Field epidemiological studies on malaria in a low endemic area in the Philippines.

Field epidemiological studies were conducted to examine factors affecting endemicity in an area with a low prevalence of malaria. Two annual cross sectional surveys were done to estimate parasite prevalence rates at two periods in time, to determine the distribution of the parasitemic population and to describe the serological status of the population. A longitudinal study of a sample of infected people was used to measure reinfection rates and antibody dynamics. A 2 year passive case detection was done to estimate the number and distribution of people with symptomatic infections. Malaria was found in all age groups, with marked clustering of cases. Active and passive case detection and serological surveys all gave a similar pattern of malaria distribution: generally low prevalence with small foci of relatively high endemicity. The infection frequencies were generally similar in all age groups, measured by both active and passive case detection. There was a high frequency of P. falciparum gametocytemic infections in the asymptomatic cases found through active case detection. Twenty to 39 year old males had the highest frequency of infection by active case detection, and 10-19 year old males by passive case detection. These two groups were also more likely to be gametocyte positive than their female counterparts, suggesting that in this community, this portion of the population acts as the main reservoir of infection.