West Nile virus in birds and poultry and its implication on public health

Objectives: Birds are the main reservoir for West Nile virus (WNV) while mammals, particularly humans and horses, represent dead end hosts. Infected birds usually do not show symptoms, although few avian species can suffer from severe disease even with lethal outcome. Geese and ducks are particularly sensitive to the disease and develop high viremia and severe symptoms. Mortality in young geese can be as high as 60%. On the other hand, infection in chickens and turkeys usually remain subclinical but with development of high titre antibodies and very low viremia. Hence, chickens can be used as sentinels for seasonal incursions of WNV which is usually introduced by migratory birds and transmitted by mosquitoes. WNV can cause neurological disease and death in humans as well as in horses. The objective of this study was to use chickens and horses as sentinels for early detection of WNV in order to timely conduct appropriate anti-epidemic measures in Croatia. Materials and methods: During a three-year period (2013-2015), a total of 1, 717 serum samples from sentinel outdoor chickens and 8, 131 serum samples from horses were tested for WNV antibodies using IgG ELISA. In addition, 278 serum samples from humans suspected of WNV infection and 1, 778 serum samples from asymptomatic subjects were tested for WNV IgM and IgG antibodies. WNV positive human samples were confirmed using virus neutralization test. Positive horse sera were retested using IgM ELISA in order to confirm current/recent WNV infection. Results & Conclusion: 198 (11.53%) chickens, 665 (8.18%) horses as well as 22 (7.9%) humans suspected of WNV infection and 15 (0.84%) asymptomatic subjects were found to be positive. Of 585 positive horse sera, WNV IgM antibodies were found in 34 (5.81%) samples. A significant correlation in geographical distribution of high WNV seroprevalence in tested animals with human WNV infections was found. In addition, this coincided in regions contiguous to countries where high incidence of human neuroinvasive WNV infection was reported during the investigated period. Coordinated extensive monitoring of WNV infection in poultry and horses throughout Croatia, as an early warning system, enabled timely anti-epidemic measures, primarily thorough disinsection in affected areas. This resulted in prevention of mass occurrence of human neuroinvasive WNV infections as reported in certain neighbouring countries.