It is a pleasure to discuss Dr. Oliver-González' paper on serological responses to schistosomiasis. Several observers have found, when studying this disease under natural conditions, that there is a decrease in the percentage of individuals who have eggs in their stools as the age of the individuals increases. Presumably, these people are still exposed to infection and many believe that the worms live for an indefinite period of time in the host. Also, the percentage of individuals with enlarged livers decreases with age. This has encouraged the idea that some kind of immunity following repeated infection operates in these hyperendemic areas.
A number of workers, including several within this audience, have developed techniques for studying this problem. They have shown that there is a moderate reduction in the number of worms which developed in animals on reinfection as compared to those receiving the first infection.