Chronic wasting, TB a challenge for elk breeders

Chronic wasting disease and tuberculosis are two concerns elk breeders face, but according to Tom Gidlewski, with the U.S. Department of Agriculture and Animal and Planth Health Inspection Service, they are testing as many animals as possible to keep the numbers down.

Chronic wasting disease and tuberculosis are two concerns elk breeders face, but according to Tom Gidlewski, with the U.S. Department of Agriculture and Animal and Plant Health Inspection Service, they are testing as many animals as possible to keep the numbers down.

At the 2008 Minnesota Elk Breeders Association annual conference in Mankato in January, Gidlewski spoke about CWD and TB symptoms and testing. CWD is a transmissible spongiform encephalopathy of deer, elk and moose.

When CWD was first recognized, Gidlewski said people thought its cause might be a mineral deficiency. Now, protein misfolding, which also occurs in human diseases such as Alzheimer’s and cystic fibrosis, is most likely the culprit. The abnormal proteins, called prions, in their normal state have been discovered to be necessary for healthy function.

According to the University of Northern British Columbia, CWD is similar to other nervous system diseases such as scrapie in sheep, bovine spongiform encephalopathy in cattle and Creutzfeldt-Jakob disease in humans. Although there is no scientific evidence that CWD can affect humans, the university advises using caution.

Currently, it is not known exactly how the disease is transmitted. “We really can’t rule out feces and urine at this time,” Gidlewski said. Close contact with infected animals is the most likely means of transmission.

Early in the disease, as few as one neuron can demonstrate the immunohistochemical staining associated with chronic wasting disease. As the disease progresses, however, staining may be widespread throughout the brain.

“The animals get skinnier and skinnier, and they get holes in their brains,” Gidlewski said. Other symptoms include excessive thirst, pneumonia, salivation, lack of coordination and paralysis.

In England, a method for diagnosing a similar illness, scrapie, through lymphoid tissue by the rectum was discovered, known as the rectal mucosal biopsy test.

“There are not supposed to be any pain receptors in the lower bowel,” Gidlewski said. “Validation of the rectal test will be slow because of the small number of positive herds in North America. It may be a better test for young animals. In older animals, there may be very little lymphoid tissue left.”

Bovine TB, a contagious and communicable disease caused by bacteria, is another concern for elk breeders. TB also affects cattle, bison, deer, goats and other species, as well as humans. Discovered in 1882 by Robert Koch, there is still no vaccine that is effective for all target populations.

“BCG (Bacille Calmette Guerin) vaccine has been out for a long time, but it does not appear to be the answer,” Gidlewski said. “A good vaccine would be an important tool for controlling TB.”

Finding a vaccine is difficult, Gidlewski said, because “they have not yet discovered the antigen or group of antigens that would stimulate lasting immunity.”

Skin tests, blood tests and slaughter surveillance are all ways of detecting the disease. While the gamma interferon test is being used more extensively for cattle, the interferon test for cervids is not reliable, Gidlewski said, although there are several new antibody-based tests in development. The VetTBStatPak and multiple antigen print immunoassay, for example, are currently under evaluation.

“The MAPIA test is really good for trying to decide which antigens to look for,” Gidlewski said. “It looks at a large number of antibodies, as opposed to one or two in most tests.”

When determining effectiveness of TB tests, Gidlewski said they look for high sensitivity and specificity. Sensitivity is the test’s ability to detect infected animals, and specificity is the test’s ability to minimize misidentifying uninfected animals as positive, sometimes called false positives.

The skin test has a sensitivity of about 75 percent. However, a major drawback of the skin test is the need to handle animals twice, once for injection and again three days later to read the reaction.

“That’s a big stickler at this point in time,” Gidlewski said. One advantage of blood tests is that the animals are handled once and that the specificity of these tests are between 94 to 99 percent.