What happens when we transmit human diseases to animals?

The COVID-19 pandemic has exposed the profound risks that can arise when an animal pathogen jumps to people. But what about the other way around? What are the possible pathways and risks of human pathogens “reversing” and making the leap into the wild?

A team of researchers recently published a review of the literature evaluating cases of human-to-wildlife transmission events and characterizing the factors of spread.

“Having seen things like SARS-CoV-2 being able to spread on mink farms, to tigers in zoos, and to wild white-tailed deer, we reviewed the literature and we found 100 cases in which there has been a contagion. It is likely more common than we think, but we are missing a lot of data due to sampling bias,” the authors comment.

As they explain, most of the documented cases are biased towards non-human and long-lived primates, captive animals and those that are handled by or accustomed to humans. “Studies are needed in animals with shorter lives, ungulates, rodents and bats”they report.

More than 25% of reported cases involved the bacteria that cause tuberculosis. It has been found in elephants, ungulates, captive birds, carnivores, and a rodent. Few examples led to illness or death, and even fewer showed evidence of a human pathogen remaining constant in a population of wild animals.

The researchers say this is key because “when a human pathogen remains in a population of wild animals, it has the potential to spread to people, perhaps with new changes that make it more transmissible and capable of producing more serious disease.

TWO WAYS OF TRANSMISSION

During their review, they examined two main routes of transmission. In the first, a human pathogen spills over into wildlife and causes significant illness or death. But ultimately, animals are an evolutionary dead end for the pathogen, and cannot be maintained in the non-human population.

In the second pathway, a human pathogen is shed into an animal population and remains, but then potentially sheds back into people. Additional pathways exist, the researchers note. For example, there may be regular or frequent transmission between hosts of different species, providing more opportunities for the pathogen to take hold or to mutate and acquire new characteristics.

In their literature search, the authors found more evidence for the first pathway than for the second. “This does not mean that it is more likely to occur in real life, just that it is better documented. For example, reports involving captive primates or primates that are habituated to people, through ecotourism, are likely to be overrepresented in the literature”.

It seems unlikely that long-lived, charismatic species are actually more susceptible to pathogens that infect humans; instead, these species are likely known to harbor human pathogens because they live more often alongside humans and are therefore more frequently exposed, or because they have simply been more intensively monitored and sampled. .

Conversely, there may be many cases of animals infected with human pathogens that have had few or no symptoms. These cases can go unnoticed and unreported. “The comparative lack of evidence for this pathway is interesting because reservoirs of human pathogens in wildlife pose a problem for efforts to eliminate the disease“.

“It is a double-edged problem. How do we know when the spill threatens the conservation of species? When human diseases escape and harm existing animal species. But also, how could that then affect us and cause future pandemics? We found little evidence for the reservoir hypothesis. But just because we haven’t found a lot of data doesn’t mean it’s not happening.”