Manageable risk factors associated with the lactational incidence, elimination, and prevalence of Staphylococcus aureus intramammary infections in dairy cows

Staphylococcal mastitis in dairy cows

Bovine mastitis is one of the most common diseases of dairy cattle. Even though different infectious microorganisms and mechanical injury can cause mastitis, bacteria are the most common cause of mastitis in dairy cows. Staphylococci, streptococci, and coliforms are the most frequently diagnosed etiological agents of mastitis in dairy cows. Staphylococci that cause mastitis are broadly divided into Staphylococcus aureus and non-aureus staphylococci (NAS). NAS is mainly comprised of coagulase-negative Staphylococcus species (CNS) and some coagulase-positive and coagulase-variable staphylococci. Current staphylococcal mastitis control measures are ineffective, and dependence on antimicrobial drugs is not sustainable because of the low cure rate with antimicrobial treatment and the development of resistance. Non-antimicrobial effective and sustainable control tools are critically needed. This review describes the current status of S. aureus and NAS mastitis in dairy cows and flags areas of knowledge gaps.

Associations between early lactation intramammary infections and udder health and performance during the first 180 days in milk in first-lactation organic dairy cows

Prior data from our group showed that first-lactation cows under organic management in United States have a high prevalence of Staphylococcus aureus, Streptococcus spp., and Staphylococcus chromogenes intramammary infections (IMI) in early lactation. Nonetheless, the relationship between IMI, udder health, and milk production in organically reared primiparous cows remains elusive. The objectives of this observational study were to investigate the relationship between presence and persistence of IMI in the first 35 d in milk (DIM) and somatic cell count (SCC) and milk production during the first 6 mo of lactation on first-lactation organic dairy cows. The analysis included a total of 1,348 composite milk samples collected during the first 35 DIM that were submitted for milk culture and 1,674 Dairy Herd Improvement Association (DHIA) tests during the first 180 DIM from 333 heifers in 4 organic dairy farms, enrolled between February 2019 and January 2020. The association between IMI in the first 35 DIM and new high SCC (SCC > 200,000 cells/mL) and milk production during the first 6 mo of lactation was investigated using Cox proportional hazards regression and mixed linear regression, respectively. The association between IMI persistence (harboring the same microorganism as reported by the laboratory for 2 or more samples) in the first 35 DIM and number of DHIA tests with high SCC during the first 6 mo of lactation was modeled using negative binomial regression. The presence of IMI by Staph. aureus (hazard ratio [HR] [95% confidence interval {CI}]: 3.35 [2.64, 4.25]) or Streptococcus spp. (HR [95% CI]: 2.25 [2.12, 2.39]) during the first 35 DIM was associated with an increased risk of new high SCC during the first 6 mo of lactation. Milk production was reduced when Streptococcus spp. were identified in milk samples. However, there was no evidence of a difference in milk production in Staph. aureus IMI. Isolation of non-aureus staphylococci and mammaliicocci was related to a mild increase in the hazards of high SCC (HR [95% CI]: 1.34 [0.97, 1.85]) and a decrease in milk production during one or more postpartum tests. Presence of gram-negative or Streptococcus-like organisms IMI was not associated with either high SCC or milk production. Presence of Bacillus IMI was associated with a lower hazard of new high SCC (HR [95% CI]: 0.45 [0.30, 0.68]), and higher milk production during the first 180 d of lactation (overall estimate [95% CI]: 1.7 kg/d [0.3, 3.0]). The persistence of IMI in the first 35 DIM was associated with the number of tests with high SCC during the lactation for all microorganisms except for Staphylococcus chromogenes. Therefore, our results suggest that the persistence of IMI in the first 35 DIM could be an important factor to understand the association between IMI detected in early lactation and lactational SCC and milk production in organic dairy heifers. Our study described associations between IMI, udder health, and milk production in first-lactation organic dairy cows that are consistent with findings from conventional dairy farms.

