328 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



ROLE OF THE SNAIL 



The key to lasting preventive measures against the blood-fluke dis- 

 ease is the elimination of the moUusk, for without this one kind of 

 snail the disease could not continue to exist. Of the several hundred 

 species of fresh-water mollusks living in the Philippines, only the 

 Oncomelania snail has been known to serve as the intermediate host. 

 Although this species of moUusk is always found in endemic areas, 

 there are localities where the disease is unknown, yet where the snail 

 is present. One such region discovered by the Navy exists in the Lake 

 Lanao area of central Mindanao Island. Formerly an American mili- 

 tary site, and perhaps in the future to be used as a Philippine Army 

 training camp, the country bordering the north end of the lake is 

 cool, relatively free from tropical diseases, and not unlike the country- 

 side of Connecticut. Yet the tiny creeks flowing into the northeastern 

 end of the lake are heavily colonized by Oncomelania snails. Until 

 these snails are eliminated, the area must be considered a potential 

 breeding ground for the blood fluke. It is probably only a matter of 

 time before the disease is introduced and established there. 



Surveying Philippine jungles and swamps for Oncomelania snails 

 was, before the war, a particularly difficult task as is evidenced by the 

 fact that Philippine workers, familiar with the country, found the 

 snail in only 25 percent of localities where the disease was endemic. 

 Recognition of the disease-carrying mollusk was hampered by the 

 presence of similar-looking, harmless shells, and because the habitat 

 preference of the gtiilty mollusk was not completely understood. 

 Military medical men, armed with malacological information, were 

 able to track down the snail in every case. 



Recognizing the guilty species of snail is not done by only studying 

 the shell, but principally by observing the snail animal which is housed 

 within. Oncomelania snails were found to possess a combination of 

 animal characters not present in any other Philippine or Oriental 

 snail — two delicate gray tentacles at the bases of which is a small black 

 eye surmounted by a bright lunar splotch of yellow color gran- 

 ules. This last distinctive feature was referred to for convenient 

 identification purposes as "yellow eyebrows." 



Studies on habitat preference had been hampered from the be- 

 ginning. The Japanese who had been working on the problem had 

 reported many years ago on the partially successful eradication 

 method of scattering hot, unslaked lime in snail-ridden creeks. This 

 had been misinterpreted in fairly recent American literature as mean- 

 ing that crushed limestone and the changing of creek water from a 

 slightly acid to a slightly alkaline condition would eliminate the snail. 

 It took many months of water testing to show that temperature of the 

 water, with the causative amount of shade and resultant amount of 



