132 INFECTION 



body, believes that the parasites, after attaining sufficient size, lodge in 

 the capillaries of the body, especially where the blood-current is weakest, 

 and where slight obstruction occurs as the result of the protruding in- 

 ward of nuclei of the endothelial cells. Here they remain and develop 

 until segmentation takes place. In the meantime other red corpuscles 

 are forced against them, and if the opening in the infected cell is in a 

 favorable location, one or more merozoites pass directly into another 

 cell; if it is not, the merozoites are discharged into the blood-stream and 

 are speedily killed. 



INFECTION WITH ANIMAL PARASITES 



Infection with animal parasites is similar in many respects to in- 

 fection with bacteria. Owing to the difficulty of isolating and culti- 

 vating these parasites in vitro, our knowledge of their toxic properties 

 is somewhat meager. Most attention has been given to a study of 

 their life history and the modes of transmission. 



Modes of Infection. Primary infection with animal parasites is 

 often facilitated by, or in some instances only rendered possible through, 

 the intervention of special carriers, usually various species of the Ar- 

 thropoda. Thus we now know that malaria is transmitted through the 

 bite of infected mosquitos; African relapsing fever and Texas cattle 

 fever, through the bite of certain infected ticks; trypanosomiasis, 

 through biting flies. The ova of various intestinal parasites may re- 

 quire residence in certain of the lower animals before they can infect 

 man. 



Infection may occur along the same routes as bacterial infection, 

 and is governed in general by the same factors of local selection, tissue 

 susceptibility, etc. Biting insects usually deposit the .parasite directly 

 in the subcutaneous tissues or in the circulatory fluids. Abrasion of 

 the epithelium may be necessary in order to produce infection with 

 Treponema pallidum, as in the majority of the bacterial infections. 

 The ova or larva of other parasites may be swallowed or find lodgment 

 in the upper or lower air-passages or accessory sinuses. 



It would appear that our natural defenses against infection with 

 animal parasites are much weaker than those against bacteria; this is 

 probably due to the greater resistance offered by animal parasites to 

 such physical destructive influences of the host, as the acidity and ger- 

 micidal activity of the secretions, temperature, etc., as well as to a 

 general lack of natural antibodies in the body-fluids of the host, and 

 inability of leukocytes and other phagocytic cells to deal successfully 