Investigation of intramammary infections in primiparous cows during early lactation on organic dairy farms

Previous studies have shown that organically raised dairy cows have an increased prevalence of Staphylococcus aureus compared with conventionally raised dairy cows. However, little information exists about the dynamics of intramammary infection (IMI) in primiparous cows during early lactation on organic dairy farms. The objective of this study was to describe the IMI dynamics of primiparous cows on certified organic farms during early lactation. This longitudinal study enrolled 503 primiparous cows from 5 organic dairy farms from February 2019 to January 2020. Quarter-level milk samples were collected aseptically on a weekly basis during the first 5 wk of lactation. Samples were pooled by cow and time point into composite samples inside a sterilized laminar hood and submitted for microbiological culture. For each of the different microorganisms identified, we estimated the prevalence in each postpartum sample, period prevalence (PP), cumulative incidence, and persistence of IMI. Logistic regression models were used to investigate whether the prevalence of IMI differed by farm or sampling time points and whether IMI persistence differed between detected microorganisms. Our findings revealed a high prevalence of Staphylococcus aureus (PP = 18.9%), non-aureus staphylococci and closely related mammaliicoccal species (PP = 52.1%), and Streptococcus spp. and Streptococcus-like organisms (PP = 32.1%) within the study population. The prevalence of these microorganisms varied significantly between farms. Staphylococcus aureus and Staphylococcus chromogenes exhibited significantly higher IMI persistence compared with other detected bacterial taxa, confirming the divergent epidemiological behavior in terms of IMI chronicity across different microorganisms. This study improves our understanding of the epidemiology of mastitis-causing pathogens in organically raised primiparous cows, which can be used to tailor mastitis control plans for this unique yet growing subpopulation of dairy cows.

Association between management practices and estimated mastitis incidence and milk losses on robotic dairy farms

This study aims to describe the relation between farm-level management factors and estimated farm-level mastitis incidence and milk loss traits (MIMLT) at dairy farms with automated milking systems. In this observational study, 43 commercial dairy farms in Belgium and the Netherlands were included and 148 'management and udder health related variables' were obtained during a farm visit through a farm audit and survey. The MIMLT were estimated from milk yield data. Quarter-level milk yield perturbations that were caused by presumable mastitis cases (PMC) were selected based on quarter-level milk yield and electrical conductivity. On average, 57.6 ± 5.4% of the identified milk yield perturbations complied with our criteria. From these PMC, 3 farm-level MIMLT were calculated over a one-year period around the farm visit date: (1) the 'average number of PMC per cow per year', (2) the 'absolute milk loss per cow per day', calculated as the farm-level sum of all milk losses during PMC in one year, divided by the average number of lactating cows and the number of days, and (3) the 'relative milk loss', calculated as the farm-level sum of milk losses during PMC in one year, divided by the estimated total production in the absence of PMC. The 'average number of PMC per cow per year' was on average 1.81 ± 0.47. The PMC caused an average milk loss of 0.77 ± 0.26 kg per lactating cow per day, which corresponded to an average production loss of 2.38 ± 0.82% of the expected production in the absence of PMC. We performed a principal component regression (PCR) analysis to link the 3 MIMLT to the 'management and udder health related variables', whilst reducing the multicollinearity and the number of dimensions. The first principal component was mainly related to 'milking system brand, maintenance and settings'. The second component mainly linked to average productivity and somatic cell counts, whereas the third component mainly contained variables linked with mastitis management, treatment, and biosecurity. The 3 PCR models had R² ranging from 0.46 (for absolute milk loss per cow per day) to 0.57 (for relative milk loss). For all models, the second PC had the largest effect size. This analysis raises awareness of the impact of management factors on a factual basis and provides handles to take management actions to improve udder health.

Antimicrobial and teat sealant use and selection criteria at dry-off on Canadian dairy farms

Infections with antimicrobial resistant pathogens are a major threat to human and animal health worldwide. Further, reduction of livestock-associated antimicrobial use (AMU) is often identified as an area of focus. Selective dry cow therapy (DCT) warrants consideration as an important way to decrease AMU on Canadian dairy farms. In addition, teat sealants (TS) are a nonantimicrobial alternative for prevention of intramammary infection during the dry period. Therefore, objectives of this study were to determine how antimicrobials and TS are used at dry-off on Canadian dairy farms to determine selective DCT uptake and enacted selection protocols. It was expected that these data will provide a baseline understanding of DCT practices and highlight areas for future intervention to further reduce AMU. An observational study was conducted utilizing 2 in-person questionnaires conducted between July 2019 and September 2021 on 144 participating dairy farms in 5 Canadian provinces (British Columbia = 30, Alberta = 30, Ontario = 31, Québec = 29, and Nova Scotia = 24). Overall, 45 farms (31%) reported adopting selective DCT, 95 (66%) enacted blanket DCT, and 4 (3%) did not provide antimicrobial DCT. Farms enacting selective DCT had approximately 50% less intramammary antimicrobials used at dry-off compared with blanket DCT farms. Cow somatic cell count history was the most common criterion for selective DCT decision-making, followed by previous clinical mastitis history, bacteriological culture, and milk production. A slight majority of farms (56%) applied TS to all cows at dry-off, whereas 17 farms (12%) used TS selectively, and 46 farms (32%) did not use TS. Larger herds more often used TS, and farms with an automatic milking system more often used TS selectively than applied to all cows. Results highlighted the variability in antimicrobial treatment and TS use protocols at dry-off on Canadian dairy farms, and the potential for further antimicrobial reduction with increased adoption of selective DCT.

