Seroepidemiology of malaria in northern Thailand: II. Focal and temporal variations in endemicity.

Four population groups from regions of Northern Thailand were surveyed for the presence of antibodies to Plasmodium falciparum using the indirect immunofluorescent antibody (IFA) test. Each of the four populations was selected from areas known to represent different patterns of malaria transmission. Group 1 was from an area where there had been no malaria transmission for approximately 30 years. Individuals in this group below age 40 showed an extremely low prevalence of malaria antibodies. Group 2 was chosen from an area where low levels of transmission have continued despite more than 30 years of DDT spraying. In this group the age related pattern of malaria antibodies varied from village to village but in all villages there was a sharp increase in the prevalence of IFA positive tests in individuals over 30. The third group has had continuously high levels of transmission. Although there are differences in the age related prevalence of IFA positives when individual villages are compared, there is a greater prevalence at all ages than in the first two groups. The fourth group was selected from an area where transmission had recently resumed after freedom from indigenous cases for approximately six years. There was little difference in the prevalence of IFA positive individuals below the age of 25 but above that there was a steady increase in prevalence with age. The correlation of IFA antibody positives with known patterns of malaria transmission in these four areas demonstrates the usefulness of this serological technique in assessing malaria endemicity and the effectiveness of control measures as well as in the interpretation of other malaria statistics.

Seroepidemiology of malaria in northern Thailand.

The seroepidemiology of malaria in two areas of Northern Thailand was studied by means of the indirect immunofluorescent antibody tests (IFAT). In one transmission has been interrupted completely for over 30 years, but in the other area transmission still occurs at a low level. Results of the survey for antibodies confirm existing parasitological findings and reflect both the recent and past history of malaria in both areas. The IFAT is less sensitive to recent malaria infection in very young people in an area of low endemicity than is the number of reported slide proven cases, but as age and cumulative exposure increase the IFAT reflects period prevalence in adults of ages 20--40. Thus, the serological data are complementary to other available information. This study also confirms the observation that malaria antibodies may last for over 30 years in people who formerly lived in hyperendemic areas and had multiple infections with the malaria parasite.