Thrush is usually (but not always) the result of horses being kept in wet dirty ground conditions, which make maintaining good hoof hygiene and providing good hoof care very difficult. It is an infection of the hoof’s frog, frog grooves and heel region caused by a variety of bacteria and fungi, with one particularly aggressive species of bacteria called Fusobacterium necrophorum being commonly involved.
Thrush in horses may affect one or multiple feet and one or more of the three clefts of frog — medial, lateral and central — may be affected. It is more commonly found in the hind feet, especially if your horse has deep narrow frog clefts. Thrush is characterised by a black, foul-smelling discharge. A horse with thrush may well be lame with a foul smelling, slimy, dark discharge around the frog. Pressing on the frog with your thumb often causes pain. The frog itself may be growing abnormally with loose rubbery flaps. The infection causes degeneration of the frog tissue and the frog tissue may be under-run (separated). In severe cases, the bacteria may have eaten away at the frog to cause open sores into the deeper sensitive tissues.
Thrush is usually quite easy to spot in horses, but occasionally it only becomes apparent when a hoof pick is advanced deep into the affected cleft. Luckily most cases are superficial and limited to the frog tissue. In more severe cases the infection may extend to the digital cushion, hoof wall and heel bulb regions, resulting in lameness. Occasionally swelling extends up the lower part of the leg to the knee or hock. The condition is usually recognised by your vet or farrier on signs alone, but occasionally biopsies are taken of the frog tissue.
Symptoms and diagnosis of thrush
Inadequate hoof hygiene and poor stable management — often involving horses standing for prolonged periods in soiled bedding or muddy fields — are the most likely causes of thrush. Softening of the hoof tissues is also a predisposing factor.
Infrequent farriery and allowing a horse’s feet to become so overgrown that the frog and bars almost meet, combined with a lack of oxygen (fresh air) to the deeper frog tissue, sets up an an aerobic environment in which the bacteria implicated in thrush can thrive.
Thrush can also develop in horses shod with pads, especially when moisture and dirt become trapped underneath the pad. Occasionally, though, thrush develops even when the horse’s management and foot care is exemplary, and owners often find this difficult to accept. In such patients foot conformation and sheared heels are likely contributory factors, but some horses just develop thrush for no apparent reason.
Preventing thrush in horses
Thrush causes the hoof and frog to become soft and crumbly, allowing the bacteria and fungi to penetrate and establish themselves leading to infection. Wet and muddy fields or soiled damp bedding are particular culprits with deep litter style bedding being amongst the worst. For this reason thrush is much more common in winter than summer.
Prevention is of course better than cure so picking out your horse’s feet properly twice a day and maintaining clean dry bedding is essential. It can be difficult during the wetter months but avoiding having your horse standing in a badly poached field for long periods is important.
In addition, regular trimming of hooves and frog by a farrier is important to maintain good hoof conformation and frog health. Intermittent use of a disinfectant such as iodine to scrub out the hooves about once a month would also be a good idea. It can be difficult to adequately clean the frog and clefts under and around bar shoes and impossible under sole pads, putting horses who wear these shoes for other foot problems at increased risk of suffering from thrush.
If left untreated, a thrush infection will progress and may affect the deeper structures within the foot causing serious distortion of the frog and ongoing lameness. The first treatment step is to look at the horse’s environment and management. Avoid deep litter beds and use an absorbent bedding with soiled areas removed twice a day. Doing the following is also good practice:
- If the horse lives out in a wet or muddy field, bringing them in will give the feet a chance to dry.
- Hooves, including the clefts of frog, should be picked out twice a day.
- Your farrier should attend as soon as possible to trim the feet and remove overgrown horn, frog and under-run tissue.
- This opens the area to fresh air and helps to reduce the anaerobic environment. If there is heel instability in a barefoot horse, shoeing may be recommended.
- Bar shoes and pads should be removed and the frog, its grooves and the sole trimmed and paired back to visually healthy tissue by a vet or farrier allowing air to reach the affected tissue.
- The foot should then be picked out carefully twice daily before scrubbing the frog and sole with dilute iodine solution. Once washed the horse should be stood on a clean dry concrete area for about an hour to allow it to dry.
- In terms of medical management of thrush, the foot and frog tissue should first be cleaned using a stiff brush and Hibiscrub (chlorhexidine) solution.
- After this a topical treatment may be applied, such as oxytetracycline spray, iodine, copper sulphate or 10% formalin, to name but a few.
- It is very important that the horse is kept in a clean dry stable where immaculate hygiene is maintained during the treatment period. Painkillers may be given by mouth if the horse is lame, but antibiotic powders are rarely required.
- Tetanus cover must be given to an unvaccinated horse.
This regime should be maintained until the feet are back to normal – probably two-three weeks, but sometimes longer. There is no one-size-fits-all therapy for thrush, and different vets and farriers will recommend different treatments. Exercise in a clean, dry environment is also important to strengthen the foot and facilitate natural cleaning of the foot.