Zoonotic diseases travel between humans and animals. Find out which ones pose a real threat.

Is your dog making you sick? Can you contaminate your pet when you have the flu? Diseases that can potentially pass from animals to humans are called zoonotic diseases, and according to the World Health Organization, approximately 70 percent of emerging infectious diseases are zoonotic. These diseases are transmitted by parasites, bacteria, viruses and fungi. Zoonotic diseases significantly impact human and animal health.

“Every animal is carrying something that could be transmissible to a person in certain situations,” says J. Scott Weese, D.V.M., DVSc, Dipl. ACVIM, a professor in the department of pathobiology at Ontario Veterinary College at the University of Guelph in Ontario, Canada. “The likelihood of disease transmission in a household is low compared to [that of someone] in the hospital. The risk gets higher for a person with a compromised immune system.”

Movement of bacteria between people and their dogs is a regular occurrence. Although most bacteria are harmless, sometimes even beneficial, some bacteria species are pathogenic and cause infectious disease. “Considering the type of close contact we have with our pets that we really don’t have with many other individuals, it’s not surprising that things move back and forth,” Weese says. “Still, the actual amount of diseases that occurs in your average household with healthy people is extremely low.”

Therapy dogs and pathogens
In 2007, Weese was part of a Canadian research team that studied 26 therapy dogs and their handlers. Twelve of the therapy teams visited hospitals, and 14 visited long-term care facilities. The groups were tested before and after each visit to determine if pathogens commonly found in healthcare facilities can transfer between patients and dogs.

Before each visit, the dogs’ paws and the handlers’ hands were sampled for three different strains of bacteria that are main contributors to hospital infections: methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci and Clostridium difficile. To detect if the dogs carried germs on their fur, one of the study’s investigators also sanitized her hands, handled each dog and tested her hands for the pathogens.

MRSA is a staph infection that is resistant to certain antibiotics. Serious staph infections can lead to blood infection, pneumonia, heart-valve infection and more. Organ failure and death may result from untreated MRSA infections.

Vancomycin-resistant enterococci is a type of bacterium that has developed resistance to many antibiotics, especially vancomycin. Enterococci bacteria live in human intestines and on the skin, usually without causing problems. The bacteria become a problem when they cause infection. These infections can occur anywhere in the body, but often occur in the intestines, urinary tract and wounds. For some people, especially those who are weak or ill, these infections can become serious.

The bacterium Clostridium difficile can cause symptoms from diarrhea to life-threatening colon inflammation. Illness from it most commonly affects older adults in hospitals or in long-term care facilities, and typically occurs after use of antibiotic medications.

None of the tested pathogens were found on the dogs, handlers or investigators prior to entering the facilities. Testing was repeated after leaving each location.

Two of the dogs tested positive for bacteria contamination.

After visiting an acute-care facility, one dog was found to have C. difficile on its paws. During the visit, the Greyhound had been encouraged to “shake hands” with several patients.

In a separate test, MRSA was detected on the hands of the investigator after petting a Pug that visited a long-term care facility. During the stay, the dog was allowed onto patients’ beds and was observed being kissed by several residents.

The results of the two-month study were published in an online paper titled, “Contamination of pet therapy dogs with MRSA and Clostridium difficile, on March 28, 2009, in the Journal of Hospital Infection.

The study suggests that germ-free dogs might pick up bacteria and transmit them to people while participating in hospital visitation programs. Compared to human visitors, therapy dogs typically visit a larger number of patients and walk bare-pawed on hospital floors, making them more likely to pick up germs.

“This finding is important because it means that screening a therapy dog for disease before entering a hospital or nursing home is not enough to prevent infectious-disease transmission,” Weese says.

Additionally, the study indicates that dogs can serve as carriers for certain infectious agents. Staphylococcus aureus is a common bacterial germ that people encounter every day, but MRSA is a more serious form of this bacterium, one that is resistant to antibiotics. “MRSA is not a zoonotic disease, but animals can be a short-term vector for it,” Weese says. “If a person is carrying MRSA in their nose or on their skin and pets a dog, he could transfer MRSA to the dog’s coat. The next person that comes along and pets that same spot can pick up the bacteria.” The dog itself is just a carrier of the bacteria.

