9 Dec 2019
Adele Williams guides readers through recognition and various control methods – new and existing – in horses and donkeys.
Adele Williams
Job Title
Image © Alexia Khruscheva / Adobe Stock
This article reviews pain recognition and existing analgesics used in equine patients. Additional measures for pain control – such as acupuncture, physiotherapy and appropriate diet – are also discussed. The differences for analgesic dosing between horses and donkeys are highlighted, while potential future developments for equine pain assessment and management are discussed. Doses, routes of administration and dosing intervals for commonly used analgesics in equine patients are given.
Recognition and assessment of pain in horses is a key to any pain control protocol.In some instances, pain is obvious – such as non-weight bearing lameness or rolling with colic. However, more often, since equids are prey species, they conceal pain, making identification difficult.
Furthermore, pain responses in horses are sometimes incorrectly identified as behavioural problems; examples include trigeminal-mediated head shaking pain and gastric ulcer pain.
Facial expression has been identified as a way of assessing pain in horses, and can be a useful adjunct for assessing pain and monitoring response to therapy (Dalla Costa et al, 2014; Gleerup et al, 2015). Lameness locator technology is now available to aid diagnostics for equine orthopaedic pain.
Composite pain scales can also be useful for equine pain assessment (van Loon and Van Dierendonck, 2018). A mathematical model for assessing objective measures of the grimace response in relation to degree of pain in horses has been developed (Dalla Costa et al, 2018).
These methods of objective pain assessment could pave the way to development of automated programmes – such as computer facial recognition software – to aid equine pain assessment. This could be used to monitor a patient’s response to therapy, whether in a hospital or via images acquired by an owner remotely and submitted to the vet for evaluation.
Pain may be acute or chronic, and of nociceptive or neuropathic origin. Inflammation is intrinsically linked to pain (Omoigui, 2007). Therefore, most analgesics are aimed at reducing biochemical inflammatory mediators or reducing nociception.
It is important to adhere to the prescribing cascade; unlicensed products should only be used when no suitable equine alternative exists, with owner consent and following cascade guidelines (GOV.UK, 2019).
It is equally important to ensure section IX of the patient’s passport has been filled in; otherwise only medications permitted in food-producing animals may be used. Guidance should be given to owners on medicine information entry into passports.
A holistic approach to equine pain includes not only use of multimodal analgesia, but also treatment/removal of the source of pain and supportive therapy of the connected body systems, with optimum nutrition and environmental conditions to maximise repair and recovery.
A multimodal approach to analgesia results in the most effective pain relief for a patient, including different classes of analgesic appropriate for the type of pain. Where possible, the source of inflammation should be treated and removed. Targeted analgesia, such as intra-articular injections or epidurals for hindlimb pain, are very effective in appropriate cases.
Complementary supportive therapies, such as physiotherapy and acupuncture, should be considered – particularly for relieving muscle tension associated with pain. The importance of rest and controlled movement for repair and recovery from painful conditions should not be underestimated.
Pain relief should be tailored to the individual patient and client, as dictated by the type and degree of pain, and the clients’ ability to administer medications. Cost is not a justifiable reason on the cascade for using an off-licence product when a licensed alternative is available.
It is worth noting donkeys have a different metabolism to horses, with generally a quicker metabolism of most drugs, and therefore dosing of analgesics is different in this species (Grosenbaugh et al, 2011).
NSAIDs inhibit cyclooxygenase (COX) pathways’ inflammatory mediators production, and are therefore a cornerstone of pain relief for inflammatory conditions. Several licensed options exist for horses in the UK, including flunixin, phenylbutazone, ketoprofen and meloxicam in various preparations.
The most common side effect of NSAIDs in horses is colonic (rather than gastric) ulceration, with subsequent diarrhoea and weight loss associated with protein loss. Clients should monitor horses for diarrhoea, even if only a short course of NSAID is prescribed. Nephrotoxicity is a less common side effect of NSAID use in equines. It is logical to periodically monitor serum protein levels and renal parameters with chronic NSAID use.
Both flunixin and phenylbutazone are licensed for once-daily dosing, but are frequently given twice daily for painful conditions in horses. Phenylbutazone has a more rapid clearance in donkeys compared to horses, so should be administered twice daily to standard-sized donkeys and three times daily to miniatures (2.2mg/kg IV to 4.4mg/kg IV; Grosenbaugh et al, 2011). It is not advisable to use meloxicam in donkeys as it has a very short half-life (Grosenbaugh et al, 2011). Carprofen (used under the cascade) has a slower metabolism in donkeys, so should be given as a single IV dose if used.
Although unlicensed, paracetamol is useful under the cascade in some equine patients that aren’t suited to licensed NSAID alternatives (20mg/kg twice daily by mouth). It has 91% oral bioavailability in horses and has been shown to be safe dosing twice daily for 14 days in healthy adult ponies (Mercer et al, 2019).
The author finds it particularly useful in laminitis cases that remain painful on other NSAIDs, or that can’t have NSAIDs due to associated colonic ulceration or renal impairment (West et al, 2011). When used in combination with other analgesics, it can have a synergistic effect on pain relief. No published research exists to date on IV paracetamol use in equids.
Corticosteroids include prednisolone and dexamethasone licensed for systemic therapy. They are potent anti-inflammatories acting on the COX and lipoxygenase pathways, and therefore indicated in some painful inflammatory conditions
They can be given alongside an NSAID for better pain relief. Despite concerns regarding laminitis development with exogenous corticosteroid use, research examining medical records has shown laminitis does not occur more frequently in animals treated with corticosteroids than untreated controls or the general equine population (Jordan et al, 2017; Potter et al, 2019).
