Interdigital dermatitis is a necrotizing condition of the interdigital skin and usually precedes or accompanies footrot. In Australia, it may be caused by less virulent strains of Dichelobacter nodosus and is termed benign footrot. It is commonly caused by other organisms, such as Trueperella pyogenes and Fusobacterium necrophorum, and there is evidence that this initial infection is required for the development of virulent footrot. Wet weather, damp pastures, and mud are predisposing factors. In milder cases, the interdigital skin is red, hairless, swollen, and moist. In severe cases, the integrity of the interdigital skin is compromised, exposing subcutaneous tissues. Suppuration and swelling of the deeper interdigital tissues may develop. Lameness can be a sign in many as 90% of affected sheep, and all four limbs may be affected. Healing is rapid when conditions dry out, but the disease may recur when conditions again become wet.
Because interdigital dermatitis usually precedes a footrot outbreak, it is prudent to treat the condition as if it were footrot. Other differential diagnoses include dermatophilosis (strawberry footrot), which affects the hairy skin of the coronet and pastern. Viral diseases such as ulcerative dermatitis, contagious ecthyma, and foot-and-mouth disease may be excluded based on flock history, clinical signs, electron microscopic examination, and serologic analysis. Currently, the treatment of choice consists of external application of 10% wt/vol zinc sulfate disinfectants via a footbath or aerosol.
