Considering the widespread nature of the current H5N1 outbreak in Asia and the capability of influenza viruses to jump the species barriers, it is inevitable that H5N1 virus will be detected in some pigs.
Pigs can be infected with both avian and human influenza A viruses -- for instance, human influenza H3N2 viruses have been detected in pigs in Asia, Europe and Africa.
Some of these human and avian influenza viruses might become adapted to pigs and then begin circulating in pig populations. The co-circulation of avian, human, and pig viruses in pigs is of significant concern because of the potential for a genetic exchange, or "reassortment," of material between these viruses. Such an occurrence has the potential to produce a new, pandemic influenza strain.
Last week, a researcher at China's Harbin Veterinary Research Institute announced that pigs from farms in parts of China had been infected with the H5N1 strain of avian influenza (see previous report)
http://www.who.int/csr/don/2004_08_20/en . China's Ministry of Agriculture has since confirmed the researcher's findings. What is unclear from the few studies that have been conducted is whether the H5N1 virus has already become established in pig populations in China.
Because the findings remain preliminary and are not necessarily indicative for widespread infection among pigs, assessing the consequences of this information for public health is difficult. Providing a detailed risk assessment of the current situation requires an understanding of the main factors influencing the potential for the emergence of a pandemic influenza strain: the prevalence of H5N1 and human H3N2 virus in pigs in Asia, and the likelihood of a reassorted virus and its possible pathogenicity.
The role of pigs in genetic reassortment is not fully understood. While there has been no known natural occurrence of reassortment of influenza viruses in pigs that resulted in a new pandemic strain, the probability of this occurrence is not negligible.