Tropical Disease Bilharzia and Irrigation Systems in Puerto Rico

In many tropical countries the construction of irrigation and hydroelectric systems has caused outbreaks of the parasitic disease, bilharzia, or schistosomiasis, transmitted to man from a fresh-water snail and caused by the parasitic schistosome. The history of irrigation systems in Puerto Rico shows that transmission occurred in the South Coast System soon after it went into operation. The prevalence of infection in children from Guyama, a town in the irrigated zone, was zero and rose to 25% soon after irrigation began. Other irrigation systems constructed later did not have this problem, apparently because they had better drainage systems and did not have the small night-storage ponds that served as snail habitats and transmission sites in the South Coast System. A snail control program was started in this System in 1954 and the prevalence in children dropped to zero by 1966.