Bumblefoot is a necrotizing or purulent infection involving the distal interphalangeal soft tissues and sometimes the joint. The incidence is usually sporadic, but as much as 25% of the flock may be affected.
The two organisms most consistently recovered from bumblefoot are Fusobacterium necrophorum and Trueperella pyogenes. Foot abscesses may develop as a complication of ovine interdigital dermatitis by extension of the necrotic process into the subcutis and then into the distal interphalangeal joint. They also commonly develop after penetration of the interdigital skin by sharp objects (eg, crusted snow, frozen or stiff stubble of alfalfa and grain), bruising of the foot and injury of the skin when slipping on frozen rocks, or even careless paring of the hoof. This joint is vulnerable to infection on the interdigital aspect where the joint capsule protrudes above the coronary border as the dorsal and volar pouches. At these sites, the joint capsule is protected only by the interdigital skin and a minimal amount of subcutaneous tissue.
Foot abscesses often develop when the soil and pastures are wet or frozen. The disease causes an acute lameness that is usually restricted to one foot, which the sheep will not place on the ground. It may be possible to express necrotic material through an opening in the interdigital skin caused by the bacterial invasion, but more commonly the swollen sinuses break open and drain at one or more points above the coronet. If this does not occur, the swelling will have to be lanced. In some instances, movement of the affected digit is exaggerated, indicating that the ligaments about the distal interphalangeal joint have ruptured. Displacement of the digit during locomotion and permanent deformity are likely in those cases.
Acute lameness with the sheep packing a foot, swelling of one digit, and discharging sinuses distinguish foot abscess from footrot.
Early treatment with parenteral long-acting antibiotics is sometimes effective and may prevent joint infection. The therapy aims to maintain the integrity of the joint ligaments by draining the abscess and applying an antibacterial preparation and a self-adhesive bandage. This reduces stress on the ligaments, keeps the lesion out of the mud, and counters the bacterial infection. Although the prognosis for complete recovery is poor, in most cases the foot heals sufficiently to allow adequate locomotion. Once the infection becomes established in the joint, conservative treatment is not effective. However, a toe can be surgically removed (if the other digit is healthy) with relatively good success.
Control depends on early treatment and moving the sheep to avoid conditions that lead to ovine interdigital dermatitis or other causes of abscess. Although F necrophorum vaccines are available, they have not proved to be very effective.