Lameness is a huge concern for equine athletic performance. Means of controlling musculoskeletal pain are critical strategies for equine practitioners and horse owners. Typical approaches include the use of NSAIDs and/or intra-articular joint injections.
A study reviewed the efficacy of oral acetaminophen in horses with mechanical lameness as an alternative or adjunctive therapy [Mercer, M.A.; McKenzie, H.C.; Byron, C.R.; et al. Pharmacokinetics and clinical efficacy of acetaminophen (paracetamol) in adult horses with mechanically induced lameness. Equine Veterinary Journal, May 2022; DOI: 10.1111/ evj.13601].
The journal article explained that in a 2015 U.S. national survey, “over 65% of equine premises with greater than three horses had an animal experience lameness in the previous 12 months, and on any given day, 7.1% are expected to be lame.” As well, 54.2% were treated with an NSAID. In some cases, NSAIDs were only partially effective or were not effective at all. In other cases, NSAID administration is contraindicated for a variety of medical reasons.
Acetominophen has fewer detrimental gastrointestinal effects compared to other NSAIDs, and it has a high safety margin. Therefore, it might serve an effective role in equine pain amelioration.
An acetaminophen study had nine healthy mares ages 14-23 years that were fitted with a horseshoe with reversible sole pressure that induced pain and Grade 4 lameness (on the 5-point AAEP). The mares were treated orally with one of the following compounds mixed with water and dark corn syrup in each 10-hour trial period:
- 20 mg/kg PO acetaminophen
- 30 mg/kg PO acetaminophen
- 2.2 mg/kg PO phenylbutazone
- An oral placebo
All horses received all treatments following a three-day washout period between each treatment. Feed was not withheld during the studies. Blood samples were taken at time 0, 15, 30 and 60 minutes, and at two, four, six and eight hours after drug or placebo administration.
Peak acetaminophen concentrations occurred at a mean of 0.66 hours for the lower dose, and a mean of 0.43 hours for the higher dose. Half-life elimination was relatively comparable for both doses—3.5 hours (20 mg/kg) vs 5.3 hours (30 mg/kg). Heart rate and heart rate variability were measured, with the higher dose acetaminophen eliciting a significantly lower heart rate than the lower dose.
Videos of straight-line trot in hand on asphalt were used to assess lameness and response to treatment using a 10-point lameness scale that tracks more subtle changes than the 5-point lameness scale.
The study demonstrated a hysteresis loop of a time delay between measured plasma concentrations and drug effect. This pilot study was unable to “determine a therapeutic plasma concentration for analgesia.”
That said, the authors reported that the higher concentration of 30 mg/kg significantly improved lameness compared to 20 mg/kg when used as a monotherapy.
In this study, oral phenylbutazone at 2.2 mg/kg did not elicit significant differences in lameness scores until eight hours post-treatment. It is thought that because oral phenylbutazone binds to intestinal digesta, there is a prolonged absorption time to take effect.
In conclusion, the research showed that a single dose of acetaminophen given at 30 mg/kg “produces a more rapid onset of greater improvement in lameness scores and heart rate compared to the other treatments used in this study.”