Avian Influenza (Bird Flu)

Avian influenza, commonly referred to as bird flu, describes a group of viral infections primarily affecting birds, particularly domestic poultry. However, the term can be misleading, as it encompasses zoonotic infections in humans caused by strains of the influenza A virus. Influenza A is an orthomyxovirus characterized by a segmented RNA genome, which allows for genetic reassortment and antigenic shifts. These characteristics make avian influenza a potential and unpredictable threat to human health. ^[17]

Influenza viruses are classified into three main types: A, B, and C. Among these, influenza A is the most significant in terms of zoonotic potential and pandemic risk. The influenza A virus is further categorized into various subtypes based on two surface proteins: hemagglutinin (H) and neuraminidase (N). There are 16 known H subtypes and 9 N subtypes, leading to a variety of combinations, such as H5N1 and H7N9. Historically, strains like H5N1 have posed significant threats to human health due to their high mortality rates and potential for transmission. ^[10]

Historical context and key events

The history of avian influenza is marked by several significant outbreaks that have raised public health concerns:

• 1997: The first severe human cases of H5N1 were reported during an outbreak in Hong Kong. This outbreak was notable for its high mortality rate, resulting in the culling of approximately 1.5 million chickens. Eighteen human infections were reported, with six fatalities. This event marked the beginning of heightened awareness regarding the potential for avian influenza to infect humans. ^[5]

• 2013: The H7N9 strain emerged in China, leading to a series of human infections linked to direct contact with infected domestic birds. The initial outbreak involved 126 reported cases, and subsequent smaller outbreaks have continued to occur. As of November 2024, over 1,500 human infections have been reported, underscoring the ongoing threat posed by this strain. ^[22]

• 2020-November 2024: A global outbreak of H5N1 clade 2.3.4.4b viruses has severely impacted wildlife, particularly pinnipeds (seals and sea lions). Reports indicate that over 30,000 sea lions have died in South America due to this outbreak. The emergence of a novel marine mammal clade capable of infecting both mammals and birds raises significant concerns for wildlife conservation and public health. ^[19]

Mechanisms of spread and spillover

Avian influenza viruses primarily circulate among bird populations, but certain strains have the ability to infect mammals, including pigs and humans. The reassortment of avian strains with mammalian strains can produce chimeric viruses that may be transmissible between mammals. This genetic mixing can lead to the emergence of new strains that are capable of infecting humans, raising concerns about potential outbreaks.

The spillover of avian influenza into other species is facilitated by several factors:

Genetic Reassortment:  The segmented nature of the influenza A virus genome allows for genetic reassortment when a host is co-infected with multiple strains. This can result in the creation of new viral strains with altered antigenic properties, potentially evading the immune response of existing hosts. ^[10]

Host Susceptibility:  Certain strains of avian influenza have shown a preference for specific receptors found in different species. For example, the receptor usage of the virus can determine its ability to infect mammals. The presence of sialic acid receptors, which bind to hemagglutinin, plays a crucial role in the virus's ability to infect various hosts. ^[18]

Environmental Factors:  The movement of migratory birds can facilitate the spread of avian influenza viruses across geographic regions. These birds can act as reservoirs for the virus, introducing it to new populations of domestic poultry and other animals. ^[19]

Recent findings and concerns

Recent findings have highlighted the ongoing threat of avian influenza, particularly the H5N1 strain, to both animal and human health:

Dairy Cattle Infections:  In 2024, H5N1 was confirmed in dairy cows across 13 US states, affecting nearly 200 herds. This spillover raises significant concerns for the dairy industry and highlights the potential for the virus to adapt to new hosts, including humans. The presence of H5N1 in dairy cattle poses risks not only to animal health but also to food safety and public health. ^[28]

Infections in Domestic Cats:  Infections also have been confirmed in indoor pet cats, with six cases reported in Colorado. This suggests that the virus is spreading among various animal species, increasing the risk for mutations that could lead to severe human illness. ^[30]

Human Cases:  On November 22, 2024, the CDC confirmed a human infection with avian influenza A(H5N1) in a California child, marking the first pediatric case in the United States. The child experienced mild symptoms and is recovering. Initial tests showed low levels of the virus, with follow-up tests negative for H5N1 but positive for other respiratory viruses. As of November 25, 2024, there have been 55 reported human cases of H5N1 in the United States during 2024, with 29 cases in California. ^[30]

Public health implications

The ongoing spread of avian influenza, particularly the H5N1 strain, poses significant risks not only to poultry but also to other species, including mammals. The potential for mutations that could enhance transmissibility among mammals raises serious public health concerns. The emergence of H5N1 in dairy cattle and domestic pets underscores the need for increased surveillance and biosecurity measures to prevent further spread. ^[31]

The interconnectedness of human, animal, and environmental health highlights the importance of a One Health approach to managing avian influenza. This approach emphasizes the need for collaboration among public health officials, veterinarians, and environmental scientists to monitor and control the spread of the virus. ^[27]

Surveillance and response strategies

To mitigate the risks associated with avian influenza, several strategies are essential:

Enhanced surveillance:  Continuous monitoring of avian influenza in both wild and domestic bird populations is crucial. Surveillance also should extend to mammals, particularly those in close proximity to poultry farms or known avian influenza outbreaks. ^[31]

Biosecurity measures:  Implementing strict biosecurity protocols on farms can help prevent the introduction and spread of avian influenza. This includes controlling access to poultry facilities, monitoring for signs of illness, and ensuring proper sanitation practices. ^[28]

