Avian influenza in mammals and dairy cattle
How H5N1 clade 2.3.4.4b spread from birds to mammals and US dairy cattle, what it means for milk, workers and pandemic risk, and the One Health response.
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For decades, highly pathogenic avian influenza (HPAI) H5N1 was mainly a poultry problem. It caused periodic outbreaks that led to the culling of millions of birds, and occasional, often severe human infections in people who handled sick poultry. Since about 2020, a new lineage, clade 2.3.4.4b, has behaved differently. It spread through wild birds to every continent except Australia, caused mass deaths in sea lions, seals and other wild mammals, and in March 2024 was confirmed in dairy cattle in the United States, a host never seen before for this virus.
The spread of H5N1 into mammals, and especially into a farmed species in close daily contact with people, is a textbook One Health problem: animal, human and environmental health are intertwined.
The virus
Influenza A viruses are named by two surface proteins:
- haemagglutinin (H), which binds to host cells;
- neuraminidase (N), which releases new virus particles.
Wild waterbirds, such as ducks, geese and gulls, are the natural reservoir of most subtypes. Highly pathogenic strains, mainly of the H5 and H7 subtypes, kill most infected chickens within days.
H5N1 first emerged as a highly pathogenic strain in China in 1996 (the "goose/Guangdong" lineage) and has evolved into many clades since. Clade 2.3.4.4b spread rapidly through migratory birds from 2020 and is now endemic in wild birds across Europe, Africa, Asia and the Americas.
Spread into mammals
Mammals have been infected mainly by eating sick or dead birds. Infections have been recorded in foxes, otters, bears, mink and ferrets on fur farms, and in cats. Clade 2.3.4.4b reached South America in late 2022, where tens of thousands of sea lions and elephant seals died along the coasts of Peru, Chile and Argentina in 2023. The pattern of deaths suggested mammal-to-mammal spread in some colonies.
Dairy cattle: a new host
In early 2024, dairy herds in Texas showed a puzzling drop in milk production, thick yellowish milk, and reduced appetite. In March 2024, tests confirmed H5N1. Genetic analysis showed:
- a single spillover from birds into cattle, probably in late 2023, of a genotype called B3.13, with at least one further independent spillover of another genotype (D1.1) detected in early 2025;
- spread between herds through cattle movement, shared equipment, people and vehicles;
- spread within herds mainly through milk and milking equipment. The virus replicates in the udder and reaches very high levels in raw milk.
By 2025, the virus had been confirmed in more than 1,000 herds in over a dozen US states. Most infected cows recovered, though with milk losses, and mortality was low. Cats on affected farms died after drinking raw milk. Infections were also found in poultry flocks near affected dairies.
Human cases
Dozens of human infections were reported in the USA in 2024–2025, most in dairy and poultry workers. Most were mild, often conjunctivitis from milk splashing into the eyes, though a severe case and at least one death, linked to backyard poultry, were reported. There has been no sustained human-to-human transmission.
Is milk safe?
- Pasteurised milk is safe. Studies of retail milk found fragments of viral genetic material, but no infectious virus, in pasteurised products. Standard pasteurisation (72 °C for 15 seconds, or equivalent) inactivates the virus.
- Raw milk from infected cows can contain very high levels of infectious virus, and has killed cats. Health authorities strongly advise against drinking raw milk.
- Milk from sick cows is diverted from the food supply, and in the USA there is national testing of milk from processing plants to track the virus.
Biosecurity on a dairy farm
A dairy farm near an affected area takes several steps. It tests bulk-tank milk regularly, and it requires pre-movement tests before buying or moving cattle. Milking clusters are cleaned and disinfected between cows. Milk from sick cows is discarded safely, and raw milk is never fed to calves or cats. Visitor and vehicle movement is limited. Workers wear eye protection, gloves and aprons in the milking parlour, and anyone with conjunctivitis or flu-like symptoms is tested. Wild birds are kept out of feed and water. These steps protect both the herd and the people who work with it.
