Published online 1 July 2008 | Nature | doi:10.1038/news.2008.926

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Prions' great escape

Disease-causing proteins can evade sewage treatment procedures.

sweage treatmentPrions may end up in the 'biosolid' byproducts of sewage treatment.Punchstock

Prions, the infective particles behind diseases such as bovine spongiform encephalopathy (BSE), can breach standard sewage treatment methods, new research shows.

The discovery raises the possibility that the rogue proteins, which are infamously hard to detect, can jump one of the most important barriers that safeguard human health.

Prions are abnormally structured proteins that can transform normal proteins into a form that matches their own. They are infectious and cause a range of neurodegenerative diseases including BSE, also called mad cow disease, and its human equivalent, Creutzfeldt-Jakob disease. Prions are very difficult to destroy, remaining intact in the presence of radiation, disinfectants and extreme heat.

Scientists have long wondered whether prions entering sewers from slaughterhouses and meatpacking facilities could survive the processes used in conventional sewage treatment plants, and unfortunately it now appears that they can, although they are still unlikely to turn up in treated tapwater.

Sewage watch

Researchers led by Joel Pedersen, of the University of Wisconsin–Madison, collected sewage from the nearby Nine Springs wastewater treatment plant. They tested this sewage for the presence of proteins that could be turned into prions, and the initial results came up negative. They then added prions collected from the brains of infected hamsters.

The team took this contaminated material and ran it through a recreated miniature sewage treatment facility that they constructed in their laboratory.

The treatment process first used aerobic bacteria to digest the contaminated sewage for seven hours, breaking down organic material. The treated wastewater was then separated from sewage sludge, which settled at the bottom of the treatment tank. The sludge was collected and further digested by anaerobic bacteria at 37 °C for 10 days. They then tested the wastewater and sludge generated by the treatment process.

Although the wastewater generated from the experiment was clear of prion protein presence, the biosolids were not. The team’s analysis revealed that the sewage treatment process had done almost nothing to reduce the concentration of prions that they had added in the beginning. The results are reported in the journal Environmental Science and Technology1.

Because biosolids are often used as crop fertilizer, this raises the prospect of small amounts of prions being present on the surfaces of the crop plants - and without careful washing, they could therefore be ingested when the food is consumed.

Early warning

Although Pedersen admits that the high prion concentrations his team used in the lab would never be found in a real sewage treatment situation, the findings make it clear that sewage treatment facilities cannot be counted on to handle prion contamination. “If at all possible, prions should be prevented from entering wastewater treatment plants,” Pedersen says.

“I would not have predicted this outcome,” says molecular neurovirologist Robert Rohwer, of the University of Maryland, Baltimore. The complex protease composition of microbial flora [used in treatment plants] that has evolved to extract every last amino acid from the waste matter of the living world should have made relatively short work of prion proteins, Rohwer explains.

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At this stage, it may be useful to perform a full quantitative risk assessment of the environmental spread of prion diseases to identify the most important risk factors. With respect to wastewater, it might help to assess whether there was a significant risk of wastewater being contaminated in the first place, explains biochemist Robert Somerville at the University of Edinburgh, in the UK. “The question that intrigues me most is what molecular properties are responsible for making prions so resistant to inactivation in the first place,” comments Somerville.

“The good news appears to be that [prion] infectivity is relatively securely locked up in the sludge...rather than the water. The bad news is that it may take years or even decades to be rid of [the] contaminations,” Rohwer says. 

  • References

    1. Hinckley, G. et al. Environ. Sci. Technol. doi:10.1021/es703186e (2008).
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