Swine Influenza

Although enzootic in pigs, zoonotic viral transmission of influenza A viruses from animals to humans (pig-to-human spread) is rare. Infected swine shed the virus in their nasal secretions and can transmit viruses to humans in the same way seasonal influenza viruses spread among people. Once infected, humans can transmit infection to each other (person-to-person transmission), but such transmission is rare. Even more rarely (given the disease in enzootic in pigs), infected humans can infect uninfected swine (human-to-pig transmission). Swine influenza has not been shown to be transmissible to people through consumption or handling of properly prepared pork.

In 1918, swine influenza was first clinically observed in pigs independently in the United States, China, and Hungary (1). It was first formally isolated in 1930 (2). Since then, sporadic porcine outbreaks have occurred throughout the 20th and beginning of the 21st centuries. The disease became enzootic in pigs in the United States in 1998. As of 2025, the virus is enzootic in many pig populations worldwide and is highly prevalent in the Americas, Europe, and Asia; some cases have also been reported in Africa.

An H1N1 flu pandemic occurred in 2009–2010; the variant subsequently spread to > 70 countries and throughout the United States (3). Most of the deaths occurred in Mexico. The variant was initially referred to as a swine influenza virus, but it was a combination of swine, avian, and human influenza viruses. The infection is not acquired through ingestion of pork, and is acquired very rarely through contact with infected pigs. Subsequently, the virus name was standardized to influenza A(H1N1)pdm09 to denote the pandemic and to distinguish that variant from seasonal H1N1 strains and from the 1918 pandemic H1N1 strain. Since 2009, influenza A(H1N1)pdm09 has circulated as a seasonal influenza strain.

Because variant influenza A viruses warrant national notification, the United States Centers for Disease Control and Prevention (CDC) has established national surveillance of confirmed novel influenza A virus infections. Fifty-six cases of novel influenza viral infection were reported to the CDC in 2024–2025. The World Health Organization has established the Global Influenza Surveillance and Response System to provide global and regional surveillance for influenza A.

Comprehensive epidemiologic investigation of each identified human infection is warranted to (4):

• Identify the source of exposure and transmission.

• Implement appropriate control measures to prevent further zoonotic and anthroponotic (human-to-animal) transmission.

• Evaluate the potential for ongoing person-to-person spread.

The incubation period for variant influenza A virus infection is typically 1 to 4 days after exposure; however, the period can extend to 7 days (1). Illness duration generally parallels that of uncomplicated seasonal influenza, ranging from 3 to 5 days (2).

The clinical presentation of variant influenza A virus infection in humans typically resembles uncomplicated seasonal influenza, characterized by fever, cough, pharyngitis, rhinorrhea, myalgia, and headache.

Gastrointestinal manifestations, including vomiting and diarrhea, have been documented in children. Milder presentations may occur, including afebrile illness, and conjunctivitis has been observed.

Although presumed to be comparable to seasonal influenza A virus infection, the kinetics of viral replication and the infectious period for variant influenza A virus infection remain inadequately characterized.

Similar to seasonal influenza, swine influenza is associated with a wide range of complications (3).

• History and physical examination

• Rapid detection

• Molecular methods (eg, reverse transcriptase–polymerase chain reaction [RT-PCR] testing)

• Sometimes genomic sequencing

The diagnosis of swine influenza is first clinically suspected in patients who have appropriate clinical manifestations; symptoms are often indistinguishable from nonvariant seasonal influenza. Patients often have a history of relevant epidemiologic exposure. Relevant exposure includes direct (within 2 meters [6 feet]) or indirect (eg, with swine environments, contaminated surfaces, agricultural fair attendance) contact with potentially infected swine or humans in the week before illness onset (1).

