Zoonoses are diseases transmitted from animals to humans, or vice versa. But don’t panic – taking the right precautions can keep you both healthy.
Many of us assume that because dogs and people are different species, diseases can’t spread from one to the other. But the fact is, about 60% of the nearly 1,500 diseases recognized in humans are classified as zoonoses, meaning they can be transmitted between species (not just canines), while about 75% of emerging infectious diseases are zoonotic. But don’t get overly alarmed! Most of these zoonoses are rarely seen. In this article, we’ll just focus on the more common diseases that can be passed from dogs to people.
8 common zoonoses
This ubiquitous parasite is a leading cause of acute and chronic diarrhea in humans and dogs. It is often acquired from contaminated water and sewage. This zoonoses can be transmitted from dogs to humans and vice versa. In dogs, it causes diarrhea that can be short-lived or long lasting, and can wax and wane.
This is the most widespread zoonosis in the world. The source of infection in humans is usually direct or indirect contact with the urine of an infected animal. The incidence is significantly higher in countries with warm climates, and seasons of hot, humid weather. Leptospirosis infects domestic animals as well as livestock and wild or feral animals. The usual portal of entry is through abrasions or cuts in the skin, via the conjunctiva of the eye; it can also be contracted through prolonged immersion in water.
The disease causes chronic kidney infection in maintenance hosts. Infection in nature is endemic and transferred from animal to animal by direct contact. The most important maintenance hosts are small mammals, which may transfer infection to farm animals, dogs and humans. Humans are accidental (incidental) hosts and become infected by indirect or direct contact with a maintenance host, which can include dogs.
3. Lyme disease
Caused by Borrelia burgdorferi, a spirochete Lyme disease is transmitted by a bite from an infected deer tick. It occurs in humans and dogs and occasionally in other domestic animals. Many wild animals and birds become sub-clinically infected and serve as reservoirs for deer tick infection. Lyme disease is now the most common arthropod-borne disease of humans in North America, and one of the most common in dogs.
In contrast to the severe systemic illness this zoonoses it causes in humans, Lyme disease causes acute or sub-acute arthritis in dogs. Some have speculated that transmission to people can occur through the saliva or urine of infected dogs, but the organisms are rarely found in kidneys, and deteriorate quickly in urine and saliva. Thus, the chance of zoonotic infection is minimal.
4. Methicillin-resistant Staphylococcus aureus (MRSA)
Although MRSA is a bacterial disease in people, it can also be found on the skin, nose and throat of some dogs, and can be passed quite easily to a person via a scratch, bite or other lesion. Fortunately, only some dogs harbor this organism.
This fatal viral disease is usually transmitted through the saliva and bite of a rabid animal. Rabies virus infects the central nervous system, ultimately causing brain disease and death. Transmission can occasionally occur via other routes: contamination of mucous membranes (i.e., eyes, nose, mouth), wounds, aerosol transmission, corneal and organ transplants, and even between infected and non-infected people. The majority of reported rabies cases in the wild occur in raccoons, skunks, coyotes, bats and foxes.
This fungal disease appears as a circular rash on the skin or scalp. It can be wet or dry, scaly or crusty, and may be itchy. Ringworm is passed from dogs to humans by physical contact with the infected area.
Diagnosis is confirmed with a Woods lamp, which fluorecses a bright apple green if ringworm is present.
Caused by salmonella bacteria, this disease commonly afflicts people and animals with gastrointestinal symptoms (fever, abdominal cramps, diarrhea). It’s usually caused by eating contaminated food or by accidentally ingesting contaminated feces of any species. Insects can also pick it up on their feet and bodies and transmit it that way. In dogs, signs can include bloody stool, diarrhea, vomiting, lethargy and fever. Keep in mind that salmonella bacteria are everywhere, and most of us and our animals harbor them at once time or another, with no ill effects.
8. Worms (hookworm, roundworm, tapeworm)
a) Canine hookworms belong to the Ancylostoma family and attach to the bowel wall of infected dogs. They may also infect cats, foxes, and rarely, humans. Despite their small size, hookworms can siphon a large volume of blood and cause serious illness, especially in young dogs. Hookworms infect humans by burrowing through the skin. People are most often infected when lying, sitting or standing in or on moist soil or sand. Lesions appear as red lines under the skin and can break open on the skin’s surface. The condition causes severe itching, and usually disappears within several weeks as the larvae die.
b) Canine roundworms belong to the Ascarid worm family, which can infect dogs, foxes, cats and humans. These worms live and feed in the small intestine. Though all dogs are susceptible, puppies are generally hardest hit by infestations and may become seriously ill. The most common way for a dog to become infected is through his mother. Worm larvae incubating in the female migrate through the body and invade developing fetuses. Puppies are then born with worms already living in their intestines.
In addition, larvae may also be passed from an infected mother to her offspring via milk. Roundworms can infect children that ingest the eggs found in soil, dog feces, or other contaminated substances. Once in the body, the eggs can hatch and cause visceral larval migrans, a disease arising from the larvae migrating through the liver, eyes or nervous system. Fortunately, such infections are rare.
c) Canine tapeworms belong to the Cestode parasite family. As the worms mature in a dog’s intestines, they shed mobile segments that are then passed in the feces and may be seen around the anus, on the dog’s coat, or in bedding. The segments dry out and release their eggs, which are then eaten by flea larvae on the dog or in the environment. Dogs chewing and licking themselves can ingest the fleas, setting up a cycle of re-infection. Human infection is rare and unlikely, however, since an infected flea must be eaten.
How to protect yourself and your dog
• Protect yourself when handling any animal suspected to have a zoonotic disease, especially if you work at a vet’s office or shelter; use gloves, a mask and goggles. Always practice general cleanliness such as hand-washing with hot soapy water. Keep pet food and water dishes clean (don’t wash them in the kitchen sink), and thoroughly wash any area contaminated with canine fecal matter, even after it has been cleaned up and disposed of.
• Flea control is important for preventing tapeworm infestation. For giardia and other parasites, have routine fecal examinations done on your dog, and de-worm if necessary.
• Canine vaccines are available for leptospirosis and Lyme disease (currently, West Nile virus vaccine is only available for horses). If the former two diseases are endemic in your area, ask your veterinarian about these vaccines. Rabies vaccination is currently required by law in most areas.
Last but far from least, ensure that you and your dog enjoy healthy lifestyles that will keep your immune systems strong and less susceptible to zoonotic and other diseases. A good diet, plenty of pure water, adequate exercise and rest, regular checkups, and minimal exposure to vaccines, stress and environmental toxins will go a long way towards keeping you both well.
Dr. Jean Dodds received her veterinary degree in 1964 from the Ontario Veterinary College. In 1986, she established Hemopet, the first non-profit national blood bank program for animals. Today, Hemopet also runs Hemolife, an international veterinary specialty diagnostics service. Dr. Dodds has been a member of many committees on hematology, animal models of human disease and veterinary medicine. She received the Holistic Veterinarian of the Year Award from the AHVMA in 1994, has served two terms on the AHVMA’s Board of Directors, chairs their Communications Committee, and currently serves on the Board of the AHVMF, as well as its Research Grant and Editorial Committees.