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Variation in species distribution and diversity of staphylococci and mammaliicocci (SaM) causing intramammary infections in dairy cattle is associated with different management practices. Disparate selective pressures on organic dairies could potentially result in population differences of these mastitis-causing bacteria. The species-specific effect on quarter SCC of SaM for a population of certified organic dairies has not been described previously. The current study presents data from a longitudinal study of 10 certified organic dairy farms. The objective was to estimate how quarter milk somatic cell count (qmSCC) varied as a result of infection with the most frequently isolated SaM species. Aerobic culture of quarter milk samples to identify IMI was conducted in parallel with determination of qmSCC. A linear hierarchical repeated measures mixed model was used to estimate qmSCC for quarters with an IMI caused by a given SaM species, compared with culture-negative quarters. The model included DIM at time of sampling to adjust qmSCC estimates for each SaM species. The final dataset consisted of 648 quarters with an IMI due to 10 different SaM species and 1,972 culture-negative quarters. Staphylococcus chromogenes was the most frequent species, followed by S. aureus, S. haemolyticus, and S. simulans. A large amount of variability was observed in the SCS for culture-negative quarters and those infected with many SaM species, especially S. chromogenes, S. haemolyticus, S. simulans, and S. aureus. Somatic cell score was significantly higher in quarters infected with S. agnetis, S. aureus, S. chromogenes, S. devriesei, S. haemolyticus, S. hyicus, S. simulans, S. warneri, and S. xylosus compared with culture-negative quarters. The highest cell count was for quarters infected with S. warneri, followed by S. aureus, S. agnetis, and S. hyicus. The relative distribution of various SaM species and their effect on qmSCC in this population of small to midsize organic farms was similar to previous studies. Although the increase in qmSCC was modest for most SaM species observed, the widespread prevalence of these intramammary pathogens could potentially contribute to sizable increases in bulk tank SCC., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Our hypothesis was that the accuracy of a commercial milk pregnancy-associated glycoproteins (PAG) ELISA test using the same thresholds at all stages of gestation would be improved by adjusting the cut-points according to the number of days since last breeding (DSLB) between 23 and 90 d after insemination in Holstein dairy cows. Our objectives were to develop a DSLB-specific set of thresholds that would provide better test performance under field conditions and provide more information for the inconclusive test results, by dichotomizing these into "probably open" and "probably pregnant" categories. Milk samples (n = 182,738) submitted to the Lactanet (Canadian Dairy Herd Improvement) laboratory from 2013 to 2021 for pregnancy testing using a commercial PAG ELISA test were compared with records on insemination outcomes. The data were separated randomly into a training dataset used to develop the DSLB-specific interpretation grid and a validation dataset to quantify its test characteristics and compare the performance of the DSLB-specific and fixed thresholds. Our aim was to achieve negative predictive value >0.99 at all stages of test use and positive predictive value (PPV) ≥0.95 at ≤59 d and ≥0.99 between 60 and 90 d after insemination. Neither approach met these targets between 23 and 25 DSLB. The DSLB-specific interpretation grid had greater PPV than the fixed threshold between 26 and 49 DSLB and met the targets. Both approaches were very near the targets of performance between 50 and 90 DSLB. The DSLB-specific interpretation grid has a similar prevalence of inconclusive test results compared with the fixed threshold but provided additional information on the likelihood of the cow being pregnant or open. Classification of milk PAG results using DSLB-specific cut-points improved the predictive value of pregnancy diagnosis between 23 and 49 d after insemination., (© 2025, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Microbial infections of the mammary gland often cause mastitis, and it can lead to substantial economic losses within the dairy industry due to its direct negative impact on milk production and composition and the associated treatment costs. Somatic cell count has emerged as a critical indicator in monitoring udder health, and recently, the large-scale availability of differential cell count analysis potentially offers new insights into underlying physiological processes. Therefore, the main objective of this study was to estimate the variation of DSCC and SCC of individual quarter-level milk samples of cows according to 1) their intramammary