A community-based study of subclinical flavivirus infections in children in an area of Tamil Nadu, India, where Japanese encephalitis is endemic

Abstract

A characteristic feature of the epidemiology of Japanese encephalitis (JE) is the occurrence of a large number of subclinical infections. The reporting of only overt cases underestimates the total level of virus transmission, a knowledge of which is essential for the evolution of control strategies. We carried out a 3-year prospective serological study between 1989 and 1991 in a primary health centre in Tamil Nadu where JE is endemic. Each year paired specimens, taken before and after the transmission season from a cohort of schoolchildren aged 5-9 years, were tested for haemagglutination inhibition (HI) antibody titres in order to study seroconversion. The seroconversion rates in the successive years were 37.5, 42.1 and 25 percentage points, and in a third of such seroconversions it was possible to establish a specific diagnosis. Seroconversion was attributable predominantly to JE virus and minimally to West Nile virus. Relatively high dengue virus activity occurred only in 1991. There were statistically significant differences in seroconversion rates between villages and this was related to variations in the ratio cattle:humans:pigs. Very high seroconversion rates occurred among children who were negative for HI antibodies before the transmission season. HI antibodies declined to undetectable levels 6-8 months later in half the children who had seroconverted. The average net annual increase of 16.2 percentage points in seropositivity was nevertheless much higher than values reported from other areas of endemicity. The overall incidence of JE cases was 15 per 10,000 children aged 5-9 years, and the estimated ratio overt:inapparent infection was 1:270.

PIP: 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.