The bottom-line question is: Can deer carcasses with CWD be disposed of safely in the landfill environment?

By C. Douglas Goldsmith Jr.

In the past, landfill managers rarely had to consider the need to become disease specialists or that they might have to make decisions that could forever impact the status of their landfills - and any solid, liquid, or gas associated with it. But the situation has changed in light of recent outbreaks of chronic wasting disease (CWD) in deer populations. Does this cause you some alarm? It should.

Many landfills are being asked to accept the carcasses of deer that might be infected with CWD. The decision to accept or reject deer carcasses is a difficult one that might not easily be made even by those individuals trained in pathology. Several approaches to a discussion of the CWD subject were considered. A strict scientific review of the problem from a molecular biology or biochemical aspect would be beyond the scope of this article. Just type in the initials CWD, TSE, or BSE and click on "Search" using any engine available to your Internet service, and the references and information are voluminous. As a compromise, a decision was made to include the biochemistry necessary to understand the gravity of the problem while providing enough information on the potential difficulties associated with the acceptance or rejection of solid waste in the form of carcasses carrying or potentially carrying a neurological disease that is transmitted or initiated by an unusual pathogen. The bottom line question is: Can deer carcasses with CWD be disposed of safely in the landfill environment?"

Background

Transmissible spongiform encephalopathies (TSEs) are a group of central nervous system diseases that can afflict various different mammalian species (Williams, Miller, and Thorne, 2002). TSEs leave the brains of their victims with lesions that result in a spongelike appearance upon postmortem histological examination. Symptoms are manifested in the loss of coordination and movement and an emaciated appearance and are fatal. CWD is the specific type of TSE that impacts white-tailed deer, mule deer, black-tailed deer, and elk (Rocky Mountain Elk Foundation, 2002; USDA, 2002). Feared since its discovery in 1967, CWD has been monitored for several decades by the wildlife management community and such entities as the Centers for Disease Control (Brown et al., 2001). A recent outbreak among the wild deer population of Wisconsin has drawn renewed attention to the disease (Wisconsin Department of Natural Resources, 2002a; 2002b). The causative agent of TSEs, and CWD in particular, is now believed to be a prion ("pree-on"), an abnormal form of a normal protein known as cellular prion protein. The normal form of the cellular prion protein is referred to as PrP^c or PrP sensitive (PrP^sens) since it is susceptible to digestion by enzymes (proteinase K) that can break down protein. The pathogen-associated form is referred to as PrP resistant (PrP^res) due to its resistance to proteinase K digestion (Aranha and Larson, 2002a). A pioneer in this arena of pathology was Stanley Prusiner, M.D., who won the 1997 Nobel Prize in medicine for his work in the TSE field based on research with TSE responsible for the fatal disease scrapie in sheep (Madson, 1998). Prusiner determined that the causative agent contained no DNA or RNA. These particles were not a virus, virino, or bacteria. He believed that a proteinaceous infectious particle could transform other proteins into its own image (Prusiner, 2002). It followed that if one prion could be responsible for one form of TSE, why not others? He was correct.

Many diseases affecting humans and animals belong to the TSE family and now are being attributed to prions. The earliest recorded TSE was found in sheep and goats in 1732. Sheep with scrapie tend to lose weight, become itchy, and will scratch until their fleece is worn off often through the skin, which is the origin of the name (Madson, 1998). More recently, bovine spongiform encephalopathy (BSE) or mad cow disease, drew huge press worldwide upon its discovery in United Kingdom cattle herds. First detected in 1986, the epidemic peaked in 1992 and has gradually declined in a true bell curve fashion to only a few cases presently. Ultimately more than 180,000 cattle were lost to BSE, and the slaughter and disposal of nearly 4.5 million cattle followed in an attempt to eradicate the disease (Brown et al., 2001). The infection was determined to be from contaminated meat and bone meal present in cattle feed. Precautionary intervention measures have nearly eliminated BSE. Although very rare (0.5‚1.0 case per million), humans also can be affected by TSE (Aranha and Larson, 2002a). These diseases include fatal familial insomnia, Gerstmann-Straussler-Scheinker, Creutzfeldt-Jacob disease (CJD), and the variant of CJD. Table 1 summarizes the spongiform-type diseases found in man and animal. Based on the problems with BSE in the cattle population, naturally the fear exists that a similar situation could develop in the wild-animal populations. Why the concern? Primarily, it is based in the fact that the mode of transmission is not all that well understood. Moreover, after the discovery of BSE in cattle the new form of CJD was found in humans and termed variant CJD, or vCJD. The strong similarities between the BSE prion and vCJD raised concern that these cases might have been caused by the consumption of BSE containing beef products (Brown et al., 2001). Prions have been demonstrated to show an altered host range after passage through another species (Aranha and Larson, 2002b; Horwich and Weismann, 1997). Even those who considered the risks to humans to be low expressed some concern over long-term changes that might not have yet been observed.

