Lichens

Abstract

The prion diseases sheep scrapie and cervid chronic wasting disease are transmitted, in part, via an environmental reservoir of infectivity; prions released from infected animals persist in the environment and can cause disease years later. Central to controlling disease transmission is the identification of methods capable of inactivating these agents on the landscape. We have found that certain lichens, common, ubiquitous, symbiotic organisms, possess a serine protease capable of degrading prion protein (PrP) from prion-infected animals. The protease functions against a range of prion strains from various hosts and reduces levels of abnormal PrP by at least two logs. We have now tested more than 20 lichen species from several geographical locations and from various taxa and found that approximately half of these species degrade PrP. Critical next steps include examining the effect of lichens on prion infectivity and cloning the protease responsible for PrP degradation. The impact of lichens on prions in the environment remains unknown. We speculate that lichens could have the potential to degrade prions when they are shed from infected animals onto lichens or into environments where lichens are abundant. In addition, lichens are frequently consumed by cervids and many other animals and the effect of dietary lichens on prion disease transmission should also be considered.

Keywords: :

• chronic wasting disease
• degradation
• environment
• lichen
• prion
• protease
• transmissible spongiform encephalopathy

Acknowledgments

This work was funded by USGS Wildlife: Terrestrial and Endangered Resources Program. We thank Dr. Dennis Heisey, Dr. Christopher Brand, Christina Carlson and Nicole Gibbs for their valuable comments and the photographers whom we have listed for the use of their images. Any use of trade, product or firm names is for descriptive purposes only and does not imply endorsement by the US Government.

Figures and Tables

Figure 1 Lichens can be a predominant ground cover in various, diverse environments. (A) Lichens are one of the few types of vegetation capable of surviving in tundra and are found covering soil. Photographer: Daniel Ruthrauff (USGS). (B) In boreal forests, lichens form thick mats covering the soil. Photographer: Cephas. (C) Lichens in deciduous forest breaks can be found in abundance as ground cover, as seen on the mine tailings in the photo, or growing on trees as epiphytes (inset). Photographer: James Bennett (USGS). (D) Biological soil crusts (dark surfaces) form on desert soils and are composed of a community of organisms, including lichens. Photographer: Charles Schelz (National Park Service).

Figure 2 Hypothetical points where lichens could interrupt CWD transmission cycles. Transmission of CWD occurs directly through animal-to-animal contact and through indirect routes of exposure. Soil and other fomites are thought to maintain CWD infectivity in the environment and cause disease following oral exposure to prions. Should CWD agent be shed from infected animals or released from infected carcasses onto lichen surfaces, protease activity from the lichens may degrade the prions. Lichen leachates containing protease activity may also be able to degrade prions bound to soil or other fomite surfaces. The consumption of lichens by healthy animals may block both direct and indirect CWD transmission by promoting degradation of prion in the gastrointestinal tract.

Table 1 Activity of various lichens to degrade PrP from infected hamsters (HY strain)