Researchers have found that the immune systems of some carnivorous animals appear to be defective; they don't express immune genes that typically are used to identify an infection and respond to it. When large numbers of carnivorous animals are brought together, such as on a mink farm, many may be infected with microorganisms but not showing symptoms. That may lead to the creation of a large disease reservoir, in which the animals harbor pathogens that may mutate, spread to other species, and potentially infect humans. Infectious bacteria, fungi, or viruses that move from animals to humans are called zoonotic, and many zoonotic diseases, like anthrax, Salmonellosis, COVID-19 or Ebola, are dangerous.
"We've found that a whole cohort of inflammatory genes is missing in carnivores. We didn't expect this at all," said senior study author Professor Clare Bryant of the University of Cambridge's Department of Veterinary Medicine.
Publishing in Cell Reports, the researchers found that three critical genes that are involved in maintaining the health of the gut are inactive. They are still in the DNA sequence, but they aren't used. In other species, these genes encode for proteins that are involved in the inflammasome complex, which helps fight off pathogens and trigger inflammation.
So why did carnivores stop expressing these genes? The researchers suggested that the high-protein, all meat diet has an antimicrobial effect on the body, which may be making up for the loss of immune functions that would normally be important. In these animals, gut infections can be eliminated with diarrhea. However, other microbes may be infecting other parts of the body that are not so easy to flush out. More research will be needed to confirm this hypothesis.
In some animals, another gene that plays a role in gut health now carries a mutation that has resulted in a fusion of enzymes called caspases. The function of the molecules is altered, and it may not be able to react to some infections.
"We think that the lack of these functioning genes contributes to the ability of pathogens to hide undetected in carnivores, to potentially mutate and be transmitted becoming a human health risk," added Bryant.
Carnivores carry many zoonotic pathogens, and while dogs and cats are carnivores, the researchers said domestic pets are not a cause for concern. The danger is when huge numbers of these animals live tougher in close quarters.
"When you have a large population of farmed carnivorous animals, like mink, they can harbor a pathogen like SARS-CoV-2 and others, and it can mutate because the immune system of the mink isn't being activated. This could potentially spread into humans," said Bryant.