Research and analysis

HAIRS risk assessment: influenza of avian origin in lactating livestock

Published 27 March 2025

This document was prepared by the UK Health Security Agency (UKHSA) on behalf of the joint Human Animal Infections and Risk Surveillance (HAIRS) group.

HAIRS is a multi-agency cross-government horizon scanning and risk assessment group, which acts as a forum to identify and discuss infections with potential for interspecies transfer (particularly zoonotic infections). Its work cuts across several organisations, including:

• UKHSA
• Department for Environment, Food and Rural Affairs (Defra)
• Department for Health and Social Care (DHSC)
• Animal and Plant Health Agency (APHA)
• Food Standards Agency (FSA)
• Food Standards Scotland (FSS)
• Public Health Wales (PHW)
• Public Health Scotland (PHS)
• Department of Agriculture, Environment and Rural Affairs for Northern Ireland (DAERA)
• Welsh Government
• Scottish Government
• Public Health Agency of Northern Ireland
• Department of Agriculture, Food and the Marine, Republic of Ireland
• Health Service Executive, Republic of Ireland
• Infrastructure, Housing and Environment, Government of Jersey
• Isle of Man Government
• States Veterinary Officer, Bailiwick of Guernsey

Information on the risk assessment processes used by the HAIRS group can be found at HAIRS risk assessment process.

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Date of this assessment: March 2025

Version: 1.0

Reason for the assessment: First detection of avian influenza A(H5N1) in a lactating ewe in the UK.

Completed by: Helen Roberts (Defra), Michael Reynolds (UKHSA), Christopher Williams (PHW), Andrew Frost (APHA), Lara Harrup (Defra) with review from HAIRS Members.

Non-HAIRS group experts consulted: Anissa Lakhani (UKHSA), Anthony Wilson (FSA), Richard Puleston (UKHSA), Rudolf Reichel (APHA), Raquel Jorquera (APHA), Ashley Banyard (APHA), Richard Hepple (APHA).

Information on the risk assessment processes used by the HAIRS group can be found online.

Executive summary

Overview

In this assessment, the risk associated with human exposure to all lactating livestock is being considered, not just dairy animals that are habitually kept for the production of milk for commercial reasons. This follows the finding of avian influenza (AI) virus in a lactating ewe in March 2025, which had been in close contact with captive birds at a backyard premises, which had previously been confirmed as being infected with highly pathogenic AI A(H5N1). On initial testing, the ewe was positive for serological activity for the AI A(H5) antigen but was negative for influenza viral RNA from nasal and rectal swabs. On retesting, the blood from this single ewe was again seropositive for the AI A(H5) antigen and its milk was also PCR positive. Further testing at postmortem gave negative PCR results for all organs and tissue samples, but again a PCR positive for milk was detected and seropositivity in blood detected for this single animal. The ewe was confirmed as the first livestock infection in the UK with AI A(H5N1) on the 21 March 2025. The risk of spread from this single premises is considered negligible, because affected animals were culled and there were no tracings to other livestock establishments or trade (domestic or international) of live animals or products.

When mammals are infected with an avian influenza virus (AIV) this is known as influenza of avian origin (IOAO). This assessment considers the risk lactating livestock infected with IOAO presents to the UK human population, assuming infection was a more regular occurrence in lactating livestock. The assessment considers exposure to infected animals, their contaminated environments and animal products.

Should further detections of IOAO in lactating livestock be identified in the UK, the most likely pathways of human exposure are occupational/husbandry and include backyard keepers, farm and dairy workers, abattoir workers, veterinary professionals, and laboratory exposures. Unprotected contact with infected animals and/or contaminated tissues and fluids from an infected animal present the greatest risk of exposure to humans, particularly to those who have regular contact. Previous human infections of AIVs have been reported in the UK, all of whom had close contact with infected birds. There is no evidence of AIV being transmitted from infected non-avian livestock to humans in the UK.

This assessment is a live document and will be updated as more information emerges.

Assessment of the risk of infection in the UK

The probability of infection with IOAO from lactating livestock is considered very low for the general population. For individuals interacting with infected lactating livestock the probability would be considered low.

The impact on the general UK population would be considered very low, while it would be considered low for higher risk groups (which may include immunocompromised, pregnant, young or elderly individuals)

Level of confidence in assessment of risk

Satisfactory.

Given human detections of IOAO are generally rare, there is a paucity of evidence on what risk factors may increase disease susceptibility, severity and poorer clinical outcomes in human cases. For those cases reported, clinical disease has ranged from mild to severe. There is possible under-ascertainment of cases, particularly in instances where mild disease manifests.

Further testing of the individual seropositive ewe at the premises showed that there was no evidence of a systemic infection in the positive animal, with no gross lesions in any organs, that the mammary tissue did not test positive by PCR but the detection of viral Ribonucleic Acid (vRNA) was limited to the milk in the udder. Environmental contamination of the samples was ruled out through milk samples testing positive for vRNA across 3 different assays and 2 different time points. The lack of any detection of AI viral material in the rest of this small backyard flock of sheep suggests this is an uncommon event associated with very close contact with infected birds or their contaminated environment. Highly pathogenic avian influenza (HPAI) A(H5N1) had been detected by both PCR and virus isolation in the captive birds present at the premises. There was no evidence of spread to other sheep; the lambs of the infected ewe were negative by accredited tests, although an unaccredited test gave a weakly positive serological reaction.