For most patients with suspected infection, nasopharyngeal swabs, nasal aspirates, or a combined nasopharyngeal and oropharyngeal swab should be obtained for testing. If these specimens cannot be collected, a nasal swab or oropharyngeal swab is considered an acceptable alternative. In patients who are mechanically ventilated, endotracheal aspirates or bronchoalveolar lavage fluid may also be obtained. Collection personnel should follow enhanced infection control measures when obtaining specimens from patients with suspected infection. In the United States, only the Centers for Disease Control and Prevention (CDC) and state public health laboratories can confirm variant influenza A virus infections. Specimens should be placed in viral transport media and the state or local health department should be immediately contacted to arrange transport and testing at a public health laboratory. 

Although rapid (typically antigen-based) diagnostic tests are commercially available, they cannot distinguish between seasonal influenza A, variant influenza A, or other novel influenza A viruses and may be falsely negative or yield a nontypeable result. Commercially available molecular (nucleic acid-based) influenza assays have higher sensitivity for influenza A viruses than rapid antigen-based detection tests (2).

The confirmatory test of choice for variant influenza A infection is a specialized RT-PCR test, which is done only at specific public health laboratories. In the United States, clinicians involved in the care of patients with suspected variant influenza should contact their state health department to arrange for transport of specimens to the laboratory for novel influenza A virus testing. Outside the United States, clinicians should contact their local health department or the World Health Organization (WHO).

Other available diagnostic tests are similar to those used to diagnose seasonal influenza and include antigen-based rapid influenza diagnostic tests, multiplex assays that detect both influenza and SARS-CoV-2, immunofluorescence assays, and viral culture. However, these tests are often limited by logistic considerations (ie, time-consuming, varying sensitivities, lack of subtype specificity).

• Supportive care

• Antiviral medications

The treatment of swine influenza generally follows the same principles as for seasonal influenza (1). For most patients, supportive care is sufficient. Measures include timely administration of supplemental oxygen for hypoxia or mechanical ventilation for respiratory failure; vasopressors for shock; and renal replacement therapy for acute kidney injury. Aspirin should be avoided in children because of the risk of Reye syndrome.

Antiviral treatment is recommended for patients who are high risk, hospitalized, and/or have severe or progressive disease. Antiviral treatment should not be delayed while waiting for confirmatory laboratory testing results from specialized laboratories.

Most novel influenza A viruses, including variant swine influenza viruses, remain susceptible to neuraminidase inhibitors (eg, oseltamivir, zanamivir, peramivir) and endonuclease inhibitors (eg, baloxavir marboxil). Similar to other influenza A viruses, variant influenza viruses are resistant to the antiviral medications amantadine and rimantadine; these should not be prescribed. If secondary invasive bacterial infection is suspected, empiric antibiotic therapy should be initiated promptly. 

For more detailed information, see Treatment of Influenza.

People at high risk of severe swine influenza complications (including animal caretakers) should avoid pigs and swine barns (particularly sick pigs and/or during an outbreak) if feasible (1). If exposure cannot be avoided, people at high-risk should wear a well-fitting mask that covers their nose and mouth (N95 or KN95 respirator preferred; surgical mask if respirators unavailable) and wash their hands with soap and water before and after pig or barn exposure (or use alcohol-based hand sanitizer if soap is unavailable).

Unlike seasonal influenza, antiviral chemoprophylaxis (either before or after swine exposure) is not routinely recommended; however, it may be considered for patients who are at high risk of complications.

There are no specific human variant influenza A vaccines available. Seasonal influenza vaccines are not designed to confer protection against swine influenza. However, seasonal influenza vaccination may reduce the spread of human seasonal influenza and lower the risk of co-infection with both human and swine influenza A viruses in people or pigs (2).

Specific swine influenza vaccines are licensed for use in pigs. For more information on porcine vaccination, see Treatment and Control of Influenza in Pigs. See also CDC: What People Who Raise Pigs Need To Know About Influenza (Flu) and CDC: Risk and CDC Interim Guidance for Workers Who are Employed at Commercial Swine Farms: Preventing the Spread of Influenza A Viruses.