Threat of Reverse zoonosis : “One Health” Approach to Animals & Human Wellbeing
Zoonosis, the transmission of an infectious disease from animals to humans, represents a major public health issue.While Reverse zoonosis, in which a disease transmits from humans to animals, also poses threats to animal health as well as public health, due to the potential for animal disease reservoirs to form.
Cross-species diseases
The animal origins of many infectious diseases are well characterized and can range from bacterial, viral, parasitic, or fungal in nature. However, for these diseases to cross the species barrier and persist in a new host population, certain conditions must be met.
Firstly, the contact between the original host and the potential new host must be sufficient for transmission events to occur (interspecific interactions). Secondly, there must be enough compatibility between the pathogen and recipient host to establish an infection and promote ongoing transmission (host-virus interactions). Finally, the network of the recipient species must be structured in such a way as to allow the infection to spread through the population (host-host interactions).
Much of the scientific focus of infectious diseases that cross-species barriers are those that jump from animals to humans. In fact, almost 62% of pathogens affecting humans are estimated to have multi-species infectivity. Because of this, humans also carry the potential to transmit diseases to animals with which they come into close contact.
A recent example of this is reports of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of COVID-19, being detected in domesticated and farmed animals.
From humans to animals
Reverse zoonosis can have a severe impact on the health of the animals affected. In 2020, several mink farms in the Netherlands reported cases of severe respiratory disease. Analysis of those minks that had passed away identified SARS-CoV-2 as the causative pathogen, which was likely introduced into the population after contact with infected farmworkers.
Humans are also able to pass on diseases to their pets. There have been several documented cases of domesticated dogs, cats, and ferrets displaying flu-like symptoms, which were identified as being caused by an influenza A virus that had likely been transmitted from the pet’s owners. Some of these animals, particularly the cats and ferrets, progressed to severe respiratory distress and even death.
The threat that human-derived pathogens pose to endangered species is also an important topic. In Tanzania, an outbreak of human metapneumovirus led to fatalities in a wild chimpanzee population. The virus was believed to have been transmitted by researchers and visitors to the national park where the chimpanzees resided.
Another potential human-derived emerging disease in an endangered species is caused by the parasite Giardia duodenalis. This parasite has been found in the stool samples of both wild and captive African painted dogs and is thought to have derived from the open defecation of infected humans contaminating the local area.
Reverse zoonosis and disease reservoirs
A major concern with reverse zoonosis events is the potential for disease reservoirs to form, whereby reintroduction of pathogens into human populations could occur. This can be demonstrated by the incidents at the mink farms in the Netherlands. After the initial reverse zoonotic transmission of COVID-19, the disease then spread mink-to-mink before spilling back over into the human farmworkers.
One of the best-studied reverse zoonoses is that of human-to-pig transmission of influenza A viruses. Farmed pigs are considered an important reservoir and a potential source of an emerging influenza pandemic, due to the fact that they can contract both human and avian influenza viruses. Because of this, there is the possibility for the genetic exchange of viral segments (reassortment) to occur, whereby a new viral strain emerges.
In 2009 there was a global outbreak of an H1N1 influenza A virus subtype (pH1N1), dubbed the swine flu pandemic. Although originating in Mexico due to a pig-to-human zoonosis event, a large-scale reverse zoonosis of pH1N1 back into pigs then occurred globally.
As well as being a concern for swine herd health, the sustained pig-to-pig transmission of the human-origin pH1N1 created the potential for disease reservoirs or genetic reassortment to occur. This was born out in the United States of America when the reassortment of pH1N1 with a human seasonal H3N2 influenza A virus led to sporadic outbreaks of the novel H3N2v influenza from 2011 onwards.
The importance of reverse zoonoses
Reverse zoonosis is not just an interesting concept; it is an important global issue. Animals bred for food are transported far and wide, interacting with wild species that they would never naturally have encountered. With a rapid growth in animal production and an increase in the movement of both animals and people, a human pathogen within an animal could potentially move thousands of miles in just 24 hours.
For instance, during the H1N1 influenza pandemic of 2009, the virus was able to travel the breadth of the planet and from pigs to humans in a matter of months.
