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\nInfluenza A viruses circulating in pigs that have infected humans are referred to as \u201cvariant\u201d viruses. Human infections with an influenza A (H3N2) variant (H3N2v) virus that contains the M gene from the influenza A(H1N1)pdm09 virus (2009 H1N1 pandemic virus) were first detected in 2011. Notably, a large increase in cases of H3N2v virus infection has been identified since July 2012. (This virus has been circulating among pigs in the U.S. since 2010, has been detected in pigs in many states, and appears to be circulating widely in swine in the U.S.) The majority of H3N2v cases have been in children, although some adults have been infected, and linked to recent direct or indirect exposure to pigs. Almost all of the H3N2v cases in 2012 were epidemiologically linked to agricultural fairs, either through exhibiting pigs or walking through a swine barn. Occupational exposure to pigs was reported in one H3N2v case. Limited, non-sustained human-to-human transmission of H3N2v virus was first reported in 2011, including in households and a child care setting. Some H3N2v case-patients have been hospitalized, including previously healthy people and persons with chronic underlying conditions, and one death has been reported. (MMWR, September 27, 2012: Influenza A (H3N2) Variant Virus-Related Hospitalizations \u2013 Ohio, 2012. MMWR 2012; 61(38);764-767.)
\nFor updated information on confirmed H3N2v cases, hospitalizations and deaths, see Case Count: Detected U.S. Human Infections with H3N2v by State since August 2011.
\nCDC is monitoring the situation closely with state health departments and will issue updates accordingly as additional information is available.
\nClinical characteristics of human cases of influenza A (H3N2) variant virus infection generally have been similar to signs and symptoms of uncomplicated seasonal influenza, including fever, cough, pharyngitis, rhinorrhea, myalgia, and headache. Vomiting and diarrhea have also been reported in some pediatric cases. Milder clinical illness is possible, including lack of fever. The duration of illness in most cases appears to be similar to uncomplicated seasonal influenza, approximately 3-5 days. While assumed to be similar to seasonal influenza virus infection, the duration of viral replication and possible infectiousness in H3N2v cases has not been studied.
\nExacerbation of underlying conditions (e.g. asthma) has occurred. The same people at increased risk for complications of seasonal influenza are likely at high risk for serious complications from H3N2v virus infection, including children aged <5 years, pregnant women, people 65 years and older, those who are immunosuppressed, and persons with chronic pulmonary, cardiac, metabolic, hematologic, renal, hepatic, neurological or neurodevelopmental conditions, as well as those with other co-morbidities, including morbid obesity.
\nLimited serologic data suggest that children born after 2001 (age ?9 years at 2010) lack immunity to H3N2v virus and therefore are likely to be most susceptible to H3N2v virus infection. Adolescents and younger adults may have cross-protective antibodies, but some would be expected to be susceptible. Middle-aged adults and elderly may have lower levels of cross-protective antibodies and may also be susceptible to H3N2v virus infection. Seasonal influenza vaccination does not provide protection against H3N2v virus infection.
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\nH3N2v virus infection cannot be distinguished by clinical features from seasonal influenza A or B virus infection, or from infection with other respiratory viruses that can cause influenza-like illness (fever and either cough or sore throat). Therefore, the key to suspecting H3N2v virus infection in an ill patient at this time is to elicit an epidemiological link to recent swine exposure in the week prior to illness onset:
\nA patient with influenza-like illness and an epidemiological link to recent swine exposure should be considered a probable H3N2v case.
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\nClinicians should notify the local public health department regarding any probable H3N2v virus cases as soon as possible.
\nAntigen detection tests, such as commercially available rapid influenza diagnostic tests (RIDTs) and immunofluorescence assays [e.g. direct fluorescent antibody staining (DFA)] are likely to detect H3N2v virus in respiratory specimens, although some RIDTs may NOT detect this virus (e.g. false negative results). False negative result can also occur with other influenza viruses. While some H3N2v cases have tested positive by RIDTs, other confirmed H3N2v cases have tested negative by RIDTs. In addition, DFA and RIDTs CANNOT specifically identify H3N2v virus infection, and a positive test result does not differentiate between seasonal influenza A virus infection or H3N2v virus infection. In summary:
\nA variety of commercially available molecular assays, including RT-PCR assays, that can detect influenza viruses are available. All of the available assays are likely to detect influenza A virus infection. However, commercially available molecular assays CANNOT differentiate H3N2v virus from seasonal influenza A (H3N2) virus, and the sensitivity and specificity of molecular assays to detect H3N2v virus are unknown. Some medical center laboratories may utilize non-commercially available molecular assays for influenza (\u201chome brews\u201d); the sensitivity and specificity of home brew molecular assays to detect H3N2v virus is unknown. Therefore:
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\nAll state health department laboratories and some other public health laboratories can perform the CDC FLU real-time reverse transcription polymerase chain reaction (rRT-PCR) Dx Panel influenza assay [CDC FLU rRT-PCR Dx Panel assay] on respiratory specimens. Positive results for influenza A, pandemic A, and H3 (Influenza A+, H1-, H3+, pandemic A+, pandemic H1-) by testing respiratory specimens with the CDC FLU rRT-PCR Dx Panel on respiratory specimens are considered presumptive positive for H3N2v virus.
