Foot abscess (heel abscess, infectious bulbar necrosis, digital suppuration) is a necrotic then suppurative condition of the ovine foot predisposed by water maceration or other interdigital skin damage followed by a synergistic infection with the bacterium Arcanobacterium pyogenes (previously Corynebacterium pyogenes) and Fusobacterium necrophorum. In Australia, lambing ewes are most susceptible although the disease is also seen in lambs, wethers and rams (Roberts et al 1968, Egerton 1981, West 1983, West 1990). West (1983) considered that foot abscess was an infection of the distal interphalangeal joint and surrounding tissues rather than a primary infection of the bulbar soft tissues of the foot. He therefore considered that the term 'foot abscess' was preferable to infectious bulbar necrosis as proposed by Roberts et al. (1968). Foot abscess has also been used as a generic term to cover both heel abscess and toe abscess (Egerton 2007). The term foot abscess will be used here rather than heel abscess or infectious bulbar necrosis in part because it is now widely used in both the veterinary literature and in lay parlance.
Foot abscess was identified as an important contributor to the above average ewe mortality observed in ewes during the winter and spring of 2010 in the central tablelands of NSW (Watt, Eppleston and Dhand 2011). We conducted a trial looking at iodine supplementation pre-lambing as a preventative, because of anecdotal support for this treatment and because iodine has been demonstrated to reduce the prevalence of experimentally induced bovine footrot (Berg et al. 1984). We planned to study the prevalence, pathogenesis and risk of foot abscess in three sheep flocks, two of which were seriously affected by foot abscess in 2010. The project consists of two parts, an on-farm iodine response trial (presented here) and risk factor study (to be presented later).
This paper reports the prevalence of a range of foot conditions including foot abscess, shelly toe, overgrown hooves and ovine interdigital dermatitis during lambing in three specialist lamb producing flocks on the central tablelands of NSW during winter 2011. The impact of an iodine drench administered about 10 days prior to the commencement of the lambing period is also reported.
Flocks Three specialist prime lamb producing flocks running first cross or composite ewes near Blayney (altitude of 900 m) were used in the study. Two of these flocks were selected because they experienced a high prevalence of foot abscess during lambing in August - September of the previous year, while the owner of the third flock has drenched his ewes with iodine pre-lambing over the past two decades and considered that he had fewer cases of foot abscess consequently. All flocks grazed similar improved perennial pastures on volcanic derived soils (andesite or basalt) that had been top dressed regularly with superphosphate.
Design Multiple bearing, 4 year old ewes were identified at pregnancy scanning six weeks after the completion of joining for use in the trial. Approximately 10 days before the expected onset of lambing the trial ewes were tipped and had their feet inspected for lesions. Concurrently, each ewe was fitted with a numbered neck tag to enable identification from a distance during lambing. Half of the ewes, selected at random, were drenched with a solution containing 237 mg of ethylenediamine dihydriodide (EDDI, equivalent to 213 mg of iodine) and all ewes were weighed and condition scored. The ewes had their feet inspected again at lamb marking approximately 8 weeks later. Following the first foot inspection, the ewes were moved to their lambing paddocks in mobs of approximately 100. In one flock, 10 ewes from each treatment group were bled 7 days after the EDDI drench and again at the final foot inspection at marking. The blood samples were assayed for free T3 and free T4 levels.
Foot lesion scoring Each foot inspected was examined for the presence and degree of active foot abscess (FA), evidence of previous foot abscess (old foot abscess, OFA), overgrown hooves (OG), shelly toe (ST), interdigital dermatitis (OID) and interdigital fibromas (corns). No previously published scoring system for these foot conditions (other than OID) could be found in the literature so we developed a system for use in this study. The results of our findings, shown in Table 1, simply indicate the presence or absence of these conditions rather than including data on degree. Active FA was diagnosed when sheep presented with one or more hot, swollen digits with a purulent discharge. OFA was diagnosed when one or more digits were abnormally thick and sometimes deformed and the phalanges ankylosed but without evidence of current infection. Feet were scored as OG when the hoof projected beyond the end of the toe more than 1-2 cm. ST as described by Egerton (2007) was recorded when more than about one third of the lateral hoof wall was separated from the underlying hoof. OID, an acute infection of the interdigital skin has been previously described (Parsonson et al 1967). Interdigital fibromas (corns) were recorded when a firm swelling of otherwise normal tissue between the toes was observed. Each week during the lambing period, the ewes were inspected from a vehicle that was driven slowly through each paddock ensuring that each ewe was required to walk. Neck tag numbers and affected feet were recorded for all lame sheep. To avoid mismothering, lame ewes were not caught for closer inspection so no information was collected on the cause of lameness.