Pandemic risk
The main fear is that H5N1 could acquire the ability to spread efficiently between people. Key changes would include receptor binding: shifting from bird-type receptors (α2,3-linked sialic acid) to human-type receptors (α2,6) in the upper airway. Every infection of a mammal gives the virus more chances to adapt, and co-infection with seasonal flu could allow reassortment of gene segments.
So far, the assessment of WHO and national agencies is that the risk to the general public remains low, while the risk to exposed workers is higher. Candidate vaccine viruses and stockpiles of human H5 vaccines exist, and antivirals such as oseltamivir remain effective against most strains.
The One Health response
- Surveillance across species: wild birds, poultry, cattle, pigs, cats and people, with genetic sequencing shared openly.
- Animal control: culling and movement restrictions for poultry (in India, culling within 1 km of an outbreak and surveillance within 10 km), herd quarantine and milk testing for cattle, and poultry vaccination, which some countries such as China, Egypt and France use under controlled programmes.
- Worker protection: protective equipment, testing, and in some countries vaccination of high-risk workers.
- Food safety: pasteurisation and diversion of milk from sick animals.
- Coordination: the Quadripartite of FAO, WHO, WOAH and UNEP, and national One Health missions, including India's National One Health Mission.
India has had repeated H5N1 outbreaks in poultry since 2006. It maintains an Action Plan for Preparedness, Control and Containment of Avian Influenza, and is strengthening surveillance in livestock and wildlife.
Key points
- H5N1 clade 2.3.4.4b spread worldwide through wild birds from 2020, killing poultry and many wild mammals.
- In 2024 it was confirmed in US dairy cattle (genotype B3.13, later D1.1), spreading mainly via milk and milking equipment. More than 1,000 herds were affected by 2025.
- Infected cows shed very high levels of virus in raw milk; pasteurisation makes milk safe.
- Human cases, mostly mild conjunctivitis in farm workers, have occurred without sustained human-to-human spread.
- Pandemic risk depends on adaptation to human receptors and reassortment; the public risk is currently assessed as low.
- The One Health response combines cross-species surveillance, animal control, worker protection, food safety and coordination.
What the exams ask
- Explain the naming of influenza A viruses and the meaning of highly pathogenic.
- Describe the spread of H5N1 into mammals and dairy cattle.
- Explain why pasteurised milk is safe and raw milk is risky.
- List biosecurity measures for poultry and dairy farms.
- Explain the One Health approach with avian influenza as an example.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Has H5N1 been found in Indian cattle?
As of the time of writing, H5N1 infection of dairy cattle has been reported in the USA, and authorities elsewhere, including India, have increased surveillance of milk and cattle. Outbreaks in Indian poultry occur periodically and are controlled by culling and zoning. Check current reports from the Department of Animal Husbandry and Dairying for the latest situation.
Why are cows getting bird flu now?
Clade 2.3.4.4b is widespread in wild birds and has evolved the ability to infect many mammals. Exposure of cattle to infected birds or contaminated feed and water, followed by efficient spread through milking, allowed it to establish in herds. Why cattle infection appeared only recently is still being studied.
Should I stop eating eggs and chicken?
No. Properly cooked poultry and eggs are safe, because cooking to normal temperatures kills influenza viruses. Infected flocks are culled and removed from the food chain. The main risk is to people who handle sick birds without protection.
Sources
- WHO, FAO and WOAH. Updated Joint FAO/WHO/WOAH Public Health Assessment of Recent Influenza A(H5) Virus Events in Animals and People. 2024–2025 editions.
- Caserta, L. C. et al. "Spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle." Nature 634, 669–676 (2024).
- Burrough, E. R. et al. "Highly pathogenic avian influenza A(H5N1) clade 2.3.4.4b virus infection in domestic dairy cattle and cats, United States, 2024." Emerging Infectious Diseases 30, 1335–1343 (2024).
- Uyeki, T. M. et al. "Highly pathogenic avian influenza A(H5N1) virus infection in a dairy farm worker." New England Journal of Medicine 390, 2028–2029 (2024).
- Department of Animal Husbandry and Dairying. Action Plan for Preparedness, Control and Containment of Avian Influenza. Government of India, revised 2021.
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