3IO READINGS IN BIOLOGICAL SCIENCE 



and water constitute an important health factor; in tropical countries they 

 are usually one of the most important health factors. In countries where 

 the level of sanitation is high and where the standards of hygiene are exact- 

 ing, parasitism that spreads ordinarily through soil pollution tends to dis- 

 appear, more particularly in urban communities. In rural sections, how- 

 ever, including those of this country, there is always a greater or lesser 

 residuum of parasitic infection of one kind or another, and urbanites who 

 visit the country for rest and recreation may acquire a few unwelcome 

 guests, such as hookworms, ascarids, whipworms, dysentery-producing 

 amoebae and other parasites, which occur as infective eggs, cysts or larvae 

 in contaminated soil. 



By and large, however, human beings hving in cities and towns are 

 in most cases adequately protected from acquiring parasitic infestations 

 to which rural inhabitants may be exposed as a result of contact with the 

 soil. The nation-wide campaign against soil pollution, undertaken in this 

 country on a large scale in the beginning of the twentieth century, has 

 done much to reduce the danger of acquiring parasitic infestations, even 

 in rural areas. Several years ago the annual report of the Rockefeller 

 Foundation contained the statement that hookworm disease, for years an 

 important factor in the physical and mental retardation of the population 

 of rural areas in certain parts of the South, had been practically eradicated. 

 While this statement was open to challenge at the time that it was published 

 and was challenged vigorously, the fact remains that the hookworm in- 

 cidence and intensity in the United States have been greatly reduced, thanks 

 to the activities of such agencies as the U.S. Public Health Service, the 

 Rockefeller Foundation, the state boards of health and local health units in 

 the South. 



While progress in the control of human parasitic infestations traceable 

 to soil pollution has been steady and on the whole satisfactory, that relat- 

 ing to the control of parasites of man that are acquired from consuming 

 animal food still leaves much to be desired. Actually, the available evidence 

 shows that one human tapeworm infestation acquired from certain species 

 of fresh-water fish is spreading in the United States, although its distribu- 

 tion is still rather limited. Trichinosis, a serious, painful and sometimes a 

 fatal disease of man, is apparently gaining headway. Whether the increase 

 in the number of cases of human trichinosis is only apparent because of the 

 greater vigilance on the part of physicians in making a correct diagnosis, 

 or whether the increase in the number of such cases is real, is difficult to 

 determine on the basis of available evidence. The extent of beef tapeworm 

 infestation, in so far as this can be determined from the data on the preva- 

 lence of the larval stages of these parasites in cattle slaughtered under federal 

 inspection, shows that during the past ten years or so this parasite has been 

 holding its ground, although the data of previous years showed a down- 

 ward trend. 



The parasites mentioned, namely, the fish tapeworm, the beef tapeworm 



