A subsolar abscess, or pus in the foot, is an infection beneath the sole of the foot. Subsolar abscesses cause severe pain to your horse due to the build-up of pressure under the hoof capsule that cannot expand. The pain can probably be compared to an infection or blood blister under your fingernail, with the additional problem of the horse having to bear weight on the hoof capsule.
How do subsolar abscesses occur?
Hoof abscesses can occur following a penetrating injury to the sole of the foot, which introduces contamination and bacteria in the tissues under the sole. Hoof abscesses can also occur after bruising of the sole with secondary infection. Solar bruising can be the result of thin soles or flat feet (particularly in Thoroughbreds) or poorly fitting shoes; riding with or without shoes on an irregular surface can also lead to bruising of the feet.
When a horse has a subsolar abscess, the resulting lameness is usually severe and cannot be distinguished from other injuries to the foot, eg fractures. Your horse will often bear almost no weight on the affected limb and will just touch the toe to the ground. The foot can feel warmer than the other feet and a strong digital pulse, in the blood vessels (digital arteries) at the back of the fetlock, can often be felt. These blood vessels may also be enlarged and more easily seen.
How are abscesses diagnosed?
Subsolar abscesses are one of the most common reasons for acute onset severe lameness. Any horse that is severely lame should immediately be examined and treated by your veterinarian. After trying to identify the local signs described above, your veterinarian will usually apply hoof testers over several areas of the foot in an attempt to localize the area of pain more precisely.
Occasionally, abscesses drain spontaneously or are identified when the foot is being picked out. However, paring the foot with a hoof knife is often required to find the infected area. First, the entire sole is very lightly pared to trim back to a clean pale solar surface. This allows the identification of any suspicious dark areas which may represent the surface of the abscess; these suspect areas are then carefully pared further. An infected cavity will either be identified, or the discolored solar tissue is shallow and will be completely removed. In some cases, no obvious abscess is identified, or only blood is released.
If the infection has expanded, a larger portion of the sole may be undermined and detached from the tissues of the foot (underrun sole). In these cases, all the underrun solar tissue should be removed.
It is very easy to create excessive additional trauma and prolong the healing process with improper use of a hoof knife, so this should be left to your veterinarian or farrier who are experienced at investigating subsolar abscesses.
How are abscesses treated?
The key to treating a subsolar abscess is establishing drainage, which is accomplished with a hoof knife and the creation of a patent outflow tract from the bottom of the foot as described above. Once the abscess can drain, the lameness will usually improve substantially, but it will not completely resolve until all the residual infection has subsided.
To encourage drainage from the tissue, the foot will often be soaked in a solution of warm water and Epsom salts. The addition of the salt will draw fluid out of the foot and into the salt solution. This may need to be done once or twice a day for several days. The foot should then be bandaged to prevent further contamination of the sole. Often, a poultice is placed directly on the foot under the bandage and acts in the same way as the salt solution by drawing fluid out of the foot.
It is very important that the foot remains as clean as possible while the abscess is healing. If the foot is contaminated again by mud or bedding, the infection will not be eliminated.
Antibiotics are not usually required for the treatment of foot abscesses, unless the infection has reached the soft tissues above the coronary band. Antibiotics do not penetrate the tissues of the foot well and are therefore not very effective. However, anti-inflammatory drugs/pain killers are always warranted.
Make sure your horse is up to date with tetanus vaccinations. Tetanus following a subsolar abscess can happen in non-vaccinated animals.
The solar tissue at the bottom of the foot takes longer to heal than other areas on the body. The larger the defect created in the sole to facilitate drainage the longer the healing process will take. Once the infection has been successfully treated, your veterinarian may recommend fitting your horse with a protective shoe and hospital plate or pad. Rubber boots that fit over a hoof bandage are also useful and keep the foot watertight. Occasionally, the defect in the bottom of the foot is packed with swabs of sterile cotton soaked in iodine solution or antibiotics before a bandage or hospital plate is fitted.
How long will it take my horse to recover?
The length of time it takes for recovery varies depending on the severity of the infection, and the size of the defect created in the foot when the abscess was drained.
Uncomplicated abscesses usually heal within a week, or even sooner.
Severe cases on the other hand, can take months to completely resolve.
What else do I need to know?
If there is ongoing drainage and the defect in the foot is not healing, your veterinarian may recommend obtaining x-rays of your horse's foot.
In a small number of cases, the pedal bone, which lies directly under the sole, may become involved in the infection. Bone infection and/or an area of the pedal bone that has died off (bone sequestrum) can perpetuate an infection. It takes approximately three weeks before bone sequestrum becomes obvious on x-ray. This usually needs to be surgically removed under sterile conditions. Alternatively, a benign tumor (growth) of the horn -producing cells (keratoma) can result in recurrent abscess formation and hoof wall deformation.