Diseases from Animals: A PrimerA is for animals, Z is for zoonoses. By Daniel DeNoon
Reviewed By Charlotte Grayson 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." Diseases from Cats and Dogs 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:
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:
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:
Other Pets, Other Diseases We humans have other friends besides cats and dogs. And with these other friends come other diseases:
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 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:
Equine Encephalitis and West Nile Viruses 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. Ebola Virus 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. SARS 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." Back to From Animals to Humans: Tracing the Path of Infectious Disease. Published July 9, 2003. SOURCES: Emerging Infectious Diseases, March 2003. Lawrence T. Glickman, VMD, DrPH, professor of veterinary epidemiology and environmental health, Purdue University School of Veterinary Medicine, West Lafayette, Ind. George A. Pankey, MD, director, infectious disease research, Ochsner Clinic Foundation, New Orleans. T.G. Ksiazek, PhD, DVM, chief, special pathogens branch, CDC, Atlanta. Last Editorial Review: 1/31/2005 6:29:50 AM © 2005-2013 WebMD, LLC. All rights reserved. |
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