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Steady Improvements Made in Tuberculosis Blood Test

April 8, 2014

By Owen L. Henderson, DVM, Staff Veterinarian, TB Eradication Program, USDA/APHIS

On February 4, 2013, APHIS implemented official program testing of cervids with the CervidTB Stat-Pak/DPP serologic tests in captive and free-ranging North American elk, white-tailed deer, red deer, fallow deer, and reindeer.
Testing is being conducted at the National Veterinary Services Laboratories in Ames, IA. Testing of 5216 cervids through August 2013 found 16.2% of the tests to be positive on the CervidTB Stat-Pak and 2.28 % positive on visual reading the VetTB DPP secondary test.  Forty three necropsies were performed on suspect and reactor animals.  Subsequent lab tests found no bovine TB in any of these test positive animals.

Since testing was started, 11,048 cervids of the five species have been tested. Eighteen percent of animals tested have been elk.  The reactor rate in elk has been 0.6% of all tested animals.

VS recognized early that a larger number of positive tests and false positives had occurred than expected based on our data from the test validation studies and previously published scientific papers. Our statisticians determined that at least 30 necropsies were needed to have a statistically sound data set to make any changes to the testing protocol. We analyzed the data statistically to provide a sound scientific determination of what could be done to improve the performance of these tests.  An optical density reader was used to provide a numerical readout of the color saturation of the indicator lines on the DPP test cassette.  The optical density of the lines corresponds directly to the amount of antibody against Mycobacteria present in the serum.  The optical density readout data corresponding to each test was accumulated for the first 5216 animals tested negative or positive and compared to the follow-up laboratory test findings of necropsied reactor animals.  All data was analyzed statistically and optical density cutoff values were determined for each species.  Since September 1, 2013 through March 1, 2014, 5832 cervids have been tested using the established DPP optical density cut off values on Stat-Pak positive tests.  Tests from 10 animals were found to be positive on the first DPP and 5 were positive on the follow-up post 30 day DPP.   Necropsy and follow-up laboratory testing found no bovine TB infection in any of these reactor animals.

The manufacturer of the CervidTB Stat-Pak has recently discontinued production of this test and NVSL is currently exhausting its remaining test inventory.  APHIS VS has initiated the necessary changes to the guidance document and regulatory language to eliminate the Stat-Pak as the primary cervid TB test and make the DPP as the primary test and also the follow-up post 30 day test.  This change in the cervid TB testing laboratory protocol are expected to take place in the spring of 2014.  

If the primary DPP test is positive the animal is called a suspect, quarantined and may be retested in not less than 30 days which requires a new blood sample.  As there is not another serologic test to use, the DPP will be the stand-alone test for the first and if necessary, the follow-up test.  (Experimental testing by NVSL of a statistically derived number of negative Stat-Pak samples with the DPP showed complete agreement of the negative results.)

If the follow-up DPP is negative the animal is cleared and called negative.  If the follow-up DPP is positive, the animal will be classified as a reactor and kept under quarantine until a necropsy and lab tests are completed to determine the infection status of the animal.  The cost for the DPP test has not been set by NVSL yet, but it is projected to be the same or a little more than the Stat-Pak test.  If a second post 30 day test is necessary, there is no charge to the submitter for that. There is still indemnity available for animals classified as reactors and sent to necropsy.  The cap on the indemnity is $3,000 per animal but appraisals must be completed to verify the value of the animal.

For more information contact Dr. Owen L. Henderson, APHIS VS at 970-494-7317 or owen.l.henderson@aphis.usda.gov

 

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