428 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



tapeworm eggs with feed and water. Children become infected in 

 a similar manner by soiling their hands or contaminating their food 

 in one way or another with the excreta of infested persons. There 

 is reason to believe that there is a human strain of this parasite wliich 

 passes from person to person. It is still problematical whether chil- 

 dren acquire this parasite from rodents. 



A related but larger tapeworm, H. dmunuta^ also occurs in rodents 

 and, rarely, in human beings. This parasite has an indirect life cycle, 

 fleas and other external parasites of rodents, and cereal-inliabiting 

 insects serving as intermediate hosts. Children acquire this tapeworm 

 by accidentally swallowing the infected intermediate host. 



In studies made in the 1930's in Kentucky, Mississippi, North 

 Carolina, and Teimessee, the incidence of infestation with H. nana 

 ranged from a fraction of 1 percent to 2.7 percent, with a peak in- 

 cidence of 3.6 percent in children 5 to 9 years old in Kentucky. R. 

 diminuta was found only occasionally, and not in all the States 

 surveyed. 



The dog tafewoimi. — DipylidAwn caninum, which lives in the dog, 

 cat, and in a nmnber of wild carnivores, sometimes occurs also in 

 children, localizing in them, as in its animal definitive host, in the 

 small intestine. The dog louse, the dog flea, and the human flea serve 

 as intermediate hosts. Dogs and cats become infected by gnawing at 

 their fur to relieve the irritation produced by ectoparasites. Children 

 acquire this tapeworm either by accidentally crusliing the infected 

 ectoparasites and then licking their hands, or by contaminating their 

 food with the larval tapeworms, known as cysticercoids, or by being 

 licked by a dog on the tongue of whicli these cysticercoids are present. 



INTESTINAL ROUNDWORMS 



Ascaris lumhricoidesy an intestinal parasite of man known since 

 ancient times, has a world-wide distribution and is undoubtedly one 

 of the most common worms affecting human beings, especially persons 

 living in rural areas and elsewhere where sanitation is inadequate. 

 Ascaris is the parasite to which people everywhere usually refer 

 when they speak of intestinal "worms." It has been estimated that 

 about 30 percent of the world's population harbors this worm and that 

 about 45 millions of those so parasitized live in the Western Hemi- 

 sphere, mostly in the warm portions thereof. A closely related but 

 morphologically indistinguishable worm, sometimes referred to as 

 Ascaris suum, occurs in pigs the world over. The porcine worm, 

 though considered by most parasitologists to be specifically identical 

 with the human ascarid, is sometimes designated as a host variety 

 thereof under the name Ascaris luinbricoides var. suis (pi. 4, fig. 1). 



Like that of some other nematodes of the family Ascaridae, the 

 early development of Ascaris involves a roundabout journey of the 



