CASE NOTES

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Footrot research update from the University of Sydney (2018-2021)

Om Dhungyel, Richard Whittington, Andrew McPherson and Karen Smith, Sydney School of Veterinary Science, The University of Sydney

Posted Flock & Herd September 2021

The University of Sydney continues to offer a referral service and laboratory diagnostic service for footrot. This is quite a busy service assisting field veterinarians and farmers in the southern states.

Completed research projects

• Development of a website on footrot
  The objective was to provide online technical resources to field veterinarians / biosecurity officers / farmers. Funded by McGarvie Smith Trust.

Please visit www.footrotsydney.org

This is a fully illustrated, detailed, technically-focussed website covering all aspects of footrot from clinical presentation on farm through to explaining how the various laboratory tests work. It deals with the complex issues of diagnosis as well as treatment, control and prevention. The information provided is evidence-based and draws on decades of research at the University of Sydney. It is intended to keep the website up to date and to include case studies which are both educational and add to the repository of knowledge.

• Aspects of the aetiopathogenesis and diagnosis of ovine footrot
  PhD research of Andrew McPherson funded by the Ian Peter Wrigley Award. The following outcomes will be of benefit to farmers through improved understanding of the diagnosis and development of the disease at individual sheep and flock level.
• Validation of genotypic and phenotypic protease virulence tests for Dichelobacter nodosus infection, in particular provision of objective evidence to challenge the use of the AprV2/B2 qPCR tests in NSW. This test, developed in Europe and adopted in Victoria and Western Australia, was shown to result in an unacceptably high rate of false positive test outcomes that if believed would result in enforceable undertakings for farmers with benign footrot in their flocks. The results were later confirmed in independent studies completed by EMAI.
• Detection and serogrouping of D. nodosus infection using direct PCR from lesion swabs. This advance in technology for testing sheep with virulent footrot will reduce the time to confirmation of serogroups present in the flock to support outbreak-specific vaccination.
• While footrot is caused by D. nodosus, there are many other bacterial species that contribute to lesion development. They are poorly understood. In a metagenomic study of the footrot lesion, the changing bacterial community in the hoof was characterised during the course of the disease.
• Development of a conventional polymerase chain reaction test for detection of D. nodosus serogroup M. This is a relatively newly discovered serogroup that is of increasing importance. A new test for it has been added to the suite of tests used to detect the other serogroups.

• Development of footrot vaccine best practice Funded by AWI as Project ON-00545. This project addressed the need to understand how and when vaccine should be used to prevent and control footrot. It encompassed both multivalent vaccine and outbreak-specific vaccination strategies.
• Four novel multivalent vaccine formulations were compared with commercial bivalent vaccines. The antibody responses stimulated by the commercial bivalent vaccines were greater than those of all the novel multivalent vaccines. However, some novel formulations stimulated antibody responses that were deemed sufficient to provide some control over the disease on-farm. The best multivalent formulation containing nine serogroups (A to I) was evaluated in a field trial in four commercial Merino flocks in Tasmania. Virulent strains of D. nodosus were detected in each flock, with between two and five serogroups per flock. Three-hundred 1.5-year-old Merino ewes from each parent flock were included in the trial, and were allocated to one of three treatment groups (control, bivalent vaccine, multivalent vaccine). Sheep in the control group were also foot-bathed in a 10% zinc sulphate solution on a monthly basis for welfare reasons. It was not feasible to fully evaluate the protective effect of any of these treatments because the disease may have self-cured or not been expressed due to a lack of rainfall and unusually dry conditions in the trial area. It is recommended that a further evaluation of the multivalent vaccine is undertaken when environmental conditions are conducive to transmission and expression of footrot. Regardless, the duration of protection afforded by the multivalent vaccine was almost certainly less than that of bivalent vaccine as antibody titres induced in sheep by the former were lower and less persistent than for bivalent vaccine.
• Inter-vaccination intervals of two- and three-months between successive bivalent vaccines were compared. Antibody levels were similar for sheep that received a second bivalent vaccine after a two- or three-month interval, which suggests that a two-month interval is sufficient to avoid antigenic competition. This has important practical advantages for farms where more than two serogroups are present and sequential bivalent vaccines are therefore needed.
• Study of lesser virulent forms of footrot in NSW project 1
  Funded by NSW Rural Assistance Authority. (PhD research of Karen Smith).
• The severity of footrot lesions induced by aprV2-positive strains of D. nodosus was shown to vary between strains. This helps to explain why diagnostic tests based on aprV2 are fundamentally flawed.

