Diseases From Animals: A Primer

A is for animals, Z is for zoonoses.

Medically Reviewed by Charlotte E. Grayson Mathis, MD on July 08, 2003
Written by Daniel J. DeNoon
12 min read

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.

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:

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."

By far they're our best friends. And that means cats and dogs are common sources of disease.

Cats often carry a germ called Bartonella henselae. Some 40% of cats are infected at least once in their lives -- usually when they're kittens -- but they don't look sick. Humans get infected only when they are bitten or scratched by an infected animal -- cat-scratch fever

Other bacterial infections humans can get from cats and dogs include:

  • Plague. Rodents carry the plague bacteria. Very rarely, cats get fleas from infected rodents and pass the disease to humans.

  • Q fever. People are much more likely to get Q fever from barnyard animals than from cats. But it does happen. Half of infected people get symptoms that include fever, headache, chest or stomach pain, diarrhea, and/or vomiting. It can also cause temporary swelling of the heart -- a dangerous event for people who already have heart disease.

  • Campylobacter infection. Found in animal feces, this germ causes gastrointestinal symptoms. It's usually not dangerous, but can cause severe illness in people with weakened immune systems.

  • Leptospira infection. Humans get infected via contact with water, food, or soil containing urine from infected animals. Left untreated, leptospirosis can be quite serious. It can lead to liver failure, trouble breathing, kidney damage, brain and spinal cord infection, and, rarely, death. Symptoms vary widely but can include high fever, severe headache, chills, muscle aches, and vomiting. There may also be yellow skin and eyes, red eyes, abdominal pain, diarrhea, or a rash.

  • Salmonella infection. People get this often-severe gastrointestinal infection via contact with animal feces. It can cause severe kidney damage to young children.

Both cats and dogs sometimes get parasites that infect humans. One of the most common is roundworm. Left untreated, nearly all puppies and kittens pick up this parasite. Its egg-like form -- the oocyst -- can survive for years in soil.

When humans ingest oocysts, tiny worms hatch in the gut and move through the body. Symptoms include fever, coughing, asthma, and/or pneumonia. Once in a while, the tiny worms enter the eye and scar the retina. This results in permanent partial vision loss. "Some 750 to 1,500 kids go blind each year with roundworm infection [of the eyes] passed from dogs through feces to children, " Glickman says.

Other parasites of cats and dogs:

  • Toxoplasmosis. See above.
  • Tapeworm. A person gets infected by swallowing an infected flea -- a relatively rare event, but it happens.
  • Hookworm. Hookworms are common in tropical and subtropical areas. They infest soil contaminated by animal feces. Humans get infected by direct contact, usually by walking on contaminated soil. Heavy infections can be serious.
  • Cryptosporidiosis. This parasite cause mild to severe intestinal symptoms like diarrhea. It's not usually a dangerous infection, except to people with weakened immune systems.

Ringworm isn't a parasite, but a fungal infection that forms a ring-shaped rash on the skin or a bald patch on the scalp. People can get it from direct contact with an infected animal.

Cats and dogs get viruses, too. Rabies is the most dangerous one. Be sure to keep up with your pet's rabies vaccination.

To protect yourself from diseases carried by house pets:

  • Wash your hands with soap and running water after touching feces.
  • Take your pet to the vet on a regular basis and keep up with all vaccinations recommended for your area.
  • Avoid rough play with cats.
  • If your cat or dog bites you, wash the area with soap and water right away.
  • Wash your hands after handling your pet -- especially before eating or preparing food.
  • People with weakened immune systems should take special precautions. These include never letting pets lick them on the face or on an open cut or wound, never touching animal feces, and never handling an animal that has diarrhea.
  • Don't let your pet drink from toilet bowls or eat feces.

We humans have other friends besides cats and dogs. And with these other friends come other diseases:

  • Birds. Pet birds, including parrots and parakeets, can spread psittacosis. It's a relatively rare disease, with about 50 U.S. cases each year. Symptoms include fever, chills, headache, muscle aches, and a dry cough. There's often pneumonia, which can be quite serious and even fatal. Untreated infections can lead to serious heart, liver, and nerve problems.

  • Reptiles and amphibians. Snakes, turtles, lizards, frogs, toads, and salamanders -- like other animals -- can carry Salmonella bacteria. Wash your hands after handling them. Keep them in their habitat; don't let them wander your room. Keep reptiles and their equipment away from the kitchen. Don't clean reptile cages in sinks or tubs used by people. Don't kiss your reptile -- it won't like it, anyway. And keep reptiles and amphibians away from children younger than 1 and people with weakened immune systems.

  • Exotic animals. Yes, some people make pets of animals like African pygmy hedgehogs. These tiny, antisocial animals that roll up into spiky balls were a fad not too long ago. And they came with salmonella. More recently, pet Gambian giant rats brought monkeypox into the U.S. Similar to smallpox -- but fortunately milder and not as contagious -- monkeypox lurks in small mammals in the African rainforest.

George A. Pankey, MD, director of infectious disease research at New Orleans' Ochsner Clinic Foundation, thinks the trend toward exotic animals has gone too far. He points out that we've evolved along with more common domestic animals, so that they carry relatively few diseases we can't handle. Who knows what bizarre disease might lurk in the next fad pet?

Wild animals should stay that way. Enjoy them from a distance. Even so, they're a rich source of human disease. Here are a few:

  • Raccoon roundworm. This is the best reason not to feed wild raccoons. The feces of an infected raccoon carry millions of roundworm eggs. These eggs become infectious in two to four weeks and can survive for years in the environment. They are very difficult to kill -- the CDC recommends cleaning contaminated decks or porches with boiling water or a propane flame gun (with proper caution, of course).