Public awareness and education:  Raising awareness about the risks of avian influenza and the importance of biosecurity measures among farmers, veterinarians, and the general public is vital. Education campaigns can help inform individuals about safe practices when handling poultry and other animals. ^[31]

Research and development:  Ongoing research into the genetic characteristics of avian influenza viruses and their potential for spillover into humans is essential. Understanding the mechanisms of transmission and mutation can inform vaccine development and treatment strategies. ^[27]

The ongoing threat of avian influenza, particularly the H5N1 strain, underscores the need for vigilance and proactive measures to protect both animal and human health. The potential for spillover into other species, including mammals, raises significant concerns for public health. Continuous surveillance, enhanced biosecurity measures, and a comprehensive One Health approach are essential to mitigate the risks associated with avian influenza and prevent future outbreaks. As the situation evolves, it is crucial for public health officials, veterinarians, and researchers to work collaboratively to monitor and respond to the challenges posed by this unpredictable virus. ^[31]

For additional information on influenza, see Medscape's Influenza Resource Center.

The pathophysiology of avian influenza differs from that of normal influenza. Avian influenza remains primarily a respiratory infection but involves more of the lower airways than human influenza typically does. This likely is due to differences in the hemagglutinin protein and the types of sialic acid residues to which the protein binds. Avian viruses tend to prefer sialic acid alpha(2-3) galactose, which, in humans, is found in the terminal bronchi and alveoli. Conversely, human viruses prefer sialic acid alpha(2-6) galactose, which is found on epithelial cells in the upper respiratory tract. One group has reported that ex vivo cultures of human tonsillar, adenoidal, and nasopharyngeal tissues can support replication of H5N1 avian influenza. ^[1]

Although this results in a more severe respiratory infection, it probably explains why few, if any, definite human-to-human transmissions of avian influenza have been reported; infection of the upper airways probably required for efficient spread via coughing and sneezing. Many are concerned that subtle mutation of the hemagglutinin protein through antigenic drift will result in a virus capable of binding to upper and lower respiratory epithelium. The 1918 pandemic strain was so lethal partially because the receptor utilization of the hemagglutinin differed from that of other strains, and H5N1 has that potential to acquire that same biology through mutation.

Differences in the PA, NP, M1, NS1, and PB2 genes tend to correlate with human is strains of influenza, including human infections with avian influenza. ^[2]  The functional role of these genetic markers has yet to be determined but likely involves replication enhancement and immune suppression.

Unlike with human influenza, most deaths associated with avian influenza have been due to primary viral pneumonia, with no evidence of secondary bacterial infection.

United States statistics

Seasonal influenza disease burden varies each year. The overall burden of influenza for the 2021-2022 US season was an estimated 9 million influenza illnesses, 4 million influenza-related medical visits, 100,000 influenza-related hospitalizations, and 5,000 influenza deaths. According to the US Centers for Disease Control and Prevention (CDC), influenza activity has been lower that before the COVID-19 pandemic began.

The first case of an avian influenza A(H5N1) virus in a human in the United States was reported on April 28, 2022. 

International statistics

As of August 2024, 893 cases of H5N1 have been reported worldwide, with 463  deaths (case fatality rate, 52%). ^[18]  Most cases were in eastern Asia; some cases have been reported in Eastern Europe and North Africa. Underreporting has been a concern, particularly in China, but the prevailing attitude about the need to suspect, test, and report cases of avian influenza is growing. There have been 1,568 reported cases of H7N9 influenza, mostly from China, with other cases in Taiwan, Malaysia, Hong Kong, and Canada.

Although the risk remains largely theoretical, the ease of global travel emphasizes the possibility of international spread. The risks have been highlighted with the rapid spread of a low-pathogenicity H1N1 swine-origin influenza in early 2009. The risk of a successful recombination event occurring between swine-origin H1N1 and a pathogenic avian influenza cannot be easily assessed. A mutation in avian influenza that rendered it permissive for sustained human-to-human transmission without affecting its pathogenicity in humans could be extremely dangerous. ^[4]

Mortality/morbidity

The extraordinarily high mortality rate of avian influenza (>60% for H5N1; approximately 30% for H7N9) is worrying and reasonably accurate. There have been very few instances of seropositive individuals without clinical signs of infection. In most instances, the policy is to test exposed individuals around an outbreak (human and avian). Therefore, a large population of exposed but untested people is unlikely.

Race

Race appears to be a factor only to the extent that geographic differences in the rate of HPAI among birds and the degree of bird-to-human contact are significant.

Sex

In Egypt, 90% of fatalities due to avian influenza have involved women, a pattern that has not been readily apparent elsewhere. ^[5] Most cases of H7N9 have been reported in men.

Age

Avian influenza has the highest case-fatality rate among persons aged 10 to 39 years. Unlike seasonal influenza, which disproportionately affects very young and very old individuals, young adults make up a large proportion of the avian influenza cases.

Fifty percent of reported cases have been in people younger than 20 years. Forty percent of cases involve persons aged 20 to 40 years.

In Egypt, avian influenza has been associated with a relatively low mortality rate, which seems to be associated with a high rate of infection in young children (< 10 y); as of May 2009, the mortality rate in this subpopulation was zero. The significance and reproducibility of these findings remains to be seen. ^[5]

The prognosis of confirmed human cases of avian influenza is related to the degree and duration of hypoxemia. The cases to date have exhibited a 60% mortality rate. The risk for mortality depends on the degree of respiratory disease rather than the bacterial complications (pneumonia).

Little evidence regarding the long-term effects of disease among survivors is available.