Parasites Common to Animals and Man' 



By Benjamin Schwartz 



Parasitologist, Animal Disease and Parasite Research Branch 



Agricultural Research Service 



U. S. Department of Agriculture 



[With 4 plates] 



Of the more than one hundred species of protozoan and worm 

 parasites reported from man, with the exception of those of doubtful 

 status, few are sojourners in Homo sapiens exclusively. As a matter 

 of fact, man shares with the lower animals a large number of endo- 

 parasites. Some of them are primarily inhabitants of animals and 

 occur in persons only occasionally. Others, however, are in the main 

 human parasites which sometimes occur in, or can be experimentally 

 transmitted to, animals. 



Among the parasites of man are species that undergo their entire 

 development in human beings. Others occur in man as adult parasites 

 only, and still others as larval parasites. In either case the life cycle 

 of the parasites falling into these two categories involves an alterna- 

 tion between the human and animal host, the one harboring the adult 

 parasite being known as the definitive host and the other, harboring 

 its immature stages, the intermediate host. Man serves, therefore, as 

 the definitive host for some internal parasites, the intermediate host 

 for others and, occasionally, both as intermediate and definitive host 

 for the same species. 



Most of the parasites that find a haven in the human body have been 

 acquired apparently as a result of man's long and intimate associa- 

 tion with certain mammals, namely, dogs, cats, and rats, and with ani- 

 mals raised for food and fiber, namely, domestic ruminants and hogs. 

 It is interesting but by no means surprising to note that man apparent- 

 ly has acquired more parasites, especially kinds that have become 

 well adapted to him, from carnivorous and omnivorous animals, whose 

 feeding habits he shares, than from herbivores. Among the parasites 

 that man shares with domestic ruminants are certain nematodes known 



* Based on an article entitled "Parasites of Domestic Animals Transmissible 

 to Man," which appeared in the Southwestern Veterinarian, vol. 7, No. 4, 1954. 



410 



