Role of Fusobacterium necrophorum in sheep and the environment in the severity and persistence of footrot (PhD)

Ovine footrot is an infectious cause of lameness in sheep that has significant economic impact for the UK sheep farming industry. It is also a major concern for animal health and welfare. The causal agent is Dichelobacter nodosus, and Fusobacterium necrophorum is an opportunistic secondary pathogen that increases disease severity.

The primary reservoirs for F. necrophorum in sheep were believed to be sheep faeces and the environment, however, no studies had demonstrated the presence of F. necrophorum at either of these sites.

Two longitudinal studies (Study A and Study B) were conducted to determine reservoir sites of F. necrophorum in ovine footrot. Study A included 10 sheep sampled on four occasions at two week intervals. Study B included 40 sheep sampled weekly for 20 weeks.

Samples collected from sheep and their environment were foot swabs, mouth swabs, faeces, soil and grass. Quantitative PCR was used to detect and quantify F. necrophorum.

A multiple locus variable number tandem repeat analysis (MLVA) community typing scheme for F. necrophorum was developed and validated, and used to analyse samples from Study A and Study B.

Contrary to prior assumption, the environment was not a significant reservoir of F. necrophorum. F. necrophorum persisted in sheep, primarily on feet with footrot. MLVA indicated that the strains of F. necrophorum found on the feet of sheep were closely related, and they may therefore share characteristics that make them well adapted to feet and footrot. Mouths and faeces were an intermittent reservoir for the strains of F. necrophorum involved in footrot. Mouths and faeces may therefore facilitate persistence of F. necrophorum in the absence of footrot, or facilitate transmission of F. necrophorum between flocks. Mouths were a persistent reservoir for strains of F. necrophorum not involved in footrot.

Industry Message

Fusobacterium necrophorum is an opportunistic pathogen that increases the severity of footrot. It was previously believed that the main sources of F. necrophorum in footrot were the environment and sheep faeces. This study provided evidence that F. necrophorum is rarely present on sheep pasture. There was evidence that different strains of F. necrophorum could be found in sheep, and only some of these were involved in footrot. The strains involved in footrot were most often found on the feet of sheep, and feet with footrot had higher numbers of F. necrophorum for longer periods of time. Feet with footrot may therefore be important for transmission of F. necrophorum within the flock. The strains of F. necrophorum involved in footrot were transiently present in the mouths of sheep and were intermittently shed in faeces of a small proportion of sheep. Mouths and faeces of healthy sheep may be a potential route for F. necrophorum to spread between flocks when sheep are bought and sold.

The Challenge

Footrot is an infectious condition that makes sheep severely lame. It is common in the UK, and has important consequences for both welfare and production. Severe lameness can result in poor body condition, and therefore reduced fertility and greater susceptibility to other diseases. If ewes are affected during pregnancy and around lambing time it may also cause increased lamb mortality and reduced lamb growth rates.

There are two types of bacteria involved in causing footrot. The main bacteria involved is called Dichelobacter nodosus, but a second type of bacteria called Fusobacterium necrophorum is also important in causing the typical signs.

Dichelobacter nodosus can only survive in the environment for a maximum of 2 weeks, yet disease can reappear in a flock several months after the last outbreak. It is not yet fully understood where these bacteria survive when they are not causing disease in sheep, and what the exact role of Fusobacterium necrophorum is. One suggestion is that it increases the severity of lesions on the feet, and through doing this may increase the contamination of the environment with Dichelobacter nodosus.

The Project

During my project I will sample sheep and their environment (including soil and faeces) over several months in order to establish where Fusobacterium necrophorum can be detected, where it persists, and how any changes in location over time correlate to levels of disease in the flock. I will use a combination of laboratory and epidemiological techniques to achieve this.

My results could potentially help us to understand the spread of the disease, and why it reappears in flocks after several months. We may therefore be able to design better programmes to control the disease within flocks in order to minimise losses due to lameness.

Student

Rachel Clifton