CDC Guidance for Swine Influenza Virus Infection

James C. Kile, DVM, MPH, DACVPM, CPH


May 09, 2019

Editorial Collaboration

Medscape &

Swine Influenza

Swine influenza, a respiratory disease of pigs, is caused by type A influenza viruses that commonly circulate in swine and cause periodic outbreaks. Although rare, human infections with swine flu viruses do occur and can be serious.

In the United States, most swine flu virus transmission takes place during agricultural fairs. Exhibitions of swine are common at county and state fairs, putting pigs from different farms into close contact with each other and with people. Less commonly, swine influenza virus infections have been detected among workers in the swine industry. Infected animals may or may not show signs of being infected.

Since 2010, more than 460 human infections with swine influenza viruses have been detected, caused by the H1N1, H3N2, and H1N2 swine flu virus subtypes. Human infections with swine flu viruses are considered "variant" virus infections. In some cases, multiple people have become sick after being exposed to infected swine or virus-contaminated environments. In rare cases, variant influenza virus infections have occurred following person-to-person transmission. Each human infection with swine influenza viruses should be fully investigated to ensure that these viruses are not being transmitted efficiently among humans. If infected animals are identified, they should be isolated to limit further human exposure.

It's important for healthcare professionals to be familiar with how to evaluate, test for, treat, and report human cases of swine influenza. The Centers for Disease Control and Prevention (CDC) provides Interim Guidance for Clinicians on Human Infections with Variant Influenza Viruses.

Signs and Symptoms of Swine Influenza

Signs and symptoms of swine influenza infection in humans are similar to those of uncomplicated seasonal influenza and include:

  • Fever

  • Cough

  • Pharyngitis

  • Rhinorrhea

  • Myalgia

  • Headache

  • Vomiting and diarrhea (in children)

Mild illness is similar in duration to that of uncomplicated seasonal influenza, (3-5 days) and may be managed on an outpatient basis, with close monitoring for clinical progression and development of complications. Hospitalization may be required for severe complications or clinical progression to severe illness.

People at increased risk for complications of seasonal influenza are likely to be at high risk for serious complications from swine influenza infection as well. These people include:

  • Children younger than age 5 years

  • Pregnant women

  • People aged 65 years and older

  • People who are immunosuppressed

  • People with underlying conditions (chronic pulmonary, cardiac, metabolic, hematologic, renal, hepatic, neurologic, or neurodevelopmental conditions)

  • People with extreme obesity

Diagnostic Testing for Swine Influenza

The clue to clinical diagnosis of swine influenza virus is whether the patient had a recent exposure to infected swine or to a virus-contaminated environment in the week before illness onset. Exposure can be defined as:

  • Direct contact with swine (eg, showing, raising, or feeding swine, or cleaning swine waste);

  • Indirect exposure to swine (eg, visiting a swine farm or walking through a swine barn), especially if swine were known to be ill; or

  • Close contact (within 2 meters/6 feet) with an ill person who had recent swine exposure or is known to be infected with a variant virus.

Respiratory samples should be taken to test any ill person who may have been exposed to swine influenza viruses according to these criteria. To make a diagnosis, clinicians should:

  • Take a nasopharyngeal or oropharyngeal swab (or a combined nasal and throat swab) or aspirate of the ill patient;

  • Place the swab or aspirate into viral transport medium; and

  • Contact the state or local health department to arrange transport, and request a timely diagnosis from a state public health lab.

Only CDC and state public health laboratories can confirm variant virus infections. Specimens taken for testing at a hospital or clinic should be split into two samples, or two specimens should be obtained so that one sample can be sent immediately to the health department for testing.

Clinicians should notify the local public health department of any suspected variant virus infections as soon as possible. The health department can arrange for appropriate testing of clinical specimens at the state public health lab.

Antiviral Drugs for Swine Influenza

Antiviral treatment recommendations for variant virus infection are based upon those for seasonal influenza.

Oral oseltamivir, inhaled zanamivir, intravenous peramivir, or oral baloxavir is recommended for the treatment of variant virus infections within 48 hours of symptom onset. Most variant viruses are resistant to the antiviral drugs amantadine and rimantadine; therefore, these agents should not be prescribed.

For persons suspected of having a variant influenza virus infection who are hospitalized, have severe or progressive illness, or are in a high-risk group, empiric antiviral treatment should be started as soon as possible, without waiting for the results of influenza testing.

For hospitalized patients and those with severe or complicated illness, treatment with oral or enterically administered oseltamivir is recommended. Inhaled zanamivir and oral baloxavir are not recommended because of the lack of data for use in patients with severe influenza disease. Data to support the efficacy of intravenous peramivir and oral baloxavir for hospitalized influenza patients are also insufficient.

Although early antiviral treatment (within 48 hours of illness onset) is generally most effective, antiviral treatment may still be effective in patients with moderate and severe illness when administered later.

Antiviral treatment with oral oseltamivir, inhaled zanamivir, intravenous peramivir, or oral baloxavir is recommended for outpatients with suspected influenza, including variant virus infection, if they are in a group considered to be at high risk for complications from influenza. Antiviral treatment also can be considered for previously healthy, symptomatic outpatients who are not at high risk. On the basis of clinical judgement, patients with confirmed or suspected variant virus infection can be treated with antiviral drugs if started within 48 hours of illness onset.

Antiviral chemoprophylaxis (before or after swine exposure) is not routinely recommended but can be considered in limited instances, according to the Infectious Diseases Society of America's Influenza Clinical Practice Guidelines, for persons who are at high risk for influenza complications. High-risk persons who become ill should seek medical care as soon as possible, and early antiviral treatment should be started if influenza, including variant virus infection, is suspected.

Preventing Swine Influenza Virus Infection in Humans

People who are at high risk for influenza complications (including young children) should avoid exposure to swine and to ill persons with swine exposure. If exposure to swine cannot be avoided, persons at high risk for influenza complications should consider wearing appropriate personal protective equipment. CDC has guidance for people who work with or raise swine.

Ill persons who are suspected of having or have confirmed variant virus infections and who do not require hospitalization should be isolated at home, away from other family members as much as possible. Household members who are at increased risk for influenza complications should avoid coming within 2 meters (about 6 feet) of ill persons.

Healthcare personnel who collect respiratory specimens from ill persons for influenza testing should follow standard and droplet precautions during patient care.

No vaccine specifically targeted against variant viruses is available at this time. Seasonal influenza vaccines do not provide protection against infection with variant viruses. A strong recommendation from a healthcare professional is a critical factor that affects whether a patient will get a seasonal influenza vaccine. Please see the CDC website for complete guidance on human infections with swine influenza viruses.