Phylum N emathelminthes and Related Phyla 435 



be fatal. Trichinella may also infect cats and rats as well as human 

 beings and pigs. 



The infection is passed from animal to animal by the indiscriminate 

 tastes of these domestic forms. It is further encouraged by the 

 human habit of feeding garbage to hogs. Prevention of human in- 

 fections is comparatively simple. If pork is heated above a tempera- 

 ture of 131° F., the cysts are destroyed. Freezing and cold storage 

 likewise destroy them. 



It has been estimated that about 10 per cent of the population of the 

 United States has at least a mild infection of trichinosis. These in- 

 fections cause little or no trouble and are likely to be undetected by 

 a person. As pointed out, a heavy infection can be fatal; thus trichi- 

 nosis is one of the really dangerous parasitic infections. 



Geographically, trichinosis is commonest in the northeastern sec- 

 tion of the United States where garbage feeding of hogs is most com- 

 mon. Through the Middle West, where hogs are often fed on grains, 

 infections are not so prevalent. 



The Pinworm. — The commonest nematode parasite of human 

 beings through the temperate areas of the world is the "pinworm" or 

 "seatworm," Enterobius vermicularis (Fig. 141, C). These are small 

 worms, the females being from 8 to 13 mm. long and the males 2 to 

 5 mm. They live in the region of the junction of the small and 

 large intestines and often concentrate in the caecum and appendix. 

 The gravid females make nightly forays from these sites to the rectum 

 and out the anus, and then crawl around the skin to deposit their 

 eggs. These movements of the worms cause extreme itching in the 

 anal region. The eggs are very tiny (55 by 30 microns) and are easily 

 scattered by the host. The commonest manner of reinfection is, of 

 course, that of getting the eggs under the fingernails and transferring 

 them to the mouth. The usual symptoms of these infections are gastro- 

 intestinal disturbances. 



As these tiny eggs can even be air borne, the spread of the in- 

 fection can easily attain tremendous proportions. This also makes 

 control rather difficult as the eggs can be found in such unlikely spots as 

 the dust on furniture. It has been estimated that from 33 to 50 per 

 cent of children of school age have this nematode as a parasite! 



The Whipworm. — Another nematode parasite of the caecum 

 and large intestine is the whipworm, Trichuris trichiura (Fig. 141,5). 

 This is found in areas where there is abundant warmth and moisture 



