Human cases of bird flu associated with outbreak in poultry and 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 16 July 2024, over 9500 wild birds in 50 jurisdictions have tested positive for the virus, as well as over 99 million poultry across 48 states, and 157 herds of dairy cows across 13 states. Four cases of human infection associated with dairy cows in this outbreak have been reported.[22]
In April 2024, one case of human infection was reported in Texas in a dairy worker.[36]
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, two cases of human infection were reported in Michigan in dairy workers who worked on dairy farms where H5N1 virus had been identified in cows.[37][38]
Similar to the case in Texas, the second case only reported eye symptoms.
The third case reported more typical symptoms of acute respiratory illness associated with influenza virus infection (i.e., cough, eye discomfort with watery discharge).
In July 2024, a fourth case of human infection was reported in Colorado in a dairy worker who worked on a dairy farm where H5N1 virus had been identified in cows.[39]
The person reported eye symptoms only and recovered with treatment.
Prior to this outbreak, the first and only 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 ongoing outbreak, a further four cases of confirmed human infection associated with exposure to poultry were reported in Colorado in July 2024. All cases were in farm workers involved in the depopulation of poultry at a facility experiencing a H5N1 outbreak. The workers reported conjunctivitis and eye tearing, as well as more typical influenza symptoms.[40]
Updated situation summaries are available from the US Centers for Disease Control and Prevention (CDC), and are being updated regularly:
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. Between February 2022 and May 2024, at least 9500 people with potential exposure have been monitored in the US. The CDC currently considers the public health risk to be low. In the UK, the UK Health Security Agency (UKHSA) considers the risk to the public to be very low.[41]
In light of the outbreak, the CDC has published interim guidelines for prevention, monitoring, and public health investigations.[42]
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 faeces, bedding/litter, unpasteurised (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 who has a relevant exposure history.
People are advised to monitor themselves for new respiratory illness symptoms, including conjunctivitis, for 10 days after their last exposure if they meet relevant epidemiological criteria. Antiviral chemoprophylaxis may be considered in certain people to prevent infection.
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 hospitalised 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.[43]
Sporadic human cases of HPAI A(H5N1) infection have been reported since 1997, with a case fatality rate of more than 50% in reported cases. 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 non-sustained human-to-human transmission has not been reported since 2007.
Summary
Definition
History and exam
Key diagnostic factors
- presence of risk factors
- cough
- influenza-like illness
- conjunctivitis
- dyspnoea
- fever
- rales, rhonchi
- wheeze
- decreased breath sounds
- tachypnoea
Other diagnostic factors
- abdominal pain, vomiting, diarrhoea
- altered mental status
- seizures
Risk factors
- close contact with infected birds
- close contact with other animals
- 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
Diagnostic investigations
1st investigations to order
- FBC 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)
Investigations to consider
- viral culture
Treatment algorithm
close contact of confirmed or probable case
suspected or probable or confirmed infection
Contributors
Authors
Timothy M. Uyeki, MD, MPH, MPP
Chief Medical Officer
Influenza Division
National Center for Immunization and Respiratory Diseases
Centers for Disease Control and Prevention
Atlanta
GA
Disclosures
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.
Acknowledgements
Dr Timothy M. Uyeki would like to gratefully acknowledge Dr Justin R. Ortiz, a previous contributor to this topic.
Disclosures
JRO declares that he has no competing interests.
Peer reviewers
Richard Martinello, MD
Professor
Departments of Internal Medicine and Pediatrics
Yale School of Medicine
New Haven
CT
Disclosures
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
Belgium
Disclosures
ADS declares that she has no competing interests.
Differentials
- Coronavirus disease 2019 (COVID-19)
- Community-acquired pneumonia
- Atypical pneumonia
More DifferentialsGuidelines
- Highly pathogenic avian influenza A(H5N1) virus in animals: interim recommendations for prevention, monitoring, and public health investigations
- Interim guidance on the use of antiviral medications for treatment of human infections with novel influenza A viruses associated with severe human disease
More GuidelinesPatient information
Flu
Pneumonia
More Patient information
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