DISEASES OF CATTLE 351 



organs. In some the wings are lost or, if present, are very rudi- 

 mentary, while in others the mouth parts are greatly developed 

 for the purpose of biting or piercing the skin. In others the clasp- 

 ing organs are exceedingly well developed for the purpose of hold- 

 ing. The eyes and antennae are also often greatly modified. 



Each group of animals has its particular forms of parasites, 

 and each parasite has its particular host. In fact, there is a form 

 of parasite for every form of animal life, from the highest to the 

 lowest, and each form has its particular group of animals for ita 

 host. With reference to the external parasites, a parasite of one 

 species of animal will riot survive on a different species. With ref- 

 erence to the internal ones, however, there are many which are more 

 or less cosmopolitan, since several of them are content to gain an 

 existence in the tissues, or organs, of varied hosts, at least during 

 some portion of their life cycle. 



The rapidity with which parasites multiply is astounding, 

 and, were it not for the fact that there are numerous causes of de- 

 struction surrounding them, animal life would be next to an im- 

 possibility. Leuwenhoeck calculated that the female louse may be- 

 come the grandmother of 10,000 lice in eight week's time. Other 

 authors assert that the second generation of a single individual may 

 furnish 2,500 progeny, and the third generation 125,000. Gerlach 

 asserts that a couple of the itchmite of man yield six generations 

 in three months, the last generation comprising 1,500,000 individ- 

 uals. It is estimated that a tapeworm may throw off in one year 

 150,000,000 eggs. The state of surroundings in which an animal 

 is kept may, however, hasten or retard the multiplication of para- 

 sites. Well kept and regularly groomed animals are less liable to 

 parasitic attacks than poorly kept ones. Certain kinds of parasites 

 are also more numerous during certain seasons of the year. For 

 example, the flies, mosquitoes, ticks, etc., are more numerous during 

 the warm summer months, while lice are in greater evidence in the 

 winter time, when cattle and other animals are crowded together in 

 stables and sheds. Butchers' dogs are more frequently infested 

 with tapeworms than those which have less raw meat in their diet. 

 Certain classes of parasites are also more numerous in some coun- 

 tries than others. The trichina, especially, is more common in 

 Germany, notwithstanding the claims made by the Germans that 

 American pork leads the list in this direction. The influence of 

 parasites on the health of their host has been a subject of more or 

 less dispute, insomuch as some have advanced the theory that the 

 host was not only not injured, but benefited, by the presence of the 

 parasite. Others have claimed, and with good reason, that para- 

 sites are not only unnecessary for the health of the host, but actu- 

 ally injure it. It may, however, be stated that certain forms of 

 parasites, when in comparatively small numbers, do no special 

 injury, and are only discovered by accident, or by a most careful 

 examination, or on post-mortem. In one or two instances, it has 

 been stated that a certain parasite (liver fluke of sheep) , for a time, 

 improves the condition of the host, since the primary stimulation 



