Zoonoses (zo-o-NO-seez) are diseases that people can catch from animals. Well-known examples include rabies and plague.
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Many organisms cause disease only in people. Others affect only certain animals. Zoonotic organisms are those that can spread from animals to humans. There are more than 175 kinds of organisms that can cause zoonoses. They include viruses, bacteria, fungi, and such parasites as mites and worms. Infection can take place by touching an infected animal, by eating undercooked meat, through insect or animal bites, or through contact with infected feces or urine.
Which Animals Cause Zoonoses?
Most zoonoses are caught from animals closely connected to people's lives, such as household pets or farm animals, but wild animals can also spread disease to humans.
Rabies is a rare but fatal viral disease that can be caught from dogs, usually from a bite. Also rare but less serious is a disease called toxocariasis (tok-so-ka-RY-a-sis), a worm infestation of dogs and cats. Ticks, mites, and fleas from dogs are also common problems.
Cat scratch disease sometimes develops after the scratch or bite of a cat. It may cause swollen glands and fever and occurs mostly in children. Toxoplasmosis (tok-so-plaz-MO-sis), caused by a protozoan (a type of single-celled organism), is picked up by contact with cat feces. It can cause serious medical problems in pregnant women and in people with weakened immune systems. Ringworm is a skin disease caused by a fungus that can be caught from cats.
Success in the Germ
For a disease-causing germ, adaptability bring success. The same is true in animals. The raccoon, for example, is considered successful because it can thrive in areas of human habitation as well as in the wild. The panda, by contrast, can live only in certain remote bamboo forests or in a zoo with controlled conditions, and it is a very rare and endangered species.
In much the same way, germs are more successful—that is, they have a better chance of living and reproducing—if they can infect many different species of animals ratherthan just one. The zoonoses, therefore, are really success stories in the world of germs.
Parrot owners sometimes get psittacosis (sit-a-KO-sis) by inhaling dust from dried droppings. This pneumonia-like illness, also called "parrot fever," is caused by the bacterium Chlamydia psittaci (kla-MID-e-a si-TA-kee).
Diseases that humans get from such farm animals as cattle, sheep, hogs, and poultry usually come from eating meat. For example, eating undercooked or contaminated meat can cause food poisoning and typhoid fever from Salmonella bacteria. Trichinosis (trik-i-NO-sis) is usually contracted by eating undercooked pork. Some forms of encephalitis can be caught from horses (equine encephalitis) or from the bites of infected mosquitoes.
Many of the diseases that may be transmitted to humans from pets and farm animals are also found in wild animals. Rabies, for example, also occurs in raccoons, foxes, skunks, and bats. Other zoo-noses are transmitted only by wild animals. These include:
- Lyme disease, which causes flulike symptoms and joint pain, is a bacterial infection caused by the bites of ticks that feed on the blood of mice carrying the bacteria.
- Hantavirus, which is passed to humans through infected rodent urine or droppings, causes severe respiratory disease.
Some of the most severe viral infections in humans are known or believed to be connected in some way to infections in monkeys and chimpanzees. These include yellow fever, Ebola, and AIDS. The best-known zoonosis historically is probably the bubonic plague, a devastating bacterial infection transmitted from rodents to humans by flea bites.
How Are Zoonoses Treated and Prevented?
Diagnosis and treatment depend upon the specific illness, and doctors should always be consulted following an animal bite. Children should be taught not to approach or pet wild animals or stray dogs, and proper vet-erinary care is important for household pets.
Good personal hygiene can go a long way toward preventing zoo-noses. It is important to wash the hands after using the bathroom and both before and after handling food. Meat should be cooked thoroughly until juices run clear and the inside is no longer pink. Utensils and plates should be washed frequently to avoid recontaminating cooked meat.
Echinococcosis (ek-KINE-o-kok-o-sis), also called hydatid (hy-DAD-id) disease, is an infection by the larvae of Echinococcus tapeworms. Tapeworm eggs or larvae can infect field mice, sheep, and other plant-eating animals. Foxes and coyotes, or household dogs and cats, may pick up the tapeworm eggs or larvae by eating infected animals. And people may pick up the tapeworm eggs or larvae by eating food contaminated with animal droppings (greens or fruits) or by contact with infected farm animals or pets.
Echinococcosis eggs are similar to the Taenia eggs that cause other tapeworm infections, and the cyst-like tumors formed by the larvae can infect the liver, the lungs, the brain, and other organs. People may be infected for many years without any symptoms, but if left untreated Echinococcosis infection can be fatal.
This infection is most common in sheep-raising areas, including Australia, New Zealand, South America, and parts of the Mediterranean. It also occurs in Canada, Alaska and other northern states, Russia, China and central Asia, and Japan, especially among hunters, trappers, and veterinarians.
Cockrum, E. Lendell. Rabies, Lyme Disease, Hanta Virus: And Other Animal-Borne Human Diseases in the United States and Canada. Tucson: Fisher Books, 1997.
The World Health Organization posts fact sheets about many different
zoonotic diseases at its website.
The U.S. Food and Drug Administration's Center for Food Safety
and Applied Nutrition posts a
Bad Bug Book
at its website with information about foodborne bacteria, parasites,
viruses, and toxins.
U.S. Centers for Disease Control and Prevention (CDC). CDC posts a fact
sheet about alveolar hydatid disease (echinococcus) at its website.