Current evidence gaps

Evidence gaps include:

• it is unclear how many other countries have implemented AIV surveillance in livestock, including sheep
• routes of infection into sheep other than directly via the teat and infectious dose level
• unknown susceptibility of sheep to AIV, or of sialic acid receptors in sheep udders: whether they are avian or mammalian like
• as there has only been one detection of AIV in a sheep in the UK, it is unclear whether the viral load observed in the sheep’s milk would be representative of all sheep
• how the health status of a sheep, including co-infection with other pathogens, influences susceptibility to AIV infection
• unknown duration of viral presence (or shedding) in milk samples from affected animals
• pasteurisation studies with sheep milk; persistence of the virus in raw cheese
• diagnostic tests available are not all validated for sheep
• unknown level of raw milk consumption on or from affected premises, including farmgate sales. The magnitude of possible human exposure is unknown
• there is no routine surveillance from bulk milk tank testing of any samples from cattle, sheep or goats for AIV. A cross-sectional survey was carried out over a period of 6 weeks from May to June 2024, testing 508 bulk milk samples from 455 dairy farms distributed across England, Scotland and Wales. All samples were negative to HPAI A(H5N1) real time polymerase chain reaction (RT-PCR) tests
• unknown if the presence of AIV in sheep milk (or the milk of other lactating livestock) would lead to visible changes such as mastitis, discoloured or clotted milk
• there are gaps in official reporting of sheep mastitis to the Animal and Plant Health Agency (APHA) as causes can be multifactorial and it is not a notifiable or reportable condition in livestock; mastitis alone is not an indicator of infection with AIV
• there is a lack of data relating to specific risk factors for increased susceptibility of humans to AIV infection, through different exposure pathways

Actions and/or recommendations

For animal health and veterinary professionals

The guidance is to:

• be aware of the case definition for mammals and requirements to report
• continue close monitoring and testing of animals where risk pathways for potential cross species infection are identified in non-avian species including lactating livestock, and the possible transmission pathways to humans
• provide guidance to people in contact with lactating animals and their offspring, particularly around lambing times and open farm attractions
• provide advice and guidance on appropriate biosecurity on premises where different species co-mingle

For public health professionals

The guidance is to:

• continue to raise awareness regarding consumption of raw dairy products including colostrum
• consider the rapid investigation and testing of farm workers with respiratory or compatible symptoms, particularly of conjunctivitis, on affected premises or where close contact between lactating animals and a contaminated environment is evident
• ensure timely sharing of data between animal and human health agencies to enable a coordinated, effective One Health approach to issues on farms where lactating livestock are present

Step 1. Assessment of the probability of infection in the UK human population

This section of the assessment examines the likelihood of an infectious threat causing infection in the UK human population. Where a new agent is identified there may be insufficient information to carry out a risk assessment and this should be clearly documented. Please read in conjunction with the probability algorithm found at Annexe A.

Is this a recognised human disease?

Outcome: Yes.

Quality of evidence: Satisfactory.

Avian influenza viruses (AIV) typically infect a large range of avian species, but in the current epizootic of avian influenza (AI) A(H5N1) clade 2.3.4.4b (Eurasian lineage), the ability to infect mammalian animal hosts and thus pose a potential zoonotic risk to humans has been observed with multiple species. Gradual changes in AIV genomes over time, through mutation or genome reassortment, have resulted in several AIV subtypes that are either circulating or newly emerging with the potential to trigger global health threats to mammals (1). Although AIVs rarely infect people, human cases have occurred following direct contact with infected animals, by inhaling droplets or dust containing the virus and/or contact with surfaces contaminated by an infected animal’s saliva or secretions (for example, milk) (2).

AIVs are classified into subtypes according to the combinations of different viral surface proteins hemagglutinin (H) and neuraminidase (N). They are further categorised into either high pathogenicity (HP) or low pathogenicity (LP), indicated by the severity of disease and mortality caused in chickens in a laboratory setting (3). Most AIVs cause no or mild illness, such as fever or conjunctivitis, in humans. However, subtypes including Asian lineage AI A(H5N1), A(H5N6) and A(H7N9) are known to have the potential to cause severe disease in humans, with mortality rates of up to 50%. While these subtypes do not easily infect people and have not acquired the ability to cause sustained transmission among humans (4), they are considered subtypes of public health concern.

The presence of conjunctivitis in many of the human cases reported in 2024 suggest infection through the conjunctiva and this is upheld by experimental evidence in ferret infections (5). There is currently no evidence of AIVs being transmitted to humans through the consumption of pasteurised dairy or cooked meat and eggs. In 2005, 2 human cases of AI A(H5) reported in Vietnam possibly became infected after consuming raw duck blood as part of a traditional dish, although other routes of exposure could not be ruled out (6). The risk of AIV infection via food is explored thoroughly in several recent Food Standards Agency (FSA) publications (see 7, 8, 9).

Between 2021 and March 2025, UKHSA reported 7 human detections of AIV; all in exposed persons on premises in England where AI A(H5N1) was also confirmed in birds on site (10, 11). Six detections were identified as part of an enhanced surveillance study of workers exposed to premises infected with AIV. It is likely that the detections in at least some of the asymptomatic individuals who tested positive reflected deposition of viral particles on the mucosal surface (in other words, not causing infection) rather than true infection with active viral replication. However, there was one infection in a symptomatic individual and several others where the status could not be determined.

Where severe cases of clinical disease have been associated with AIV infection in the USA, often there were co-morbidities or underlying health conditions, although, co-infection with other respiratory pathogens was frequently ruled out.

There is no evidence of sustained human-to-human transmission of AIVs and human cases are rare given the number of exposures which are likely to occur globally.

Is this a zoonosis or is there zoonotic potential?

Outcome: Yes.

Quality of evidence: Good.

Since 2003, and as of 20 January 2025, the World Health Organization (WHO) reported 964 human cases of HPAI A(H5N1), including 466 deaths, from 24 countries (10). Almost all human cases of AIV infection have been associated with contact with live or dead infected animals (predominantly poultry) or their contaminated environments, highlighting the zoonotic potential of AIVs (12). Based on current knowledge and understanding, AIVs do not easily infect or spread between humans.

In the context of AIV infected lactating livestock, the USA has reported detections of AI A(H5N1) clade 2.3.4.4b genotype B3.13 in dairy cattle since March 2024, and a further viral strain, genotype D1.1 since January 2025. By 19 March 2025, the US Department of Agriculture had confirmed 989 detections of AI A(H5N1) in dairy herds across 17 states. For up-to-date figures, including a map of affected states, (13). Between April 2024 and March 2025, 70 human cases of AI A(H5) virus infection, including one death, have been reported in the USA (14). Of these, 41 cases were exposed to sick dairy cattle, 24 were exposed to poultry farms or culling operations, and 2 were exposed to backyard flocks. The source of the exposure in 3 cases could not be determined (15).

