A subsolar abscess occurs after a penetrating injury to the sole. It is the most common cause of a sudden (acute) lameness and can occur within 12 hours of the injury. Affected horses are often found unable to bear weight on the affected limb with pointing of the toe (grade 5/5). Some are less severely affected but still remain with an obvious lameness (grade 3-4/5). According to the AAEP lameness scale a grade 3 lameness is consistently observable at a trot and a grade 4 is obviously lame at the walk.
The penetrating injury can occur around the white line or sole, through hoof and sole cracks, or via nail holes. In many cases an entry point may not be apparent. If the abscess is not draining there is often concurrent swelling of the distal limb. This is due to a local cellulitis. It is not uncommon for infection to track up the hoof causing a soft painful swelling at the coronary band that will eventually break open and drain.
Diagnosis
The diagnosis should be suspected in horses with a sudden onset of moderate to severe lameness in a single limb. Digital pulses in the affected limb will be elevated and there may be an increase in surface temperature of the hoof capsule. Hoof testers are used to identify focal regions of pain, although this may be difficult in horses with a hard sole or if the abscess is located in the heels. Suspect painful areas over the sole or frog should be carefully pared with hoof knife looking for cracks or dark spots. Black, foul smelling fluid will be found. If no drainage is identified then soaking twice daily with magnesium sulphate (Epsom salts) in warm water and application of a poultice may facilitate spontaneous drainage.
Treatment
The establishment of drainage is critical. Paring with a hoof knife is important to create good drainage, but blood or pink coloured tissue is a warning to stop to avoid damaging healthy tissue. A wet poultice (such as Animalintex) can be applied to the area(s) of drainage on the sole or at the coronary band, along with once to twice daily soaking in warm Epsom salts.
Antibiotics are rarely indicated. Non-steroidal anti-inflammatory drugs (such as phenylbutazone) can be used to alleviate pain, although once drainage is established it is rarely needed. Tetanus protection is critical.
Most horses will have an excellent prognosis.