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EVOLUTION 



May, 1932 



Parasitology Shows Kinship of Monkey and Man 



By ROBERT HEGNER 

 Professor of Protozoology in the Johns Hopkins University School of Hygiene and Public Health 



npHE association of two types of animals in nature is a 

 ■*■ very common phenomenon. In many cases this associa- 

 tion does no harm to either party and may even be mutually 

 beneficial, but occasionally, just as in human society, one 

 member of the association lives at the expense of the other. 

 Such an organism is known as a parasite and the animal it 

 lives on is called the host. 



Animal parasites belong principally to three groups in the 

 animal kingdom, protozoa, worms, and insects. Everyone is 

 familiar with worms and insects, but protozoa are invisible 

 to the naked eye and hence are never seen except through a 

 microscope. I shall refer in the following paragraphs only 

 to the protozoa that live in monkeys and man. 



Protozoa are the most primitive of all animals. They dry 

 up very quickly and die if they are deprived of water, hence 

 they are to be found orJy in ponds, streams, lakes, oceans, 

 etc., and in places that are always moist. Vast numbers of 

 protozoa live in both fresh water and salt water. These are 

 called free-living protozoa. Other protozoa, the parasites, 

 live inside of the bodies of animals that live on land, and 

 both inside and outside of animals, such as fish, turtles and 

 whales, that live continuously in the water. 



Every species of animal that has been carefully studied has 

 been found to harbor protozoa within its body. In some cases, 

 every individual animal belonging to certain species is para- 

 sitized. For example, certain white ants or termites, have 

 their intestines loaded with protozoa that aid in the digestion 

 of their food. This may be considered the normal condition 

 for the white ants. The white ants die if they are deprived 

 of their protozoa, hence every living termite of this type 

 must be a host to large numbers of protozoa. 



Among other species of animals, parasitic protozoa may or 

 may not be present. Usually the life of the host does not de- 

 pend on the protozoa nor do the protozoa injure the host 

 perceptibly. As a matter of fact, most protozoan parasites 

 appear to be harmless, a sort of equilibrium having become 

 established between the protozoa and their hosts which allows 

 the protozoa to live and reproduce successfully but does not 

 inconvenience the host to any extent. A few protozoa are 

 harmful to their hosts and are said to be pathogenic. Among 

 the pathogenic protozoa of man may be mentioned those that 

 are responsible for amoebic dysentery, malaria, kala-azar, and 

 African sleeping sickness. Domesticated animals are injured 

 by such protozoa as those that cause coccidiosis in chickens, 

 Texas fever in cattle, and sarcosporidiosis in sheep. 



The protozoan parasites of man have been studied more 

 thoroughly than those of any other animal. They can be 

 separated conveniently into two types: those that live in the 

 digestive tract and are commonly called intestinal protozoa 

 and those that live in the blood and are referred to as blood- 



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inhabiting protoza. The protozoan parasites of monkeys and 

 many other lower animals have also been carefully studied. 

 The following table presents in orderly fashion the names 

 and location in the body of the protozoa that are known to 

 live in monkeys and man. 



This list is not final since we are still studying these or- 

 ganisms and continually learning more about them. How- 

 ever, on the basis of our present knowledge, we may state 

 that of the twenty-eight species of protozoa listed, all but 

 three have been reported from man and all but four have 

 been found in monkeys. 



This condition is very different from that encountered when 

 one compares the protozoa of man with those of any other 

 animal. For example, the protozoa that have been reported 

 from the pig include the following: 



1) Eimeria debliecki — Coccidium 



2) Endamoeba polecki — Amoeba 



3) lodamoeba suis — Amoeba 



4) Trichomonas suis — Flagellate 



5) Balantidium coli — Ciliate 



6) Trypanosoma brucei — Flagellate 



7) Trypanosoma evansi — Flagellate 



8) Babesia sp. — Sporozoon 



9) Sarcocystis sp. — Sporozoon 



Only one of these is known with certainty to occur in man, 

 namely Balantidium coli, in spite of the fact that man and 

 pig are very closely associated and must often be inoculated 

 with each other's parasites. A similar condition is met with 

 when the protozoan parasites of man are compared with those 



