Prion Fact Sheet
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|Issue: Can the potential presence of prions in land applied biosolids result in food chain contamination with the subsequent development of animal and human disease?
Answer: Based on a review of available information and assessments made to date, it is unlikely that significant levels of prions enter the wastewater collection system and the risk of prion transmission directly to animals and indirectly to humans from biosolids management and effluent land application is practically zero. The following reviews current understanding of prions and potential risks of infection due to the land application of biosolids.
What Are Prions?
Prions are small folded protein molecules containing no genetic information, which are made up of amino acids, the essential building blocks of all proteins. Prion-like proteins that are found naturally in many (perhaps all) plants and animals are folded differently than normal proteins due to slight amino acid changes in certain regions of the protein. The term prion is often used to refer to such abnormally folded proteins (also referred to as “proteinaceous infectious particles”), when they have the ability to affect other proteins, causing them to change from the normal form to the abnormal form. In their normal, non-infectious state, prions are believed to be involved in cell-to-cell communications and other important cell functions. Throughout this fact sheet the word “Prion” is used to indicate the abnormal, infectious form of prions.
Why Are Prions a Concern?
Prions are associated with, and may cause transmissible spongiform encephalopathy (TSE) diseases in animals and humans. Brain tissues affected by these diseases have tiny holes in them making them resemble a sponge when viewed under a microscope. TSE diseases are untreatable and fatal. The most common examples of TSE diseases are Chronic Wasting Disease (CWD) in deer/elk/moose, BSE or Mad Cow Disease in cattle, and Scrapie in sheep. Prions are thought to cause a variant form of Creutzfeldt-Jakob Disease (vCJD), a fatal neurodegenerative illness, and Kuru in humans.
Can Prion Diseases Be Transmitted From Animals to Humans?
With the exception of BSE (Mad Cow Disease), there is no scientific evidence that other animal TSE diseases can be transmitted to humans through exposure to contaminated animals or consumption of animal products. In the United Kingdom, persons who ate tissues from BSE-contaminated cows are known to have developed vCJD. While no such cases have been linked to consumption of American beef, the United States Department of Agriculture (USDA) has a comprehensive monitoring program to identify diseased cattle and to prevent these animals from entering the human food chain. As of November 2005, only four animals have been identified as having BSE in the U.S., and none of these has entered the human food chain.
Why Are Prions of Interest to Wastewater Treatment Operations? There have been concerns raised that prions present in animal remains could enter the wastewater stream, survive the treatment process and be released into the environment in discharged effluents and/or biosolids. Most recently, attention has focused on the management of infected deer and elk carcasses to avoid the release of CWD prions.
What is the Likelihood of Prions Entering POTWs?
Prion-containing waste is carefully managed to avoid the release of prions to POTWs and the environment.
• Control of leachate from landfills used for disposal of diseased animal carcasses. Carcasses of diseased animals are not typically sent to landfills. In areas where CWD has been detected, carcasses of infected animals are incinerated or treated with an alkaline digestion process that destroys the prions. Similarly, the carcasses of the animals in the U.S. known to have BSE were incinerated.
• Waste products from meat processing plants. Guidelines and pretreatment standards promulgated by the United States Environmental Protection Agency for the Meat and Poultry Products Point Source Category, coupled with USDA testing, make it unlikely that a significant amount of prion infected material could enter the wastewater collection system.
Reported cases of Creutzfeld-Jakob Disease (CJD) are extremely rare, occurring at a rate of about one case per million population per year. Variant CJD occurred even less frequently: through 2002, only one case was reported in the United States. There is no evidence that exposure to urine or feces from patients suffering from CJD poses a risk for disease transmission, although recent evidence suggests that blood transmission of variant CJD is possible. However, the low occurrence of all forms of CJD, coupled with controls on blood donations and various guidelines in place for operating rooms, autopsy rooms, research laboratories, and mortuaries, make it highly unlikely that these materials would enter the wastewater collection and treatment system in significant amounts.
Is There a Direct Test for Measuring Prions in Biosolids or Effluent?
There are currently no validated analytical methodologies available for detecting and directly measuring the levels of prions in such complex mixtures as wastewater, effluent or biosolids. Research by the US EPA is underway to develop such analytical procedures.
Do Prions Pose a Risk in the Management of Biosolids or Effluent?
No. Based on a review of available information and assessments made to date (including a quantitative risk assessment for BSE in biosolids by Gale & Stanfield in the U.K.), it is unlikely that significant levels of prions enter the wastewater collection system and the risk of prion transmission directly to animals and indirectly to humans from biosolids management and effluent land application is extremely low (i.e., practically zero). Prion transmission via biosolids land application seems less likely than other potential food chain pathways.
Is Prion Related Research Being Conducted?
Yes, the beginning stages of research in this area are being conducted and will hopefully provide additional insight on the potential presence of prions in wastewater, biosolids, or effluents and effective treatment processes to eliminate them. The Water Environment Federation (WEF) will provide updates on this research as information becomes available.
Additional information regarding prions and biosolids can be found in the Prion White Paper.