326 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



the head end, one for holding to the walls of the blood vessel, the other 

 for ingesting blood cells. The greatest damage to the host, however, 

 does not come from the small loss of blood or the obstruction caused 

 by the worm, but rather from the vast number of tiny, spined eggs 

 being constantly liberated by the female. A large proportion of the 

 eggs are carried to the liver by the blood stream, and as protective 

 encystment takes place, hepatic congestion or liver blockage develops. 

 Ultimately, with increasing deposition of eggs and the introduction of 

 toxins given off by the adult flukes, the liver becomes a gigantic mass 

 of scar tissue. Accompanying symptoms are daily fever, extreme 

 weakness, diarrhea, loss of appetite and weight, emaciation, and, in 

 untreated cases, death from exhaustion. 



A number of the eggs laid by females in the mesenteric blood vessels 

 work themselves through the intestinal wall and pass out into the 

 open, thus permitting the life cycle of the worm to continue. The 

 eggs must reach fresh water directly or bo rain-washed within a day 

 or so to a nearby creek. There, the eggs break open to liberate a 

 minute, free-swimming miracidium. Unless this microscopic larval 

 form can reach an Oncomelania snail within 35 hours it will die. The 

 miracidium will swim rapidly through the water in erratic and un- 

 directed courses until by chance it passes within a few inches of an 

 Oncomelania snail. At that moment, it turns and takes a straight 

 course to the mollusk and plunges its head into the flesh of the snail. 

 In a matter of a few minutes it will have bored itself through the skin. 

 In time, the tiny invader burrows to the liverlike digestive gland 

 of the snail, and there, during 8 weeks of complicated transformation, 

 multiplies into hundreds of small, fork-tailed cercariae. The more 

 miracidia that invade a single snail, the greater will be the production 

 of cercariae; in heavy infections, the reproductive ability of the 

 molluscan host will be impaired. 



It was this fork-tailed cercaria that infected our troops. During 

 the early hours of morning thousands are shed from the ruptured 

 tissues of the Oncomelania snails. Invisible to the naked eye, they 

 lie on the quiet surface of the creeks and marsh waters and await the 

 first person that comes to bathe or swim. Cercarial penetration of the 

 skin is painless and often goes unnoticed, although occasionally in 

 heavy infections an itchy rash or cercarial dermatitis is produced. 



Two to three months may pass before the victim shows any dis- 

 tressing symptoms of the disease. During this incubation period the 

 larval worms will have migrated from the small blood vessels and 

 lymphatics to the heart through the venous system and will have 

 reached the lungs. From here, the maturing worms penetrate through 

 the tissues to the vessels near the liver, pair up in sexes, and begin their 

 deadly production of eggs. 



