Reverse zoonoses: from humans to animals

There are more than 827,000 viruses in the animal kingdom that have the potential to infect humans. This figure is all the more alarming given that just 13 zoonoses are responsible for 2.2 million deaths each year. It’s so staggering that we might almost forget that the reverse can also happen. While humans fear diseases transmitted from animals, animals can also contract infections from humans. Transmission works both ways. This is referred to as reverse zoonosis or zooanthroponosis. Humans can transmit 21 bacteria, 12 viruses, and 7 fungi to animals. Wild animals are the most affected, followed by livestock and domestic animals. For example, hepatitis A can infect non-human primates, the herpes virus can affect tamarins and marmosets, and Salmonella can affect farm animals and pets. The most recent example involves COVID-19, which spread from bats to pangolins and then to humans, after which it reportedly infected cats, among other animals. This represents a transmission of the disease from humans to animals. Although relevant on a global scale, reverse zoonoses are of particular interest to Indian scientists. Indeed, the proximity of humans to wildlife in India is exacerbated by growing economic and political inequalities, habitat fragmentation, and environmental degradation, coupled with a very high concentration of livestock. Humans, livestock, and wildlife therefore often interact with one another. And these interactions come at a cost. Livestock-raising and grazing practices, along with environmental conditions in India, increase the likelihood of disease spread in both humans and animals—whether viral, bacterial, or fungal. For example, 15 to 28 percent of cattle tested in a 2005 study were found to be infected with M. tuberculosis—likely of human origin and often fatal. And while the infection spreads to animals living in close proximity to humans, it also affects wildlife. Elephants are also believed to be susceptible to M. tuberculosis and prone to mycobacterial infections transmitted by humans. In 2018 and 2014, infections in children and cows suffering from diarrhea showed similarities that, based on genetic studies, were confirmed to be genuine. The infection in cows is of human origin. And this type of event is far from unique; there is no shortage of evidence. Following the 2009 swine flu pandemic, a study highlighted the transmission of the virus from humans to pigs. In 2012, another study also examined the transmission of influenza virus subtype A from humans to pets. Even fungal zoonoses are not unheard of. In 2009, researchers identified Microsporum gypseum, a pathogenic fungus that had been transmitted to a dog by its owner. There are therefore numerous cases of reverse zoonosis, particularly in India. And reducing contact between humans and animals through environmental conservation is key to reducing the risk of disease transmission. We must rethink our interaction with nature, as well as the movement of goods, livestock, and people, in a context where globalization must now take into account zoonotic and zooanthroponotic events with pandemic potential.