Determining the organisms and pathways of infection leading to mastitis in sheep (PhD)

The aims of the first study were to estimate the incidence rate of clinical mastitis (IRCM) for suckler ewes (ewes rearing lambs for meat production) in England and to identify potential risk factors. A postal questionnaire was sent to 999 randomly selected English sheep farmers in 2010 to gather data on the IRCM and flock management practices for the calendar year 2009. The mean IRCM per flock was 1.2 / 100 ewes / flock / year (0.0-19.1). The IRCM was 2.0, 0.9 and 1.3 / 100 ewes / year for flocks that were always housed, always outdoors and a combination of both respectively. Six mixed effects over-dispersed Poisson regression models offset by flock size were developed to identify management practices associated with IRCM. Of particular interest were environmental factors, as indicated by the affect flooring and indoor versus outdoor rearing had on the IRCM, and host susceptibility as indicated by the association of IRCM with udder conformation (a potentially heritable trait). Based on these findings, the role of the environment was first considered by investigating the potential sources of mastitis-causing pathogens.

Milk samples and udder skin swabs were taken from 27 pedigree Texel ewes with clinical mastitis and 3 ewes showing no clinical signs in order to identify whether udder skin could act as a bacterial reservoir for intramammary infections. Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS) was used to identify isolates to species level and compared to pulsed-field gel electrophoresis (PFGE) to assess the capabilities of MALDI-ToF-MS as a strain differentiation tool. MALDI-ToF-MS had good correspondence with PFGE for all species tested in this study including Staphylococcus warneri, Staphylococcus equorum, Rhodococcus corprophilus, Bacillus pumilus and Micrococcus luteus. The same strain type was found in mastitic milk and on ewe udder skin for Staphylococcus warneri and Rhodococcus corprophilus indicating a possible reservoir of bacteria that might enter the mammary gland.

The third study built upon the findings from the second study by identifying the role of lamb mouths and ewe udder skin as potential transmission pathways of mastitis causing bacteria by sampling these environments and ewe milk over time in a longitudinal study of 21 ewes over 10 weeks. Twenty-six bacterial species were found in more than one location (lamb mouth, ewe udder skin and/or ewe milk), many of which have previously been associated with mastitis. Potential transmission events and persistence of the same bacterial strains between and within lamb mouths, ewe udder skin and/or ewe milk over time were identified for the first time in suckler ewes using MALDI-ToF-MS for a variety of bacterial species. Having identified udder conformation as the most significant factor associated with IRCM in the first study, the final study aimed to investigate whether udder and teat conformation are heritable traits that affect mastitis.

A total of 968 records were collected from 10 Texel flocks over a period of 3 years (2012-2014) from England, Wales and Scotland to assess whether udder and teat conformation were heritable traits associated with higher levels of chronic mastitis. Univariate quantitative genetic parameters were estimated using individual animal and sire models. The heritabilities for teat length and teat placement were greatest (0.42 and 0.35, respectively). The remaining traits (traits that generally describe the volume of the xv udder) were of moderate to low heritability. Univariable logistic regression was used to identify the phenotypic association between udder traits and chronic mastitis.

The work in this thesis has addressed several gaps in the knowledge in mastitis epidemiology by providing the first estimate for the IRCM in suckler flocks in England, and generating hypotheses for factors that may affect the IRCM. The role of the ewe’s environment and heritable traits that could potentially affect a ewe’s likelihood of getting mastitis were subsequently investigated. The results have contributed to our understanding of the factors associated with the development of mastitis.

Industry Message

Further work could include testing the hypotheses linked to the ewe’s microbial environment generated in Chapter 1, in particular the effect of flooring type on the IRCM and whether this is linked to bedding saturation and therefor microbial colonisation.

The effect of ewe movement between environments on the IRCM was another interesting hypothesis that could be tested further by investigating bacterial communities in different environments for example individual lambing pens, group lambing pens and each field.

Further evidence for the potential transmission pathways highlighted within this thesis could be gathered using shorter sampling periods using single nucleotide polymorphisms (SNP). In addition, the somatic cell count could be compared to this dataset in order to observe whether somatic cell count (indicating an immune response) increases after a potential transmission event.

The Challenge

Mastitis occurs when a bacterial infection in the udder causes inflammation. It can be caused by a variety of bacteria. It is a disease with both welfare and economic implications. Income can be lost due to decreased milk production which will result in reduced weight gain in lambs. To date, little is known about infection in the udder of ewes.

The Project

• Investigating the colonisation of the udder with certain strains of bacteria and their role in the development of mastitis.
• Determining the links between environmental strains of pathogens and intramammary (udder) infections.
  Identifying management and environmental factors that might affect a ewe’s risk of developing mastitis

Ultimately this will inform us when and how the udder becomes infected and whether certain strains of bacteria are responsible for disease.

To address the aims of this project a combination of molecular microbiological and epidemiological techniques are being used. The results will contribute to our understanding of mastitis and eventually contribute to reduction in the levels of mastitis, thus improving ewe and lamb welfare and increasing the net return of meat producing sheep.

Student

Selin Cooper, University of Warwick