TB Blood Test Update
September 19, 2013
Veterinary Services (VS) is changing the protocol for interpreting the Stat-Pak and DPP serologic tuberculosis (TB) tests for cervids. The change will reduce the unusually high number of false positives that have occurred since testing began in February 2013.
Since these tests were implemented, over 5200 animals have been tested, with 16.2 percent Stat-Pak positive and 2.3 percent DPP positive (suspects) on the first sample tested. On the second, post-30 day testing of suspects, 1.08 percent (58 animals) were still DPP positive, and should have been submitted for necropsy. This large number of reactors was unanticipated and created an economic hardship to the cervid producers due to quarantines and the loss of valuable animals. To date 45 animals have been submitted for necropsy and no lesions typical of TB have been found. In addition, 29 cultures have been completed and no Mycobacterium bovis has been isolated.
VS analyzed the test data and determined that a colorimetric optical density (OD) reader could be used on the DPP test to determine a numerical value for a positive test, rather than a visual test as in the original protocol. These OD values could then be compared to a statistically and scientifically determined cutoff value for each species, improving the specificity of the testing without a significant loss of sensitivity. The U.S. Animal Health Association’s TB Scientific Advisory Subcommittee has concurred with our findings.
Stat-Pak/DPP tests are performed exclusively at the National Veterinary Services Laboratories, and the changes in protocol are effective immediately. In addition, all cervids currently classified as suspects and reactors will be reevaluated using the new DPP protocol on their previously submitted test samples. VS will notify States within 1 week of the reclassification of any suspect or reactor animals so that owners may be notified and quarantines released. We anticipate that some animals will remain suspects and require a secondary DPP test, and some animals will remain reactors that will need to go to necropsy.
Thank you for your patience as we work to improve TB testing. If you have any questions regarding this change in the implementation of the Stat-Pak/DPP protocol, you may contact Dr. Owen Henderson at Owen.L.Henderson@aphis.usda.gov or 970-494-7317.