response latencies, but these were longer at temperatures of 78C. We suggest that the method is a useful way of measuring cattle’s sensitivity to nociception as the animals need not be restrained
and the distance to the animal need not be closely controlled. However, to obtain accurate, valid Ž
. and reliable measures it is necessary to use a high power setting 4.5 W and take at least three
consecutive measures of the response latency. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Cattle; Welfare; Pain sensitivity; Nociception
1. Introduction
Aversive events are known to produce a variety of behavioural and physiological responses, generally referred to as ‘‘stress responses’’. Among these, a reduced sensitiv-
ity to pain, known as ‘‘stress-induced analgesia’’, has been repeatedly found, particu- Ž
larly in rats submitted to acute stressors, such as electric shocks or restraint Fanselow, .
1984; Maier and Watkins, 1991; Gamaro et al., 1998 . This analgesia is dependent not only upon the physical characteristics of the aversive events, but also upon psychologi-
cal components such as conditioned fear, mental representations of an aversive event Ž
and learned helplessness Tazi et al., 1987; Fanselow et al., 1991; Maier and Watkins, .
1991 . The effects of stress on pain sensitivity suggest that measures of nociceptive responses may be useful to assess the degree of stress that farm animals are under, and
stress-induced analgesia has been recently examined in farm animals as a potential Ž
method of assessing their responses to stress Rushen and Ladewig, 1991; Rushen et al., .
1999; Cook et al., 1996; Jarvis et al., 1997 . Measures of nociception threshold of farm animals are also needed to test the efficacy
Ž of analgesics or anaesthetics Detweiler et al., 1993; Kyles et al., 1993; Lin et al., 1994;
. Main et al., 1995, 1997; Grant et al., 1996 , and to measure chronic pain by examining
Ž . Ž
decreased nociceptive thresholds hyperalgesia Ley et al., 1989, 1995, 1996; Whay et
. al., 1997 .
In animals, nociception responses are usually assessed by applying a painful stimulus to the animal and measuring the animal’s latency to respond and the nature of its
reaction. Nociceptive responses have been measured most often in sheep, and the Ž
measures have been based on withdrawal responses to electrical Lin et al., 1994; Cook .
Ž et al., 1996 , thermal Nolan et al., 1987; Ley et al., 1989; Main et al., 1995, 1997; Cook
. Ž
et al., 1996 and mechanical stimulation Detweiler et al., 1993; Kyles et al., 1993; Chambers et al., 1994; Welsh and Nolan, 1995; Cook et al., 1996; Grant et al., 1996;
. Main et al., 1997 . Cattle’s nociceptive responses has not been measured as often.
Ž .
Pinheiro Machado et al. 1997, 1998 measured leg withdrawal responses to a hand-held Ž
. source of radiant thermal energy, while Whay et al. 1997 recorded ear or head-flick
responses to a thermal resistor attached to the ear, and leg-lift responses to mechanical pressure applied to the foreleg.
The methods used so far to measure nociceptive thresholds in farm animals suffer from a number of problems. Often the animals need to be restrained, and a device must
be attached to the animal or a person must touch or be very close to the animal to Ž
. deliver the stimulation e.g., in Pinheiro Machado et al., 1998 . These could affect the
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
