Until recently the basin of Sentani Lake and the Grimé Valley which are representative of many parts of Netherlands New Guinea and also of areas of Australian New Guinea (Avery, 1946; Bern and Hansen, 1950; Hairston et al., 1947; Mackerras and Aberdeen, 1946) were both highly malarious due to the presence of potent vectors of the Anopheles punctulatus group and also, in the Sentani area, of A. kawari.
In 1954, the houses of the Sentani area comprising the villages on and around the lake where observations were made, and a surrounding area where the villagers have their gardens, were sprayed with DDT, leaving the Nimboran, the western part of the Grimé Valley, as control since surveys had shown malaria to be hyperendemic in both valleys.
The spraying did not completely prevent transmission as a few sporozoitepositive mosquitoes were found and a few new infections occurred during the following year in infants who had not been outside the sprayed area, amounting however to less than 1/10 the expected number before spraying. Surveys made one year after the spraying showed considerable improvement in the malaria situation, with a much lower parasite rate, which dropped from 46 to 32 per cent in the hyperendemic lake area and from 18 to 8 per cent in the mesoendemic lake area, while that of the control area remained approximately the same. In the sprayed area the sporozoite rate in the Anopheles dropped from 1.2 to 0.2 per cent. Improvements in the spleen rates were less marked, due perhaps to splenic fibrosis. Also the high spleen rate in malaria-free infants is striking and should be investigated, though palpable spleens in infants need not be of pathological significance.
Since February, 1955, the houses in a large territory around the experimental area, including Hollandia and the Grimé Valley, have been sprayed so that the risk of infection both inside and outside the Sentani basin is now equalized and minimized. Eradication of the local vectors and of the malaria parasites in New Guinea cannot be anticipated by residual spraying, but a much lower degree of endemicity seems reasonable to expect.