The potential role of Clostridium botulinum toxin in the treatment of equine laminitis

Abstract

Laminitis is a severe and painful disease in the horse that may lead to loosening of the attachment of the distal phalanx to the horny hoof capsule. Although a large variety of treatment options has come and gone since the first descriptions of laminitis, there is certainly a thus far unmet need for therapies that are successful in preventing dislodging of the distal phalanx or in creating favourable conditions for healing. Clostridium botulinum toxin has been successfully used in equine medicine to reduce the activity of several skeletal muscles. It was therefore hypothesised that the toxin, when used to reduce the muscle tone of the deep digital flexor, could have potential to reduce the traction of the deep digital flexor on the distal phalanx which might be a useful supportive therapy in case of laminitis. First, the effectiveness and safety had to be tested in the healthy horse: Clostridium botulinum toxin injections proved to reduce the muscle tone of the deep digital flexor muscle significantly without severe side-effects or inducing lameness in the healthy horse. Therefore, further research into the clinical application of the substance was justified. It has been suggested that laminitis may result in increased muscle tone or even contracture of the deep digital flexor. This suggestion was investigated, as the outcome could further substantiate the hypothesis.The increased firing rate of the deep digital flexor muscle in laminitic horses, as evidenced by EMG interference pattern analysis, suggests an increased muscle force which provides some support for the claim that reduction of the muscle tone by use of Clostridium botulinum toxin might be a successful supportive therapy. However, further in-depth research into this item is warranted, as two other parameters, the number of turns/second and amplitude/turn, were not dissimilar, indicating that larger motor units with larger-amplitude motor unit potentials were not recruited. It proved possible to determine the muscle volume of the deep digital flexor in the living animal quite accurately. This way of determining muscle volume can be used in future clinical studies to estimate individual Clostridium botulinum toxin dosages as precise as possible. Not only for efficacy and safety reasons, but also taking into account the high costs involved, this is beneficial. Realization of a large-scale clinical study into the application of Clostridium botulinum toxin in cases of naturally occurring acute laminitis proved difficult. The main reason is that recruitment of a sufficiently large population of acutely laminitic horses and ponies without distal phalanx displacement and complying with strict inclusion criteria is problematic. However, the beneficial outcome as seen in two animals suggests that putting effort in the design and execution of such a (multi-centre and probably multinational) trial using a uniform and objective evaluation protocol is justified. As further research into the application of Clostridium botulinum toxin in acute and chronic laminitis seems justified, suggestions for future research include i.a. its use in chronic laminitis and in the prevention of supporting limb laminitis.