220 PARASITES OF THE DOMESTIC ANIMALS 



of their effect does not depend upon this wholly. A relatively light 

 invasion with forms which elaborate toxins possessing a high degree of 

 toxicity may have a more deleterious influence upon the health of the 

 animal than a heavier infestation with worms from which the elimina- 

 tions are less toxic. Again independent of numbers, adult worms or 

 their larvae can, by their migrations, set up in their unusual locations 

 serious inflammatory and degenerative changes which may be of an 

 infective character due to the bacteria which they transport. 



Intestinal worms which attach to the mucosa are far more capable 

 of producing serious effects than those which live free in the intestinal 

 contents. The former live upon the tissues of their host and cause at 

 their attachment a wound through which infection may readily enter, 

 while the latter obtain their nourishment from the partly digested 

 alimentary material and do not directly lacerate the mucosa. 



Location is a main pathogenic factor. This may be accidental by 

 active or passive migration, as in the case of adult or larval filariae, 

 which seem capable of wandering to most any part of the body, or it 

 may be specific, certain nematodes normally infesting only the intestines, 

 others the respiratory tract, while some occupy the blood vascular 

 system in their larval state or both as larvae and adults. Again, Trich- 

 inella spiralis causes its most serious disturbance during the migration 

 of the embryos through the musculature of its host. In general, it may 

 be said that nematode invasion of the intestines is less serious than that 

 of the respiratory tract. The injurious effects from verminous parasitism 

 of the blood are usually due to injury to the vascular walls, or, if the 

 worms are numerous and massed, to interference with the blood flow. 

 Following upon this there may be the production of a thrombus and 

 formation of emboH with the subsequent development of aneurism. 

 While parasites in the blood in any case constitute a serious infection, 

 the greater number of specific conditions due to such parasitism are 

 caused by blood-invading Protozoa. 



The specific limitations as to host of the parasitic worms is probably 

 much influenced by the character of the nutriment with which they are 

 supplied in each particular case. Certain hosts having no more than a 

 class relationship may harbor intestinal worms of the same species, but 

 are more likely to do so if there is a measure of similarity in the char- 

 acter of the hosts' alimentation. This is exemplified in the distinctly 

 omnivorous animals, man and the pig, each furnishing hostage for the 

 intestinal worms Ascaris lumbricoides {A. suis) and Gigantorhynchus 

 hirudinaceus, while, again, the carnivorous dog and cat both harbor 

 Belascaris marginata and Ankylostoma canina. 



Opportunity is also a factor. Animals of similar diet are alike exposed 

 to infection by food specific for or most likely to be contaminated with 

 larvae or eggs of certain species of parasites. Such parasitism as the 