It is, however, prudent to avoid corticosteroid use in animals with a laminitis risk factor (endocrine disease, obesity, previous laminitis) – particularly in ponies (Potter et al, 2019). Corticosteroids are contraindicated in hyperlipaemic animals (Grosenbaugh et al, 2011).
Opioids are potent analgesics, although their use in equines has often been avoided due to concern over reduction of gut motility and subsequent large colon impaction. Pain itself can be a cause of reduced gut motility, and the analgesia offered by opioids may be of benefit over and above their effect on motility. Butorphanol, buprenorphine and pethidine (all licensed), and morphine, fentanyl and tramadol (all unlicensed) are opioids that have been used in equids.
Butorphanol is frequently used in colic cases and can be used in combination with NSAIDs to provide extended periods of analgesia between NSAID doses. Buprenorphine’s effects last 6 to 12 hours, and it is useful in needle-shy horses with good absorption across the oral mucosa (6μg/kg). Pethidine must be given IM to avoid histamine release-associated seizures, and is only useful for short-term analgesia, with a duration of action of 1 to 4 hours.
Morphine can be given IM or IV, or via epidural, but needs to be administered every four hours as it has a short duration of action. Excitement can be seen with IV administration, therefore it is normally given alongside sedation IV. A common side effect is pruritus, which has caused problems for the author in patients with epidural catheters that have rubbed the catheter out.
Fentanyl patches, whereby fentanyl is absorbed transdermally over time, provide longer lasting pain relief, with patches needing to be changed once every three days in horses. This can be invaluable in severely painful laminitis cases. It should be noted that peak serum concentration can take more than eight hours to be achieved; therefore additional analgesia will be required while waiting for the first applied patch to take effect. More frequent fentanyl patch changes are required in donkeys compared to horses (Grosenbaugh et al, 2011). The area for fentanyl patch attachment should be shaved and clean, to maximise skin contact and absorption.
Tramadol has a poor oral bioavailability (11%) in donkeys and is, therefore, not recommended (Grosenbaugh et al, 2011). In horses, the bioavailability seems to be variable, and it has a short half-life. Therefore, the author finds little use for this opioid in equine practice.
Gabapentin (used under the cascade) is thought to work by inhibiting neurotransmitter release. The neurological side effects commonly associated with its use in other species do not seem to occur in horses. It has been used in horses at doses of 10mg/kg to 20mg/kg by mouth two to three times daily (every 12 to every 8 hours).
Refractory laminitic patients and burns patients can benefit from this, as can some head shakers, since it acts on neuropathic pain. It can also be useful in painful colicking colitis cases, which NSAIDs are contraindicated in. However, its expense limits its long-term use.
Systemic lidocaine can be useful in postoperative colic and colitis cases for its anti-inflammatory – and, therefore, pain-relieving – effect. A loading dose of 1.3mg/kg is followed by a continuous rate infusion at 0.05mg/kg/min, delivered by an infusion pump. If a patient develops neurological signs while on lidocaine, the infusion should be stopped; this will normally resolve the neurological effects.
Microdosing of ketamine at sub-anaesthetic doses can provide excellent short-term analgesia. The author finds it useful in extremely colic cases to permit examination, at a dose of 0.2mg/kg IV, alongside alpha-2 agonist sedation such as xylazine or detomidine. The effect is short-lived, lasting 15 to 20 minutes, meaning pain indicating the need for exploratory laparotomy surgery won’t be masked for long. It is metabolised more quickly in donkeys than horses, so is less helpful in this species.
The use of cannabinoids for treatment of various painful conditions is gaining traction in human medicine. The two main active substances from cannabis plants are the psychotropic tetrahydrocannabinol (THC) and cannabidiol (CBD). Cannabinoids have a complexity of interactions on mammalian homeostasis, but at least theoretical potential exists for their use in pain relief (Landa et al, 2016).
Products containing CBD, but no THC, are now legally available in the UK for human use. This has sparked a wide interest by owners in the use of CBD products in pets, including horses. To date, no scientific evidence exists to support the use of CBD products in horses. Furthermore, pet-specific CBD products are illegal in the UK.
The VMD has released a statement stating a veterinary surgeon may prescribe a human CBD-based product for animal use under the cascade since no veterinary licensed products exist in the UK (GOV.UK, 2018). Use of CBD products in horses without a veterinary prescription is illegal in the UK. The psychotropic properties of THC-containing products may influence locomotion and would, therefore, be a safety concern in equids, even if they were legal.
Acupuncture has value as an adjunct therapy in some horses with chronic pain, such as laminitis (Faramarzi et al, 2017). However, in other studies it has not been found to have any positive effect on pain relief and more good-quality evidence is needed to support its use in equine medicine (Robinson and Manning, 2015). Electro-acupuncture and percutaneous electric nerve stimulation (PENS) treatment have both shown promise in treating trigeminal-mediated head shaking pain in horses (Devereux, 2019; Roberts et al, 2016).
Dietary supplementation with omega-3 fatty acids has been shown to have positive anti-inflammatory properties in the equine respiratory system, and as more research is done, this supplement may prove worthy for use in chronic orthopaedic painful conditions in horses (Nogradi et al, 2015).
Most oral drugs have optimum absorption when given on an empty stomach, so this would coincide with first thing in the morning for most horses. Owners should be educated to orally dose their horse in the morning half an hour before feeding any concentrate feeds or forage, to maximise absorption, rather than the common practice of giving medication in a concentrate feed mix.
