Surveillance for Nipah virus (NV) was conducted in Thailand's bat population. Immunoglobulin G antibodies to NV were detected with enzyme immunoassay in 82 of 1,304 bats. NV RNA was found in bat saliva and urine. These data suggest the persistence of NV infection in Thai bats.

After 12 serial Nipah virus outbreaks in humans since 1998, it has been noted that all except the initial event in Malaysia occurred during the first 5 months of the year. Increasingly higher morbidity and mortality have been observed in subsequent outbreaks in India and Bangladesh. This may have been related to different virus strains and transmission capability from bat to human without the need for an amplifying host and direct human-to-human transmission. A survey of virus strains in Pteropus lylei and seasonal preference for spillover of these viruses was completed in seven provinces of Central Thailand between May 2005 and June 2007. Nipah virus RNA sequences, which belonged to those of the Malaysian and Bangladesh strains, were detected in the urine of these bats, with the Bangladesh strain being dominant. Highest recovery of Nipah virus RNA was observed in May. Of two provincial sites where monthly surveys were done, the Bangladesh strain was almost exclusively detected during April to June. The Malaysian strain was found dispersed during December to June. Although direct contact during breeding (in December to April) was believed to be an important transmission factor, our results may not entirely support the role of breeding activities in spillage of virus. Greater virus shedding over extended periods in the case of the Malaysian strain and the highest peak of virus detection in May in the case of the Bangladesh strain when offspring started to separate may suggest that there may be responsible mechanisms other than direct contact during breeding in the same roost. Knowledge of seasonal preferences of Nipah virus shedding in P. lylei will help us to better understand the dynamics of Nipah virus transmission and have implications for disease management.

Surveillance for lyssaviruses was conducted among bat populations in 8 provinces in Thailand. In 2002 and 2003, a total of 932 bats of 11 species were captured and released after serum collection. Lyssavirus infection was determined by conducting virus neutralization assays on bat serum samples. Of collected samples, 538 were either hemolysed or insufficient in volume, which left 394 suitable for analysis. These samples included the following: Pteropus lylei (n = 335), Eonycteris spelaea (n = 45), Hipposideros armiger (n = 13), and Rousettus leschennaulti (n = 1). No serum samples had evidence of neutralizing antibodies when tested against rabies virus. However, 16 samples had detectable neutralizing antibodies against Aravan virus, Khujand virus, Irkut virus, or Australian bat lyssavirus; all were specifically associated with fruit bats P. lylei (n = 15) and E. spelaea (n = 1). These results are consistent with the presence of naturally occurring viruses related to new putative lyssavirus genotypes.

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