Bovine TB Diagnostics

Bovine TB Diagnostics

In the U.S., the bTB status of captive cervids has been monitored primarily through individual animal or whole herd TB skin testing and   by voluntary inspection at slaughter.

The 1999 Uniform Methods & Rules (UM&R) for Bovine Tuberculosis Eradication and 9 CFR Part 77 have detailed information on tb testing in farmed/captive cervids. Approved tests include the single cervical tuberculin test (SCT), the comparative cervical tuberculin test (CCT) and the Dual Path Platform VetTB Assay (DPP).

Skin testing.  The APHIS bTB regulations for farmed/captive cervids authorize the use of the single cervical tuberculin test (SCT) as a primary test and comparative cervical tuberculin test (CCT) as a supplemental test as official tests for bTB in captive cervids. These tests require intradermal administration while the cervid is held in place or tranquilized. This can present testing challenges since physical or chemical restraint of farmed/captive cervids often is difficult, may result in misapplication of the test, or significant risk of injury to personnel and risk of injury or death for the restrained animal.

Serological testing. In 2012, two new bTB serum-based diagnostic tests, the CervidTB Stat-Pak® and the Dual Path Platform VetTB Assay (DPP), were developed for use in bTB testing of farmed/captive cervids. These tests only require a blood sample to be drawn from the cervid during temporary restraint. This results in a much lower risk of misapplying the test, and a greatly decreased risk of injury to personnel, and risk of injury or death of the restrained animal.

The CervidTB Stat-Pak® test was initially approved for use as the primary test for bTB in cervids with the DPP as a secondary test after 30 days on those animals with a primary positive test result. However, in 2014, the manufacturer stopped producing the Stat-Pak for cervids. Following scientific review and public comment, APHIS approved the DPP as both the primary test and subsequent secondary test for official program testing to diagnose bTB in farmed and captive elk, red deer, white-tailed deer, fallow deer, and reindeer when the test is conducted at the National Veterinary Services Laboratories (NVSL) or an APHIS-approved laboratory.

If the primary DPP test on an animal is positive, the animal is classified as a suspect and that animal or herd is quarantined by the state and a secondary DPP test may be done after 30 days. If the secondary DPP is negative, the animal is then negative and released.  If the secondary DPP is again positive the animal is considered a TB reactor.  According to regulations TB reactors must be depopulated and submitted to a diagnostic necropsy.  Lesions suggestive of TB and certain lymph nodes are harvested and submitted to NVSL for analysis.  Histopathology of the tissues is performed and if positive a PCR test is then done.  Tissues are submitted for culture whether the histopathology and PCR are negative or positive and incubated for at least 60 days.  If the culture is positive for TB the animal is infected and further testing of the herd must be done.  NVSL is now using a whole genome sequencing process and can identify if a particular TB organism is related to others in the library and perhaps identify where it was transmitted from.

The availability of a rapid serology test is a benefit to the cervid industry and an incentive to additional bTB testing towards eradicating bTB in U.S. domestic livestock and minimizing zoonotic disease transmission.

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