Is multistrain infection by Dichelobacter nodosus important in the severity of footrot and in the management of disease?

Footrot causes lameness in sheep. This debilitating disease causes sheep to lose weight and produce less milk so that their lambs grow slowly. Footrot is very common in the UK, affecting >98% of sheep flocks, with ~3 million sheep becoming lame/year. Footrot is caused by Dichelobacter nodosus, an anaerobic bacterium, that causes inflammation of the skin in a sheep's foot: this causes lameness. In some cases damage is so severe that the hoof horn separates from the living tissue inside the foot and exposes this flesh (severe footrot, SFR). It is an infectious disease that spreads between sheep, and sheep can become lame repeatedly because immunity is poor. Consequently footrot is an important cause of reduced productivity and poor welfare in sheep. Footrot costs the UK industry ~£80 million/year and is one of the most costly diseases in sheep farming globally.

Whilst we know D. nodosus causes footrot we also know that it is present on healthy feet and so does not always cause disease. Some D. nodosus strains are more able to cause severe disease than others. In the laboratory strains are labelled virulent (can cause SFR) or benign (cannot cause SFR) because of genetic markers in the bacteria, however, this is classification does not match disease that we see in sheep.

Footrot is present all year round (endemic) in most areas of the UK, appearing as a series of mini-epidemics throughout the year. Yet, in some parts of the world farmers have eradicated footrot from their flock. This is difficult in the UK because footrot is present in so many flocks, persists in our temperate climate and is spread between flocks by movement of sheep between farms, which is common in the UK. Control is therefore a more feasible option for the UK. This can be done by separating diseased sheep, treating sheep promptly to prevent disease spread, by preventive footbathing in disinfectant and by vaccines (although the currently licensed vaccine is not effective if used alone). These do not prevent disease.

Studies of all strains of D. nodosus on the feet of on healthy, inflamed and diseased feet show that increasing numbers of D. nodosus on inflamed skin is linked to increasing risk of the development of severe disease.

Here we propose that for disease to occur at least one virulent D. nodosus must be present and in sufficient numbers on a foot and conversely that control can be gained by reducing the number of D. nodosus and reducing the number of virulence factors in the population of D. nodosus.

To test these ideas we will investigate the population of D. nodosus on the feet of sheep from flocks that are free from footrot, sheep that are changing from the diseased to non-diseased state and in flocks that are part of a clinical trial to reduce lameness by implementing a variety of new managements. In total ~11,000 swab samples of feet will be collected for analysis.

The analysis will use modern molecular techniques to determine whether D. nodosus is present in healthy flocks, whether different strains of D. nodosus (both virulent and benign) are present at different points in the progression of disease and whether the number of D. nodosus that are present on feet is linked to the disease state of the foot. We will then use these data to model what is happening on the feet, on sheep and within a flock to understand how the disease spreads and persists and how this might be prevented. The models will use sophisticated statistics to determine what factors are most important in disease progression and powerful mathematics to determine how these lead to disease spread and persistence.

This project includes scientists working to understand how communities of microbes change in time and space, with veterinary epidemiologists, who investigate how livestock diseases spread and can be controlled and modellers who determine how data we collect today can be used to most effectively treat and hopefully prevent disease in the future.

Footrot, caused by Dichelobacter nodosus (Dn), costs the UK sheep industry >£80million pa: 98% of flocks have footrot and 3 million ewes are diseased pa. The current licensed vaccine includes 9/10 Dn FimA serogroups, offering 60% efficacy for 4 months. As vaccines contain fewer serogroups efficacy increases; flocks often have several serogroups.

Dn isolates are defined as benign or virulent based on specific virulence genes. Disease is also defined as benign or virulent, however, disease and isolate virulence are not well correlated: virulent isolates are detected in benign disease and vice versa. We propose that confusion has arisen because culturable isolates rather than whole community Dn have been studied and because isolate virulence has not been studied in longitudinal studies where disease dynamics would inform on temporal relationships between virulence of bacteria and disease.

Recent work in our group indicated that Dn load drives pathogenesis and that flocks with no disease still have detectable Dn. We hypothesise that total load and the whole community of virulent and benign Dn strains are key to understanding disease dynamics and to identify effective flock specific managements that provide resilience to disease. We will study total community, FimA and virulence of Dn using non-culture techniques: PCR, qPCR, MLVA fingerprinting and high-throughput sequencing. We will combine lab results with disease and management data from 3 epidemiological studies and use competing risks models to elucidate strain and virulence factors that drive disease progression and severity. A framework of stochastic mathematical models will be used to simulate optimal control strategies including the role of varying number of serogroups / vaccine. Should our hypothesis be correct then in 3-5 years the sheep industry will have a strategy to control footrot using a combination of flock specific vaccine together with other managements to reduce load and increase resilience.

Endemic and chronic bacterial diseases are a major health and welfare issue for both humans and animals and many appear to be associated with either polystrain or polymicrobial infections. In our project, we aim to understand the role of polystrain infections with Dichelobacter nodosus on the development and severity of disease (footrot) in sheep and gain insights into the pathogenesis and best control of footrot at flock level. Footrot has major economic, environmental and welfare impacts. Therefore the results from this project have potential to impact across a number of sectors, from fundamental and applied science to the actual management of foot health on sheep farms. The Pathways to Impact of this proposal describes how end-users and other stakeholders will be engaged.


Understanding chronic bacterial diseases: the context of current management of footrot.

This project will provide evidence that explains how the current management of footrot perturbs the D. nodosus community and particular attention will be paid to effective managements and possibly novel managements should the data suggest these may be possible. Suitable partners within the ARC (levy bodies, pharmaceutical companies) will be sought to exploit these ideas.

If our hypothesis proves to be correct and we clarify the interaction between D. nodosus and footrot by estimating load and the community of virulence factors independent from culture, then our results will have direct impact on the sheep industry (vets, agricultural advisors and farmers). Vets and farmers will understand that control of disease can be achieved by driving down load and that this might be achieved using a variety of routes identified from the project. Farmers and vets can identify those suitable for their specific farm and implement them immediately.


Polystrain diseases and the role of bespoke vaccines

In the short to medium term, specifically for footrot, the veterinary pharmaceutical companies will be provided with simulated scenarios for the likely value of bespoke FimA vaccines in control of footrot. These can be produced for a flock once the serogroups present are known and the project will elucidate whether and when in a flock production cycle they would be most effective. The antigens are developed but clinical trials would be necessary before such products were used on farms.

In the longer term, all parties interested in polystrain diseases with poor immunity will have a novel strategy to consider investigation and management of polystrain infections.

This project, while based in the strategic science of understanding and controlling footrot, has the potential to impact both fundamental and strategic science across many fields. It will provide a model for investigating, understanding and potentially exploiting polystrain diseases and in the use of cutting-edge scientific approaches to tackling real-world problems in a coherent multidisciplinary fashion.