Tissue digester’ is new CWD testing tool


Madison The state has a new “tissue digester” that can now help
the DNR dispose of deer heads and carcasses safely, and with less
cost in its chronic wasting disease testing effort.

The digester is located at the Wisconsin Veterinary Diagnostic
Laboratory (WVDL) in Madison, and is necessary in the state’s
efforts to eliminate CWD.

“This digester is designed to be a self-contained unit, and is
the world’s only large-scale mobile tissue digester,” said Dr.
Robert Shull, WVDL director. “It is the most proven and safe way of
inactivating CWD prions and disposing of tissues that are infected
with those prions.”

Shull said the digester also will be used for future disease
surveillance activity. Besides CWD, it could be used to destroy
tissue from any intentional or accidental introduction of foreign
animal diseases.

“The primary use is to get rid of materials that we know or
suspect might be infectious,” Shull said. “The tissue digester is a
proven way of inactivating prions.”

Currently there are only 30 tissue digesters available, but this
is the first mobile, large-scale digester in the nation. It is
built on a trailer that can be trucked to the site of disease
outbreaks around the state, or the Upper Midwest.

Gordon Kaye, chairman of Waste Reduction by Waste Reduction,
Inc., the company in Indianapolis, Ind., that built the digester,
described it as a giant rendition of, “Your mother’s pressure
cooker filled with lye.”

The digester is a heated, pressurized vessel that breaks down
infectious microorganisms and prions, into a sterile solution of
amino acids, sugars, and soaps. It will be used to help the WVDL
dispose of as many as 15,000 samples of deer tissue being tested
from the 2003 deer season for the presence of CWD.

During an earlier test of the digester using dead cows, a small
crane hoisted a 1,000-pound cow and lowered it into a large
stainless steel tank. The crane put two more cows in the tank, and
then the huge lid was secured, chemicals and water were added, and
the boilers started up. The cows hoofs, hide, bones and all were
soon reduced to little more than a sterile slurry.

Kaye said that the residue can go into any sewage treatment
system. The digester destroys all infective prions, viruses, or
bacteria, including brucellosis and tuberculosis, Kaye said.

The digester uses heat and 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. It can hold
4,000 pounds. The process of heating and digesting the materials
takes about six hours.

Earlier it was thought the prion was resistant to most standard
disinfecting agents, but it has been shown that a combination of
heat and alkali could destroy the infectivity.

Kaye said that he believes this is currently the “gold standard”
method to destroy prions. He hopes to receive approval soon from
the European Union to use similar digesters to destroy materials
infected with bovine spongiform encephalopathy (known as mad cow

Laurie Osterndorf, DNR Division of Land administrator, said the
digester will help the DNR save money in its fish and wildlife
account that otherwise would have been used to incinerate
CWD-infected deer carcasses.

“We forecast a savings of $80,000 this year alone in disposal
costs,” she said.

Previously the DNR paid 65 cents per pound for incineration of
CWD-positive carcasses and heads. With the digester, the DNR
per-pound disposal costs are expected to drop by 60 percent.

At the end 2003, the DNR had 150,000 pounds of carcasses in cold
storage. The digester will allow the DNR to save money on carcass
storage, as well.

The digester was part of a multi-agency cooperative effort, with
the U.S. Department of Agriculture providing funds for purchasing
the $900,000 digester. The state DNR, Department of Agriculture,
Trade and Consumer Protection, and UW-Madison provided $300,000 for
construction of a building adjacent to the veterinary laboratory to
house the digester.

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