Factsheet on swine influenza in humans


Epidemiological background

Evidence of person-to-person spread of SIVs is very limited and most swine-to-human transmissions have been epidemiologically dead-end events, i.e. onward transmissions are very rare.  One known exception is the A(H1N1)pdm09 influenza virus. Although this virus is likely of swine origin, it has not (to date) been detected in pigs prior to its appearance in humans. This could simply reflect that, as mentioned above, there is absence (or limited presence) of programmes of surveillance for SIVs in many parts of the world.

Human infections with swine influenza have been sporadically detected (or at least published in the literature) since the late 1950s. These findings usually occurred in people with direct and indirect exposure to pigs i.e. people working in pig farms, etc. Since 1958, a total of 17 cases of swine influenza in humans have been reported in Europe. The journal Eurosurveillance published an article related to one of these cases in Spain in 2008, accompanied by an editorial. Some information has also emerged about three more cases of swine influenza in humans in Switzerland, and this has been included in an ECDC Rapid Risk Assessment published in November 2011. Also this year ECDC has published two Public Health Developments describing parallel descriptions of such occurrences in US & Germany and in the state of Iowa (United States).

ECDC Risk Assessment  

In view that some transmission, albeit limited, occurred in the American instance, ECDC also published a Rapid Risk Assessment in relation to it, accessible through the following link:

Swine-origin triple reassortant influenza A(H3N2) viruses in North America

The 2009 pandemic influenza A(H1N1)

The A(H1N1)pdm09 influenza virus contained genes from pig, bird and human influenza viruses, in a combination that was never reported before 2009  in any part of the world. Following the emergence of the 2009 pandemic, this virus was also isolated from pigs in multiple areas of the world (including Europe), and it is currently circulating in pigs in several European countries. The infections in swine were initially due to transmission from infected humans, but subsequently through pig-to-pig spread.

Triple Reassortant A(H3N2) viruses in USA from 2005 onwards

As of November 2011, thirty-one cases of swine influenza infection in humans have been reported in the United States between December 2005 and November 2011, of which 11 had a clear history of exposure to pigs. Twenty-three cases occurred in children (persons 18 years or younger) and eight in adults. Single generation person-to-person transmission has been reported in the literature but longer chains of transmission have not been recorded to date.

See ECDC Rapid Risk Assessment (November 2011).

The 1976 A(H1N1) Incident – Fort Dix (United States)

An exception to the rule is a well known 1976 outbreak of swine influenza among young and healthy adult military recruits at a basic-training centre in New Jersey (Fort Dix), in the United States. This was caused by an A(H1N1) SIV and resulted in at least 230 infections, 13 of whom experienced severe disease and a previously healthy man died. After intense local transmission over a one-month period that virus was never observed again. Hence, the A(H1N1)pdm09 influenza virus is, so far, the only SIV that has shown the capacity to spread rapidly between humans.