    Symptoms depend on where the roundworms travel in the body. They can include nausea, fatigue, enlarged liver, and symptoms of brain infection (poor coordination, inattention to one's surroundings, loss of muscle control, coma, and/or blindness). Some infections have been fatal. Diagnosis is difficult. If you are having symptoms after contact with raccoons, be sure to tell your doctor. There is no specific cure, but early treatment can limit the extent of disease.

  • Giardia infection. This microscopic parasite is the hiker's bane. It's one of the main reasons why you should always purify water taken from a stream, no matter how far from civilization you're camping. An infected animal sheds Giardia in its feces. It can survive for a long time in water and in soil.

    Symptoms include loose or watery diarrhea, stomach cramps, and stomach upset. People with Giardia infection are contagious and easily spread the parasite to others. Fortunately, there are excellent curative treatments.

  • Hantavirus. This deadly virus is carried by some strains of mice, especially deer mice. People get the infection by breathing dust contaminated with mouse droppings. If you need to clean an area that's been infested with mice, DON'T sweep it up in a big cloud of dust. Instead, put on latex gloves, wet the area with detergent or diluted bleach, wipe with damp towels, and then mop. Burn all contaminated materials. And be sure the mice are gone -- call an exterminator.

  • Lymphocytic choriomeningitis (LCM). This is a virus spread by the common house mouse. The virus can infect the linings of the brain and spinal cord. It's a serious disease, although many people get only mild infections. Mice shed the virus in their urine, saliva, and feces. People get infected by eating contaminated food or by inhaling aerosolized mouse urine or feces. LCM has two phases. The first lasts about a week and begins with fever, loss of appetite, head and muscle aches, nausea, and/or vomiting. There may be other symptoms as well.

    The second phase happens just as the first one gets better. It may begin with symptoms of meningitis: fever, headache, and stiff neck. It may also begin with symptoms of encephalitis: sleepiness, confusion, and movement problems. There's no cure, but most people recover completely with supportive treatment. However, some people are left with permanent nerve or brain damage. About 1% of people with LCM die.

  • Tularemia (rabbit fever). People usually get tularemia from direct contact with rabbits. A person can also get it via the bite of an infected tick or deerfly, by eating contaminated food, by drinking contaminated water, or by breathing in F. tularensis, the bacteria that causes rabbit fever. It's very infectious: Fewer than 10 microscopic germs can cause a lethal infection. This is why tularemia was studied during World War II as a germ warfare agent.

    The kind of disease one gets depends on how one is infected. The inhaled form is most severe, with a 30%-60% fatality rate in untreated cases. It causes pneumonia with sudden fever, chills, muscle and joint aches, dry cough, and progressive weakness. In severe cases there is bloody spit with difficulty breathing.

The equine encephalitis and West Nile viruses are transmitted from wild birds to humans -- and to horses -- by mosquitoes. We read a lot about West Nile virus, because of its recent arrival in the U.S. and its rapid spread across the nation. It can cause a very dangerous infection of the brain and spinal cord. So can equine encephalitis, which has long been firmly rooted in the U.S. In fact, eastern equine encephalitis is considered a much more serious disease. About 30% of people who get it die, and another 30% have lasting nerve damage.

Most years there are very few cases of equine encephalitis. But some years are much worse than others -- and there's no way to predict in advance when there will be an outbreak. It's too soon to know whether West Nile virus will be the same every year or come in cycles in the U.S.

It's hard to think of a more horrible disease than Ebola hemorrhagic fever. Ebola virus is spread by contact with the blood or body fluids of an infected person. Does it come from animals? Probably. Monkeys and great apes get it -- and people can get it from them when they butcher them for food. But monkeys die of Ebola, so they can't be the ultimate host. Most researchers think there's an animal out there harboring the virus. They just haven't found it yet.

That SARS emerged in China's Guangdong province seems sure. What's not sure is where it came from. SARS is a coronavirus, but it's not like any other member of the coronavirus family. Some researchers think it may have come from an endangered animal known as a masked palm civet -- like most exotic animals, a culinary delicacy in parts of China. Others find the evidence weak. Whether SARS evolved in animals or humans remains a matter of debate.

One disease that's definitely evolving in animals is influenza. And one place it's evolving is none other than Guangdong, China, where animals are kept in close proximity to one another. Flu viruses tend to arise in ducks and geese. They spread to chickens and to pigs. Pigs can also get infected with human flu viruses, so they make a good mixing pot for new flu. When an animal or a person is infected with two different flu viruses, the viruses like to swap parts. Voilà! A new virus emerges.

Infectious disease specialists don't wonder whether there will be a new worldwide flu epidemic. They only wonder when it will happen. There have been two recent close calls.

In 1997, lethal bird flu arose in the poultry markets of Hong Kong. People got infected and died, but the slaughter of millions of chickens stopped the virus before it learned how to spread from person to person. In 2001 and 2002, similarly bird flu viruses evolved in Hong Kong chickens. Fortunately, they didn't spread to humans.

Robert G. Webster, PhD, is director of the World Health Organization collaborating center on influenza viruses in lower animals and birds.

"We don't want this in humans or the world will be in deep, deep trouble," Webster told WebMD in a 2002 interview. "What will you do if one of these gets away? You haven't got anything to do. Are we going to be prepared for this? It is going to happen sooner or later."