There have been 3 other incidents of livestock species testing positive for AI A(H5N1) in the USA: in goats, alpacas and pigs. However, in the case of the pigs, consumption of infected birds was implicated as the source, and the animals did not develop systemic infection. For the alpacas and goats, in both cases, mammals were co-located with infected poultry or backyard birds and were exposed to a heavily contaminated environment (16 to 19). Clinical signs in the neonate goats led to the animals being tested, as it occurred soon after the poultry were culled. Ten goats aged 5-9 days old died. However, these signs could have been caused by co-infection, but included neurological signs. The follow-up tests at the USDAs National Veterinary Services Laboratory in Ames, Iowa identified the virus as AI A(H5N1) B3.6 genotype, the same strain as circulating in wild birds, in brain and tissue samples of 5 of the 10 goats.

In a separate incident, alpacas sharing pasture and water sources with poultry infected with AI A(H5N1) B3.13 genotype tested positive for the same virus, and exhibited weakness, depression, and mild respiratory signs.

Since the emergence of AIV infection in cattle in the USA, in Great Britain (GB) all mammals kept on poultry premises where HPAI has been confirmed are risk assessed and may be triaged for diagnostic testing of samples, if there are clear epidemiological links to the infected poultry.

In mid-February 2025, HPAI A(H5N1) was confirmed on a backyard mixed captive bird premises in Yorkshire, England. Birds included ducks, chickens, geese and turkeys, all kept for non-commercial purposes. They had been ‘housed’ as per the requirement of the Avian Influenza Prevention Zone (AIPZ) (20), but biosecurity on the premises was considered very poor. There were 26 sheep present, including 1 ram and some ewes that were heavily in lamb. A local risk assessment considered if the sheep were exposed over and above the background risk on the premises or area, to determine testing. All testing was undertaken at the UK’s AI national reference laboratory at the APHA Weybridge site. Ten out of 26 sheep were sampled on the 10 March 2025 – the others were heavily in lamb, so sampling them was not pursued for welfare reasons. One of the 10 sampled animals gave a UKAS accredited haemagglutination inhibition test (HAIT) (1/80) positive serological result and tested positive on 2 commercially available but unaccredited ELISAs targeting anti-NP and anti-H5 antibodies. The other 9 ewes tested negative on serology and all negative on PCR of rectal and nasal swabs.

The single sheep was resampled on 17 March 2025, again for nasal and rectal swabs, blood and this time milk (from both sides of the udder). Serology was 1/160 by HAIT and again positive by both ELISAs. Although swabs were negative, the pooled milk sample tested positive by PCR across all 3 frontline UKAS accredited PCR assays targeting the M gene, HPH5 and N1 with CTs being weakly positive. The animal was culled on 19 March 2025, and a postmortem undertaken. There were no unusual pathological signs; the udder on one side was slightly hard and reddened. Over 50 samples from organs, swabs, blood, and milk were taken. All organs were negative by PCR including mammary tissue. The swabs were negative. Serology was positive again with an HAIT titre of 1/80 and both ELISA assays also being positive as before. This is within the test variation. The milk was positive again on PCR with CT values in the 30s, with a weaker detection across all 3 assays. Additionally, the owner reported a history of mastitis on 3 March 2025 in one half of her udder. Swabs and bloods were taken from the ewe’s lambs, which were all negative on accredited tests, although one lamb had a weak positive reactivity on the unaccredited ELISAs.

Nineteen of the remaining sheep were sampled on 21 March 2025 (nasal and rectal swabs, as well as milk samples from 3 lactating ewes) and tested by RT-PCR for HPAI A(H5). Bloods were also taken for serology. All results were negative.

The current hypothesis is that the ewe became infected directly into her udder from contamination, possibly via the suckling lambs, resulting in a localised infection. Infected ducks had been in the stable with the sheep, with chickens in the adjoining area, but there was no separation from wild birds or vermin. The ewe would have lambed around the second week in February 2025, within the high-risk period for disease incursion onto the premises. The ewe reportedly developed mastitis around the 3 March 2025 and therefore it could be estimated that her milk may have low levels of virus for up to 3 weeks based on the PCR results. At the time of publication virus has not been isolated from the milk samples, but the fat and protein content of ewe’s milk may interfere with routine virus isolation techniques impairing their sensitivity. Sequencing of the weakly positive milk samples is underway although initial attempts using the frontline pipeline was unsuccessful. Research approaches are being undertaken.

A localised infection of the udder, with prolonged virus presence in an environment conducive to viral replication is more likely where the udder has receptors which bind avian-like viruses, as seen with the cattle udder. While it is not known yet whether the sheep udder has similar receptors, and if so in similar proportions to those in cattle, the higher CT values seen with the sheep compared to reported cattle infections may suggest that the udder receptors are not as efficient, possibly due to different glycosylation attributes of the sialic acid receptors. Under the Defra UKRI funded programme, FluTrailMap, the attributes of sheep udder receptors will be investigated, but initial evidence is they are similar to the attributes of the bovine udder receptors (Pers Comm, various).

All HPAI A(H5N1) viruses circulating in GB wild birds and poultry belong to the Eurasian 2.3.4.4b clade, and predominantly the DI strain, although there is a pocket of infection in wild birds with spill over to poultry and other captive birds with the 2.3.4.4b BB strain in Southwest England and HPAI A(H5N5) has been detected in wild birds and wild mammals in GB. The genomic constellation of the GB viruses is quite different to those seen in the USA poultry, cattle, and other mammal cases.