Statistical analyses Data on the proportion of ewes exhibiting feet conditions were compared using chi-squared analysis, while data on peripheral T3 and T4 concentrations were analysed by ANOVA using Genstat.
At the pre-lambing inspection the mean liveweight and condition score of the twin bearing trial ewes were 78.3 - 0.3 kg and 3.30 - 0.02, respectively, (73.5 - 0.4 kg and 3.0 - 0.03, 75.0 - 0.4 kg and 3.2 - 0.02 and 85.8 - 0.43 kg and 3.6 - 0.03 for Flocks A, B and C respectively). The proportion of ewes at the pre-lambing inspection with at least one foot affected by each foot lesion type monitored is presented in Table 1.
|Flock||Ewes inspected||FA||Old FA||Shelly Toe||Overgrown||OID||Corns|
|A||351||0 (0%)||41 (12% A)||78 (22%)||86 (25%)||0 (0%) A||8 (2% A)|
|B||296||0 (0%)||65 (22%) B||54 (18%)||78 (26%)||13 (4%) B||6 (2%) A|
|C||353||0 (0%)||88 (25% B)||80 (23%)||80 (23%)||0 (0%) A||28 (8%) B|
Means within columns with different superscript are different at the 1% level.
No cases of FA were detected at the pre-lambing inspection, however lesions remaining from previous, healed FA (OFA) were found in 194 of the 1000 (19.4%) ewes inspected. The prevalence of OFA varied between flocks (X22 = 21.52; P<0.001) with the prevalence in Flock A being lower than in the other 2 flocks.
The prevalence of shelly toe and overgrown hooves averaged 21.2% and 24.4 % and was not different between flocks. Interdigital dermatitis was observed in Flock B only where 4.4% were affected, while 4.2% of sheep were identified with corns and the level was greater in Flock C compared with the other 2 flocks.
Post-lambing prevalence of active foot abscess. The effect of iodine treatment on the prevalence of active FA is shown in Table 2. At the second inspection 4/370 (1.1%) iodine-treated ewes developed FA compared to 10/360 (2.9%) untreated ewes (X21 = 3.26; P=0.20; NS). There was no difference between flocks in the prevalence of FA (X21 = 2.79; P=0.10; NS).
|No||5/137 (3.7%)||4/119 (3.4%)||1/104 (1.0%)||10/360 (2.8%)|
|Yes||2/141 (1.4%)||2/123 (1.6%)||0/106 (0.0%)||4/370 (1.1%)|
|Total||7/278 (2.5%)||6/242 (2.5%)||1/210 (0.5%)||14/730 (1.9%)|
Of the 14 feet identified with FA at inspection 2, two had been identified with an OFA, two with OG and one with OID at inspection 1, suggesting that these other conditions do not predispose feet to the development of FA. However, the 2 feet that had previously been identified with OFA suggests the possibility that in some instances FA could develop from bacteria resident in OFA lesions.
During the lambing period, 146 ewes were assessed as lame (Table 3). Only 2 of the 14 ewes observed with FA at the second inspection were not recorded as lame during the lambing period.
|Flock||Proportion of lame ewes (%)||Duration of lameness (weeks)|
Peripheral plasma thyroxin response to iodine drenching
The mean peripheral concentrations (pmol/L) of free T3, free T4 and T3/T4 ratio before and after lambing were 5.02 and 6.89, 13.4 and 14.3, 0.41 and 0.55, , respectively. There was no significant interaction between iodine treatment and time for any thyroxin measure, indicating that drenching with EDDI had no impact on circulating thyroxin levels.