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\nClinical management of H3N2v virus infection is similar to management of seasonal influenza A or B virus infections. Patients with uncomplicated H3N2v virus infection can be managed on an outpatient basis, with close monitoring for clinical progression and development of complications. Early neuraminidase inhibitor antiviral treatment is indicated for all hospitalized patients, severe and progressive illness, and for any high-risk patients with suspected or confirmed H3N2v. Management of mild to moderate complications such as non-severe exacerbation of underlying co-morbidities may be managed on an outpatient basis. However, hospitalization may be required for severe complications or clinical progression to severe illness. At hospital admission, antiviral treatment should be started as soon as possible for previously untreated patients. If secondary invasive bacterial infection is suspected, appropriate empiric antibiotic therapy should be started promptly. Additional care includes supportive care of potential complications (supplemental oxygen for hypoxia; mechanical ventilation for respiratory failure; vasopressors for shock; renal replacement therapy for renal failure, etc.).
\nH3N2v viruses tested to date are susceptible to the neuraminidase inhibitor drugs oseltamivir and zanamivir. These drugs can be prescribed to treat H3N2v infections. However, H3N2v viruses are resistant to the antiviral drugs amantadine and rimantadine; therefore, amandtadine and rimantadine should not be prescribed.
\nAntiviral treatment recommendations for H3N2v virus infection are based upon those for seasonal influenza (note age approved indications and dosing, and contraindications [433 KB, 22 pages]); see also Antiviral Drugs.
\nAntiviral chemoprophylaxis (before or after swine exposure) is not recommended, including for persons who are at higher risk for influenza complications. If such high-risk persons become ill, they should seek medical care as soon as possible and early antiviral treatment should be started if influenza, including H3N2v, is suspected.
\nIn most cases, variant flu viruses have not shown the ability to spread easily and sustainably from person to person. In general, even for influenza viruses that are transmitted easily from person to person, CDC does not recommend widespread or routine use of antiviral medications for chemoprophylaxis, so as to limit the possibilities that antiviral resistant viruses could emerge. Please see chemoprophylaxis recommendations for seasonal influenza at Influenza Antiviral Medications: Summary for Clinicians.
\nSystemic corticosteroids should not be routinely administered to patients with suspected or confirmed influenza, including H3N2v virus infection, except for patients on chronic corticosteroid therapy for other co-morbidities (COPD, asthma).
\nClinicians should remind parents that aspirin or aspirin-containing products should not be given to children with influenza-like illness, including suspect or confirmed cases of H3N2v virus infection, because of the risk of Reye\u2019s syndrome.
\nLimited, non-sustained human-to-human transmission of H3N2v viruses has been reported. While very limited data are available, it is assumed that H3N2v viruses may be transmitted from person-to-person similarly to seasonal influenza viruses. Therefore, in health care settings, infection control recommendations are the same as for seasonal influenza, including standard, droplet (surgical mask), and contact precautions. For aerosol-generating procedures, a fit-tested N95 respirator or equivalent should be used. See Prevention Strategies for Seasonal Influenza in Healthcare Settings and also Infection Control in Health Care Facilities.
\nHealth care personnel who collect respiratory specimens from ill persons for influenza testing should follow standard, contact, and droplet precautions, as recommended for patient care.
\nIll persons who are suspect or confirmed H3N2v cases 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 (or approximately 6 feet) of ill persons. For seasonal influenza guidance, see The Flu: Caring for Someone Sick at Home.
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\nNo specific H3N2v vaccine is available at this time. Immunization with seasonal influenza vaccine does not provide protection against infection with H3N2v virus.
\nSeasonal influenza viruses are expected to circulate during this fall and winter. Therefore, seasonal influenza vaccination is recommended for all persons aged 6 months and older to prevent seasonal influenza. For more information, see Recommendations of the Advisory Committee on Immunization Practices (ACIP).
\nPersons who are at higher risk for influenza complications, including young children, are not routinely recommended for antiviral chemoprophylaxis \u2013 they should avoid exposure to pigs and to ill persons with swine exposure. If exposure to pigs cannot be avoided, persons at higher risk for influenza complications should consider wearing appropriate personal protective equipment. Guidance for people who work with or raise pigs is available at Guidance Documents Related to Preventing the Spread of Influenza A Viruses. Other information related to H3N2v is available at Fact Sheet: Protect Yourself Against H3N2v.
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