PROBLEMS IN HOST-PARASITE SPECIFICITY 



number of parasites that gain access to a host may be 

 an important factor; that is, a few specimens may not 

 succeed in bringing about an infection, whereas large 

 numbers of specimens might. The necessity for the pres- 

 ence of large numbers of parasites may account for the 

 great number of clinical cases of amoebiasis that occur 

 in the tropics, where the ingestion of large numbers of 

 cysts is favored by meteorological and insanitary condi- 

 tions. 



The method of entrance of the parasites may play a 

 role in the initiation of an infection. For example, cats 

 apparently do not often become parasitized by Enda- 

 moeha histolytica in nature but may be infected in the 

 laboratory in several ways. The method that results in 

 the greatest success seems to be that of Sellards and 

 Theiler (1924), who produce stasis by surgical ligature 

 of the large intestine and then inoculate cysts anterior 

 to the ligature. Their experiments show that excystation 

 occurs at the point of stasis and that no excystation might 

 take place if stasis was not induced. Many infections are 

 no doubt prevented in nature by the rapid passage of the 

 cysts through the digestive tract. 



We can thus account for laboratory infections in for- 

 eign hosts by the bringing together of a host and parasite 

 that do not ordinarily become associated in nature; or 

 by the use of very large numbers of parasites ; or by pro- 

 cedures not possible in nature. 



(5) What conditions are responsible for differ- 

 ences IN susceptibility between young and adult 

 ANIMALS? The greater susceptibility of young animals 

 to infection has been abundantly demonstrated in the 



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