The comparative cervical test (CCT) is a critical component of the Cooperative State–Federal Tuberculosis Eradication Program. To conduct the test, the animals are handled two times. The first is for the veterinarian to measure the skin and to give the tuberculin injections, which consist of an avian purified protein derivative (PPD) and a bovine PPD. The second time the animals is at 72 (± 6) hours after the injections for reading the test results by palpation and by measuring for changes in skin thickness. An animal that has been exposed to Mycobacterium bovis or M. avium will respond with swelling and induration at the corresponding injection site. This swelling may be associated with inflammation and thus, skin temperature changes. Infrared thermography (IRT) remotely measures skin surface temperatures and may be an alternative technique to reading the CCT that reduces the need to handle the animals.
The objective of this study was to determine if IRT could be used to correctly classify cattle that were sensitized to Mycobacterium bovis or M. avium through their responses to the avian and bovine PPDs. Over 90 days before conducting the CCT and taking any IRT images, 15 domestic cattle (Bos taurus) received one of three treatments: sensitization to M. bovis, sensitization to M. avium, or nothing (control). Thermal images, using a Forward Looking Infrared camera, were taken at the time of the injections of the avian and bovine PPD and at 24 (±3) hour intervals until 72 hours after the PPD injections. The animals were restrained in a chute for images taken at 0 hours and 48 hours, but for images at 24 hours and 72 hours, the animals were loose in their pen.
The images taken at 72 hours were used to classify the animals. To classify an animal, both injection sites were identified in the same image, and the area max measurement mode was used to cover both sites. A reactor classification for M. bovis was given to the animal if the max temperature was located within the shaved area of the bovine PPD injection site and the temperature was above the temperature cut-off of 7°C. If the max temperate was above the cut-off, but located at the avian PPD injection site, the animal was classified as a reactor to M. avium. If the max temperature was under 37°C, the animal was classified as non-reactor to both injections. Results from the traditional method of palpation and skin thickness measurements were also collected and compared to the IRT results.
Using IRT, 86% of cattle were correctly classified to the sensitization treatment the animals’ received. From the traditional skin thickness measurements, 80% of the cattle matched the sensitization received by the animal. The bovine sensitized cattle and control cattle could also be correctly classified at 24 hours using IRT if the temperature cut off was raised to 7.5°C. One avian-sensitized animal was not correctly classified at 24 hours using IRT.
Infrared thermography was able to correctly classify over 86% percent of the animals and all of the bovine-sensitized cattle were correctly classified. These results indicate that IRT may be a non-invasive and objective way to read the CCT. It may also provide results for the CCT within 24 hours instead of the typical 72 hrs. Further research is still needed to understand how other factors, such as weather and breed, may affect skin temperature and influence the process used to identify reactors.
The paper “Use of Infrared Thermography as an Alternative Method to evaluate the Comparative Cervical Test (CCT) in Cattle Sensitized to Mycobacterium bovis or M. avium” by Shylo R. Johnson and Mike R. Dunbar, National Wildlife Research Center, USDA-APHIS-Wildlife Services, was presented at the 2008 United States Animal Health Association annual conference.
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