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Parasites and Hosts Relations Over Evolutionary Time Research Paper

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Updated: May 5th, 2022


Most parasites become less toxic to their hosts over evolutionary time. According to the scientific research conducted on parasites today, an evolutionary examination concludes that the parasites would virtually be harmless to the principal host organism. This research has been backed up by the scientific notion that it is because the host serves as the environment where the parasite generally survives and sexually copulates (Price, 1980). Consequently, when the parasite causes any toxic harm to its host organism, it would be counterproductive to its survival as well. As a result, the characteristics of the parasite that is relatively harmless to the prime host, for example as found in cats, would be anticipated to evolve. This is because the parasites that behave in that manner would have a better rate of reproduction.

On the other hand, the parasite would equally be anticipated to be rather harmful to the secondary host organism. This is explained by the fact that the parasite will never get to the main host unless the secondary host is consumed by the main host. Subsequently, within the secondary host, the behavior of the parasite that makes the secondary host more at vulnerable risk to being consumed by the primary host would be anticipated to evolve. The parasites that act as a result of that would therefore have a better opportunity of getting to the main host and hence reproducing into it. Therefore, within the secondary hosts, which more often than not comprises rodents such as mice and rats, it is evident that the parasite attacks the host’s nervous system and creates odd modifications in the behavioral characteristics of the host. An example of such an odd modification is illustrated in the infected mice and rats’ behavior of becoming attracted to the smell of cats instead of being repelled away. This lethal attraction in rats is caused by the infection of Toxoplasma Gondii parasites.

Currently, human beings are equally victims of the evolutionary strategy that is conducted by the parasite on the host. Similar to rats and mice, human beings act as secondary hosts to the parasites. Most infected people fruitfully battle out the parasite, but in others, the nervous system is invaded, thus creating odd behavior in them as well (Roger, Pedro & Gonzalez, 2006).

Hosts’ Reactions to Parasites

Hosts react to parasites in various ways varying from their behavioral and morphologic characteristics. For instance, plants create certain substances to prevent them from harmful parasites. Vertebrates have intricate and well-advanced immune systems. They produce a corporeal fluid that aims to battle out the parasites in the course of physical contact with them. This exhibits the morphologic characteristics of animals (Tinsley, 1999). Animals are also identified to respond to parasites using behavioral counter actions. For instance, roundworm eggs get collected during the preceding year hatch all together as a whole. Sheep also exhibit behavioral counter actions by avoiding grazing on the open meadow during the spring season. In addition to this, the infected fruit flies consume alcohol to act as self-medication against parasites that are transmitted through the blood. Human beings produce Immunoglobulin E antibodies. These act as resistance to the parasites (Viney, Read, & Chappell 2002).

The Evolutionary Aspects of Parasites

Parasitism is a general form of life that takes place autonomously and numerous times in the course of evolution. More than fifty percent of all living organisms have a minimum of one parasitic period in their life spans. This is equally common in fungi and plants. Furthermore, nearly all independently living animals serve as hosts to one or more parasites (Lewis & Campbell, 2002).

Parasites undergo evolution to counteract the mechanism of their host species. As the consequence of the resistance of the hosts, some parasites evolve adaptations that are precise in certain hosts, therefore specializing until the level where they can infect only one distinct species. This kind of constricted host specialization by the parasites can be costly to them during the evolutionary period though this only occurs when the host species becomes extinct. For this reason, many parasites can infect numerous host species that are more or less strongly interrelated, with diverse rates of accomplishments (Stearns & Hoekstra, 2005).

The counteractive mechanisms in the hosts equally evolve in reaction to the nature of the infections made by the parasites. Hypothetically, parasites may have a benefit in this evolutionary defense mechanism because of their extra fast reproduction timeframes. This is due to the fact that hosts procreate less fast than parasites, and hence have less probability to adjust than how their parasites do during a particular life span (Hurd, Lane, & Chappell, 1998).

In certain cases, parasite species may also co-evolve with their host species. Long-term coevolution from time to time leads to a reasonably firm relationship between the host species and the parasite species which leads to commensalism or mutualism. Equally important is the fact that it is in the evolutionary significance of the parasite that its host flourishes. A parasite may evolve to develop into being less detrimental to its host or a host may evolve to deal with the inevitable existence of the parasite (Webster, 2009). The eradication of certain parasites that coexist with the host can cause the host’s immune systems to become irregular and unbalanced. This can even lead to the point where the parasite’s non-existence causes harm to the host. For instance, this is illustrated by the actuality that even though animals that are infected with parasitic worms are identified to be evidently harmed, and for that reason parasitized, such infections may also decrease the predominance and outcomes of inflammatory autoimmune diseases in the animal host species, including in human beings (Lively & Dybdahl, 2000).

The co-relationship between parasites and their hosts have huge autoimmunity affiliations in ailments such as Crohn’s disease and asthma. These theories were proved to be of clinical significance when several patients with Crohn’s disease and ulcerative colitis were treated with pig whipworm; Trichurus Suis. More than two-thirds of the patients showed improvement. Researchers have begun establishing a secure dosage of human hookworm for asthma patients due to their findings (Clark & Jean, 2012).

The rivalries between parasites have a propensity to favor quicker procreation and as a result this leads to the creation of more dangerous parasites. The parasites whose life span entails the fatality of the host, which involves departing from the current host and then getting into the next, evolve to be more powerful and dangerous even changing the behavior or other characteristics of the host species. This is done to make the host species more susceptible to their predators. The parasites that procreate mainly to the progeny of the preceding host are predisposed to become less dangerous and hence carry out symbiotic mutualism so that their host can procreate more efficiently as well (Hughes, Brodeur, & Thomas, 2012).


Clark, D. P., & Jean, N. P. (2012). Molecular biology. 2nd ed. Waltham, MA: Academic Press.

Hughes, D. P., Brodeur, J., & Thomas, F. (2012). Host manipulation by parasites. London: Oxford University Press.

Hurd, H. Lane, R. P., & Chappell, L. H. (1998). Parasite-insect interactions: reciprocal manipulation. British Society for Parasitology Journal, 116 (35), 48-59.

Lively, C. M., & Dybdahl, M. F. (2000). Parasite adaptation to locally common host genotypes. Nature Journal, 405 (19), 128-145.

Lewis, E., & Campbell, J. F. (2002). The behavioral ecology of parasites. Wallingford: CABI Publishers.

Price, P. W. (1980). Evolutionary biology of parasites. Princeton, NJ: Princeton University Press.

Roger, M. J., Pedro, N., & Gonzalez, L. (2006). Allelopathy: a physiological process with ecological implications. Dordrecht, Netherlands: Springer Publishers.

Rosza, L., Reiczigel, J., & Majoros G. (2000). Quantifying parasites in samples of hosts. Journal of Parasitology, 86 (7) 228-232.

Stearns, S. C. & Hoekstra, R. F. (2005). Evolution: an introduction. Oxford: Oxford University Press.

Tinsley, R. C. (1999). Parasite adaptation to environmental constraints. Cambridge, Cambridge University Press.

Viney, M. E., Read, A. F., & Chappell, L. H. (2002). Parasite variation: immunological and ecological significance. Cambridge University Press.

Webster, J. P. (2009). Natural history of host-parasite interactions. London: Academic Publishers.

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