What really causes footrot in sheep (PhD)
About this project
Dichelobacter nodosus causes footrot, a disease that affects 90% of flocks. D. nodosus also causes two thirds of foot lesions in sheep. It is a painful disease for the sheep and is therefore a welfare issue. It also has economic consequences in terms of reduced sheep productivity. There have been recent improvements in management and control however the disease remains a significant problem.
Sheep have poor natural immunity to D. nodosus and although they usually recover with correct treatment they remain susceptible to footrot. Vaccination however can be effective in giving immunity. There are ten serogroups of D. nodosus and antibodies are serogroup specific, therefore vaccines will only protect against the serogroups they target. The current commercial vaccine, Footvax, contains nine of the ten serogroups, however it has limited efficacy. This is possibly due to antigenic competition because the multiple serogroups result in lower levels of antibody production, which persist for a shorter time than vaccines with fewer serogroups.
The objectives of the project were to: • investigate the prevalence of lameness and change in management of lameness in ewes in England monitored from 2013 to 2015 • estimate the population attributable fractions that would inform on key management practices that could reduce the national global flock prevalence of lameness • identify if all serogroups (A – I) of D. nodosus are still present in England • identify if there are regional differences in serogroup diversity and distribution in D. nodosus in England • identify if serogroup diversity and distribution are associated with biosecurity practices, including vaccination • identify if the prevalence, diversity and distribution of serogroups are such that a vaccine with fewer serogroups could be used nationally, and this may offer better protection than the current commercial vaccine against footrot.
Seven hundred and forty farmers were invited to participate in the study. Each farmer had completed detailed questionnaires on lameness in their flock in 2013 and most in 2014, and had indicated that they were willing to participate in further research. A three-page questionnaire on the average period prevalence of, and management practices for, lameness in sheep was developed for the calendar year 2015. Upon the receipt of a reply, farmers were sent a swabbing kit and asked to swab the interdigital skin of one foot of each of eight sheep, ideally all lame with footrot. Detailed instructions and pre-paid packaging were provided. DNA was extracted from the swabs and tested in triplicate for the presence of D. nodous with a qPCR that detected gene loads. Positive samples were tested for the nine serogroups (A-I) using single serogroup PCRs. Data from swabs and questionnaires were analysed.
A group of 164 English sheep farmers completed questionnaires regarding the prevalence of, and managements for, lameness in their flock in 2015, and submitted interdigital swabs from the feet of eight sheep.
Key findings from this project were:
The prevalence of lameness in ewes in England in 2015 was 4.1%, significantly higher than the previous two years, 3.3% in 2013 and 3.2% in 2014. This increase was associated with a change in management practices by farmers towards flock-based managements and away from individual prompt (within 3 days) treatment of lame sheep. Although there has been an increased uptake in flock-level best management practices of culling lame sheep, vaccinating against footrot and not routine foot trimming, there has been a detrimental change with fewer farmers treating lame sheep promptly (within 3 days) and this is not enough to counteract the effect of neglecting this highly effective management for footrot.
Prompt treatment of lame sheep, within 3 days of becoming lame, is the change in practice that would have the highest immediate impact on reducing the prevalence of lameness. Identifying both the reasons why some farmers stop treating promptly and practical methods to make prompt treatment easier will be useful in encouraging farmers to treat sheep promptly.
All serogroups were detected nationally. There is random heterogeneity in the communities of serogroups between flocks. The dissimilarity in communities of serogroups between flocks is stochastic, with no geographical patterns. Whilst serogroups H and B are the most prevalent, they are dominant in only 38.2% of flocks and so a vaccine with fewer serogroups is unlikely to be effective nationally.
Serogroups are clustered within flocks rather than homogenously spread with fewer than expected flocks positive for each serogroup. Each flock has a distinct population of D. nodosus serogroups.
The number of serogroups present in a flock is higher than previously reported, probably due to the increased sensitivity in the detection methods used in this study. The number of serogroups present in a flock is associated with stocking density, with more serogroups present in flocks with a stocking density of greater than 4 ewes/acre.
Creating flock-specific vaccines is an option for improving vaccination against footrot, and is currently the method of vaccination in Australia. However the prevalence of flocks with multiple serogroups, and the frequent movement of sheep between flocks will make flock-specific vaccination more challenging than it is in Australia. A vaccine with fewer serogroups would be unlikely to be more efficacious nationally than that currently available based on the assumptions in this project. One or two bivalent vaccines targeting the two and four most prevalent serogroups would only protect up to 27.1% and 65.3% of flocks respectively.
Further details of this project can be found in the final report.