Brief History and Outline of Techniques

Counterirritation techniques have long been a part of veterinary therapy, particularly in horses. Major veterinary texts, such as those by Percivall,¹ Wooldridge,² and even Stashak³ as late as 1987, devoted considerable attention to indications and techniques. To some this now seems misguided and even cruel, but one has to read only a few of the older texts to admire the authors’ inquiring minds and their thorough observations on matters medical and particularly orthopedic. They were not far from the truth, but they did not have the benefit of modern anesthetics or imaging equipment to learn and apply more direct and rational therapies. Techniques currently used include hot or cold firing and blisters, but use of those therapies is much reduced because of changes in horse ownership and veterinary attitudes. This chapter considers both British (DRE) and North American (SPD) perspectives.

Blistering

Limited availability of ingredients and ignorance of preparation and safe use has restricted blistering of horses in the United Kingdom. Although veterinary surgeons or equine wholesalers are the source, the horse owner is usually the user. More severe blisters, such as those containing cantharides or croton oil, are used less commonly than the working blisters that are iodine or mercuric iodide based and mainly used on splints, sore shins, or curbs. The technique involves rubbing or brushing the liquid preparation onto the clipped skin once daily until a scale forms. The horse is kept in light exercise, such as walking, trotting, or hack cantering, and this phase usually is undertaken after the initial rest and antiinflammatory treatment have removed heat, swelling, and pain (i.e., 7 to 10 days after initial clinical signs). When the heat and soreness from counterirritation has settled, which takes 2 to 3 weeks, the horse resumes work.

Irritation from working blisters is minimal, and protective measures such as antiinflammatory treatment, keeping the horse crosstied, or fitting a bib or cradle are not necessary.

Strong blister was made mostly from cantharides (Spanish fly) or croton oil, but nowadays these ingredients are difficult to obtain and, because they are not licensed drugs in the United Kingdom, are probably illegal for a veterinary surgeon to import without a permit. As a consequence the strong blister that is used most commonly is made as a cream using red mercuric iodide. The skin is clipped and the blister rubbed into the area for 5 minutes. Excess is removed and petroleum jelly is applied to flexor surfaces distal to the blistered areas. The horse is kept shod for blistering, in case it paws the ground, and a neck cradle is applied until the initial inflammatory phase has subsided. It is essential that the horse continues walking exercise, usually 30 minutes twice daily. A substantial risk of laminitis or lymphangitis exists if blistered horses do not have this regular exercise. When blisters form and burst, they are left alone and sprayed with antibiotic powder. Rarely is the reaction severe enough to warrant antiinflammatory treatment. After 10 days the blistered areas of the legs are smeared with petroleum jelly, which is left on overnight and then washed off with warm water and soft soap. This softens and lifts the scabs free, without causing bleeding or further irritation. Any blistered areas that are still moist are dressed with antibiotic wound powder, and walking is continued. Scabs that persist after 7 to 10 days are removed by repeating this procedure. After 6 weeks the horse can be turned out to pasture or start light ridden exercise. The program for a return to training depends on the nature and progress of the original injury.

Areas treated most commonly with strong blister are the digital flexor tendon region of forelimbs and the fetlock joints. Indications are tendonitis, tenosynovitis, suspensory desmitis, and chronic effusion of fetlock joints.

In North America, curbs frequently are treated by blistering (see Chapter 78), and both hindlimbs are usually treated. The area is clipped and rubbed aggressively for 3 minutes with medical-grade turpentine and then with kosher salt for 3 minutes, alternating for a total of 15 minutes. This is repeated once daily for 3 days. The horse is exercised immediately after each treatment for at least 20 minutes. The skin excoriates and remains thickened and inflamed for some time, but therapeutic effects are almost immediate.

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