How Do You Kill It?

The resistance of prions to most of the commonly accepted sterilization techniques is unrivaled. Dry heat, boiling, radiation (microwave, ultraviolet, ionizing), chemicals (e.g., alcohol, ammonia, and acids) and gaseous disinfectants (e.g., ethylene oxide and formaldehyde) have all been cited as ineffective. Chlorine dioxide, glutaraldehyde, iodophores, sodium dichloroisocyanate, sodium meta-periodate, and autoclaving at 250°F for 15 minutes or boiling in 3% sodium dodecyl sulfate have been listed as variable or partially effective disinfection methods (Aranha and Larson, 2002c). Essentially, no single decontamination method has proven to be totally effective against TSE infectious agents. Additionally, the presence of organic matter has been shown to enhance the survivability of the prion. The sobering reality of these laboratory disinfection studies is highlighted by a few real-world examples as reported by Madson (1998). Deer and elk holding pens in Sybille, CO, were left empty for six months to one year after all animals were removed and killed. New animals were introduced that had no previous contact with infected deer and elk. Elk in the pens came down with CWD within five years after the attempt at facility sterilization. A similar experience was seen at Ft. Collins, CO. The pen soils were plowed and everything was sprayed with disinfectant. Twelve young elk were taken from the wild and placed in the sanitized pens. Two of these died from CWD over the next five years.

The Extent of the Problem

CWD was at one time thought to be limited in the wild to northeastern Colorado, southeastern Wyoming, and southwestern Nebraska but has recently been found in other parts of Colorado and Nebraska, as well as affecting wild deer in Illinois, New Mexico, South Dakota, Wisconsin, and Saskatchewan (USDA, 2002; Chronic Wasting Disease Alliance). Commercial game farms, where the disease usually results in tremendous animal losses after onset of the initial cases, have tested positive in Colorado, Nebraska, South Dakota, Minnesota, Montana, Okalahoma, Kansas, Wisconsin, Saskatchewan, and Alberta. As a result, many of these states have intensive testing programs underway. For example, Colorado has examined more than 22,000 deer and elk (Gerhardt, 2002b) and Wisconsin tested approximately 37,000 white-tailed deer in 2002 (Wisconsin Department of Natural Resources, 2002b).

CWD-impacted areas in North America (taken from the Chronic Wasting Disease Alliance Web site

Identifying CWD

The testing procedure is painstaking and must be performed postmortem by qualified individuals. Previously the determination of TSE was made from a section of brain tissue examined under a microscope. Today there are several more sophisticated biochemical methods that include, but are not limited to, Western Blot, dot blots, ELISA (enzyme-linked immunosorbent assay), and immunohistochemistry (IHC) that can detect the prion protein (Aranha and Larson, 2002c). A discussion of each test being used or under development would require a separate review text. Unfortunately, there is no diagnostic test currently available for identifying animals in the early stages of CWD. Therefore an accurate count of diseased animals in the wild is not available. With the presently used methods, the highest percentage of diseased wild animals is found to be less than 5% for deer (2.8% in new Wisconsin outbreak) and 1% of elk in affected areas (Beringer, 2002). ELISA has found favor in Colorado at the Colorado State University Laboratory since IHC takes approximately 53 hours and ELISA takes only five hours to produce a result (Gerhardt, 2002a; 2002b). However, recent research has provided a new method of blood screening for prions, the results of which are available in 15 minutes (BioPharm International, 2002). This type of research is of tremendous importance for early detection and monitoring purposes.

