Interim Guidance on Case Definitions for Investigations of Human Infection with Highly Pathogenic Avian Influenza A H5 Viruses in the United States
This guidance pertains to newly detected avian influenza (H5) viruses in the U.S. (H5N2, H5N8, and the new reassortant H5N1 virus).
This document provides interim guidance for state and local health departments conducting investigations of possible human infections with highly pathogenic avian influenza (HPAI) H5 viruses in the United States. These HPAI H5 viruses were first identified in wild birds and domestic poultry in December 2014 in the United States. Separate guidance describing case definitions for avian influenza H7N9 and the HPAI H5N1 virus that has caused severe human illness in Asia, Africa, and other parts of the world can be found elsewhere. The following definitions are for the purpose of investigations of confirmed cases, probable cases, and cases of HPAI H5 virus infection under investigation in the United States. CDC is requesting notification of all confirmed cases of HPAI H5 virus infection, including presumptive positive cases, within 24 hours of identification.
Confirmed Case: Highly pathogenic avian influenza (HPAI) A H5 virus infection in a person that is confirmed by CDC’s Influenza Division Laboratory. Presumptive positive identification of infection with HPAI A H5 viruses may be made by public health laboratories using the CDC Human Influenza Virus Real-Time RT-PCR Diagnostic Panel for detection of Asian-origin HPAI A/H5 viruses; however, specimens from presumptive positive cases should be sent to CDC for confirmatory testing.
Probable Case: Illness compatible with influenza1 in a patient meeting any of the exposure criteria below and for whom laboratory test results indicate influenza but do not provide a sufficient level of detail to confirm HPAI A H5 virus infection. Examples of such test results include: results that are presumptive positive for HPAI H5 virus (see Confirmed Case definition above), an influenza real-time reverse transcription polymerase chain reaction (RT-PCR) test that is positive for influenza A but cannot be subtyped (i.e., an influenza RT-PCR test that is positive for influenza A, negative for H1, negative for H1pdm09, and negative for H3); and a rapid influenza diagnostic test (RIDT2) that is positive for influenza A. Specimens from probable cases should be sent to CDC for confirmatory testing.
Case Under Investigation: Illness compatible with influenza1 in a patient meeting any of the exposure criteria below and for whom laboratory test results are not known or are pending.
- Patients who have had recent contact3 (within <10 days of illness onset) with birds potentially infected with HPAI H5 virus (i.e., sick or dead birds, or flocks where HPAI H5 virus infection has been confirmed).
- Patients who have had recent close contact (within <10 days of illness onset) with confirmed or suspected4 cases of human infection with HPAI H5 virus. Close contact may be regarded as coming within about 6 feet (2 meters) of a confirmed or suspected case while the case was ill (beginning 1 day prior to symptom onset and continuing until resolution of illness). This includes healthcare personnel providing care for a confirmed or suspected case, family members of a confirmed or suspected case, persons who lived with or stayed overnight with a confirmed or suspected case, and others who have had similar close physical contact in a community or workplace environment.
- Unprotected exposure to live HPAI A H5 virus in a laboratory.
1 Illness compatible with influenza may present as influenza-like illness (ILI) [fever ≥100°F plus cough or sore throat] or other signs and symptoms associated with influenza such as rhinorrhea, fatigue, myalgia, arthralgia, headache, and difficulty breathing. Note that influenza may not cause fever in all patients (especially in patients under 5 years of age, over 65 years of age, or patients with immune-suppression), and the absence of fever should not supersede clinical judgment when evaluating a patient for illness compatible with influenza. Atypical presentations of influenza may include nausea, vomiting, or diarrhea. While a rare sign of seasonal influenza, conjunctivitis has been reported as a sign of avian influenza virus infection.
2 Note that commercially available RIDTs cannot distinguish between influenza A virus subtypes (i.e., they do not differentiate between human and animal influenza A viruses); thus, a positive RIDT test result cannot confirm HPAI A/H5 virus infection. Commercially available RIDTs also may not detect HPAI A/H5 viruses in clinical specimens; therefore a negative RIDT result does not exclude infection with HPAI A/H5 virus.
3 Contact may include: direct contact with birds (e.g., handling, slaughtering, defeathering, butchering, culling, preparation for consumption); or direct contact with surfaces contaminated with feces or bird parts (carcasses, internal organs, etc.); or prolonged exposure to birds in a confined space.
4 Suspected cases of HPAI H5 virus infection include probable cases, cases under investigation, and other patients for whom available clinical and epidemiologic information support a diagnosis of infection with HPAI H5 virus.
- Page last reviewed: September 21, 2015
- Page last updated: April 19, 2016
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
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