For zoonotic risk to increase from these detections in animals, both a route of exposure and genetic adaptation of the virus to enable efficient cellular entry and replication in human cells needs to occur. The E627K mutation in PB2 is considered a very early adaptive mutation but is required in tandem with several other mutations in PB2 for optimal adaptation to a mammalian host.

More importantly than changes to the polymerase though, to efficiently infect mammals, changes are needed in the HA gene that drives receptor binding and entry. The 3 main areas where changes are needed to drive efficient species adaptation are generally considered to be:

1. Adaptation of polymerase genes to allow efficient replication in human systems:

• includes adaptation to use host cell proteins (human ANPR32) requiring E627K, Q591K/R, D701N and M631L in PB2

2. Previous mammalian mass mortality events have seen some of these mutations including: E627K in human infection in US with conjunctivitis; M631L in US cattle; E627K in several Finnish mink farms; Q591K and D701N in South American marine mammals:

• alteration of surface glycoproteins to efficiently bind human receptors in the upper and lower respiratory tract (URT / LRT)
• avian viruses utilise α2,3 sialic acid residues for attachment and entry and these are abundant in avian tissues but not in the human URT
• α2,3-linked glycans are present in the human LRT and conjunctiva

3. Adaptation of haemagglutinin receptor genes to enable better fusion with cells and increased stability at more acid conditions is needed to enable fusion events at lower pH required for human infection:

• N224K, L226 and G228S with combinations of these mutations are required for efficient airborne transmission in ferrets

Therefore, for effective mammal-to-mammal respiratory transmission, AIVs would need to adapt to use the α2,6 sialic acid residues found in the mammalian URT, increased replication through polymerase adaptation and adapt to enable activation of membrane fusion at a lower pH. This would require multiple mutations across different genes.

Where AIVs circulate in animals, including lactating livestock, then sporadic human cases should not be unexpected in people with close, unprotected contact or high levels of exposure resulting in infection of the conjunctiva or the lower respiratory tract.

In most of the UK, unpasteurised milk can be sold to consumers for drinking, and is also key in making certain cheeses. The exception is Scotland, where the commercial sale of unpasteurised milk is not permitted. In addition to selling milk, a proportion of producers will consume raw milk from their bulk tank and/or an individual animals. Therefore, some members of the UK population do drink raw milk (and consume cheese made from it) so if infected this could be a tenable transmission route. If such milk appears mastitic (or in some other way unusual) it may not be sold for human consumption. However, this may then be fed to calves or other youngstock of various ruminant species, so could potentially multiply the number of infected ruminants on a specific farm – although whether this is a viable transmission route remains unclear.

No information is currently available on the level of infectious virus in milk from infected sheep. The single case identified had high CT values in the late twenties to low thirties, but the animal may have been infected for up to 2 weeks before being sampled. Virus could not be isolated from the milk but that is not necessarily because of low viral levels. Unpasteurised milk from infected US cattle showed a high level of viral RNA (21) even after dilution in bulk milk tanks (22), which would potentially indicate a high level of infectious virus if untreated. Even assuming (as a reasonably foreseeable worst-case scenario) similarly high levels to those in US cattle, and similar effectiveness of pasteurisation on viral infectivity, we assess that the probability of exposure to infectious levels of virus via pasteurised products is considered negligible (high uncertainty by the ACMSF scale (23)), but medium from dairy products made with unpasteurised milk (again with high uncertainty).

Postmortem testing of samples from the infected sheep suggests dissemination in the infected animal was minimal, which is consistent with the results obtained by testing muscle tissue from infected US dairy cattle displaying severe clinical signs (24). Although no information is available on the inactivation of influenza viruses in sheep meat, recent studies in beef and other cow meat products suggest heat is highly effective at reducing viral infectivity even at rare cooking temperatures (9, 25). We therefore assess that the probability of exposure via meat or offal from infected sheep will be very low when cooked rare or cooked thoroughly, and low if raw meat or offal is consumed, with high uncertainty (again via the ACMSF scale; 23).

Is the disease endemic in humans within the UK?

Outcome: No.

Quality of evidence: Good.

In January 2022, a laboratory confirmed human case of AI A(H5) (neuraminidase confirmation unknown) was reported for the first time in the UK. The case lived in the South West of England with a large number of captive birds (mostly waterfowl) that were allowed access to the dwelling house. The birds displayed onset of illness on 18 December 2021 and subsequently tested positive for HPAI A(H5N1). Genomic analysis of isolates from the infected birds demonstrated no strong correlates for specific increased affinity for humans. The human case remained clinically asymptomatic, and no subsequent human-to-human transmission was detected (26).

Between 2021 and March 2025, the UKHSA reported 7 human detections of AIV in exposed persons on premises in England where AIV was also confirmed in birds on site (27, 28). Six detections were identified as part of an enhanced surveillance study of workers exposed to premises infected with AIV.

It is likely that the detections in at least some of the asymptomatic individuals who tested positive reflected deposition of viral particles on the mucosal surface (in other words, not causing infection) rather than true infection with active viral replication. However, there was one symptomatic individual and several others where the carrier or infected status could not be determined.

Although there is limited seroprevalence data available on AI A(H5) subtypes and clades in humans, the WHO states that population immunity against AI A(H5N1) clade 2.3.4.4b virus haemagglutinin is expected to be minimal (29).

Is the disease endemic in animals within the UK?

Outcome: No.

Quality of evidence: Satisfactory.

During the AI epidemic in GB between 2020 and 2025, there have only been a small number of wild or kept mammals that have tested positive for AIV infection (confirmed findings of OOAO in kept mammals and in wild mammals (30, 31)). While ascertainment might be poor for some of the susceptible carnivore species which have relatively low population sizes and solitary behaviours, with animals such as red foxes, which regularly scavenge birds, there have not been reports of large numbers of dead animals. The expected neurological signs seen in some carnivore species would raise suspicion of other notifiable diseases such as rabies, but there have been no such reports. There has only been a single seal colony where the number of dead animals was still below the usual annual mortality rate, yet suspicion of AIV infection was raised due to the presence of dead sea birds. Out of nearly 200 dead seals, 40 were sampled and tested, with 15 testing positive for AI A(H5N5).

