434 The Animal Kingdom 



Ancylostoma diiodenale has an identical life history. Dogs are 

 often infected by another hookworm, A. hraziliense. At times, larvae 

 of this latter species burrow under the skin of human beings and 

 cause considerable irritation which is known as "creeping eruption." 

 Of course, A. braziliense is unable to complete its life cycle in human 

 beings. 



Again, control measures consist mainly of improving sanitation; 

 wearing shoes, naturally, would cut down the threat of infection. 



The Trichina Worm. — Another important parasitic nematode 

 is Trichinella spiralis (Fig. 141,F), the causative organism of trichi- 

 nosis. Unlike the two parasitic forms just discussed, the chief dam- 

 age of this form is done by the immature stages which encyst in vol- 

 untary muscles. Infection occurs when improperly cooked, infested 

 pork is eaten. In rare instances, 100,000 encysted larvae may be 

 present in a single ounce of badly infected meat. As at least half of 

 these are probably females, it is possible to have an infection of this 

 type result in 100,000,000 larvae, a fatal dose. The meat and cysts 

 are digested away from the larvae by the action of the host's digestive 

 juice. Once released, the larvae find their way to safe sites between 

 the villi of the intestine. Here a series of molts occur, but copulation 

 may take place within forty hours after an infection. The small 

 females (3 to 4 mm. long) retain the developing eggs within the uterus 

 until the embryos are about 0.1 mm. long. The males (about 1.5 mm. 

 long) often pass from the intestine after they copulate, but may remain 

 as long as the females. When the embryos are sufficiently developed, 

 the female burrows into the mucosa and deposits the eggs in such a 

 manner that they quickly enter the blood stream. Soon they are car- 

 ried to the voluntary muscles. Once they reach a proper site, the 

 embryos grow rapidly to about 1 mm. in length, differentiate sexually, 

 roll up, and a cyst forms around them. Most commonly, the worms 

 encyst in muscles such as the diaphragm, tongue, and chest muscles 

 where a rich blood supply is ensured. The cysts become calcified 

 gradually and persist throughout the lifetime of the person. The 

 worm usually dies after a few months. 



The symptoms of a trichina infection begin with slight diarrhea, 

 nausea, and abdominal pains. Upon penetration of the muscles, there 

 are intense muscular pains. Often there is puffiness under the eyes 

 as the worms invade those muscles. At this stage, trichinosis may 



