ANIMAL PARASITISM 443 



Nematodes. — Of the thousands of species of parasitic nematodes, 

 space permits mention of only a few which are particularly impor- 

 tant because of their danger to man. 



Hookworms. — In the United States the most important human 

 nematode parasite, from the public health viewpoint, is the American 

 hookworm, Necator americanus. Although called the ''American hook- 

 worm" this species probably came originally from Africa and was 

 introduced into America by the negro slaves. The pioneer work on 

 hookworm in the United States was done by Dr. Charles W. Stiles 

 in 1901. Hookworms are slender threadlike nematodes about one- 

 half inch long; the females are tapered to a point at each end, while 

 the slightly smaller males have on the posterior end a fanlike ex- 

 pansion, the copulatory bursa, with curved riblike supports. Both 

 sexes have a large mouth containing hooklike chitinous teeth by means 

 of which they tear holes in the walls of the intestine and start blood 

 flowing from the w^ounds. A muscular esophagus leading back from 

 the mouth cavity gradually broadens into a large muscular bulb ; by 

 means of rhythmic contractions and expansions of the bulblike esoph- 

 agus blood is drawn into the mouth and forced down into the straight 

 intestine where some of it is digested and the rest passes on through 

 and out of the anus near the posterior end. Because of the large 

 number of worms present there is a serious loss of blood, resulting in 

 anemia and lack of energy ; in children the growth is stunted or re- 

 tarded by hookworms, and often there is also a lack of proper mental 

 development. Individuals, very heavily infected during childhood 

 and early youth, may fail to develop sexually. Treatment is fairly 

 easy, hookworms being easily killed by doses of anthelmintics, such 

 as carbon tetrachloride and hexylresorcinol (which are poisonous and 

 should be taken only under doctor's supervision). 



Each female hookworm produces -9,000 eggs per day; these eggs 

 pass out with the feces of the host ; if the infected person defecates 

 on the ground, the eggs hatch and the larvae crawl around in the 

 soil; there they develop into infective larvae w^hich live for several 

 months on the surface of the ground. If bare human skin comes in 

 contact with these microscopic worms they bore through it to the 

 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 increas- 



