Host-Parasite Relations

A knowledge of parasite life cycles is crucial in the understanding of the ways infection is acquired and spread, the pathogenesis of disease, and the ways in which disease might be controlled. Some parasites which infect only humans, such as Enterobius vermicularis (pinworm), have a narrow host specificity, whereas others such as Trichinella spiralis infect numerous species. When other animals harbor the same parasite stage as humans, these animal species may serve as reservoir hosts. Humans infected with a parasite stage usually seen in other animal species are referred to as accidental hosts. 

TABLE 1.1 INCIDENCE OF INTESTINAL PARASITES IN 322,735 FECAL SPECIMENS EXAMINED BY STATE HEALTH DEPARTMENT LABORATORIES 

In some helminth infections, a migration through various body tissues is essential for maturation, as in ascarasis or schistosomiasis, whereas in other infections, the larva leaves the egg and simply matures in the intestinal tract, as in trichuriasis and enterobiasis. Host tissues involved vary depending upon the parasite. In severely immunocompromised patients, sites may be involved that are not involved in normal hosts. 

Parasites of humans proliferate tremendously at certain stages, with thousands or even millions of forms being produced for every one that survives to perpetuate the parasite. Parasites may be quite hardy. For example, certain stages, particularly eggs and cysts, may survive for weeks or months in the environment. 

Parasites have often developed unique ways of protection from the defense mechanisms of the host. 

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Dobson, C. and Tang, J. (1991) Genetic variation and host-parasite relations Nematospiroides dubius in mice. Journal of Parasitology 77, 884-889. 

Dropkin, V.H. and Boone, W.R. (1966) Analysis of host-parasite relations of root-knot nematodes by single larva inoculations of excised tomato roots. 

Muller M, Lindmark DG, McLaughlin J (1976) Biochem Parasites Host Parasite Relat 537-544... 

Kirby EJM. Host-parasites relations in the choke disease of grasses. Trans Br Mycol Soc 44 493-503, 1961. 

A number of mathematical models for host-parasite relations have been published. For references to literature appearing before 1977, see Fredrickson ( ). A more recent model has been given by Levin al. (76). 

Bernard J, Riou G 1976 Biochemical action of ellipticine derivatives in Trypanosoma cruzi. In H. van den Bossche (ed) Biochem Parasites Host-parasites Relate. Proc Int Symp. Orth-Holland, Amsterdam p. 477... 

In a relationship of parasitism the population that benefits, the parasite, normally derives its nutritional requirements from the population that is harmed, the host. The host-parasite relationship is characterized by a relatively long period of contact, which may be directly physical or metabolic. Normally, the relation is as specific as for example the viruses, which are obligate intracellular parasites of bacterial, fungal, algal, and protozoan populations. 

Watson D Host-parasite factors in group A streptococcal infections. Pyrogenic and other effects of immunologic distinct exotoxins related to scarlet fever toxins. J Exp Med 1960 111 255—284. 

In parasitic flatworms, 5-HT serves a variety of functions from control of carbohydrate metabolism to intra- and interorganism (host-parasite) communication (36). Rhythmical movement is stimulated by 5-HT and related indoleamines (e.g., lysergic acid diethylamide) (7,8,37). This stimulatory effect, like the ACh inhibitory effect, can be obtained in the absence of the central ganglia suggesting a peripheral site of action (37). The stimulation of motility (increased contraction amplitude and frequency) in vitro suggests that 5-HT or a closely related indoleamine is an excitatory transmitter in both the trematodes F. hepatica (9, 37, 38), S. mansoni (11, 12, 39, 40), Clonorchis sinensis... 

In addition to the parasite-related considerations described above, there are many additional issues which must be addressed before any potential chemotherapeutic agent can be considered a candidate for clinical studies. These generally come under the headings of pharmacokinetics and pharmacodynamics. The former generally deals with the rates of absorption and elimination of a compound whereas the latter is concerned with the distribution of the compound in the body, its effects on particular organ systems, and other interactions with host metabolic processes. 

One of the early vaccine candidates was directed against sporo2oites, the form of the parasites that is first injected into the host by a mosquito. With recent development of recombinant techniques, several circumsporo2oite proteins or its related peptides were proposed as the vaccine candidates. Clinical trials have been carried out. The vaccines were immunogenic, but did not provide sufficient protective efficacy (90,91). 

Gasser, R.B. and Hoste, H. (1995) Genetic markers for closely-related parasitic nematodes. Molecular and Cellular Probes 9, 315—320. 

A number of attempts have been made to select on nematode immune-related traits. Lines of Heligmosomoides polygyrus have been selected in hosts with different histories of previous exposure (naive, once previously exposed, or multiply previously exposed). This resulted in parasite lines that differed in fitness when tested in semi-immune animals, with the lines selected in the hosts with greatest previous exposure surviving best and... 

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