A false positive rate of 1-2% is expected during a whole herd test. Acidfast staining allows visualization of the bacteria. If positive, the animal is classified as a responder and requires further testing. The bacteria appear as short red or pink rods. CDFA, USDA and approved private veterinarians may draw blood from the CFT responder. Recent research has led to the development of a blood test (gamma interferon), a test that measures cell–mediated immunity in cattle. Suspect animals can be re-bled within 30 days of the CFT injection. PCR is the amplification of portions of the M. bovis DNA. This laboratory-based test is used alongside the skin test in known infected herds under TB restrictions. This test quantifies the amount of cytokine interferon gamma produced by live lymphocytes (white blood cells) in response to stimulation by PPD tuberculin. The TB skin test is the common name for the Single Intradermal Comparative Cervical Tuberculin (SICCT) test. Reactions and Interpreting Test Results. Animal Health Branch, Instructions for Completing a TB Test Record for. Fingerprinting is of great importance to determine the source of TB infections in cattle herds. This test has recently been approved as a confirmatory test for cattle that may be used instead of the CCT. If an animal is exposed to the antigens present in bovine TB, a tuberculin injection results in a delayed hypersensitivity reaction manifested by swelling and induration at the injection site. PCR is used to detect the presence of specific M. bovis genetic material. Once Mycobaterium is isolated, the sample undergoes biochemical testing to ensure the species is M. bovis. Samples are cultured under specific conditions, which favor M. bovis growth. The tissue is ground into small pieces, treated to destroy nonmycobacteria, concentrated by centrifugation, and applied to a microscopic slide to be examined. This is commonly known as the tuberculin skin test. M. bovis is a slow growing organism and takes from 8-16 weeks to grow. The main screening test for TB in cattle in Great Britain is the single intradermal comparative cervical tuberculin test (SICCT). PCR results are available in approximately 48 hours. A positive response usually begins within 8 to 12 hours and peaks about 72 hours after injection. Samples include lymph nodes of the head, thorax, and abdomen, as well as any other abnormalities found. The bovine TB skin test is supplemented by the interferon-gamma blood test, or gamma test for short. If necropsy and subsequent sample testing indicate the animal is not infected with TB, the herd must still be evaluated and may be tested a minimum of 60 days after removal of the reactor. If negative, no further action is required. This pattern represents the fingerprint of that particular M. bovis isolate. The test is expensive ($8–10/head) and the blood is difficult to handle. The owner/producer, along with government agencies involved, can apply for whole herd depopulation OR develop a herd test and removal plan. An alternative to the CCT is a whole-blood based test, the gamma interferon test, which must be performed within three (3) to 30 days following CFT injection. Invalid results due to poor lymphocyte function can occur, and a CCT may be necessary, as retests often prove repeatedly invalid. Histological examinations of the granulomas are performed to identify the accumulation of macrophages and bacteria. Animal necropsy and laboratory testing of samples (including microbiology, molecular biology, and histopathology) are needed to definitively diagnose bovine TB.
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