February 16, 2010
Birds are important in the natural history of Influenza and flu, but there is another character besides us humans. The pigs.
Our knowledge of Influenza in pig dates back to at least 1918 when it was observed that they could also catch the flu during a time when the human flu caused an uneven pandemic. In 1930 the virus was isolated in pigs, an H1N1 called the classic lineage, close to the human H1N1 and from the same origin: the H1N1 influenza from birds.
This virus was practically the only one circulating in pigs of North America until the end of the last century. It also circulated in pigs from Europe and Asia through a contamination in Italy in 1976, but in 1979 a new avian H1N1 completely substituted it. Since then, the history of the swine influenza and our influenza has been intercalating.
The recent world attention was more focused on the avian viruses, mostly due to the deadliness and fear caused by the H5N1, but pigs were also considered a possible source of pandemic viruses. There are several reasons that concern us about pigs, and the main ones are related to the swine physiology.
Pigs have both types of receptors for Influenza in the respiratory system, the sialic acid α2,3 and α2,6. While the virus circulating in birds have difficulties to infect us – because it uses mostly α2,3 and we only have this receptor in the lower respiratory tract (lung region), which makes the spread by cough or sneeze difficult – if this virus enters pigs it will find the α2,3 in the entire respiratory system, including the upper respiratory system. It will also find the α2,6 that, if it is able to use it, it will guarantee a higher chance of transmission among humans.
There is also the issue of temperature. Birds have a more active metabolism than ours, chickens for instance have an average temperature of 42ºC, in such a way that a virus adapted to replicate in birds generally has its enzyme functioning with less efficiency in humans. Pigs, however, have an average temperature of 39ºC, very close to ours, a convenient intermediary between birds and humans.
From an ecological point of view, the possibility of a same pig being infected by two different viruses, giving origin to a new rearranged lineage must be accounted. The chances are high given that, as pigs are able to be infected with the avian and human virus and live with both in farms, the introduction of human strains in pigs are often. Avian strains are also common in restricted cases of contamination by viruses such as H9N2, H3N3, H4N6, H1N1 and others.
This has already occurred in 1997 when the swine virus gained genes from an avian influenza and another from humans (our H3N2). This triple rearrangement circulates until today and it was one of the two viruses in pigs that gave origin to the Influenza A (H1N1) in 2009, directly demonstrating the potential of transmission to humans.
The 2009 pandemic has brought back attention to an important question. Pigs are transported around the world, bred in locations with a high density of animals, and – in many less developed places – are in direct contact with poultry birds and their owners. These animals have to be monitored and bred with some control, if we wish to reduce the chances of the appearance of new dangerous lineages.
Olsen, C. (2002). The emergence of novel swine influenza viruses in North America Virus Research, 85 (2), 199-210 DOI: 10.1016/S0168-1702(02)00027-8
Landolt, G., & Olsen, C. (2007). Up to new tricks – A review of cross-species transmission of influenza A viruses Animal Health Research Reviews, 8 (01) DOI: 10.1017/S1466252307001272