M. J. Green, K. A. Leach, J. E. Breen, L. E. Green, A. J. Bradley

An intervention study was carried out on 52 dairy farms in England and Wales to determine whether the implementation of a well-specified mastitis control plan in herds with an incidence of clinical mastitis of more than 35 cases per 100 cows per year would reduce the incidence of clinical mastitis, and also reduce the incidence of increases in the somatic cell counts of individual cows. A clearly defined plan for the diagnosis and control of mastitis was developed by two veterinary specialists from the research literature. The herds were randomly allocated to receive the plan either at the start of the study (intervention herds) or after one year (control herds). Data on mastitis management and the farm environment were collected during farm visits. After one year there was a significant 22 per cent reduction in the proportion of cows affected with clinical mastitis on the intervention farms compared with the control farms. There were also significant reductions of approximately 20 per cent in the incidence of clinical mastitis and in the occurrence of increases in the somatic cell counts of individual cows from below, to above 200,000 cells/ml.

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A. J. Bradley, K. A. Leach, J. E. Breen, L. E. Green, M. J. Green

A survey of clinical and subclinical mastitis was carried out on 97 dairy farms in England and Wales, selected at random from members of a national milk recording scheme. The farmers were asked to collect aseptic milk samples from five consecutive cases of clinical mastitis and from five quarters with high somatic cell counts using a defined protocol, and they completed a questionnaire that included information on the cows sampled, the herd and the history of mastitis in the herd. The samples were collected throughout the year. The mean incidence of clinical mastitis was 47 cases per 100 cows per year (estimated from historic farm records) and 71 cases per 100 cows per year (estimated from the samples collected). Streptococcus uberis and Escherichia coli were isolated in pure culture from 23·5 per cent and 19·8 per cent, respectively, of the clinical samples; 26·5 per cent of the clinical samples produced no growth. The most common isolates from the samples with high cell counts were coagulase-negative staphylococci (15 per cent), S uberis (14 per cent) and Corynebacterium species (10 per cent). Staphylococcus aureus and coagulase-positive staphylococci together accounted for 10 per cent of the samples with high somatic cell counts; 39 per cent produced no bacterial growth.

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A. J Bradley and M. J. Green

The presence of cells in bovine milk, so-called 'somatic cells', has been recognised and studied for many years. More than 95 per cent of somatic cells are leucocytes, including neutrophils, macrophages and lymphocytes. The somatic cell count (SCC) – that is, the number of somatic cells per millilitre of milk – is therefore a useful proxy for the concentration of leucocytes in milk. SCCs in milk are used as indicators of mammary health on the basis that they reflect an immune response and thus the presence of infection. An SCC of <100,000 cells/ml is often considered to be 'normal', reflecting a healthy mammary gland, whereas an SCC of >200,000 cells/ml is suggestive of bacterial infection. Although a raised SCC is an accepted indicator of an existing bacterial infection, a very low SCC has been associated with an increased subsequent susceptibility to clinical mastitis. This suggests that somatic cells may provide protection from bacterial colonisation as well as being a marker of infection. This article focuses on how SCCs may be used as a management tool for gauging current infection status – both at the individual cow and herd level.

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Part 2. Product Selection

A. J. Bradley, J. Huxley and M. J. Green

The use of antibiotics in food-producing animals is an increasing area of public concern. In particular, the prophylactic use of antibiotics is coming under scrutiny - and antibiotic dry cow therapy is one such use. The aim of this two-part article is to provide an insight into the most important pathogens during the dry period and the best approaches to minimising their impact, through a rational approach to the prescription of dry cow therapy. Part 1, published in the last issue (November/December 2002, pp 582-587), discussed udder health priorities during the dry period. This article discusses aspects of the design, prescription, use and monitoring of antibiotic dry cow treatments.

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Part 1. Udder health priorities during the dry period

M. J Green, J. Huxley and A. J. Bradley

In all areas of medicine, the appropriate use of antimicrobial agents has become a subject of much interest and debate. Concerns over the misuse of antibiotics centre on the possible build up of bacterial resistance and a fear of residues entering the food chain. The veterinary surgeon plays a pivotal role in the way in which antibacterial agents are used on the dairy farms under his/her care and dry cow therapy is probably the most commonly prescribed antibiotic product, with almost 4 tonnes of active ingredients being used in the year 2000. As with all medicines, it is essential that dry cow therapy is prescribed on a rational basis. The aim of this two-part article is to describe current knowledge on udder health in the dry period and to use this to develop a logical approach to prescribing dry cow therapy.

Part 2, which will be published in the next issue, will provide guidance on product selection.

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Andrew J Bradley

Mastitis remains a major challenge to the worldwide dairy industry despite the widespread implementation of mastitis control strategies. The last forty years have seen a dramatic decrease in clinical mastitis incidence but this has been accompanied by a change in the aetiology of mastitis and the relative and absolute importance of different pathogens. Escherichia coli and Streptococcus uberis are now the two most common causes of bovine mastitis and are an increasing problem in low somatic cell count herds.

