526 



EVOLUTION OF ANIMALS 



Part V 



stance surrounds the developing embryos must be moderately warm and moist, 

 must contain bacteria and be well mixed with air. Temperature between 70° F. 

 and 85° F. is the optimum; if it is much lower or higher than that, the embryos 

 are injured or destroyed. Direct sunshine and drying kills them. Wriggling 

 through soil is rugged business and clay or salty ground injures them. Hook- 

 worms are not long-lived, at most about five years. Infections tend to die out 

 unless repeated, the chemical environment having changed, and immunity be- 

 ing established. Such obstacles as these are set against the daily litters of 

 10,000 eggs. 



Trichina. Adult trichinae {Trichinella spiralis) are parasites of the intestine. 

 But it is young ones, not the adults, which are responsible for the serious dis- 

 turbance called trichinosis. Unlike most parasitic worms, they live in temperate 

 climates and are almost completely absent from the tropics; they occur mainly 

 in Europe and the United States. According to data of 1947 and more recent 

 estimates, the United States had three times as much trichinosis as all other 

 countries combined. 



Trichinae most often parasitize man and pigs but can live in other animals 

 (Fig. 26.5). Rats and cats are easily infected, dogs are less so, and birds are 

 resistant to them. Human infections usually come from eating imperfectly 

 cooked pork, hurriedly cooked roasts with red parts left in the center, and ham 

 improperly cured and cooked. In the United States at this date, the prevalence 

 of infections in man and pigs is highest in the Atlantic States, especially in 



Original Source 



of 



Infection 



for Hogs 



(Usually Garbage) 



Infected Rats 



Infected Hog 



U^ 



Infected Cats, Dogs 

 & Other Animals 



Fig. 26.5. Diagram illustrating the common methods of exposure to trichinosis 

 (caused by Trichinella spiralis) in the continental United States. (Courtesy, Craig 

 and Faust: Clinical Parasitology, ed. 5. Philadelphia, Lea and Febiger, 1951.) 



