Pain can be defined as an aversive sensation or feeling associated with actual or potential tissue damage (Broom, 2001). Pain is part of everyday life and essential for survival, and people with congenital insensitivity to pain have a high morbidity and mortality rate (Anderson and Muir, 2005a). Pain is a protective mechanism that ensures an animal moves away from damaging influences; it signals the potential for or development of tissue damage, thereby protecting the animal from further injury (Anderson and Muir, 2005a).
The pain response
Noxious (harmful) stimuli evoke a range of responses, including behavioural, autonomic nervous system, neuroendocrine and immune responses. The proportion of these responses depends on the severity of the noxious stimulus.
Nociception is the detection of a noxious stimulus and the subsequent transmission of this information to the brain. Pain is the perception of the sensual experience induced by a noxious stimulus (Muir and Woolf, 2001). Pain is, therefore, subjective.
Types of pain – is it adaptive (nociceptive) or maladaptive (clinical)?
Pain can be either adaptive or maladaptive. Adaptive pain increases the survival potential of an animal, protecting them from injury and promoting healing. Maladaptive pain is a disease created by a pathological process which results in the persistence of pain for long after the initial cause is removed (Anderson and Muir, 2005a).
While nociception is the normal physiological process by which animals perceive pain (Constable et al., 2017), nociceptive pain (physiological pain) occurs when a stimulus that induces minimal or no tissue damage activates high-threshold sensory nerve fibres, warning the organism of events with the potential to cause tissue damage. Physiological pain is well localised, peripheral and transient and plays a vital role in the body’s normal defence mechanisms by initiating protective reflexes and physiological and avoidance responses. Typically, nociceptive pain is more acute.
Clinical pain […] occurs when excessively intense or prolonged stimuli induce tissue damage, resulting in extended discomfort and abnormal sensitivity
Clinical pain, by contrast, occurs when excessively intense or prolonged stimuli induce tissue damage, resulting in extended discomfort and abnormal sensitivity. Clinical pain may arise spontaneously and is characterised by:
- hypersensitivity – a low threshold to noxious stimuli
- hyperalgesia – an exaggerated response to noxious stimuli
- primary hyperalgesia – pain at the site
- secondary hyperalgesia – pain beyond the area of primary tissue
- extraterritorial pain – nerve injury into surrounding uninjured tissue
Clinical pain is caused by inflammation associated with tissue damage (inflammatory pain) or central or peripheral nerve injury (neuropathic pain) and may be induced by stimuli that do not normally cause any pain (allodynia) (Muir and Woolf, 2001). This pain can have a more chronic character when central sensitisation has taken place. It can also influence how active an animal is and alter memory processes and animal behaviour (Anderson and Muir, 2005a).
| 1) Suffering and stress, which results in delayed healing 2) Increased catabolism and decreased feed intake 3) Prolonged recovery and longer recumbency, with a greater risk of post-operative complications 4) Ability to cause ineffective respiratory ventilation 5) Self-mutilation 6) The possibility of acute pain, which leads to chronic pain |
Pain assessment
There are three ways of assessing pain in cattle (Constable et al., 2017):
- Observation of behaviour
- Measurement of physiological parameters that change due to the activation of the sympathetic nervous system, for example an increase in heart rate, blood pressure, sweating and breathing
- Measurement of plasma concentrations of factors indicating the activation of the sympathetic nervous system, such as cortisol and ephedrine levels
Pain-related behaviour changes in cattle
Pain-related behaviour changes can be linked directly to the area of pain or include unrelated activities. In most cases, somatic pain (musculoskeletal or skin) is quite specific. Changes in gait (such as lameness or shifting weight from one leg to another) and postural changes are common indicators of somatic pain.
Visceral (organ) pain, on the other hand, is often a lot less specific to the area of origin. In cattle, visceral pain can be observed by behavioural changes comprising: looking at the abdomen, kicking, rolling, pawing, crouching, grinding teeth, vocalisation and falling on the floor (Constable et al., 2017).
A decreased appetite can be a general sign of pain (Constable et al., 2017).
Measuring pain in cattle
Measuring pain in animals is difficult. Nevertheless, numerous studies have been published on the measurement of pain in animals in the last 20 years; many of these studies are on specific painful conditions, such as mastitis or lameness.
