In the interest of animal welfare and public health, researchers make the case for improving farm animal conditions

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Potential for mixing of avian influenza viruses and emergence of new variants among different animal species. Potential mixing vessel hosts are grouped based on frequency of infection, proximity to humans, high population numbers, and presence of both avian and human receptors. A diverse range of type A subtypes can be found among different animals within each group. “Highly likely” mixing vessel hosts include humans, pigs, minks, ferrets, seals, dogs, cats, and birds, especially turkeys, chickens, quail, and ducks. “Moderately likely” hosts are non-human primates, raccoons, camels, pikas, zoo animals including tigers and lions, and horses. “Low probability” receivers include foxes, bats and whales. After Abdelwhab and Mettenleiter (2023). credit: Frontiers in Veterinary Science (2023). DOI: 10.3389/fvets.2023.1310303

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Potential for mixing of avian influenza viruses and emergence of new variants among different animal species. Potential mixing vessel hosts are grouped based on frequency of infection, proximity to humans, high population numbers, and presence of both avian and human receptors. A diverse range of type A subtypes can be found among different animals within each group. “Highly likely” mixing vessel hosts include humans, pigs, minks, ferrets, seals, dogs, cats, and birds, especially turkeys, chickens, quail, and ducks. “Moderately likely” hosts are non-human primates, raccoons, camels, pikas, zoo animals including tigers and lions, and horses. “Low probability” receivers include foxes, bats and whales. After Abdelwhab and Mettenleiter (2023). credit: Frontiers in Veterinary Science (2023). DOI: 10.3389/fvets.2023.1310303

Research shows that three-quarters of emerging infectious diseases are transmitted from animals to humans; a disease of this type is known as a zoonosis (plural: zoonoses). Farm animals, especially pigs and poultry, pose a high risk of zoonotic infection.

Zoonoses involve many species of animals. Intensive industrialized animal agriculture (IIAF) destroys wildlife habitats, leading to closer contact between wildlife and humans, thereby increasing the risk of transmission of infection from wildlife.

However, while existing research indicates that approximately 75% of zoonoses can be traced to wild animal species, the remainder originate from domestic animals, including farmed animals. In many farms around the world that raise poultry and pigs for meat, the animals are kept in crowded, unsanitary conditions, which increases the zoonotic risk.

In a new review, researchers from the University of Winchester in the UK and Griffith University in Australia examine the risks of raising chickens and pigs in intensive agricultural environments and in crowded facilities. The work was published in Frontiers in Veterinary Science.

Zoonoses and high-risk conditions

Zoonotic agents include bacteria, fungi, parasites, protozoa, and viruses that incubate in animal hosts. Of these agents, the most serious risk is posed by viruses – especially influenza viruses, which infect the digestive and respiratory tracts of their animal hosts and humans.

Zoonotic spread often occurs through direct human contact with the saliva or feces of an infected animal or through environmental contamination.

In general, conditions that increase the risk of zoonosis worldwide include:

  • Record number of people (8 billion by November 2022); farmed chickens (25 billion at any given time) and farmed pigs (1 billion at any given time)
  • Close proximity of people with farmed animals
  • Suboptimal conditions for breeding and living of farmed chickens and pigs
  • IIAF’s global presence and internationalization
  • IIAF disruption of ecosystems due to excessive land and resource demands
  • IIAF Specific Sites of Operations
  • Misconceptions that farmed animals both represent and are subject to low risk

The new study discusses the ramifications of each of these factors in detail.

Well-known examples of zoonoses include outbreaks of Asian influenza (H2N2 virus) in the late 1950s and Hong Kong influenza (H3N2 virus) in the late 1960s, both of which were transmitted to humans via infected chickens; and swine flu (H1N1 virus transmitted to humans through infected pigs), which originated in 1919 and is still in circulation.

Using antimicrobials makes things worse. 2023 survey PLoS Global Public Health shows that globally farmed animals consume 73% of all antimicrobials used and that by 2030 this percentage is expected to increase by 8%.


Global number of slaughtered chickens and global human cases of H5N1 between 1961 and 2022. H5 vaccine was introduced in 2017. Data on H5N1 cases are from CDC with records beginning in 1997; the number of chickens slaughtered are from the UN FAO. After Kessler et al. Viruses (2021). credit: Frontiers in Veterinary Science (2023). DOI: 10.3389/fvets.2023.1310303

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Global number of slaughtered chickens and global human cases of H5N1 between 1961 and 2022. H5 vaccine was introduced in 2017. Data on H5N1 cases are from CDC with records beginning in 1997; the number of chickens slaughtered are from the UN FAO. After Kessler et al. Viruses (2021). credit: Frontiers in Veterinary Science (2023). DOI: 10.3389/fvets.2023.1310303

A brief overview of influenza viruses

Article published in Viruses in 2023 outlined four types of influenza viruses, each with a unique genome and affecting a specific group of hosts:

  • Type A (wide variety of birds and mammals)
  • Type B (humans and pigs)
  • Type C (humans, pigs and seals)
  • Type D (cattle and several other ruminants, horses and pigs)

The researchers of this new study focused their work on type A because it is the most common. It is divided into multiple “H” and “N” subtypes based on the presence of hemagglutinin and neuraminidase proteins on its surface, leading to specific strain names such as the H2N2 virus.

Influenza type A dominates mainly because it can both shift and deviate antigenically. The researchers explain that when two different strains infect a host, an antigenic shift can take place, possibly resulting in a new strain of type A virus. Meanwhile, if the host has developed some immunity to the virus, it can mutate for a longer period of time and overcome this immunity. This is antigenic drift.

Viruses and mixing vessels

Animal hosts that can be simultaneously infected by two or more influenza strains are known as mixing vessels, existing as prime sites for virus mutation and the emergence of new strains containing genetic material transmitted from existing strains. Infections of this type have great potential to lead to a public health emergency.

“All four major influenza pandemics originated from avian influenza that formed new types in humans, pigs, or some other currently unspecified host mixing vessel. Further mixing can also occur through reverse zoonotic infection, where influenza in humans can be transmitted to other animals, allowing further mixing with any other influenza strains present in those animals,” the paper states.

Recommended mitigation measures

Advocating definitively against the idea that there is a single easy way to adequately reduce zoonotic risk among farmed poultry and pigs, the researchers conclude by discussing, in the context of current conditions, specific recommendations for risk reduction. They include:

  • Biodefense
  • vaccinations
  • Reduction in consumption of farm animals by the population
  • Restructuring poultry and pig farming to increase animal welfare by improving breeding practices while reducing antibiotic use and stocking density, and
  • Phasing out approvals of applications for new and expanded industrialized intensive livestock farms.

More info:
Jenny L. Mace et al, Influenza risks arising from mixed intensive pig and poultry farms with a focus on the United Kingdom, Frontiers in Veterinary Science (2023). DOI: 10.3389/fvets.2023.1310303

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