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Avian influenza A (H5N1) virus infection

Last reviewed: 25 Jun 2024
Last updated: 24 May 2024
24 May 2024

Human cases of bird flu associated with outbreak in dairy cattle in the US

Avian influenza A(H5N1) virus is currently causing a multistate outbreak in poultry and dairy cows in the US. While outbreaks have previously been reported in poultry, this is the first time the virus has been detected in cows. Cases of human infection linked to the outbreak in dairy cows have been reported, and these cases represent the first instances of likely mammal-to-human transmission globally.

As of 22 May 2024, over 9000 wild birds in 50 jurisdictions have tested positive for the virus, as well as over 90 million poultry across 48 states, and 52 herds of dairy cows across 9 states. Two cases of human infection associated with this outbreak have been reported.[22]

Situation summaries are available from the Centers for Disease Control and Prevention (CDC), and are being updated regularly:

In April 2024, one case of human infection was reported in a dairy worker in Texas.[35]

  • The person tested positive for highly pathogenic avian influenza (HPAI) A(H5N1) virus, and had a history of exposure to dairy cattle presumed to be infected with the virus.

  • The patient reported conjunctivitis (eye redness) as their only symptom, and was isolated and treated with antiviral therapy.

In May 2024, a second case of human infection was reported in a dairy worker in Michigan.[36]

  • Similar to the case in Texas, the person worked on a dairy farm where H5N1 virus had been identified in cows, and only reported eye symptoms.

  • While a nasal swab from the person tested negative, an eye swab tested positive for influenza A(H5) virus.

  • The worker was being monitored because of their work exposure to H5N1 virus-infected cattle, and reported their symptoms to local health officials.

Prior to this, the first human case of infection ever reported in the US was in 2022 in Colorado, and was associated with direct exposure to infected poultry during a culling process.

As part of the current outbreak, people with exposure to infected (or potentially infected) birds, cows, or other domestic or wild animals are being monitored and tested if they develop influenza-like symptoms. To date, no other people have tested positive. The CDC currently considers the public health risk to be low.

In light of the outbreak, the CDC has published new interim guidelines for prevention, monitoring, and public health investigations.[37]

Members of the public are advised to:

  • Avoid close, long, or unprotected exposures to sick or dead animals, including wild birds, poultry, other domesticated birds, and other wild or domesticated animals (including cows)

  • Avoid unprotected exposures to animal feces, bedding/litter, unpasteurized (raw) milk or cheeses, or materials that have been touched by, or closely exposed to, birds or other animals with suspected or confirmed H5N1 virus infection.

Clinicians are advised to consider the possibility of HPAI A(H5N1) virus infection in anyone showing signs or symptoms of acute respiratory illness, including conjunctivitis, who have a relevant exposure history.

  • People are advised to monitor themselves for new respiratory illness symptoms for 10 days after their last exposure if they meet relevant epidemiologic criteria. Antiviral chemoprophylaxis may be considered in certain people.

  • Symptomatic people are advised to seek prompt medical care for diagnosis and treatment, and isolate themselves away from others.

The CDC has also updated its antiviral treatment guidelines to recommend oseltamivir as the first-line treatment in both outpatients and hospitalized patients. The agency no longer recommends inhaled zanamivir or oral baloxavir for avian influenza A(H5N1) infection due to a lack of efficacy data in these patients.[38]

Sporadic human cases of HPAI A(H5N1) infection have been reported since 1997, with a case fatality rate of more than 50%. Most human infections have occurred after unprotected exposure to sick or dead infected poultry. There has been no evidence of sustained human-to-human transmission, and limited nonsustained human-to-human transmission has not been reported since 2007.

See Epidemiology

Original source of update



History and exam

Key diagnostic factors

  • cough
  • influenza-like illness
  • conjunctivitis
  • dyspnea
  • fever
  • rales, rhonchi
  • wheeze
  • decreased breath sounds
  • tachypnea
Full details

Other diagnostic factors

  • abdominal pain, vomiting, diarrhea
  • altered mental status
  • seizures
Full details

Risk factors

  • close contact with infected birds
  • recent travel to a county where HPAI-A(H5N1) virus has been detected in birds
  • environmental exposure to H5N1 virus
  • close contact with infected humans
  • laboratory work with H5N1 virus
  • close contact with other animals
Full details

Diagnostic tests

1st tests to order

  • CBC with differential
  • LFTs
  • chest x-ray
  • pulse oximetry
  • sputum Gram stain
  • sputum and blood bacterial culture
  • real-time reverse transcription polymerase chain reaction (rtRT-PCR)
Full details

Tests to consider

  • viral culture
Full details

Treatment algorithm


close contact of confirmed or probable case


suspected or probable or confirmed infection



Timothy M. Uyeki, MD, MPH, MPP

Chief Medical Officer

Influenza Division

National Center for Immunization and Respiratory Diseases

Centers for Disease Control and Prevention




TMU declares that he is an unpaid member and clinical Chair of the World Health Organization Guidelines Developmental Group: Clinical Management of Influenza Guidelines 2023-2024.


Dr Timothy M. Uyeki would like to gratefully acknowledge Dr Justin R. Ortiz, a previous contributor to this topic.


JRO declares that he has no competing interests.

Peer reviewers

Richard Martinello, MD


Departments of Internal Medicine and Pediatrics

Yale School of Medicine

New Haven



RM declares that he has no competing interests.

An De Sutter, MD, PhD

Associate Professor

Department of General Practice and Primary Health Care

Ghent University



ADS declares that she has no competing interests.

  • Differentials

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    • Community-acquired pneumonia
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  • Guidelines

    • Interim guidance on the use of antiviral medications for treatment of human infections with novel influenza A viruses associated with severe human disease
    • Avian influenza in humans (bird flu)
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  • Patient information



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