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The main cause of ovine footrot is the anaerobic bacterium Dichelobacter nodosus (Green and George 2008, Winter 2008, Kennan and others 2011). D nodosus is an obligate pathogen of sheep that does not survive long in the environment (Green and George 2008, Cederlof and others 2013), so the main reservoir of infection is in sheep. In theory, the organisms could be eliminated from a flock, through a combination of vaccination and/or antibacterial treatment (parenteral or topical) and/or culling of infected animals coupled with good biosecurity (Winter 2009). Elimination of footrot, using these methods, has been described in Australia (Egerton 1991, Abbott and Egerton 2003, Mills and others 2012) and Norway (Vatn and others 2012).
Gamithromycin is a macrolide antibiotic licensed for cattle in the treatment and control of bovine respiratory disease (Zactran, Merial, an injectable solution of 150 mg gamithromycin/ml). Though not currently licensed for use in sheep, gamithromycin has been used in the field in the control of footrot (Stamphoj 2011, Sargison 2012) and in treating an outbreak of lameness associated with Bacteroides melaninogenicus in sheep (Sargison and Scott 2011).
This paper describes attempts to eliminate D nodosus and footrot from one German and 48 Danish sheep farms through flock treatment with gamithromycin. In both countries, local veterinary authorities were consulted in order to obtain guidance and agreement for this off-label use (Strobel and others 2012).
In the German study, a flock of Merinolandschaf ewes was selected for footrot elimination based on a high prevalence (20–80 per cent) over the previous five years. On day 1, 184 sheep, comprising 177 ewes, two rams and five ewe lambs, were enrolled. Each animal was examined in dorsal recumbency and footrot lesions scored (0 (normal) to 5) based on previously described systems (Egerton and Roberts 1971). Samples were taken from the interdigital skin of the feet of 12 lame sheep at random using sterile wooden sticks, pooled and submitted for PCR testing (Frosth and others 2012); from the subsequent analysis the presence of D nodosus was confirmed.
At the start of the study, 117 of the 184 animals had a footrot score of ≥1, of these, 98 had lesions scores 3–5. All sheep were given Zactran at a standard dose of 4 ml to the ewes, 5 ml to the rams and 2 ml to the ewe lambs, equivalent to ∼6 mg gamithromycin/kg live weight, the dosage licensed for cattle. The injection was given subcutaneously in the axilla while the sheep were restrained in the turnover crush.
On day 23, when the sheep were re-examined, eight sheep were lame and had lesion scores of >1; a further 11 had warm feet, possibly indicating low-grade inflammation. Samples were taken from all these sheep and submitted for PCR analysis, the results of which subsequently showed that the lame sheep were positive for D nodosus, while samples from the warm feet were negative. All 19 sheep were re-treated with gamithromycin.
On day 45, the whole flock was reinspected and no lame sheep or positive lesion scores in the feet were observed. Over the following six months five cases of lameness were observed, which responded to topical treatment with an antibiotic. PCR analyses of samples from these cases were negative for D nodosus. At the time of writing, this flock has remained free of footrot for 18 months.
The Danish study followed the appearance of footrot in several sheep flocks in eastern Denmark in 2009; of 115 flocks tested for D nodosus with PCR, 48 (42 per cent) were positive. For various reasons, monitoring and treatment of clinical cases proved incompatible with farm practices and facilities, hence several farmers elected to manage the disease through farm-level elimination.
Forty-eight flocks comprising around 9000 sheep were enrolled, and the elimination protocol was initiated following disposal of the lambs so that antibiotic treatment was limited to adult sheep. Pretreatment confirmation of the presence of D nodosus was based on samples taken from the interdigital region of all four feet of eight sheep in each flock using sterile sticks. This sampling regimen has been shown to be adequate to detect a flock prevalence of 32 per cent.
Following an initial assessment and the collection of samples, a whole flock treatment was undertaken using gamithromycin, administered by subcutaneous injection on the side of the neck at a dosage of 6 mg/kg live weight. The flocks were reinspected one and six months after treatment, when eight sheep were again sampled for the presence of D nodosus.
Of the 48 participating flocks, 44 remained free of footrot and D nodosus more than one year later. In a subset of five pedigree flocks, every animal has been regularly tested with PCR and all remain free of D nodosus more than two years after the original antibiotic treatment.
These results show that whole flock, systemic antibiotic treatment can eliminate D nodosus and footrot. In the Danish series, a single treatment with gamithromycin was sufficient; in the German flock, two treatments at an interval of 22 days were required in some animals. The treatment with gamithromycin was well tolerated, though a few individual sheep developed transitory injection site reactions that resolved quickly without treatment. In the four flocks where D nodosus reappeared, it seemed that the initial treatment was successful, but that reinfection occurred subsequently. It is a matter of speculation whether the source was a cryptic infection reservoir in some of the resident animals or premises, introduction via infected sheep from other properties or indirect carriage on vehicles or people.
The veterinary use of antibiotics for group treatments needs to be justified under the aegis of responsible use. Theoretically, there should be no selection of resistance genes in the target organism if no D nodosus survive following treatment, as appeared to be the case in 45 of the 49 participating flocks. Furthermore, the relatively short survival time of D nodosus in the environment should limit the dissemination of any genes for resistance. The possible transfer of macrolide resistance genes from non-target (enteric) bacteria to human beings would be mitigated through targeting adult sheep for treatment, following which there would normally be a long interval (>6 months) to slaughter, minimising any risks of resistance-gene transfer into the human food chain. An assessment of the potential impact on human therapy arising from the use of macrolide antibiotics in livestock concluded that the risks of compromising macrolide treatment of bacterial enteritis (particularly campylobacteriosis) in man are minimal (Hurd and others 2004).
In conclusion, these field studies provide support for the use of the macrolide antibiotic gamithromycin, administered as a flock treatment, in the control and possible elimination of footrot from infected flocks under commercial farming conditions. The rationale for this approach needs to be assessed at the farm level and justified on economic and ethical grounds, but the benefits of a successful outcome on animal welfare are manifest.
Zactran is a registered trademark of Merial.
The cooperation of all the farmers who participated in this work is gratefully acknowledged. Oestein Angen, Technical University of Denmark, National Veterinary Institute, Copenhagen and Martin Ganter, Clinic for Small Ruminants, Veterinary School, Hannover, are thanked and acknowledged for carrying out the Dichelobacter nodosus PCR analyses.
Provenance: Not commissioned; externally peer reviewed
Competing interests Dr Forbes was a full-time employee of Merial. Funding of the observations in Germany was provided by Merial SAS, 29 Avenue Tony Garnier, Lyon 69007, France.
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