infectious status; 2) parity of the cow; 3) quarter location; and 4) DIM at the time of sampling. A convenience sample of 5 dairy herds using an automated milking system was selected and visited every 2 weeks for milk sample collection. The determination of SCC and DSCC was performed by Lactanet using a CombiFoss 7 DC instrument. The bacteriological culture was performed according to the National Mastitis Council standards. The different types of colonies (up to 10 colonies) were counted and identified MALDI-TOF. Given the hierarchical structure of the data, a 4-level, linear mixed model with herd, cow, and quarter as random intercepts was built with either SCS or DSCC as the outcome. Differential SCC varied broadly in the SCS range 2 to 12 but tended to have a narrower variation at higher SCS levels. The effect of DIM on DSCC depended on the parity. Early in lactation, primiparous cows tended to have lower DSCC than older cows. Following 230 DIM, the DSCC in primiparous exhibited an upward trend, whereas, in older cows, it tended to decline. The quarter position did not affect either DSCC or SCS. Quarters infected with S. chromogenes, S. dysgalactiae, S. aureus, S. epidermidis, "other major," and "other minor," had an increase in DSCC by approximately 10.2%, 9.9%, 9.8%, 9.2%, 6.0%, and 4.9% when compared with quarters with no growth. Our findings highlighted that IMI, parity, and DIM influenced DSCC. Therefore, these parameters should be considered when interpreting DSCC., (© 2025, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Mastitis poses significant economic challenges for dairy farms. Therefore, enhancing the accuracy of diagnostic methods for detecting intramammary infections (IMI) can potentially improve prevention, control and treatment strategies. The somatic cell count (SCC) is a well-established parameter for identifying inflammation resulting from IMI. Given the recent introduction of differential somatic cell count (DSCC) for routine milk sample screening, limited research has been conducted to assess its additional benefits for diagnosing IMI. Therefore, our main objective was to evaluate the diagnostic accuracy of SCC, DSCC and SCC-DSCC combinations in detecting IMI caused by any pathogen or by major pathogens using quarter milk samples. Five dairy herds using automated milking systems were selected using convenience sampling in Québec, Canada. Determination of SCC and DSCC was performed by Lactanet (Ste-Anne de Bellevue, QC, Canada) using a CombiFoss 7 DC instrument. A 5-populations 2-tests Bayesian latent class models was used, with bacteriological culture employed as the imperfect reference test. Posterior estimates for sensitivity (Se), specificity (Sp), and the predictive values for 2 hypotheticals IMI prevalences due to any pathogen or major pathogens were computed. The proportion of quarters positive for any pathogen or major pathogen using milk culture was 31.7% (5,125/16,176) and 5.4% (871/16,176), respectively. For the detection of IMI by any pathogen, using a serial interpretation for the combination of SCC ≥100,000 and DSCC at ≥65% increased the Sp from 0.71 (95BCI: 0.70, 0.72) to 0.84 (95BCI: 0.83, 0.86) compared with SCC alone at the cut-off ≥100,000 cells/mL, although resulting in a slight decrease in Se from 0.49 (95BCI: 0.43, 0.54) to 0.46 (95BCI: 0.42, 0.50). Moreover, for detecting IMI caused by major pathogens, combining SCC at the threshold of ≥100,000 cells/mL and DSCC at the threshold of ≥65% using serial interpretation increased the Sp from 0.68 (95BCI: 0.67, 0.69) to 0.80 (95BCI: 0.79, 0.81) compared with SCC alone at the ≥100,000 cells/mL threshold. Our findings suggest that DSCC could be combined with SCC to provide a modest improvement in Sp with minimal compromise in Se for identifying IMI caused by any or by major pathogens. In addition, DSCC combined with SCC provided a small improvement in Se for detecting any pathogen using the parallel interpretation. However, no improvements in Se were observed when using the combination of SCC and DSCC for detecting major pathogens., (© 2025, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Mastitis, an inflammation of the udder primarily caused by an intramammary infection, is one of the most common diseases in dairy cattle. Somatic cell count (SCC) has been widely used as an indicator of udder inflammation, assisting in the detection of subclinical mastitis. More recently, differential somatic cell count (DSCC), which represents the combined proportion of lymphocytes and polymorphonuclear leukocytes, has become available for routine dairy milk screening, though it was not yet widely studied. Therefore, the objective of this study was to assess and compare the usefulness of quarter-level somatic cell score (SCS) or DSCC to predict the probability of subclinical mastitis. Additionally, our goals included estimating the sensitivity (Se) and specificity (Sp) of SCS and