A foot abscess can create a painful build-up of pus — and the prospect of a more
serious problem if treatment is not prompt. Richard Stephenson MRCVS explains
They may be a frequent cause of lameness in the horse, but this does not mean that foot abscesses are easy to diagnose or treat.
A number of pitfalls can complicate a simple case of “pus in the foot”. While most horses recover quickly after developing an abscess, the potential for further damage should never be underestimated.
An abscess most typically occurs when bacteria carrying dirt penetrate the white line of the foot, the area where the sensitive laminae of the pedal bone interlink with the insensitive laminae of the hoof wall. Dirt and/or gravel can be forced through this weak point into the more sensitive tissues.
The response of the blood supply in the hoof is to try to remove the foreign tissue by surrounding it with white blood cells. This can lead to a build-up of pus in the area. The abscess which forms creates increased pressure within the hoof, resulting in pain.
Another common cause is penetration of the foot by a sharp object, typically a nail.
What to watch for
The signs of a foot abscess can be varied and sometimes confusing. The affected horse classically displays:
Lameness — which may start as slight but can rapidly become severe. Foot abscesses can be extremely painful and may even be mistaken for a fractured limb by an owner.
Sweating — due to pain.
Mild colic-like symptoms — the horse may be restless or anxious to lie down.
Heat — in the foot.
A bounding pulse — at the level of the proximal sesamoids, the two small bones located at the back of each fetlock.
Swelling of the fetlock — and distal [lower] third of the cannon, often confused with a tendon injury.
Who’s at risk?
Hoof conformation and management can be contributing factors. Collapsed, weak heels often seem susceptible to abscesses if not sympathetically shod.
Surveys have shown that unshod horses are affected by foot abscesses almost twice as frequently as those with shoes. Leaving shoes on too long, however, can result in the heels of the shoe resting within the seat of corn to either side of the caudal frog, provoking infection. Long shoeing intervals (of more than six weeks) should therefore be avoided.
Previous laminitis attacks make a horse very much more prone to pus in the foot. While it has been suggested that certain hoof supplements containing high levels of sulphur encourage bacterial propagation in the white line, there is as yet little scientific evidence to support this.
Other factors aside, there can be no doubt that wet, muddy conditions make foot abscesses much more likely.
Radiograph of a foot with a probe following a track left by a nail
Tracking infection
Simple but thorough treatment is usually effective. Occasionally, an abscess is not detected early enough or proves difficult to drain. A pool of pus can then form under the sole and infection may begin to eat into the pedal bone.
The hoof wall must then be resected (cut out) and the infected bone scraped away. The wall grows continuously from the coronary band, replacing itself every six to eight months, so the foot will eventually heal.
Another complication is that the abscess may burst out through the coronary band. This is usually very painful for the horse: for the preceding 24 hours he may be almost unable to stand on the affected limb.
While the eruption of an abscess through the coronary band is not desirable, there is usually very little long-term damage, and recovery is often good once the pus has been drained.
Immediate veterinary investigation is required if the suspected cause of an abscess is penetration of the sole by a sharp object. Permanent or even life-threatening lameness can result if damage occurs to the vitally important structures within the hoof — deep penetration can damage the digital flexor tendon or, worse, the navicular bursa.
The ability to take MRI (magnetic resonance imaging) scans of the feet has added a powerful new tool to the vet’s armoury when dealing with foot penetrations, enabling puncture wounds to be tracked through the hoof and mapped out.
A foot abscess provides an ideal environment for the bacteria that cause tetanus to grow, so keep your horse’s vaccinations up to date.
Further reading
• ”Presenting signs of foot abscessation — a practice-based survey of 150 cases” ,UK Vet Vol. 16 October 2011, Richard Stephenson
• www.horseandhound.co.uk/tag/hoof-problems
Ref: Horse & Hound; 10 September 2015