Controlling CWD

The length of time from exposure until noticeable symptoms develop is still unknown and is probably the most likely reason for underestimates of infectivity in wild populations. At this time one could assume that there is no advance knowledge of whether or not the deer being consumed and disposed of are CWD infected. The management of BSE in Great Britain was effective in most part due to identifying the infective source (i.e., feed containing contaminated animal protein), careful control of feedstock constituents, and total eradication of any cattle herds testing positive (Brown et al., 2001). Following suit, the wildlife managers of Wisconsin have embarked on an eradication program for white-tailed deer at the CWD epicenter and for a radius of several miles to control spread of the disease. These type of activities might not be that uncommon in the future and will place the landfill manager in a difficult decision-making position. The confirmed CWD-positive animals are generally incinerated in facilities permitted for the incineration of medical wastes. The remaining are being requested for acceptance into landfills. Once again the problem with the nonpositives is that the exact time period from infection until apparent symptoms has not been determined in naturally occurring cases. The disease course has been shown to be greater than 25 months in experimentally infected deer and 34 months in elk (Williams, Miller, and Thorne, 2002). There has never been a reported case of CWD in humans, and the disease is not believed to cross species barriers; however, there is a group of knowledgeable scientists that urge extreme caution based on the many unknowns associated with the prion agent.

Disposal

It is understandable that all CWD impacted states are looking for more cost-effective disposal methods as compared to incineration. Arguments have been developed to justify the placement of CWD-infected deer carcasses or other cervids into a landfill. A preliminary risk analysis was prepared by the Wisconsin Department of Natural Resources in September 2002 and submitted to sanitary landfills and public sewage treatment operations. Most arguments for land filling in Subtitle D landfills are based on the assumption that prions can be expected to adsorb to organic material and the anticipated prion concentration in the leachate would be quite low. Further, it was assumed that these concentrations would be even less for land application of biosolids associated with leachate treatment.

Certainly this might be true to some extent in new or young landfills. As a landfill ages, however, the organic matter is lost over time as anaerobic digestion processes in the landfill convert the organic material to methane and carbon dioxide. As previously discussed, prions are immune to enzymatic action and would be attached to residual recalcitrant particles found in the leachate as total suspended solids. Therefore, problems associated with landfilling carcasses might not be expressed for several years in traditional Subtitle D landfills. More importantly, those landfills operating as a leachate recycle landfill or bioreactor landfill will achieve refuse biodegradation and methane production at a much faster rate due to the presence of adequate moisture. Changes in leachate quality could then be expected to occur more rapidly. It stands to reason that the infectious agent might be recycled to the environment where it could start the cycle of destruction once again.

Summary

What is known? Prions, although they have been on the planet for quite some time, are a newly identified form of infectious agent that are made of protein and contain no DNA or RNA. They are resistant to most standard disinfection techniques. Their mode of infection in the ecosystem is undefined but assumed to be a result of consumption of contaminated foodstuffs or animal-to-animal contact. There are some scientists who believe the BSE prion might be responsible for the variant form of CJD. CWD has never been reported to infect humans; however, the ability of prions to generate fatal neurological diseases in man, sheep, cattle, cervids, cats, and mink, as well as some exotic zoo animals in the UK, has created an expected fear response in the general public. The demonstrated resiliency of prions in soil environment in terms of reinfection is cause for concern. The infectious dose or inoculum size is not defined and the incubation time from exposure to visible symptoms might be quite long, allowing some infected animals to go undetected.

The incorporation of CWD-contaminated materials into a landfill might be proven a harmless exercise at some time in the future. The number of unknowns is great, however, and the long-term liability to the landfill owner or operator might prove to be significant. The ability to detect prions in landfill media (i.e., leachate or MSW) in a timely manner would be of considerable importance to the landfill industry for long-term monitoring and worker safety. It would be prudent for any responsible party to read more about TSE in order to make an educated decision regarding the acceptance of suspect carcasses to a landfill. To minimize the time required for further research on this topic, all references were chosen based on Internet availability.

References

Aranha, Hazel and Robert Larson. "Prions: Mayhem and Management, Part 1, General Considerations." BioPharm Supplement, pp. 11-17. www.pall.com/applicat/bio_pharm, click on Published Articles. May 2002a.

