Footrot is a highly contagious bacterial disease of sheep and other small ruminants. The essential causative agent, Dichelobacter nodosus, is a parasite of the hoof and does not survive on pasture or in soil or faeces for longer than 14 days (Beveridge, 1938). Upon review of data from many experiments, it was concluded that “The maximum period of survival in the environment, even under the most favourable conditions, is not more than 2 weeks, and after only one week insufficient of the infection survives to produce foot-rot in sheep exposed to it naturally” (Beveridge, 1941).
Footrot is an economic burden in most sheep-rearing countries, including Australia and the U.K. (Lane et al., 2015; Nieuwhof and Bishop, 2005). In Australia, footrot costs the sheep industries ~$45 million per annum, primarily due to production losses and the cost of controlling the disease (Lane et al., 2015).
Footrot presents as a spectrum of conditions, ranging from a mild dermatitis of the interdigital skin, through to complete separation of the hoof from the underlying tissues (referred to as “underrunning”) (Beveridge, 1941; Stewart and Claxton, 1993; Stewart, 1989). The mild forms of the disease are not economically significant.
The different forms of footrot all commence with interdigital dermatitis and are described in detail on the pages that follow – use the dropdown menu to access these pages, or click on these links :
Intermediate (low virulent) footrot
The different types of footrot
For descriptive and regulatory purposes, two distinct clinical forms of the disease are generally recognised in Australia: virulent footrot and benign footrot. Both commence on the interdigital skin as mild dermatitis. In benign footrot, this is unlikely to progress to underrunning except in a small proportion of sheep (Stewart, 1989; Stewart and Claxton, 1993). In contrast, in virulent footrot, mild dermatitis progresses and a large proportion of sheep can develop severe, underrun lesions (Stewart, 1989; Stewart and Claxton, 1993). There are forms of the disease between these two extremes: intermediate or low-virulent footrot is described in the literature but is not currently recognised by Australian animal health agencies (Buller and Eamens, 2014).
Foot scoring is a means of objectively describing the severity of footrot lesions. It has been used in diagnosis and to study the impact, treatment, and control of footrot. A simple scoring system was developed by Egerton and Roberts (1971), ranging from Score 0 (healthy, with no signs of disease), through Scores 1 and 2 (interdigital skin lesions) to Scores 3 and 4 (severe lesions with underrunning of the sole of the hoof). Score 3 lesions were later subdivided into 3a, 3b and 3c according to the degree of underrunning (Stewart et al., 1982b; Stewart et al., 1985) while Score 5 (more severe than score 4, with separation of the walls of the hoof) was defined later (Stewart and Claxton, 1993; Anonymous, 1990). For more information about foot scores please click on this link to go to the page: Introduction to foot scores.
Score 3, 4 and 5 lesions are regarded as severe and cause lameness. Differentiation of virulent and benign outbreaks of footrot has been based on the prevalence of severe lesions. For example, in one classification system, clinically virulent footrot is characterized by a high prevalence of sheep with severe lesions (>10% score 4), while such lesions are rare (<1%) in outbreaks of clinically benign footrot (Egerton, 1989; Stewart and Claxton, 1993). Importantly, this classification scheme acknowledges that virulent and benign strains of D. nodosus are both capable of inducing severe lesions.
The clinical approach to diagnosis depends on environmental conditions being adequate for full expression of the disease. For more information on diagnosis, please click on this link to go to the page: Introduction to footrot diagnosis.
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.
There are many strains of D. nodosus. They have been classified as virulent or benign according to their microbiological characteristics as well as their potential to cause disease (Beveridge, 1941; Stewart and Claxton, 1993; Stewart, 1989; Stewart et al., 1986b). In general, so-called benign strains are associated with mild disease while so-called virulent strains are associated with severe disease. Intermediate footrot is thought to be caused by D. nodosus strains of intermediate virulence (Stewart et al., 1986b; Abbott and Egerton, 2003a). These correlations are not absolute and can lead to great misunderstanding.
Transmission and expression of footrot is completely dependent on the environment, with mild air temperatures and high, evenly distributed rainfall favouring progression of the disease (Graham and Egerton, 1968).
Susceptibility to footrot varies between sheep breeds; the Merino is more susceptible than British breeds and cross-bred sheep (Emery et al., 1984).
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.