136 ILLINOIS STATE ACADEMY OF SCIENCE 



SOME OF THE FACTORS INFLUENCING THE 

 DISTRIBUTION OF ANIMAL PARASITES* 



H. J. Van Cleave, University of Illinois 



Many things influence the geographical distribution of 

 parasitic animals. In the past, it has been a common 

 fallacy to assume that the geographical distribution of 

 a parasite is coincident with the range of its host. This 

 idea fitted well with the belief in host specificity, but re- 

 cently there have been numerous demonstrations that 

 many animal parasites are capable of utilizing various 

 species as hosts. Furthermore, parasites common in a 

 species of one locality may be entirely wanting in indi- 

 viduals of the same species in other parts of its range. 



Especially in those forms which involve an alterna- 

 tion of hosts in the course of their development, do we 

 find circumstances which stand as limiting factors to the 

 distribution of parasites. Through the operation of 

 these limtiting factors the indefinite spread of some par- 

 asitic species is prohibited. In some instances species 

 artificially introduced into new territory are prevented 

 from becoming established there through the operation 

 of the same agencies, even though organisms suitable 

 as hosts may be available. In the following paragraphs, 

 some of these limiting factors are discussed, though no 

 attempt has been made to give an exhaustive classifica- 

 tion or treatment. 



FACTORS OF THE PHYSICAL ENVIRONMENT 



There are some organisms which pass their entire ex- 

 istence as parasites within other organisms. In some 

 such instances, the eggs or larvae never leave the body 

 of the host individual which shelters the mature parasite. 

 Obviously, under these circumstances, conditions of the 

 environment outside the body of the host could have no 

 influence upon the parasite and its chances for develop- 

 ment. More frequently, however, animal parasites pass 

 through one or more stages when the parasitic organism 

 is liberated from the body of the host. This period of 



• Contributions from the Zoological Laboratory of the University of 

 Illinois No. 230. 



