Study of Japanese encephalitis virus in Chiangmia Valley, Thailand. V. Animal infections.

Abstract

Johnsen, D. O., R. Edelman, R. A. Grossman, D. Muangman, J. Pomsdhit and D. J. Gould (SEATO Medical Research Laboratory. Bangkok, Thailand). Study of Japanese encephalitis virus in Chiangmai Valley, Thailand. V. Animal infections. Am J Epidemiol 100: 57–68, 1974.—In this study, we examined the temporal nature of animal infections and sought to determine which animal species play an important role in the transmission of Japanese encephalitis virus (JEV) in the Chiangmai Valley. Sera were collected from a sample of domestic mammals and fowl residing in four study villages before the start of the 1970 Japanese encephalitis epidemic and three times during the epidemic year. Antibody-free pigs, placed in each village as sentinels, were bled every month from April 1970 until June 1971. Tree sparrows and bats were also trapped and bled in the villages. Attempts to isolate JEV from domestic and wild animal specimens were unsuccessful. Census data revealed that the turnover rate in one year for pigs was much higher than for other domestic mammals. JEV transmission to sentinel pigs was detected in each of the 14 consecutive months, except for two months of the cool-dry season; transmission rapidly increased with the start of the seasonal rains. The prevalence of JEV antibody (HAI) was high in dogs, bovines and indigenous pigs, but, except for a small group of horses, the geometric mean titer was much higher in pigs than in other mammals. Antibody to Tembusu virus, rather than to JEV, was prevalent in high titer in domestic fowl. Paired serum samples collected from indigenous pigs, bovines and dogs confirmed that primary JEV infections occurred during the human epidemic in these species; animals with prior JEV antibodies were not infected. Only a small percentage of tree sparrow and bat sera neutralized JEV. One pig infected with JEV under laboratory conditions developed a viremia of high titer and long duration. Buffalo and cattle, on the other hand, failed to develop detectable viremia and had a poor HAI antibody response following exposure to JEV-infected Culex tritaeniorhynchus and repeated inoculations of JEV. One of three dogs developed low level viremia after being bitten by JEV-infected mosquitoes. The serologic responses of these experimentally infected animals mimic the serologic patterns observed in domestic animals in the field. All of these results together suggest that pigs are important in the epidemiology of JEV in the Chiangmai Valley; the role of other domestic mammals is less certain. A pig-mosquito-man transmission sequence may be sufficient to account for human infections that occur throughout the year.