Malaria in tree crop plantations in south-eastern and western provinces of Thailand.

During the past three decades almost half of the existing natural tropical forests in Thailand were destroyed and replaced by cash crops, rubber, coffee, fruit orchards (durian, rambutan, mangosteen) and other commercial plantations. In order to determine the proportion of malaria cases contracted from such commercial plantations, an epidemiological study was conducted between June 1996 to May 1997 in two districts, one in Pong Nam Ron, located in a south-eastern province near the Cambodian border and another in Sai Yok, in a western province along the Myanmar border. Data were collected by passive case detection from patients attending the existing malaria clinics and active case detection by monthly malariometric survey in selected villages. All malaria cases were thoroughly investigated and classified according to exposure to different ecotypes prior to onset of malaria symptoms in the preceding two weeks. Malaria cases acquired from commercial plantations accounted for 35.2% and 11.2% in Pong Nam Ron and in Sai Yok districts respectively. In such plantations, most of the malaria cases were contracted from fruit orchards and to a lesser extent from rubber and teak plantations. From this study it is evident that commercial plantations provide a significant site of malaria transmission in addition to the forest and foothills areas in Southeast Asia where efficient vectors such as An. dirus and An. minimus are prevalent and have adapted to such changed ecosystems.

Behavior of people with respect to malaria in endemic areas with different levels of malaria transmission.

The present survey was carried out in two villages of the same malaria endemicity but different levels of transmission in Lamae District, Chumphon Province, southern Thailand with the aim to study whether behavior of people to malaria was similar or not. The total populations in low (village 11) and high (village 13) transmission areas were surveyed twice during low (February 1990) and high (June 1990) transmission periods. All subjects were interviewed by trained interviewers using a structured interview form. Among 410 and 614 people in villages 11 and 13, respectively, it was revealed that during the low transmission period behavior related to chemoprophylaxis and use of bed nets was similar in both villages, whereas risk behavior of night work was significantly higher in village 11 than village 13. This was likely due to their different principal occupations: farming of rubber plantation in village 11 and of coffee plantation in village 13. Concerning pattern of seeking malaria treatment, the malaria clinic and the district hospital were the common places attended by people in both villages. During the second survey in high transmission period, a cohort of 277 in village 11 and 430 in village 13 were available for follow up to study changes in their behavior between low and high transmission seasons. There were significant decrements related to night work and significant increments related to bed net use in both villages.

Comparative study of various serological criteria in malaria survey.

The determination of the most appropriate serological criterion related to the blood slide examination in malaria surveys was carried out in western Thailand, an area of low persistent malaria transmission. Six different serological criteria were assessed against the blood slide results among 5001 paired blood samples of one-month interval. The indices used in the analysis were sensitivity, specificity and kappa coefficient and the blood slide examination was used as the reference test. It was apparent that a titre of greater than or equal to 1:1024 was the most appropriate serological criterion related to the blood slide results. This serological criterion can be used to indicate the high or low malaria exposure group, particularly in an area of low transmission.

The effect of sulphadoxine/pyrimethamine on gametocytes in falciparum malaria.

A study of the effect of sulphadoxine/pyrimethamine (Fansidar) on P. falciparum's gametocytes in peripheral blood was carried out in Western Thailand. One group of 77 patients with asexual form P. falciparum sensitive to Fansidar were followed weekly to detect the appearance and the duration of gametocytes in peripheral blood after Fansidar treatment on the basis of thick blood film examination. Another group of 14 patients with sexual form P. falciparum was not given any antimalarial treatment and also followed up weekly. No significant difference of average duration of detectable gametocytes was observed between the groups. The average number of days that gametocytes appeared after asexual form in patients receiving treatment was the same as in the untreated group. It is unlikely that Fansidar has the stimulating effect on gametocytogenesis as previously reported.