Swine flu: a challenge for livestock and human health

What is swine influenza? 

Swine influenza, commonly known as swine flu, is a highly contagious viral respiratory disease. It is very common and affects pigs of all ages on all types of farms. 

It is caused by influenza A viruses. Like all flu viruses, the ones causing swine flu are constantly evolving. Overall, the viruses circulating in the swine population belong to three subtypes: H1N1, H3N2 and H1N2. However, there are many distinct genetic lineages (or genotypes) within each of these subtypes.

Swine influenza virus can have a major economic and health impact on the swine sector and can also affect humans. 

What are the clinical signs in pigs? 

Flu of normal intensity causes moderate clinical signs, including hyperthermia, apathy and mild respiratory symptoms. The mortality rate is low and the animals usually recover within five to seven days. Some infections may even go unnoticed, as they do not cause easily identifiable flu-like symptoms.

However, the severity of the disease can vary depending on the viral strain involved, the age of the animals and their immune status. Highly intense flu is characterised by severe clinical manifestations, affecting a large proportion of animals and potentially leading to death. Concomitant infections with other pathogens affecting the respiratory system can cause complications. 

Moreover, a recurrent form of the disease is regularly reported on certain farms. These repeated flu episodes lead to the destabilisation of infected farms and may promote co-infections with several strains of influenza virus.

Why does the circulation of influenza viruses in pigs pose a risk to humans? 

Swine influenza viruses are zoonotic, meaning that they can infect humans. The symptoms are usually similar to those of seasonal human flu. In most cases, the virus cannot be transmitted between humans.

Pigs can be infected with swine influenza viruses as well as human or avian influenza viruses. They can therefore act as intermediate hosts in the transmission of avian influenza viruses to humans. Moreover, when a pig is infected by several flu viruses at the same time, their genes can combine, which can result in "reassortant" viruses, which possess genes derived from viruses of different origins. Some of these viruses may then be capable of infecting humans. 

For example, the H1N1pdm09 virus, which caused a human pandemic in 2009, was derived from the reassortment of several swine influenza viruses. This virus then spread to numerous swine populations around the world and led to the emergence of other reassortant viruses. Some were responsible for cases of human infection. 

How is the virus transmitted among pigs? 

The viruses are shed in oro-nasal secretions. Transmission within a herd is mainly airborne. The disease is often transmitted between farms through the movement of pigs from infected herds, but airborne inter-farm transmission, particularly in areas with a high density of farms, cannot be ruled out.

What measures are taken to limit transmission?

Knowledge of the circulating virus strains is an essential prerequisite for preventing and controlling the disease in pigs, as well as for assessing the zoonotic risks. Biosecurity measures are designed to limit both the introduction of influenza viruses (including human viruses) into livestock farms and also the transmission of swine influenza to humans and other animal species. 

These measures include, for example:

• managing incoming and outgoing air flows on farms,
• installing air filtration systems that are effective against viruses,
• ensuring that people working on farms wear appropriate personal protective equipment. 

Prevention also includes flu vaccination, for both animals and humans. Vaccination against seasonal flu is recommended for people who work with animals, in order to reduce the risk of emergence of reassortant viruses.

What is ANSES doing about swine influenza?

National Reference Laboratory

ANSES's Ploufragan-Plouzané-Niort Laboratory has been the National Reference Laboratory (NRL) for swine influenza since August 2009. It develops and monitors diagnostic tools for detecting and identifying influenza A viruses circulating in pigs. The NRL leads the network of official veterinary laboratories performing the molecular diagnosis of these infections. 

Surveillance of swine influenza virus

The Ploufragan-Plouzané-Niort laboratory plays a key role in monitoring swine influenza viruses on farms. It is part of the national Résavip network, which has been monitoring the genetic diversity and distribution of influenza A viruses in pigs in metropolitan France since 2011. Specific work may also focus on influenza D virus infections, which are less common in pigs. Occasional surveys are also conducted for wild boars.

Research activities

ANSES's research on swine influenza has several objectives:

• Monitoring changes in swine influenza viruses

The Agency analyses the characteristics associated with the pathogenicity of viruses and those that could facilitate their transmission to other species, particularly humans. This knowledge ensures that the French authorities are notified of any new emerging risks.

• Studying the transmission and pathogenicity of influenza viruses in pigs

The Agency investigates the factors associated with the various epidemiological forms of influenza in pigs, as well as the means of transmission within farms. 

• Studying the role of influenza viruses in respiratory syndromes in pigs

Swine influenza sometimes occurs in conjunction with infections caused by other respiratory pathogens, which can lead to complications. Research explores the mechanisms underlying the synergies or, conversely, the interference between these pathogens. 

Participation in national, European and international networks

ANSES’s research into swine influenza viruses is shared within the RésaFLU research group, which brings together French teams working on human and animal flu. 

At European level, ANSES participates in the European Swine Influenza Network (ESFLU), which supports surveillance and proposes measures for controlling swine influenza viruses. 

ANSES is also involved in the international expert network OFFLU, which focuses on animal influenza viruses. This network promotes exchanges on the global circulation of viruses. The data exchanged provide the WHO with information for detecting potential pandemic viruses and selecting strains for the development of new vaccines in humans.