba coli and Trichomonas spp.

The next two terms both refer to potentially pathogenic parasites. Parasitosis is the association between two organisms in which one injures the other, causing signs and lesions of disease. Parasitiasis is the association between two organisms in which one is potentially pathogenic but does not cause signs of disease.

The difference between parasitosis and parasitiasis is quantitative. In parasitiasis the host is able to repair the damage caused by the parasite without noticeable injury, while in parasitosis it cannot. As Whitlock (1955) put it, "Parasitiasis is a state of balance. Parasitosis is a state of imbalance". Applying the concept to ruminant helminths, Gordon (1957) said, "Helminthiasis is almost universal and continuous, helminthosis is more restricted and sporadic. However, one shades imperceptibly into the other in subclinical infestations". The same organism can cause either parasitosis or parasititiasis, depending upon the number present or upon the nutritional condition, age, sex, immune state, etc. of the host. Failure to recognize this distinction may cause many false diagnoses - the mere presence of a potentially pathogenic species of parasite does not necessarily mean that it is causing disease.

The carrier state furnishes a good example of parasitiasis. Carriers are animals which have a light infection with some parasite but are not harmed by it, usually due to immunity resulting from previous exposure, but which serve as a source of infection for susceptible animals. Thus, adult sheep and cattle may be lightly infected with gastrointestinal nematodes without noticeable effect, but their lambs and calves may become heavily parasitized from grazing with them. The condition in the adults is parasitiasis; that in the young is parasitosis. Adult chickens rarely suffer from coccidiosis because they have recovered from a clinical or subclinical attack when young. However, they are usually still lightly infected and continue to shed a few oocysts; they have coccidiasis. Cattle which have aborted as a result of Brucella infection may continue to shed the bacteria in their milk without ordinarily suffering further clinical attacks. The aborting cow has brucellosis, while the carrier has brucelliasis.

These endings can also be applied to the names of the disease agents, as has already been done above. Thus, Haemonchus contortus may cause haemonchosis or haemonchiasis, Taenia may cause taeniosis or taeniasis, Histomonas meleagridis may cause histomonosis or histomoniasis, depending on the circumstances.

It was mentioned earlier that the solutions different parasites have made of their problems of living have varied in satisfactoriness. We might consider this in regard to type of parasitism. Symbiosis is a highly specialized type of association which occurs only in certain groups. Mutualism is a much looser association, also fairly uncommon. It could well be a step on the road to symbiosis. The most common types of parasitism are the last three. Of these, commensalism is clearly the most desirable, both from the standpoint of the host (which isn't harmed) and of the parasite. Parasitosis, which harms the host, is in the long run harmful to the parasite also. By injuring their hosts, parasites harm their environment, and if they are so indiscreet as to kill their hosts, they die too. Parasitiasis is intermediate between parasitosis and commensalism in some cases, but not in all.