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Interim Guidance on Case Definitions for Investigations of Human Infection with Highly Pathogenic Avian Influenza A (H5N1) Virus in the United States

This document provides interim guidance for state and local health departments conducting investigations of human infections with highly pathogenic avian influenza A (H5N1) viruses. The following definitions are for the purpose of investigations of confirmed and probable cases, and cases of highly pathogenic avian influenza A (H5N1) virus infection under investigation. CDC is requesting notification of all confirmed and probable cases of highly pathogenic avian influenza A (H5N1) virus infection within 24 hours of identification. State health departments are encouraged to investigate all persons who meet testing recommendations for avian influenza A (H7N9) virus infection in the United States, to determine their case status.

Case Definitions

Confirmed Case: Highly pathogenic avian influenza A (H5N1) virus infection in a patient that is confirmed by CDC’s Influenza Laboratory or a CDC certified public health laboratory using methods agreed upon by CDC and CSTE. Confirmation of infection with avian influenza A (H5N1) viruses may be made by public health laboratories following CDC-approved protocols for detection of avian influenza A (H5N1) virus, or by laboratories using an FDA-authorized test specific for detection of avian influenza A (H5N1) virus.

Probable Case: Illness compatible with influenza in a patient meeting the exposure criteria below and for whom laboratory diagnostic testing is positive for influenza A, negative for H1, negative for H1pdm09, and negative for H3 by real-time reverse transcription polymerase chain reaction (RT-PCR) and therefore unsubtypable.

Case Under Investigation: Illness compatible with influenza in a patient meeting any of the exposure criteria below and for whom laboratory confirmation is not known or pending or for whom test results do not provide a sufficient level of detail to confirm avian influenza A (H5N1) virus infection.

Exposure Criteria

  • Patients with recent travel (within <10 days of illness onset) to areas where human cases of highly pathogenic avian influenza A (H5N1) virus infection have become infected or to areas where highly pathogenic avian influenza A (H5N1) viruses are known to be circulating in animals.1

OR

  • Patients who have had recent close contact (within <10 days of illness onset) with confirmed or suspected3 cases of human infection with avian influenza A (H5N1) 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 illness 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.2

OR

  • Unprotected exposure to live highly pathogenic avian influenza A (H5N1) virus in a laboratory.

1 Outbreaks of Highly Pathogenic Avian Influenza (subtype H5N1) in poultry notified to the OIE from the end of 2003 to 2 January 2014 [143 KB, 1 page] and Cumulative Number of Confirmed Human Cases for Avian Influenza A(H5N1) Reported to WHO, 2003-2013 [40 KB, 2 pages].

2 Limited non-sustained person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus has been reported in several countries only following close, prolonged unprotected physical contact with a severely ill H5N1 patient.

3 Patients suspected of having infection with a novel influenza A virus can include probable cases, cases under investigation for infection with highly pathogenic avian influenza A (H5N1) virus, and other patients for whom available clinical and epidemiologic information support a diagnosis of infection with highly pathogenic avian influenza A (H5N1) virus.

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