Reverse Zoonosis: Can You Make Your Pet Sick?

For good reason, there is a great deal of interest in the transmission of diseases from animals to humans. Recently, however, medical researchers have started to ask the opposite question: can we make animals sick?
[Kitten being checked by doctor]
The transmission of diseases from humans to animals is a growing area of concern.

Swine and bird flu are two of the most recent and startling examples of animals passing diseases to humans.

Other unpleasant pet-to-human medical problems include ringworm, roundworm, and hookworm, as well as beaver fever, toxoplasmosis, and rabies.

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

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

[Yorkshire terrier panting]
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 human borne 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.

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.

Cat Scratch Fever: Everything You and Your Cat Need to Know

Cat scratch fever occurs when a person is bitten, scratched, or licked by a cat infected with the bacteria Bartonella henselae.

The infection doesn’t usually cause severe complications. However, it’s possible that it can in people with weak immune systems. Knowing the causes and symptoms can ensure a person receives swift treatment.

Cats can transmit several types of infections to humans. Some of these diseases can be severe. Carrying out routine care for a cat often reduces the risk of many of these diseases.

Causes of cat scratch fever

A person can get cat scratch fever if they are scratched or bitten by an infected cat. The B. henselae bacteria live in a cat’s saliva, and can also be passed to a person through an open area of skin.

People are most likely to experience cat scratch fever in the fall and winter when they’re inside and play with their cats. Kids are more likely than adults to have the condition. They can play with cats more roughly, making them more likely to be scratched.

Symptoms of cat scratch fever

Cat scratch fever doesn’t usually cause symptoms in the first few days after a person is exposed. During this time, the bacteria are multiplying in the body.

About 3 to 10 days after a person is scratched, they may notice a small bump or blister on the affected area. Doctors call this an inoculation lesion. These lesions are commonly seen on the:

A cat has scratched a hand.
Cat scratch fever symptoms appear a few days after the bite, lick, or scratch has happened.
  • Arms
  • Hands
  • Head
  • Scalp

A few weeks later, a person will usually see the lymph nodes near the lesion swollen or tender.

Lymph nodes are responsible for filtering bacteria and other particles as well as creating immune system cells. They usually feel like small, spongy, round or oval bumps.

If a person was bitten or scratched on the arm, the lymph nodes under the arm or near the elbow may be especially tender.

Sometimes, the lymph nodes swell as much as 2 inches across. They may be warm to the touch, pus-filled, or red in color. The lymph nodes may remain swollen for anywhere from 2 to 4 months after the initial infection.

Most people only have swollen lymph nodes as a symptom. Other symptoms associated with cat scratch fever include:

  • Abdominal pain
  • Loss of appetite
  • Fever, typically no higher than 101°F
  • Tiredness
  • Headache
  • Joint pain
  • Rash
  • Sore throat

Complications of cat scratch fever

Cat scratch fever doesn’t usually cause severe symptoms. However, some people may develop a high fever that doesn’t seem to go away with time.

Some people can also experience infections in the bones, joints, liver, lungs, or spleen. According to the Centers for Disease Control and Prevention (CDC), the most severe symptoms usually occur in children ages 5 and under.

While cat scratch fever isn’t a condition that usually requires emergency care, there are always exceptions. A person should contact their doctor immediately if they experience the following symptoms:

  • A cat bite or scratch that is not healing or is getting worse
  • The red area around a bite or scratch is enlarging
  • A high fever that lasts more than 2 days after being bitten or scratched
  • High levels of pain

Diagnosing cat scratch fever

Cat scratch fever can be hard to diagnose as the symptoms are similar to a lot of other conditions. A doctor will ask about a person’s medical history and any interactions a person may have had a cat.

A doctor will then conduct a physical examination, looking at the scratched area and any swollen lymph nodes. Examination and medical history are often enough to make a diagnosis.

The doctor may order additional tests to make sure another condition isn’t causing the symptoms. They could take a sample of blood and send it to a lab to determine what type of bacteria is growing.

Doctors can also order a blood test that specifically tests for cat scratch fever.

Treatments for cat scratch fever

As most cases of cat scratch fever are mild, a doctor won’t always prescribe a treatment. If symptoms are moderate to severe, they may prescribe an antibiotic.

At-home treatments for the condition include bed rest and an over-the-counter pain reliever if the lymph nodes are painful or especially tender.

While children don’t have to stop playing, they should avoid hitting or interfering with the affected lymph nodes.

