Diseases From Animals: A Primer

They come from giant Gambian rats and fuzzy bunnies. They come from puppies and pythons. Whether the animal is friend or food or foe, it can carry dangerous diseases.

There are at least 39 important diseases people catch directly from animals. There are at least 48 important diseases people get from the bite of bugs that bit an infected animal. And there are at least 42 important diseases that people get by ingesting or handling food or water contaminated with animal feces.

Some are as old as memory: rabies, bubonic plague, food poisoning. Others have only recently emerged: monkeypox, West Nile encephalitis, Legionnaires' disease. And some, such as highly lethal bird flu, we fear even though they haven't -- yet -- spread in humans.

People have lived with animals for eons. There's a reason for that. They don't just make us feel better. They actually contribute to our health. People who keep pets tend to have lower cholesterol levels and lower blood pressure. They tend to get more exercise and to feel less lonely.

On the other hand, pets and other animals can get sick. And some of these illnesses can be quite dangerous. This article offers an overview of these diseases -- and how to avoid them.

Why Animal-Borne Diseases Matter

Diseases passed to humans from animals are called zoonoses. What makes one of these diseases important? Two things, says zoonosis expert Lawrence T. Glickman, VMD, DrPH, professor of veterinary epidemiology and environmental health at Purdue University School of Veterinary Medicine, West Lafayette, Ind.

"If you ask Americans in general what is the most important zoonosis, most would say rabies," Glickman tells WebMD. "It is something they fear, it is in the news. But in terms of risk, there are only zero to two human cases a year in the U.S. It's one of those zoonoses that are important because of their seriousness, but not their frequency: rabies, tularemia, plague, monkeypox, listeria, anthrax. These are diseases that are very serious if one gets them but which are relatively uncommon."

On the flip side, Glickman notes, are animal-borne diseases that are important because they are fairly common even if not often fatal. Cat-scratch fever, for example, infects as many as 20,000 Americans a year. And an estimated 4%-20% of U.S. kids get roundworm from dogs and cats.

"Even these diseases can be quite serious," Glickman says. Here's a roundup of a few important zoonoses:

Toxoplasmosis

Cats allowed to roam outdoors often pick up a parasite known as Toxoplasma gondii. Most of the time, the cat will fight off the infection before it becomes contagious. However, sometimes cats shed egg-like forms of the parasite in their feces. That's why pregnant women, small children, people with damaged immune systems, and people on cancer chemotherapy should avoid cleaning cat litter boxes.

Usually, a person who gets toxoplasmosis gets very few symptoms. But when a person does get the disease, it causes a flu-like illness and/or muscle aches and pains lasting for a month or even longer. "A very sizeable proportion of humans -- 30%-40% -- have been infected with toxoplasmosis, usually by eating undercooked meat," Glickman says. "Most people never had a symptom or had very mild disease. But in people [with weakened immune systems] it can be fatal. And the worst infections may be in pregnant women. The organism can go to the fetus and, if the baby doesn't die, cause lifetime illness."