Veterinary column
Laminitis
22nd April 2009
Veterinary Column by David Bardell MRCVS
Laminitis is a complex, systemic disease producing pain and distress, which despite extensive research it is still incompletely understood. It is difficult and frustrating to treat, and can result in the animal having to be destroyed for humane reasons. Although often associated with ponies kept on rich pasture in the spring, horses are also susceptible and it can occur at any time of year.
There are many potential ‘trigger factors’ for the development of laminitis, but they all lead to a final common sequence of events which produce the signs of pain associated with the feet. The bone within the foot of the horse, through which all the weight bearing forces are transmitted is called the pedal bone. Within the normal foot, this bone is attached to the inside surface of the hoof by hundreds of microscopic interlocking folds called laminae. In laminitis, blood flow to the foot is generally increased but diverted away from these laminae through special ‘bypass vessels’. As a result they start to suffer from lack of oxygen and nutrients which compounds the circulatory disturbances. This weakens the bone to hoof attachment allowing the bone to tear away from the inside of the hoof wall due to bearing forces and the pull of the deep digital flexor tendon. Depending on how much of this attachment is lost the pedal bone may rotate or sink.
Feed related laminitis is the type most often encountered in the UK and is due to excessive dietary carbohydrate such as sudden access to high energy feedstuffs or lush grass. Significant seasonal variations in grass growth and sugar content can occur, putting horses at risk without obvious feeding or management changes having occurred.
Other factors predisposing to the development of laminitis include severe foot trimming, excessive exercise on hard ground or roads, Cushing’s disease and endotoxaemia following an infection, colic or retained placenta. Lame horses which persistently rest one leg may develop laminitis in the opposite leg from repeatedly overloading it. Any horse which fits into one of the above categories must be managed with laminitis in mind. Corticosteroid administration for treatment of other conditions has been associated with laminitis and these horses should be closely monitored.
It is important to appreciate that changes at a microscopic level to cells and blood vessels may have been ongoing for two to three days before clinical signs become evident. Sometimes this is as far as things progress if the initiating cause is removed and therefore no clinical signs develop. If symptoms do develop, the disease is then divided into acute or chronic forms, depending on how it progresses.
Acute phase
This is generally considered to be the first 3 days from appearance of clinical signs. These may be severe or very subtle such as a slightly stiff gait only evident if the animal turns sharply or walks across a hard surface, whilst being undetectable on soft ground. Often there is reluctance in allowing one foot to be picked up because this increases the weight bearing and discomfort in the opposite foot. Using hoof testers to squeeze the foot will elicit an increased reaction in the toe region. An increase pulse in the digital arteries may be detected by feeling over the sesamoid bones at the back of the fetlock joint. Changes may be subtle or the pulse may be prominent and ‘bounding’. Generalised increased heat in the feet may be felt but this can be misleading. The horse may lean back on its heels when standing and adopt a typical ‘heel to toe’ placement of the foot when walking to limit load bearing on the toe. Typically one or both front feet are affected, but all four feet may be involved and the horse may lie down and be difficult to persuade to stand. This can initially lead to confusing the signs with those of a horse with colic. Horses may also sweat up and become anxious and distressed. Extensive irreversible damage can occur within 24 hours of the onset of acute laminitis so these are true emergency cases.
First the inciting cause must be identified and removed. This usually involves preventing further grazing by confining in a stable on a soft deep bed. Non-steroidal anti-inflammatory drugs such as phenylbutazone (‘bute’) are the mainstay of pain relief by reducing the inflammatory reaction. Restoration of normal circulation in the foot is often attempted using acepromazine (‘ACP’ or ‘Sedalin’ gel), or ointments containing the compound glyceryl trinitrate applied to the skin over the digital arteries. The ability of these drugs to open up the blood vessels is controversial, however the sedative action of acepromazine may help in relieving the anxiety that these horses feel. Other drugs to improve circulation and prevent blood clots forming may also be used.