Once a person has had cat scratch fever once, they’re unlikely to get the disease again.

Preventing cat scratch fever

While cats can transmit cat scratch fever to people, people don’t usually pass it to others. If one family member is affected, others should practice caution around the family cat as the cat could infect them too.

An episode of cat scratch fever also doesn’t mean a family should necessarily get rid of their pet. However, they can practice the following preventive techniques:

A cat scratches itself.
Preventing a cat from getting fleas can help reduce the risk of cat scratch fever.
  • Adopting a cat that is older than 1 year if a person is at high risk for adverse symptoms of cat scratch fever (kittens are most likely to carry the disease)
  • Avoiding rough play around a cat or kitten
  • Never allowing a cat to lick wounds or open areas of skin
  • Never petting stray or feral cats
  • Washing hands and any other affected areas after playing with a cat
  • Vacuuming a home frequently to avoid fleas
  • Practicing flea prevention to reduce the risk a cat could get the infection
  • Contacting a pest control company if a lot of fleas have been identified in a home

Recognizing the condition in your cat

According to the CDC, an estimated 40 percent of cats carry the B. henselae infection at some point in their lives. Most of the time, cats that carry the infection don’t show signs of illness.


Cats get the infection when they scratch and bite at fleas that infect them or fight with cats that are infected. If a cat has fleas or visible scratches, these could be signs a person should practice caution when handling their cat. Once a cat is infected, it can carry the disease for several months.

In rare cases, cat scratch disease can cause severe symptoms in cats, including inflammation of the heart. Cats may have difficulty breathing due to this. Upon examination, a vet may also identify inflammation in the eyes, mouth, or urinary system.

Diagnosis and treatment

A vet can inspect a cat for fleas and make recommendations regarding flea prevention and avoiding scratches and bites.

While there is a blood and fluid test available for the Bartonella bacteria, doctors don’t usually recommend it for cats that don’t have symptoms. The bacteria are very common, and the test can be unreliable.

Cats aren’t usually treated with antibiotics unless they have noticeable symptoms.


Taking steps to reduce fleas in a cat can reduce the likelihood of cat scratch fever. People can care for their cats by doing the following:

  • Applying or administering a vet-approved flea treatment on a regular basis
  • Keeping a cat indoors to avoid contact with stray or infected animals
  • Keeping a cat’s nails trimmed and neat
  • Scheduling and maintaining regular check-ups with a vet

Vaccines aren’t currently available against cat scratch disease bacteria.

Other conditions cats can spread

Cats can carry and spread additional diseases besides cat scratch fever. These diseases include:

  • Campylobacteriosis: An intestinal infection caused by bacteria
  • Cryptosporidiosis: A parasite that causes diarrhea and abdominal cramping
  • Plague: This condition isn’t common in the United States, but can occur if a cat is taken to another country
  • Rabies: According to Seattle and King County Public Health, cats are the domestic animal most likely to experience a rabies infection
  • Ringworm: Kittens are especially likely to carry this disease that causes bald patches on the skin
  • Tapeworm: Most common in children, this infection occurs when a person swallows a flea from a cat that is infected with tapeworm larvae
  • Toxocara infection: While the condition doesn’t always cause symptoms, it can be associated with serious complications like blindness
  • Toxoplasmosis: Toxoplasmosis is of special concern to pregnant women because it can cause complications like miscarriage, affected fetal growth, and eye problems

Smoking Is Damaging Your Pet’s Health, Researchers Warn

If you made a New Year’s resolution to stop smoking and are already struggling to stick to it, a new study may offer a further incentive: quitting the habit can benefit your pet’s health as well as your own.
[A cat paw on an ashtray]
Pets in smoking households are at greater risk for weight gain, cell damage, and some cancers, according to researchers.

Smoking is the leading cause of preventable disease and death in the US, accounting for around 1 in 5 deaths annually.

According to the Centers for Disease Control and Prevention (CDC), smoking causes around 90% of all lung cancer deaths in men and women, and it is also a risk factor for heart disease, stroke, and numerous other illnesses.

But it is not only smokers themselves who are at risk of such conditions; since 1964, around 2.5 million non-smokers in the US have died from exposure to secondhand smoke.

With this in mind, it is perhaps unsurprising that pets living in households where someone smokes are at greater risk for poor health.

Previous research from Clare Knottenbelt, professor of small animal medicine and oncology at the University of Glasgow in the UK, and colleagues has shown that dogs living in a smoking household ingest a high amount of tobacco smoke.

