Avian influenza A (H5N1) virus infection

Clinical criteria

In the absence of a more likely alternative diagnosis or cause, an acute illness characterized by either:
- One or more of the following: cough, sore throat, fever (measured or subjective), shortness of breath or difficulty breathing, conjunctivitis (red eye, discharge from eye), OR
- Two or more of the following: headache, myalgia, arthralgia, fatigue, rhinorrhea or nasal congestion, diarrhea, vomiting.

Laboratory criteria

Confirmatory laboratory evidence
- Category 1 (novel influenza virus detection): positive confirmatory molecular test result (e.g., reverse transcriptase polymerase chain reaction [RT-PCR]) for novel influenza A subtype, OR genetic sequence indicative of novel influenza A virus.
- Category 2 (viable virus): isolation of a novel influenza virus from a clinical specimen.
- Category 3 (evidence of infection): significant immunoglobulin G (IgG) antibody rise to novel influenza A (i.e., at least a fourfold rise in a quantitative titer or seroconversion) in paired acute and convalescent serum IgG in the absence of another explanation (such as vaccination).
Presumptive laboratory evidence
- Category 1: presumptive positive for novel influenza virus on tests specifically designed to detect novel influenza viruses, such as H5 or H7.
- Category 2: virus testing results indicative of variant influenza virus, such as H1v or H3v, as determined in consultation with subject matter experts at CDC.

Epidemiologic linkage

Close contact with a confirmed human case of novel influenza A virus infection, OR
Shared a common exposure (such as an agricultural fair or live animal market) with a confirmed novel influenza A case, OR
Direct or indirect contact (such as touching an animal, their environment, or their raw or unprocessed animal products) with animals with confirmed influenza A, OR
Inadequate use or breach of personal protective equipment and exposed to novel influenza A virus in a laboratory.

Case classification

Suspect
- Meets clinical criteria AND epidemiologic linkage criteria AND laboratory testing results are positive for influenza A virus, but no laboratory evidence is available that would rule out novel influenza A.
Probable
- Meets confirmatory laboratory evidence category 1, OR
- Meets clinical criteria AND presumptive laboratory evidence category 1, OR
- Meets clinical criteria AND epidemiologic linkage criteria AND presumptive laboratory evidence category 2.
Confirmed
- Meets clinical criteria AND confirmatory laboratory evidence category 1, OR
- Meets confirmatory laboratory evidence category 2, OR
- Meets confirmatory laboratory evidence category 3.

Criteria to distinguish a new case from an existing case

A person should be enumerated as a new case of a novel influenza A virus infection if:
- The virus is distinguishable from the individual’s previous novel influenza A virus infection, OR
- The virus is indistinguishable from the individual’s previous novel influenza A virus infection, AND: the person has recovered fully or returned to baseline health, OR it has been >30 days since symptom onset date (if available) or first positive specimen collection date.

Suspected influenza A(H5) case definition

A person presenting with unexplained acute lower respiratory illness with fever >100.4ºF (>38ºC) or cough, shortness of breath, or difficulty breathing or conjunctivitis AND one or more of the following exposures in the 14 days prior to symptom onset:
- Close contact (within 3 feet [1 meter]) with a person (e.g., caring for, speaking with, or touching) who is a suspected, probable, or confirmed avian influenza A(H5) case
- Exposures in an area where avian influenza A(H5) virus infections in animals or humans have been suspected or confirmed, such as: close contact (within 3 feet [1 meter]) with live, sick, or dead infected animals or animal products, or consumption or handling of raw uncooked meat, unpasteurized milk, or other raw animal meat or products; direct exposure to surfaces that could be contaminated with infected animal products or with water contaminated with such products (e.g., wastewater from a live bird market or slaughtering facility); or visiting or working at a live animal market, farm, zoo, or other setting with infected animals
- Handling samples (animal or human) suspected of containing avian influenza A(H5) virus in a laboratory or other setting

Confirmed influenza A(H5) case definition

A person with a laboratory-confirmed infection (i.e., positive result from polymerase chain reaction [PCR], virus isolation, or serologic testing of paired acute and convalescent serum) with an avian influenza A(H5) virus. Serologic testing of a single convalescent serum specimen may be used to confirm infection when certain criteria are met (see full case definition).

Possible cases must fulfill the clinical AND exposure criteria below.

Clinical criteria

Fever >100.4ºF (>38ºC); or
Acute respiratory symptoms (e.g., cough, hoarseness, nasal discharge or congestion, shortness of breath, sore throat, wheezing, or sneezing); or
Other severe or life-threatening illness that is suggestive of an infectious process.

Exposure criteria

Close contact (within 3 feet [1 meter]) with live, dying, or dead domestic poultry or wild birds (including live bird markets), in an area of the world affected by avian influenza, or with any confirmed infected animal, in the 10 days before the onset of symptoms; or
Close contact with a confirmed human case of avian influenza, or human case(s) of severe unexplained respiratory illness from avian influenza-affected areas, or human case(s) of unexplained illness resulting in death from avian influenza-infected areas, in the 10 days before the onset of symptoms.
- Close contact includes handling laboratory specimens from cases without appropriate precautions, within 3 feet distance, directly providing care, touching a case, or within close vicinity of an aerosol-generating procedure, from 1 day prior to symptom onset and for duration of symptoms or positive virologic detection.

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Criteria