While AIVs in general are considered to have a natural host in wild waterfowl, for the highly pathogenic viruses it is not possible to say whether the pattern of small numbers of found dead positive wild birds is indicative of endemicity or not. However, the lack of evidence for infection in indigenous wild birds during the summer months is also indicative of the lack of endemicity. Whilst uncertainty exists due to a lack of evidence with regards to the number of birds exposed, those without clinical signs and case fatality rates, HPAI A(H5N1) is not currently considered endemic in GB.

For poultry, in GB there are more than 60,000 poultry premises with over 380 million birds. In addition, over 35 million gamebirds are initially kept and reared before being released each year. Yet there are only a very small percentage of premises reporting AIV infection each year (in the 2024 to 2025 season, just 45 poultry premises where HPAI A(H5N1) was confirmed in poultry or other captive birds and 1 premises where HPAI A(H5N5) was confirmed in poultry or other captive birds at the time of publication). For gamebirds, as they are wild birds once released, they may not be reported when they are found dead, but we also do not see high numbers of wild carnivores becoming infected due to predation, during the release season. Gamebirds are considered ‘indicator’ species for HPAI which means infection will lead to obvious clinical signs, and high morbidity and mortality.

In terms of the GB sheep population there are around 67,000 holdings with approximately 21 million sheep, of which 13.8 million are female breeding animals (32). Mastitis is only rarely reported to or diagnosed by the APHA (mastitis on its own is not a notifiable or reportable condition in livestock). In the last 2 years, just 73 reports compared to 215 from cattle (where the GB national herd consists of 1.8 million dairy and 1.3 million beef cattle) (33). However, mastitis is consistently the primary cause in sheep of premature culling, loss of udder function, and reduction in milk yield. In worst case scenarios, it can lead to death of the ewe. Industry guidance recommends that as any infection causing mastitis can be spread through hands and contaminated clothing, so this should be avoided by washing hands regularly and wearing gloves. When stripping ewes to check for mastitis, milk should be collected in a container, rather than onto bedding, to prevent contaminating the environment further. Teat trauma caused by lambs suckling hard if the ewe is not producing much milk can cause further ingress of bacteria or viruses, leading to mastitis (34, 61).

Are there routes of introduction into the UK?

Outcome: Yes.

Quality of evidence: Satisfactory.

In terms of incursion of HPAI into GB from one year to the next, it is generally driven by the migration of wild waterfowl to their wintering sites. The season will start in September and peak in December to January, then the numbers start to reduce as the birds leave for their summer breeding sites. In some years, infection may continue to circulate in colony breeding sea birds over the summer, around the coast of GB. The main migratory flyway into England and then to other parts of GB, the Atlantic Flyway, is used by waterfowl from Northwest Europe, and very rarely from Iceland and Greenland into Scotland. The strain involved in the single sheep case in England is one which is circulating in Northwest and Central Europe as well as in GB.

Other pathways for incursion, such as live animals and products of animal origin are less important because of trade rules which prevent the movement from disease restriction zones in Europe or other affected regions.

Mammals can then be exposed through one of several pathways: consumption of infected material, for example birds or eggs, including as raw pet food; consumption of contaminated products, which may include vermin or contaminated feed or water; aerosol transmission from infected birds and/or contact with secretions and excreta from infected birds.

What is of most interest for this assessment is how a mammalian udder may become infected, if not using a consumption or aerosol pathway. In this case, the theory is that somehow the udder has become infected directly, through introducing the virus via the teat, perhaps during suckling, or when the keeper is stripping the udder or stimulating the lamb to suckle. The fact that the affected ewe co-mingled with ducks in which HPAI A(H5N1) infection was confirmed may be relevant and several experimental studies have demonstrated that infected ducks contaminate their environment considerably more than infected chickens, although this may vary by genotype. The use of poultry litter in either ruminant feed or as ruminant bedding is not permitted in the UK, but where there is poor biosecurity then bird faeces may be present in a stable and if the animal is recumbent, there is a small possibility of transfer of contamination (62). The lack of evidence for any spread within the other sheep on the premises is also suggestive that this is a rare occurrence, resulting in a localised and limited infection.

For mammal-to-mammal transmission to occur, it will depend on the level of contact between the mammals, the susceptibility of the mammalian species involved to infection with AIVs, the effective population size for viral circulation as well as adaptation of the virus as indicated above.

Are effective control measures in place to mitigate against these routes of introduction?

Outcome: No.

Quality of evidence: Satisfactory.

There are no measures for preventing AIV infection in wild birds. There are control measures in place for domestic poultry outbreaks. For areas where an Avian Influenza Prevention Zone (AIPZ) is in force it means it is a legal requirement for all bird keepers (whether they have pet birds, commercial flocks or just a few birds in a backyard flock) to follow strict biosecurity measures to limit the spread of and eradicate the disease. This includes a requirement to keep their birds housed. Additional requirements apply in any disease control zones in force surrounding infected premises. Biosecurity guidance for bird keeper’s to help them comply with these requirements is provided by Government, in addition to key sector organisations. Imports of poultry would need to comply with freedom of notifiable avian disease and originate from a country or region approved and listed with an open date in accordance with Regulation No. 798/2008.

There are no specific testing requirements for AIV in imports of live mammals, although if sourced from a premises that has had HPAI then restrictions served on that premises may impact on their being imported.

For products of animal origin then a much wider range of permitted supplying countries and less clarity of what (if any) pertinent conditions might apply, although the requirements may reflect AIV presence for products of avian origin.

Do environmental conditions in the UK support the natural vectors of disease?

Outcome: Yes.

Quality of evidence: Good.

AIVs can readily be transmitted between wild and farmed birds in the UK, and there are no climatic limitations on the potential for it to spread to mammals if a high enough viral dose is given. This is most likely in an area with a heavy weight of infection and close continual contact as has recently been seen in infection of seals in north Norfolk (30), or previously identified in some wildlife rehabilitation centres where avian and mammal species were kept together.