On top of the increasing animal trade, we have an ever-growing pet industry. An estimated 68 percent of people in the United States owned a pet in 2015 and 2016, up from 56 percent in 1988. Humans, animals, and disease are more entwined than ever.
Understanding how diseases work across all scenarios is essential for the future success of the human food chain and our survival as a species.
Although guidelines, protocols, and legislation attempt to keep on top of the increased movement of animals across the planet, the size of the issue is immense. Above and beyond legal farms and markets, zoos and aquariums, there is also an illegal meat trade that has the potential to affect the situation significantly. For instance, some estimateTrusted Source that 5 tons of illegal bushmeat move through Paris’ Roissy-Charles de Gaulle airport every week in personal luggage.
Early research into human to animal pathogens
The fact that diseases can pass from humans to animals is, perhaps, not such a surprise. An estimated 61.6 percentTrusted Source of human pathogens are regarded as multiple species pathogens and are able to infect a range of animals. Also, over 77 percent of pathogens that infect livestock are multiple species pathogens.
Although investigating these interactions is not a new endeavor, interest in the field has grown and developed over recent years. One of the earliest studies demonstrating reverse zoonosis was conducted in 1988 and looked at dermatophytesTrusted Source – fungi that cause superficial infections of the skin, nails, and hair – including Microsporum and Trichophyton. The authors found that these fungi could be transmitted from animal to animal, human to human, animal to human, and human to animal.
In the mid-1990s, focus moved from fungal reverse zoonoses to bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and Mycobacterium tuberculosis.
In the late 1990s, interest in viruses picked up, peaking during the 2009 H1N1 swine flu pandemic. From 2000, studies began to emerge investigating the ability of certain parasites to pass from human to animal, including Giardia duodenalis (the parasite responsible of giardiasis) and Cryptosporidium parvum (a microscopic parasite that causes the diarrheal disease cryptosporidiosis).
Below, we outline a selection of pathogens that have been observed jumping the gap between human and animal.
MRSA transferred from humans to their pets
MRSA is sometimes called a “superbug” because of its resilience to antibiotics. Infections caused by MRSA are notoriously difficult to treat and have the potential to be fatal.
Although cases of MRSA in the U.S. appear to be decliningTrusted Source, it is still a significant public health concern.
A study, published in the journal Veterinary Microbiology in 2006, looked at MRSA in pets and its transmission between humans and animals. They concludedTrusted Source that:
“Transmission of MRSA between humans and animals, in both directions, was suspected. MRSA appears to be an emerging veterinary and zoonotic pathogen.”
The paper mentions a specific case in which a couple was repeatedly infected with MRSA. The re-infections only stopped once their dog was identified as the source and treated. It is presumed that the dog was initially infected by the couple and then passed the infection back to them each time they had been successfully treated.
With the inherent difficulties of treating MRSA, it is a genuine concern if animals – and particularly pets – are able to contract and transmit the pathogen. As the authors write: “The emergence of MRSA in household pets is of concern in terms of animal health, and perhaps more importantly, the potential for animals to act as sources of infection or colonization of human contacts.”
Tuberculosis in a Yorkshire terrier
A paper, published in 2004, describes the case of a 3-year-old Yorkshire terrier who arrived at the University of Tennessee College of Veterinary Medicine with anorexia, vomiting, and a persistent cough.
After running a barrage of tests – including, sadly, an eventual postmortem – the authors concluded that it had contracted tuberculosis (TB) (Mycobacterium tuberculosis). The dog’s owner had been receiving treatment for TB for 6 months. This was the first documented transmission of TB from human to canine.
Cats are also susceptible to TB, but they most commonly catch cattle TB (M. bovis) or, more rarely, a version of the disease carried by birds (M. avium).
Dogs are not the only animals that can be affected by humanborne TB. There have been a number of documented cases of elephants contracting TB from humans, including threeTrusted Source from an exotic animal farm in Illinois.
Cats catching flu from humans
In 2009, the first recorded case of fatal human-to-cat transmission of the H1N1 flu virus occurred in Oregon. The owner of the cat had a severe case of influenza and had to be taken to the hospital. Her cat – an indoor cat with no exposure to other people or animals – later died of pneumonia caused by an H1N1 infection. Details of the case were published in the journal Veterinary PathologyTrusted Source.
