The epidemiology of malaria in a Karen population on the western border of Thailand

Abstract

From November 1991 to November 1992 a prospective, descriptive study of malaria epidemiology was conducted in a Karen population on the western border of Thailand. Two study groups were selected at random and more than 80% of the subjects were followed for one year. In Group 1, comprising 249 schoolchildren (aged 4–15 years), daily surveillance for illness was combined with fortnightly malaria surveys. These children experienced 1·5 parasitaemic infections per child-year (95% confidence interval [CI] 1·3–1·7), of which 68% ( 193 285 ) were symptomatic ( Plasmodium falciparum 84%, P. vivax 57%). The estimated pyrogenic densities were 1460/μL for P. falciparum and 181/μL for P. vivax. In Group 2, comprising subjects of all age from 428 households, malaria was diagnosed during two-monthly surveys, at weekly home visits, and otherwise by passive case detection. Malaria and splenomegaly prevalence rates were low in all age groups (spleen index 2–9%; P. falciparum prevalence rate 1–4%; P. vivax 1–6%). Group 2 subjects had 1·0 infections per person-year (95% CI 0·9–1·1), most of which were symptomatic ( 312 357 ; 87%). Malaria infections clustered in households. Overall, P. vivax caused 53% and P. falciparum 37% of the infections (10% were mixed), but whereas P. vivax was most common in young children, with a decline in incidence with increasing age, P. falciparum incidence rates rose with age to a peak incidence between 20 and 29 years, although the risk of developing a severe malaria decreased with increasing age. There was no death from malaria during the study. P. falciparum infections were more common in males, subjects with a history of malaria before the study, and in those who had travelled outside their village. These findings suggest a higher transmission rate for P. vivax than P. falciparum, although adults still suffered symptomatic malaria due to both species. The 2 malaria parasites found in this area contribute approximately 50% of infections each, but their clinical epidemiology is very different.