Summary
Recently, cases of Highly Pathogenic Avian Influenza A(H5N1) virus transmission from dairy cows and poultry to humans have been reported in the United States. Between March and October 2024, 46 human infections were identified, mostly presenting with mild symptoms such as conjunctivitis, fever, and mild respiratory issues. Contact with infected animals was the primary source of infection, and inadequate use of personal protective equipment (PPE) increased the risk of exposure. While no evidence of human-to-human transmission has been observed so far, continued monitoring of the virus’s evolution and enhanced protection measures for farm workers are necessary.
1. What is A(H5N1) Virus?
A(H5N1) is a highly pathogenic avian influenza (HPAI) virus that can infect poultry, mammals, and humans. First reported in Hong Kong in 1997, it has since caused over 900 human infections globally with a fatality rate of approximately 50%【5】. In 2024, the virus has been increasingly detected in dairy cows and poultry in the United States, leading to human infections【5】.
2. Analysis of 2024 U.S. Human Infections
(1) Infection Pathways
According to the U.S. Centers for Disease Control and Prevention (CDC), 46 human infections were reported between March and October 2024. The primary exposure routes were direct contact with infected animals【5】.
- Dairy cow exposure (25 cases): Handling infected cows and raw milk
- Poultry exposure (20 cases): Contact with infected birds during depopulation and processing
- Unknown source (1 case): The individual was diagnosed through routine surveillance with no clear exposure history
(2) Common Symptoms
Among the 46 infected individuals, 93% developed conjunctivitis (eye redness and inflammation), while 49% had fever and 36% experienced respiratory symptoms【5】.
- Conjunctivitis: 42 cases (93%)
- Fever: 22 cases (49%)
- Cough & sore throat: 16 cases (36%)
- Muscle aches & fatigue: 10 cases (22%)
- Diarrhea & vomiting: 2 cases (4%)
Most symptoms resolved within 4 days, and no hospitalizations were reported【5】.
(3) Treatment & Prevention Measures
- 87% of infected individuals received antiviral treatment (Oseltamivir/Tamiflu), with an average treatment duration of 5 days【5】.
- A total of 97 household contacts were monitored, but no secondary infections were identified【5】.
3. Case Studies & Key Takeaways
Case 1: Outbreak at a Dairy Farm
In September 2024, an outbreak of A(H5N1) infections at a dairy farm in California led to four workers contracting the virus. All experienced conjunctivitis and fever, and the primary cause was inadequate PPE use. Following the outbreak, the farm implemented mandatory use of face masks and goggles【5】.
Case 2: Poultry Processing Workers Infected
In Colorado, nine poultry workers contracted A(H5N1) while handling infected birds. These individuals did not use adequate eye protection or face masks while participating in depopulation activities. The incident highlights the importance of PPE and hand hygiene in preventing transmission【5】.
4. Practical Prevention Tips
(1) For Farm Workers
✅ Minimize direct contact with infected animals
✅ Wear PPE: masks, goggles, gloves, waterproof clothing
✅ Maintain proper hand hygiene
✅ Boil milk and cook poultry properly before consumption
(2) For the General Public
✅ Avoid consuming raw milk, raw eggs, or undercooked poultry
✅ Wash hands after visiting poultry farms or live markets
✅ Seek medical attention immediately if experiencing fever, cough, or conjunctivitis
5. Conclusion: How Should We Prepare for A(H5N1)?
Although human-to-human transmission has not been observed, farm workers and individuals exposed to infected animals face a higher risk of infection. Proper hygiene, PPE use, and cooking practices are essential in reducing the risk. Continued surveillance of A(H5N1) mutations and rapid public health response will be necessary to prevent further outbreaks【5】.
References
【5】 New England Journal of Medicine (NEJM), February 27, 2025, “Highly Pathogenic Avian Influenza A(H5N1) Virus Infections in Humans,” DOI: 10.1056/NEJMoa2414610.
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