Will there be human exposure?

Outcome: No: general population. Yes: for individuals interacting with infected lactating livestock and their products (for example, farm workers or veterinarians).

Quality of evidence: Satisfactory.

Should further detections of AIVs in lactating livestock be identified in the UK, the most likely pathways of human exposure are occupational or husbandry and include backyard keepers, farm and dairy workers, abattoir workers, veterinary professionals, and laboratory exposures. Previously, human infections of AIV have been predominantly attributed to close contact with infected poultry, wild birds, dairy cattle, or their contaminated environments. Unprotected contact with infected animals and/or contaminated tissues and fluids from an infected animal present the greatest risk of exposure to humans, particularly to those who have regular contact, such as farm workers or veterinarians. Farm workers involved in lambing to help with dystocia and getting lambs to take colostrum and suckle may be at increased risk of exposure. The risk of exposure may be mitigated if appropriate personnel protective equipment (PPE) and respiratory protective equipment (RPE) is worn (see 35, 36). Current evidence suggests that the likelihood of human infection and onwards spread is very low, and no sustained human-to-human transmission of AIVs has ever been reported. Pregnant women should not be in contact with animals giving birth, because of other more serious zoonotic disease risks (36).

The general UK population is unlikely to be in close, prolonged, unprotected contact with AIV infected lactating livestock. For premises with unknown infection, transient contact between an infected animal and a member of the public is possible, for example at visitor attractions or on open farms (with young children and adult females tending to be the most exposed (37)). Guidance does exist in the form of the Industry Code of Practice, aimed at ensuring visitor health and safety by providing sensible, practical, and proportionate guidance on preventing or controlling ill health at visitor attractions (38). However, this does not preclude bottle feeding lambs, kids or calves, a popular activity at such premises.

It is our opinion that the probability of exposure of UK consumers to a potentially infectious dose of influenza virus via milk from infected sheep will be negligible if pasteurised but medium from dairy products made with unpasteurised milk (high uncertainty), and that the probability of exposure via meat and offal from infected sheep will be very low if cooked rare or cooked thoroughly, and low if raw meat or offal is consumed (high uncertainty). Further detail is given in several recent FSA risk assessments (7, 8, 9).

Are humans highly susceptible?

Outcome: No.

Quality of evidence: Satisfactory.

Despite the high number of outbreaks and likely human exposures to AIVs at the human-animal environment interface, from 2020 to the end of 2024, only 102 AI A(H5N1) detections in humans were reported globally (10). A large proportion of the confirmed human cases were associated with exposure to infected dairy cattle as part of an ongoing outbreak in the USA, with splashing of milk into eyes documented as one exposure route, and research suggesting exposure could occur via contaminated milk and fomites (such as milking equipment (39 to 41)).

Most cases of AI A(H5N1) reported globally had no or mild symptoms. Fatal human cases of AI A(H5N1) have been reported from Cambodia (6 cases), China (1 case), India (1 case) the USA (1 case) and Vietnam (1 case) (10). Humans are likely to be susceptible through lack of immunity to AIVs, but the risk of initial transmission appears to be very low.

UKHSA works with APHA, Defra and the public health agencies of the 4 nations to investigate the risk to human health of AIV infection in the UK. Human exposures to AIVs are managed by local health protection teams (HPTs). Human cases of AIV remain rare compared to the number of exposures that are likely to occur. Between 2021 and March 2025, 7 confirmed human cases of AIV infection have been reported in the UK, all of which were asymptomatic or had mild symptoms (10, 11).

Transmission of AIV from lactating livestock to humans has never been reported in the UK, and there is a paucity of data relating to specific risk factors for increased susceptibility of humans to AIV infection.

Outcome of probability assessment

The probability of infection with influenza of avian origin from lactating livestock is considered very low for the general population. For individuals interacting with infected lactating livestock the probability would be considered low.

Step 2. Assessment of the impact on human health

The scale of harm caused by the infectious threat in terms of morbidity and mortality: this depends on spread, severity, availability of interventions and context. Please read in conjunction with the impact algorithm found at Annexe B.

Is there human-to-human spread of this pathogen?

Outcome: No.

Quality of evidence: Good.

Human cases of AIV have been sporadic, isolated incidents as a result of close, unprotected contact with infected animals and/or their contaminated environments (29). Although people have been infected with AIV, sustained human-to-human transmission has never been reported. However, the systematic testing of human AIV cases will vary with global region and it cannot be assumed that human-to-human transmission of a mild infection would be detected quickly. Where limited human-to-human transmission of AIVs has been reported, this has only involved a few individuals and was not sustained (42). However, because of the possibility that AIVs could mutate and gain the ability to transmit more readily between people, monitoring for human infection and onward transmission is essential for protecting public health.

Is there zoonotic or vector borne spread of this pathogen?

Outcome: Yes.

Quality of evidence: Satisfactory.

Human infections with AIV have been linked to unprotected close contact over a prolonged period with infected animals, including lactating livestock, or their contaminated environments. To date, there have been no reported transmission events of AIV from lactating livestock to humans in the UK.

For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Outcome: Yes.

Quality of evidence: Good.

As with the total cattle distributions, the highest densities of dairy population and holdings tend towards the west of GB (43).

Sheep holdings and the sheep population of GB are mainly distributed across Wales, northern England and eastern and southern Scotland, with additional areas of high population density in the midlands, south-west and south-east England. However, unlike dairy cattle, sheep may be moved considerable distances over a given 12-month period, so population densities in some regions will vary significantly over a year (32).

Goat holdings are diffusely spread across England and Wales with areas of greater goat holding density in the south-west, south-east and western parts of England and in south Wales. There are relatively few goat holdings in Scotland. There are a few areas of high goat population density but low holding density in the south-west and northern parts of England, which are likely to be due to a small number of large dairy goat units. Outside of these areas the goat density is less than one goat per km² (32).

