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The epidemiology of influenza A viruses in pigs in Thailand and Cambodia

Netrabukkana, Punnaporn (2015) The epidemiology of influenza A viruses in pigs in Thailand and Cambodia. PhD thesis, Murdoch University.

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In April 2009, a novel influenza H1N1 virus (A(H1N1)pdm09 virus) emerged in humans in California and Mexico and quickly spread worldwide through human-to-human transmission, resulting in the World Health Organization declaring a phase 6 pandemic. The virus was found to be a swine–human–avian triple reassortant virus, with a unique combination of gene segments derived from North American and Eurasian swine lineages. Soon after its detection in humans, the A(H1N1)pdm09 virus was isolated from pigs around the world. Transmission of influenza A viruses between humans and pigs is of public health concern because reassortment can take place in the pig reservoir and further cross-species infections may facilitate creation of a new reassortant influenza virus with pandemic potential.

The first study in rural Thailand demonstrated that farmers’ and traders’ practices may constitute substantial risks for interspecies influenza virus transmission, thereby posing a threat to pig populations and public health. A cross-sectional knowledge, attitude and practices (KAPs) survey of pig smallholders showed that most farmers had limited knowledge of influenza A in pigs. This has significant public health implications and will be important when developing influenza prevention and control programs in the future. Virus isolation was attempted from a number of small and commercial pig farms in Thailand. No positive samples were recovered from pigs on small farms, whereas 8 isolates of influenza A virus were isolated from weaning pigs from commercial farms. Two viruses isolated in the study were found to be reassortant H3N2/A(H1N1)pdm09 viruses. The other six influenza A viruses isolated were all the A(H1N1)pdm09 virus strain. Virological surveillance for influenza A viruses in the Thai pig population should be continued to study virus evolution and improve the understanding of virus ecology to benefit the pig industry and public health.

Results from a study in Cambodia indicated sustained transmission of human influenza virus infections in pigs. Antibodies against seasonal H1, H3 and A(H1N1)pdm09 subtypes were common in Cambodian pigs. Despite numerous outbreaks in chickens in Cambodia, infection with the H5 subtype related to avian influenza was not detected in pigs. The historical seroprevalence data for human influenza viruses in Cambodian pigs were further analysed. Associations between seroprevalence against seasonal H1N1 influenza virus in pigs and the population densities of humans and pigs were not significant. However a positive association was found between anti-H3 antibodies in pigs and the human population density. In contrast, there was a negative association between seroprevalence of H3N2 in pigs and the pig population density. These findings highlighted the difficulty in identifying epidemiological risk factors when a limited dataset is used for analyses and thus recommendations on data collection for future epidemiological analyses were provided.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Fenwick, Stan
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