Carnivorous animals are thought to have a deficient immune system that allows pathogens to evolve incognito and acquire potentially dangerous mutations that can be transmitted to humans.
In Denmark, more than 17 million mink had to be killed after the discovery of a dangerous mutation that could be transmitted to humans. And this is no coincidence. According to a WHO report, “there is a high risk of the Covid-19 virus spreading from mink farms”. Another study in the journal Science explains that mink or raccoons (usually bred in China) may be an intermediate host for SARS-CoV-2 coronavirus transmission between bats and humans. But why are these animals particularly at risk?
49% of all carnivores carry one or more unique zoonotic pathogens.
A new study published in the journal Cell Reports sheds new light on this mystery: carnivorous animals are thought to have a deficient immune system, making them an ideal reservoir for a wide range of pathogens. “49% of all carnivores (minks, dogs, cats, etc.), the largest percentage of all mammals, carry one or more unique zoonotic pathogens,” explains Clare Bryant, a veterinary researcher at the University of Cambridge and lead author of the article.
Dogs and cats are often asymptomatic carriers of salmonella, and during the pandemic, several cases of SARS-CoV-2 infected pets were reported. “We, therefore, investigated whether this is because carnivores are a large group of animals that carry many pathogens, making them proportionally more zoonotic carriers, or whether it is due to other factors, such as differences in immune systems.”
Lack of inflammatory response
By comparing the distribution and evolution of inflammatory genes in carnivores, scientists found that, during evolution, the three main genes responsible for gut inflammation gradually lost their functionality in carnivores. These genes activate protein complexes called inflammasomes, which trigger the secretion of anti-inflammatory cytokines in response to microbial detection, as well as a programmed cell death called pyroptosis, which is responsible for the rapid elimination of infectious agents. “The absence of these functional genes facilitates the ability of pathogens to lurk undetected in muscle cells, transforming and transmitting themselves, thereby becoming a threat to human health,” said Clare Bryant.
A protein-rich diet has a natural antimicrobial effect
But how do these animals avoid getting sick? Researchers hypothesize that protein-rich diets, such as carnivore diets, would have antimicrobial properties that would compensate for the loss of immunity caused by a lack of genes.
So should we get rid of our cats, dogs, and ferrets? Not necessarily. “It is only when large numbers of carnivores are kept together that a reservoir of pathogens can accumulate and potentially mutate among them,” reassures Clare Bryant. In other words, the zoonotic risk is mainly posed by mink farms or perhaps large dog farms. Yet the risk is not zero: in 2018, one patient was infected with recombinant canine coronavirus and developed atypical pneumonia.
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