For this latest study – which is ongoing – the team set out to investigate how tobacco smoke exposure impacts the health of cats and dogs.

Cats at greatest risk from smoke exposure

Prof. Knottenbelt and colleagues analyzed the nicotine levels in the animals’ fur and looked at whether such levels were associated with any health problems. Additionally, they assessed the testicles of dogs following castration in order to identify any signs of cell damage.

Compared with pets living in non-smoking households, the researchers found that those living in smoking households may be at greater risk of cell damage, some cancers and weight gain.

Cats are most at risk, according to the researchers, because they ingest more smoke than dogs – regardless of whether or not they have access to outdoors. The team speculates that this may be down to the extensive self-grooming cats engage in, causing them to ingest more tobacco toxins.

When analyzing the testicles of castrated dogs from smoking households, the researchers identified a gene that represents a sign of cell damage that is related to some cancers.

Furthermore, they found that dogs that lived in smoking households gained more weight after being neutered than dogs from non-smoking households.

Stopping smoking completely ‘best for pets’ health and well-being’

However, the researchers also found that these risks reduced when owners smoked outside, therefore reducing the amount of smoke their pets ingested.

While owners who reduced the number of cigarettes they smoked per day did reduce pets’ smoke exposure, it was not eliminated completely; cats from households that reduced their cigarette intake to less than 10 daily still had higher nicotine levels in their fur than those from non-smoking households.

The team suggests that pets may even be at greater risk of health problems from smoke exposure than children in smoking households, noting that because pets are lower in height, they are more likely to ingest third-hand smoke – that is, tobacco chemicals present in carpets and other surfaces.

While the research is ongoing, the team believes the results to date should act as a warning to smokers with pets. Prof. Knottenbelt says:

“As well as the risk to the smoker, there is the danger of secondhand smoke to others. Pet owners often do not think about the impact that smoking could have on their pets.

Whilst you can reduce the amount of smoke your pet is exposed to by smoking outdoors and by reducing the number of tobacco products smoked by the members of the household, stopping smoking completely is the best option for your pet’s future health and well-being.”

So, the next time you get the urge to light up and break that New Year’s resolution, just spare a thought for the health of your four-legged friend.

Sharing A Bed With Your Pet Could Help You Sleep

Do you allow your pet to snuggle up with you in bed? If not, you might want to reconsider; new research finds that, for most people, the presence of a pet in the bedroom could benefit sleep.
[A dog sleeping on its owners bed]
Letting your four-legged companion sleep on the bed could help you sleep, according to new research.

Lead author Dr. Lois Krahn, of Mayo Clinic’s Center for Sleep Medicine in Scottsdale, AZ, and colleagues publish their findings in Mayo Clinic Proceedings.

It goes without saying that the US is a nation of animal lovers; almost 65% of American households own a pet, the most common companions being dogs and cats.

There have been numerous studies hailing the benefits of pet ownership. A recent study, for example, found that children with pet dogs experience less stress.

But according to Dr. Krahn and colleagues, there is limited quality research on how the presence of a pet in the bedroom may impact an owner’s sleep.

Pets in the bedroom may offer sense of security, relaxation

To address this research gap, the team surveyed 150 patients at the Center for Sleep Medicine, of whom 74 reported owning at least one pet – mostly dogs and cats.

The researchers gathered various information, including whether they allowed their pet to sleep in the bedroom and on the bed, and whether their pet is disruptive to their sleep.

Around 56% of pet owners reported allowing their pets to sleep in the bedroom or the bed.

Disruptive behaviors – including wandering, whimpering and snoring – were reported by 20% of owners who allowed their pet to sleep close by.

However, 41% of owners said their pets were not disruptive, with some – particularly individuals who were single – saying their presence even helped them sleep by providing security, companionship or relaxation.

One woman described her two small dogs as “bed warmers,” while another woman described her cat as “soothing” when it slept on her bed. A single 64-year-old woman said she felt more content when her dog slept under the covers by her feet.

“The value of these experiences, although poorly understood, cannot be dismissed because sleep is dependent on a state of physical and mental relaxation,” say the authors.

These findings may help doctors counsel patients with sleep problems, according to the researchers:

“Health care professionals working with patients with sleep concerns should inquire about the home sleep environment, and companion animals specifically, to help them find solutions and optimize their sleep.”

The authors note some limitations to their study. For example, they did not gather data assessing whether individuals being treated for sleep disorders – such as sleep apnea – find a pet sleeping nearby beneficial or more disruptive.

