Biology and Medicine 461 



occurrence of the disease in man. In 1904 a case of human 

 plague occurred a short distance east of San Francisco, and 

 in 1906 a boy near Oakland was attacked by the disease after 

 handling some ground squirrels which he had shot a few 

 days previously. In July, 1908, two cases of human plague 

 were found in the same region in California, and an examina- 

 tion of 425 ground squirrels collected in the vicinity showed 

 the presence of the disease in four. In August of the same 

 year a boy was stricken with plague in Los Angeles after 

 being bitten by a sick ground squirrel, and a dead squirrel 

 taken in the vicinity was found to be infected with plague. 

 The woodchuck has been suspected as a plague spreader, but 

 its relation to the disease has not been proven as yet. 



Not only are ground squirrels a source of danger in the 

 spread of plague, but the loss they cause in the destruction 

 of grain is very serious. Their destruction therefore is of 

 first importance for both sanitary and purely economic rea- 

 sons. To the accomplishment of this task both the U. S. 

 Public Health Service and the Biological Survey are devot- 

 ing their energies. Many methods are employed at present 

 for the extermination of both rats and ground squirrels, 

 which have been discussed in a previous chapter. In the de- 

 struction of rats, trapping, poisoning, inoculation with 

 viruses or bacterial cultures, shooting and rat-proofing have 

 all been employed more or less successfully. Space does not 

 permit a detailed discussion of the use and merits of these 

 various methods, but it may be said in a general way that the 

 rat-proofing of buildings and systematic trapping are the 

 most effective means of control. 



There are many diseases of man and lower animals, some 

 of them among the worst scourges of the human race, which 

 are caused, not by microscopic organisms, either plant or ani- 

 mal, but by parasitic worms. One of the most terrible of 

 these diseases is trichinosis, caused by a minute worm, about 

 1/20 inch in length, the Trichina spiralis. The devas- 

 tations of this disease have been greater among some of the 

 poorer classes of Europeans, who were accustomed to eat- 

 ing raw pork, than among Americans. Nevertheless the dis- 

 ease is not unknown in this country, and the occasional oc- 

 currence of the parasite in American hogs led about 1880 to 

 the prohibition of their import into several European coun- 

 tries. The worm is a parasite of man, the hog and the rat, 

 its life history being similar in each. Let us trace this, start- 

 ing with a pair of adult worms living in the intestine of the 

 hog. Here the female is fertilized and gives birth to some 

 thousands of progeny, which promptly bore their way through 



