Researchers are reporting that they have solved a longstanding mystery about the rapid spread of a fatal brain infection in deer, elk and moose in the Midwest and West.

The infectious agent, which leads to chronic wasting disease, is spread in the feces of infected animals long before they become ill, according to a study published online Wednesday by the journal Nature. The agent is retained in the soil, where it, along with plants, is eaten by other animals, which then become infected.

The finding explains the extremely high rates of transmission among deer, said the study’s lead author, Dr. Stanley B. Prusiner, director of the Institute for Neurodegenerative Diseases at the University of California, San Francisco.

First identified in deer in Colorado in 1967, the disease is now found throughout 14 states and 2 Canadian provinces. It leads to emaciation, staggering and death.

Unlike other animals, Dr. Prusiner said, deer give off the infectious agent, a form of protein called a prion, from lymph tissue in their intestinal linings up to a year before they develop the disease. By contrast, cattle that develop a related disease, mad cow, do not easily shed prions into the environment but accumulate them in their brains and spinal tissues.

There is no evidence to date that humans who hunt, kill and eat deer have developed chronic wasting disease. Nor does the prion that causes it pass naturally to other animal species in the wild.

Besides mad cow and chronic wasting disease, the prion diseases include Creutzfeldt-Jakob, which leads to dementia and death in humans. Each of these diseases is caused by a different strain, and all strains behave somewhat differently.

In the case of chronic wasting disease, “it turns out prions exploit the oldest trick in the book used by pathogens and parasites,” said Mike Miller, a veterinarian at the Colorado Division of Wildlife who is an expert on chronic wasting disease.

“Fecal-oral transmission is very effective,” Dr. Miller continued.