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Reverse Zoonosis – When Humans Pass Diseases on to Animals

Reverse Zoonosis – When Humans Pass Diseases on to Animals

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The media is overflowing with news on zoonosis, the medical term for diseases that are transmitted from animals to humans. Debates are ongoing about which wild animal bears ultimate responsibility for the current pandemic. However, we often forget that humans also transmit diseases to animals. In a global survey, whose results were published in 2014, humans were found to be able to transmit 21 bacterial, 12 viral and seven fungal pathogens to animals. Wild animals are the most affected, followed by livestock and pets.

For example, the hepatitis A virus is known to cause hepatitis in non-human primates. The herpes viruses may infect tamarins and marmosets. Salmonella bacteria could infect farm and pet animals, and the mumps virus is known to infect dogs. Infectious disease spillover between animals and humans is a two-way street – but few scientists focus on disease transmission from humans to animals, a phenomenon called reverse zoonosis or anthropozoonosis. A recent example of reverse zoonosis is the possible transmission of the novel coronavirus from humans to cats, both wild and domestic.

While it is globally relevant, reverse zoonosis holds particular interest for Indian scientists, given the proximity of humans and wildlife across India’s diverse landscapes.

The Indian perspective

In the 21st century, India is one of the world’s most rapidly evolving countries. Its economist growth poses multiple challenges for biodiversity conservation. Aided by increasing economic inequality (which forces the less well-off to live in more crowded areas), habitat fragmentation and environmental degradation, India’s policies and activities have increasingly downplayed natural barriers between humans and the ‘wild’.

In India is home to 17% of the world’s livestock. Its protected areas network covers about 5% of its land area, and human settlements often populate the buffer zones of these regions. These three realities together mean humans, livestock and wildlife often interact with each other. Other modes of spillover events can be found in the pet trade and livestock farming, both of which have only been becoming more popular.

Livestock rearing and grazing practices increase the probability of disease spillover from both humans and non-human animals. A 2015 study by the World Wildlife Fund for Nature India noted that livestock in the Anaimalai Hills had transmitted foot-and-mouth disease to wild Nilgiri tahrs.

A 2005 study conducted in Meerut documented infections of Mycobacterium tuberculosis and Mycobacterium bovis passed between humans and cattle. In this study, 15-28% of the 29 animals tested were found to be infected with M. tuberculosis, likely from humans. In 2011, a study in Himachal Pradesh found evidence that M. tuberculosis infections had been responsible for the death of several cattle. Another study published three years later corroborated these findings.

In the forested Muthanga range of Wayanad Wildlife Sanctuary, wild elephants live close to human settlements. A study in this range in 2017 found that three wild elephants had died of M. tuberculosis infections transmitted from humans. According to the researchers, captive elephants are actually prone to human-transmitted mycobacterial infections.

Even fungal anthropozoonoses are not unheard of. In 2009, researchers reported that Microsporum gypseum, a pathogenic fungus that lives in soil and, in the bodies of mammals, affects their inner skin, had been passed on to a pet dog by its owner.

Scientists in India studied rotavirus infections in Vellore in 2014 and Haryana in 2018, and found similarities in the infection type seen in children and cows affected with diarrhoea. Based on further genetic studies, they were able to conclude that the infection in cows was of human origin. Following the 2009 swine flu pandemic, a study conducted by researchers in Uttar Pradesh found evidence of the virus having passed from humans to pigs. In 2012, a study also discussed the transmission of influenza virus subtype A from humans to companion animals.

Such specific cases require the attention of medical and veterinary health professionals before we can implement measures to prevent future spillovers.

Also read: Why You Should Care About the ‘One Health’ Approach to Human Wellbeing

The elephant in the room

It seems likely that there are many cases of reverse zoonosis to be found in India, but we need more research on this topic, and more researchers to go looking for them. Living in close quarters threatens the wellbeing of humans and animals alike, plus given the importance of livestock farming and the domestic pet trade, we need details of the specific ways in which reverse zoonoses can threaten animals. Another lesser-known threat is the case of reverse zoonosis from humans to invasive species.

There is enough evidence that habitat conservation is a key component of reducing interactions between humans and wildlife, thus reducing the risk of disease transmission  as well. Studies have recommended that villages within the buffer zones of protected areas be relocated to reduce the stress on forests and wildlife, and reduce negative interactions.

An expanding body of research has also been finding signatures of ‘pathogen pollution’ – the introduction of a disease to a new area – due to unethical tourism in protected areas.

One 2010 study found that a strain of Giardia duodenalis, a parasite that causes a diarrhoeal infection called giardiasis, had infected an endangered African painted dog population in South Africa. The strain was of human origin, and  the researchers who conducted the study suspected tourists and local residents who had defecated in the open to be the source. Even Antarctica hasn’t been spared. One report, published in 2018, discussed the discovery of the traces of human pathogen strains in birds on the icy continent, and speculated that depending on the pathogens’ deadliness, endemic species could be rendered extinct.

The exploitation of forest resources and animal products, from the wild-meat trade and poaching to mass-farming of livestock for consumption, can increase the risk of contracting diseases from animals. In exposing ourselves to these diseases, we also open doors to transmitting diseases back.

In sum, the current pandemic has given us ample opportunities to rethink the ways in which we interact with nature, especially in India. And the mass movement of goods, livestock, food supplies and people assisted by globalisation and opportunities in trade, employment and investment could create ideal conditions for more reverse zoonoses in the years to come.

The views expressed here are the authors’ own and don’t represent those of the organisations the authors are affiliated with.

Priya Ranganathan is a wetland ecologist and geologist. Nobin Raja works at the Ashoka Trust for Research in Ecology and the Environment, Bengaluru.

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