ANIMAL PARASITISM 753 



blood vessels, are carried by the blood to the lungs, then migrate up to 

 the trachea and pharynx, into the esophagus, then down through the 

 esophagus and stomach to the small intestine; meanwhile they in- 

 crease in size, so that on arrival in the small intestine tliey are ready 

 to attach themselves to the wall of the intestine, feed on the blood of 

 the host, and become adults. 



Since the larval hookworms must go through part of their develop- 

 ment in the soil, and a person can become infected only by direct 

 contact with contaminated soil, the distribution of hookworm in the 

 United States is determined by the following factors: (1) freezing 

 of soil in winter (kills the larvae) ; (2) texture of soil (hookworm 

 larvae live best in light, sandy loams) ; (3) moisture (hookworm 

 larvae can live only in damp soil) ; (4) customs of the people in dis- 

 posal of feces ; the bad hookworm districts are sections in which sani- 

 tation is very primitive and sanitary toilets are not in universal use ; 

 deposit of feces on the ground is particularly conducive to spread of 

 these animals. 



From the public health standpoint hookworm disease is a social 

 problem rather than a medical problem. Few if any people are killed 

 by hookworm, and infected individuals are easily cured if they go 

 to a physician for treatment. On the other hand, such a large pro- 

 portion of the population, in hookworm territory, are kept in bad 

 health and a listless condition that the social welfare of the whole 

 community is injured. Prevention of hookworm disease is theoreti- 

 cally easy; hookworms could be killed out of a community in a few 

 months if everyone would defecate only in sanitary toilets, if every- 

 one would take treatments for hookworms at tlie same time, or if 

 everyone would wear good shoes ; but so far, it has been impossible to 

 get the cooperation of all the people in hookworm districts. The work 

 of the medical profession, with the help of certain state agencies, has 

 reduced hookworm disease in the United States, but there are still 

 considerable districts in which over 20 per cent of the populations are 

 infected. In parts of East Texas 33 per cent of the people examined 

 have hookworm, even now. 



Trichina. — TricJiinella spiralis, commonly knowai as Trichina, is an 

 example of a nematode with an alternation of hosts and a passive 

 means of transmission. The microscopic lar\^ae are encysted in the 

 muscles of various meat-eating animals, being particularly common 

 in hogs and rats ; within the cyst, the larva is coiled in a tight spiral, 



