The health risks and benefits of having a pet

The research associating pet ownership with health benefits goes back over 25 years. Without too much trouble, you can find study results linking lower blood pressure, improved recovery from heart disease, and even reduced rates of asthma and allergy in children. Cats and dogs dominate the pets-for-health literature. Data on the benefits of consorting with other sorts of creatures are sparse to nonexistent.

Many of these studies have been cross-sectional snapshots, or retrospective, so it’s hard to know whether animal companionship truly makes people healthier, or if people who are healthier to begin with are more likely to take on the responsibility of having a pet. It’s a problem that bedevils a lot of health research: how to separate the effect of a behavior from the qualities of the sorts of people who have it.

Researchers have made some progress untangling pet cause from pet effect. A Canadian research group recently reported that people who own dogs walk more than people who don’t. Maybe that’s not so surprising, but when they didn’t include dog walking in the exercise tallies, the non-dog owners were found to be more active. One possible explanation is that sedentary types who might not exercise for themselves will do so for the sake of a pet.

Another recent study showed that pets have a remarkably direct impact on heart rate and blood pressure when the owner is under stress. The study included 120 married couples who had pets. The researchers asked them to solve math problems and put their hands in cold water (although not at the same time!) as a way of inducing controlled stress under experimental conditions. They completed the tasks in one of four different situations: alone, with their pet, with their spouse, or with their spouse and their pet. The results are good news for pets but maybe not for spouses: blood pressure readings and rates stayed the most stable and returned to normal the fastest when people were with their pets.

An Infectious Appeal

Unfortunately, pets don’t just have positive health effects. They can also pass on diseases. The medical term for a disease that can spread to people from animals — any animal, not just pets — is zoonosis. Rabies is probably the best known of the zoonotic diseases. Widespread vaccination of dogs and cats in more advanced countries has meant that wildlife has now become the main source of rabies in those countries, although dogs remain the main rabies threat in many parts of Africa and Asia, including the Philippines.

Most people have heard of cat scratch disease (or at least the Ted Nugent song Cat Scratch Fever). Dogs can transmit Bartonella henselae, the bacterium that causes the disease, but it’s most often transmitted by a cat bite or a scratch. Kittens are more likely than older cats to carry and spread it, so health officials advise against hands-on roughhousing with kittens — and cats in general. Although B. henselae has been found in fleas, health officials say that so far, there’s no evidence that flea bites transmit the bacteria to humans.

But we may have more to fear from feces than from tooth or claw: many zoonotic diseases are transmitted by people inadvertently ingesting pet feces. Ingesting feces? That may seem so gross as to be unlikely, but it only takes a microscopic speck of fecal matter to make someone sick. Less-than-diligent handwashing creates opportunity enough. Young children frequently stick things in their mouths, so they could get sick if the sandbox sand, toys, or other objects were contaminated. And sometimes, the possibility that feces might be present isn’t so obvious; gardeners may not realize, for example, that the flower bed they’re digging in had a recent visitor.

Examples of zoonotic diseases spread through ingestion of dog and cat waste include toxoplasmosis, a disease caused by the Toxoplasma gondii protozoa, whose microscopic eggs are shed by cats in their feces; roundworm, hookworm, and tapeworms; and several sorts of bacteria, including Campylobacter, Salmonella, and Escherichia coli.

The spiral-shaped bacteria that cause leptospirosis are spread through animal urine. Pets — dogs appear to be more susceptible than cats — and people can become infected with the bacteria through contact with contaminated soil and water (such as wading in contaminated flood- waters!). It’s also possible to pick up some infections through the skin. The larvae that cause hookworm can get into cuts and scrapes, for example. People who come in contact with larvae-infested soil include electricians, plumbers, and other workers who crawl under raised buildings.

How sick people get from these infections varies quite a bit. Despite the scary-sounding name, toxoplasmosis usually doesn’t cause any symptoms, although some people have a bout of illness that’s like mononucleosis. Doctors usually tell pregnant women not to change the litter box or garden without gloves because, although the risk is small, if they were to pick up a toxoplasmosis infection from cat feces, it could spread to the fetus and cause birth defects.

Roundworm larvae tend to cause few, if any, symptoms, partly because they aren’t capable of completing their life cycle in humans and so they remain as larvae. If hookworm larvae penetrate the skin, they sometimes burrow along just under the surface, which causes a raised, wormlike pattern in the skin that is red and itchy (“creeping eruptions,” or cutaneous larvae migrans).

Small But Dangerous

The risk is there, but the chance of getting an infection from a dog or cat is actually small. But other sorts of pets, especially exotic ones, are a Petri dish of problems. In 2003, an outbreak of monkeypox in the US Midwest was traced back to infected prairie dogs. African pygmy hedgehogs can be sources of Salmonella infections. People may also come down with parrot fever, or psittacosis, a disease people get by breathing in dried secretions from birds infected with Chlamydophila (formerly Chlamydia) psittaci bacteria.

Pets that are small and handled a lot, particularly by young children, pose an especially high risk of transmitting the Salmonella bacteria found in feces. Last year, more than 100 salmonellosis cases abroad have been traced to small pet turtles, and in 2007, health officials connected about a dozen cases of infection with antibiotic-resistant Salmonella to pet hamsters, mice, or rats. The researchers noted that prophylactic use of antibiotics is widespread in the “pocket pet” industry, a practice that may make treatment-resistant organisms more likely.

Still Worth It

Whether pet ownership really produces health benefits is something that researchers may never fully resolve. In the meantime, long and varied experience has shown that a pet can bring joy into your life and keep you company. A dog, in particular, can get you out on walks and help you socialize with your neighbors, both of which are good for health and overall well-being.

True, pets are capable of giving us some nasty disease. But when you consider the large number of people who have animals and the small number of people who get sick from them, the risk is tiny — and worth it.

A few sensible precautions will make the small infection risk even smaller. Here are five suggestions:

1) Wash your hands. Those three words are easily the most important piece of advice. Handwashing is especially important for people with compromised immune systems and for anyone dealing with animal waste. People with small rodents and reptiles should get in the habit of washing their hands whenever they touch these animals.

2) Keep up with vet care. Rabies vaccinations are obviously important. There’s a vaccine against leptospirosis, although it’s not completely effective.

3) Treat for worms. Tapeworm segments can be seen in feces, so you can keep a lookout to see if your pet is infected. Your vet should also test a fecal sample at least once a year and anytime your animal is having gut symptoms. Deworming medicines are effective.

4) Don’t feed raw meat. The raw meat diet is a big trend in dog care right now. Raw meat, however, can infect your pet with E. coli and Salmonella — and you, too, if you don’t wash your hands and anything the meat touches. Besides, dogs didn’t evolve to just eat raw meat, so an all-meat diet isn’t nutritionally sound for the animal.

5) Stick to the basics. If you want to see exotic creatures, channel-surf your way to an animal show on cable TV, visit a zoo, or go on a safari. Keeping them as pets is often no kindness to the animals themselves and may be a danger to you.