Examining the effect of intramammary infections with minor mastitis pathogens on the acquisition of new intramammary infections with major mastitis pathogens - a systematic review and meta-analysis

Major mastitis pathogens such as Staphylococcus aureus, Streptococcus agalactiae, Streptococcus uberis, Streptococcus dysgalactiae, and the coliforms are usually considered more virulent and damaging to the udder than minor mastitis pathogens such as Corynebacterium bovis and coagulase-negative staphylococci (CNS). The current literature contains several studies detailing analyses with conflicting results as to whether intramammary infection (IMI) with the minor pathogens decreases, increases, or has no effect on the risk of a quarter acquiring a new intramammary infection (NIMI) with a major pathogen. To investigate the available scientific evidence regarding the effect of IMI with minor pathogens on the acquisition of NIMI with major pathogens, a systematic review and meta-analysis were conducted. The total extant English- and French-language literature in electronic databases was searched and all publications cited by relevant papers were investigated. Results from 68 studies were extracted from 38 relevant papers. Random-effects models were used to investigate the effects of CNS and C. bovis on acquisition of new IMI with any of the major pathogens, as well as individually for the minor pathogens and Staph. aureus. Significant heterogeneity among studies exists, some of which could be accounted for by using meta-regression. Overall, observational studies showed no effect, whereas challenge studies showed strong and significant protective effects, specifically when major pathogens were introduced into the mammary gland via methods bypassing the teat end. Underlying risk can account for several unmeasured factors, and studies with higher underlying risk found more protective effects of minor pathogens. Larger doses of challenge organisms reduced the protective effect of minor pathogens, and studies with more stringent diagnostic criteria for pathogen IMI identified less protection. Smaller studies (those utilizing fewer than 40 cows) also showed a greater protective effect than larger studies

Antibiotic dry cow therapy, somatic cell count, and milk production : Retrospective analysis of the associations in dairy herd recording data using multilevel growth models

Antibiotic dry cow therapy (DCT) is an important part of most mastitis control programs. Updating DCT recommendations is an ongoing topic due to the global problem of antimicrobial resistance. Finland, along with other Nordic countries, has implemented selective DCT for decades. Our study analyzed Dairy Herd Improvement (DHI) information from 241 Finnish farmers who participated in a survey about their drying-off practices. The aim was to evaluate herd-level associations between milk somatic cell count (SCC), milk production, and various antimicrobial DCT approaches both cross-sectionally in 2016 and longitudinally in 2012 - 2016. The three DCT approaches in the study were selective, blanket, and no DCT use. An additional aim was to evaluate whether dynamic changes occurred in herd-average SCC and annual milk production over five years, and whether these potential changes differed between different DCT approaches. The method for the longitudinal analyses was growth modeling with random coefficient models. Differences in SCC and milk production between farms with different DCT approaches were minor. Regardless of the farm's DCT approach, annual milk production increased over the years, while average SCC was reasonably constant. The variability in SCC and milk production across all DCT groups was low between years, and most of the variability was between farms. Compared to other milking systems, farms with automatic milking system (AMS) had higher SCC, and in 2016 higher milk production. The results of this study suggest that it is possible to maintain low herd-average SCC and good milk production when using selective DCT and following the guidelines for prudent antimicrobial use. Average SCC and milk production varied across the herds, suggesting that advice on DCT practices should be herd-specific. The methodology of growth modeling using random coefficient models was applicable in analyzing longitudinal data, in which the time frame was relatively short and the number of herds was limited.Peer reviewe

Diagnosing intramammary infections: Evaluation of definitions based on a single milk sample