The main significant species (in terms of milk for human consumption) of milk-producing mammals in the UK are cattle, goats and sheep. There may be a much lower number of other species that may be milked by their keepers, such as buffalo, American bison, llama, alpaca, horses, and other equids. If all species of mammals’ mammary tissues have the necessary receptors (as demonstrated in cattle in the USA in the last year) then milk from any mammal could be a risk if it were to be infected by AI, but human contact might be minimal and confined to such activities as getting new born offspring to start to suckle and investigating or treating issues such as mastitis identified in kept animals.

Is the UK human population susceptible?

Outcome: Yes.

Quality of evidence: Poor.

Human infections with AIV are usually the result of close, prolonged, and unprotected (for example, absence of PPE) contact with infected animals or their contaminated environments. Although people have been infected with AIV, subsequent human-to-human transmission is very rarely observed (42).

There are no data available to suggest that the general UK population would not be susceptible to AIV infection. However, human cases of AIV remain rare compared to the number of exposures that are likely to occur globally.

Does it cause severe disease in humans?

Outcome: Yes and no. Disease severity is dependent on AIV lineage.

Quality of evidence: Poor.

Disease severity in humans infected with AIV varies depending on lineage. For example, Asian lineage AI A(H5N1) can cause severe disease in humans, with mortality rates of up to 50%. While this lineage does not easily infect people and has not acquired the ability to cause sustained transmission among humans (4), it is considered a public health concern. It is unclear whether AI A(H5N1) viruses in certain geographical regions differ in their pathogenicity. Clade 2.1 viruses (Indonesia) appear more pathogenic than clade 1 viruses (Cambodia, Thailand, Vietnam) and 2.3 viruses (China) (44).

Eurasian lineages have been associated with milder disease in humans. Out of 6 Eurasian lineage AI A(H5N1) clade 2.3.4.4b human infections reported between January 2020 and December 2022, 4 cases presented with no or mild symptoms (1 case each in the UK and the US, and 2 cases in Spain) (45). The absence of symptoms in 2 poultry workers in Spain, together with the laboratory results which showed a very low viral load and the absence of specific H5 antibodies against the A/H5 virus, suggested that the positive results in the PCR were most likely due to environmental contamination, rather than legitimate infections (46). Of human detections of AI A(H5N1) in the US associated with the outbreak in dairy cattle, the majority of cases have reported mild symptoms such as conjunctivitis or mild respiratory symptoms, have not required hospitalisation and have fully recovered (29).

Severe disease has been reported in human cases of AI A(H5) internationally, including a fatality in a case in China and severe disease in a case in Vietnam who subsequently recovered. In January and March 2023, human cases of AI A(H5) were reported in Ecuador (47) and Chile (48) respectively. Both cases, neither of which had known comorbidities, required admission to intensive care units for medical treatment and mechanical ventilation. The latter case was subsequently confirmed as being infected with AI A(H5N1) clade 2.3.4.4b infection (49). In November 2024 the first domestically acquired human case of AI A(H5N1) clade 2.3.4.4b, genotype D.1.1 was reported in Canada, in an individual with asthma and an elevated body mass index, who experienced severe disease. No source of exposure was identified, however sequencing indicated that the virus was related to viruses from an ongoing outbreak in poultry in the same Province (50). While the majority of human cases of AI A(H5N1) in the US have experienced mild symptoms, in January 2025 a fatal human case of AI A(H5N1) was reported from Louisiana in an individual with co-morbidities and exposure to backyard and wild birds (51) This individual was infected with a genotype of AI A(H5N1) closely related to that detected in the Canadian case. More recently, a poultry culler in Ohio was tested positive for the D1.3 strain of AI A(H5N1) (52). He was hospitalized with both respiratory and non-respiratory symptoms. The patient had initially tested negative on upper-respiratory samples but was positive on lower respiratory tract sampling.

Estimation of the proportion of cases that are severe is difficult as the denominator of all infections is not known. Testing and case detection is likely to be biased towards more severe cases, and to those known to be exposed to affected poultry or mammals. Prior to the current epizootic, a review of serological evidence of AI A(H5N1) infections found an overall low prevalence (53), which was higher but still low in those exposed to human cases and infected birds (0.4 to 1.8%).

Monitoring of exposed workers provides an estimate of overall risk and severity. During the current USA incident, 5,126 workers across 29 dairy farms in California were monitored and 171 tested (based on symptoms) for AI A(H5N1) from relevant swab material. Of these, there were 37 cases confirmed by US CDC, comprising 21.6% of those tested and 0.7% of the exposed population. All 37 had mild illnesses, most commonly involving eye irritation or redness (97%), muscle aches (34%) and fever (29%) (54). In a UK study of poultry workers exposed to AI A(H5N1) infected flocks, of around 300 workers followed up, there were 6 detections of AI A(H5N1) from appropriate swabs, of which 2 (0.7%) were classed as confirmed cases (note that some participants were exposed more than once to affected flocks) (UKHSA internal communication).

Of 2 dairy farm workers in Michigan who had exposure to AI A(H5N1) infected dairy cows, one reported mild illness and conjunctivitis, tested positive for AI A(H5N1) clade 2.3.4.4b and developed neutralising antibodies. The other individual tested positive for AI A(H5) and did not develop neutralising antibodies, however this study shows that mild illness from AI A(H5N1) infection is able to generate an immune response in humans (55). In a study of 115 dairy workers in Michigan and Colorado from 2024 (56), 7% had serological evidence of AI A(H5N1) infection, with over half not reporting any illness, and those with symptoms had a mild illness. Further, in a study of seroprevalence of AI A(H5) among 150 bovine veterinary practitioners, evidence of recent infection was found in 3 individuals, including 2 without known exposure to infected animals (57).

Between 2021 and March 2025, 7 confirmed human cases of AIV infection have been reported in the UK, all of which were asymptomatic or had mild symptoms. Given human cases are sporadic and infrequent, there is a paucity of evidence on what risk factors may increase disease severity and poorer clinical outcomes in human cases with AIV infection.

