Swine influenza is a highly contagious respiratory disease caused by infection with influenza A virus (IAV). Field isolates are of variable virulence and clinical signs of disease may be due to secondary infections. Pigs play an important role in the overall epidemiology of influenza because they are able to replicate influenza viruses of avian Avian Influenza Avian influenza is a viral infection found in domestic poultry and a wide range of other birds. Wild waterfowl and shorebirds are often asymptomatic carriers. In poultry, low pathogenicity strains... read more and human origin, playing a possible role in the emergence of zoonotic strains with pandemic potential. Pigs can infect people, but swine strains of IAV do not appear to easily spread in humans. However, deaths have occurred in immunocompromised people. In contrast, it is not uncommon that human seasonal influenza viruses become established in pigs, contributing to the overall genetic diversity of IAVs found in pigs. In 2009, an H1N1 IAV strain of swine origin spread globally; it infected humans, swine, poultry and fewer dogs, cats, and other animals. IAV in pigs is widespread globally and is reported mostly in swine-producing regions.
Etiology of Influenza A Virus in Swine
Swine IAV is an enveloped, negative-stranded RNA virus with a segmented genome and belongs to the Orthomyxoviridae family. IAV has two main antigenic proteins, named hemagglutinin and neuraminidase. These proteins are responsible for viral attachment to cells, release of virions from infected cells, and determination of the virus subtype. There are three main subtypes responsible for influenza infections in pigs (H1N1, H1N2, and H3N2), with multiple strains within each subtype. Other subtypes have been reported sporadically, and influenza B and C viruses have been isolated from pigs but have not been reported to cause the classic disease. Coinfections of IAV with other viruses such as porcine reproductive and respiratory syndrome virus Porcine Reproductive and Respiratory Syndrome Porcine reproductive and respiratory syndrome is a viral disease first reported in 1987 in the USA and now found throughout North and South America, Asia, Africa, and Europe. There are two distinct... read more , and bacteria such as Glaesserella parasuis (Glässer disease Glasser Disease ), Actinobacillus pleuropneumoniae Pleuropneumonia in Pigs Pleuropneumonia is a severe respiratory infection caused by Actinobacillus pleuropneumoniae. Clinical signs include fever, anorexia, reluctance to move, respiratory distress, and sudden... read more , and Mycoplasma hyopneumoniae Mycoplasmal Pneumonia in Pigs Mycoplasma hyopneumoniae is a common cause of pneumonia in pigs worldwide. It also frequently leads to subclinical infection that causes lung lesions that can be detected post-mortem... read more are common. The mixing of carrier or subclinically infected animals and susceptible pigs is an important predisposing factor. The virus is readily inactivated by disinfectants.
Transmission and Epidemiology of Influenza A Virus in Swine
In North America, outbreaks of IAV in swine occur during all seasons but are most common in fall or winter, often at the onset of particularly cold weather. In warmer areas of the world, infection may occur at any time. Usually, an outbreak is 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, and evidence of IAV infection, antibodies, or circulating virus has been well documented in Europe, North America, South and Central America, and Asia. Seroprevalence and viral titers vary among countries, but in general, IAV is considered ubiquitous in the global pig population. All virus subtypes—H1N1, H1N2 and H3N2—are known to circulate in most countries, and cocirculation of subtypes and strains is common on individual farms. Until the 2009 pandemic, very few countries (eg, Norway) had documented records of influenza seronegativity in pigs.
IAV becomes endemic in herds because of the ongoing recruitment of susceptible hosts. In breeding, a herd's piglets, prior to weaning, have been identified as a reservoir of influenza infection capable of spreading IAV to other farms after weaning. Carrier pigs are usually responsible for the introduction of IAV into previously uninfected herds and countries. In addition, lack of strict all-in/all-out practices and movement of pigs between crates or pens are associated with the spread of new influenza infections. In antibody-positive herds, outbreaks of infection recur as immunity wanes. Up to 40% of herds may contain antibody-positive pigs.
Pathogenesis of Influenza A Virus in Swine
The spectrum of IAV infection ranges from subclinical to acute. In the classic acute form, the virus multiplies in bronchial epithelium within 16 hours of 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 seen 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 outbreaks appear to be secondary to the high fever response, because IAV very rarely causes systemic infection.
Clinical Findings of Influenza A Virus in Swine
A classic acute outbreak of IAV is characterized by sudden onset and rapid spread through the entire herd, often within 1–3 days. The main signs are:
anorexia, fever (up to 108°F [42°C]), prostration
nasal and ocular mucous discharge
Mortality is generally 1%–4%. The overt course of the disease is usually 3–7 days in uncomplicated infections, with clinical recovery of the herd almost as sudden as the onset. However, virus may continue to cycle among pigs when clinical signs are suppressed by immune responses because pigs may become reinfected with similar or distinct strains. In many herds the main economic loss is from coinfections or secondary infections resulting in stunting and delay in reaching market weight. Some increase in piglet mortality has been reported, and effects on herd fertility, including abortions in late pregnancy and embryonic resorptions, may follow outbreaks in naive herds.
