The signs of footrot commence on the interdigital skin with dermatitis, then progress to underrunning of the soft horn of the heals, and progress to underrun the hard horn, directionally from the heal, across the sole, to the outer wall and toe. The lesions can be scored (see pages on Foot Scores). They are described below in terms of the main clinical entities.
Interdigital dermatitis (a diagnosis at individual and flock level)
Uncomplicated and non-progressive ovine interdigital dermatitis is due to superficial infection of skin with F. necrophorum. It predisposes feet to subsequent infection with D. nodosus (Egerton et al., 1966; Parsonson et al., 1967). Ovine interdigital dermatitis is clinically indistinguishable from the early stages of virulent footrot and benign footrot, when lesions are confined to the interdigital skin, i.e. the Score 1 lesion of footrot.
Footrot
Following infection with D. nodosus, the interdigital dermatitis intensifies and a cleft appears in the keratin at the inner margins of the heal, at the skin-horn junction. When this occurs a diagnosis of footrot is likely. This is an assessment at the level of the individual sheep. It is still not possible to determine whether there is an outbreak of virulent footrot – more sheep must be examined.
Secondary effects of 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). Presentation of sheep with these signs is a clue that footrot is present.
Virulent footrot (a diagnosis at flock level)
Beveridge (1941) provided an excellent account of the clinical signs of severe 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.”
Benign footrot (a diagnosis at flock level)
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).
The non-aggressive, so-called benign strains of D. nodosus can cause severe lesions in a small proportion of sheep under certain environmental conditions. Such sheep are considered to be highly susceptible. For this reason a diagnosis of benign footrot is made by assessing the proportion of sheep with severe lesions during favourable environmental conditions (Figure 1).
Intermediate footrot (a diagnosis at flock level)
Intermediate footrot is also referred to as low-virulent or chronic benign footrot). The loss of body weight and the decrease in the rate of wool growth is less than in virulent footrot, underrunning lesions occur in fewer feet and lameness is mild (Depiazzi and Richards, 1985; Stewart et al., 1982; Stewart et al., 1984; Stewart et al., 1986a).
Clinical differentiation of the forms of footrot
Classification of outbreaks of footrot as virulent or benign is often possible based on clinical examination alone. By examining the feet of a sufficiently large, randomly selected sample of sheep, a statistical estimate of the prevalence of severe (underrun) lesions can be made. If the upper confidence limit for the observed prevalence of severe lesions exceeds 5% a diagnosis of intermediate or virulent footrot can be made with some confidence. If most sheep have lesions confined to the interdigital skin the likely diagnosis is benign footrot (Egerton, 1989b). This procedure was recommended in 1990 in the NSW Footrot Strategic Plan (Anonymous, 1990) as the principal method of diagnosis. Although confidence limits for observed prevalence were not given, guidelines for interpreting the observed prevalence of severe (underrun lesions were included: up to 1% of sheep with severe lesions = benign footrot; >1 to 10% = intermediate footrot; >10% = virulent footrot, based on Egerton (1989a).
These assessments require that there is full expression of the disease, which requires conducive pasture/environmental conditions. But how can one be sure this is the case?
Egerton (pers. comm. 1993) cited in Stewart and Claxton (1993) considered that footrot has had an opportunity to express itself fully if at least 25% of the flock are affected with score 2 lesions or greater.
If the predominant lesions are interdigital, and if there is doubt about the opportunity for full expression, for example there has been a sub-optimal environment or a very recent introduction of infected sheep, the general recommendation for many years has been to re-examine the flock in 10-14 days provided environmental conditions are now favourable, or place affected sheep on wet straw or wet foam rubber mats (kept moist, ambient temperature > 10oC) and re-examine after 10-14 days (Stewart and Claxton, 1993).
Clinical diagnosis remains the preferred option in NSW but in Western Australia, diagnosis is based solely on the results of laboratory tests.
The current Australia and New Zealand Standard Diagnostic Procedure (Buller and Eamens, 2014) includes this information:
“In some Australian states the policy for the diagnosis of virulent footrot is based on flock assessment of the extent and progression of lesion severity (Seaman, 2007; Stewart and Claxton, 1993). The lesion scores of a minimum of 100 randomly-selected sheep from a suspect mob are recorded and an assessment is based on the prevalence of sheep with lesions of score 2 or greater, and prevalence of sheep with lesion scores of 4 or greater (Seaman, 2007). The guidelines suggest a flock in which the cohort of 100 has more than 1% sheep with severe lesions (score 4 or 5) (Seaman, 2007; Stewart and Claxton, 1993) is a useful guide, although not an exclusive criterion, for considering virulent footrot. Under favourable conditions more than 10% of a flock with virulent footrot will usually have score 4 or 5 lesions, and can be up to 90%. In benign footrot, sheep will have score 1 and 2 lesions (interdigital dermatitis without underrunning) and a small percentage (not stated in the guidelines) will have score 4 lesions; however, these will regress without treatment.”
Figure 1: Cut-points, based on the prevalence of score 4 lesions, can be used to differentiate outbreaks of virulent and benign footrot, as proposed by Egerton (1989a). This is applicable only when environmental conditions are favourable for disease expression.
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The diagnostic dilemma
There is a spectrum of clinical severities of footrot. Under appropriate environmental conditions, severe lesions (underrunning of the horn) may occur in a large proportion of sheep in a susceptible flock if Dichelobacter nodosus strains of sufficient virulence are involved. A diagnostic dilemma may exist where severe lesions are observed in only a small proportion of sheep or where mild lesions (no or slight underrunning) are seen in many sheep. In genetically resistant sheep, the lesions may not progress beyond the interdigital skin and these interdigital lesions may resolve quickly even where virulent strains of Dichelobacter nodosus are present (Emery et al., 1984). Early in an outbreak, or when environmental conditions are unfavourable, or where clinical signs have been deliberately masked by treatments, the true nature of the outbreak may be unclear (Whittington et al., 2016). The strategies to deal with this are:
- to examine sheep a few weeks later, after lesions have had a chance to progress
- to wait for more favourable environmental conditions, eg several weeks after a rain event, and re-examine the same sheep
- to place the sheep on irrigated pasture (or wet straw or rubber mats, see above) and examine them again after a few weeks
- to collect samples for laboratory examination
The principal concern after establishing that footrot is present in a flock is the accurate assessment of the potential significance of the outbreak in the context of homebred livestock and livestock on other farms given that infected sheep may be translocated. For these reasons, laboratory tests may be required to assist in diagnosis.
The main problem with laboratory tests for virulence is that all are subject to a certain proportion of false positive results. Some tests are associated with high rates of false positive test outcomes. For this reason it is very important to properly screen flocks through history and clinical examination before submitting samples to a laboratory.
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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Footrot @ Sydney 