Two of the three properties used in this study had a high prevalence of foot abscess in the winter and spring of 2010. The owner of the third property routinely uses iodine pre-lambing and so had an interest in its efficacy. However, in the winter and spring of 2011, a wet autumn was followed by a dry winter. Prevalence of foot abscess in the ewe flocks studied were therefore low (14 new cases in 730 ewes examined at the marking visit or 1.9%). An examination of ewes at marking may under estimate the prevalence of FA as some cases may have healed quickly (Egerton J pers. comm.). It is also possible that the high prevalence of FA in previous years may have induced some immunity providing protection in 2011. On all properties, the prevalence of foot abscess was lower in the treated than the control ewes but this difference was not significant. Therefore, no conclusions can be drawn on the effect of parenteral prelambing iodine on foot abscess from this study.
The presence of a range of other feet conditions of sheep was recorded. The ewes were also weighed and condition scored. It was intended to determine the relationship between bodyweight, these feet conditions and FA. However, the prevalence of FA was low during this trial and so it was considered pointless to attempt to correlate FA with these conditions. However, it is interesting to note that OFA was detected in from 12-25% of the ewes examined pre-lambing in 2011. These findings support the observation that FA was a major problem in the past and presumably mainly in 2010. These prevalence figures may also underestimate the true prevalence of previous FA, as some cases will presumably have healed without permanent deformities (Egerton J, pers. comm.). A finding that was not anticipated was that two ewes with OFA developed active foot abscess in the same foot. It is possible that these were new cases that progressed from IOD as is usual with FA (Parsonson et a.l 1967, West 1983). However, the lesions appeared to be unusually extensive and chronic suggesting that recurrence from bacteria harbouring within chronically infected tissues is a possibility.
Bovine interdigital necrobacillosis (footrot) is a similar condition in cattle to FA in sheep. It is a synergistic invasion of Fusobacterium necrophorum and Porphyromonas levii into the deeper structures of the foot originating from an interdigital dermatitis (Vermunt et al 2010). While Berg et al (1984) and Maas et al (1984) found that iodine supplementation (as EDDI) was effective in reducing experimentally induced or naturally occurring bovine footrot, it was administered daily and presumably achieved therapeutic blood levels. In this trial, iodine was administered as a single dose about two weeks before the commencement of lambing. It is doubtful that this regimen would have been sufficient to maintain therapeutic levels of iodine but an equivalent quantity of potassium iodide (280 mg equivalent to 213 mg of iodine) has been shown to be adequate to alleviate iodine deficiency in perinatal lambs and kids (Caple 1990). No difference was found in T3 and T4 levels between the treated and control ewes.
Foot abscess remains a challenge to manage. It occurs under conditions otherwise consistent with good management and productivity (fecund ewes in good body condition on abundant winter pastures or forage crops). Some of the management recommendations that would alleviate foot abscess therefore risk compromising productivity and profitability. However, allowing high-risk sheep (especially mature twin bearing ewes) access to dry areas can sometimes be managed. Foot bathing with zinc sulphate can also reduce interdigital dermatitis but mustering sheep and putting them through wet yards is usually counterproductive. The second part of this project, still underway aims to quantify some of the risk factors associated with the expression and severity of FA.
Foot abscess occurs sporadically associated with favourable seasons although it can be a common problem for some producers. Egerton (1981) commented that there was (at least in 1981) no evidence that F. necrophorum vaccines are of any value in sheep. However, vaccines have been shown to reduce F. necrophorum infections in cattle (Clark et al. 1986). It is intriguing that foot abscess, while a common problem in young rams in New Zealand is much less of a problem in lambing ewes under circumstances that would otherwise appear conducive to foot abscess (D West pers. comm.).
Meat and Livestock Australia funded this trial. We also acknowledge the assistance of graziers Bob Stanbridge, Tony Gordon and Bruce Gordon, farm manager Tim Jamieson and TLHPA rangers Colin Somerset, Neville Collins and John Foran in the conduct of this trial. Murray Grant from Chemvet Australia Pty. Ltd. kindly donated the EDDI used in this trial and John Egerton made valuable suggestions both prior to the conduct of the trial and subsequently.