MOLLUSKS AND MEDICINE — ABBOTT 329 



oxygen in the water, was the chief factor. It was also discovered that 

 the ideal condition of the water for the shedding of cercariae was on 

 the slightly alkaline side. Today, the fact is almost universally ac- 

 cepted that a creature which builds its shell of calcium carbonate will 

 grow best in slightly alkaline waters. The Oncomelania snail is am- 

 phibious, and although a gill-breather, is as much at home on the moist 

 earth bordering the creeks as it is under water. If the water is cool, 

 the mollusk will make no attempt to leave. If trapped in a small 

 puddle of water from a previous flood, and if the temperature rises 

 above 88° F., it will either crawl out over moist ground or, if the water 

 is still slowly draining to the main creek, will come to the toj), cup its 

 foot to the surface film and float to cooler safety. 



This matter of temperature and habitat preference is not without its 

 far-reaching military aspects when it comes to locating safe areas for 

 encampment. Two similar-appearing valleys can, by the nature of 

 their drainage and terrain, possess totally different mollusk popula- 

 tions. On Leyte Island nearly any flat stretch of fairly well-shaded 

 and creek-drained land may be looked upon as good breeding grounds 

 for the Oncomelania snail. The bordering hillside streams are, with- 

 out exception, perfectly safe. However, on Samar Island, at the 

 northeastern end, a small valley tucked away in the hills was found 

 which presents a totally different type of snail distribution. It is an 

 area much like one that our troops might have entered had an assault 

 on Japan been necessary. The center of the valley had every outward 

 appearance of being snail-ridden country, yet a thorough survey there 

 would have given a clean bill of health. The bordering hills, lush in 

 protective woodland and near a source of clean, loinning water, would 

 have been automatically recommended as an ideal site for encampment. 

 Yet, unlike Leyte conditions, the areas around these hillside streams 

 were heavily colonized by Oncomelania snails. The valley proper 

 was ideal as a habitat in every respect except one — the temperature of 

 the water was above 88° F. Had the inhabitants of our hypothetical 

 Japanese valley fled, so that no survey could have been made of the 

 natives, 1 to 2 months might have elapsed before the presence of the 

 disease had become known. By that time our infected troops might 

 have been holding an important sector of a battle front. 



Locating small endemic areas of schistosomiasis has been made diffi- 

 cult by the migration of thousands of people during and after World 

 War 11. This has been particularly the case in the Philippine Islands 

 and China. Medical surveys brought to light cases of the disease in 

 areas which had always been thought to be free. Questioning usually 

 revealed the fact that the patient had lived in or passed through an 

 endemic area. The Navy unit employed an accurate and easy method 

 of mapping out small endemic areas by trapping and inspecting wild 

 rats. In the Philippines the rats which forage in the brush do not 



