Asymmetry in horses: Is it always a result of lameness?

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Is asymmetry caused by pain or disease, or is it simply an expression of natural biological variations? Photo: File
Is asymmetry caused by pain or disease, or is it simply an expression of natural biological variations? Photo: File

A significant number of horses in training considered sound by their owners have been shown in studies to have uneven movement that raises suspicions of mild clinical lameness.

It raises an important question: Is this asymmetry caused by pain or disease?

Or is it possible that it is simply an expression of natural biological variations?

Researchers in Sweden devised an experiment to determine whether movement asymmetries in riding horses in training were affected by treatment with the non-steroidal anti-inflammatory drug meloxicam.

Emma Persson-Sjodin and her colleagues used a crossover design for their study, in which 66 horses were treated with meloxicam or a placebo for four days, with a 14 to 16 day washout period between treatments.

They used a mix of warmblood riding horses either privately owned or belonging to two equestrian centres and two riding schools. All were stabled near the University of Agricultural Sciences in Uppsala, where the study was centred.

To be used in the study, the horses had to be in full training and considered free from lameness by their owner. Any horse treated for lameness within the preceding two months was ruled out.

The horses were assessed for asymmetry using inertial sensors, which detected any uneven movement in the head and pelvic region at a level that might be considered indicative of mild clinical lameness.

In all, 66 horses were used with average mean asymmetries greater than 6mm for the head or more than 3mm for the pelvis.

The body-mounted accelerometers were then used to measure any asymmetry on a hard and a soft surface before and on day four of each treatment protocol (the meloxicam or placebo) at the preferred speed of each horse trotted in-hand on the straight.

The researchers applied several models in analyzing the data, and found that treatment with meloxicam did not significantly affect the movement asymmetry.

“These results raise new questions,” the study team, writing in the open-access journal PLOS ONE, said.

“Are the movement asymmetries in riding horses in training simply expressions of biological variation or are they related to pain/dysfunction that is non-responsive to meloxicam treatment?

For the study, a total of 140 horses were initially screened. Out of these, 32 did not present with motion asymmetries and five were excluded due to being too difficult to handle.

Further exclusions were made, for reasons ranging from being unable to comply with the study plan, to development of lameness judged too serious to continue with training.

The final 66 animals comprised 41 geldings and 25 mares.

The researchers, discussing their findings, said meloxicam is commonly used in equine practice for the treatment of lame horses with orthopaedic disorders or for analgesic testing.

“It is … expected to be effective in alleviating pain of inflammatory origin via reduction of prostaglandin synthesis.

“The non-responsiveness of the horses in the present study thus makes pain of acute inflammatory origin a less likely cause of the movement asymmetries.

“However, it does not exclude other types of pain being present, such as chronic or neuropathic pain.”

Meloxicam has also been shown to be ineffective in reducing pain in a hoof pressure lameness model, which unrelated research has suggested can be treated with a combination of phenylbutazone and flunixin meglumine.

“This suggests that pain in chronic lameness, non-responsive to one single class of nonsteroidal anti-inflammatory drug (NSAID), may be alleviated by combination treatment.

“Thus, treatment with only meloxicam, as in the present study, may not be potent enough to obtain a group level effect in chronic lameness cases.”

Nevertheless, the findings warranted further investigation into the underlying causes of movement asymmetries, they said.

If underlying painful pathologies are identified in a large proportion of horses, the presence of movement asymmetry should be regarded as a severe welfare problem, they wrote.

“If, on the other hand, no underlying pathologies linked to pain are identified, the conclusion may be that these asymmetries are due to biological variation as motor laterality or conformational asymmetries.

“In order to avoid unnecessary lameness examinations or to avoid erroneously failing horses during pre-purchase examinations, this remains an important research question.”

The authors stressed that, while their study showed that treatment with four days of meloxicam did not significantly decrease the movement asymmetry in the horses in the study, it did not exclude the possibility that pain may be present.

The full study team comprised Persson-Sjodin, Elin Hernlund, Pia Haubro Andersen and Marie Rhodin, all with the Swedish University of Agricultural Sciences; Thilo Pfau, with the Royal Veterinary College in England; and Karin Holm Forsström, with the equine clinic at the University Animal Hospital in Uppsala.

Persson-Sjodin E, Hernlund E, Pfau T, Haubro Andersen P, Holm Forsström K, Rhodin M (2019) Effect of meloxicam treatment on movement asymmetry in riding horses in training. PLoS ONE 14(8): e0221117. https://doi.org/10.1371/journal.pone.0221117

The study, published under a Creative Commons License, can be read here

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