As physicians, we try to keep an eye on infectious diseases that might pose broader risks. H5N1, also called highly pathogenic avian influenza or bird flu, remains one of those, and the virus mainly infects birds but has crossed into humans with severe outcomes. The first human cases were seen in Hong Kong in 1997. Since then, human infections have been sporadic, usually tied to direct contact with infected poultry or contaminated environments, and unlike seasonal influenza, it doesn’t spread efficiently from person to person.

WHO’s cumulative count puts the case fatality rate at about 52 percent, 463 deaths among 888 confirmed cases worldwide since 2003, in data current as of March 2024. Human cases identified in the United States during the 2024 dairy and poultry outbreak ran far milder than that historical average, and many were mild. But the potential for severe disease is still there.

Recent Human Cases

In the United States, poultry wasn’t the only exposure route. In March 2024, transmission from cows to humans was confirmed, and cats were infected too, after drinking raw milk from infected cows. Between March 28 and October 31, 2024, there were 45 human cases in six states, all in adults, 25 linked to infected cows and 20 to poultry. By January 2025, there had been about 90 reported human cases nationwide.

A common thread was inconsistent use of protective equipment, and infection rates ran higher when gloves, masks, and goggles weren’t used consistently. Among those infected during the March-October window, 71 percent reported wearing gloves, 60 percent eye protection, 47 percent face masks, and only 36 percent reported both eye protection and masks together. PPE seemed to make a real difference, but adherence was incomplete.

Symptoms

The most common presentation was conjunctivitis. About 93 percent of patients developed viral pink eye, and fever was reported in about half, with headaches and muscle aches close behind. Sore throat, cough, and fatigue were less frequent. Diarrhea and nausea were rare. The median duration of symptoms was four days. Severe disease can still progress quickly to respiratory failure, multi-organ involvement, and death, though fatality figures reflect real biases in surveillance and reporting, since mild and subclinical infections are the ones most likely to go undetected.

Diagnosing H5N1

Diagnosis depends on history and testing. Exposure to poultry, cows, or contaminated settings should raise suspicion in anyone with severe flu-like illness. Nasopharyngeal or conjunctival swabs are typically used. PCR testing is the gold standard. Culture is rarely done. Biosafety concerns. Serology is sometimes used for retrospective surveillance.

Monitoring and Management

Close surveillance is essential for people exposed to infected birds or animals, with daily checks for fever and respiratory symptoms during the incubation period, about 10 days, recommended. Public health reporting remains critical for containment.

Treatment relies on antivirals like oseltamivir or baloxavir if started early, and severe cases often require oxygen therapy or mechanical ventilation, with supportive care for complications like ARDS central to management.

Human-to-Human Spread

What keeps H5N1 from being a global pandemic threat is the lack of efficient human-to-human spread. There have been family clusters and caregiver cases, mostly in the mid-2000s, where limited transmission was suspected, including a mother in Thailand caring for her sick daughter and family members in Indonesia in 2006, cases that involved close, unprotected exposure where the virus didn’t transmit beyond those immediate contacts.

H5N1 lacked the genetic adaptations that would make it easily transmissible like seasonal flu or COVID-19, and the concern was that mutations or reassortment with human influenza strains could change that, which is why surveillance continued.

Where the Risk Stands

As of early 2025, the risk to the general public remained low, with most cases stemming from direct animal exposure and person-to-person spread limited. For clinicians, though, awareness matters: we need to consider H5N1 in patients with severe respiratory illness who also have a relevant exposure history. For patients, the focus should be on minimizing contact with infected poultry, avoiding raw milk, and following public health guidance on protective measures.

H5N1 was still circulating in birds, and occasionally in other animals, and it occasionally infected people, though it was not, at that point, spreading widely between humans. The threat was real but contained. The critical piece remains vigilance: watching for changes that would signal the virus adapting toward easier human spread.

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Scott Rennie, D.O.

Sources

Garg S, Reinhart K, Couture A, et al. Highly Pathogenic Avian Influenza A(H5N1) Virus Infections in Humans. N Engl J Med. 2025;392(9):843-854. PMID 39740051.

Centers for Disease Control and Prevention. Avian Influenza A (H5N1) Virus. https://www.cdc.gov/bird-flu/situation-summary/index.html

World Health Organization. Avian Influenza Weekly Update. https://www.who.int/emergencies/disease-outbreak-news

Uyeki TM, Peiris M. Novel Avian Influenza A Virus Infections of Humans. Infect Dis Clin North Am. 2019;33(4):907-932. PMID 31668198.

Board Certified in Obesity Medicine and Family Medicine

This blog is for educational purposes only and does not constitute individual medical advice. Always consult your own physician before making changes to your health, medications, or treatment plan.