Abscesses are probably the most commonly observed disorder of the hoof in equine practice. A hoof abscess is a focal accumulation of purulent exudate that most often occurs between the germinal and keratinized epithelium of the hoof. Also known as septic pododermatitis, hoof abscesses can occur in both shod and unshod horses.
Etiology and Pathophysiology of Hoof Abscesses in Horses
For a hoof abscess to develop, bacteria must gain access through the keratinized epithelium via either naturally occurring defects in the hoof capsule or traumatic damage that exposes the underlying sensitive dermis of the hoof. Traumatic damage can result from penetration by a foreign body, penetration of the dermis by a horseshoe nail, or bruising of the sole.
Usually, the specific cause of a hoof abscess cannot be identified, and the bacteria are presumed to have been introduced through small defects in the hoof capsule. These defects include microcracks and separation of the white line.
Environmental factors such as excessive moisture can soften the hoof and allow bacteria to enter through gaps in the white line.
Extreme moisture changes from wet to dry can cause brittleness of the hoof capsule, leading to microcracks that allow the entry of bacteria.
Predispositions for recurrent hoof abscess formation in horses include concurrent disease, such as laminitis, and poor hoof structure, such as thin soles that are more prone to bruising.
The pain that is responsible for the lameness associated with hoof abscesses in horses is due to the rapid increase in pressure within the low-compliance environment of the hoof capsule. With time, the hoof capsule can separate from the germinal layer of epithelium, allowing the abscess to extend further under the sole (subsolar), under the frog (subcuneal), or proximally under the wall (submural).
An abscess that extends proximally deep to the hoof wall and drains at the coronary band historically has been referred to as a “gravel.” The infection can also extend through the germinal layers of the integument and dermis to affect deeper structures.
Clinical Findings of Hoof Abscesses in Horses
Most horses with foot abscesses have a history of acute onset of severe lameness in one limb. However, clinical signs can vary depending on the duration and extent of the abscess.
Diagnosis of Hoof Abscesses in Horses
Findings on physical examination that indicate hoof abscesses in horses often include an increased digital pulse or higher palpable temperature of the hoof capsule as compared with the other limbs. Examination of the coronary band and heel bulbs may reveal swelling, a painful response to palpation, or a purulent discharge. Application of hoof testers usually produces a dramatic withdrawal response. A focal response to hoof testers may help localize the abscess.
Courtesy of Dr. Brian Beasley.
The use of a wire brush and light paring with a hoof knife can reveal areas of dark discoloration of the horn (indicative of a tract or injury site through which bacteria may have entered). If a tract is found in the area of hoof tester response, further exploration often exposes the abscess. An abaxial sesamoid perineural block is sometimes necessary to facilitate exploration.
If no tract is visible after exploration but clinical signs generally suggest a hoof abscess, further exploration may be needed; however, excessive paring of the foot is contraindicated. If further exploration still fails to reveal the abscess, the hoof capsule should be poulticed for 24–48 hours to soften it, enabling easier exploration if the abscess has not already started draining spontaneously. During this time, the use of NSAIDs is indicated for pain management.
If the lameness is still present and the abscess cannot be revealed after physical exploratory efforts have been exhausted, radiography is warranted to exclude other causes of acute foot lameness and to detect radiographic signs of a hoof abscess (see hoof abscess radiographic images).
Differential diagnoses of hoof abscesses in horses include coffin bone fracture, navicular bone fracture, sepsis of a deep digital structure, severe bruising, and severe injury to a ligament or tendon within the foot.
Treatment and Management of Hoof Abscesses in Horses
The goals in treating hoof abscesses are to facilitate drainage while preserving as much normal horn as possible, and to prevent further infection. Generally, a small hole (less than 1 cm in diameter) provides adequate drainage, regardless of the size of the undermined area. The foot should then be bandaged for 3–5 days with a medicated poultice pad, to facilitate drainage and prevent debris from entering the site.
The administration of NSAIDs for pain management should enable increased weight bearing on the affected limb, effectively pushing out more of the purulent material. Antimicrobials (systemic or regionally perfused) are not required unless the abscess has extended deep to the dermis. Tetanus prophylaxis is indicated for horses without a recent history of vaccination (ie, within 3 years).
Within 12 hours of establishing drainage, the lameness caused by the abscess should be markedly improved. For subsolar abscesses where drainage was established through the sole, the shoe can be replaced after drainage has ceased and the horse is no longer sensitive to hoof testers.
If clinical signs fail to improve or the hoof abscess recurs, radiographic images should be obtained. If the radiographs do not reveal other pertinent lesions, further exploration is justified.
The prognosis for simple hoof abscesses that are not associated with a predisposing cause or infection of a deeper structure of the foot is good. If the infection involves a deeper structure, the prognosis varies depending on which structures are affected.
See Regional Anesthesia in Horses for discussion of specific anesthetic techniques used to localize lameness.
Key Points
A hoof abscess is the most common reason for a non-weight-bearing lameness in horses.
Creation of ventral drainage with sole debridement should markedly improve a horse's lameness.
Horses with recurrent hoof abscesses in the same general area of the same foot should have imaging performed to evaluate whether there is an underlying reason for the abscess, including a keratoma or sequestrum.
For More Information
Fürst AE, Lischer CJ. Other clinical problems of the equine foot. Vet Clin North Am Equine Pract. 2021;37(3):695-721. doi:10.1016/j.cveq.2021.08.005
Also see pet health content regarding sole bruises and abscesses in horses.
