Swine influenza is a highly contagious respiratory viral infection in pigs caused by influenza A viruses. The morbidity rate, i.e. the number of animals that contract swine influenza in a given period, is usually high in pigs, while the mortality rate is low and the symptoms in pigs usually subside within 7-10 days. Infections can lead to respiratory symptoms (such as coughing and pneumonia), weight loss and fever and indirectly cause fertility problems such as abortions in pregnant sows.

There are four main influenza A virus subtypes circulating in pigs worldwide: A(H1N1), A(H1N1)pdm09, A(H1N2) and A(H3N2). Infections with influenza A viruses (SwIAV) originally from pigs also occur in poultry and humans. However, this transmission between different species occurs only rarely.

Pigs are also susceptible to infection with avian influenza viruses, which originate from birds, and with human influenza viruses. If these different influenza viruses meet in infected pigs (co-infection), they can exchange genetic information (reassortment). This can result in swine influenza strains with new characteristics. Pigs are therefore regarded as so-called "mixing vessels" for influenza viruses. In 2009, a variant of the swine influenza virus emerged in humans, which was transmitted from person to person, but also between humans and pigs. Due to its pandemic spread, this variant was named (A(H1N1)pdm09 or pandemic ("swine") influenza A(H1N1)). It carried components of swine influenza viruses, but also of avian and human influenza viruses.

The disease is currently widespread worldwide and affects millions of pigs every year

Pigs are the natural reservoir of the pathogen. Infections with swine influenza A viruses (SwIAV) also occur in wild boar, poultry and humans.

The virus is excreted via secretions and spread by droplets and aerosols. The main route of transmission of the virus is indirectly via the air or through direct contact.

The first symptoms of the disease usually appear 24 hours after infection. In the majority of cases, viruses are no longer excreted 7-10 days after infection.

The course of the disease depends on the duration of virus circulation on the farm. The symptoms can also be exacerbated by co-infection with other viruses and/or bacteria.

In the case of a new infection:

Up to 100 % of pigs fall ill

• Mortality lower than 5 % (without co-infections)
• High fever
• Dry cough
• Reduced feed intake
• Nasal discharge
• Conjunctivitis

In the case of continuous infection between the different (age) groups (endemic infection), the disease often progresses subclinically without any recognisable symptoms.

In sows, fertility problems (abortions) can also occur due to fever and circulatory disorders.

Vaccination is an important preventive measure against SIV outbreaks in pig farms. Several commercial vaccines against SIV are authorised in Europe. All are based on inactivated whole virus preparations that are administered by intramuscular injection and require at least one booster vaccination. These vaccines have the major disadvantage that they do not consistently provide robust cross-immunity against new virus subtypes. The veterinarians in charge and all persons working with pigs should be vaccinated against seasonal human influenza to minimise the risk of infection and the emergence of new virus variants.

There are no current prevalence data from Austria.

The detection of influenza A viruses in pigs is not notifiable in Austria. There is hardly any global structural surveillance. Due to the risk of the emergence of new influenza viruses with pandemic potential, swine influenza is one of the 10 most important zoonoses according to EFSA andECDC , for which improved surveillance is indicated.

As part of the EU project "United4Surveillance", AGES is conducting pilot studies (2023-2025) on different surveillance approaches to monitor the spread of swine influenza viruses.

In addition, some European initiatives aim to improve public awareness of zoonotic influenza (transmission of influenza viruses between animals and humans) and to establish European networks for the surveillance of swine influenza. In 2022, a European Swine Influenza Network (ESFLU) was established with the aim of setting up an interdisciplinary European network for the swine influenza A virus. The aim is to improve the exchange of information, awareness-raising and global surveillance in preparation for a pandemic.

Various swine influenza viruses are circulating in Europe: A(H1N1), A(H1N1)pdm09, A(H1N2) and A(H3N2).

A(H1N1): The classical influenza A(H1N1) virus - a direct descendant of the human influenza pandemic virus of 1918 that is still present in pigs in America and Asia - has not been detected in European pigs for over 15 years. The avian type H1N1av (wild ducks) is often endemic in pig populations. The 2009 pandemic H1N1 virus (A(H1N1)pdm09) and corresponding genetically modified mixed types have also been frequently detected and currently appear to persist in some pig populations.

A(H1N2): Several H1N2 viruses circulate temporarily or continuously in Austria.

A(H3N2): The H3N2 viruses were originally introduced into the pig population by humans. Since then, however, the H3N2 viruses circulating in pigs have changed. The H3N2 viruses now circulating in pigs are very different from the seasonal H3N2 viruses circulating in humans.

Further information

Zeller et al, (2018). ISU FLUture. BMC bioinformatics, 19(1), 397

Chapter 3.9.7 - Influenza A viruses of swine, WOAH Terrestrial Manual 2023 (PDF)

Diagnostic methods and protocols for the detection of currently occurring swine influenza viruses are well established and in use at AGES.

Real-time RT- PCR and sequencing: Direct pathogen detection is only possible in the acute phase (up to max. 5 days p. i.). Suitable sample materials are nasal or tonsil swabs (dry or with special transport medium) or lung tissue. The pathogen is routinely detected directly using PCR. Selected samples are subtyped using specific H1/H3 and N1/N2 PCR methods, and whole genome sequencing (WGS) can also be carried out.

ELISA: The enzyme-linked immunosorbent assay is an antibody-based detection method. Antibodies present in the serum bind to a specific antigen and are measured qualitatively or semi-quantitatively using an enzymatic colour reaction. These methods can be used to detect vaccine and infection antibodies.