It is often said that the equine hoof is a miracle of bioengineering, which has to fulfil a variety of functions. One of the roles of the sole, hoof wall and frog is to protect the more sensitive structures within the foot. Under certain conditions, however, these protective elements can become subject to problems and conditions.
Two particular equine hoof disorders warrant attention at this time of year: thrush, a very common infection, and canker, a rare but serious condition that often results in euthanasia on humane grounds or due to financial constraints.
Thrush is thought to be related to long hours spent standing in unhygienic stables or to the wet winter months, yet cases do occur in animals kept in the most meticulously clean and dry yards. Poor yard management cannot, therefore, be the only contributing factor for a thrush infection.
It appears that certain hoof conformations, especially those with poor frog function, play their part in allowing the development of the disease. Deep grooves on either side of the frog or a split central sulcus (groove) can form an ideal environment for the development of microorganisms, which cause thrush.
It is more difficult to attribute the occurrence of canker to specific risk factors. The exact cause of the condition has still not been determined. Although it was commonly assumed that heavy horse breeds, such as draught horses, were more susceptible to the disease, experience has revealed that canker can affect any horse, of any breed, sex or age.
One similarity with thrush, however, is that the condition appears to be more common in poorly functioning feet. Tall and contracted heels, with deep grooves which are barely exposed to air, offer an ideal environment for microbes to flourish.
The lowdown on thrush
Thrush is a predominantly bacterial infection of the frog. The condition occurs in the grooves on the side of the frog or in the central cleft, or both. Bacteria cause degeneration and breakdown of the frog tissue.
The main characteristic sign is the presence of a black, foul-smelling discharge, coming from the infected areas. An affected horse is not usually lame, unless the infection is so severe that it has reached underlying structures of the foot.
Although a number of bacteria and possible fungal elements can be involved, the bacterium fusobacterium necrophorum is most commonly identified in thrush. These are so-called “opportunistic” bacteria, which will not normally cause a pathological infection but can gain a foothold in less than healthy, damaged or poorly functioning hooves.
Treatment involves the removal of the diseased tissue by a vet or hoof-care provider. The frog should be thoroughly cleaned with an antiseptic wash and dried well. The application of astringent or antimicrobial products can be beneficial, as well as the exposure of the infected tissue to air – as the fusobacterium necrophorum is anaerobic and lives in the absence of oxygen.
This process alone may not always be sufficient to manage the infection in the long term. Often, the key to success is identifying and rectifying the reasons why the infection has gained access in the first place.
A management programme should be established to help keep the horse’s feet as clean and as dry as possible. Significant improvement will be achieved by addressing inappropriate foot balance, such as tall and contracted heels which lead to a lack of frog stimulus, or sheared frogs which appear as a fissure of the central cleft of the frog that extends to the sensitive tissues of the heel.
A well-functioning frog and optimal hoof mechanics improve blood circulation and increase the growth of healthy tissue, which helps to keep infections at bay. Furthermore, the benefits of adequate exercise of the horse for the establishment of a healthy frog area should not be underestimated.
If thrush has affected deeper, sensitive tissues of the hoof and causes further signs such as lameness, swelling, heat and inflammation, your vet may consider the use of topical antibiotics.
In most cases of thrush the prognosis is good. If infection persists, regardless of treatment, any possible underlying causal factors should be reassessed.
A complex condition
Canker is a relatively rare infectious disease of the hoof, which leads to “hypertrophic” (abnormal and uncontrolled) growth of the cells of the frog and hoof tissue. Typical signs are frog tissue that produces cauliflower-like material, often of a yellow, creamy-white or orange colour, with a soft, cheese-like texture. In advanced cases, finger-like tendrils can grow out of the frog. A foul odour is often present and the abnormal tissue has a tendency to bleed easily when manipulated.
Canker may not only affect the frog but also the bars, sole and hoof walls, down into the deeper sensitive part of the hoof. Lameness does not usually occur until the disorder becomes more severe or long standing.
Treatment of this complex disease can be challenging. Early recognition and immediate involvement of a vet are undoubtedly crucial. Research has shown that canker is often misinterpreted as persistent thrush in the early stages and mistreated as such, which hugely influences the possibility of a successful outcome.
Different treatment approaches have been tried, though none seem to have been consistently successful. Treatment usually starts with careful removal of diseased tissue, often with hoof knives, scalpels, electrocautery or cryotherapy.
Maggot debridement therapy (MDT) can also be used (pictured, below). After surgical removal of obviously diseased tissue, sterilised fly larvae are applied to the hoof and kept in place with sealed bandages. The maggots feed on dead, infected tissue and microorganisms, further disinfecting the tissue by secreting antimicrobial molecules and stimulating healthy growth.
The topical application of antibiotics such as metronidazole or astringent products has been shown to be beneficial. Recent studies have tried to identify microorganisms that are present in canker tissue, in order to find the most effective treatment. Treponemes (a type of anaerobic bacteria), which have been isolated from diseased tissue, are involved in causing foot rot in sheep and cattle. These bacteria are sensitive to certain antibiotics, which may be useful in the treatment of horses with canker.
Bovine papillomavirus particles have also been isolated from canker tissue. This virus is believed to be a causal agent for equine sarcoids, suggesting a relationship between the two disorders. Studies suggest that the application of cisplatin, as a local chemotherapeutic agent, may be a promising therapy. This is very toxic, however, and must only be used in very controlled circumstances and under strict veterinary guidance.
Early diagnosis and treatment of canker greatly increases the chances of a successful outcome. Cases that are longstanding, with the involvement of multiple limbs and disease that invades not only the frog but other areas, still have a poor prognosis.
- Try to establish a clean and hygienic environment for your horse.
- Check his hooves regularly for any discharge, or irregular changes in and around the frog area.
- Focus, together with your vet and farrier, on trying to achieve optimum foot balance, minimising any distortions that may result in a poorly functioning rear half of the hoof.
- Give your horse adequate exercise on a surface that allows frog engagement — grass or a good school surface. If possible, allow him time on a dry pasture to walk around freely.
- Be aware of the different signs of thrush and canker.
- Involve your vet or farrier immediately if you suspect a hoof disorder. Attempting to treat conditions yourself can waste valuable time and jeopardise the chances of your horse’s recovery.
Ref Horse & Hound; 10 November 2016