Introduction

Influenza viruses have many animal reservoirs that reduce the possibility of effective infection prevention in humans due to the risk of transmission. This article will explore the importance of animal reservoirs in influenza transmission and infection, and its implications on public health.

Influenza viruses are a group of viruses responsible for the contagious, respiratory infection known as the ‘flu’. Viruses are small, non-living infectious agents transmitted by the droplets in coughs and sneezes of those infected.¹ There are four types of influenza virus (A, B, C and D), but only types A and B cause the seasonal epidemics observed in humans.² 

Animals that harbour and transmit pathogens, such as influenza, are referred to as ‘animal reservoirs’³ and can have significant public health consequences such as during the Swine Flu pandemic of 2009.⁴ Type A influenza viruses are known to have many animal reservoirs that act as zoonotic sources of influenza infection, with people becoming infected by the virus through direct contact with the infected animals (such as via handling) or indirect contact such as environmental exposure to the animals’ bodily fluids.²

The natural reservoirs of influenza in animals make it impossible to eradicate. The World Health Organisation (WHO) suggests that the continuous monitoring of influenza occurrence in animal and human populations is critical to maintaining public health.^2,5

Animal reservoirs for influenza

Avian and swine reservoirs

Avian (birds) and swine (pigs) reservoirs are prevalent zoonotic sources of influenza and pose significant risks to public health due to the risk of the virus obtaining a mutation that enables it to ‘cross’ the species barrier and infect humans.

Wild aquatic birds are the natural reservoir for many type A influenza viruses, denoted as ‘avian reservoirs’. Since October 2021, UK birds have been affected by an epidemic of Highly Pathogenic Avian Influenza (HPAI) with tens of thousands of birds dying due to the disease.⁶ 

Although there is limited evidence for sustained person-to-person infection with avian influenza, new ‘strains’ of influenza arise rapidly within animal reservoirs during epidemics, posing a constant risk to human public health.⁷ Four strains of bird flu have been identified as having the possibility of infecting humans, and so these are continuously monitored by government bodies in the UK.

‘Swine’ reservoirs refer to the natural reservoirs of influenza viruses that persist in pig populations and were most notably responsible for the 2009-2010 influenza type A (H1N1) pandemic.⁸ Pigs are commonly held as livestock, with 1.7 million breeding animals kept in the UK in 2023 alone ⁹, increasing the risk of direct or indirect contact with potentially infected animals. 

Other potential reservoirs

There are several mammalian hosts to avian influenza including seals, mink, horses, and dogs that may act as an intermediate host between transmission from the avian reservoir to human hosts.¹⁰ The number of mammalian hosts has particularly increased in the last fifteen years due to the emergence of two canine influenza viruses circulating amongst domestic dogs since 2006 when avian influenza first crossed the species barrier.¹¹ Despite this, avian and swine influenza reservoirs remain the most prevalent globally.

Role of animal reservoirs

Animal reservoirs play a critical role in the evolution, maintenance, and transmission of influenza viruses by providing a breeding ground for zoonotic strains that can cross interspecies barriers to develop.¹² Due to their non-living nature, viruses rely on their host’s cells to replicate and infect other organisms, enabling them to rapidly evolve into new ‘strains’ that may be more pathogenic or able to cross an interspecies barrier. Consequently ‘zoonotic spillover’ events occur where new strains of influenza infect a new organism for the first time and establish new influenza lineages in animal populations.¹³

Transmission dynamics

As mentioned previously, viruses like influenza can be transmitted via infectious droplets in coughs and sneezes between people, or through direct and indirect contact with infected animals. Viruses require host cells to replicate themselves and survive, and when a cell is co-infected with two different influenza viruses, genetic reassortment can occur. 

Genetic reassortment and zoonotic transmission

Genetic reassortment involves swapping gene segments in viruses with a segmented genome to create genetic diversity that may affect the virus’ transmission and pathogenicity.¹² Influenza is an example of an RNA virus with a segmented genome that can undergo genetic reassortment during the co-infection of a host cell. 

