INTRODUCTION xili 
majority of parasites are acquired by transmission from 
other animals, either by mediate or immediate con- 
tagion, though a few may be hereditary—that is to say, 
transmitted to the young animal through the genital 
organs of the mother. The piroplasms and trypano- 
somes are examples of this method of infection. Mange 
parasites and lice are transmitted by actual contact or 
by mediate contagion, as from harness, grooming 
utensils, or even from attendants. In the case of a 
great number of parasites, ova or embryos from one 
animal may be ingested by another, or, as in the case 
of Hvpoderma bovis (the warble-fly), the eggs may be de- 
posited on, or in some cases under, the skin of the host. 
Some parasites, such as leeches, and probably some 
of the Nematode worms, may be taken in with the 
drinking-water. 
It may be taken for granted that an animal possesses 
a certain degree of immunity against parasitic invasion, 
dependent upon the general constitution and state of 
health, and upon the condition of the various organs. 
Worms usually infest the intestines of an animal already 
in low condition, and in which the intestinal tract is 
already probably deranged and weakened. Ina similar 
manner lice and mange Acari infest the skin of an 
emaciated animal or one suffering from some chronic 
debilitating disease. Tuberculous cattle in the stage 
of emaciation are usually affected with lice, and it is 
more reasonable to suppose that emaciation and loss of 
tone afford an opportunity for the parasites to take up 
their abode than that they infect a healthy animal and 
produce these symptoms. It cannot be denied that in 
many cases, after the invasion of the particular parasite, 
the previous symptoms are exaggerated, and sufficiently 
so sometimes to bring about death. Sclerostomum 
armatum and S. tetrvacanthum of the horse afford good 
evidence of this. : 
