Serological evidence of widespread West Nile virus and Japanese encephalitis virus infection in native domestic ducks (Anas platyrhynchos var domesticus) in Kuttanad region, Kerala, India

Highlights

• This is the first report of high seroprevalence of WNV infection in Chara and Chemballi domestic ducks (Anasplatyrhynchosvardomesticus) in Kuttanad region, Kerala, India.

• Detection of WNV specific and JEV specific neutralizing antibodies in domestic ducks in wide geographical area indicate co-circulation of these viruses in the region.

• Domestic ducks may be useful captive sentinels in monitoring WNV/JEV activity.

Abstract

Birds can act as reservoirs of West Nile virus (WNV) with a key role in its epidemiology. WNV lineage 1 associated fatal cases of human encephalitis in 2011 and acute flaccid paralysis in 2013 were reported in Alappuzha district, Kerala, India. But no information is available on WNV circulation in domestic ducks, which are abundant, cohabit with humans and occupy wetlands and water bodies in the region. To determine the extent of WNV infection, we investigated 209 sera, 250 oral and 350 cloacal swab samples from local Chara and Chemballi domestic ducks (Anas platyrhynchos var domesticus) in the districts of Alappuzha, Kottayam, Kollam and Pathanamthitta collected during January and March 2015. The serum samples were tested for WNV antibodies first by a competition ELISA and then by a micro virus neutralization test (micro-VNT), while oral and cloacal swabs were subjected to WNV real-time RT-PCR. Ninety five ducks showed evidence of flavivirus antibodies by ELISA. End point neutralizing antibody titre against WNV and Japanese encephalitis virus (JEV) revealed WNV specific antibodies in 24 (11.5%) ducks in 3 districts, JEV specific antibodies in 21 (10%) ducks in 2 districts and flavivirus specific antibodies in 19 (9%) ducks. However, no WNV genomic RNA could be detected. The results of this study demonstrate evidence of widespread WNV and JEV infection in domestic ducks in Kuttanad region, Kerala with a higher seroprevalence to WNV than JEV. Additionally, it highlights the utility of domestic ducks as a surveillance tool to detect WNV/JEV circulation in a region.

Introduction

West Nile virus (WNV) is one of the most important transboundary mosquito-borne zoonotic pathogens around the world. It is associated with outbreaks of acute encephalitic syndrome (AES) and hence constitute a significant threat to global human health. WNV belongs to the Japanese encephalitis virus (JEV) serogroup of the genus Flavivirus in the family Flaviviridae [1]. WNV is widely distributed in Africa, Asia, Europe and the Americas. WNV and JEV are antigenically related and are prevalent in India.

WNV infection in humans is mostly subclinical, while febrile illness develops in about 20% of infected persons and neurological disease develops in <1% cases. In India, serological evidence of WNV infection was first reported in humans in Bombay in 1952 [2]. Later, febrile illness with clinically overt cases were observed in southern, central and western India [3]. Although most of the human flavivirus encephalitis outbreaks in India have been associated with JEV, WNV has been associated with some of the recent outbreaks of encephalitis with mortalities in the states of Kerala and Assam [4], [5]. Co-infection of WNV and JEV associated with AES has also been reported in Assam [4].

WNV is maintained in nature in a mosquito-bird-mosquito (enzootic) transmission cycle which primarily involves Culex mosquitoes as vectors and birds as natural reservoir (amplifying) hosts. Humans and horses are considered as dead-end hosts and are usually infected when spillover occurs from the enzootic cycle. More than 150 species of wild and domestic birds are susceptible to WNV infection with reports of significant mortalities in North American land birds [6] including captive and farmed ducks and geese in the USA and Canada [7], [8]. However, WNV infection in wild and domestic birds remain mostly subclinical. Only limited information is available on prevalence of WNV infection in wild and domestic birds in India. Prevalence of WNV antibodies has earlier been reported in ardeid local migrant birds in Andhra Pradesh state [9], in water frequenting and terrestrial wild birds in Karnataka state [10] and recently in nine species of wild birds including Mallards and Spot-billed ducks in eastern and northern India [11].

An outbreak of AES was reported in Alappuzha district, Kerala in 2011 with 300 cases and four deaths and WNV clade 1a was isolated from a clinical case indicating reemergence of WNV lineage 1 activity in India [5]. WNV associated acute flaccid paralysis (AFP) in adults has also been reported in Kerala during pre-monsoon period in 2013 [12]. Kerala state is geographically surrounded by the Arabian Sea on the West and is bound by the Western Ghats in the East. The Kuttanad region, which covers parts of Alappuzha, Kottayam and Pathanamthitta districts of South Kerala is known as the rice bowl of Kerala due to the presence of several large water bodies and vast stretches of paddy growing in wetlands. Duck farming has been a traditional ancillary occupation for rice farmers in this region where domestic ducks (local breeds) reared for meat and eggs have a symbiotic relationship with the rice farms.

The experience in Europe has shown that domestic ducks can act as sentinel birds for monitoring of WNV activity [13]. Although, outbreaks of WNV in humans have recently been reported in Alappuzha district, Kerala, no information is available on WNV activity in domestic ducks in this region. Here we report serological evidence of widespread WNV infection in Chara and Chemballi domestic ducks (Anas platyrhynchos var domesticus) in not only Alappuzha but also in neighboring districts of Kottayam and Pathanamthitta, Kerala.