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French Agency for Food, Environmental and Occupational Health & Safety

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Updated on 25/08/2016

Swine influenza and influenza viruses: Questions & Answers

Keywords : Swine influenza, H1N1 (Swine), Zoonosis, Animal diseases, Animal health

What was the origin of the virus that caused the 2009 human A(H1N1) flu pandemic?

The genome of the A(H1N1) virus responsible for the 2009 swine flu pandemic has a novel gene constellation derived from several swine influenza viruses, but scientists have not been able to determine the date or place where this viral reassortment process took place, nor the host (human or animal).


Do pigs carry the virus causing the 2009 pandemic?

The A(H1N1)pdm09 virus had never been detected in pigs before being isolated in humans for the first time in April 2009. However, as soon as it emerged, its specific genomic structure led to fears that it may easily cross the species barrier from humans to pigs (reverse zoonosis). Experimental inoculations confirmed the susceptibility of pigs, and swine farms around the world were declared infected as of May 2009. Today, it appears that this virus has adapted to pigs and is still circulating on swine farms in many countries, on every continent. It specifically affected swine populations that were previously free from swine influenza, such as those in Réunion Island and New Caledonia. In mainland France, the virus is circulating in the central regions of the country, but is practically asymptomatic.


Can people be contaminated by the 2009 A(H1N1) virus via the digestive tract?

The A(H1N1)pdm09 virus is not infectious via the digestive tract and can therefore not be transmitted from pigs to humans through food. However, like other swine influenza viruses, it can be transmitted through contact with affected live pigs (nasal secretions, aerosols), and as such poses a risk to public health (zoonosis). The biosafety measures to be applied on farms to limit inter-species transmission have been reiterated by the Agency and by the Directorate General for Food.


Influenza viruses in pigs

What flu viruses can infect pigs?

Like all influenza viruses, those that cause swine influenza change constantly. Moreover, pigs can be infected by both human and avian influenza viruses. They can therefore become intermediate hosts for the transmission of avian flu viruses to humans, but can also serve as "vessel hosts" providing a setting in which new reassortant viruses, specifically with segments of avian and human viruses, can emerge as a result of co-infection. As a result, a large number of different strains have been isolated over the years thanks to surveillance carried out on swine farms. There are three major families of viruses that circulate in the swine population: H1N1, H3N2 and H1N2, but many genetic strains of varying origins are found within each of these viral subtypes.

What are the signs of infection in animals?

Swine influenza is a respiratory disorder of viral origin. It is caused by various type A influenza viruses classified as swine influenza viruses (SIVs). The disease that develops in swine following infection with the virus is similar to human flu, but less pronounced. It is a highly contagious disease but mortality rates in affected animals are low and symptoms resolve quickly, within 5 to 7 days. Transmission is primarily direct, via aerosols that form during coughing or sneezing. Unlike human flu, outbreaks occur all year round on swine farms.

Can humans be infected with swine viruses?

Yes. Swine influenza is a zoonosis. Transmission of swine influenza viruses to humans is usually benign but can sometimes have more serious consequences. Cases of infection are usually reported in people who are in direct contact with pigs. Transmission was still considered sporadic a few years ago. However, it appears that new swine viruses, derived from reassortment between the A(H1N1)pdm09 virus and other SIVs, easily cross the species barrier. Some of these reassortant viruses in fact led to a number of new cases of human infection in the United States in 2011-2012.