Feeding needs to be carefully controlled. Energy intake must be reduced but fibre intake maintained. Reducing feed too much can lead to another serious condition developing called hyperlipaemia. Hay is usually used although if it is very high quality, soaking may be advisable to reduce the sugar content. Commercially formulated diets are now available which make feeding much easier.
Exercising these animals is contraindicated, both on humane grounds and because this will increase the risk of further damage, encouraging rotation or sinking of the pedal bone. Commonly four to six weeks of box rest are required to ensure adequate stability has returned to the affected feet.
Support and cushioning of the feet is vital and normal shoes should be removed. Greater support under the frog and heel region is required. Initially this might be something as simple as rolled up bandages taped onto the frog or specialised high density foam support pads designed to crush and conform to the shape of the foot which are then trimmed to a custom fit. Taking radiographs (‘x-rays’) of the feet may well be recommended within the first few days if initial treatment does not appear to be working. This allows your vet to see whether rotation or sinking of the pedal bone has occurred and if so how severe, permitting a more accurate prediction of treatment length or success to be made.
Once the foot is comfortable, bar shoes made by your farrier will provide more permanent support. Padding and corrective shoeing also contribute to improving blood flow within the foot.
Re-shoeing and trimming may have to be performed as frequently as fortnightly in the early stages as gradual changes are much safer and more comfortable for the horse. Radiographs may be repeated to monitor progress and assist the farrier in making and fitting the shoes correctly. If no structural changes take place, as in milder cases, this progresses to the subacute phase where the condition is brought under control and treated successfully. This may still require several months of treatment and careful management. Reintroduction to grass needs to be undertaken with great care and usually will only be started once the horse is completely comfortable when pain relieving medication has been stopped. Initially only 5 to 10 minutes a day are permitted and this period will be gradually built up as long as the animal remains free of symptoms. A small paddock of short or poor quality grass is ideal for this. Access to a sand paddock or ménage is very useful and can be used to increase the time out of the stable, without the danger of the animal being able to increase its food intake.
Chronic phase
This describes the course of the disease following rotation and/or sinking of the pedal bone. The sole of the foot will lose the normal concave profile and become flat or convex. In some cases part of the hoof wall may require cutting away to remove pockets of fluid or abnormal tissue. In extreme cases the pedal bone can perforate through the sole of the foot. As well as managing the pain, inflammation and providing support to the foot, the condition now becomes complicated by other factors. Poor quality hoof horn and abnormal growth patterns develop with widening of the white line making recurrent foot abscesses common. Acute flare ups can happen due to the damage already present in the foot. If these are frequent or treatment is unsuccessful, euthanasia on humane grounds may be required. Chronic cases may require treatment for 12 to 18 months and permanent changes are often required in the way the horse is managed.
As damage occurs early in the course of the disease, and treatment is difficult, expensive and requires a committed, coordinated approach from owner, vet and farrier, prevention is key with laminitis. Identifying at risk animals such as overweight ponies and managing their grazing and diet strictly is vital. Spotting early signs and acting promptly will maximise the chances of recovery. The complex nature of the disease means that there are multiple ways of treating and managing these cases. However whichever methods are applied, they all aim to alleviate pain, restore normal blood flow and support the structures within the foot to minimise further damage. If in any doubt, always consult your vet.
Illustrations
Typical `leaning back` stance adopted by a laminitic animal.
A. Lateral radiograph of mild/early case of laminitis. A marker has been taped onto the dorsal hoof wall running from the coronary band to the toe to assess alignment and sinking of the pedal bone. Note the front surface of the pedal bone is parallel with the hoof wall. Also the top of the pedal bone is level with the top of the marker.
B. This radiograph shows a more severe/advanced case of laminitis. Note that the pedal bone has rotated and is no longer parallel with the hoof wall and the pedal bone has sunk; the top is now lower than the top of the marker.
