Diagnostic Test for Cattle Tuberculosis
Using the new test for cattle tuberculosis, biological
technician Dennis Orcutt loads an agar gel with PCR-amplified tissue material
from the thermal cycler (foreground). He will know within 3 days if the culture
contains Mycobacterium bovisbacteria.
A new test that detects cattle tuberculosis bacteria within 3 days is a
major improvement over current methods that can take up to 3 months.
Mycobacterium bovis--the culprit in cattle tuberculosis--is similar
to two other bacteria, M. avium and M. paratuberculosis. The
inability to distinguish among the three has been a problem in eliminating the
The new test, developed by ARS
veterinarian Janice M. Miller at the National Animal Disease Center (NADC) in
Ames, Iowa, allows researchers to quickly tell which organism has infected an
animal. Miller developed the test at the request of USDA's Animal and Plant
Health Inspection Service (APHIS). That agency needed the test to aid in joint
efforts with state animal inspection agencies and U.S. livestock producers to
eradicate cattle TB.
The test uses polymerase chain reaction (PCR), a technique that makes many
copies of certain genetic material found only in M. bovis. Making so
many copies of the targeted DNA allows easy identification of M. bovis,
which couldn't be seen before PCR amplification. Extensive tests in other
laboratories have proven that this piece of DNA isn't present in M.
avium or M. paratuberculosis. Researchers recently developed
new tests using the same technology to identify both of these organisms.
Miller and others at NADC and at APHIS' National Veterinary Services
Laboratory in Ames validated the PCR test by examining 99 known cases of TB in
cattle and elk. In 93 percent of the cases, they could make an accurate
diagnosis within 2 to 3 days after receiving the tissue samples.
The speedier diagnosis will allow APHIS officials to take immediate action
to identify the most common sources of cattle tuberculosis: imported Mexican
steers, the captive elk and deer population, and large dairy herds with low
levels of infection.
TB spreads when coughing releases the bacteria into the air. Although human
tuberculosis in the United States is ususally caused by a different
bacterium--M. tuberculosis--the M. bovis organism can cause TB in
humans, and M. tuberculosis can cause TB in livestock and other animals.
Humans don't get TB from eating meat or drinking milk from infected animals
because pasteurization and appropriate cooking temperatures kill the disease
organism. When milk pasteurization standards were developed in the 1920s, M.
bovis and M. tuberculosis were considered the most resistant
pathogens then known.--By Linda Cooke
McGraw, Agricultural Research Service Information Staff.
Janice M. Miller is in the
USDA-ARS Respiratory and
Neurologic Diseases Research Unit, National Animal Disease Center, P.O. Box
70, Ames, IA 50010; phone (515) 239-8349, fax 239-8458.
"Diagnostic Test for Cattle Tuberculosis" was published in
the September 1998 issue of Agricultural Research magazine. Click here to see this
issue's table of contents.