PARASITES — SCHWARTZ 429 



newly hatched, microscopic larvae. In this case the journey is initiated 

 in the intestine where the infective parasite eggs, which have been 

 swallowed with contaminated food or water, hatch. The emerging 

 larvae penetrate the intestinal wall and migrate to the liver by way 

 of the blood stream, and thence to the lungs. Here the larval worms 

 become free in the alveoli or air spaces (pi. 4, fig. 2) by rupturing and 

 then escaping from the capillaries. The larvae get to the bronchi and 

 trachea by upward migration in the lungs. They continue their 

 migration to the phaiynx and from there back to the intestine, after 

 being swallowed. In the intestine they develop in about two months 

 or less to egg-laying maturity, and ultimately reach a length up to 

 about a foot or so and a thickness exceeding that of an ordinary lead 

 pencil. 



The migratory mstincts of ascarids are by no means satisfied, how- 

 ever, even after their merry-go-round through the body during their 

 early larval life. The grown worms, which are among the most in- 

 jurious parasites of aimals and man, also exhibit a strong tendency 

 to wander. Some pass into the stomach and are vomited up, and oth- 

 ers continue their migration to the esophagus and from there into the 

 trachea and bronchi where they can produce asphyxiation. Deaths of 

 children in the Tropics by asphyxiation so produced is by no means 

 rare. Some ascarids migrate to the pharynx and get into the nasal 

 cavities. Ascarids have been known also to leave the body through 

 the external ear after having perforated the middle ear. In fact, 

 they have a tendency to push into narrow spaces and ducts within 

 their migratory paths, and frequently get into the bile ducts and even 

 the biliary canals and obstruct the flow of bile. In pigs this obstruc- 

 tion results in icterus or yellowish coloration of the flesh, a cause of 

 condemnation of entire carcasses imder Federal meat inspection. 

 Moreover, a large number of worms can become so inextricably en- 

 tangled with one another in the intestine that the resultant coiled 

 mass can produce intestinal obstruction. These and other mechani- 

 cal injuries produced by the large ascarids are in addition to those they 

 produce by eliminating their metabolic products into the host's in- 

 testine. Some of these products, toxic in nature, have been ex- 

 tensively investigated by chemists and pharmacologists. 



Whether the pig Ascans can reach egg-laying maturity in the hu- 

 man intestine is still a moot question. The evidence at hand affords 

 conclusive evidence that the pig Ascaiis can undoubtedly infect man 

 to the extent of undergoing its larval migrations through the liver 

 and lungs, as it does in its porcine host. However, the worms are 

 apparently eliminated from the human intestine before reaching 

 maturity. The bulk of the experimental work involving the ingestion 

 by human volunteers of Ascans eggs of porcine origin points to these 



