In the interest of animal welfare and public health, researchers are advocating for improved conditions for farmed animals.

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

Zoonoses affect many types of animals. Intensive factory farming (IIAF) destroys wild animal habitats, leading to closer association between wild animals and humans, thereby increasing the risk of transmission of infections from wild animals.

However, while existing research shows that approximately 75% of zoonoses can be traced to wild animal species, the remainder comes from domestic animals, including farmed animals. On many farms around the world that raise poultry and pigs for meat, the animals are kept in overcrowded and unsanitary conditions, increasing zoonotic risk.

In a new study, researchers from the University of Winchester in the United Kingdom and Griffith University in Australia examine the risks of raising chickens and pigs in intensive agricultural environments and crowded premises. The work is published in Frontiers of Veterinary Science.

Zoonotic agents and high-risk conditions

Zoonotic agents include bacteria, fungi, parasites, protozoa, and viruses that incubate in animal hosts. Among these agents, viruses, particularly influenza viruses, which infect the digestive and respiratory tracts of their hosts, animals and humans, represent the most serious risk.

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

Overall, conditions that increase the risk of zoonosis worldwide include:

• Record number of humans (8 billion in November 2022); farmed chickens (25 billion at a time) and farmed pigs (1 billion at a time)
• Human proximity to farm animals
• Suboptimal breeding and living conditions for farmed chickens and pigs
• The global presence and internationalization of the IIAF
• Disruption of ecosystems by the IIAF due to its inordinate demands for land and resources
• Specific locations of IIAF operations
• Misconceptions that livestock are both present and at low risk

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

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

The use of antimicrobials makes the situation worse. A 2023 study in 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%.

A Brief Overview of Influenza Viruses

An article published in Virus in 2023 delineates four types of influenza viruses, each having 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 some other ruminants, horses and pigs)

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

Influenza type A has dominated primarily because it can both move and drift antigenically. The researchers explain that when two different strains infect a single host, an antigenic shift can take place, eventually leading to the appearance of a new strain of type A virus. Meanwhile, if the host has developed immunity to the virus, it can mutate over a longer period and overcome this immunity. This is an antigenic drift.

Viruses and mixing containers

Animal hosts that can be infected with two or more influenza strains simultaneously are known as mixing vessels, which provide 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.

“The four major influenza pandemics originated from avian influenzas that formed new types in humans, pigs, or another currently undetermined mixing container host. Further mixing may also occur via reverse zoonotic infection, whereby “Influenzas in humans can be transmitted to other animals, allowing additional mixing to occur with any other influenza strains present in these animals,” the document states.

Recommended mitigation measures

Categorically opposing the idea that there is a single, simple way to adequately reduce zoonotic risk in farmed poultry and pigs, the researchers conclude by discussing, in the context of currently existing conditions, the specific risk mitigation recommendations. These include:

• Biosecurity
• Vaccinations
• Reduction in consumption of livestock by the public
• Restructuring poultry and pig farming to improve animal welfare by improving breeding practices while reducing antibiotic use and stocking density, and
• Phase out approval of applications for new and expanded intensive industrialized livestock farms.

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