Swine and bird flu are two of the most recent and startling examples of animals passing diseases to humans.
Although these animal-to-human transmissions are relatively well described, pathogenic traffic in the opposite direction is much less well understood.
In this Spotlight feature, we will investigate whether pathogens can travel from humans to animals in a process referred to as reverse zoonosis, or anthroponosis.
A review of current literature on this topic, published in PLOS One in 2014, identified a wealth of examples. They found cases of bacteria, viruses, parasites, and fungi jumping from human hosts to animal-kind to occur across 56 countries on every continent with the exception of Antarctica.
The importance of reverse zoonoses
The transmission of diseases from humans to animals is a growing area of concern.
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 estimate 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 percent 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 dermatophytes - 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 declining, 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 concluded 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
Humans are capable of spreading TB across the species barrier.
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 three 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 Pathology.
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 result.
African painted dogs
African painted dogs are an endangered species of wild dog. As part of the conservation effort, a study 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.
It is thought that human parasites put African painted dogs at an increased risk of extinction.
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.