Seroepidemiological study of infection with West Nile virus in Karachi, Pakistan, in 1983 and 1985.

The prevalence of West Nile (WN) virus infection in Karachi, Pakistan, was unknown until 1982. It had been noticed that there were more than a few patients with encephalitides in Karachi, and it was supposed that Japanese encephalitis (JE) cases would be found among them. Therefore, a seroepidemiological study was conducted to define the prevalence of WN virus infection and the possible occurrence of JE virus infection in the Karachi area. Prevalences of haemagglutination inhibition (HI) and neutralization (NT) antibodies against WN virus were studied among 81 serum samples (in July, 33 samples; in September, 48) during 1983, and among 156 paired serum samples that were collected twice, in July and October of 1985. Nearly the same antibody-positive rates were obtained in July of both years (1983: HI 55%; 1985: HI 53%; NT 50%); the rates increased slightly during September/October (1983: HI 65%; 1985: HI 59%, NT 54%). Among 156 paired samples in 1985, 20 (13%) and 12 (8%) showed positive- or negative-antibody conversion between July and October. Two serum samples from each of 156 residents obtained in July had a significantly higher NT antibody titre against JE virus than against WN virus (in case 1, JE 1:80, WN less than 1:10; in case 2, JE 1:40, WN less than 1:10). This is the first report to show the prevalence of WN virus infection in Karachi, Pakistan.