A few days ago it was confirmed that pigs on a farm in Taitung were infected with novel A(H1N1) swine flu, probably having caught the illness from contact with a human carrier. This follows similar outbreaks among swine in nine other countries — Canada, Argentina, Australia, Britain, Ireland, Norway, Japan, Iceland and the US. As novel flu strain infections spread through schools and society, this group infection among pigs has scared many people.
The novel influenza virus that has been spreading among humans belongs to the influenzavirus A genus, which can infect and sicken many animals, including humans, pigs, horses, cats, dogs and other pets, many other mammals and most bird species. It is a disease that is shared by humans and other animals. The World Organization for Animal Health has stated that the novel influenza virus contains components derived from avian, swine and human flu, so it should be possible for pigs and humans to infect one another. It is therefore no surprise that pigs in Taiwan have caught the flu virus that has been spreading among humans.
Pigs, being one of the important reservoir hosts of A-type influenza viruses, are susceptible to influenza strains associated with birds and humans as well as pigs. If pigs are infected with more than one kind of influenza at the same time, their bodies can act like mixing machines in which viruses multiplying within them undergo genetic reassortment, giving rise to new variant strains (or mutations).
Occasionally such a mutant strain of virus will jump between species and infect humans. If that were to happen in the case of the current epidemic, would the new strain be more deadly than the original one? Would it interact differently with the immune system, making currently available flu vaccines ineffective? Could it even cause a new pandemic among humans? No one can answer these questions definitively.
In June 1969, when Taiwan was struck by an epidemic of the Hong Kong strain of type A(H3N2) virus, researchers isolated the same virus from pigs. This was the first known case of pigs in Taiwan being infected with human H3N2 influenza.
Between 1976 and 1994, there were many occasions on which pigs in Taiwan were found to be infected with the human A(H1N1) virus. In 2006, a Taiwanese research institute investigated influenza cases from 73 pig farms and isolated 132 strains of flu virus from among them. Apart from the H3N2 and H1N1 subtypes, they also confirmed for the first time the presence of two other novel subtypes — H1N2 and H3N1. Today H1N2 has become the most prevalent type of flu among pigs in Taiwan. Now that the novel H1N1 strain behind this year’ epidemic has spread to pigs, what kinds of virus might evolve from it? It is something to be worried about.
For pigs infected with influenza, if they are not also infected with other pathogens, they usually have only mild symptoms and recover quickly. It is very rare for pigs to die from flu. For these reasons, pig breeders are not particularly worried.
I doubt that the breakout in Taitung is the only current case of human-to-pig H1N1 infection. It may well be that pigs on the majority of Taiwan’s pig farms may have been infected, without being discovered. The health and agricultural departments should quickly work out a plan for monitoring the situation. They need to be prepared to deal with the situation if a mutant virus strain emerges from pig farms that is highly virulent in humans.
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