Serological evidence of duck Tembusu virus infection in free-grazing ducks, Thailand.

Duck Tembusu virus (DTMUV) has been reported in ducks raised in farming system since its emergence in 2010. No information is available on DTMUV infection in free-grazing ducks, which are commonly raised and widespread in several Asian countries. To determine the presence of DTMUV infection in free-grazing ducks in Thailand, retrospective serum samples collected from 1,000 free-grazing ducks during 2008-2015 were tested for DTMUV infection. Our result showed that 91 (9.10%) were positive for DTMUV neutralizing antibodies and DTMUV seropositive ducks have been detected in Thailand since 2008. To further investigate the seroprevalence and geographic distribution of DTMUV infection in free-grazing ducks in Thailand, a cross-sectional serological survey of DTMUV was conducted in 2016. Of 1,200 free-grazing ducks in the 60 flocks from 20 provinces located in the major free-grazing duck raising areas of Thailand, 365 (30.42%) were positive for DTMUV neutralizing antibodies and 56 flocks (93.33%) had at least one DTMUV seropositive duck. Additionally, DTMUV seropositive ducks were observed in all provinces tested. In conclusion, our data demonstrated the presence of DTMUV infection in free-grazing ducks since 2008 and widespread DTMUV infection in free-grazing ducks in Thailand with a relatively high seroprevalence. These findings suggest the potential role of free-grazing ducks in the dissemination of DTMUV and highlight the necessity of systemic DTMUV surveillance in free-grazing ducks in addition to farm ducks for early detection, prevention, and control of this emerging disease.