Malaria in a rural area of eastern Thailand: baseline epidemiological studies at Bo Thong.

Malaria is still a serious health problem in Thailand. Present attempts at controlling the disease by drug treatment and other means remain unsatisfactory. Thus, development of vaccination against malaria is a major research goal of malaria immunology. The objective of this study was to acquire epidemiological base line data for subsequent vaccine trials. A cross-sectional descriptive survey was conducted among 253 local inhabitants during the beginning of the transmission season in July 1989 at Bo Thong District, Chonburi Province, Eastern Thailand where malaria transmission was likely to be moderately high. Following the cross-sectional survey weekly morbidity surveillance was started to detect new cases of malaria by using active and passive case detection at the district hospital, local health centers and at neighboring malaria clinics. Fifty-four percent of the population were male and forty-six percent female: nearly a half (48.3%) were under the age of 15 and 17% under the age of 5 years. Eighty percent of the adults were married. Seventy percent of the subjects interviewed gave a history of malarial illness in the past. Malaria, malnutrition, anemia abnormal hemoglobin diseases and parasitic infestations were the main health problems in the study area. The annual parasite incidence of malaria was 169.4/1,000 population and 77% of parasitemic individuals were asymptomatic, indicating the existence of a semi-immune condition among these subjects. Antibody level to crude parasite antigen increased with age. It is hoped that the information obtained from these field studies may be useful in malaria vaccine trials in the near future.

Seroepidemiologic studies of humoral immune response to the Plasmodium falciparum antigens in Thailand.

We have investigated seroreactivity against Plasmodium falciparum crude parasite antigens, the P. falciparum ring-infected erythrocyte surface antigen (Pf155/RESA), as well as against two synthetic peptides (EENV)6 and (EENVEHDA)3 that represent important epitopes of Pf155/RESA. The study population consisted of 421 children and adult Thais living in an area with moderate malaria transmission. We related these serologic findings to some important epidemiologic baseline data collected in the study area. The parasite rate in study subjects was 18.76%. Sixty-two percent were seropositive to crude P. falciparum antigens, 30.3% to the Pf155/RESA antigen, 23.05% to (EENV)6, and 20.17% to (EENVEHDA)3. Antibody responses to crude P. falciparum antigens and to Pf155/RESA were age dependent and increased with exposure. There was evidence that Pf155/RESA antibodies might play a role in protective immunity in this population. Since Pf155/RESA is a potential vaccine candidate antigen, the information obtained from these field studies will provide some seroepidemiologic baseline data for subsequent vaccine trials.