What is Reverse Zoonosis? | Is It Possible For Humans To Make Animals Sick?

Many of us know that animals or our pets can make us sick or pass certain illnesses to us. However, experts are now questioning if we humans can make our pets or animals sick.⁽¹⁾

Some of the common diseases passed from animals to humans are bird flu, swine flu along with toxoplasmosis, ringworm, hookworm, roundworm, rabies and beaver fever. A lot has been said and described about the transmission of diseases from pets to humans, but a lot has to be understood about humans passing diseases to their pets.

In this article, we will discuss about the possibility of transmission of pathogens from humans to animals, which is known as anthroponosis or reverse zoonosis.

A review done in 2014 discovered many cases of parasites, viruses, bacteria and fungi transmitting from humans to animals across 56 countries.⁽⁴⁾

The passing of diseases from humans to animals is a new developing concern. Reverse zoonosis is an important worldwide issue along with it being an interesting notion too. Animals which are bred for the purpose of food are carried or moved far and wide, mingling with wild species which normally wouldn’t happen. With an increase in the movement of both human and animals along with the swift growth in animal production; the result of a pathogen from human residing in an animal can likely move thousands of miles within 24 hours.

Along with the increasing animal trade, there is also the growing pet business. About 68% of Americans owned a pet between the years 2015 and 2016, which was a total increase of 56% since 1988.⁽⁵⁾ There is more intermingling of humans, animals and diseases than before.

In order for our survival as species and for success of the human food chain in the future, one needs to have a better understanding of the manner of transference of diseases in all types of scenarios. Even though there are many protocols and guidelines to supervise and to assess the increased movement of animals all over the world, there is no denying the huge scale this issue has become. Along with the legal markets, farms, aquariums and zoos, one also has to deal with the illegal meat trade which can have a significant effect on the process of reverse zoonosis.

The concept of reverse zoonosis shouldn’t be that much of a surprise as there is an estimated 61.6% human pathogens that are considered to be pathogens to a range of species with the ability to infect many animals.⁽⁶⁾ About 77% of pathogens which infect livestock belong to the category of multiple species pathogens.

With changing times, there has been increased interest in the field of reverse zoonosis with one of the earliest studies conducted in 1988 regarding reverse zoonosis looking at dermatophytes, which are fungi (including trichophyton and microsporum) responsible for causing superficial infections of the hair, nails and skin.⁽⁷⁾ It was found that these fungi could be transmitted from human to animal; animal to animal; animal to human and also human to human.

The focus was shifted from fungal reverse zoonosis to bacteria in the mid-1990s, such as Mycobacterium tuberculosis and Methicillin-Resistant Staphylococcus aureus (MRSA).

The interest in viruses regarding reverse zoonosis developed in the late 1990s, during the 2009 pandemic of H1N1 swine flu. Studies began to emerge from 2000 onwards regarding the ability of certain parasites that can get transmitted from human beings to pets or animals including Cryptosporidium parvum and Giardia duodenalis.

Given below are some of the pathogens that have been known to pass between humans and animals.

Humans can transmit tuberculosis across the species barrier. A paper that was published in 2004 reported an instance of three-year-old Yorkshire terrier suffering from vomiting, anorexia and a persistent cough presenting at the University Of Tennessee College Of Veterinary Medicine. After doing a battery of tests including an eventual postmortem, it was concluded that the dog has contracted tuberculosis from its owner who was undergoing TB treatment for about six months. This was the first ever verified transmission of tuberculosis from human to a canine.

Among other animals, cats are also susceptible to tuberculosis; however, they commonly get cattle tuberculosis and rarely the variation of disease which is carried by the birds. Other than dogs, there have been various documented cases of elephants getting tuberculosis from humans.

MRSA is also referred to as “superbug” because of its stubborn resistance to antibiotics and the infections caused by this pathogen are notoriously difficult to treat and are mostly fatal.⁽¹⁰⁾ MRSA remains a significant public health concern even though its cases in America seem to be declining.⁽¹¹⁾

A study in 2006 which assessed MRSA in pets and the transmission of this from humans to animals concluded that there is a great likelihood of transmission of MRSA between humans and animals; and animals to humans too. MRSA seems to be an emerging zoonotic and veterinary pathogen.⁽¹²⁾

This article describes a particular case where a couple was getting repeated infected of MRSA and these re-infections stopped after their dog was discovered as the source of the MRSA and was treated. It was also thought that the dog got initially infected from this couple and then transmitted the infection back to its owners time and again after their successful treatment.

MRSA is already an extremely difficult infection to treat and is often fatal. So, it is a great concern if animals or pets can not only contract this pathogen, but become a source and transmit this pathogen back to humans.

Among all the animals, chimpanzees and gorillas seem to be the most susceptible to human illness because of the resemblance in their genetic and physiological constitution. Chimpanzees and gorillas are thought to be prone to various human diseases, including influenza, pneumonia, measles, variety of parasites, bacteria and viruses.

Due to habitat loss, poaching, bushmeat hunting, zoos and wildlife parks, we human beings come in frequent contact with the primates. Due to this, the cross-species transmission of diseases is a genuine and serious concern.

In the years 2003, 2005 and 2006; there were occurrences of fatal respiratory disease in the wild chimpanzees in Tanzania at the Mahale Mountains National Park. The cause was thought to be influenza and measles, but there was no concrete proof to support those as the cause. According to research where the stool samples from the non-affected and affected individuals was analyzed, it was found that the cause was a human-related metapneumovirus responsible for causing upper respiratory infection.

Something of similar happened in the year 2009, where there was an outbreak of human metapneumovirus infection in Chicago that got transmitted from the infected zookeepers to some captive chimpanzees, all of which became sick with one dying.

The first documented case of fatal transmission from human-to-cat of the H1N1 flu virus occurred in 2009 in Oregon. The cat owner suffered from acute influenza and had to be hospitalized. The pet cat, which had no exposure to animals or other people, later suffered from pneumonia due to H1N1 infection and died.

There were more than 13 cats and one dog recognized in the years 2011 and 2012, in the pandemic of H1N1 infection, which seem to have some type of human contact. The symptoms of these animals also resembled the symptoms of human carriers, such as appetite loss, fast developing respiratory disease and death in some cases.

An endangered species of wild dog are the African painted dogs and to conserve them, a study in 2010 examined the parasites present in the feces of these dogs. Giardia duodenalis is a parasite, which dwells in small intestine and caused infection in about 62% of captive animals and 26% of wild animals.

It was thought that the African painted dogs were at an increased risk for extinction due to the human parasites. G. duodenalis is often found in domestic dogs and cats, but is not usually found in African painted dogs. Other than this, the parasite strain present in feces of the dogs belonged to a subtype, which is commonly related with humans. Symptoms of this disease consist of decreased appetite; nausea; abdominal discomfort and diarrhea.

It was concluded that the parasites had entered via human-dog interactions and from there it was transmitted from dog to dog thus creating a new threat to their already doubtful future.

The studies about reverse zoonosis are very less; however, it remains an urgent and important field of study. If the human pathogens can infect other species, and then these species further mingle with humans and travel vast distances, then it is a pandemic waiting to explode.

There is rapid mutations seen in the flu virus and by spreading and staying in various other species, the flu virus has the opportunity to change and mutate in different ways which it cannot do in human beings. As these changes in these pathogens continue, some can become increasingly deadly or less dangerous to humans. Whatever the result may be, more dedicated research and studies need to be done of reverse zoonosis.

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