Reverse Zoonoses

Yes, animals can give us diseases. But it works both ways.

Could you make your pet sick?


Portrait of female rag doll cat , Chloe, relaxing on a couch whilst her owner, checks her phone. Brighton, Victoria, Australia. April 2020. Model released.





With COVID-19 dominating headlines around the world, concerns about zoonoses (infectious diseases that jump from animals to humans) have never been higher. And rightly so. It’s only natural that we would want to defend ourselves against potential threats.

But it’s also necessary to ask the opposite question: can humans make animals sick? As it turns out, the answer is a resounding yes. In fact, an estimated 61.6 percent of human pathogens are capable of infecting other non-human hosts. However, these so-called reverse zoonoses – and the ways they impact wildlife – seldom make the headlines.




There are several known examples of diseases being transmitted from humans to animals.

For instance, in 2010, it was reported that African painted dogs had been infected with human strains of Giardia duodenalis (a gut parasite). Researchers concluded the pathogen entered the dog population through open defecation by tourists and locals in and around national parks. From then on, the infection spread dog to dog, becoming a new potential threat to an already endangered species. Then, in 2017, a research paper suggested that genetic transmission from human Giardia to captive Tasmanian devils could also be occurring. More recently, there have been documented cases of elephants and also a pet dog contracting tuberculosis from humans. And in 2019, evidence emerged that human bacteria Salmonella and Campylobacter were being spread to Antarctic seabirds – with potentially catastrophic consequences.

[BELOW] Human diseases have been documented in African wild dogs, Tasmanian devils and elephants, as well as Yorkshire terriers, Anatrctic seabirds and chimpanzees.

Of all animals, primates are perhaps the most vulnerable to human diseases. Because of the similarity of their genetic makeup, gorillas and chimpanzees are vulnerable to measles, pneumonia, influenza, and more. For example, in Tanzania, several fatal outbreaks of human metapneumovirus in wild chimpanzees have been attributed to researchers and tourists viewing the animals in the wild. A similar outbreak took place in a Chicago zoo, where infected keepers spread metapneumovirus to a group of captive chimpanzees. As a result, one of the apes died.


Antibiotic Resistance


One of the most concerning aspects of reverse zoonoses is that some human pathogens carry antibiotic resistance. In fact, this issue is considered to be one of the world’s most pressing health concerns. At the forefront of research into reverse zoonoses and antibiotic resistance is Professor Michelle Power from Macquarie University, Australia. Power is revealing just how far antibiotic resistant bacteria have spread into Australia’s wildlife.

‘There is clear evidence that shows the closer our wildlife gets to people, the more likely they are to carry antibiotic resistant bacteria,’ Power says. ‘Yet another form of pollution—microbial pollution—impacting our wildlife.’

[BELOW] Associate Professor Michelle Power from Macquarie University, Australia, plates out a culture of E.coli taken from faecal samples of sea lions. Power’s research revealed that bacteria from humans are making their way into wildlife, even in places such as the Antarctic wilderness.

Power’s research focuses on animals that live in close proximity to humans, as well as those in wildlife care or rehabilitation facilities. So far, she has found antibiotic resistant bacteria in everything from little blue penguins and sealions, to possums, flying foxes and wallabies. While many wildlife species in Australia would not usually require antibiotics, more and more are coming into care, particularly in the wake of worsening heat waves and bushfires. Consequently, if a patient carries antibiotic resistant bacteria, it can leave carers without effective treatments.

‘This is a critical issue for koalas, where Chlamydia is decimating their numbers,’ Power says. ‘The emergence of antibiotic resistant Chlamydia would compound the human impacts to this iconic species. Let’s hope that does not happen’

[BELOW] Researches prepare to take a faecal sample from a Little Blue Penguin in St. Kilda, Australia. An estimated 3% of the wild population carry antibiotic-resistant bacteria. Such pathogens are also present in grey-headed flying foxes and brushtail possums. Both species spend time with humans in animal care facilities.

There is also the risk of drug-resistant pathogens spreading throughout the population, and potentially back to humans again. For example, one study published in 2016 found evidence for the transmission of MRSA (a superbug notoriously resistant to antibiotics) between humans and household pets, and vice versa. What’s more, gut health can impact things like an animal’s immune development, protection from other diseases, cognitive ability and behaviour. Which begs the question: do these proverbial ‘weeds in the garden of the gut’ affect the host’s chances of survival?




For good reason, medical researchers are now concerned about COVID-19 spilling back from humans into wildlife. Could we infect bats? Or a beloved family pet? What about farmed animals? Not only could those infected animals become ill (or even die), the population of animals in question could become a virus reservoir, from which reintroduction into humans can take place.

Already, there are several reports of reverse zoonosis events associated with the ongoing COVID-19 pandemic. So far, they involve domestic cats and dogs, tigers, lions, and mink. As a result, Denmark culled all farmed mink in the country – some 17 million animals. The mink first caught COVID-19 from farm workers. But subsequently, the virus spilled back from the animals into humans. In other words: a reverse-reverse zoonosis event. What’s more, during its passage through the mink, the virus had accumulated mutations in the spike protein gene (the part targeted by vaccines). The example of Denmark is therefore a stark warning. Could a variant that mutates in an animal host undermine our progress in the fight against COVID-19?

[ABOVE] A Pomeranian in Hong Kong was one of the first dogs to test positive for the coronavirus, and a female Siamese cat became the first animal in the UK to catch COVID-19. Meanwhile, tigers at the Bronx Zoo and lions at Barcelona Zoo have also tested positive, as well as farmed mink in several countries.

[BELOW] Bats are widespread in urban areas and come into close contact with domestic animals and humans. They are natural reservoirs for several pathogens which can cause human disease. However, it can also work the other way around. Free-ranging bats are a primary concern for spillover of COVID-19 from humans back to wildlife. Scientists are examining the risk of humans inadvertently passing the infection into bat colonies.



The spread of human pathogens could have potentially catastrophic impacts on highly sensitive animal populations. A species which is exposed to a novel human germ could subsequently suffer local collapse or even extinction. Moreover, infected animals could pass the disease back to humans.

Nevertheless, human-animal contact is escalating, and with it the threat of disease transmission. That’s partly due to our increasing tendency to exploit natural habitats. We deforest, we mine, we build roads—therefore reducing natural barriers between animals and humans. But we also use animals for food, sport, entertainment and companionship, among other things. If human pathogens are able to infect other species, and these species are able to interact with humans and travel great distances, then it’s a pandemic waiting in the wings.

[BELOW] Rapid urbanisation and population growth push people and wildlife closer together, creating opportunities for disease transmission.


The global export of animals for food means that a human pathogen within an animal could potentially move thousands of miles in just 24 hours. For example, during the H1N1 influenza pandemic of 2009, the novel virus was able to travel across the globe and from humans to pigs in less than two months. What’s more, busy animal markets create the ideal conditions for emerging diseases.

Clearly, reverse zoonoses require more attention. However, the research remains thin on the ground. Many common and dangerous pathogens are unstudied as reverse zoonotic threats. Moreover, by living in different species, there is a risk they could mutate in ways that they would not in humans. In the process, they may become less dangerous. Or, perhaps, they would become more deadly.

As we continue to push back against COVID-19, we should therefore be asking ourselves two questions. Firstly, how we can avoid infections from wildlife? And secondly, how can we prevent susceptible wildlife from getting infections from us?