Puncture Wounds of the Foot in Horses
(Subsolar abscess, Septic navicular bursitis)
Puncture wounds are common in horses and are the most common cause of subsolar sepsis. Most puncture wounds result only in sepsis of the subsolar soft tissue (ie, subsolar abscess) but can be catastrophic when the puncture is in the frog and travels deep enough to enter synovial structures such as the navicular bursa, the distal interphalangeal joint, or possibly the deep digital flexor tendon sheath.
Puncture of the sole by a foreign body is associated with introduction of pathogenic microorganisms that lead to subsolar abscess formation. Lameness is usually severe; the degree of lameness may be similar to that of a fracture. The horse may stand pointing the affected foot. There is commonly a prominent digital pulse in the affected limb. If allowed to progress, the abscess may travel proximally to rupture at the coronary band; there will usually be edematous swelling proximal to the coronary band before rupture. Diagnosis is made by confirming the site of pain by pulling the shoe, using hoof testers, and picking or paring the suspect area to locate the foreign body or its dark tract. If a foreign body is found in the frog, it may be best to obtain a lateral radiograph of the foot to assess the structures penetrated before removing the offending object. If a tract is found in the frog, it should be probed and a radiograph taken with the probe in place. Because puncture wounds in or near the frog commonly enter a synovial structure, they constitute a serious problem requiring rapid, aggressive diagnosis and therapy. If a synovial structure is entered, the horse should be placed on broad-spectrum antibiotics and transported to a facility capable of advanced surgical and medical techniques; the affected synovial structure should be lavaged with sterile polyionic solution as soon as possible (within hours).
If a puncture wound is noted in the solar area, ensuring adequate drainage from the site helps prevent abscess formation. If there is a suspected abscess but no tract is found, the foot can be poulticed in an attempt to promote organization of the abscess for localization. If a tract is found that leads to a subsolar abscess, adequate drainage should be established with a hoof knife; the drainage hole should be kept as small as possible (~0.5–1 cm diameter) to avoid a prolapse of sensitive corium. Some farriers and veterinarians prefer to drain the abscess through the hoof wall (instead of the sole) if possible. Once the abscess has been entered, it should be probed to determine its extent; a palmar digital nerve block will usually be necessary before probing and lavage of the area. If the abscess underruns a large area of the foot, another small ingress/egress hole can be made at the farthest extent of the abscess from the original entry point, and lavage can be performed by placing a 14-gauge catheter or teat cannula into the affected subsolar space and flushing with saturated Epsom salt solution (made by adding Epsom salts to boiling water or saline until some salt crystals sit on the bottom of the container and will not go into solution). If a chronic subsolar abscess has developed, this treatment may have to be repeated daily to every other day for several treatments. The foot should be protected until there is full healing/epithelialization of the solar region entered; healing appears to be much more rapid and is easier to treat if the abscess is entered through the wall instead of the sole. All horses with puncture wounds should be immunized against tetanus. Local and systemic antibiotic therapy are not necessary for a sole abscess but should be considered with swelling above the coronary band (indicating septic cellulitis) and must be used aggressively if sepsis of a synovial structure is present.