Schistosomiasis has long been recognized as a public health problem of the first magnitude. Nevertheless, it has gone essentially unchecked in most of the endemic areas. The schistosomes have not only been able to maintain themselves, but there is evidence that even now, while great strides are being made toward the control of many parasitic diseases, schistosomiasis is actually increasing in prevalence and spreading to new areas. In some areas schistosomiasis is now a more serious problem than it was some years ago. For instance, in a number of localities where schistosomiasis has long been endemic, the recent introduction of crop irrigation has been followed by increase in the prevalence of the disease (Khalil and Azim, 1935, 1938; Alcay et al., 1942; and Stephenson, 1947).
In other areas there is evidence that the disease is spreading. In Brazil, a focus of the disease has appeared at Forlandia in the Amazon Basin (Machado and Martins, 1951; Sioli, 1953).