In 2011 and 2012, researchers identified more than 13 cats and one dog with pandemic H1N1 infection that appeared to have come from human contact. Interestingly, the animals’ symptoms were similar to those experienced by human carriers – rapidly developing respiratory disease, a lack of appetite and, in some cases, death.
Fatal respiratory illnesses in chimpanzees
Of all the animals, gorillas and chimpanzees are perhaps most susceptible to human ailments, thanks to their similar genetic and physiological makeup. They are known to be vulnerable to a number of human diseases, including measles, pneumonia, influenza, a range of viruses, bacteria, and parasites.
Due to poaching, habitat loss, wildlife parks, zoos, and bushmeat hunting, humans more frequently come into close proximity with primates. Because of this, cross-species transmission of diseases is becoming a pressing concern.
In 2003, 2005, and 2006, outbreaks of fatal respiratory disease struck the wild chimpanzees at the Mahale Mountains National Park in Tanzania. Although measles and influenza were both considered, no evidence to support them as the cause could be found.
Researchers analyzed stool samples from affected and nonaffected individuals, and they identified that a human-related metapneumovirus – a virus that causes an upper respiratory infection – was to blame.
This dwindling population of chimpanzees was being decimated by a cold transferred to them by humans.
Similarly, in 2009, an outbreak of human metapneumovirus infection in Chicago, IL, spread from infected zookeepers to a group of captive chimpanzees. All seven became ill, and one died as a resultTrusted Source.
African painted dogs
African painted dogs are an endangered species of wild dog. As part of the conservation effort, a studyTrusted Source published in 2010 investigated the parasites present in the species’ feces.
Infection by Giardia duodenalis, a parasite that lives in the small intestine, was found in 26 percent of wild animals and 62 percent of captive animals.
Although common in domestic cats and dogs, G. duodenalis is not a parasite naturally found in African painted dogs. Additionally, the strains of parasite found in the dogs’ feces were of a subtype commonly associated with humans, rather than the subtypes usually seen in pet dogs.
Symptoms of the disease can include diarrhea, nausea, abdominal discomfort, and reduced appetite.
The authors concluded that the parasites had entered the population from human-dog interactions and, from then on, were passed from dog to dog, becoming a new potential threat to their already uncertain future.
Although research into reverse zoonosis is relatively scant, it is an important and urgent field of study. If human pathogens are able to infect other species, and these species are able to interact with humans and travel great distances, it is a pandemic waiting in the wings.
We already know that the flu virus can mutate quickly, and by living in different species, it has the chance to change and mutate in ways that it could not in humans. As these pathogens change, they might become less dangerous to humans. On the other side of the coin, however, some might become increasingly deadly.
The Indian perspective
The Indian Council of Medical Research (ICMR) is planning to launch an extensive study to check the prevalence of bovine tuberculosis (TB) in household cattle across the country after it found evidence of human-to-cattle (reverse zoonosis) TB transmission in farm animals in southern India — an alarming development, which could potentially adversely impact India’s goal of eliminating TB by 2025, as it poses a risk of cross infection. ICMR’s Chennai-based National Institute for Research in Tuberculosis (NIRT) submitted the research proposal to Union ministry of health and family welfare in February this year to assess the magnitude of the spread. To screen animals on a large scale for the study, the institute has also sought funding from the National Institutes of Health in the United States.
“This is the first time we have seen TB strains in cattle that are usually found in humans, such as mycobacterium tuberculosis. Their handlers were also tested and the same TB strain was found in them. On the face of it, there is strong evidence of human-to-animal transmission as these strains are not usually found in animals. But we need to conduct a larger study to understand the magnitude of the problem as these samples were lifted from four farms in Tamil Nadu…now we need to screen household animals to see if the disease is also found in that subset, and at different sites across the country, to get a pan-India picture,” said Dr P Kannan, senior scientist at NIRT who studies zoonotic diseases.
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.
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.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
Image-Courtesy-Google
Reference-On Request.
