NYTimes- Study Spells Out Spread of Brain Illness in Animals
Study Spells Out Spread of Brain Illness in Animals
By SANDRA BLAKESLEE
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.
Each deer excretes about two pounds of fecal pellets a day. As wild herds move around, or captive herds are trucked between states, more soil becomes infected.
In captive herds, up to 90 percent of animals develop the disease, Dr. Prusiner said. In wild herds, a third of animals can be infected.
This is an important finding, said Judd M. Aiken, a leading prion expert who is director of the Alberta Veterinary Research Institute in Canada and who was not involved in the new study. Most of us suspected that prions might be spread in feces, but we needed proof.
The fact that prions are shed at a preclinical stage of the disease is very significant, Dr. Aiken added.
The study was carried out in two parts. First, Dr. Miller and his team infected five mule deer by feeding them brain tissue from an infected animal. They took fecal samples before infection and at three to six months afterward. The deer came down with chronic wasting disease 16 to 20 months later.
Four to nine months after infection, the deer began shedding prions in low levels in their feces, even though they had no symptoms. Surprisingly, an infected deer could shed as many prions at this stage as would accumulate in its brain during terminal disease.
In the second part of the experiment, Erdem Tamguney, an assistant professor at Dr. Prusiner’s institute, created a strain of mice with deerlike prions in their brains.
When Dr. Tamguney inoculated the brains of these mice with feces from infected but asymptomatic deer, half developed symptoms of chronic wasting disease. Fourteen out of 15 fecal samples transmitted the disease to some of the mice.
Dr. Aiken said prions tended to bind to clay in soil and to persist indefinitely. When deer graze on infected dirt, prions that are tightly bound to clay will persist for long periods in their intestinal regions. So there is no chance chronic wasting disease will be eradicated, he said. Outside the laboratory, nothing can inactivate prions bound to soil. They are also impervious to radiation.