Tissue digester to help dispose of CWD-infected materials

Dec. 15, 2003

by Terry Devitt

In a test of bovine proportions, the small crane hoists a one-thousand-pound cow and sets it neatly into a massive stainless steel tank. Two more cows follow, a massive lid is secured, chemicals and water are added, boilers are revved up, and the cows - hoofs, hide, bones and all - are soon reduced to little more than a sterile slurry.

Performed recently at the Wisconsin Veterinary Diagnostic Laboratory (WVDL) in Madison, the test was designed to put a state-of-the-art mobile tissue digester through its paces. The digester, which works like an enormous pressure cooker, is intended to help the lab dispose of as many as 15,000 samples of deer tissue to be tested in the coming months by the lab for the presence of chronic wasting disease (CWD).

Purchased by the U.S. Department of Agriculture (USDA) and housed in a new state-built facility on Madison's west side, the $900,000 digester promises a safe and efficient way to dispose of materials that may contain pathogens - prions, viruses or bacteria - and might otherwise be difficult to dispose of.

"It is designed to be a self-contained unit. It's the world's only large-scale, mobile tissue digester," explains Robert Shull, director of the WVDL and the person responsible for overseeing the lab where state's testing for CWD takes place.

Built by the Indianapolis-based WR2, Inc., the digester uses heat and caustic chemicals - sodium hydroxide or potassium hydroxide - circulated through the 8-foot diameter tank to reduce animal tissues and microorganisms to a thick broth that can be safely transported to the sanitary sewer system. The digester replaces high-temperature incineration, previously the method of choice for disposing of materials that may be infected with pathogens, including the CWD agent.

"This digester comes as a result of the continued cooperation between the USDA and the state of Wisconsin," says Ulysses J. Lane, associate northeast region director for the USDA's Animal and Plant Health Inspection Service. "It will enhance our capability to work with diagnostic transmissible spongiform encephalopathy specimens, such as CWD."

A safe, efficient and cost-effective means of disposing of tissue that may have the CWD infectious agent - a rugged protein known as a prion and that defies conventional deactivation - is critical, says Shull, who also serves as a UW-Madison clinical professor of pathology.

"Prions are as tough as they come," explains Shull. "There's no nucleic acid to deactivate or viral envelope to attack. They are very resistant to most methods of deactivation. This (the tissue digester) is a proven way of inactivating prions and anything else of an infectious nature that might be in there."

The digester, which has the appearance of a massive pressure cooker on wheels, not only makes short work of the prion, but, from an environmental perspective, is a better solution than incineration, notes Shull. It releases nothing into the atmosphere and the sterile solution it produces provides nutrients for waste water microbes in sewage treatment plants and alkalinity to counter the acidity the results from the biodegradation of sewage.

What's more, the digester's mobility, according to Shull, provides a capacity to deal with outbreaks of animal disease that may occur anywhere in Wisconsin or the upper Midwest: "The reason this was provided to the state of Wisconsin is that we are one of the core laboratories of the National Animal Health Network. If there is ever an outbreak of a foreign animal disease such as foot-and-mouth disease, this mobile digester will be a critical resource."

Shull says the entire digester project was made possible by a partnership among the USDA, which provided funds for its purchase; the state, including the Department of Natural Resources (DNR), and the Department of Agriculture, Trade and Consumer Protection; and UW-Madison. State agencies provided an estimated $300,000 for a building to house the digester.