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Cervical Tuberculin Test (CT)
The CT test is the most sensitive test used for identifying live cattle infected with bovine TB. The CT test is performed by the interdermal injecting 0.1 ml of USDA bovine cervical PPD tuberculin in the cervical region. A determination as to the possible presence of bovine tuberculosis (Mycobacterium bovis) is made by visual observation and palpation of the injection site in 72 hours (+/- 6 hrs) following injection. Cattle showing any response to the CT test can only be classified as a reactor and are subject to federal regulations overseeing reactor classification. A full-time State or Federal regulatory veterinarian can only perform the CT test. A response to the CT test indicates that an animal has mounted an immune response capable of recognizing Mycobacterium bovis. This immune response may be caused by infection with Mycobacterium bovis. However exposure or infection with other closely related bacteria, such as Mycobacterium avium (avian tuberculosis) and Mycobacterium paratuberculosis (Johne's disease), could also cause a CT test response. This is referred to a false positive test. The CT test is performed with USDA bovine cervical PPD tuberculin, which is more concentrated than the tuberculin used in the caudal-fold (CFT) tuberculin test. This increases the ability of the test to detect cattle infected with bovine TB, also known as sensitivity. However, the number of potential false positive reaction, or specificity, also is increased.
Any animal that does not reveal an observable or palpable response on the CT test is classified as bovine TB negative.
A response to the CT test indicates that the animal has mounted an immune response capable of recognizing Mycobacterium bovis. These animals are automatically classified as reactors. Cattle classified as bovine TB reactors are quarantined to the premises where they are disclosed until a State or Federal movement permit can be obtained. The animals are submitted to appropriate diagnostic laboratories within 15 days for necropsy and further diagnostic testing.