Japanese Encephalitis in Swine in San Jose, Tarlac, Philippines

Abstract Background A systematic review of multidisciplinary studies on Japanese encephalitis (JE) in the Philippines indicated that endemic foci may be found in all 17 administrative regions in the country. To establish the etiology of the disease virus isolation and seroprevalence surveys in 198 pigs were conducted in 2010–2011 in four barangays (villages) in the Municipality of San Jose, Tarlac. Prior to this present study Japanese encephalitis virus genotype III (JEV GIII) was recovered from the mosquito, Culex tritaeniorhynchus in the same municipality where backyard hog-raising and wet rice-cultivation were common practices among households located within one kilometer radius from paddies. Results JEV GIII was detected from serum and nasal swabs from pigs, 3–5 months old, from barangays Pao, Moriones and Villa Aglipay. IgM and IgG were measured by ELISA in pigs less than 4 to more than 8 months old, with an overall total of 17.2% and 62.1%, respectively. The presence of these antibodies in all pigs during four observation periods indicated year-round transmission starting with the rainy season which encompasses the months of July and September 2010. IgG increased correspondingly with age with repeated infections in older pigs. IgG prevalence rates remained high in all barangays. The proportions of positive households with any one of the markers: IgM, IgG, RT-PCR were above 80% or an average of 82.5% of households where all pigs were examined. This report contributes to knowledge on JE where incidence in humans may be linked to its epizootic spillover from pigs. Conclusion The study has shown that four barangays, representing a rice-farming community, supported the enzootic cycle of JE in swine, with mosquitoes previously found to be infected with JEV GIII, in San Jose. Thus, infected pigs, rainfall and proximity of human habitation to breeding sites of vector mosquitoes constituted the risk factors for JE as it were in other endemic countries in Asia. The finding of viral RNA in nasal swabs suggests the possibility of direct transmission among pigs via oronasal route. From the standpoint of public health JE immunization of children and periodic surveillance in swine are recommended.