Testing and Vaccination Closing In On Bovine TB And Johne's

Testing and Vaccination Closing In On Bovine TB And Johne's

More accurate tests help identify these similar diseases

Bovine tuberculosis and Johne's disease are both are caused by mycobacteria and progress is being made toward better testing for and control of both.

The pathogen responsible for Johne's disease is M. avium, subspecies paratuberculosis, and the cause of bovine TB is M. bovis.

Scientists at the Agricultural Research Service National Animal Disease Center in Ames, Iowa, are developing and improving diagnostic tests, vaccines and other technology to detect and prevent spread of these diseases.

Great progress has been made to eradicate bovine TB in US cattle, and infection rates are low. However, M. bovis still persists in wildlife, which can transmit it to cattle.

More accurate tests help identify these similar diseases and deer vaccination could cut further into

In addition, more reliable tests are needed to detect TB-positive cattle from Mexico before they are imported into the United States.

"While the diagnostic tuberculin skin test for cattle is helpful in slowing the spread of bovine TB, it is not sensitive enough and requires a 72-hour waiting period for results," says NADC veterinary medical officer Mitch Palmer.

Also, the common skin test may not be able to detect all TB-positive animals in a large herd, he says. When an animal is infected with TB, the entire herd typically is euthanized.

So Palmer and his colleagues are working to develop a better test which will allow producers to identify and remove infected cattle and keep TB-free animals. They are investigating antigens, which are components of foreign bacteria or viruses within the body that cause the immune system to produce a response.

BADLY INFECTED: This TB-infected bovine lung shows extreme "nodulation" from the tuberculin damage caused by Mycobacterium bovis, the bacteria which causes TB. (Photo by Armed Forces Institute of Pathology, the Center for Food Security and Public Health at Iowa State University and College of Veterinary Medicine.)

It appears their work is already paying off. A new serum TB diagnostic test was recently developed by IDEXX Laboratories, Inc., in Westbrook, Maine, based on NADC scientists' findings that an antigen called MPB83 is useful in bovine TB antibody-based tests.

This was a team effort, says NADC veterinary medical officer Ray Waters. IDEXX used samples from ARS experimental infection trials and ARS helped the company validate and optimize the test. ARS also assisted with worldwide field analyses with colleagues in the United Kingdom, New Zealand and Ireland to verify the test's sensitivity and specificity.

The new test is more convenient and could potentially be used in combination with other tests to identify undetected TB-infected animals, Waters says.

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Another type of test, based on polymerase chain reaction analysis of DNA, has been developed by NADC microbiologist Tyler Thacker. The PCR test detects M. bovis in fresh tissues.

Many of the current bovine PCR assays used to detect bovine TB were shown to detect mycobacteria that were not M. bovis. The new PCR test distinguishes between M. bovis and environmental mycobacteria that can cause a false-positive reading, Thacker says. It also speeds up the process by confirming M. bovis in fresh tissue.

"What's most important is the specificity of the new PCR assay," Thacker adds. "The fact that it's faster than a traditional PCR assay is an added benefit."

In studies of the new assay, the specificity for detecting M. bovis was 100%, and it was detected 67% of the time in samples from infected animals.

Reduce TB in deer?
A century-old vaccine, Bacillus Calmette-Guerin, is being tested in deer and may provide a missing piece of the puzzle in eradicating bovine TB.

Palmer says ARS researchers are trying to determine whether it will protect deer, whether there any undesirable side effects, and whether it safe to use in deer that often become food for hunters.

In some countries, BCG is still used to vaccinate humans where TB persists, but it is not used in the US. The main reason is because people who are vaccinated with BCG might test positive when given a TB skin test, even if they don't have the disease.

In experiments with captive deer, they were fed a BCG oral bait vaccine at a standard dose and 10 times the standard dose. Deer fed standard doses showed no traces of the vaccine after one to 12 months. However, the vaccine was found in deer that received higher doses.

"Importantly, the vaccine was never found in cuts of deer meat or tissues commonly used for food by humans, regardless of whether it was given orally or by subcutaneous vaccination, in any of our safety experiments," Palmer says. "It seems to be safe in deer."

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Detect Johne's
For scientists in NADC's Infectious Bacterial Diseases Research Unit, Johne's disease is a priority. The disease can cause diarrhea, reduced feed intake, weight loss, and sometimes death in infected animals. Yearly estimates in losses to the U.S. dairy industry alone exceed $220 million. It is much less common in beef cattle, but a 1997 USDA study suggests 10% of beef cattle herds could have Johne's disease present.

In the past, testing was inconclusive because any antibody used to detect the pathogen which causes Johne's disease also reacted to other environmental mycobacteria, including the pathogen responsible for bovine TB.

Microbiologist John Bannantine has discovered an antibody that's 100% specific in detecting the Johne's organism. A patent has been awarded for the new antibody and scientists are working to develop diagnostic tests that will confirm the presence of the Johne's disease bacterium.

Vaccination versus test
Although vaccines reduce the severity of Johne's disease, it is important they do not cross-react with tests for other cattle diseases.

"Producers were concerned that if they vaccinated against Johne's, they would not be able to tell whether their herd had bovine TB or Johne's disease," says microbiologist Judy Stabel.

To test for cross-reactivity of Johne's vaccines and TB tests, calves were vaccinated with a commercial vaccine against Johne's disease. Then blood samples were taken for more than a year. Novel serologic TB tests, which will soon be commercially available, were used to measure animals' responses.

"No reactivity was found using the TB tests with vaccinated calves, which is good," Stabel says. "This means that these tests will not wrongly identify cattle with Johne's disease as being positive for bovine TB."

Scientists found similar results using skin tests in other experiments.

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