Welfare of dairy cows

This Scientific Opinion addresses a European Commission's mandate on the welfare of dairy cows as part of the Farm to Fork strategy. It includes three assessments carried out based on literature reviews and complemented by expert opinion. Assessment 1 describes the most prevalent housing systems for dairy cows in Europe: tie-stalls, cubicle housing, open-bedded systems and systems with access to an outdoor area. Per each system, the scientific opinion describes the distribution in the EU and assesses the main strengths, weaknesses and hazards potentially reducing the welfare of dairy cows. Assessment 2 addresses five welfare consequences as requested in the mandate: locomotory disorders (including lameness), mastitis, restriction of movement and resting problems, inability to perform comfort behaviour and metabolic disorders. Per each welfare consequence, a set of animal-based measures is suggested, a detailed analysis of the prevalence in different housing systems is provided, and subsequently, a comparison of the housing systems is given. Common and specific system-related hazards as well as management-related hazards and respective preventive measures are investigated. Assessment 3 includes an analysis of farm characteristics (e.g. milk yield, herd size) that could be used to classify the level of on-farm welfare. From the available scientific literature, it was not possible to derive relevant associations between available farm data and cow welfare. Therefore, an approach based on expert knowledge elicitation (EKE) was developed. The EKE resulted in the identification of five farm characteristics (more than one cow per cubicle at maximum stocking density, limited space for cows, inappropriate cubicle size, high on-farm mortality and farms with less than 2 months access to pasture). If one or more of these farm characteristics are present, it is recommended to conduct an assessment of cow welfare on the farm in question using animal-based measures for specified welfare consequences.

Evaluation and application of a milk antibody ELISA for assessing the prevalence and incidence of bovine tuberculosis in dairy herds in Hubei Province, China

Bovine tuberculosis (bTB) is a chronic zoonotic disease that is endemic in China. Current in-vitro tests for bTB are mainly based on blood assays. Collection of samples results in some stress to the sampled cattle and associated economic losses for the herd owner. This study was designed to investigate the relationship between milk and serum antibody tests for bTB in dairy cows using 85 cows with milk and corresponding blood samples. Totally 4,395 milk samples were used to assesse the apparent (test) prevalence and incidence of bTB using the milk antibody ELISA. The association between levels of bTB milk antibody and milk quality was also evaluated. Milk and serum antibody tests showed a good correlation with a 87.5% (95% CI: 61.7%, 98.4) positive agreement and 98.7% (95% CI: 95.4, 99.8) negative agreement. The animal level lactoprevalence ranged from 0.3% (95% CI: 0, 1.2) to 33.3% (95% CI: 26.6, 40.6) in different farms and the incidence rate ranged from 0 head/cow-month (95% CI: 0, 0.02) to 0.04 head/cow-month (95% CI: 0.02, 0.07). Twenty percent of sampled farms met the criteria for bTB control in China. The prevalence on large-scale farms was lower (p < 0.001) than on small farms. The bTB milk antibody levels had a negative correlation with milk yield and a positive correlation with somatic cell count (SCC), milk protein percentage (MPP) and percentage of total solids (TS). According to this research, milk ELISA could be used as a supplement of blood samples to assist in the surveillance for bTB and for alerting control and eradication of bTB.

Staphylococcus aureus adlb gene is associated with high prevalence of intramammary infection in dairy herds of northern Italy: A cross-sectional study