DSCC across all potential cut-off values. The current study was an observational study conducted on commercial dairy farms. Five dairy herds were selected using a convenience sampling. A Gaussian finite mixture model (GFMM) was applied to investigate the latent quarter subclinical mastitis status with either measurement, i.e., SCS or DSCC. Posterior values for SCS and DSCC obtained from the GFMM were used for predictive estimation of the parameters. The estimated SCS distribution for healthy quarters had a mean (standard deviation) of 1.4 (1.3), while, for quarters with subclinical mastitis, it was 4.5 (2.4). For DSCC, the estimated mean was 55.6% (15.2) for healthy quarters, whereas it was 80.4% (6.4) for quarters with subclinical mastitis. The most discriminant cut-off for SCS, as indicated by the Youden index, was 3.0, corresponding to exactly 100,000 cells/mL. At this threshold, the Se and Sp of SCS were 0.73 (95% Bayesian Credible Interval [BCI]: 0.70-0.77) and 0.90 (95% BCI: 0.89-0.91), respectively. The most discriminant cut-off point for DSCC was 70.0%, with corresponding the Se and Sp values of 0.95 (0.93, 0.96) and 0.83 (0.81, 0.85), respectively. For the SCS analysis, we obtained predictive probabilities of subclinical mastitis approaching 0 and 100%, with only a narrow range of SCS results yielding intermediate probabilities. On the other hand, predictive probabilities ranging from 0 to 90% were obtained for DSCC analysis, with a large range of DSCC results presenting intermediate probabilities. Thus, SCS seemed to surpass DSCC for predicting subclinical mastitis. These findings provided a foundation for future studies to further explore and validate the efficacy of GFMM for diagnostic tests yielding quantitative results., (© 2025, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Antimicrobial resistance (AMR) is one of the greatest threats to global health worldwide and is threatening not only humans, but also animal production systems, including dairy farms. The objective of this paper was to describe risks factors associated with AMR on dairy farms in Québec, Canada. This observational cross-sectional study included 101 commercial dairy farms and took place over a one-year period between the spring of 2017 and the spring of 2018. We explored risk factors such as farm practices and producer knowledge (measured using a questionnaire), antimicrobial use (quantified using veterinary invoices), and the presence of Salmonella Dublin (tested by serology). We evaluated AMR with fecal Escherichia coli retrieved from preweaning calves and lactating cows using the following outcomes: the presence of extended-spectrum β-lactamase/AmpC resistance and the number of resistances to antimicrobial classes. We used logistic regression models to evaluate the association between each risk factor and the 2 outcomes for the 2 types of samples (preweaning calves and lactating cows). Furthermore, we explored the relationships between these risk factors utilizing data dimensionality reduction and hierarchical clustering. Outputs of these analyses were used as regressors for AMR in regression models. Although the results for univariate analyses were ambiguous, the unsupervised analysis naturally categorized the sample of farms according to their health and treatment status (dimension 1, explaining 12.9% of the variance) and herd size (dimension 2, explaining 7.8%). Three clusters of farms were identified (cluster 1: mainly healthy herds and low ceftiofur users, cluster 2: relatively high ceftiofur users, cluster 3: farms with a higher incidence of diseases and higher antimicrobial treatment rates). Dimension 1 and cluster membership were statistically associated with the presence of extended-spectrum β-lactamase/AmpC resistance in lactating cows and in preweaning calves. Dimension 1 was also statistically associated with the number of resistances in lactating cows and in preweaning calves. This study highlights the complexity of analyzing risk factors associated with AMR. Our results suggest that the herd health status and the antimicrobial use-related practices used are associated with AMR in dairy farms. However, prospective studies are needed to confirm a causal relation., (© 2024, The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Information is needed on vaccination protocols used by veterinarians and dairy producers to prevent and control infections in dairy herds. This observational study described farm's vaccination standard operating procedures (SOP) developed by veterinarians in collaboration with dairy producers in Québec. Data pertaining to vaccination protocols and dairy producer practices were collected as part of the biosecurity component of the National Mandatory Quality Assurance Certification Program (proAction). Generalized statistical mixed-effects models were used to assess associations between dairy herd characteristics and the vaccination