Influenza in pigs is a highly contagious respiratory disease that results from infection with influenza A virus. Clinical signs include anorexia, depression, fever, sneezing, coughing, mucous nasal discharge, and lethargy. In affected pregnant sows, fever can lead to abortion. Diagnosis is based primarily on PCR assay or virus isolation. Vaccination is the main means of control in pigs; however, antimicrobials may be used to treat secondary bacterial infections.
Influenza is a highly contagious respiratory disease of pigs that is caused by infection with influenza A viruses. Influenza A virus is recognized as part of the porcine respiratory disease complex as a primary pathogen, and infection can lead to pneumonia caused by opportunistic secondary pathogens.
Pigs play an important role in the overall epidemiology of influenza because they are able to replicate influenza viruses of avian and human origin, playing a possible role in the emergence of zoonotic strains with pandemic potential. Influenza viruses from humans can be established in pigs. and vice versa, but swine strains of influenza A virus do not appear to be transmitted easily between humans. However, deaths have occurred in immunocompromised people.
In contrast, it is not uncommon for human seasonal influenza viruses to become established in pigs, contributing to the overall genetic diversity of influenza A virus strains found in pigs. In 2009, an H1N1 influenza A virus strain of swine origin spread globally; it infected humans, swine, and poultry, as well as (in lesser proportion) dogs, cats, and other animals.
Influenza A virus in pigs is widespread globally and is reported mostly in swine-producing regions. Subtype H5N1 has been detected in migratory waterfowl, with direct transmission to dairy cattle and feline species, as well as spillover into humans, resulting in a true One Health crisis (1).
Also see Equine Influenza, Canine Influenza, and Avian Influenza in Poultry and Wild Birds.
Etiology of Influenza in Pigs
Influenza is an respiratory disease of pigs caused by type A influenza viruses.
Influenza viruses are enveloped, negative-stranded RNA viruses that have a segmented genome and belong to the Orthomyxoviridae family. Influenza viruses have two main antigenic proteins, named hemagglutinin (H) and neuraminidase (N). These proteins are responsible for viral attachment to cells, release of virions from infected cells, and determination of the virus subtype.
Influenza viruses are divided into four types (A, B, C and D). Although pigs are susceptible to infection with influenza viruses of types B, C, and D, pigs are not the main host, and disease is typically mild. Influenza A viruses are the only species of major clinical relevance in swine, causing regular outbreaks of severe respiratory disease.
Three main influenza A virus subtypes cause influenza infections in pigs (H1N1, H1N2, and H3N2), and each subtype has multiple strains.
Field isolates are of variable virulence, and secondary infections can be the main contributors to clinical signs in affected pigs.
Coinfections of influenza A virus with other viruses, such as porcine reproductive and respiratory syndrome (PRRS) virus, and with bacteria such as Glaesserella parasuis, Actinobacillus pleuropneumoniae, and Mycoplasma hyopneumoniae are common.
The mixing of carrier or subclinically infected animals and susceptible pigs is an important predisposing factor for influenza A infection.
Pearls & Pitfalls
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Pathogenesis of Influenza in Pigs
The spectrum of influenza A virus in pigs infection ranges from subclinical to acute. In the typical acute form, the virus multiplies in bronchial epithelium within 16 hours after infection and causes focal necrosis of the bronchial epithelium, focal atelectasis, and gross hyperemia of the lungs.
Bronchial exudates and widespread atelectasis, grossly appearing as plum-colored lesions affecting individual lobules of apical and intermediate lobes, are evident after 24 hours. The lesions continue to develop until 72 hours after infection, after which the virus becomes more difficult to demonstrate.
Losses in reproduction associated with influenza outbreaks in pigs appear to be secondary to the high fever response, because influenza A virus very rarely causes systemic infection.
Transmission and Epidemiology of Influenza in Pigs
In North America, outbreaks of influenza in pigs occur during all seasons but are most common in autumn and winter, often at the onset of particularly cold weather. In warmer regions of the world, infection can occur at any time. Usually, influenza A virus infection outbreaks are preceded by one or two individual pig cases; the infection then spreads rapidly within a herd, mainly by aerosolization (limited distance), direct pig-to-pig contact, and fomite transfer.
Influenza is endemic and widespread in pigs worldwide. Seroprevalence and viral titers vary among countries; in general, however, influenza A virus infection is considered ubiquitous in the global pig population.
All influenza A virus subtypes—including H1N1, H1N2 and H3N2—are known to circulate in most countries, and cocirculation of subtypes and strains is common on individual farms.
Influenza A virus infection becomes endemic in swine herds because of the ongoing recruitment of susceptible hosts. In breeding programs, piglets often become infected before weaning and can spread influenza A virus infection when shipped to other farms after weaning. Carrier pigs are usually responsible for the introduction of influenza A virus into previously uninfected herds and countries.
Lax adherence to all-in all-out practices and movement of pigs between crates or pens are associated with the spread of new infections in pigs. In antibody-positive herds, outbreaks of infection recur as immunity wanes.
Clinical Findings of Influenza in Pigs
A typical acute influenza outbreak in pigs is characterized by sudden onset and rapid spread through the entire herd, often within 1–3 days. The main clinical signs are these:
depression
anorexia
fever
prostration
coughing
dyspnea
weakness
nasal and ocular mucous discharge
The overt course of influenza in pigs is usually 3–7 days in uncomplicated infections, with clinical recovery of the herd almost as sudden as onset. However, the virus can continue to cycle among pigs when clinical signs are suppressed by immune responses, because pigs can become reinfected with similar or distinct strains.
