EIGHTH ANNUAL YEAR BOOK-PART VI. 239 



In or on this animal, and do not thrive in or on different species or 

 soon leave them. 



The propagation of parasitic disease is subordinate to the condition of 

 the existence of the parasites. The excremental contents of the intestinal 

 canal contain Ihe eggs deposited by the worms living within the canal. 

 The parasites of the respiratory system are expelled by coughing, and 

 fragments or even entire worms may be thrown out, disintegrate and yield 

 the ova or eggs, to external agents. The majority of these are destroyed 

 by drying and many of them may have to remain for months or even 

 years before they can reach the body of the new host, sometimes it may 

 be the ovum itself, and at other times the hatched embryo. 



The length of time the embryo is confined in the egg varies with the 

 different species. The ova of certain parasites do not develop if they are 

 kept in a moist medium, while others on the contrary die when they are 

 placed in a dry medium. The thickness of the shell enveloping the ova 

 may be very thin or thick and resistant, and in the first case the ova 

 usually hatch in the surrounding media, and infection takes place by the 

 embryo. While on the other hand the eggs possessing a thick shell pass 

 into the digestive canal of the host, where hatching takes place. When 

 the embryo is thus liberated it either remains in the digestive tract or 

 passes directly, or through the circulation, to the particular tissue or 

 organ which favors its future development. 



Parasites may be transmitted from an infested animal to a healthy 

 animal, either by immediate contract, or some intermediate bearer. Cer- 

 tain predisposing conditions favor the propagation of parasites, and few 

 of them are common to several species of animals. Young animals with 

 delicate and non-resistant tissues favor the development of certain para- 

 sites, old animals are less able to defend themselves from attacks, and 

 the contraction and secretion of the digestive organs are less capable of 

 expelling the organisms which may be found within. 



The multiplication of parasites may be favored or hindered by the state 

 of surroundings. Crowding and dirty habitations aid in the propagation 

 of parasites, the different seasons of the year have a direct influence, and 

 this is especially noticeable in animals affected with mange and lice. 



Parasites have a varying influence on the health of their host. 

 Autopsies performed on animals showing every sign of health may re- 

 veal the presence of large numbers of parasites in the intestines. The 

 damage done by parasites is not so much due to the blood abstracted as 

 to the effect of the bite or sting, and at other times, as with the mange 

 parasites, the nutrition of the skin is interfered with, and consequently 

 the general health, and if the parasite infest the ear we may have serious 

 nervous disturbances. The intestinal parasites usually exert their dele- 

 terious effect by mechanically obstructing the intestines, and at times 

 impairing digestion, and in extreme cases causing perforation or rupture 

 of the organ. 



Parasites living in the stomach and intestines are nourished by the 

 more or less modified alimentary matters contained in these organs. All 

 the others derive their sustenance from the substance of their host. The 

 majority of the parasites derive the materials for their development and 

 maintenance from the morbid products they excite the secretion of. Those 



