Introduction
A form of disease called intermediate footrot (also referred to as low-virulent or chronic benign footrot) is described in the literature (Stewart et al., 1982; Stewart et al., 1984; Stewart et al., 1986a; Stewart et al., 1986b; Depiazzi and Richards, 1985). In intermediate footrot, the loss of body weight and the decrease in the rate of wool growth was less than in virulent footrot, underrunning lesions occurred in fewer feet and lameness was mild (Depiazzi and Richards, 1985; Stewart et al., 1982; Stewart et al., 1984; Stewart et al., 1986a). Intermediate footrot has been the focus of relatively few studies since the 1980s.
The 1990 NSW Footrot Strategic Plan recognised intermediate footrot as a clinical entity (Anonymous, 1990). However, in 1992, an Animal Health Committee (AHC) Working party on the co-ordination of footrot control in Australia recommended that footrot be described as virulent or benign, and that strains of D. nodosus of intermediate virulence be grouped with virulent strains (Stewart and Claxton, 1993). Consequently, only two clinical forms of footrot are currently recognised by most Australian animal health regulatory bodies: virulent footrot and benign footrot (Buller and Eamens, 2014). However, there is renewed interest in low-virulent footrot because as virulent footrot has been brought under control, lesser forms of the disease have become more obvious.
Clinical characteristics
The term “intermediate footrot” is vague and describes a spectrum of disease ranging from that which is somewhat more severe than benign footrot, through to that which approaches the severity of virulent footrot.
Egerton (1989) defined outbreaks of intermediate footrot as those in which 2 to 9% of sheep presented with at least one score 4 lesion (Figure 1).
Note that the percentage of the flock affected and the prevalence of underrun lesions varies depending on climatic conditions (Stewart and Claxton, 1993).
Lesions of intermediate footrot can spontaneously regress with the advent of unfavourable conditions, although they can persist in the worst affected sheep.
The term “low-virulent footrot” has come into use recently and describes a form of the disease that is close to virulent footrot, may be economically significant, and therefore may warrant further attention. A cut-point has not been determined to differentiate low-virulent footrot from the less severe forms of intermediate footrot (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.
Microbiological characteristics
Strains of D. nodosus isolated from flocks with clinically intermediate footrot share some microbiological characteristics with virulent strains of D. nodosus. Intermediate strains of D. nodosus are typically elastase-positive, but exhibit reduced or delayed activity. In contrast to virulent strains of D. nodosus, which are typically elastase positive after 4 to 12 days of incubation, intermediate strains are often elastase positive after 16 to 24 days (Stewart et al., 1986b; Stewart et al., 1986a; Liu and Yong, 1997; Liu and Yong, 1993). The gelatin gel test and the zymogram cannot reliably differentiate intermediate strains (Liu and Yong, 1997), as both virulent and intermediate strains produce thermostable proteases (Stewart and Claxton, 1993), with similar electrophoretic patterns.
Impact
The economic impact of low-virulent footrot is unknown but is commensurate with the severity of disease in a flock and the prevalence of severe, underrun lesions.
Intermediate footrot does not progress to virulent footrot if sheep are transferred to an area with ideal climatic conditions for disease expression (Abbott and Egerton, 2003a). Merino wethers with intermediate footrot in the Monaro district of NSW, which has moderately unfavourable climatic conditions, were transported to a second property located near Tarcutta, in the south-west slopes district of NSW where climatic conditions were more favourable for footrot; the proportion of sheep with footrot lesions remained low.
Prevalence
The prevalence of intermediate/low-virulent footrot is unknown. This is due, in part, to the lack of a clear case definition, and the fact that intermediate footrot is no longer differentiated in most Australian States. Stewart and Claxton (1993) reported that intermediate strains of D. nodosus are commonly found in footrot control regions where quarantine programs are in effect. Recent anecdotal evidence suggests that the prevalence of low-virulent footrot has increased, particularly in New South Wales.
Control and elimination
There has been limited research on control of intermediate footrot. In a replicated experiment, the disease was eradicated either by inspection and culling alone, or when combined with vaccination (Abbott and Egerton, 2003b). However, eradication failed when the sheep were treated with parenteral antibiotics before inspection and culling. The use of parenteral antibiotics appears to be contraindicated in the eradication of intermediate footrot (Abbott and Egerton, 2003b).
Current research
There is renewed interest in low-virulent footrot. A number of research projects are currently being undertaken by the Farm Animal Health group in the Sydney School of Veterinary Science, The University of Sydney.
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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