Biosolid pathogen issues: survival of prions and helminths

Pepper, I., Miles, S., Williams, D. and Gerba, C., The University of Arizona, USA



In the United States, domestic wastewater is treated to produce either Class A or Class B biosolids, 60% of which is land applied (NRC, 2002). By definition Class B biosolids contain detectable concentrations of pathogens capable of infecting humans. Many pathogens including bacteria such as Salmonella and viruses such as Rotovirus have been studied in depth in terms of their fate and survival during wastewater treatment and land application of biosolids. Biological entities that have not been extensively studied include infectious prions and helminths. In this paper we evaluate the survival of prions during wastewater treatment and the survival of helminths following land application of Class B biosolids.

Key words

Infectious prions, helminths, land applied biosolids

  1. Survival of Infectious Prions During Wastewater Treatment

Transmissible spongiform encephalopathies (TSE) are a group of neurological prion diseases of mammals which in humans include Kuru, Creutzfeldt-Jakob disease (CJD), sporadic Creutzfeldt-Jakob disease (spCJD), and variant Creutzfeldt-Jakob disease (vCJD). In animals TSE includes scrapie in sheep and goats, bovine spongiform encephalopathy (BSE) in cattle, and chronic wasting disease (CWD) affecting deer, elk, and moose. TSE in humans have been the subject of increased attention and concern. Usually, TSE is characterized by an extremely long incubation period through vCJD is a new type of CJD that has been reported in the United Kingdom (U.K.) where research has demonstrated that the incubation time of diagnosed BSE and vCJD was similar in inbred mice strains. The specific geographic occurrence and chronology of vCJD proved that BSE prions can be transmitted to humans. The known consumption of contaminated meat and concerns about cross contamination of cattle feed with prohibited material (meat and bone meal [MBM]) intended for other species prompted the introduction of a specified bovine offal ban by the Food Standard Agency in the U.K. in 1990. However, disease-causing prions or hereafter termed infectious prions referring to any prion isoform that is infectious may still enter the food chain undetected and expose humans to a TSE.

One route of exposure to infectious prions is through raw wastewater via animal rendering and meat processing operations that may have contained prion infected cattle or sheep. Furthermore, there is the possibility of discharged contaminated urine, feces and blood from CJD or vCJD patients. Most conventional wastewater treatment (WWT) facilities rely on anaerobic digestion to reduce the mass and volume of activated sludge biomass production. However, recently it was reported that the conventional treatment, mesophilic anaerobic digestion, does not reduce infectious prion numbers due to the extremely resistant properties of the infectious prion proteins. In addition, infectious prions can subsequently partition out into biosolids. Also called sewage sludge, biosolids are the solid, semi-solid, or liquid generated during the treatment of domestic sewage. Although biosolids have many agricultural uses such as land application which is good for recycling natural resources and beneficial for the environment, it may cause concern as this could expose humans to infectious prions.

Here we evaluate the fate of prions during wastewater treatment using a new assay that only detects infectious prions. An infectious prion (PrPsc) has a high degree of β sheet structure as compared to normal prions (PrPc) which contain an ” helix structure.  Of greater concern is the fact that the presence of an infectious prion causes a conformational change in PrPc resulting in the infectious form PrPsc. Disease occurs when a PrPsc threshold concentration is reached. Infectious prions causes Transmissible Spongiform Encephalopathy (TSE), which infect deer, elk, sheep, goals and cows. A “mad cow” disease outbreak in the 1990s resulted in the slaughter of hundreds of thousands of animals in Britain. Previous studies have suggested that prions are highly stable in the environment and resistant to degradation. In addition a previous study reported that prions survived wastewater treatment, however, the assay used was Western Blot technology which did not distinguish between infectious and normal prions (Kitchmayer et al., 2006).

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