Pigs are exposed to IAV-S primarily through:

  • Direct pig-to-pig contact (nose-to-nose)
  • Droplet or aerosolized transmission: when the virus spreads through the air over a short distance
  • Indirect transmission: contaminated objects or materials can carry infection, such as equipment, clothes, or boots; also known as fomites

IAV-S cannot be transmitted through the placenta or by colostrum/milk. It is only carried in the respiratory tract of infected pigs.1

IAV cannot survive long in the environment; it is continuously infecting new animals.

  • IAV-S has been detected on fomites following contact with infected pigs2
  • The more positive cases of IAV-S there are in an airspace increase the chances of detecting aerosolized virus3
  • Viable virus has been found 70 to 100 meters outside the pig barn3

In contemporary swine operations, the disease can linger for a prolonged period as it finds susceptible animals.4 It is an endemic disease, meaning it occurs regularly in a particular area or population, that is present in most production systems in the US.5

How the virus enters a host

IAV-S enters a host through the upper respiratory tract. The virus then binds and infects cells that line the airway and begins replication.

Infected cells cause local inflammation, which helps the host mount an immune response. The infection and immune response causes a pneumonia along the airways, called bronchopneumonia.6

Hosts and reservoirs

Wild, migrating waterfowl are natural reservoirs for influenza A virus (IAV). Other natural hosts of IAV include humans, pigs, horses, dogs, and sea mammals.7,8

Influenza-like disease was first reported in pigs in 1918 during the human Spanish influenza pandemic and was isolated in pigs as early as 1930.9 Influenza A virus in swine (IAV-S) identifies the influenza viruses that have become adapted to swine as the primary host. Swine are important hosts for IAV because they are susceptible to strains of different species and foster the reassortment of influenza A viruses that could be transmitted between other pigs and other species.8

IAV-S is zoonotic, which is a term referring to a disease-causing agent capable of being transmitted from animals to humans.10 IAV-S is capable of being transmitted between pigs and humans as well as between humans and pigs.11

Transmission of IAV between species*

Transmission of IAV Between Species
* Adapted from Ma et al.12 IAV can be transmitted between species, as shown in the diagram above.10,11

IAV-S can change and adapt over time

Although viruses are not ‘alive’, they behave like all other pathogens—existence is based on ‘survival’. To that end, IAV can withstand many genetic changes.

Antigenic drift describes the genetic changes that occur naturally during virus replication because of selection pressure often due to host immunity or environmental conditions. Antigenic shift occurs when an animal is co-infected with more than one IAV, and the genetic material re-assorts within infected cells.5

Transmission Antigenic Shift
Adapted from Sandbulte et al.13 Antigenic drift involves random mutations occurring naturally during virus replication.14
Transmission Antigenic Drift
Adapted from Sandbulte et al.13 Antigenic shift happens when genetic material from multiple viruses re-assorts within infected cells.14

References: 1. Mastin A, Alarcon P, Pfeiffer D, et al. Prevalence and risk factors for swine influenza virus infection in the English pig population. PLoS Curr. 2011;3:RRN1209. 2. Allerson MW, Cardona CJ, Torremorell M. Indirect transmission of influenza A virus between pig populations under two different biosecurity settings. PLoS ONE. 2013;8(6):e67293. 3. Corzo CA, Romagosa A, Dee SA, Gramer MR, Morrison RB, Torremorell M. Relationship between airborne detection of influenza A virus and the number of infected pigs. Vet J. 2013;196:171–175. 4. Pitzer VE, Aguas R, Riley S, Loeffen WLA, Wood JLN, Grenfell BT. High turnover drives prolonged persistence of influenza in managed pig herds. J R Soc Interface. 2016;13:20160138. doi:10.1098/rsif.2016.0138. 5. Romagosa A, Allerson M, Gramer M, et al. Vaccination of influenza A virus decreases transmission rates in pigs. Vet Res. 2011;42:120. 6. Vincent AL, Ma W, Kelly M. Lager KM, Janke BH, Richt JA. Swine influenza viruses: a North American perspective. Adv Vir Res. 2008;72:127–154. 7. Taubenberger JK, Kash JC. Influenza virus evolution, host adaptation and pandemic formation. Cell Host Microbe. 2010;7(6):440–451. doi:10.1016/j.chom.2010.05.009. 8. Thacker E, Janke B. Swine influenza virus: zoonotic potential and vaccination strategies for the control of avian and swine influenzas. J Infect Dis. 2008;197(suppl1):S19–S24. 9. Vincent AL, Lager KM, Anderson TK. A brief introduction to influenza A virus in swine. Methods Mol Biol. 2014;1161:243–258. 10. van der Meer FJ, Orsel K, Barkema HW. The new influenza A H1N1 virus: balancing on the interface of humans and animals. Can Vet J. 2010;51:56–62. 11. Short KR, Richard M, Verhagen JH, et al. One health, multiple challenges: the inter-species transmission of influenza A virus. One Health. 2015;1:1–13. 12. Ma W, Kahn RE, Richt J. The pig as a mixing vessel for influenza viruses: human and veterinary implications. J Mol Genet Med. 2009;3(1):158–166. 13. Sandbulte MR, Spickler AR, Zaabel PK, Roth JA. Optimal use of vaccines for control of influenza SA virus in swine. Vaccines. 2015;3:22–73. 14. Rejmanek D, Hosseini PR, Mazet JAK, Daszak P, Goldstein T. Evolutionary dynamics and global diversity of influenza A virus. J Virol. 2015;89:10993–11001. doi:10.1128/JVI.01573-15.