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Avian influenza: A threat to human and animal health

  • Ecommercy
  • 2026-09-22
  • 15:57
  • 0 comments

Introduction

Avian influenza virus is an important emerging zoonotic virus with pandemic potential. It has caused significant economic losses in the poultry industry and disruptions to food security. The highly pathogenic avian influenza A/H5N1 virus has attracted particular scientific attention.

It was first isolated on a goose farm in southern China in 1996. One year later, the virus crossed the species barrier and caused human infections in Hong Kong. The outbreak resulted in 18 human cases and six deaths. Infections were identified in people who had direct contact with poultry affected by avian influenza, which at the time was a widespread problem in Asia.

In 2003, outbreaks of A/H5N1 re-emerged in poultry in China and rapidly spread across Southeast Asia, Europe and Africa. According to the World Organisation for Animal Health, 18,620 outbreaks in poultry were reported in 76 countries between 2005 and 2019. More than 3,000 additional outbreaks were recorded in 2020 and 2021 alone. The spread of highly pathogenic avian influenza therefore represents a serious and growing public health threat.

Avian influenza virus is an influenza A virus belonging to the family Orthomyxoviridae. Influenza A viruses are classified into genetic and antigenic subtypes based on combinations of their surface glycoproteins: haemagglutinin (HA) and neuraminidase (NA). To date, 19 HA subtypes (H1–H19) and 11 NA subtypes (N1–N11) have been identified. Avian influenza viruses are classified as highly pathogenic avian influenza viruses (HPAIVs) or low pathogenic avian influenza viruses (LPAIVs).

Wild waterbirds, primarily species belonging to the order Anseriformes, are the natural reservoir of the virus. Some viral subtypes, including H5N1, have crossed species barriers and caused epizootics in mammals. Of particular concern is the susceptibility of dairy cattle in the United States to infection with avian influenza A/H5N1. The number of cases among both animals and humans continues to increase.

According to the World Health Organization, between 1 January 2003 and 27 May 2025, a total of 976 human cases of H5N1 infection were reported in 25 countries, with a case fatality rate of 48.2%. The main routes of viral transmission include direct contact with animals and exposure to contaminated environments, including aerosols, raw milk, equipment, feed and water. In humans, infection most commonly occurs following direct contact with infected poultry or contaminated environments.

The A/H5N1 virus has a very broad potential host range. It occurs in numerous species of wild birds and poultry. Among mammals belonging to the order Carnivora, it can infect members of the Felidae, Canidae, Phocidae, Procyonidae, Mustelidae and Viverridae families.

In 2020, clade 2.3.4.4b emerged and subsequently caused the largest highly pathogenic avian influenza epizootic recorded in Europe during the 2021/2022 season, affecting 37 countries. Since 2022, influenza A/H5N1 has resulted in the loss of more than 70 million poultry across five continents and has even reached Antarctica.

The virus is currently present on virtually every continent and infects numerous species of wild birds. Unfortunately, this also facilitates infections in wild mammals that are exposed to the virus by feeding on infected birds or their carcasses. Of increasing concern is the fact that the virus continues to mutate.

Infections in companion animals – cats

Cats were previously considered not to be susceptible to influenza virus infections. It is now known that they can become infected with certain influenza A viruses.

Several outbreaks of A/H5N1 infection in felids have been described to date. The first was reported in 2003 at a zoo in Thailand, where two tigers and two leopards died. The animals had been fed infected poultry carcasses.

One year later, 14 domestic cats died on a farm in Thailand, with A/H5N1 infection laboratory-confirmed in three of them. During the same period, a clouded leopard at a zoo in Chonburi, Thailand, also died as a result of infection. In 2009, infections were reported in lions, Asian golden cats and clouded leopards in Cambodia.

Between 2004 and 2024, 607 infections, including 302 fatal cases, were reported in 12 felid species across 18 countries. A dramatic increase in the number of feline infections was observed in 2023 and 2024, coinciding with the emergence of clade 2.3.4.4b. This clade has been confirmed in cats in Finland, Poland, the United States, Peru, South Korea, Italy and France.

Interestingly, subclinical A/H5N1 infection was identified in three cats at an animal shelter in Austria. Following the emergence of clade 2.3.4.4b, the first documented case of seroconversion in companion animals was confirmed in Italy in 2023. Antibodies were detected in five dogs and one cat despite the absence of clinical signs.

In Poland, numerous cat deaths occurred between late June and early July 2023. All affected cats showed respiratory, gastrointestinal or neurological signs, with some displaying signs involving all three systems simultaneously. Using RT-qPCR, Stefańska et al. confirmed that these clinical signs were caused by infection with highly pathogenic avian influenza A/H5N1.

