Influenza virus is not transmitted through pork or pork products. It appears that natural infections do not produce a viraemia and therefore the meat is not infected through the bloodstream.
Affected pig farm
The North American H1N1 influenza virus was detected in a swine herd in Canada one month ago. The Alberta farm is still the only farm to have tested positive as far as we know. The virus was believed to have been carried to the farm by a human who travelled to Mexico and returned on April 12th 2009.
Over 450 of the pigs were believed to have clinical signs and 19/24 pigs tested were positive for the matrix gene, an structutal gene of the virus, and 15/24 for the H1 gene with 100% and 99% similar identity to the Mexican isolate. The pigs have recovered without incident from the initial disease which was seen as inappetance, fever, and respiratory signs with no deaths attributable to influenza.
The Alberta farm has 220 sows with their piglets and about 1800 growers in 4 barns. Asymptomatic infections are perfectly possible
On the Alberta pig farm that was found to be infected about 500 of the animals out of a population of 2200 were culled and the rest remain under quarantine (1700). The culling was carried out to reduce the overcrowding.
This measure is neither a complete cull or just treating it as an endemic disease on one unit, basically because such a situation has not occurred before and is not covered by food safety legislation.
The influenza virus
In North America the circulating swine viruses since the 1990s have been triple re-assortants containing genetic material from avian, swine and human viruses with the general movement from humans and avians to pigs. These are from H1N1, H3N2 and H1N2 sub-types.
Between December 2005 and the early part of 2009 in the USA there have been very few infections reported in which pig viruses were believed to be responsible ( 11). All patients recovered but in some cases the clinical signs in the humans were quite severe and included fever, cough, headache and diarrhoea.
This new virus is not typical of previously circulating North American swine viruses in that the matrix gene (M) and the neuraminidase gene (N) have not been found in US pigs and may be of Eurasian origin. Until the case reported from Alberta this virus has not been reported in pigs in North America.
There is no evidence as yet of a spread from pigs to humans and as yet has not been shown to be circulating in the pig population in general. It may have occurred some time before in pigs and has not been previously reported as the surveillance methods have not been specific enough but we shall see with the increased investigation from now on. As yet therefore it still stands as a reverse zoonosis.
In humans the virus is associated with a mortality around 1% which is similar to the level of mortality associated with our normal winter flu. The most susceptible population appears to be that in the age group 15-48 which suggests that there may be some residual protection from other viruses in groups either side of this.
We are all waiting to see how the infection transmitted by returning travellers to the southern hemisphere countries behaves in their winter months as it may give a clue to how this virus may behave in the winter months. In other words will we need to concentrate on vaccine production for the new virus or our usual winter viruses.
Secondly, it appears too spread more easily but produces a much less severe disease. Do not forget that the usual European winter influenza outbreaks will often cause quite severe mortality in old people, babies, or the immunologically disadvantaged or diabetics.
Thirdly, we should be thankful that as yet not too many Mexicans have holidayed in China or Chinese been on holiday in Mexico as the possibilities of recombinants between H1N1 and H5N1 might be a far more formidable combination.
Summary of swine influenza
In Europe as a whole there is not a widespread surveillance project for influenza although research teams at Ghent and at Weybridge (European reference laboratory) have kept a watching brief on the occurrence of swine influenza types. There are 8 genes in the influenza genome and the two most important of these are the H (haemagglutinin) and the N (neuraminidase) gene. There are 16 H and 9 H genes and most of these occur in chickens.
The usual pig viruses are H1N1, H3N2 and H1N2. The original H1N1 pig viruses have been largely replaced by poultry viruses in pigs. The H3N2 viruses were from humans originally. The H1N2 viruses are re-assortants from human viruses (H and N genes) and poultry viruses.
These patterns vary from country to country. For example the classical H1N1 that has since disappeared in the UK is still found in pigs in Europe at the same time as the poultry H1N1 viruses. The H3N2 found in the pig population in the mid 1980s has also disappeared in the UK but not Europe and been replaced by the H1N2. The H1N2 in Europe is not exactly the same as the H1N2 in the UK.
The supposition is that because the pig has receptors for both avian and mammalian viruses it acts as a mixing vessel for the viruses to swap genetic material.
From time to time other viruses appear in the pig population but have not been maintained.
Experimental work is being carried on with the new virus in pigs and that will determine the pathogenesis and the patterns of viral shedding.
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