Volume 43 Number 4
Waterborne Prion Disease Transmission: Assessing the Risks
Prion diseases are also called spongiform encephalopathies because they attack the brain, causing sponge-like holes in the tissue. Although prion diseases may take two to eight years or even longer to manifest, they're all fatal and widespread in animals. The most publicized prion disease is probably BSE (bovine spongiform encephalopathy), or "mad cow" disease, while the most common is scrapie, afflicting sheep and goats. Other animal prion diseases include TME (transmissible mink encephalopathy) and CWD (chronic wasting disease of mule deer and elk). Humans are also susceptible to prion diseases including: CJD (Creutzfeld-Jacob Disease), GSS (Gerstmann-Straussler-Scheinker syndrome), FFI (fatal familial insomnia), Kuru and Alpers Syndrome.
Prions, infectious particles composed simply of protein, are an entirely new class of infectious pathogens.1 They're smaller than the smallest of viruses and contain no nucleic acid material (i.e., DNA or RNA, previously thought to be required by infectious agents). In fact, they may be a modified form of a normal cellular protein found predominantly on the surface of host neurons. They're highly resistant to heat and disinfectants and don't elicit an immune response in infected hosts. Prions are known to be responsible for inherited as well as communicable diseases and there's no known treatment or cure.
Where do prions exist?
The risk of acquiring BSE from consumption of beef is difficult to determine, but there's strong evidence the agent responsible for BSE outbreaks in cattle is responsible for human cases and that specific foods are associated with transmission of prion disease from cattle to humans. In addition, health officials in the UK have announced a new variant form of CJD (vCJD). This new form afflicts a younger population range (14-to-52 years, with an average of 28 years). Data also support an association between BSE and vCJD. Ongoing research is evaluating the potential connection between prions and common neurodegenerative conditions in humans including Alzheimer's disease and Parkinson's disease.
Role of cannibalism
The U.S. Department of Agriculture (USDA) has been conducting surveillance efforts to identify the presence of BSE since 1990. In addition, in 1989, restrictions were placed on the importation of beef products into the country. As a result, no BSE has ever been identified in the United States. In 1997, the Food and Drug Administration prohibited the feeding of animal proteins to cattle. Last year, the USDA prohibited all importation of rendered animal protein products from Europe, regardless of species, due to the announcement that animal feed was potentially cross-contaminated with the BSE agent. Prion diseases detected in U.S. animal populations include: Scrapie in sheep and goats, transmissible mink encephalopathy and CWD.
The waterborne route
The good news is that prions don't multiply in the environment and are extremely "sticky" due to their hydrophobic nature, meaning they'll easily bind to particulate matter, making coagulation, filtration and granulated activated carbon (GAC) treatment effective measures for their removal from drinking water and wastewater.
Calculating the odds
According to risk analysts, even under worse case assumptions, an individual would have to consume two liters per day of tap water for 45 million years to have a 50 percent chance of infection through drinking water2. The daily risk to individuals has been estimated to be extremely low (<10-10 per person per day). These risk levels are based on assumptions that the level of BSE in the UK would continue at the 1996 level (which it hasn't) and that infectivity is accumulative (which it isn't). Others warn that the true risk of BSE in water cannot be quantified and can't be assumed to be zero, due to the great uncertainties and assumptions associated with estimating exposure levels, survivability, transport, and dose-response characteristics of prions.3
About the author
FYI-Mad about prion diseases
Several European countries have also established surveillance centers to monitor occurrence of prion diseases, or spongiform encephalopathies, in response to the epidemic of Bovine Spongiform Encephalopathy (BSE) or "mad cow disease" that occurred in the United Kingdom during the 1980s. Here are some online data sources: