Entomological and epidemiological investigations of malaria transmission in relation to population movements in forest areas of north-west Thailand.

Transmission of forest-related malaria was observed entomologically and epidemiologically for 2 transmission seasons in 1990 and 1991 in 5 villages of Mae Sariang district, Mae Hong Son Province, north-west Thailand. The entomological study included collections of mosquitos and determination of infection rate by using enzyme-linked immunosorbent assay in the residential villages and the farm huts. The epidemiological study included fortnightly visits to 30% of the households to interview and record movement activities and illness of villagers. Circumsporozoite proteins, in most cases of Plasmodium falciparum, were detected in Anopheles minimus species A, An. dirus s.l., An. maculatus s.s. and An. sawadwongporni in residential villages and/or farm huts, suggesting transmission could occur there. Movement of people away from their residences occurred throughout the year for several reasons with a sharp peak in July for agricultural activity, mainly ploughing and planting for rice cultivation. The relative risk of infection for people engaged in agricultural activity was 3 times that of people living in the residential villages. Although a higher biting density of vectors was generally evident at the farm huts, the estimated inoculation rates in the 2 settings were similar. Movement for forest activity increased after harvesting rice in the cool dry season and carried the highest malaria risk, suggesting different epidemiological and probably entomological conditions which need further investigation. The significance is discussed of discrepancies between the case classification system used by this study and that used by malaria sector staff.

Susceptibility of Thai zoophilic Anophelines and suspected malaria vectors to local strains of human malaria parasites.

Wild caught zoophilic Anopheles and suspected malaria vector species collected in northwest Thailand were experimentally infected with local human malaria parasites using a membrane feeding. One week post-feeding a number of mosquitos were dissected for oocyst examination. The remainder were kept for another one week or more, and then the salivary glands were examined for the presence of sporozoites. The results revealed that An. vagus, An. kochi and An. annularis were susceptible to both Plasmodium falciparum and P. vivax whereas An. barbirostris and An. sinensis were susceptible to only P. vivax. The non-susceptibility to P. falciparum of these two mosquito species may indicate their poor vector status of this malaria species in the field.