Eradication of ovine footrot by repeated daily footbathing in a solution of zinc sulphate with surfactant.

OBJECTIVE: To investigate the effect on ovine footrot of repeated daily footbathing in a solution of zinc sulphate with surfactant. DESIGN: Merino sheep were allocated to control and treatment groups of 119 sheep each at week 0. The sheep had a history of S1, U1, T and/or U6 types of Dichelobacter nodosus in interdigital and underrunning footrot lesions. Feet were not pared prior to treatment. PROCEDURE: Treatment sheep were footbathed in a 15 to 18% (w/v) solution of zinc sulphate with surfactant for 10 min on five consecutive days during week 1. At week 2, and fortnightly to week 52, all feet were inspected, lesion scores were recorded and samples were taken for laboratory tests. At week 53, all feet with no lesions at week 52, but with underrunning lesions prior to week 1, were pared and samples were taken. RESULTS: After footbathing, there were no lesions in any treatment sheep at any inspection to week 52. The percentage of feet of control sheep with lesions increased from 9% (391 of 4,284) between weeks 20 and 36, to 14% (593 of 4,284) between weeks 36 and 52. Ninety-five of 96 control sheep with no lesions at week 20 were still asymptomatic at week 52. D nodosus was not isolated from samples taken from 99 and 87 pared feet of treatment and control sheep, respectively. CONCLUSION: Repeated daily footbathing combined with prolonged exposure to a dry environment eradicated footrot in sheep with both interdigital and underrunning lesions in feet that were not pared prior to treatment.

Occurrence of different strains of Dichelobacter nodosus in new clinical lesions in sheep exposed to footrot associated with multi-strain infections.

OBJECTIVE: To investigate the occurrence of S1, U1 and T strains of Dichelobacter nodosus in new clinical lesions in sheep exposed to footrot associated with multi-strain infections. DESIGN: Seventy-seven donor sheep were grazed with 84 recipients for 33 weeks. The donor sheep were Merinos with a history of clinically virulent footrot associated with protease type S1, U1 and T strains of D nodosus that hybridised with gene sequences pJIR314B, pJIR318 and/or pB645-335. The recipient sheep were Merinos with no history of footrot. PROCEDURE: Each fortnight, all feet were examined, their lesion scores were recorded and samples of lesion material were taken for laboratory tests. RESULTS: Eighty-nine percent (299 of 336) of feet of recipient sheep developed new clinical lesions. S1, U1 and T strains of D nodosus were recovered from 58%, 22% and 18%, respectively, of these lesions at a ratio that remained constant during two apparent peaks in footrot transmission. Gene sequences homologous to pJIR314B and pB645-335 were detected in 56% (93 of 166) and 29% (48 of 166), respectively, of S1 strains of D nodosus at a ratio that was not constant during the experiment. CONCLUSIONS: S1 was the dominant protease type of D nodosus in new clinical lesions. The occurrence of S1 strains did not increase relative to U1 and T strains of D nodosus during the experiment. S1, U1 and T strains of D nodosus remained in equilibrium despite changes in environment, genetic types in the population of S1 strains, and host resistance to footrot.

Severity and persistence of footrot in Merino sheep experimentally infected with a protease thermostable strain of Dichelobacter nodosus at five sites.

OBJECTIVE: To test the hypothesis that ovine footrot associated with a thermostable protease strain of Dichelobacter nodosus undergoes self cure or is sustained as an annually recurring disease, depending on the environment. DESIGN AND PROCEDURE: Forty Merino sheep from a single blood line were infected with a protease thermostable strain of D nodosus at each of five sites in Western Australia. Footrot lesions and microscopic evidence of D nodosus were recorded every fortnight for 2.5 years, supplemented by laboratory culture. Rainfall, soil and air temperature, pasture quantity and composition and soil types were also recorded. Flocks that apparently self cured were relocated to a more favourable site for footrot in the final spring season. RESULTS: The maximum prevalence of feet with clinical footrot lesions was 80.6, 1.3, 14.4, 3.8 and 88.1% at the five sites. Severe footrot occurred for three consecutive spring seasons at one site that had clay loam soil and at least 3500 kg/ha total pasture dry matter annually. However, the infection was asymptomatic for up to 10 weeks between outbreaks. D nodosus was isolated from flocks for 2.5 years at only two sites, although there was microscopic evidence of the organism at other sites in the final year. A thermolabile variant (strain U6) of D nodosus was isolated from the two sites where footrot persisted. CONCLUSION: Depending on time and location, ovine footrot induced by a protease thermostable strain of D nodosus either self cured or persisted as annual outbreaks interspersed with periods of asymptomatic infection.

Comparison of gene probe and conventional methods for the differentiation of ovine footrot isolates of Dichelobacter nodosus.

In a collaborative study that involved four Australian veterinary diagnostic laboratories a gene probe test based on the recombinant plasmids pJIR318, pJIR314B, and pJIR313, which contain genomic vap or vrl regions, was compared with conventional tests used for the differential diagnosis of ovine footrot. A total of 771 clinical dichelobacter nodosus isolates were tested and designated as belonging to one of several gene probe categories. The results showed that 87% of the virulent isolates belonged to gene probe category 1, compared to only 6% of the benign isolates. It was concluded that there was good correlation between the gene probe test and the virulence designation of these isolates as well as the results of elastase, gelatin-gel and protease isoenzyme tests. Furthermore, the gene probe test was converted to a polymerase chain reaction (PCR)-based test. It is suggested that diagnostic laboratories consider carrying out both this PCR test and tests based on the extracellular proteases of D. nodosus.

The laboratory culture of Dichelobacter nodosus in a footrot eradication program.

As part of a program to eradicate virulent footrot from Western Australia, 2745 isolates of Dichelobacter nodosus were isolated from 5263 specimens from 1883 submissions. The virulence of each isolate was assessed using protease thermostability and isoenzyme zymogram. We describe changes to the materials and methods required to handle these specimens and to reduce the interval between submission and report to 8 days.