Man and beast both at risk.
"Zoonotic diseases run the whole gamut of types of organisms," points out Larry Glickman, professor of epidemiology and environmental health. "Some of these organisms don't appear to cause disease in the animal that harbors them, while others are as toxic to the animal host as they are to the human or animal to which the illness is passed."
Viruses, bacteria, parasites, and a protein form, called prions, also can cause zoonotic diseases. The latter induces bovine spongiform encephalopathy, commonly called mad cow disease, as well as its human variant, Creutzfeldt-Jakob disease--and probably chronic wasting disease in deer and elk.
Animals often provide a sentinel system for people, as was the case with West Nile virus when birds and horses were diagnosed before any human cases were reported. "When animals contract a disease, it's often a warning to people that the illness is moving into the area, and it's time to take measures to prevent its spread," cautions Leon Thacker, veterinary pathologist and head of the Indiana Animal Disease Diagnostic Laboratory.
This is true with insect-borne diseases such as West Nile virus, Lyme disease, Rocky Mountain spotted fever, and Ehrlichiosis, a long-recognized canine condition. In dogs, Ehrlichiosis can cause chronic weight loss, paralysis, blindness, and brain damage. In humans, it is an emerging zoonotic disease that results in acute respiratory distress and is potentially fatal. Yet, no evidence exists that dogs can transfer it directly to people.
Mosquitoes are the carriers, or vectors, of West Nile virus while ticks carry Lyme disease, Rocky Mountain spotted fever and Ehrlichiosis. A vector's bite infects a host, who, in turn, then transmits the bacteria or virus to another animal or person. With ticks, it can take as long as two years to complete the cycle that allows the illness to transfer to a new host. Bacteria and parasites sometimes spread disease by hiding in soil, water, and food. Bacterial food-borne diseases shared by man and animals include Campylobacter enteritis, E. coil infections, listeriosis, and salmonellosis.
A few zoonotic diseases result from direct contact with an animal. The best known is rabies, an old virus found almost worldwide. Bats are the major carriers and reservoirs of rabies in most regions. On the East Coast, through Ohio, secondary culprits are raccoons. In most of the Midwest, Texas, and much of California, skunks and, in some areas, coyotes and foxes are the biggest carriers after bats.
Examples of other zoonotic diseases include:
* Bartonella, or cat scratch disease, like rabies, can spread directly from an infected animal to humans. Fleas or lice transport the bacteria from cat to cat. A feline becomes the reservoir and the carrier of the bacteria when bitten by an infected insect. Infected cats usually do not show disease symptoms, but they can spread the illness. Most reported human cases of Bartonella have resulted from a bite or scratch. It is believed flea feces on the cat's claws facilitate transmission.
* Parasitic roundworms, or ascarids, found in dogs, cats, and raccoons, can cause larva migrans in humans. Roundworms live in the animals' intestines and usually do not harm the natural host. In humans, though, the liver, lungs, eyes, brain, or skin can be damaged. People become infected by ingesting roundworm eggs or larvae. Children are exposed by playing in an area where a dog or raccoon has defecated. This especially is dangerous for youngsters who eat dirt or other material exposed to egg-laden feces.
* Dog roundworm, or Toxocara canis, can cause two different syndromes. One is a systemic, allergic-type disease in which the larvae migrate to the lungs, liver, and muscles. In the other, the larvae go to the eye. It only takes one larva to cause blindness. The Centers for Disease Control, Atlanta, estimates that about 500 children a year are blinded by dog roundworm.
Glickman has developed a diagnostic test to detect the dog parasite in people. Eye doctors and the CDC now use it to confirm larva migrans. A slightly different test is used for diagnosing raccoon roundworm, called Baylisascaris procyonis. Glickman has determined that dog roundworm infects between five-20% of children at some time in their lives. Few, however, suffer permanent damage. Keep in mind, though, that raccoon roundworm is much more dangerous because the larvae seem to hone in on the brain. It can cause permanent mental disability and even death.
* Leptosporosis is considered the world's most widespread zoonotic disease. Its incidence in dogs is rising in the U.S., although human cases here remain relatively rare. Infected animals contaminate the soil and water with their urine. In people and animals, the bacteria can be transferred through the placenta, by contact with saliva, eating contaminated food, and from the soil and water--from which it can travel through the skin and mucous membranes. It is the number-one cause of kidney failure in dogs and can damage the human liver.
Recent cases include triathlon athletes who developed leptospirosis from swimming in a lake infected with a lepto organism. "This may have come from farm runoff or maybe from raccoons while they were drinking and feeding from the lake" Glickman concludes. If leptosporosis is diagnosed early enough, whether in a person or a dog, it can be treated with penicillin or tetracycline.
For years, dogs have been vaccinated against two types of leptosporosis but, over the past 15 years, new variants have surfaced. While any breed can be affected, it is more common in larger canines that spend a great deal of time outdoors, such as setters and retrievers.
"What makes zoonotics so fascinating is that each disease has its own dynamics in the host, in the infected individual, in the manner in which it's transmitted, how it's maintained, and how you prevent it," Glickman explains.
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|Title Annotation:||Zoonotic Diseases|
|Publication:||USA Today (Magazine)|
|Date:||Oct 1, 2005|
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