How SARS-Cov-2, the bat-sourced causative viral agent of COVID-19, emerged, whether an accidental laboratory release or a wet market (live animal market) or otherwise, is currently unknown to the global community. Regardless, infectious diseases that pass from bats, often through intermediate species, to humans has been a major health concern for decades. SARS-CoV, the virus that caused the worldwide outbreak of SARS, almost certainly passed from bats to civet cats then to humans at a wet market in Guangdong province, China in 2002.
Normally pathogens that infect one species do not easily cross over to infect an entirely different species; this is due to what scientists refer to as the species barrier. The human encouraged interaction of different species is a major contributor to the breakdown of the species barrier that can lead to an outbreak of zoonotic disease (a disease that is transmitted from animals to humans). Wet markets, where varied species of live animals are forced together, are a breeding ground for cross-species disease transmission. These animals, whether captured in the wild or brought from farms, are under significant stress just from transportation and caging alone. Stress that reduces their immune response and encourages the shedding of any pathogens they are harboring. We then add a significant human population into the mix and it is a recipe for species barrier breakdown.
Mixed agriculture has also been a source of deadly zoonotic outbreaks, including those originating in bats. In 1998-1999 there was a deadly outbreak of Nipah virus in Malaysia, through a multidisciplinary research effort (a “One Health” approach) it was determined that transmission occurred at mixed fruit plantation-swine operations. Bats were attracted to the fruit trees, they dropped partially eaten, virus-contaminated fruit and feces down to the pigs, and the pigs then became infected and passed the virus on to the farm workers. Pigs are often considered an infectious disease bridge species between other animals and humans (civets served as this bridge species between bats and humans in the SARS-CoV outbreak of the early 2000s). Pigs’ immune systems are quite similar to humans and they can be infected with animal diseases as well. In the context of influenza virus they are referred to as “mixing vessels,” as they can be infected with both avian and human strains of influenza. The genes of these viruses can mix in pigs and generate new strains which can more easily cross the species barrier. It would be prudent not to have these bridge species alive and stressed in wet markets exposed to unknown number of other species. In the context of influenza, it would also seem prudent to not to have mixed farms of birds and swine to avoid avian flu passing through pigs to humans, yet these farm practices do occur in parts of Asia and Africa, as well as some “boutique” farms in the United States.
So what can be done to mitigate the risk of zoonotic disease? The answer is modern, scientific animal agriculture and modern food systems. Here in the United States our meat predominantly comes from commercial animal agriculture operations producing a single livestock species. On these farms, animals like poultry or swine are usually raised in well-managed and controlled indoor environments. These barns are closed, not because animal producers are uncaring or are trying to hide something from the public; it is to protect their animals from disease and harm. A well-constructed poultry barn can stop a mallard from the Chesapeake Bay carrying avian influenza to a chicken in Delaware. A swine barn can stop a bat carrying Nipah from infecting a pig in Malaysia. Biosecurity and personal protective equipment (PPE) are now well entrenched in the public consciousness, but for decades they have been core to the operations of animal producers. Poultry producers will often don disposable coveralls, hairnets, boot covers, and use boot decontamination baths just to enter a barn to see their birds. This is how the small Delmarva Peninsula can raise 609 million broiler chickens a year without continuous disease outbreaks. In many swine operations in the United States farm workers must disrobe, shower, and put on sanitized clothing before entering a barn in order to protect the pigs. They must then shower out and change clothes again to protect themselves, their families and the public. This is similar to what scientists do in highly biosecure laboratories. We don’t even ask our health care professionals in hospitals and clinics to do this. Once these animals are transported to a USDA inspected processing plant, they are humanely slaughtered and are processed into cuts of meat that are transported via refrigeration or freezer truck to sanitary grocery operations. There is minimal risk of zoonotic spillover in this chain.
Some may argue that intensive livestock operations themselves, with tens of thousands of chickens, or thousands of swine or cattle, breed disease. After all, 30,000 chickens would not normally all live under one roof in nature. While it is true there are diseases in livestock animals that are only found under such conditions, almost none of these diseases are zoonotic. The diseases these animals would experience in the wild are often far more severe, both for the animal and potential human consumer. In addition, raising a single species on a farm helps keep the species barrier intact. Finally, animal scientists, animal health companies, veterinarians, and producers work tirelessly to develop vaccines, treatments and management practices to help prevent and treat diseases of livestock, this does not happen in the wild.
How do we eliminate future SARS-like outbreaks generated from live animal wet markets in China? How to we reduce the risk of zoonotic disease pandemics in general? The answer is modern, scientific animal agriculture practices that prioritize health.
Ryan Arsenault
Associate Professor
Department of Animal and Food Sciences
rja@udel.edu