Stan Fenwick

The highly pathogenic avian influenza virus H5N1 was the cause of a pandemic of avian influenza in poultry throughout many parts of the world. The role of wild birds in the transmission and cycling of this virus has been uncertain and the current study was designed to collect further data on the role of wild birds in the transmission of H5N1 in Thailand. The study site for the current study was located in Nakorn Pathom province, the central part of Thailand, where both backyard poultry and low biosecurity poultry farms are common and co-exist. The analysis of existing extensive data from the national wild bird surveillance program for HPAI H5N1 virus in Thailand, found that since 2004 the prevalence of infection with H5N1 in wild birds was low (1.0% 95%CI (0.7, 1.2). However, the annual prevalence varied considerably over this period with a peak of 2.7% (95%CI 1.4, 4.1) in 2004, which dropped to 0.5% (95%CI 0.3, 0.8) and 0.6% (95%CI 0.3, 1.0) in 2005 and 2006, respectively, and then rose again to 1.8% (95%CI 1.0, 2.6) in 2007. During this period, sixteen species of wild birds tested positive for H5N1 virus infection. All samples from juvenile birds were negative for H5N1 virus, whereas the virus prevalence in pooled samples from adult birds was 0.6% (95%CI 0.4, 0.9). The positive birds belonged to twelve species which were mainly resident species that are commensal with human activities. Infected wild bird samples were only found in provinces where poultry outbreaks had occurred. A risk factor study conducted in this project using a questionnaire for villagers on farm practices and wild birds observed in the area revealed that factors associated with disease included replacing poultry individually into households/farms, buying native chickens and/or fighting cocks from commercial hatcheries and the presence of lesser whistling ducks (Dendrocygna javanica) on farms. Selecting healthy poultry when purchasing replacement birds was identified as a protective factor in this study.

The longitudinal wild bird surveillance programs conducted in this study revealed that the serological and virological prevalence of H5N1 virus were low in the wild bird population. The seroprevalence as tested by the H5N1 serum neutralization test (NT) was 2.1% (95% CI 0.7, 3.5). Species that tested positive to NT were rock pigeon (Columba livia), Asian pied starling (Gracupica contra), spotted dove (Streptopelia chinensis), oriental magpie robin (Copsychus saularis), blue-tailed bee-eater (Merops philippinus), myna (Acridotheres spp.), and pond heron (Ardeola spp.). The prevalence of H5N1 virus detection was 0.5% (95% CI 0.0, 1.1); the two H5N1 virus -positive samples were from Asian pied starling (Gracupica contra) and white vented myna (Acridotheres grandis). Wild birds that tested positive to H5N1 virus were mostly common terrestrial birds, some of which showed no clinical signs of disease. Molecular epidemiology showed that the viruses isolated from the survey were most closely related to poultry viruses isolated in Thailand (A/chicken/Thailand/PC-168/2006, A/chicken/Phichit/NIAH606988 /2006, and A/quail/Thailand /CU-333/06). There was no evidence to support the presence of unique strains in wild birds in Thailand.

A wild bird observational study undertaken demonstrated that habitats which contain the potential for a high risk of interspecies transmission of HPAI H5N1 viruses were open system duck farms and household/backyard areas. In these areas wild birds were commonly observed feeding together and in close contact with domestic poultry and pigs. Common terrestrial birds considered as bridge species (e.g. pigeons, sparrows, mynas, starlings, and doves) were likely to be involved in the disease transmission. Moreover, a qualitative risk assessment conducted in this study showed that the risk of wild birds transmitting the disease to poultry was low with an overall risk ranking of ―Medium severity‖. For quantitative risk assessment conducted, the risk of an infected lesser whistling duck defaecating an infectious dose of HPAI H5N1 virus close to a domestic duck in an open system duck farm was 5.8 x 10-6. This risk increased to 2.5 x 10-1 when all ducks visiting an open system duck farm were considered in a year.

In conclusion, wild birds can help maintain the virus in wild and domestic bird populations through spill back and spill over. However, risk of wild birds transmitting HPAI H5N1 virus to poultry in the current study was considered to be low. Monitoring of the disease in wild birds and poultry should be performed in Thailand, and the biosecurity of small and backyard poultry farms should be improved.