Cattle behaviour can be measured objectively or subjectively. Until recently, in-practice subjective assessments have been the most practical, but technological advances, such as photography and videography, provide opportunities for objective assessments outside the laboratory (Millman, 2013). Gleerup et al. (2015) provide a good example of creating a score chart to measure pain-related behavioural changes.
Until recently, in-practice subjective assessments have been the most practical, but technological advances […] provide opportunities for objective assessments outside the laboratory
These subjective pain evaluations in dairy cattle often rely on illustrations to show the important changes in facial expression. Specifications for the “bovine pain face” and ear positions have been modelled and modified by some of the authors of the Equine Pain Face scale (Gleerup et al., 2015).
Treatment and prevention of pain in cattle
As multiple mechanisms of both the peripheral and centralised systems all work together to cause pain, the best pain management plans use several agents that treat or prevent pain in different ways (Constable et al., 2017). The prevention and treatment of pain is, however, easiest when induced by physical features, for example surgical procedures (Anderson and Muir, 2005a).
Legislation limits the availability of pain treatment for cattle. Before you use a veterinary medicine to treat pain in a food-producing animal, you must check if it is usable not only for that species but for the specific situation and the residing country.
Before you use a veterinary medicine to treat pain in a food-producing animal, you must check if it is usable not only for that species but for the specific situation and the residing country
Medication must have a maximum residue limit (MRL) to be used in the treatment of food-producing animals – this is the highest level of medicine residue allowed in the animal when its product goes for human consumption. If a medicine does not have an established MRL, you cannot give it to animals producing food for human consumption.
When pain is treated in cattle, there are a few principles worth taking into consideration (Constable et al., 2017):
- Relief of pain is a humane act. When (veterinary) treatments which cause pain are carried out (eg castrations and laparotomies), appropriate analgesia must be used
- Analgesia may obscure clinical findings necessary to observe or diagnose a case
- Control of pain is necessary to prevent animals from inflicting serious self-injury due to uncontrolled behaviour resulting from severe visceral pain
- Analgesics for visceral pain are available and effective
- Pain management for severe, slow-healing wounds of the musculoskeletal system is problematic. No effective analgesics are available at present. This can compromise animal welfare
Pain medication
Local anaesthetics such as procaine and lignocaine work by blocking the pain response in peripheral nerves. It is sometimes possible to block the pain response while maintaining the normal motor function of nerves. The main limitation of using this method is the limited duration of action: nerves can be blocked with procaine for about 45 to 60 minutes at a time, but duration of action of procaine can be extended to about 90 minutes when a vasoconstrictor such as ephedrine is added.
Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit the enzyme cyclo-oxygenase (COX). COX acts on arachidonic acid to liberate prostaglandins and other mediators of inflammation. COX inhibitors prevent the production of these factors. Flunixin meglumine and ketoprofen are non-specific COX inhibitors, licensed as NSAIDS in many countries around the world. Meloxicam and carprofen are specific COX-2 inhibitors (Anderson and Muir, 2005b). NSAIDs have good-quality anti-inflammatory, antipyretic and analgesic properties.
Opioids, alpha-2 agonists and N-methyl-D-aspartate receptor antagonists are the most commonly used sedative analgesic compounds in veterinary medicine. These compounds may act synergistically and are, therefore, increasingly co-administered.Alpha-2 agonists such as xylaxine, detomidine and romifidine offer a degree of analgesia in addition to their primary sedative function.
On a final note, a recent survey found that Canadian veterinarians used xylazine more frequently than lidocaine as an analgesic for castration. To some, administering a local anaesthetic into the testes is considered dangerous and time-consuming with unpredictable efficacy, especially when circumstances do not allow sufficient time for maximal anaesthesia to take effect. Sedative-analgesic compounds may replace the need for intra-testicular anaesthetic injections and, thus, enhance animal well-being and operator safety. A sub-anaesthetic combination of xylazine, administered at 0.02 to 0.05mg/kg and ketamine at 0.04 to 0.1mg/kg given intravenously or intramuscularly (“ketamine stun”) is reported to provide mild sedation without recumbency in cattle. Butorphanol (0.01mg/kg) or morphine (0.05mg/kg) may be included for enhanced analgesic effects (Coetzee, 2021).