Is it highly infectious to humans?

Outcome: No.

Quality of evidence: Satisfactory.

See above evidence.

Are effective interventions available?

Outcome: Yes.

Quality of evidence: Satisfactory.

There are no control measures preventing the migration of infected wild birds into the UK. Control measures to mitigate spillover events to poultry, other captive birds and lactating livestock are limited to the application of effective biosecurity on premises. National surveillance and control of AIV in poultry and other captive birds, and surveillance in wild birds is conducted annually in GB by the APHA on behalf of Defra, Welsh Government and Scottish Government and by DAERA in Northern Ireland. AIV surveillance in pigs co-located with infected kept avian species is routinely conducted. AIV surveillance in other mammalian species co-mingling with other species including poultry on AI affected poultry premises has been in place since 2024 in GB.

In the UK, there are well established pathways for the prompt diagnosis and treatment of a suspected or confirmed human case of AIV (58). Cases may be given antiviral medicine such as oseltamivir (Tamiflu) or zanamivir (Relenza), which help reduce the severity of disease, prevent complications, and improve the chances of survival (59). To date, there have been no documented incidents of AIV transmission from infected lactating livestock to humans in the UK, and so such pathways have never been applied in this scenario.

In December 2024, the UK government agreed a contract of more than 5 million doses of human avian influenza A(H5) vaccine as part of plans to boost the UK’s access to vaccine for a wide range of pathogens of pandemic potential (60).

Would a significant number of people be affected?

Outcome: No.

Quality of evidence: Satisfactory.

Between 2021 and March 2025, there have been 7 confirmed human cases of AIV infection reported in the UK. The cases had close contact with infected live birds. Although people have been infected with AI A(H5N1) clade 2.3.4.4b globally, this is rare and subsequent human-to-human transmission has not been observed.

Within the UK, well established and robust public health interventions are implemented in response to a human becoming symptomatic following contact with known AIV infection. This involves the isolation and treatment of a case, and extensive contact tracing designed at preventing further transmission.

Outcome of impact assessment

The impact on the general UK population would be considered very low, while it would be considered low for higher risk groups (which may include immunocompromised, pregnant, young or elderly individuals).

Annexe A. Assessment of the probability of infection in the UK population algorithm

Annexe B. Accessible text version of assessment of the probability of infection in the UK population algorithm

Outcomes are specified by a ☑ (tick) beside the appropriate answer. Where the evidence may be insufficient to give a definitive answer to a question, the alternative is also considered with the most likely outcome shown with ☑☑ (2 ticks) and/or the alternative outcomes with a ☑ (tick).

Question 1: Is this a recognised human disease?

Yes: go to question 3 ☑

No: go to question 2.

Question 2: Is this a zoonosis or is there a zoonotic potential?

Yes: go to question 4.

No: the probability of infection in the UK population is considered very low.

Question 3: Is this disease endemic in humans within the UK?

Yes: go to question 5. (Note: this pathway considers reverse zoonosis of a pathogen already in circulation in the human population.)

No: go to question 4 ☑

Question 4: Is this disease endemic in animals in the UK?

Yes: go to question 8.

No: go to question 5 ☑

Question 5: Are there routes of introduction into animals in the UK?

Yes: go to question 6 ☑

No: the probability of infection in the UK population is considered very low.

Question 6: Are effective measures in place to mitigate against these?

Yes: the probability of infection in the UK population is considered very low.

No: go to question 7 ☑

Question 7: Do environmental conditions in the UK support the natural vectors of disease?

Yes: go to question 8 ☑

No: the probability of infection in the UK population is considered very low.

Question 8: Will there be human exposure?

Yes, individuals interacting with infected lactating livestock: go to question 9 ☑

No: the probability of infection in the general UK population is considered very low ☑

Question 9: Are humans highly susceptible? (includes susceptibility to animal-derived variants)

Yes: go to question 10.

No: the probability of infection in the UK population is considered low ☑

Question 10: Is the disease highly infectious in humans?

Yes: the probability of infection in the UK population is considered high.

No. the probability of infection in the UK population is considered moderate.

Annexe C. Assessment of the impact on human health algorithm

Annexe D. Accessible text version of assessment of the impact on human health algorithm

Outcomes are specified by a☑ (tick) beside the appropriate answer.

Question 1: Is there human-to-human spread?

Yes: go to question 4.

No. go to question 2 ☑

Question 2: Is there zoonotic or vector-borne spread?

Yes: go to question 3 ☑

No: the impact of infection in the UK population is considered very low.

Question 3: For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Yes: go to question 4 ☑

No: the impact of infection in the UK population is considered very low.

Question 4: Is the human population susceptible?

Yes: go to question 5 ☑

No: the impact of infection in the UK population is considered very low.

Question 5: Does it cause severe disease in humans?

Yes, high risk groups: go to question 8 ☑

No: go to question 6 ☑

Question 6: Is it highly infectious to humans?

Yes: go to question 9.

No: Go to question 7 ☑

Question 7: Are effective interventions available?

Yes: the impact of infection on the UK population is considered very low ☑

No: the impact of infection in the UK population is considered low.

Question 8: Would a significant number of people be affected?

(This question has been added to differentiate between those infections causing severe disease in a handful of people and those causing severe disease in larger numbers of people. ‘Significant’ is not quantified in the algorithm but has been left open for discussion and definition within the context of the risk being assessed.)

Yes: go to question 10.

No: go to question 9 ☑

Question 9: Are effective interventions available?

Yes: the impact of infection in the UK population is considered low ☑

No: the impact of infection in the UK population is considered moderate.

Question 10: Is it highly infectious to humans?

Yes: go to question 12.

No: go to question 11.

Question 11: Are effective interventions available?

Yes: the impact of infection in the UK population is considered moderate.

No: the impact of infection in the UK population is considered high.

Question 12: Are effective interventions available?

Yes: the impact of infection in the UK population is considered high.

No: the impact of infection in the UK population is considered very high.

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