A longitudinal study of Japanese encephalitis in suburban Bangkok, Thailand

A one-year study of Japanese encephalitis (JE) in a small focus of transmission was conducted in suburban Bangkok in 1985. Monthly data were collected on weather, vector density, sentinel pig and chick JE antibody seroconversions, and epidemiology as related to human JE cases. The primary vector species were found to be Culex gelidus and Culex tritaeniorhynchus; from which one isolate each was obtained in March and June, respectively. Pig JE antibody seroconversion peaked in April (the hottest month), with secondary peaks following in July and December. Chick seroconversions were found only in June and July. Human cases (7) in the primary focus occurred from May-July, and started 2 months following the finding of the first JEV isolate in mosquitoes and 1 month following mass JEV seroconversion in pigs. Overall, the attack rate in the focus (0.83/10(5] was greater than 4 times that of the rest of Bangkok (0.19/10(5]. Attack rates were highest in 0-9 and 10-19 year-old groups, respectively. Indications are that JEV is transmitted to humans in Bangkok at least 10 out of 12 months per year, but that cases are concentrated in the May to July period.