Early researchers described “scald”, “non-progressive” or “mild” footrot which was confined to the interdigital skin, preceded or followed outbreaks of severe footrot and was difficult to eradicate (Thomas, 1962; Alexander, 1962; Littlejohn, 1967). It was formally termed “benign” footrot to acknowledge a common microbial aetiology but to distinguish it from the more severe form of the disease, “virulent” footrot (Egerton and Parsonson, 1969).
The lesions of benign footrot include hyperkeratosis of posterior interdigital skin and skin horn junction; corrugation of the skin horn junction and soft horn of the axial wall of the heels; erosive and exudative dermatitis of the interdigital skin; separation of the soft horn of the axial wall; rarely, separation of the soft horn of the heel and posterior sole, and; rarely, in a small proportion of sheep, underrunning to the sole of the toe and abaxial wall of the hoof (Stewart, 1979).
Benign footrot was shown to be due to strains of D. nodosus with lower proteolytic activity than strains associated with virulent footrot (Thomas, 1962; Egerton and Parsonson, 1969).
Benign footrot is distinguished from uncomplicated and non-progressive ovine interdigital dermatitis due to F. necrophorum which predisposes feet to subsequent infection with D. nodosus (Egerton et al., 1966; Parsonson et al., 1967). In the early stages of footrot, when lesions are confined to the interdigital skin, and in cases of benign footrot, the signs are clinically indistinguishable from ovine interdigital dermatitis.
Some sheep with benign footrot can have severe lesions.
Benign footrot is caused by non-aggressive strains of D. nodosus but a diagnosis can only be made at flock level. This is because non-aggressive strains of D. nodosus can cause severe lesions in a small proportion of sheep under certain environmental conditions. These sheep are considered to be highly susceptible. For this reason a diagnosis of benign footrot is usually made by assessing the proportion of sheep with severe lesions during favourable environmental conditions (Figure 1).
Figure 1: Cut-points, based on the prevalence of score 4 lesions, can be used to differentiate outbreaks of virulent, intermediate and benign footrot, as proposed by Egerton (1989). This is applicable only when environmental conditions are favourable for disease expression.
Note: The method and criteria for diagnosis of footrot may be subject to specific policy in particular State jurisdictions in Australia. Consult your government veterinarian.
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