INFLUENCE UPON THE HOST 11 



effect of the movement of myriads of embryos of Trichinella spiralis, is a 

 typical instance. An otherwise relatively harmless parasite may work 

 its way into a duct, or, finding lodgment in an unusual organ, set up 

 inflammatory changes and abscess formation. Again, by verminous 

 wandering, fistulous communications may be established between 

 contiguous organs normally possessing no direct connection. 



5. Age of Host. — Young animals are predisposed to endoparasitic 

 invasion. To forms which penetrate or are more or less migratory, the 

 more tender tissues of the young offer less resistance than in older 

 animals. Verminous bronchitis is a form of strongylosis observed almost 

 exclusively in animals which are immature. The reduced vitality of 

 old age invites the iavasion of both external and internal parasites; 

 there is not only a lessened ability to defend from attack, but reduced 

 activities and secretions of the intestines, skin, and other organs de- 

 crease the capability of eliminating either ecto- or entozoa. 



Such external parasites as mosquitoes, flies, ticks, and bedbugs are of 

 greatest pathologic importance as disseminators of infectious diseases, 

 acting either as direct carriers or as intermediate or definitive hosts of 

 the infecting organism. Malaria, Texas fever, and forms of trypan- 

 osomiasis are among diseases which are known to be spread only by this 

 means, while the possibilities as carriers of typhoid and other malignant 

 infections engendered by the habits of the common house fly are well 

 known. 



That Hehninthes elaborate materials toxic to their host has been 

 demonstrated in experiments with the isolated poisons. It is obvious 

 that, in cases of heavy infestation especially, this toxic effect must be 

 considerably contributed to by the products of decomposition of dead 

 worms. 



Etiology. — So varied are the conditions that surround the propaga- 

 tion and existence of parasites that the consideration of the causes of 

 parasitic diseases is best embodied in chapters devoted to their particular 

 occurrence. However, certain circumstances favoring parasitism may 

 be here briefly considered. 



Crowded and imclean housing favors the propagation and spread of 

 parasites of both man and domestic animals. For this reason lice and 

 scab mites find their most favorable season in the winter months, when 

 their transmission from animal to animal is facilitated and the reduced 

 activities of the skin offer less resistance to their invasion. Pediculosis 

 and the scab acariases are seldom seen, however, in stables that are 

 well kept, or among animals where due attention is paid to cleanliness of 

 the skin. The summer, on the other hand, is the season of attack by 

 adult parasitic Diptera, and it is during the months at pasture that 

 ticks most rapidly propagate and crawl upon their hosts. 



In helminthiasis the influences of environment as an etiologic factor 



