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Prions – Zombies of the Infectious Disease World

In the closing days of 1984, when veterinarian David Bee was called out to look at a sick cow on a farm in Sussex, England, little did he realize that he was about to examine the first case of bovine spongiform encephalopathy (BSE) – mad cow disease – in the United Kingdom. Within a few months, the disease was spreading like wildfire, eventually killing at least 150 people and 180,000 cows in the United Kingdom alone, and triggering a worldwide health crisis.

Scientists quickly mobilized to discover the agent that caused BSE, rightly believing that an infectious agent had to be responsible. They discovered that infectious prions, abnormal proteins, were the agent behind the disease. Although the cause of the BSE was identified, unanswered questions abounded about these bizarre molecules.

Chronic wasting disease – a growing threat to deer and elk

In the late 1960s, the manager of a captive mule deer herd at the Colorado Division of Wildlife Foothills Wildlife Research Facility in Fort Collins noted that some of the herd appeared to be losing weight, and in poor body condition. Initially thought to be due to inadequate food intake, subsequent monitoring identified a “new” disease, now called chronic wasting disease (CWD).

Over the next 15 years, more and more cases of this mysterious disease were identified in other states, and in several Canadian provinces. Astute researchers noted the similarities between scrapie, a disease found in sheep, and chronic wasting disease. With the emergence of BSE and the discovery of prions as the agent responsible for BSE, scientists discovered that prions also were the causative agent of CWD.

Although cases of BSE are exceedingly rare in the United States, cases of CWD are on the rise, and the disease is spreading across the United States and the world. In 2016, researchers identified the first cases of CWD in free-ranging reindeer in Norway. In the United States, the disease has now been reported in 24 states, stretching from the origins of the infection in Colorado and Wyoming east to New York and south into Virginia. 

There is no evidence that CWD can infect humans, but the Centers for Disease Control as well as numerous state wildlife agencies recommend that hunters have all deer, moose and elk carcasses tested for the disease, as animals not yet showing signs of the disease might still carry abnormal prions. Many agencies suggest not eating meat from CWD-positive animals because scientists do not have enough information yet to say if CWD can “jump” to humans. 

Morris Animal Foundation funding of CWD research

Morris Animal Foundation has been funding critical chronic wasting disease research since 2007. We have invested more than half a million dollars in studies looking at several aspects of the disease, including transmission, dynamics of disease spread, and susceptibility to the disease. However, more work needs to be done to address the possible threats posed to both wildlife species and possibly humans.

“Our lab has shown that once CWD becomes established in a population, the population can significantly decline,” said Dr. Dave Edmunds, a Morris Animal Foundation grant recipient. Dr. Edmunds, in collaboration with Dr. Todd Cornish, studied white-tailed deer in Wyoming, where CWD is prevalent. The team concluded that CWD can result in serious population declines once established and sufficiently prevalent in a herd. Given how CWD continues to spread across the United States, once plentiful deer herds could rapidly decline. 

Neither dead nor alive, prions cause deadly problems in animals and humans

Prions are simply abnormally shaped proteins – no genetic material, not “alive” in the classic sense but not exactly “dead” either. Their behavior is more akin to an infectious agent, and this fact has perplexed scientists since the first prion diseases were identified almost a century ago.

Prion proteins are actually found in all living organisms, and in many cases are not only harmless but essential. There is evidence that prion proteins are important first lines of defense against invading organisms. In the brain, they seem to be important in long-term memory function. Prion proteins also may play a role in inheritance in some species.

However, prions (the name for the infectious particle itself, not to be confused with prion proteins) cause serious, fatal diseases, all involving the nervous system. The most notorious example is BSE or mad cow disease, but there are many others, such as Creutzfeldt-Jakob disease (CJD) in people.  Although prion proteins are found in abundance in normal, healthy individuals, when they become misshapen or misfolded, they can cause disease, and are termed prions. Even more peculiar is the way these misfolded proteins somehow entice other prion proteins to misfold, helping to generate a factory assembly line of prions that overwhelm the brain and cause neurological diseases.

Attacking the nervous system

The process of infection begins when prions enter an individual through the oral cavity. For diseases such as BSE, this means ingestion of contaminated meat in feed. In the case of chronic wasting disease in deer and elk, and scrapie in sheep, prions are transferred between individuals through grazing. Transmission of CJD can occur through contaminated blood products and surgical instruments.

As more and more proteins are induced to go rogue by misfolding and clumping, they damage nerve cells (neurons), resulting in clinical signs in infected individuals, including weight loss, coordination problems and behavior changes. The process is not reversible, and is always fatal.

Prion diseases often have long incubation periods, clinically silent phases during which misshapen protein replication is thought to be taking place. As a result, it can take years from the time of exposure to the first appearance of disease. Prions also are hardy particles, resistant to disinfectants and sterilization equipment, and able to exist in the environment for many years. This makes diagnosis and management of prion diseases difficult, especially in people and wildlife. 

Ongoing research important to control of wildlife prion diseases

Although our knowledge of prion diseases has increased tremendously in the last two decades, it’s clear that more work needs to be done. We are not even in a place to manage the disease in infected populations and, without the right tools, rampant disease could have a ripple effect on many ecosystems. 

Unfortunately, the clinical signs seen in animals affected by prion diseases are similar to those seen with other neurologic diseases, making it challenging to diagnose. Laboratory testing of tissue samples (especially brain and spinal cord tissue) is the only way that the disease can be either confirmed or ruled out.  The Centers for Disease Control and local state agencies can provide information about testing in your state.

“We need to develop a test that can rapidly identify infected animals in the field,” said Dr. Edmunds. “Waiting for a week for test results is too long.”

Learning how to contain the disease, understand the underlying genetics of why some animals are infected and others aren’t, and discovering ways to eliminate prions from the environment are top research priorities. Your gift to Morris Animal Foundation can help fund researchers, like Dr. Edmunds, working on controlling this emerging health threat in wildlife. 


Categories: Animal studies, Wildlife health, Animal health
November 14, 2017