Testing for Bovine TB
Diagnosis of bovine tuberculosis (TB) in cattle involves a series of steps and may require multiple tests.
Live Animal Tests
The first test used in diagnosing bovine TB in cattle and bison is the caudal fold tuberculin (CFT) test. The CFT test must be performed by an accredited veterinarian certified to test cattle and bison for bovine TB in Minnesota.
The first step of the test requires a veterinarian to inject tuberculin into the skin of the caudal tail fold, the two folds of skin located underneath the base of the tail. Seventy-two hours after the injection, the same veterinarian examines the injection site for any discoloration or swelling. If any changes are noted, the animal is classified as a CFT test responder (also known as a suspect) and must undergo further testing.
Follow up tests
All cattle or bison that are CFT test responders must undergo additional testing. The comparative cervical tuberculin (CCT) test or gamma interferon (GI) blood test can be used to retest these animals. The CCT test must be performed on the animal within 10 days of the CFT test injection and can only be performed by a state or federal veterinarian. The veterinarian injects two different types of tuberculin into the skin of the neck at two separate sites that are shaved for the test. The skin thickness at the injection site is measured with calipers and recorded. Seventy-two hours later, the veterinarian measures the skin thickness at each injection site again. The change in skin thickness is plotted on a graph and the animal is classified as negative, a suspect, or a reactor based on the change in skin thickness.
The GI blood test requires collection of a blood sample from the CFT responder animal within 30 days of the CFT test injection. The blood must be collected in a specific tube and received by the laboratory within 30 hours of collection. The white blood cells from the sample are divided into several groups and are stimulated with the same tuberculin used in the CCT test. The cells produce gamma interferon in response to the tuberculin. The interferon production is measured and the animal is classified as negative or as a suspect based on the difference in interferon production between the groups of cells.
Animals suspected of being infected with bovine TB based on results of the CFT,CCT, or GI tests are put down and examined internally for lesions. In order to diagnose bovine TB with certainty, the bacteria that cause it must be identified in suspect animals. There are two methods used to do this: a culture and the polymerase chain reaction (PCR) test.
Polymerase Chain Reaction (PCR) Test
The PCR test is a powerful tool that is used in a variety of diagnostic procedures to detect the presence of unique DNA. The test is very sensitive and can detect a disease-causing organism at a very low level.
Positive or negative results from a PCR test can typically be obtained within two weeks. A positive test is highly suggestive that the animal is infected with TB.
Tissue samples from cattle suspected of being infected with bovine TB are cultured. Bacterial culture is a method used to grow M. Bovis, the organism that causes bovine TB. Laboratories follow strict procedures to ensure the accuracy of their culture methods.
The bacteria are slow-growing and can take eight to 16 weeks to grow. Therefore, a culture is not called negative for bovine TB until after it has been incubated for two months. If bacteria grows from cultured tissue samples, additional testing is needed to confirm the bacteria as M. bovis.