In uncomplicated infections, lesions are confined to the thoracic cavity. Affected areas of the lung are clearly demarcated, collapsed, and purplish red on gross inspection. They may be distributed throughout the lungs ("patchy pneumonia") but 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. There may be severe pulmonary edema, especially of interlobular septae, or a serous or serofibrinous pleuritis. Histologically, the lesions, when fully developed, are primarily those of an exudative bronchiolitis with some interstitial pneumonia.
Diagnosis of Influenza A Virus in Swine
Presumptive diagnosis based on clinical signs
RT-PCR and direct viral isolation
IAV is primarily diagnosed by detecting the influenza virus by RT-PCR, virus isolation, and occasionally by detecting antibodies against IAV 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 the influenza viral isolates may be needed to select epidemiologically relevant strains that may be needed for evaluating virus-specific antibodies or custom vaccine production. A clinical diagnosis (presumptive diagnosis) can be made by observing the sudden onset of a large number of pigs showing coughing, fever, and nasal secretions. However, subclinical and chronic influenza infections are common, and in those cases, cough and nasal secretions may be sporadic.
A retrospective diagnosis can be made by demonstrating a rise in virus-specific antibody titers in acute and convalescent serum samples using the hemagglutination inhibition test. Both H3 and H1 subtype antigens should be included. This test is also used for herd surveys, and an ELISA against the nucleoprotein (not subtype specific) is also available. To diagnose uncomplicated influenza infection, conditions such as pasteurellosis Pasteurellosis in Pigs Pasteurellosis is most commonly seen in pigs as a complication of mycoplasmal pneumonia, although swine influenza, Aujeszky disease, Bordetella bronchiseptica, or Haemophilus parahaemolyticus... read more , pseudorabies Pseudorabies in Pigs Pseudorabies is an acute, often fatal, viral disease with a worldwide distribution. Swine are the primary host, but other species are also occasionally infected. Clinical signs include reproductive... read more , porcine reproductive and respiratory syndrome Porcine Reproductive and Respiratory Syndrome Porcine reproductive and respiratory syndrome is a viral disease first reported in 1987 in the USA and now found throughout North and South America, Asia, Africa, and Europe. There are two distinct... read more , and chlamydial and Haemophilus infections must be excluded.
Treatment and Control of Influenza A Virus in Swine
Supportive care including antipyretics and antimicrobials to treat secondary bacterial infections
Vaccination and improved management practices are necessary for prevention
There is no effective treatment, although antimicrobials may reduce secondary bacterial infections and antipyretics may provide symptomatic relief. Expectorants also may help relieve signs in severely affected herds. Vaccination and strict import controls are the only specific preventive measures. Sow vaccination either prefarrowing or the entire herd at once (mass vaccination) are the most common vaccination protocols. Sow vaccination attempts to maximize the transfer of maternal immunity to the progeny. Piglet vaccination is possible, reduced efficacy due to maternal antibodies is an issue.
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 freedom from stress, particularly due to crowding and dust, help reduce transmission and losses.
Commercially available killed vaccines that contain both H1N2 and H3N2 subtypes appear to induce a strong protective immune response. In North America, custom-made (autogenous) vaccines prepared with the farm isolate(s) are common, and in 2017 a live-attenuated influenza vaccine became commercially available. Lastly, seasonal vaccination of personnel interacting with pigs is recommended to limit the bidirectional transmission of IAV between pigs and people.
Key Points on Swine Influenza
IAV in swine is an important disease of pigs worldwide that can also affect humans.
Vaccination is the most common strategy to control influenza infections.
Detection of influenza virus in tissues, nasal secretions, and oral fluids by RT-PCR is the most common and rapid diagnostic method to detect influenza virus.
For More Information
Zimmerman JJ, Karriker LA, Ramirez A, et al (eds). Diseases of Swine, 11th Edition. Wiley-Blackwell; 2019.
Salvesen HA and Whitelaw CBA. Current and prospective control strategies of influenza A virus in swine. Porc Health Manag 2021;7(23). https://doi.org/10.1186/s40813-021-00196-0
Gracia JCM, Pearce DS, Masic A, Balasch M. Influenza A Virus in Swine: Epidemiology, Challenges and Vaccination Strategies. Front Vet Sci. 2020(9). https://doi.org/10.3389/fvets.2020.00647