Swine influenza or swine influenza type A is endemic throughout the world and in Spain, Europe's leading producer of pigs, up to four different subtypes of this virus are circulating: H1N1, H1N2, H3N1 and H3N2. There is a certain affinity between swine and human influenza viruses and, in fact, there have been well-documented reciprocal transmissions for as long as this infectious disease has been known. When the genetic make-up of the different human and swine influenza viruses is analysed, this infectious promiscuity is clearly reflected in their respective phylogenetic trees. 

A worker at a pig farm in Lleida has just reported a punctual infection with a swine virus of the H1N1 subtype. The infection presented moderate symptoms typical of influenza and the causative virus could be identified in the nasal exudate. This is an isolated case and no transmission of the virus to other persons close to the patient has been reported. The last case similar to this one in Spain occurred in 2022 in a person from Navarra who also had contact with infected pigs. Since 2011, 19 cases have been reported in Europe and a total of 73 worldwide. It should be emphasised that these episodes come to light by chance, so the true incidence of swine-to-human virus transmission is not known exactly.   

In any case, this one-off case is a further warning that it is perfectly possible and not very unlikely that new strains of swine flu could be introduced into the human population on a stable basis, either directly or by combining with human strains, with the risk of causing a new pandemic like the one that occurred in 2009.   

It is therefore essential to implement epidemiological surveillance programmes specifically designed to detect this type of zoonosis immediately in order to be able to stop and/or prevent outbreaks leading to epidemics or pandemics with incalculable consequences.

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