Criteria for diagnosing intramammary infections (IMI) have been debated for many years. Factors that may be considered in making a diagnosis include the organism of interest being found on culture, the number of colonies isolated, whether or not the organism was recovered in pure or mixed culture, and whether or not concurrent evidence of inflammation existed (often measured by somatic cell count). However, research using these criteria has been hampered by the lack of a “gold standard” test (i.e., a perfect test against which the criteria can be evaluated) and the need for very large data sets of culture results to have sufficient numbers of quarters with infections with a variety of organisms. This manuscript used 2 large data sets of culture results to evaluate several definitions (sets of criteria) for classifying a quarter as having, or not having an IMI by comparing the results from a single culture to a gold standard diagnosis based on a set of 3 milk samples. The first consisted of 38,376 milk samples from which 25,886 triplicate sets of milk samples taken 1 wk apart were extracted. The second consisted of 784 quarters that were classified as infected or not based on a set of 3 milk samples collected at 2-d intervals. From these quarters, a total of 3,136 additional samples were evaluated. A total of 12 definitions (named A to L) based on combinations of the number of colonies isolated, whether or not the organism was recovered in pure or mixed culture, and the somatic cell count were evaluated for each organism (or group of organisms) with sufficient data. The sensitivity (ability of a definition to detect IMI) and the specificity (Sp; ability of a definition to correctly classify noninfected quarters) were both computed. For all species, except Staphylococcus aureus, the sensitivity of all definitions was <90% (and in many cases <50%). Consequently, if identifying as many existing infections as possible is important, then the criteria for considering a quarter positive should be a single colony (from a 0.01-mL milk sample) isolated (definition A). With the exception of “any organism” and coagulase-negative staphylococci, all Sp estimates were over 94% in the daily data and over 97% in the weekly data, suggesting that for most species, definition A may be acceptable. For coagulase-negative staphylococci, definitions B (2 colonies from a 0.01-mL milk sample) raised the Sp to 92 and 95% in the daily and weekly data, respectively. For “any organism,” using definition B raised the Sp to 88 and 93% in the 2 data sets, respectively. The final choice of definition will depend on the objectives of study or control program for which the sample was collected

Diagnosing intramammary infections: Evaluating expert opinions on the definition of intramammary infection using conjoint analysis

The primary purpose of this study was to develop a set of criteria to serve as a pseudo-gold standard for what constitutes an intramammary infection using data from 3 consecutive quarter milk samples taken 1 wk apart. Data from lactating cows in 90 dairy herds in 4 Canadian provinces were used to generate the data sets (profiles) used in the conjoint analysis to elicit expert opinions on the topic. The experts were selected from the participants (n = 23) in the 2007 Mastitis Research Workers’ Conference in Minneapolis and from a series of mastitis laboratory courses for bovine practitioners (n = 25) in the Netherlands. Three-week udder quarter profiles with specific combinations of somatic cell count, bacterial species isolated, and plate colony count were selected and included in the conjoint analysis based on the desire to achieve even distributions in the categories of 6 constructed variables. The participants were presented with 3 sets of cards with 20 cards in each set. On each card, they were asked to assign a probability of infection on the middle day (test day) in the 3-wk profile. Depending on the set of cards, they were asked only to be concerned with the probability of infection with coagulase-negative staphylococci, Escherichia coli, or Staphylococcus aureus. These 3 organisms were chosen to represent a minor pathogen, a major environmental pathogen, and a major contagious pathogen, respectively. The assigned probabilities for each organism were cross-tabulated according to the number of times the organism of interest was isolated in the 3-wk period, how many colonies of the organism of interest were isolated on the test day, and the somatic cell count (≤ or >200,000 cells/mL). There was considerable variation in the assigned probabilities within each of the combinations of factors. The median, minimum, and maximum values of the assigned probabilities for each combination were computed. The combinations with a median probability >50% were considered intramammary infection-positive and included as a criterion in the consensus standard. This yielded 4 possible criteria, which were condensed to the following 2 by consensus at the 2008 Mastitis Research Workers’ Conference in Toronto: 1) the organism of interest was isolated on the test day with at least 10 colonies (1,000 cfu/mL), and 2) the organism of interest was isolated at least twice in the 3-wk period