Additionally, the team notes there may have been some response bias from pet owners. “Respondents appeared eager to disclose whether they owned a companion animal and where it slept but seemed more reluctant to reveal any undesirable consequences,” they explain. “This response bias may have resulted in these data underreporting the frequency of disrupted sleep.”

Still, the researchers conclude further research investigating how having pets in the bedroom impacts a person’s sleep is warranted.

Stress In Domestic Cats: New Review Discusses Causes And Management

Pet cats can suffer from stress triggered by a variety of events and situations, including conflicts with other cats and changes to routine. While cats can adapt, sometimes the stress can be too much, with negative effects on their health.
cat hiding
When stressed, cats may stop exploring and hide away for long periods of time.
Image credit: Marta Amat, Autonomous University of Barcelona

Writing in the Journal of Feline Medicine and Surgery, a group of veterinarians from the Autonomous University of Barcelona, Spain, explains that stress can impoverish the health of pet cats and increase their risk of disease.

The authors say that stress in pet cats can lead to behavior changes that are so difficult to manage that owners end up relinquishing them or having them euthanized.

In their paper, they discuss the causes and effects of stress in pet cats and strategies on how to prevent and reduce it.

Some of the main causes of stress that they discuss include changes to the pets’ environment, a barren environment, poor relationships with humans, conflicts with other cats and lack of control and predictability.

The authors note that other new changes – such as the arrival of a new member of the household, or a change in the daily routine – may also be stressful for the family’s feline pet.

Effects of stress in cats

In cats, stress distorts normal behavior – leading to reduction or excess of it. Generally, stress causes a domestic cat to become less active and playful and engage in markedly fewer positive interactions with other cats and humans.

The authors note that stress can also cause pet cats to eat less – or more, in some circumstances – than usual.

Stress can also trigger compulsive behavior in domestic cats, such as over-grooming, to the point where the animal loses its fur, showing patches of bare skin. But sometimes, stress can have the opposite effect, causing the cat to be neglectful about grooming.

Another sign of stress in a pet cat is increased urine spraying and increased vigilance – the animal can also become a lot more vocal than usual.

Cats are naturally curious and social animals, but when stressed, they may stop exploring and hide away for long periods of time. They can also become more aggressive.

Often, owners do not realize that stress is the reason their pet is behaving like this, say the authors, especially if there are no other, more obvious, signs.

Reducing conflict

In their review, the authors cover a range of strategies that owners can use to help reduce stress in their pet cat.

For example, they describe a three-phase method for reducing conflict between cats under the same roof.

It is important, to begin with, the cats are kept in separate parts of the house – each with its own space, litter tray, food and water bowls, scratching posts, toys, and so on.

Then, the cats are introduced to each other’s territory (without the other cat being present) – primarily so they can get used to each other’s smell. In this phase, the owner may also take a clean cloth, rub it on the scent gland of one cat, and then rub the scented cloth on the cheek of the other cat. The authors call this phase “olfactory habituation.”

When the cats appear to be relaxed in each other’s territory (still in the absence of the other), the next phase, called “visual habituation,” can begin. In this phase, the cats get to see each other through a safe barrier – for example a mesh door – while they are engaging in the pleasant activity.

The duration of these “visual contact” sessions is gradually increased, until the final phase, “direct contact habituation,” when the mesh or barrier is removed and the animals are allowed to naturally approach each other physically.

Environmental enrichment

Another stress reduction approach that the authors describe is environmental enrichment, where the physical, social and complexity dimensions of the cat’s life are enriched.

In this approach, the cat is given its own space with its own resources (food and water bowls, toys, etc), where it can feel comfortable and relaxed, without feeling threatened by other cats and dogs or other pets.

As cats spend a lot of time foraging, their space should be enriched with “puzzle feeders” and by hiding food in different places.

For cats that spend a lot of time indoors, their toys should be changed frequently to pique their interest and curiosity. Toys that mimic small, moving, catchable prey are particularly effective for this.

Another way to enrich the pet cat’s environment is to install shelves cat trees or platforms so the pet can explore its space vertically as well as horizontally. Cats like using height as a vantage point, and they like to hide in places above the ground.

The authors also mention studies that suggest giving cats places to hide can reduce stress.

Finally, cats, like humans, have different temperaments, and this needs to be considered when deciding strategies for breeding and raising cats, note the authors.

The full text of the study – which details all the strategies and discussion behind them – is available free to view for a short time at this link.