SOP, encompassing various vaccination types. These included any vaccination, core vaccines only (bovine respiratory syncytial virus, infectious bovine rhinotracheitis herpesvirus, parainfluenza virus type 3, bovine viral diarrhea virus type 1 and type 2), and vaccination against diarrhea, mastitis, or clostridial diseases. These models accounted for random variations related to clustering by veterinarians and veterinary clinics. Furthermore, the variance of the outcome was partitioned into producer, veterinarian, and veterinary clinic levels to explore the proportion of the total variance attributable to each group. A total of 3,759 standardized vaccination procedures completed between 2018 and 2021 were analyzed. At least one vaccination target was included in the vaccination SOP in 90% of the dairy herds. The most frequently included vaccine in the SOP was core vaccines, comprising 88%, followed by mastitis (22%), neonatal diarrhea (18%), and clostridial disease vaccines (15%). The vaccination SOP, particularly core, mastitis, and diarrhea vaccinations, mainly varied due to the veterinarian's characteristics, followed by the clinic's characteristics. In contrast, the decision to included clostridial vaccination primarily varied with the veterinary clinic (76%). Organic producers generally included fewer vaccinations in their SOP, including core vaccines, than conventional producers. In addition, producers who were providing access to pasture had fewer vaccination SOP for vaccination against mastitis and neonatal diarrhea but more vaccination SOP for clostridial vaccination., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Mastitis is one of the most common diseases of dairy cattle. It has a high impact on-farm economy, farmers' working time, and antimicrobial usage (AMU). Selective dry cow therapy (SDCT) is an effective means of reducing AMU without negatively affecting udder health. The objective of our study was to evaluate the impact of SDCT implementation on farmer income, working time, and AMU, using a bioeconomic model. A stochastic dairy simulation model (DairyHealthSim) based on a weekly model was used to simulate herd dynamics, reproduction, milk production, culling decisions, health outcomes, and the management of health events. A specific module was developed for the simulation of quarter-level IMI acquisition and elimination during the lactation and dry-off periods, and 25 different farm settings were defined to represent herds with various udder health situations. We then defined 20 scenarios of SDCT by combining both the use of different thresholds of SCC and milk bacteriology for treatment allocation and the use of internal teat sealant (ITS). All SDCT protocols had little effect on farmer income, and we identified some protocols with a positive farm gross margin (up to Can$15.83/dried cow; at time of writing, Can$1 = US$0.72). We also found that adding an ITS to all cows led to greater economic gain. The application of SDCT had little effect on farmers' working time, except when milk bacteriology was used for decision making. Antimicrobial treatment to all cows above 200,000 cells/mL at last control, with the use of ITS on all cows, seems a good choice in most dairy farms. These findings could be used to convince farmers to adopt this strategy at dry-off., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Despite good manufacturing practices and rigorous cleaning and sanitizing procedures established in dairy processing plants, microbiological contamination remains the main cause of products being noncompliant or atypical and hence not fit for human consumption. The objective of this study was to isolate, identify, and characterize bacteria, yeasts, and molds associated with substandard dairy products in Canada and to create a collection of reference isolates. In addition to conventional microbiological characterization, each isolate was tested for biofilm-forming ability and susceptibility to heat, antimicrobial agents, and common industrial disinfectants. Among the 105 microbial strains isolated from pasteurized milk, cream, and cheese samples, 24 bacterial isolates, belonging mainly to the genus Pseudomonas, were shown to be moderate or strong biofilm producers in 96-well plates and highly resistant to peracetic acid (100 ppm, 5 min contact time) and sodium hypochlorite (70 ppm, 5 min contact time). In addition, 56 bacterial isolates, including Acinetobacter baumannii, Enterobacter bugandensis, Klebsiella pneumoniae, and Pseudomonas spp., were found resistant to ampicillin, fosfomycin or ceftriaxone, while 14 others, such as Bacillus spp. and Macrococcus spp., withstood a heat treatment equivalent to low-temperature, long-time pasteurization (63°C for 30 min). This descriptive study provides new information on potential problematic microorganisms in dairies and will guide the development of novel control strategies intended to prevent and reduce microbiological contamination and the associated economic losses., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