Like all viruses, influenza uses the cellular machinery in our cells to replicate its genetic material and viral proteins to produce new viruses to spread infection.¹⁴ Genetic reassortment and mutations during RNA replication drive the evolution of influenza viruses and are responsible for the different strains that infect the population each year.¹⁵ 

Influenza pandemics can be caused by zoonotic transmission (from an animal reservoir to humans) or through the reassortment of zoonotic influenza with a human influenza virus during co-infection.¹⁶ During these processes, influenza viruses undergo an ‘antigenic shift’ whereby novel antigenic patterns are obtained to enable the virus to evade the immune system, lowering vaccine effectiveness.¹⁵

Detection and challenges

Importance for early detection and prevention

The World Health Organisation (WHO) has run global influenza surveillance since 1952 to improve global ‘preparedness and response for seasonal, pandemic and zoonotic influenza’ by alerting public health bodies worldwide of novel influenza viruses and their potential to endanger public health.⁵ The vast range of animal reservoirs of influenza viruses, especially type A viruses that can cause pandemics, pose a continuous risk to global public health. 

The risk of a zoonotic influenza A virus becoming pathogenic to humans has been exacerbated in the last century by the worldwide increase in animal husbandry and agriculture, and the encroachment of human settlements on ecosystems.^17,18 In 2009, genetic reassortment between avian, swine and human influenza produced a novel influenza strain that infected humans, causing a pandemic.¹⁹ As described by Harrington et al, this global crisis highlighted the importance of comprehensive monitoring of influenza strains and their likelihood to emerge as a public health risk.²⁰ 

The early detection of novel influenza strains is critical to pandemic prevention, enabling the development of prevention strategies such as seasonal vaccines to disrupt human transmission of the virus.²¹

Public health implications

Most people can recover from the flu (influenza) without treatment, however, influenza can cause severe illness or death in people aged 65 and over in industrialised countries²² or children aged five years or younger in developing countries.²³ Influenza can affect people of all ages, however, some groups including pregnant women, elderly people and people with chronic illnesses are at greater risk of severe disease or complications. 

One of the difficulties in monitoring influenza and its impact on public health is the number of other respiratory viruses that cause influenza-like illness, making clinical diagnosis more challenging.²⁴ This challenge became greater more recently during the COVID-19 pandemic due to the overlap in symptoms of illness caused by rhinoviruses, COVID-19, influenza, and more. In the UK, the UK Health Security Agency (UKHSA) is responsible for tracking the spread and effect of influenza on public health in the UK, working with global partners to protect the most vulnerable people in society.²⁵

Vaccine development and one health approach

In the UK, the annual flu vaccination programme is critical to maintaining public health and reducing transmission amongst vulnerable groups.²⁶ The efficacy of vaccination against influenza is limited by the rapid development of new virus strains and their genetic variation. However, flu vaccination still prevents millions of illnesses and influenza-associated medical visits each year.²⁷ The flu vaccine is offered each autumn to vulnerable groups including the elderly, frontline healthcare workers and school-aged children. 

Furthermore, following the emergence of severe acute respiratory disease (SARS) and H5N1 influenza in the 2000s, the term ‘One Health’ was coined to recognise the link between animal and human health, and the threats that disease poses worldwide,²⁸ 

Viewing public health in the context of animal and human relationships enables international governments and organisations to conduct comprehensive surveillance of influenza risk, founded on our knowledge of the animal reservoirs mentioned in this article.²⁸ Many emergent pathogens have been linked to human-animal and human-ecosystem interfaces and have encouraged a multidisciplinary and collaborative response to diseases such as influenza.²⁹

Conclusion

To conclude, influenza viruses are responsible for acute respiratory infections known as the ‘flu’, and pose significant risks to public health and economies worldwide. Influenza is commonly found in animal reservoirs in birds, pigs, and other mammalian hosts (though these are less prevalent). Recognising the link between animal reservoirs and the human transmission of influenza has been critical to developing effective prevention and surveillance strategies worldwide.