In many swine herds, the main economic loss associated with influenza is due to coinfections or secondary infections that stunt or delay the attainment of market weight. Effects of influenza on herd fertility, including abortions in late pregnancy and embryonic resorptions, can follow outbreaks in naive herds.
Lesions of Influenza in Pigs
In uncomplicated influenza A virus infections in pigs, lesions are confined to the thoracic cavity. Affected areas of the lung are clearly demarcated, collapsed, and purplish red on gross inspection. They might be distributed throughout the lungs ("patchy pneumonia"); however, they tend to be more extensive and confluent ventrally. Unaffected areas are pale and emphysematous. The airways contain a copious mucopurulent exudate, and the bronchial and mediastinal lymph nodes are edematous but rarely congested.
Influenza in pigs can be accompanied by severe pulmonary edema, especially of interlobular septa, or by serous or serofibrinous pleuritis. Histologically, the lesions, when fully developed, are primarily those of an exudative bronchiolitis with some interstitial pneumonia (see ).
Photomicrographs of lung tissue from a pig infected with influenza A virus. A. H&E staining reveals mild bronchiolitis. B. Immunohistochemical staining of the same tissue shows the location of the virus in the bronchial epithelium. Magnification 100X for both images.
Courtesy of Dr. Phil Gauger.
Diagnosis of Influenza in Pigs
Clinical signs
RT-PCR assay and direct viral isolation
Diagnosis of influenza is based primarily on detection of influenza A virus by RT-PCR assay and virus isolation, or occasionally, by the detection of antibodies against influenza A virus in nonvaccinated animals. Virus can be isolated from nasal and oral secretions in the febrile phase, from affected lung tissue in the early acute stage, or from udder wipes collected from sows with infected suckling piglets.
Sequencing and characterization of influenza viral isolates might be necessary to select epidemiologically relevant strains potentially needed for evaluating virus-specific antibodies or custom vaccine production.
A presumptive diagnosis of influenza in pigs can be based on observation of the sudden onset of a large number of pigs showing coughing, fever, and nasal secretions. However, subclinical and chronic influenza A virus infections are common, and cough and nasal secretions can be sporadic in those cases.
A retrospective diagnosis of influenza can be made by demonstrating a rise in influenza A virus-specific antibody titers in acute and convalescent serum samples using the hemagglutination inhibition test. Antigens for both H3 and H1 subtypes should be included. This test is also used for herd surveys, and ELISA against the viral nucleoprotein (not subtype specific) is also available.
To diagnose uncomplicated influenza A virus infection in pigs, conditions such as pasteurellosis, pseudorabies, PRRS, and chlamydial infections and Glaesserella infection must be excluded.
Treatment and Control of Influenza in Pigs
Supportive care
Vaccination
Good management practices
There is no effective treatment for influenza in pigs; however, antimicrobials can decrease secondary bacterial infections, and antipyretics can provide relief from clinical signs. In severely affected herds, expectorants also can help relieve signs.
Vaccination and strict import controls are the only specific preventive measures against influenza A virus infection in pigs. Vaccination of prefarrowing sows or of the entire herd at one time (mass vaccination) are the most common vaccination protocols. The goal of sow vaccination is to maximize the transfer of maternal immunity to progeny. Piglet vaccination is possible; however, decreased efficacy due to maternal antibodies can be a problem.
Good management practices, such as strict all-in all-out procedures; limiting movement of pigs and sows within farrowing rooms and between pens, rooms, and barns; and preventing stress (particularly due to crowding and dust) help decrease influenza A virus transmission and losses from influenza.
Commercially available killed vaccines that contain both H1N2 and H3N2 subtypes appear to induce a strong protective immune response in pigs (2). In North America, custom-made (autogenous) vaccines prepared with farm isolate(s) are common, and in 2017, a live attenuated influenza vaccine became commercially available.
Seasonal vaccination of personnel who interact with pigs is recommended, to limit the bidirectional transmission of influenza A virus between pigs and humans.
As an enveloped virus, influenza A virus is readily inactivated by disinfectants.
Key Points
Influenza in swine is an important disease of pigs worldwide that can also affect humans.
Vaccination is the most common control strategy.
RT-PCR assay of tissues, nasal secretions, and oral fluids is the most common and rapid diagnostic method.
For More Information
Zimmerman JJ, Burrough ER, Karriker LA, Schwartz KJ, Zhang J. Diseases of Swine. 12th ed. John Wiley & Sons; 2026.
Salvesen HA and Whitelaw CBA. Current and prospective control strategies of influenza A virus in swine. Porcine Health Manag. 2021;7(1):23.
Gracia JCM, Pearce DS, Masic A, Balasch M. Influenza A virus in swine: epidemiology, challenges and vaccination strategies. Front Vet Sci. 2020;7:647.
References
Krammer F, Hermann E, Rasmussen AL. Highly pathogenic avian influenza H5N1: history, current situation, and outlook. J Virol. 2025;99(4):e0220924. doi:10.1128/jvi.02209-24
Mtaallah O, Germeraad EA, Puspitarani GA, et al. Influenza A virus vaccines on transmission, viral shedding and clinical signs: systematic review and meta-analysis. Prev Vet Med. 2026;253:106877. doi:10.1016/j.prevetmed.2026.106877
