1. Six rhesus monkeys were exposed to 750 cercariae of a Puerto Rican strain of S. mansoni and six to the same dose of a Tanzanian strain.
2. The Puerto Rican strain proved to be much more pathogenic and during the early stages of the infection the animals infected with this strain were obviously more severely affected than the Mwanza strain animals; two died only two weeks after eggs appeared in the faeces.
3. With the Puerto Rican strain there was a shorter prepatent period and a highly significant difference in the infectivity as determined by the worm recovery. The mean maturation rate was 81% in the two fatal infections and 63·5% in the four remaining monkeys as compared with a mean of 45·9% in the 6 monkeys infected with the Tanzanian strain.
4. There were also differences in the pattern of egg output in the faeces with higher densities with the Puerto Rican strain in the initial stages but lower densities in the later stages of the infection. But it was noted that faecal egg output was a poor index of the severity of the infection. In the severely ill monkeys the faecal egg output expressed as the total egg output in 24 hours was very misleading because the affected animals had poor appetites and they passed very small quantities of blood stained mucus containing very little faecal material.
5. Tissue egg counts were far more reliable as was shown in a preliminary experiment where during a period of one week approximately 100,000 eggs were excreted in the faeces as compared with more than a million eggs retained in the tissues.
6. Details are given of the egg densities in the various organs of the monkeys. The most important difference between the two strains was the number of eggs in the liver with a mean of 1,222 eggs per g. in the Puerto Rican strain monkeys as compared with only 158·5 eggs per g. with the Mwanza strain.
7. Both groups of monkeys developed a marked lowering of the haemoglobin levels but this was not a strain characteristic and the severity was not related to the tissue egg densities. There was raised white count which reached the highest level at the time when the haemoglobin levels were at their lowest. There was an increase in the eosinophilic count in some of the animals but again this was not a strain characteristic. There was no eosinophilia in the two animals that died.
8. Fluorescent antibody levels were recorded at different stages of the infection together with the blood protein levels. The F.A. titres were at the highest levels in the acute stages of the disease. There was a significant correlation between the peak F.A. titres and the eosinophil levels in the blood. The percentage of gamma globulins in the sera showed a peak at the end of the period of observation when the F.A. titres were decreasing.
9. These observations support the clinical impression of the relatively low pathogenicity of S. mansoni in Tanzania and other parts of Central and East Africa as compared with Egypt and South America. However, the inherent pathogenicity of the strain may not be the most important factor in accounting for the differences in the epidemiology. Differences in the reaction of the human population and in the intensity of transmission are probably equally important. In Africa there is also the possibility that in some areas worm loads are reduced due to the heterologous immunity that is acquired as a result of repeated exposure to the prevalent “nonpathogenic” schistosomes of domestic and wild animals.