The study underscores the importance of making infection control an integral part of pet-therapy programs in healthcare facilities. In 2008, animal- and human-health experts from the United States and Canada met to develop new guidelines for animal-visitation programs throughout North America. The protocols stress hand hygiene for all patients, visitors, dog handlers and healthcare workers. They also outline restrictions for therapy animals, including species, age, origin, behavior, diet and health status.

Some of the guidelines restrict suitable animal species to domestic companion animals that are household pets, deny the entry of any animal directly from an animal shelter and require that an animal be in a permanent home for at least 6 months to be considered for visiting patients.

H1N1 and pets
In 2009, an estimated 57 million Americans were infected with the H1N1 swine flu virus. (H1N1 is a disease people contract from other people, not from pigs.) With the rise of pet ownership in the United States, it’s estimated that a large portion of the people infected with HINT probably have pets. Yet, there were only three or four reported cases of household pets contracting the disease.

Dogs are susceptible to influenza and have their own circulating influenza strain, H3N8, but they rarely contract other types of influenza. “The probability of a dog being infected with a human virus is extremely low,” says Colin Parish, the John M. Olin professor of virology at the Cornell University College of Veterinary Medicine’s Baker Institute for Animal Health in Ithaca, N.Y. “Even if a dog becomes infected, the chance of it transmitting the virus back to humans is very low, as well.”

In 2009, the American Veterinary Medical Association confirmed the first U.S. case of H1N1 influenza virus in a dog in Bedford Hills, N.Y. The 13-year-old mixed-breed neutered male became ill after his owner was sick and tested positive for H1N1.

Initially, the dog was placed on antibiotics by his regular veterinarian. Two days later, when his symptoms worsened, his owner took him to an emergency clinic. The dog had developed a dry cough, was lethargic, was not eating and had a slight fever of 103.6 degrees Fahrenheit. The dog was hospitalized for 48 hours, placed on intravenous fluids and antibiotics, and given saline nebulization treatments, a medicated spray used to help treat lung congestion.

The attending veterinarian ran tests for both H1N1 and H3N8. One test confirmed the presence of HIN1 in the dog; the H3N8 test was negative. The H1N1 test results were also confirmed by Iowa State Laboratory in Ames.

Following hospitalization and supportive care, the dog made a full recovery.

“H1N1 looks like it occasionally can jump to a dog or cat, but it doesn’t maintain in those populations, meaning it doesn’t spread from dog to dog or from cat to cat,” says Katharine F. Lunn, B.V.M.S., M.S., Ph.D., MRCVS, Dipl. ACVIM, an assistant professor in the department of clinical sciences at Colorado State University in Fort Collins, Colo.

“The dog acts like a dead-end for the virus. It might get sick from it, but then it throws it off and doesn’t pass it on to other dogs. There’s only a small concern of getting the virus from a dog compared to getting it from another person.”

It’s apparent that dogs are susceptible to H1N1, but it’s still a low risk. People should be aware, but not overly worried about the potential for pets to acquire the disease. Although it’s possible for a dog to transmit H1N1 back to people, the dog most likely contracted the virus from its owner, who likely already exposed the rest of the household members.

Rabies
A serious but preventable zoonotic viral disease, rabies can affect all mammals, including dogs and humans. “There is a lot of rabies in wildlife,” Parish says. “Most rabies comes from virus-infected bats, which gets transmitted to [other] wildlife. A dog will occasionally interact with an infected rabbit, raccoon or skunk.”

In the United States, vaccinating dogs against rabies is mandatory and is extremely effective in preventing transmission of the disease. The required vaccination also protects people from contracting rabies in the event of a dog bite or exposure to a dog’s saliva.

