Beveridge (1941) provided an excellent account of the clinical signs of severe or virulent footrot: “The initial lesion is usually a mild inflammation of the skin in the interdigital space… followed by a break in the skin-horn junction on the axial aspect of one or both digits, and then a separation of the adjacent soft horn from the underlying epithelium… (which) extends to the edge of the sole or around the back of the heels … there is little or no pus visible macroscopically, but the sheep shows lameness and the foot feels warmer than normal. During the next 5 to 10 days, the infection spreads, causing separation of the horn across the whole sole… there is now a little pus with necrotic detritus present… The infection may then extend under the walls… After the infection has spread across the sole there may be a new growth of horn over the sole, but this usually again becomes involved by the destructive process extending from the skin in the interdigital space, where the infection tends to persist for long periods. On feet which have been affected for several weeks or months, the hoof becomes long and misshapen. The severity of the lesions varies considerably in different animals and in the same foot during the course of the disease.”
Secondary effects of virulent footrot include increased susceptibility to fly strike and increased mortality rate in affected sheep. Ulceration, necrosis and cellulitis of soft tissues overlying the sternum due to recumbency, reproductive failure due to reduced body weight in ewes, and loss of the entire hoof with persistent lameness and illthrift are well-known sequelae (Whittington et al., 2016).
Virulent footrot is caused by 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. For this reason a diagnosis of virulent 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.
While clinical diagnosis is the main tool used in New South Wales, in other jurisdictions the diagnosis defaults to laboratory tests. In Western Australia, virulent footrot is diagnosed when an isolate of D. nodosus that is stable (positive) in the gelatin gel test is detected in a flock, regardless of the clinical findings.
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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