The global food animal industry faces a growing concern regarding antimicrobial resistance (AMR), primarily driven by the use of antimicrobials (AM) for the treatment, control, and prevention of diseases. Addressing this challenge requires promoting responsible antimicrobial use (AMU) practices. In 2019, the province of Québec, Canada, took a significant step by implementing a regulation that limits the use of AM of very high importance for human medicine (category I AM as defined by Health Canada) in the food animal industry. However, the implementation of such regulation can significantly influence behavioral shifts among producers, contributing to the wider effort against AMR. Therefore, the objective of this observational study was to describe the perceived changes in dairy producers' knowledge and on-farm practices following the implementation of this regulation, using a cohort design. Data collection involved administering questionnaires to 87 dairy producers from 3 regions of the province of Québec (Estrie, Montérégie, Centre-Du-Québec) before (2017-2018) and after (2020-2021) the implementation of the regulation. The questionnaires explored the descriptive characteristics of farms, the knowledge of producers about the categorization of AM, the on-farm treatment practices, and the perceived effects of the regulation. Statistical analysis included t-tests and McNemar tests to compare the paired data obtained using the 2 questionnaires. The results indicated an increase in the knowledge score (the number of AM correctly categorized by the producers by their importance for human medicine) after the implementation of the regulation, suggesting an improved understanding of the categorization of AM based on their importance for human medicine. Trends in AMU practices for treating clinical mastitis and reproductive diseases suggested that category I AM were less likely to be reported as the primary treatment after the regulation, whereas category II AM were more often reported as primary treatment. Adoption of the selective dry cow therapy method significantly increased, whereas the use of teat sealants remained unchanged. Moreover, producers had divergent perceptions regarding the effect of the regulation on the cure rates and disease frequencies. This disparity emphasizes the need for comprehensive data collection to discern the risks associated with such regulatory shifts. The study acknowledges several limitations, including the potential for recall bias, confirmation bias, and desirability bias. Despite these limitations, this study shows that implementing regulations to encourage responsible AMU drives positive transformations in producers' knowledge and on-farm practices. This underscores the pivotal effect of proactive interventions in combating the escalating threat of AMR within the global food animal industry., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)

Staphylococcus aureus intramammary infections often leads to clinical and subclinical mastitis in dairy cattle. Prediction of disease evolution and treatment efficacy based on the characteristics of disease-causing strains of S. aureus would significantly improve management of dairy herds. To study the impact of biofilm production and the influence of genetic lineage, we selected S. aureus isolates from the most prevalent Canadian spa types associated with bovine mastitis. Antimicrobial susceptibility in planktonic growth and for bacteria embedded in biofilm was compared. PCR was used to detect the bap gene responsible for atypical biofilm formation. All Canadian spa types from dairy cattle were susceptible to the 8 antimicrobial agents tested. Only strain sa3493 from spa type t267 showed a resistance to pirlimycin. However, bacteria producing larger amounts of biofilms better survived the bactericidal action of antimicrobial agents even when exposed to concentrations 64 folds higher than the minimal inhibitory concentration determined for planktonic cultures. Pirlimycin was more effective on bacteria producing low to moderate levels of biofilm compared with vancomycin or ceftiofur. Antimicrobial agents did not affect the viability of spa types t13401 and t605 that were high biofilm producers. While both these spa types produced high amounts of biofilm, only t605 possessed the bap gene. We also found a close relationship between DIM at sampling and the presence of spa type t605 isolates. These results suggest that detection of S. aureus spa type may help predict the effectiveness of antimicrobial therapy and that some spa types are more likely to be retrieved toward the end of the lactation., (The Authors. Published by Elsevier Inc. on behalf of the American Dairy Science Association®. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).)