In the UK, the average dairy herd size is currently ~133 cows and the incidence rate of clinical mastitis (CM) is likely to be somewhere between 47 and 65 cases/100 cows/year. The national average bulk milk somatic cell count (BMSCC) is currently ~167,000 cells/ml and this number has been falling steadily since 2009. Clinical mastitis in the UK is caused predominantly by pathogens traditionally classified as ‘environmental’, with Strep. uberis and E. coli being the most commonly diagnosed pathogens. In contrast, pathogens traditionally classified as ‘contagious’ now account for only around 10% of diagnoses made.
In light of this UK context, much of the previous research activity by members of the Dairy Herd Health Group at Nottingham University has focussed on the design and delivery of a farm-specific mastitis control strategy that utilises in-depth data analysis methods and current mastitis literature to make farm-specific recommendations. The resulting approach was tested in a randomised controlled trial (RCT) in 52 UK dairy herds in 2004. Results from the RCT showed a mean reduction in the proportion of cows affected with clinical mastitis of 22% (having accounted for confounders) in intervention herds compared with control herds. There were also significant reductions of around 20% in the incidence of clinical and subclinical infections (Green et al., 2007). After some further developments, the Dairy Mastitis Control Plan (DMCP) was launched by the Agriculture and Horticulture Development Board (AHDB) at a national level in April 2009.
The DMCP consists of three main stages; i) analysis of the herd data to assess patterns of mastitis and categorisation of each herd according to those patterns; ii) assessment of the current farm management and, based on deficiencies identified, prioritisation of the most important management changes required and iii) frequent monitoring of the farm data to assess the subsequent impact on CM and SCC. Since the DMCP was launched at national level in 2009, over 350 plan users have been trained to deliver it and over 2,000 UK dairy herds (~20% of all UK dairy herds) have participated in the scheme. More information about the DMCP can be found on the website: www.mastitiscontrolplan.co.uk
Since the successful launch of the national mastitis control plan, the focus of mastitis research at Nottingham University has shifted more in the direction of making use of current herd data to predict what is likely to happen in the future, and to predict different farm outcomes that would be expected in light of different decisions. With advances in on-farm technologies, scientific approaches and data handling capabilities, opportunities in this field are becoming a reality. The remainder of this article will describe some of the areas where this approach is currently being applied by members of our research team in areas related to pathogen behaviour, environmental management and simulated decision making.
Predicting the behaviour of mastitis pathogens based on strain type is one area of current research within the group. Previous research has suggested that the majority of Strep. uberis strains are heterogeneous and, therefore, most likely to have been acquired from the cow’s environment. However, recent research in our group identified a number of specific clonal strains in different cows, which indicated that cow-to-cow transmission of Strep. uberis was probably occurring in the majority of herds in the United Kingdom, and may represent the most important route of infection in many herds (Davies et al., 2016). The main barriers to utilising such information are the time taken and costs incurred to strain-type mastitis pathogens. For strain typing to be effective as a predictive technology, fast throughput systems are needed that can accurately differentiate clinically important strain phenotypes. An example of such a system is Matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). MALDI-TOF-MS produces a protein ‘fingerprint’ of ribosomal proteins quantified as the amount of each protein present, defined by its molecular weight. MALDI-TOF-MS is routinely used in many microbiological laboratories for species determination of bacteria and fungi, and can also be used for the subtyping of bacteria. It has also been used successfully to differentiate bacterial sub species according to phenotype. Some recent research from our group investigated whether MALDI-TOF MS could be used to identify contagious strains of Strep. uberis and to predict future clinical mastitis risk in commercial dairy herds. This research demonstrated that it was possible to predict future high-risk periods for the transmission of Strep. uberis clinical mastitis in dairy herds based on the occurrence of specific Strep. uberis strains during the previous two weeks (Archer et al., submitted). It is now conceivable that this technology could be routinely applied to identify and predict the risk posed by specific mastitis pathogens, thereby alerting farmers prior to high-risk periods and facilitating the prioritisation of appropriate preventive measures.
In addition to predicting the risk of transmission from Strep. uberis isolates found in milk samples, there is also research on-going at Nottingham University exploring the faecal shedding patterns of Strep. uberis in dairy cows and how this relates to clinical and subclinical intramammary infections. This work is at an early stage but some preliminary results have already shown that cows were at an increased risk of shedding Strep. uberis if they were over-conditioned, in early lactation, housed or in parity one.
With limited resources available to a commercial dairy farm, it is important that potential mastitis interventions are prioritised not only according to their efficacy, but also on the likely return on investment. The efficient use of available resources requires an understanding of the opportunity costs whereby resources are allocated to fund one intervention at the expense of the potential ‘benefits’ afforded by an alternative intervention. This is the dilemma faced by veterinary decision makers and with many possible mastitis interventions making claims on farm resources, it is necessary when deciding whether to employ resources in one area to be able to compare the probability of a net benefit in that area with all other potential areas where those resources could be employed. In a recent study performed as part of a PhD project at Nottingham University, data from 77 UK dairy herds were used to explore the cost-effectiveness of specific mastitis control interventions in herds with a particular problem with intramammary infections acquired during the dry period (Down et al., submitted). The results from the Bayesian micro-simulation identified many specific mastitis control interventions that were likely to be cost-effective in many different scenarios. Important interventions likely to be cost effective for most herds included areas relating to selective dry-cow therapy, dry cow ration formulation, calving pen management and post-calving milking routines.
