Preventing transmission of swine flu viruses between animals and humans

A threat to animal and human health

Swine flu is a highly contagious viral respiratory disease that often occurs on French farms. It is generally associated with moderate clinical signs (25% of cases are severe) and is not seasonal. Pigs can be infected with swine flu as well as human and avian influenza viruses. Co-infection may then facilitate the emergence of novel influenza viruses known as reassortants. By combining the genes of several influenza viruses of different origins, they could become more virulent or spread more easily to other species.

Type A swine flu viruses can spread between animals and humans. Animal-to-human cases of transmission have been documented in several countries, including France for the first time in 2021, in the case of a pig breeder in Brittany. In view of the risk to human and animal health posed by these viruses, ANSES was asked to assess the risk of introduction and persistence of these viruses on pig farms, as well as the risk of transmission between pigs and humans.

A particularly persistent virus on pig farms

The purpose of ANSES's appraisal was to assess the risk of a type A influenza virus being introduced into a pig farm by different routes, i.e. by animals, humans or the environment. It was found that live pigs — particularly young sows — are the biggest source of introduction. In order to reduce this risk, the Agency therefore recommends that animals brought onto farms be screened before they are moved. The Agency stresses that animal movements between farms, regions, countries and/or continents are major contributors to the spread of these viruses, and therefore to the emergence of reassortants. ANSES therefore reiterates the importance of complying with biosafety measures on farms on a daily basis, but particularly when bringing in new animals. This is because once influenza viruses have been introduced into a pig farm, they are extremely persistent, especially given the current models of intensive farming. This persistence is strongly dependent upon the pigs’ immunological status, the practice of rearing them together in groups of the same age and physiological stage, and the building structure (in particular the air circulation system).

Humans are also a major source of influenza viruses on pig farms. The Agency therefore advises following the recommendations of the French National Authority for Health when it comes to vaccinating people working on pig farms against seasonal influenza. In addition, people with flu-like symptoms should not enter pig farms and, when seasonal flu is circulating, farmers and other people in contact with pigs should wear personal protective equipment: masks, goggles/visors, gloves, etc.

Greater vigilance to identify the emergence of novel type A influenza viruses as early as possible

Although the risk of human-to-human transmission of swine viruses is low with the viruses currently circulating, ANSES is calling for the surveillance of influenza viruses in pigs and humans to be stepped up. This mainly means pursuing and strengthening cooperation and information exchange between all the players, whether scientists or professionals (such as veterinarians, doctors and breeders). This will facilitate the study of all the viral strains in circulation to identify any emerging reassortants on national and international scales.

The Agency also suggests looking into the possibility of extending the SAGA avian flu surveillance protocol — designed to detect cases of animal-to-human transmission at an early stage — to swine flu in order to improve response times. This entails setting up a surveillance network to identify as early as possible the emergence of novel type A influenza viruses that could be transmitted to humans, and assess their potential for triggering human-to-human transmission.