A malaria study of a small isolated community in Brazil was made during the year 1940 before and after the eradication of Anopheles gambiae from the area. The records for the first four months of 1940 showed a rising Anophales gambiae density, and among the human population rising parasite and spleen rates and increasing symptomatology with demand for medication. The eradication of Anopheles gambiae in May by combined antilarval and antimosquito measures was accompanied by a sudden drop in the parasite rate from 85.7 per cent to about 20 per cent by June and a gradual reduction in spleen rate from 64.4 per cent to about 10 per cent by September. Although children in the age groups 1 to 4 and 10 to 14 years showed somewhat higher infection rates there was on the whole little difference in parasite rate between the young and older age groups, confirming previous information that malaria had not been endemic in the area. No consistent difference in infection intensity or in species of plasmodium, or incidence of gametocytes could be demonstrated among the age groups. Although there were many more gametocyte carriers in the months “with gambiae” than in the months “without gambiae” the ratio of gametocyte carriers to cases of malaria was found to be comparable in both periods.
The spleen rate, unlike the parasite rate, showed a distinct difference among age groups, the group over 40 years showing the lowest rate of splenic enlargement and children 1 to 14 years the highest rate. There was no correlation found between species of parasite and size of spleen.