Aranha, Hazel and Robert Larson. "Prions: Mayhem and Management, Part 2, Dealing With Danger." BioPharm Supplement, pp. 18-28. www.pall.com/applicat/bio_pharm, click on Published Articles. May 2002b.

Aranha, Hazel and Robert Larson. "Prions: Mayhem and Management, Part 3, Detection and Decontamination." BioPharm Supplement, pp. 18-28. www.pall.com/applicat/bio_pharm, click on Published Articles. May 2002c.

BBC News. "Prion diseases." http://news.bbc.co.uk/2/hi/health/medical_notes/355601.stm. August 18, 1999.

Beringer, Jeff. "Chronic Wasting Disease - Creeping Closer?" www.missouriwhitetails.com/chronic_waste_disease.htm. 2002.

BioPharm International. "German test for mad cow disease effective in 15 minutes." News Release. www.biopharm-mag.com/biopharm/article/articleDetail.jsp?id=35031. October 10, 2002.

Brown, Paul et al. "Bovine Spongiform Encephalopathy and Variant Creutzfeldt-Jakob Disease: Background, Evolution, and Current Concerns." Emerging Infectious Diseases, Vol. 7, No. 1. www.cdc.gov/ncidod/eid/vol7no1/brown.htm. National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA. January/February 2001.

Chronic Wasting Disease Alliance. "Learn about CWD." www.cwd-info.org/index.php/fuseaction/about.main.

Gerhardt, Gary. "Wasting disease testing launches into high gear." Rocky Mountain News. www.rockymountainnews.com/drmn/state/article/0,1299,DRMN_21_1484455,00.html. October 17, 2002a.

Gerhardt, Gary. "Permit extended for speedy CWD test." Rocky Mountain News. http://rockymountainnews.com/drmn/state/article/0,1299,DRMN_21_1588412,00.html. December 5, 2002b.

Horwich, A.L. and J.S. Weismann. "Deadly Conformations: Protein Misfolding in Prion Disease." Cell, Vol. 89, pp. 499-510. www.cyber-dyne.com/~tom/misfolding_review.html. 1997.

Institute for Animal Health. http://www.iah.bbsrc.ac.uk/schools/factfiles/BSE.htm. 1996.

Madson, Chris. "It's not often that a wildlife disease makes headlines. This one has." Wyoming Wildlife. http://gf.state.wy.us/HTML/hunting/chronicwast.htm. May 1998.

Prusiner, Stanley, M.D. "Itís PREE-on, Not PRI-on." Outlook on Blood Safety, Vol. 5. www.pall.com/bloodtransfusion/outlook/pree_on.asp. Winter 2002.

Rocky Mountain Elk Foundation. "What is the Elk Foundation doing about CWD?" www.rmef.org/chronic.html. 2002.

USDA. "What is Chronic Wasting Disease?" www.aphis.usda.gov/vs/nahps/cwd/index.html. 2002.

Williams, E.S., M.W. Miller, and E.T. Thorne. "Chronic Wasting Disease: Implications and Challenges for Wildlife Managers." Presented at the North American Wildlife and Natural Resources Conference, April 3-7, Dallas, TX. www.cwd-info.org/index.php/fuseaction/about.overview. 2002.

Wisconsin Department of Natural Resources. "Chronic Wasting Disease and Wisconsin Deer." www.dnr.state.wi.us/org/land/wildlife/whealth/issues/CWD/index.htm. 2002a.

Wisconsin Department of Natural Resources. Summary of CWD Statewide Surveillance.  www.dnr.state.wi.us/pls/pk_cwd_zonerpt$.startup. 2002b.

Wisconsin Department of Natural Resources. "An Analysis of Risks Associated with the Disposal of Deer from Wisconsin in Municipal Solid Waste Landfills." www.dnr.state.wi.us/org/land/wildlife/Whealth/issues/Cwd/risk_analysis.pdf. September 2002.

Author C. Douglas Goldsmith Jr. is the president and principal of Alternative Natural Technologies lnc. in Blacksburg, VA, and specializes in environmental consulting services.

MSW - September/October 2003