This research approach was repeated using data from 75 UK dairy herds that had a problem with intramammary infections originating from the cows’ environment, acquired during lactation. Important interventions that were likely to be cost effective for most herds included areas relating to space allocations for cows in the bedded lying area, use of a non-steroidal anti-inflammatory drugs, ventilation, fly control, use of drying agents in the bedding and keeping the herd closed. The results from both of these studies is being incorporated into an on-farm decision support tool that will use farm-specific input parameters to make predictions about the likely return on investment associated with specific mastitis control interventions.
Simulated decision making
Another area of our research on mastitis control uses the technique of data simulation. This is effectively a method to combine many sources of information and then to evaluate many different possible scenarios. Simulation-based studies use parameters derived from ‘real life’ (where possible) and combine these in ways to make onwards predictions in relevant areas. These methods were recently applied to assess the cost-effectiveness of using an on-farm culture (OFC) approach to the treatment of clinical mastitis compared with a conventional approach. There is increasing pressure on the agricultural sector to reduce antimicrobial drug usage due to fears over antimicrobial resistance; thus the way in which antimicrobial drugs are applied with respect to the treatment of mastitis, is therefore a sensible target for research. With the OFC system, milk samples are taken from all cases of CM and cultured ‘on-farm’ using plates designed specifically for that purpose. The plates are read after 24hrs and cases that yield a Gram-positive or mixed-culture are subsequently treated with antimicrobial drugs. Any cases that yield a Gram-negative pathogen or that fail to grow any bacteria are not treated with antibiotics, resulting in many cases of CM not being treated at all. The results of this research suggest that the proportion of Gram-positive cases and the difference in bacteriological cure rate between the two treatment approaches have a large impact on the probability that an on-farm culture approach would be more cost-effective than a standard approach for the treatment of CM. The OFC approach appeared to be suitable for herds in which Gram-negative pathogens were responsible for most CM and where the treatment of cows according to the results of an OFC approach resulted in minimal reductions in bacteriological cure rates. More research is needed in this area because results indicate that, in the UK, the use of OFC is unlikely to be cost effective for many dairy herds and could in fact be detrimental to cow welfare.
Pathogenesis, Virulence and Vaccinology
In addition to the epidemiological research, the group at University of Nottingham also has an active research programme investigating host-pathogen interactions at a molecular level. On the back of our joint project to sequence and annotate the genome of Strep. uberis (Ward et al 2009) our current research is focussed at those interactions between Strep. uberis and the bovine host that promote infection and disease. The driving force behind these studies is the need to reduce the use of antimicrobials for disease control; through the use of immunomodulatory treatments and/or vaccines. Our projects are currently aimed at identifying bacterial sequences and gene products that contribute to colonisation. The approaches being taken centre around our ability to produce bacterial mutant strains (lacking putative virulence determinants) and assessing the effect of these genetic lesions in model systems of disease, in vitro and in vivo. One such study identified the role of the transamidase, sortase A (SrtA), in anchoring proteins required for virulence at the bacterial cell surface, revealing that three distinct surface proteins were essential for Strep. uberis to fully express its virulent phenotype (Egan et al 2010; Leigh et al 2010; Egan et al 2012). More recently, we have developed the laboratory technology and a bioinformatics pipeline that enables simultaneous, rapid and statistically robust evaluation of the contribution of all genes in the bacterial genome towards virulence-related phenotype in a single experiment. Although initially developed to investigate the complex interactions of Strep. uberis with its host, this mutation mapping technology (Pragmatic Insertional Mutation Mapping System or PIMMS (Blanchard et al 2015)) has wider application in both human and veterinary medicine as it can be directly applied to any species of Streptococcus, Enterococcus or Lactococcus.
The dairy herd health group at Nottingham has expanded rapidly in recent years and consists of academics, postgraduate students and clinical residents who are responsible for the provision of farm animal teaching throughout the veterinary undergraduate programme in addition to producing high quality, high impact dairy cow research. In 2015, the group produced 29 peer-reviewed papers and won over £3 million in external grant income. High impact areas of research include the AHDB Dairy Mastitis Control Plan, use of NSAIDs in farm animals and the pathogenesis and prevention of claw horn lameness in dairy cows. A significant proportion of grant income has come from the CIEL (Centre for Innovation and Excellence in Livestock) project which is a government funded initiative managed by Innovate UK. It is a membership organisation, bringing together leading UK based academic institutions and industry partners who provide strategic direction, projects for development and commercialisation of the research carried out. The University of Nottingham was selected to be the dairy lead for CIEL. This is a significant and exciting development at Nottingham and more information can be found at: www.cielivestock.co.uk.