MOLLUSKS AND MEDICINE — ABBOTT 331 



pats them down into a protective, camouflaged jacket. With eggs 

 available, the long list of chemicals effective against the adult was 

 tested until all were eliminated save two which killed not only Oncome- 

 lania adults and their eggs but spelled death to the fork-tailed 

 cercariae. 



OTHER PARASITIC DISEASES CARRIED BY MOLLUSKS 



In other parts of the world there are two additional blood-fluke 

 diseases which have plagued the human race since the memory of man. 

 One of these, bilharzia, is mentioned in early Egyptian records, and 

 its presence has been discovered in Egyptian mummies dating from 

 1250 B. C. Napoleon's troops were infected in the Nile region, and 

 during World War I the disease became familiarly known among 

 British Tommies as "Bill Harris' disease." It has been estimated 

 that 39 million people in Africa are at present suffering from bilhar- 

 zia, with 6 million cases alone in Egypt. This almost equals the 46 

 million estimate for Oriental schistosomiasis. Bilharzia blood flukes 

 take their toll of human lives by attacking the urinary system. The 

 life cycle of the worm must include an intermediate snail host, as is 

 the case with all trematode worms. It is rather odd, however, that 

 this species of schistosome, so closely related to the Oriental type, is 

 obligated to live part of its life in a lung-bearing snail which is not 

 even remotely related to the gill-bearing Oncomelania snail. 



The third type of schistosomiasis, Manson's disease, is of more in- 

 terest to Americans, for its prevalence in Puerto Rico, the Lesser An- 

 tilles, and the northern parts of South America presents a serious men- 

 ace to tourists who enjoy fresh-water swimming. Manson's disease 

 is believed to have originated in Africa w^here today it is second only 

 to bilharzia as a trematode menace. It has been thought that its pres- 

 ence in the Western Hemisphere is attributed to early slave trade. 

 The snail responsible for the spread of this disease is related to the 

 bilharzia carrier in Africa. 



In addition to the three blood-fluke diseases, there are some half 

 dozen other snail-borne diseases which directly affect the health and 

 economy of millions of our fellow beings. These differ from schisto- 

 somiasis in being less damaging to the body, but are perhaps biologi- 

 cally more interesting in having developed strange variations in their 

 life cycles. 



The lung fluke, according to recent surveys, is at present limited to 

 about 3 million cases of infection in Asia and a few thousand in West 

 Africa. The half-inch-long adult attaches itself to the inner walls of 

 the lungs and often produces fatal tuberculosis-like lesions. No effec- 

 tive cure is known. The eggs of the fluke are coughed out of the lungs 

 into the river where they hatch into snail-seeking miracidia. This spe- 



