Kenyon College - Reveille Yearbook (Gambier, OH)

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And BIOLOGY Relation to Disease Edited for Reveille It is by this evolutionary conception of parasitism that we can find an explanation for the cause of disease and parasitic injury. The evo- lution of a parasite is guided in the direction of non-pathogenicity. Therefore, the longer that the parasite exists in a particular host species, the better adjusted it will become. A well adapted parasite will not cause disease. Any new disease may be thought of as being caused by an aberrant parasite, that is, a parasite which has strayed from its normal host to a new host to which it is not adjusted. All disease then is caused by parasites which are in the process of changing toward harmlessness after having strayed into an environment to which they are not ad- justed. Aberrancy may be brought about by the straying of a parasite from one host species to another, or by gene mutation which causes the parasite to become unadjusted and therefore pathogenic to a host species in which it was previously adjusted. Cause of disease by aber- rancy is readily seen. The Malaria parasite is almost harmless in places where it has existed for ages, but on migration to other areas it becomes deadly. Trypanosoma gambiense is harmless in a number of animals, but causes terrible sleeping sickness in man, and the Diphyllobotbrinm latum larva, rela- tively harmless in its normal host the cope- pod, produces severe effects in man causing Sparganosis. In these cases and innumerable others that can be mentioned, the parasites become harmful and produce disease only when they become aberrant and stray from their normal hosts. The adjustment process following the initial straying is revealed by the reduced pathogenicity which usually oc- curs following the outbreak of a disease. Fred W. Damn David W. Kraemcr Robert B. McFarland David M. Bell Karle I. Ellson David V. Aldrich Alhin W. Smith Page 25

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Elliot F. Ellis Richard S. Hochlcr MAJORS IN PRE-MED Plastic Evolution and Its by DAVID W. KRAEMER There are several stages in parasite-host relationships ranging from one in which the parasite is extremely harmful to one in which the para- site and the host live together in mutual benefit. The evolution of parasite-host relationships can be assumed to proceed from that of an injurious relations to a symbiotic relation, for it is to the advantage of the parasite to become innocuous to the host, or perhaps even aid the host. The mechanism guiding this evolutionary trend to beneficial para- sitism has a genetic basis since it is brought about by gene mutations. The environment, through natural selection, determines which of these mutations shall survive. In this evolutionary scheme the host is the selective or environmental agent, and it is only logical that the or- ganism will be selected for benignity. By a similar mechanism there is an evolution of the host. Here it is the parasite that is the selective agent by choosing hosts which have higher resistance. This coevolution of parasites and hosts is readily seen upon exami- nation of the history of many diseases. Among the most impressive are syphilis, leprosy, and tuberculosis. When syphilis first appeared it was a terrible disease with an exceedingly high death toll. Gradually its pathogenicity has become reduced until now it kills very few in its early stages; most deaths caused by it are due to delayed effects. The reduction in its pathogenicity can be at- tributed to the evolution of hosts which have a higher resistance to the disease and the evo- lution of the parasite in the path toward com- mensalism and symbiosis. Likewise, the sever- ity of the disease tuberculosis has fallen con- siderably, and leprosy has also shown reduced pathogenicity until now mortality caused by leprosy is at a negligible level. Richard E. Gifford John C. Young J. Philip Jayme Saul L. Sanders William K. Haas Page 24

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MAJORS IN Charles H. Dolan William R. Chadcaync Poge 26