A community-based study of subclinical flavivirus infections in children in an area of Tamil Nadu, India, where Japanese encephalitis is endemic
Gajanana, A.; Thenmozhi, V.; Samuel, P.P.; Reuben, R.
Bulletin of the World Health Organization 73(2): 237-244
1995
ISSN/ISBN: 0042-9686 PMID: 7743596 Document Number: 295868
During 1989-91 the Centre for Research in Medical Entomology followed 10,400 primary school children aged 5-9 years from villages where at least one case of encephalitis had occurred since 1986 to examine the epidemiology of flavivirus infections in Tamil Nadu, India. Paired surveys were conducted before and after the season for Japanese encephalitis (JE) transmission in August and December 1989, August 1990 and January 1991, and August 1991 and February 1992. The children's blood specimens were tested for hemagglutination inhibition (HI) antibody titres so the Centre could determine the seroconversion rate. The researchers also took blood specimens from 124 piglets aged 3-6 months to examine seroconversion in pigs. Among the pigs, 76.6% seroconverted during the transmission season. JE virus infection was confirmed in 69% of the pigs who seroconverted. Among the children, the rates for seroconversion (antibodies to flavivirus) stood at 37.5%, 42.1%, and 25% in 1989, 1990, and 1991, respectively. The type of flavivirus was established in 36% of seroconversion cases. JE was the flavivirus responsible for most seroconversions (31.4%). A low seroconversion rate in children was associated with a high cattle-to-humans ratio. For example, the seroconversion rate in children was 26.5% in Erappavur compared to 32.5% in Sepakkam and 48.5% in Kodikkalam. The corresponding cattle-to-humans ratios were 12:25, 4:20, and 4:29, respectively. The seroconversion rates among children who were negative for HI antibodies before transmission were higher than those among children who were positive for HI antibodies after transmission (e.g., 1990, 76.6% vs. 27.8%). In about 50% of the children who seroconverted, HI antibodies fell to undetectable levels 6-8 months later. The mean net annual increase of 16.2% in seropositivity was considerably higher than values from other areas of endemicity. The overall JE incidence stood at 15/10,000 children aged 5-9 years. One in 270 children who received an infective mosquito bite develop JE.