Thrush – a common condition in the wet

As the wet season approaches us in the southern states of Australia, one of the common foot complaints is the horse that develops thrush. This article aims to describe what is thrush, how it is  caused and ways that we can manage it during the wet periods of the year.

What is Thrush?

Thrush, by definition is the infection of the sensitive tissue of the foot by microbes (bacteria and fungi). The key with thrush is that for it to be a problem, the microbes must infect the sensitive tissue. The hoof capsule and the environment that horse lives is constantly inhabited by naturally occurring microbes, whether it is during dry or wet conditions. For microbes to cause thrush, there must be a defect in the resilient outer surface of the hoof capsule. Once the sensitive tissues are exposed, microbes can cause infection. Many healthy horses have a black accumulation of debris around the frog, which can sometimes be confused with thrush. As this black accumulation is not penetrating the sensitive tissue, it is not an active episode of thrush.

Figure 1: Black accumulation of debris around frog

Wearing pads, boots or pour-ins

When horses wear pads, boots or have a pour in pad applied, black debris accumulates due to the area being covered. Again, unless there is an area of the hoof capsule that allows the debris to enter the sensitive tissue, this is not an active episode of thrush.

Figure 2 (right): Black film over sole and frog after Equipak removal – this is easily trimmed away

Figure 3 (above): Filling the sole with Equipak Blue from Vettec

The sulci (ie channels that run around the frog) are the junction between three different types of hoof tissue, the frog, sole and the bars. During sudden increases in environmental moisture and subsequent increases in hoof moisture content, the sulci become more porous. If there is already debris chronically trapped in the sulci of the frog, this can lead to infection of the sensitive corium of the frog and sole. Stabling horses on urine and faeces contaminated wood shavings can also increase the risk of thrush. Wood shavings when wet and trapped in the sulci of the frog can become abrasive and work its way into the sensitive tissue.

Figure 4: Mud packed in hoof

If left untreated, thrush can penetrate deep into the sensitive tissue and can become a serious problem. Thrush will rarely infect the bony structures within the hoof capsule, however it can infect the digital cushion, which is very difficult to treat.

Sheared heels and thrush

Another way thrush can be caused, is in the horse with sheared heels. Horses with sheared heels have a deep central sulcus between both heel bulbs. In severe cases of sheared heels, the central sulcus runs down to the sensitive tissue and even into the skin. For cases of thrush that are associated with sheared heels, yes it is important to treat the microbial infection, however it is more important to treat the sheared heels to improve the outcome of the case.

Figure 5: High heel and deep central sulcus to frog

How to manage a case of Thrush

In terms of managing cases of thrush, there are a number of different approaches. Thrush appears to be associated with debris being trapped in the sulci of the frog. It is important to have the frog trimmed in a way that allows the easy passage of debris from the front to the back of foot. This usually means lightly trimming the frog at the heel bulbs, where the heel and frog intersect. Horses with overgrown frog and hoof are certainly more prone to debris being trapped, therefore it is important to have regular farrier visits at 4-6 week intervals. Also. cleaning out the foot on a daily basis, especially during wet periods is crucial in reducing the amount of mud trapped under the hoof.

Figure 6: Overgrown frog and hoof – 10 weeks since last trim

If the horse is stabled, it is very important to ensure that the stables are cleaned regularly and new shavings are used when the previous shavings start to become saturated. Depending on how frequently the horse uses the stable and the amount of mess they make will determine how often the shavings need to be changed. Also, depending on availability, you could use straw or recycled paper as an alternative to shavings. Both straw and recycled paper have a larger particle size and are less likely to become trapped in the sulci of the frog.

Managing sheared heels

As mentioned previously, in the case with thrush and sheared heels, it is important to treat the sheared heels as well. Sheared heels likely occur from un-even loading of the back half of the foot, from medial to lateral (ie inside heel to outside heel). It is important to trim the foot to encourage even loading and to support the frog. Many times, if the frog can cope with the added load, a heartbar can be used to encourage even loading of the back half of the foot. In more severe cases, a cast can be applied to stabilise the heels. Even though the thrush will be covered up, it is important to treat the underlying reason behind the development of thrush, the sheared heels in these cases.

Figure 7: Frog plate welded into steel shoe to encourage stabilisation of sheared heels

Topical treatments

There are many different anti-microbial topical treatments available to treat thrush. Ideally, the above management strategies will be the most important part of the treatment process, however in cases of active infection a broad-spectrum anti-microbial is a good option. When choosing a treatment, it is important that the product does not burn the sensitive tissue, therefore strong iodine solutions, concentrated copper sulphate and formalin are discouraged. It is also difficult to keep treatments in the foot as most are wiped away. You can use medicated wax, such as Keratex to treat thrush long term. Keratex also works well under a heartbar when applied to treat sheared heels. Specially designed boots or hospital plates can be used to cover over the thrush affected area and make it easy for the owner to treat.

Figure 8: Hospital plate

Conclusion

All in all, by addressing the underlying cause, such as the environmental conditions or sheared heels, thrush can be treated. It is important to involve your farrier and veterinarian when making a plan for the case of thrush. If you have a case of thrush you are concerned about, check out our online consulting portal