This paper reviews the changes in incidence and aetiology of mastitis in the UK over the last forty years and discusses some of the possible explanations for these changes. It focuses in particular on apparent changes in the behaviour of E. coli and its ability to cause persistent intramammary infection; which may be as a result of bacterial adaptation or the unmasking of previously unrecognised patterns of pathogenesis. The prospects for novel approaches to mastitis control are discussed, as are the current and future challenges facing the industry.

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J. N. Huxley, M. J. Green, L. E. Green, and A. J. Bradley

The efficacy of an internal dry period teat sealer containing bismuth subnitrate (Product A; Teatseal, Cross Vetpharm Group Ltd, Ireland) was compared with a long-acting antibiotic preparation containing cephalonium (Product B; Cepravin Dry Cow, Schering-Plough Ltd, UK), by assessing the number of new intramammary infections (IMI) acquired during the dry period and the number of cases of clinical mastitis during the first 100 d of lactation. Selection of study animals was based on historical data. No cases of clinical mastitis and all routine cow level somatic cell counts <= 200,000 cells/ml during the previous lactation were used to select cows likely to be uninfected with a major pathogen at drying off. Compared with the antibiotic tube, quarters that received the teat sealer acquired significantly fewer new IMI caused by Escherichia coli, all Enterobacteriaceae, and all major pathogens combined. There was no significant differences in the number of IMI caused by any other major pathogen. There was no significant difference in the severity or number of quarter or cow cases of clinical mastitis between product groups. Sixty quarters (3.2%) were infected with major pathogens at drying off, 27 (2.9%) in teat sealer and 33 (3.5%) in antibiotic tube cows. The dry period cure rate was not significantly different (63% product A, 70% product B). This is the first controlled study to demonstrate the efficacy of an internal bismuth teat sealer in protecting quarters against new dry period IMI caused by major mastitis pathogens, particularly environmental organisms, under UK field conditions.

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A. J. Bradley and M. J. Green

Clinical mastitis in six Somerset dairy herds was monitored over a 12-month period. Escherichia coli was implicated in 34.7% of all clinical cases. Forty-one percent of all clinical E. coli mastitis cases occurred in just 2.2% of the population.A total of 23.9% of clinical E. coli cases occurred in quarters suffering recurrent cases of E. coli mastitis. The genotypes of strains involved in recurrent cases of clinical E. coli mastitis were compared by DNA fingerprinting with enterobacterial repetitive intergenic consensus primers. In 85.7% of cases of recurrent quarter E. coli mastitis, the same genotype was implicated as the cause of disease, suggesting persistence of the organism within the mammary environment. The same genotype as that in the original case was also implicated in 8.5% of recurrent cases occurring in different quarters of the same cow, suggesting spread between quarters. These findings challenge our current understanding of the epidemiology of E. coli mastitis and suggest that pathogen adaptation and host susceptibility may be playing a part in the changing pattern of clinical mastitis experienced in the modern dairy herd.

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A. J. Bradley and M. J. Green

The efficacy of an intramammary antibiotic dry cow preparation with significant gram-negative spectrum (product A; Leo Red Dry Cow, Leo Animal Health, UK) was compared with a product with no gram-negative efficacy (product B; Orbenin Extra DC, Pfizer Ltd, UK) as assessed by control of coliform mastitis in the first 100 d of the lactation. The efficacy of both products was also compared for control of noncoliform mastitis and for the ability to control existing and new intramammary infections as measured by individual cow somatic cell counts. Cows treated with product A were significantly less likely to develop clinical Escherichia coli or coliform mastitis during the dry period or the first 100 d of lactation than cows treated with product B. Cows treated with product A were no more likely to develop clinical mastitis due to a noncoliform organism than were cows treated with product B. There was no significant difference between the two groups as measured by individual cow somatic cell count changes across the dry period. This study is the first to have demonstrated the clinical efficacy of an intramammary antibiotic dry cow preparation, as measured by reduction in gramnegative clinical mastitis in the subsequent lactation. These findings demonstrate that selection of a dry cow intramammary preparation with a significant gramnegative spectrum can influence the incidence of clinical coliform mastitis in the subsequent lactation. This finding should be one of the factors taken into account when selecting products.

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A. J. Bradley and M. J. Green

We assessed the incidence of enterobacterial infection of the mammary glands of 629 cows, from six commercial herds in Somerset, during the nonlactating period; samples were collected from all clinical quarters of these cows during the subsequent lactation. A rise in the incidence of intramammary enterobacterial infection was detected between drying off and before calving. Quarters infected with an enterobacterial organism during the dry period were more likely to develop mastitis due to that pathogen than were uninfected quarters. Of all enterobacterial mastitis occurring in the first 100 d of lactation, 52.6% arose in quarters previously infected, during the dry period, with the same strain of bacteria, as identified by DNA fingerprinting using enterobacterial repetitive intergenic consensus primers. When compared with unsampled controls, quarters sampled during the dry period did not show a higher incidence of infection at calving or of subsequent clinical mastitis. These findings suggest that chronic infections are important in the epidemiology of enterobacterial mastitis and that environmental management during the dry period may greatly impact the incidence of enterobacterial mastitis in the subsequent lactation.

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