Submitted to: Journal of Veterinary Diagnostic Investigation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2000
Publication Date: N/A
Interpretive Summary: Bovine tuberculosis has nearly been eradicated from the United States because of the success of eradication programs that were initiated in 1917. As the prevalence of disease declines, it becomes progressively more difficult to detect the few remaining animals and herds infected with Mycobacterium bovis, which is the organism that causes bovine tuberculosis. .Currently, skin tests are the only tests approved for diagnosis of tuberculosis in live cattle. The test requires handling animals twice and detects approximately 80% of infected cattle. Improved diagnostic tests are needed to complete the eradication of bovine tuberculosis. The gamma interferon assay is a blood test that detects immune responses of cattle to the organism. Blood samples are collected and purified protein derivatives (PPD) are used to treat the blood in the laboratory. In this study, we determined that PPD produced in either the United States or Australia can be used for treating blood samples. These findings are important because being able to use PPD from more than one source increases the flexibility of using the test. We also determined that results of the gamma interferon assay are increased if blood samples are collected between 3 and 28 days after skin testing compared to samples collected before cattle are skin tested. The boost in gamma interferon responses after skin testing will permit greater flexibility in shipping and handling blood samples. Without skin testing, samples should be processed within 12 hours of sample collection whereas samples from animals that have been skin tested can be processed for up to 30 hours after sample collection. These results indicate that detection of cattle with tuberculosis can be improved by using the gamma interferon assay in conjunction with skin testing.
Technical Abstract: We compared purified protein derivatives (PPD) prepared in the USA with PPD prepared in Australia (CSL) for use with a gamma interferon (g-IFN) assay for diagnosis of bovine tuberculosis. We also determined the effect of skin testing on results of the g-IFN assay. Twenty cattle sensitized with heat- d M. bovis were divided into 3 groups and skin tested according to the following schedule: group A, caudal fold skin test (CFT) on day 0 and comparative cervical skin test (CCT) on day 7; group B,CFT on day 0 and CCT on day 63; and group C no skin test. Blood samples were collected at various times during the 77 day study period. There were no differences in optical density (OD) values for the g-IFN assay when blood samples were stimulated with either USA avian PPD or CSL avian PPD but OD values were significantly higher when USA bovine PPD was used compared with CSL bovine PPD. OD values also were higher for blood samples collected after cattle were skin tested compared with samples collected before skin testing or from cattle not skin tested. OD values for blood samples stimulated within 2 hours of sample collection were higher than those for samples stimulated at 24 hours. Results of this study show that USA or CSL PPD can be used to stimulate blood samples for the g-IFN assay. Skin testing cattle boosts production of g-IFN in cattle that have had prior exposure to M. bovis antigens. Use of the g-IFN assay in conjunction with skin testing may improve detection of cattle infected with M. bovis. The increase in production of g-IFN after skin testing will permit greater flexibility in conducting the assay because samples can be stimulated after the samples have been shipped overnight rather than only on the day of sample collection.