Although vaccination is an important part of rabies prevention, it’s not the only part. Dog owners can take additional steps to avoid exposure to rabies by keeping bats out of the house, ensuring homes and yards are not welcoming to wild animals, and keeping pets from contact with any wildlife.

Ringworm
A skin condition caused not by a worm but by a fungus, ringworm is the most common zoonotic disease transferred from animals to humans. It can spread through contact with infected dogs or by touching objects the infected animal has touched.

Not every human who is exposed to ringworm will become infected. The person’s age, immune status, skin condition, and grooming habits influence if the fungus is actually able to infect and grow. The risks are higher with infants, elderly people and those with weakened immune systems.

Ringworm can be difficult to diagnose in dogs because not all lesions look alike. Some animals can carry the fungus in their hair coat without showing signs of itching, scaly skin or hair loss. In people, the typical lesion is a raised, reddened and itchy ring.

“An average healthy animal can carry various things in its intestinal tract, nose or respiratory tract,” Weese says. “We can reduce contact with those site areas rather easily. However, they can also carry things on their skin, like ringworm, which are harder to control because that is how we have more interaction with them.”

If your dog has been diagnosed with ringworm, eliminating the fungus from the house can be difficult because it can be found anywhere an infected animal sheds hair or skin cells. To prevent transmission, clean all bedding, grooming brushes and crates with an effective disinfectant. Throw away any items that can’t be thoroughly disinfected. Clean walls and floors in a similar manner. Remove any carpet if possible; otherwise, frequent vacuuming with immediate disposal of the vacuum bag is necessary.

Thoroughly clean and disinfect the environment every four to six weeks until all pets and people are cleared of the infection. If ringworm is identified early, cover the lesions and restrict the dog’s movement to reduce contamination in the house.

Giardia
A common intestinal parasite that can cause diarrhea in dogs, Giardia can also be found in about 7 percent of healthy dogs. Although Giardia can cause diarrhea in people, the dog and human strains of Giardia have different characteristics.

Giardia is transmitted via a fecal-oral route, which means a dog can pick up the infected cysts from eating or being near infected feces. Limiting your dog’s exposure to other animals’ feces is the best way to prevent spreading Giardia. Wild animals can also deposit Giardia into water sources, so letting your dog drink from ponds or streams is risky.

Good sanitation, such as hand-washing after playing with dogs and avoiding contact with dog feces, is the best defense against Giardia, as well as other diseases.

Stopping the spread
Prudent personal hygiene can greatly reduce the risk of disease transmission in any environment. Washing your hands before and after petting a dog is the way to stop the spread of zoonotic organisms. People with flu-like symptoms can protect their pets with the same precautions used to minimize virus transmission between humans.

“If you’re taking precautions to reduce the risk of infection with family members, you should extend that to include family pets,” Weese says, Such measures include thorough hand washing, particularly before handling a dog or preparing food; covering coughs and sneezes; and avoiding close contact with the dog during the course of illness.

Lunn stresses the importance of responsible dog ownership. “Regular parasite control, getting our pets a physical every year and keeping their vaccinations current are routine things we can do that are beneficial,” she says.

An infectious person should avoid visiting with a therapy dog, in order to protect the people who handle the dog afterward. “You’ll never make it a zero-risk situation, you just try to reduce the risk,” Parish says.

Bites and scratches can also cause disease transmission. Dogs can be potential reservoirs of MRSA, most likely acquiring it from close human contact. If a person with MRSA on their skin gets a dog bite or scratch, they can develop an infection from the bacteria entering the broken skin. Efforts to control MRSA should be directed at both the human and canine levels.

Reasonable caution
Every contact with any individual – human or animal – comes with some degree of risk. However, “we don’t want people to be so paranoid about what might be on their animal’s fur that they don’t get near them,” Weese says. “At the end of the day, that’s not going to be helpful.”

Lunn adds that numerous studies show the enormous benefits of pet ownership and of having dogs visit healthcare facilities. “We don’t want to take away from those great benefits by being overly worried about things that are minor in comparison.”