A laser-based method for measuring thermal nociception of cattle

We describe a method for measuring nociception in cattle using a CO2 laser aimed at the caudal aspect of the metatarsi. In Experiment 1, infrared ther mography showed that calves responded by lifting their legs when skin tempe ratures reached 45-55 degrees C. In Experiment 2a, the validity of the meth od was tested by comparing the response latencies of 14 calves to two power settings (2.25 W vs. 4.5 W) with each setting being applied six times. We found that both leg-lift latencies and tail-flick latencies were lower at t he higher power setting, and the calves were more likely to respond by kick ing than by simply moving the leg. The standard deviations between and with in calves were smaller at the higher power setting, and the large within-ca lf variation means that at least three tests were required to obtain reliab le measures that could differentiate between calves. In Experiment 2b, appl ication of the laser at a range of power settings (2.0, 3.0, 4.0, 4.5, 5.0 and 5.5 W) on 16 calves showed that response latencies decreased as power i ncreased up to 4.5 W, after which no further change occurred. In Experiment 3, the repeatability of the method was evaluated on nine measures with the high power setting (4.5 W). The coefficient of variation associated with r epetition of the measures was 36%. In general, we found little change in re sponse latencies with repeated use of the laser, except that responses on t he second test tended to be shorter. Experiment 4 showed that ambient tempe ratures between 16 degrees C and 27 degrees C did not affect response laten cies, but these were longer at temperatures of 7 degrees C. We suggest that the method is a useful way of measuring cattle's sensitivity to nociceptio n as the animals need not be restrained and the distance to the animal need not be closely controlled. However, to obtain accurate, valid and reliable measures it is necessary to use a high power setting (4.5 W) and take at l east three consecutive measures of the response latency. (C) 2000 Elsevier Science B.V. All rights reserved.