Avian Influenza and Influenza Viruses of Animal Origin: A Review of 2024

The 2026 epidemiological report from the European Center for Disease Prevention and Control (ECDC) on zoonotic diseases (2024 data) presents a seemingly paradoxical finding: influenza viruses of avian and swine origin circulate widely among animals, but documented human infections remain rare and, most notably, no cases were confirmed in Europe in 2024. This apparent lull in Europe contrasts with a certain increase observed in 2024 in the rest of the world, notably featuring broader transmission to mammals and enhanced surveillance.  

112 human infections linked to avian viruses

In 2024, nine countries reported a total of 112 cases (or detections) of influenza A in humans, distributed across several subtypes: H5N1, H5N2, H5N6, H5Nx, H9N2, and H10N3. The majority of cases were reported by the United States (66), followed by China (28) and Cambodia (10). Vietnam reported 3 cases, and Australia, Canada, Ghana, India, and Mexico reported 1 case each. Of these 103 cases, 30 people (29%) were hospitalized. The death toll stands at 8, representing 7% of the 112 cases. The report highlights a key factor in understanding these figures: exposure. Among the documented cases, 94% (101 out of 108) were exposed to animals or a contaminated environment. The most common sources of exposure were poultry (54%, 60 cases) and—notably in 2024— cattle (36%, 40 cases), echoing the outbreaks of influenza A (H5N1) detected in dairy herds in the United States.  

H5N1: Dairy Cattle Enter the Picture

The H5N1 subtype alone accounted for 80 human cases worldwide in 2024. No cases were reported in Europe that year, despite numerous detections and outbreaks among wild birds and domestic birds. In the United States, of the 66 cases of influenza A (H5) reported in 2024, 61% (40 out of 66) were associated with occupational exposure to dairy cattle suspected or confirmed to be infected with the H5N1 virus. The clinical presentation is predominantly mild and characterized by conjunctivitis (reported in 100% of cases where this symptom was documented), with no hospitalizations in this group. The same country also reports infections linked to operations carried out on poultry farms (particularly during depopulation). One death is mentioned: that of an elderly person with comorbidities who was exposed to a backyard flock and dead birds on their property. Finally, the report notes two cases detected without identified animal exposure, which does not prove human-to-human transmission but highlights the difficulty of tracing certain exposure chains in the field.  

H5N2 and H5N6 Viruses: Rarer, but with high case fatality

The year 2024 was also marked by the first confirmed human case of influenza A (H5N2) in a 59-year-old man who died in Mexico from complications related to his underlying comorbidities. The virus showed a high degree of similarity (99%) to low pathogenic strains detected in birds in that country. For H5N6, China reported three human cases in 2024, all of whom were hospitalized and all of whom died, following exposure to poultry (backyard flocks or markets). These incidents illustrate a key methodological point, explicitly highlighted by the ECDC: the observed case fatality rate varies greatly depending on the subtype and also depends on the surveillance system. Mildly symptomatic cases may go undetected in the absence of targeted surveillance among exposed individuals.  

H9N2: Circulation Primarily Among Children and in Asia

The H9N2 subtype, regularly associated with exposure to poultry at markets, accounted for a total of 25 reported human cases in 2024. China reported 22 of these cases, predominantly among children (86% of Chinese cases, including 13 under the age of 5). Among the documented cases, 67% were hospitalized, and exposure to poultry was common (backyard flocks or live animal markets). India, Vietnam, and Ghana each reported sporadic cases, sometimes without a formally identified avian exposure.  

Swine influenza viruses: variants under surveillance

Meanwhile, four countries have reported 12 human cases caused by the swine influenza viruses A (H1N1)v, A (H1N2)v, and A (H3N2)v. The United States accounts for 9 of these cases, while Canada, China, and Vietnam each report 1 case. The details point to a classic pattern: exposure to pigs (at agricultural fairs, in the home, or at work), often followed by mild infections, though there are exceptions. The report indicates that these infections can be severe: a total of 6 people were hospitalized, and 1 death was recorded.  

Europe Largely Spared

No human cases were reported in the European Union in 2024, neither for avian influenza nor for swine variants. The ECDC, however, emphasizes two points. The first relates to the mechanics of risk: as long as viruses circulate in animals, sporadic human infections are likely to occur in areas where unprotected exposure to infected animals or contaminated environments is possible. The second relates to viral evolution: reassortment of genetic material among influenza viruses occurs constantly. In other words, the challenge is not only to identify cases but also to detect early any signs of adaptation to mammals, which is a prerequisite for active human-to-human transmission.  

Possible Scenarios According to the ECDC

The most likely scenario in the short term remains one of low-level circulation: a few rare, scattered human cases, mostly linked to close contact (occupational or household) with infected animals, without sustained human-to-human transmission. The “breakout” scenario, meanwhile, is not described as imminent, but it warrants increased vigilance regarding the emergence of a virus better adapted to mammals, or the accumulation of consistent indicators (clusters, genetic profiles, changes in tropism) that would shift the focus from biosecurity in animal husbandry to preparedness for a pandemic risk. With this in mind, the ECDC advocates for a “One Health” approach, including the rapid sharing of information on animal outbreaks, virus characterization (via genomics), and the early detection and reporting of human infections in order to immediately investigate any suspected human-to-human transmission and implement appropriate measures.   While 2024 did not mark a shift toward a pandemic, the worrying signs are cause for sufficient concern to strengthen surveillance, regardless of species and across national borders.