Man and beast both at risk.

• Position: Zoonotic Diseases

The arrival of autumn does not lessen the threat from West Nile virus or other zoonotic illnesses--diseases that can affect people and animals--but scientists from Purdue University, West Lafayette, Ind., say most are avoidable and treatable.

"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...