response as handling and restraint can be stressful. In addition, the repeatability and the validity of the measures often have not been tested.
Behavioural responses to pain are diverse and include reflexive behaviours, such as limb withdrawal, as well as organised complex behaviours, such as paw licking,
Ž .
vocalisations and writhing Chapman et al., 1985, Dubner, 1994 . Reflex behaviours are Ž
most often used to assess pain sensitivity Ley et al., 1989, 1995; Whay et al., 1997; .
Ž .
Pinheiro Machado et al., 1998 . However, as pointed by Chapman et al. 1985 , there is a number of limitations on the use of reflexes to score pain sensitivity. In particular,
since reflex responses do not necessarily reflect higher nervous system functioning, other behaviours that are more complex and appear more purposive should be recorded
Ž .
as well in order to assess pain perception Dubner, 1994 . Lasers have been used as a source of thermal energy to examine nociception in
Ž .
humans e.g., Arendt-Nielsen and Bjerring, 1988; Pertovaara et al., 1988 and a number Ž
. of animal species e.g., Fan et al., 1995; Treede et al., 1995; Jarvis et al., 1997 . Lasers
have the advantage that the light source is coherent and so the size of the beam is constant and does not vary with the distance between the animal and the energy source.
In addition, the time of laser onset and offset can be precisely controlled, giving precise
Ž .
temporal and spatial boundaries to the stimulation Fan et al., 1995 . Finally, the neural basis of the nociceptive response to thermal energy is reasonably well understood
Ž .
Treede et al., 1995; Towell et al., 1996 . Previously, Schwartzkopf-Genswein et al. Ž
. Ž
. 1997 and Rushen et al. 1999 described a laser-based method for measuring nocicep-
tion in cattle which does not require the animals to be restrained. In this paper, we report the results of four experiments done to test the repeatability and validity of this method.
We also attempt to take into account not only reflex responses but other behaviours that can be related to nociception.
2. General procedures
All experimental procedures were approved by the Institutional Animal Care Com- mittee at the Lennoxville Research Centre, itself monitored by the Canadian Council for
Animal Care. 2.1. Description of the laser and its use
Calves’ nociceptive response was assessed by examining their reactions to a laser applied to the skin on the caudal aspect of the metatarsal. A 10-W computer-controlled
Ž .
CO laser 10600 nm, far infrared was used as the heat source, and this was attached to
2
Ž a visible, cold, helium laser, which was used to aim the CO
laser MPB Lamsor,
2
. Dorval, QC, Canada . The diameter of the CO laser beam was 5 mm. Its power, which
2
Ž .
Ž .
could be controlled manually, ranged from 0 laser off to 10 W maximum power . Ž
. According to Svensson et al. 1991 who analysed verbal reports of people subjected to
nociception tests, the sensory threshold of a laser stimulation corresponds to warmth while the pain threshold corresponds to a feeling of pinprick. Black body calibration
Ž showed a near-linear increase in surface temperature with an increase in power data
. provided by MPB Lamsor . However, since direct measurement of skin temperature
during the test would have interfered with the procedure, we report, following Fan et al. Ž
. 1995 , the actual energy supplied by the laser and the latency to respond rather than
skin temperature at the time of response. The laser was placed on a tripod approximately 2–3 m from the animal. Animals
were prepared by shaving the hair off the hind legs between the dew claws and the hock the day before the experiment. The laser was aimed at the shaved area and turned on
until the animal responded, immediately after which the laser was turned off. The response involved the animal moving the foot on which the laser was aimed, either by
kicking, lifting the leg or simply moving the foot. The laser was also turned automati- cally off within 20 s even if the animal did not respond. Tests were interrupted and
redone if the calf defecated, urinated, walked away or moved its body in response to disturbances. Testing was conducted by two persons, one acting as the laser operator, the
second as an observer. Examination of the animals’ legs throughout the experiment showed no signs of blistering or skin damage for any of the animals.
2.2. Animals and housing A total of 60 male Holstein calves were used. In experiments 1 and 4, we used
castrated calves, 6–8 months of age, which were held in individual crates, 2.5 m = 1.0 m, in a controlled environment facility. Water was available ad libitum to each animal,
Ž through an automatic water dispenser. Cubed alfalfa was presented twice daily 0800
. and 1500 h
according to NRC guidelines for the age and class of animal. In experiments 2 and 3, we used uncastrated calves between 3 and 5 months of age, which
were housed individually either in 1.8 m = 2.0 m pens, or 0.9 m = 2.0 m pens. The size of the pen was found to have no effect on the measures discussed here, and so this is not
discussed in this paper. In experiment 2b, the calves were bucket-fed a commercially available milk replacer following the recommended concentrations and volumes between
0700 and 0900 and between 1500 and 1700 h. Hay was available ad libitum. In experiments 2a and 3, the calves were fed grain ad libitum and were bucket-fed a
commercially available milk replacer following the recommended concentrations and volumes between 1500 and 1700 h. Hay was available ad libitum. In all experiments, the
calves were kept in the same room throughout testing.
3. Experiment 1