Infection was confirmed in 29 domestic cats, one captive caracal, several domestic ferrets and one dog. However, the actual number of affected animals was probably higher.

In affected cats, the disease was generally severe and rapidly progressive, with death occurring within 1–7 days after the onset of clinical signs. The most commonly observed signs of A/H5N1 infection in cats were fever and dyspnoea. Some cats also developed lethargy and loss of appetite. Severe infections may also cause vomiting and diarrhoea, seizures and disorientation.

In cats, A/H5N1 infection causes an acute and frequently fatal disease characterised by atypical pneumonia, encephalitis and multiorgan failure.

The most likely routes of infection in cats include direct contact with infected birds, consumption of raw poultry products and hunting wild birds.

A/H5N1 infections in cats are a cause for concern. Adaptation of avian influenza viruses to cats could represent a step towards changes that may facilitate transmission between humans. To date, no human infections resulting from contact with infected cats have been documented.

Interestingly, a recent report from the United States indicated possible transmission of the virus from humans to domestic cats. In 2024, A/H5N1 clade 2.3.4.4b genotype B3.13 was identified in two strictly indoor cats. Both animals developed respiratory and neurological signs, and their owners worked on dairy farms where H5N1 infection had been confirmed in cattle.

In June and July 2023, A/H5N1 infections were confirmed at two separate cat shelters in South Korea. At one of the shelters, 38 of 40 infected cats died.

Infections in companion animals – dogs

A/H5N1 infections in dogs are considerably less common than in cats and, unfortunately, are also poorly documented in the scientific literature.

The most commonly reported clinical signs include coughing, sneezing, lethargy, loss of appetite and fever. In advanced cases, vomiting and diarrhoea may occur. As in cats, severe infection can lead to multiorgan failure and even death.

Because comprehensive data are lacking, our understanding of the transmission mechanisms, clinical presentation and treatment of A/H5N1 infection in dogs remains limited.

In 2023, antibodies against H5N1 were detected in four of 194 dogs tested in Washington State. Despite the possibility of bird-to-dog transmission, the low seroprevalence and absence of disease in seropositive animals suggest that clade 2.3.4.4b H5N1 viruses circulating in North America in 2022–2023 were poorly adapted to canine hosts.

Infections in other mammals

American mink and domestic ferrets are considered highly susceptible to influenza A virus infections.

In 2022, an outbreak of A/H5N1 avian influenza occurred on a mink farm in Spain. The viruses identified belonged to clade 2.3.4.4b, which has been responsible for recent outbreaks among birds in Europe.

A rare mutation was identified in one of the viral genes that facilitates replication in mammalian cells, supporting evidence of viral adaptation to mammalian hosts. The virus may also have been transmitted between mink.

Mink are susceptible to both avian and human influenza A viruses. This raises concerns that these animals could facilitate the emergence of interspecies reassortant viruses, including reassortants involving human influenza viruses.

A/H5N1 has also been reported in house mice and cervids in New Mexico, Colorado and Washington. In October 2023, clade 2.3.4.4b was detected in several seal species on two Antarctic islands.

In March 2024, dairy cattle in Texas and Kansas tested positive for H5N1. Importantly, high concentrations of the virus were also detected in unpasteurised milk. This was the first detection of influenza A/H5N1 in cattle.

A new D1.1 variant of A/H5N1 avian influenza was subsequently identified in a dairy cattle herd in Nevada. It contains a mutation associated with more efficient replication in humans.

In March 2024, a case of H5N1 infection was confirmed in a goat in Minnesota. In March 2025, the first case of A/H5N1 infection in a sheep in the United Kingdom was reported. Antibodies were detected in the animal’s blood, while H5 RNA was identified in its milk.

Summary

The risk of cross-species transmission continues to increase. Avian influenza virus remains a serious threat to both human and animal health because of its remarkable genetic plasticity and ability to infect a wide range of animal species, from wild birds to terrestrial and marine mammals.

References

  1. Iwan A. (2025). Wirus grypy ptaków A/H5N1 – zagrożenie dla zdrowia ludzi i zwierząt. Weterynaria w Praktyce; 10.
  2. Wille M., Barr I.G. (2022). Resurgence of avian influenza virus. Science, 376, 459–460.
  3. WHO’s Western Pacific Regional Office. Avian Influenza Weekly Update 2025. 2024.

Author: Adrianna Iwan

Microbiologist, Veterinary Technician, Animal Behaviourist and postgraduate graduate in Animal Rights. Veterinary Medicine student. Member of the Polish Council for Companion Animal Parasites (ESCCAP Poland). Creator of the educational profile Mikrobiolog z przypadku.