The repeated introductions of H5N1 virus into India since 2006 have indicated the need for monitoring the wild, migratory birds and poultry population. East and Central Asian flyways of migratory birds, which include India in their path, overlap extensively in West China (around Qinghai Lake), Mongolia and Central Siberia allowing possible interchange of diseases between these areas and particularly with India
[15, 16]. Ringing data confirms that migratory birds like Bar-headed Geese and other species migrate from Mongolia to India during winter
[15, 17]. There are few reports of AI surveillance from India. The isolation and characterization of AI A(H11N1) virus from a wild aquatic bird, Eurasian Spoonbill (Platalea leucorodia) has been reported from India
[10, 18]. Though it is believed that migratory birds may have a role to play in the introduction of different AI viruses in the country, there is no data from wild, migratory birds and in ducks in West Bengal with regard to the AI viruses harboured in them. During the present study, fecal and environmental samples were collected from 76 species from 30 families of wild and migratory birds. These samples were collected during the avian winter migratory season, during which migratory birds visit India. However, no AI viruses were isolated from any of the wild bird samples. In poultry, in contrast, the present study showed that low pathogenic AI (H9N2) was the predominant AI strain circulating in the wet poultry markets and in backyard poultry in West Bengal. The partial sequencing and phylogenetic analysis of HA and NA genes of AI H9N2 showed that H9N2 viruses belonged to G1 lineage and were similar to H9N2 viruses from Iran, Saudi Arabia and Pakistan (data not shown). H9N2 viruses have been reported from India
 and Asian countries including Bangladesh, Iran, and Pakistan have reported this virus. The seroprevalence of AI H9N2 and NDV have been reported in emus (Dromaius novaehollandiae) from India
This is the first report of isolation of AI H4N6 virus from India. The partial sequencing and phylogenetic analysis of HA and NA genes of AI H4N6 showed that these viruses were similar to Eurasian lineage viruses (data not shown). Low pathogenic H4N6 viruses have been isolated from live bird markets in Thailand
. The AIV subtype H4N6 is one of the most common subtypes found through surveillance of wild waterfowl in North America
. The interspecies transmission of an avian H4 influenza virus to domestic pigs under natural conditions has been reported in Canada
. Antibodies against H4N6 have been demonstrated in wild-caught raccoon in the USA
NDV infection is considered as one of the two most important diseases of chickens along with highly pathogenic avian influenza. It is an economically important disease causing heavy production loss to the farmers besides high mortality. Depending upon the pathotype and susceptibility of birds the mortality varies from zero to 100%. The disease in India is present in endemic form with frequent outbreaks in commercial poultry. Besides commercial poultry, the disease also affects the backyard poultry and it remains as a constant threat
. A very high seroprevalence (83%) of NDV has been reported from India
. NDV vaccination is routinely practiced in poultry. The present study was performed for avian influenza, during which NDV viruses were also isolated, as they grow in embryonated chicken eggs. As the priority was to characterize AI viruses, NDV were not further characterized. Further characterization of the NDV viruses isolated from this study would throw more light about their pathotyping.
Ducks showed antibodies against the H5, H7 and H9 subtypes, indicating their exposure to these AI viruses in the past. This is the first report showing the presence of antibodies against AI H5 and H7 subtypes in West Bengal. However the number of chicken sera was few as the primary objective of this study was screening of domestic ducks. Further studies with more number of chicken sera are required.
The present study does not report any HPAI H5N1 virus from the sampled birds during the study period. There is only one report of isolation of HPAI H5N1 virus from jungle crow in Assam
. The place from which the dead crows were recovered was 8 km from the epicentre of H5N1 outbreak in poultry. Probably these wild resident birds picked up the virus from the poultry and travelled for a short distance and died. India recently reported outbreaks of HPAI H5N1 virus in crows in Jharkhand state in November 2011
Because of legal restrictions, logistics and difficulties in trapping of the wild and migratory birds, larger sample sizes of cloacal, tracheal swabs and blood samples are difficult to obtain. Therefore, during this AI surveillance mostly environmental samples were collected and screened. It has been reported that AI H5N1 virus can replicate in feather epidermal cells in asymptomatic domestic ducks and larger amounts of viruses can be isolated for a longer time from feathers than from swabs
. Therefore use of feathers for diagnostic examination of HPAI H5N1 viruses during surveillance could be considered.
Reassortment between influenza H9N2 and H5N1 in poultry have been reported
. The high prevalence of AI H9N2 in poultry market may provide the opportunity of human infections and the possibility of reassortment with the existing poultry AI viruses including HPAI H5N1, which cannot be ruled out. In immunosuppressed chickens the H9N2 virus causes severe respiratory tract infections with high mortality in young chicks and severe decline in egg production in laying chickens, which results in economic loss. The H9N2-infected birds also shed virus to non-affected flocks through fecal-oral route without showing much of severe clinical signs
Occupational exposure to infected poultry has been an important factor in AI virus transmission to humans. Human infections with H9N2 and antibodies against H9N2 have been reported in Hong Kong, China, and India
[31–33]. Therefore, surveillance in poultry workers and cullers is required to trace AI virus infections from poultry. Regular cleaning and disinfection of wet poultry markets have been found to be helpful in preventing chain of transmission of AI viruses in Indonesia
. Such attempts would also help to curtail the spread of AI viruses in wet poultry markets. This study underlines the need of continuous active surveillance in wild, migratory birds and in poultry.