Prevalence of mastitis pathogens in milk samples from dairy cows and heifers was studied over a period of 1 year (Aug 2005–Aug 2006) in ten dairy herds in Germany. Milk samples (n=8240) were collected from heifers without clinical mastitis at parturition (n=6915), from primiparous cows with clinical mastitis (n=751) and from older cows with clinical mastitis (n=574). Coagulase negative staphylococci (CNS) were the predominant group of bacteria isolated (46·8% of samples) from clinically healthy quarters of primiparous cows around parturition, followed by streptococci (12·6%), coliforms (4·7%) and Staphylococcus aureus (4·0%). Thirty-three percent of samples were negative on culture (range on farm level, 12·0–46·4%). In cases of clinical mastitis in primiparous and older cows, streptococci were the predominant finding (32·1 and 39·2%) followed by CNS (27·4 and 16·4%), coliforms (10·3 and 13·1%) and Staph. aureus (10·0 and 11·7%). Negative results were obtained from 21·3% (range, 0·0–30·6%) and 19·5% (range, 0·0–32·6%) of these samples. Results indicated substantial differences in the prevalence of pathogens among herds. There was a positive within-herd correlation between the monthly prevalences for Streptococcus dysgalactiae between the three groups of samples. This correlation was also found between clinical samples of primiparous and older cows for Staph. aureus. These correlations were not found for the other pathogens. Besides herd, prevalence of pathogens was influenced by parity, type of sample and season.

Milk culture results at approximately 6 d post calving were assessed in a 2-year retrospective single-cohort study in 178 Norwegian herds. A combined teat dipping and selective antibiotic therapy trial was performed in these herds where cows with composite milk somatic cell count (CMSCC) >100 000 cells/ml before drying-off (geometric mean of the last three CMSCC test-days) and isolation of Staphylococcus aureus or Streptococcus dysgalactiae were selected for either short-acting lactation antibiotic treatment or long-acting dry cow antibiotic treatment. Milk culture results at approximately 6 d post-calving were available from 437 treated cows and 3061 non-treated cows before drying-off and separate multivariable logistic regression models were ran for these two groups. Risk factors associated with isolation of Staph. aureus 6 d post calving for non-treated cows were CMSCC >400 000 cells/ml before drying-off v. <400 000 cells/ml (Odds ratio (OR)=2·4) and clinical mastitis (CM) in the previous lactation v. non-treated (OR=1·5). Risk factors associated with Staph. aureus 6 d post calving for treated cows was a CMSCC >200 000 cells/ml before drying-off v. <200 000 cells/ml (OR=2·3) and CM in the previous lactation versus non-treated (OR=1·7). For non-treated cows it was 1·7-times more likely to isolate Str. dysgalactiae 6 d post-calving if the CMSCC was >50 000 cells/ml compared with <50 000 cells/ml. For treated cows it was 3·7–5·8-times more likely to isolate Str. dysgalactiae 6 d post calving if given short-acting lactation formula at quarter level compared with long-acting dry cow formula used at cow level. Regular use of iodine post-milking teat disinfection (PMTD) did not influence the isolation of Staph. aureus 6 d post calving, but it was less likely to isolate Str. dysgalactiae 6 d post calving if iodine PMTD was used regularly rather than irregularly. The external teat sealant had no effect on either of the two bacteria.

This study indicates that the CMSCC limit for sampling cows before drying-off can be reduced to 50 000 cells/ml in herds with a Str. dysgalactiae problem. Iodine PMTD should also be recommended in these herds. Cows with a CMSCC >400 000 cells/ml prior to drying-off should receive long-acting dry cow formula irrespective of the milk culture result.

The economic effect of lactational antibiotic treatment of chronic subclinical intramammary infections due to Streptococcus uberis or Streptococcus dysgalactiae was explored by means of partial budgeting. Effects at cow level and herd level were modelled, including prevention of clinical mastitis episodes and the prevention of transmission of infections. Input variables for our deterministic model were derived from literature or based on 2002/2003 dairy prices and farming conditions in The Netherlands. Sensitivity analysis was used to examine the effect of uncertainty around input variables or changes in price estimates. On farms where pathogen transmission was prevented through proper udder health management, 3-d antibiotic treatment during lactation resulted in an average net profit of €+11·62 over no treatment while 8-d antibiotic treatment had an average negative net result of €−21·83. Sensitivity analysis showed that profitability depends on the probability of treatment-induced cure, pathogen transmission rates, culling rate, retention pay-off, and costs of antibiotic treatment. Three-day antibiotic treatment of chronic subclinical streptococcal mastitis is economically profitable over a range of input values for cure probabilities, transmission rates and losses due to culling. In contrast, 8-d lactational treatment is only profitable for very valuable animals, on farms where the risk of pathogen transmission is high and/or when the farmer is likely to cull a high percentage of cows with subclinical mastitis. Because bacterial flora, cow characteristics and management differ widely between farms, the economic outcome of lactational treatment of chronic subclinical streptococcal mastitis may be highly farm-dependent.