The present report is the ninth of a series of observations on malaria in a native population in Panama. This population has been under observation and treatment with various drugs since 1931.
As in previous years, no correlation with annual rainfall is shown in the monthly malaria parasite rates. The parasite rate, cumulative for the 12 months, shows that there was a small increase in the number of persons, examined at every monthly survey, who had parasites in their blood during the past year (41.4), as compared with a similar group in 1937–1938 (38.0). The slight increase is owing to the epidemic conditions which appeared during the last months of the period.
The incidence of the various species of parasites found was nearly the same as in previous years, although the percentage of P. falciparum decreased slightly, in spite of the epidemic at the close of the year. A focus of quartan malaria was discovered among the inhabitants of the former control area.
As an evidence of epidemic conditions, in which young children are not so heavily positive as in endemic areas, the parasite rate among 65 infants less than a year old was only 1.5 per cent. Among 21 infants examined an average of twice during the epidemic months of July, August, and September 1939, none was positive.
In spite of continuous treatment with three potent drugs an epidemic occurred during the last few months of the period of observation. This epidemic has been prophesied for a number of years past. Its occurrence gives additional point to the statements made in previous years concerning the necessity of having a knowledge of the local cycle of malaria, before evaluating control measures. Our monthly parasite rates show that little active malaria was present in our population before July 1939. Had our observations ceased at this time, we might have congratulated ourselves on the success of our treatment methods. But again, as in the early months of 1935, an epidemic wave swept the parasite rates twice to three times as high as they were in preceding months. Along with this increase in parasite rate, there was a concomitant rise in number of new infections, and in number of heavy infections. Clinical cases also appeared in numbers. Epidemic conditions continued at the end of the present year of observation.
For the first time since our work began, data were obtained on the relation of mosquito density to malaria rates. In one town a close correlation appeared between numbers of Anopheles and numbers of positive cases. However, this correlation did not hold true in another town, where malaria was prevalent, although Anopheles were few.
A parallel between stagnant river conditions, increase of aquatic vegetation, and Anopheles production was found in the Chagres River area during the spring months. The increase in mosquitoes observed came somewhat earlier than did the increase in malaria rates in the river villages; but no increase in mosquitoes was noted in New San Juan, although this town also experienced epidemic conditions.
As the epidemic observed during the past season occurred during the last few months of the period, it is planned to continue our observations, to obtain further information regarding its course, and the relation of mosquito density to the malaria rates in the villages under observation.