• The efficacy of a bivalent vaccines to control and eliminate lower virulent forms of footrot in sheep were studied. Although trials were hampered by drought, it appears these vaccines can play a role.
• Individual and pooled testing of footrot lesion swabs for the detection and serogrouping of D. nodosus was compared because pooled testing may be cost effective. While testing of pooled samples was successful, pooling needed to occur at a downstream stage of the test to maintain sensitivity, so there may be little practical advantage in pooling.
• Identification of risk factors and farmer knowledge towards footrot and hoof disease was assessed in a survey.
• Comparison of RNA extraction methods and expression of virulence genes from D. nodosus isolates cultured under various conditions is currently being completed as part of ongoing efforts to better understand the development of footrot lesions.

There are three ongoing projects in which results are still being collected and evaluated

• Study of lower-virulent footrot in NSW project 2 Funded by NSW Rural Assistance Authority

The aim is to study the genetic makeup (genome) of representative strains of D. nodosus causing the lower-virulent form of the disease for potential genetic markers to distinguish lower-virulent strains from virulent strains.

• Study of the role of the elastase test in supporting clinical diagnosis
  Funded by McGarvie Smith Trust. Research conducted by EMAI and University of Sydney,.

This is a longitudinal study of laboratory virulence diagnosis (Elastase test) compared to the clinical diagnosis of footrot outbreaks in NSW.

• Pilot study on the Role of Environmental Factors in footrot outbreaks

(Hons project - Sandy Nguyen)

This is a new project employing retrospective data analysis methods in conjunction with the Sydney Institute of Agriculture.

Publications in peer-reviewed scientific journals:

1. Andrew S. McPherson, Richard J. Whittington, Evelyn Hall, Emma J. Cook, Jeremy V. Jones, Yan Qi Ang, Emma L. McTavish, Om P. Dhungyel A comparison of multivalent and bivalent vaccination strategies for the control of virulent ovine footrot. Vaccine 39 (2021) 1739-1745
2. Andrew S. McPherson, Richard J. Whittington, Ruth M. Kennan, Julian I. Rood, Om P. Dhungyel. A pasture-based experimental infection model for footrot in sheep. Small Ruminant Research (2021) 195-106305
3. KJ Smith, MJ Rosser, AS McPherson, RJ Whittington, NK Dhand and OP Dhungyel. The severity of footrot lesions induced by aprV2-positive strains of Dichelobacter nodosus varies between strains. Australian Veterinary Journal (2021) doi: 10.1111/avj.13065
4. Karen J Smith, Richard J Whittington, Mark Corrigan, Belinda Edmonstone, Nic Cronin, Navneet K Dhand, Om P Dhungyel. Efficacy of bi-valent fimbrial vaccines to control and eliminate lower virulent forms of footrot in sheep. Submitted to Australian Veterinary Journal
5. McPherson, A., Dhungyel, O., Whittington, R. (2019). The microbiome of the footrot lesion in Merino sheep is characterised by a persistent bacterial dysbiosis. Veterinary Microbiology, 236, 1-10
6. Bhardwaj, V., Dhungyel, O., de Silva, K., Dhand, N., Whittington, R. (2018). An objective method for assessment of foot conformation in sheep. Small Ruminant Research, 167, 22-28
7. McPherson, A., Dhungyel, O., Whittington, R. (2018). Detection and serogrouping of Dichelobacter nodosus infection by use of direct PCR from lesion swabs to support outbreak-specific vaccination for virulent footrot in sheep. Journal of Clinical Microbiology, 56(4), 1-12
8. McPherson, A., Dhungyel, O., Whittington, R. (2017). Evaluation of Genotypic and Phenotypic Protease Virulence Tests for Dichelobacter nodosus Infection in Sheep. Journal of Clinical Microbiology, 55(5), 1313-1326