CONTROL OF ANIMAL PARASITES 



261 



Man 



pass them uninjured or may carry them to human foods 

 as dust on their feet - eggs of eighteen worm parasites 

 have been found on or in flies. It is estimated that a tape- 

 worm produces 12,000,000 eggs a year, and the flukes may 

 be equally prolific. Such powers of reproduction demand a 

 number of different hosts, or host and parasite would die 

 together. One authority 

 states the problem thus : 



If a liver fluke were to de- 

 posit its million or so of eggs 

 in the bile ducts of the sheep, 

 and these were to develop in 

 situ, the host could not with- 

 stand the increased drain upon 

 its vital resources, and host 

 and parasites would perish to- 

 gether. Hence it is clear that 

 the infection of a second host by 



trematodes is highly necessary. 



Egg 



Bovine 

 fcysticercus} 



FIG. 113. Life cycle of human tape- 

 - worm ; infection from raw beef 



So, while many of the 

 bacterial parasites " don't 

 know any better" than to 

 kill their hosts outright, 

 these animal parasites, as a rule, sap and drain slowly and 

 are the cause of prolonged misery rather than of death. 

 Reasonable cleanliness in rearing of farm animals, proper in- 

 spection of meats, and, above all, proper cooking of meats 

 on the part of everyone will finally relieve us from these 

 disagreeable pests. The life history of one or two types 

 should be generally known, and any of the following that 

 may be of local interest should be worked out to practical 

 conclusions in nature and in the books. 



Liverfluke Fasciola hepatica. The adult is most commonly found 

 in the liver of the sheep, but may occur in the horse, deer, camel, ante- 

 lope, goat, pig, rabbit, kangaroo, beaver, squirrel, and, rarely, in man. 



