Swine influenza (swine flu): overview, strains, transmission and prevention

Virology and common strains

Swine influenza viruses belong to the genus Influenza A. They are characterized by two surface proteins, hemagglutinin (H) and neuraminidase (N). Common subtypes found in pigs include H1N1, H1N2 and H3N2; less frequent types such as H3N1 and H2N3 have also been detected. These viruses can reassort—that is, exchange gene segments—when a host is infected with more than one influenza virus, producing new combinations with unpredictable properties. For background on influenza A, see Influenza A viruses, and for information about pigs as hosts see pigs and swine health.

History and public‑health importance

Interest in swine influenza rose after repeated reports of human infections associated with pig exposure and after the 2009 H1N1 pandemic, which contained gene segments of swine origin alongside avian and human components. That event highlighted how animal influenza viruses can contribute to novel human strains. Surveillance of swine populations is an important public‑health tool because changes in these viruses can signal heightened zoonotic risk. The term "swine flu" has been used both for infections in pigs and for human illnesses caused by swine-origin viruses.

Clinical signs and transmission

In pigs, swine influenza commonly causes respiratory disease: fever, coughing, nasal discharge, lethargy and reduced appetite. In humans infected with a swine-origin virus, symptoms usually resemble seasonal influenza and may include fever, cough, sore throat, muscle aches, headache, and sometimes gastrointestinal symptoms such as vomiting. Severe outcomes—pneumonia, respiratory failure or death—are uncommon but possible, particularly in vulnerable people. Symptoms listed in other contexts such as fever, vomiting and altered consciousness (see loss of consciousness) are associated with severe or atypical presentations and warrant urgent medical evaluation.

Diagnosis, treatment and prevention

Diagnosis is made by laboratory testing: RT‑PCR to detect viral RNA, virus isolation, and serology to identify antibodies. Antiviral medications effective against influenza A, such as neuraminidase inhibitors, may be used for treatment or post‑exposure prophylaxis when clinically indicated; therapy is most beneficial when started early. Supportive care—hydration, rest and treatment of complications—remains central.

Control measures and risk reduction

Prevention combines veterinary and human measures. In swine herds this means vaccination of pigs where recommended, good farm biosecurity, and monitoring. For humans, seasonal influenza vaccination does not always protect against swine-origin strains, but immunization reduces overall influenza burden and may offer partial cross-protection. People with respiratory symptoms should avoid close contact with pigs, and workers in swine facilities should use personal protective equipment and follow occupational health guidance. Public-health agencies maintain surveillance and guidance; for general information on influenza see influenza overview.

Key distinctions and notable facts

• Host range: Swine influenza viruses primarily infect pigs but can infect humans and birds under certain circumstances.
• Reassortment risk: Co‑infection with human, avian and swine viruses can produce novel viruses.
• Most human cases: Occur after close contact with infected pigs, though secondary human-to-human transmission has occurred.

Continued surveillance, prompt diagnosis of unusual influenza cases, vaccination where appropriate, and basic hygiene and farm practices remain the most effective ways to limit the impact of swine influenza on animal and human health. For more resources see authoritative public-health and veterinary sources linked above and local health agencies for region‑specific guidance.

Influenza A viruses | Pigs and swine health | Influenza overview | Fever information | Vomiting guidance | Loss of consciousness