Staphylococcus aureus is a major mastitis pathogen in dairy cattle worldwide, responsible for substantial economic losses. Environmental factors, milking routine, and good maintenance of milking equipment have been described as important factors to prevent intramammary infections (IMI). Staphylococcus aureus IMI can be widespread within the farm or the infection can be limited to few animals. Several studies have reported that Staph. aureus genotypes differ in their ability to spread within a herd. In particular, Staph. aureus belonging to ribosomal spacer PCR genotype B (GTB)/clonal complex 8 (CC8) is associated with high within-herd prevalence of IMI, whereas other genotypes are generally associated with individual cow disease. The adlb gene seems to be strictly related to Staph. aureus GTB/CC8, and is a potential marker of contagiousness. We investigated Staph. aureus IMI prevalence in 60 herds in northern Italy. In the same farms, we assessed specific indicators linked to milking management (e.g., teat condition score and udder hygiene score) and additional milking risk factors for IMI spread. Ribosomal spacer-PCR and adlb-targeted PCR were performed on 262 Staph. aureus isolates, of which 77 underwent multilocus sequence typing. In most of the herds (90%), a predominant genotype was identified, especially Staph. aureus CC8 (30%). In 19 of 60 herds, the predominant circulating Staph. aureus was adlb-positive and the observed IMI prevalence was relevant. Moreover, the adlb gene was detected only in genotypes of CC8 and CC97. Statistical analysis showed a strong association between the prevalence of Staph. aureus IMI, the specific CCs, and carriage of adlb, with the predominant circulating CC and presence of the gene alone explaining the total variation. Interestingly, the difference in the odds ratio obtained in the models for CC8 and CC97 suggests that it is carriage of the adlb gene, rather than the circulation of these CCs per se, that leads to higher within-herd prevalence of Staph. aureus. In addition, the model showed that environmental and milking management factors had no or minimal effect on Staph. aureus IMI prevalence. In conclusion, the circulation of adlb-positive Staph. aureus strains within a herd has a strong effect on the prevalence of IMI. Thus, adlb can be proposed as a genetic marker of contagiousness for Staph. aureus IMI in cattle. However, further analyses using whole-genome sequencing are required to understand the role of genes other than adlb that may be involved in the mechanisms of contagiousness of Staph. aureus strains associated with high prevalence of IMI.

Evaluating the antimicrobial use on dairy farms in Chiba Prefecture in Japan using the antimicrobial treatment incidence, an indicator based on Japanese defined daily doses from 2014-2016

The use of antimicrobial agents in food-producing animals may lead to the emergence and spread of antimicrobial resistance in bacteria of animal origin. However, there is a paucity of data on the quantity of antimicrobials use on dairy farms in Japan. This study describes antimicrobial use on dairy farms from 1 January 2014 to 31 December 2016 in five administrative districts (central, eastern, western, southern and northern) of Chiba Prefecture. The use of antimicrobial agents in dairy cattle over these three years was evaluated in terms of the antimicrobial treatment incidence (ATI; theoretical number of animals per 1,000 animal-days subjected to antimicrobial treatment) using data collected from a total of 442 dairy farms in that prefecture. Our results revealed that the average ATI on these farms for these years ranged from 38.7 to 39.4 with no significant difference between years and that the average ATI for these administrative districts varied between 32.9 and 43.2 with a significant variation between some of the districts. Approximately 84% of antimicrobials were administered intramammarily, 13–14% by injection and 1–2% orally. Scenario analyses were performed to assess the effect of changes in some of the defined daily dose (DDDjp) values used to calculate the ATI. Our results revealed that the calculated ATI is considerably affected by the changes in the long-acting factor used for assigning the DDDjp values of intramammary products for dry cows and the way in which DDD values are assigned for combination products.

Detection and Determination of Staphylococcus aureus in Camel Milk and Associated Factors in Fedis, Eastern Hararghe, Ethiopia

Background:

Camel milk is the key food for pastoralists in the arid and semi-arid areas of the eastern lowlands of Ethiopia. Unless it is handled under good hygienic conditions, milk can be a good medium for bacterial growth and can lead to foodborne diseases caused by various pathogenic microorganisms, such as Staphylococcus aureus. The current study was aimed to determine the prevalence of Staphylococcus aureus in raw camel milk and associated factors in Fedis, Eastern Ethiopia.

Methods:

A cross-sectional study was conducted from December 2, 2018 to May 26, 2019 in Fedis, Eastern Hararghe, Ethiopia. The questionnaire was used to determine the hygienic practices of camel milkers and sellers. Laboratory analysis was conducted to determine the level of Staphylococcus aureus. A total of 156 (78 from households and 78 from selling sites) milk samples were collected randomly from selected camel herd owners and selling sites for the isolation and enumeration of Staphylococcus aureus. Data analysis was carried out using SPSS software version 22.0. Finally, Chi-square and Fisher’s exact tests were used to analyze the data. A P-value of .05 was considered as the cut-off point for statistical significance.

Results:

A total of 156 samples, including 78 samples of raw milk from the households and 78 samples from the market were purchased to determine Staphylococcus aureus. Out of 156 samples, Staphylococcus aureus was detected in 60 (38.5%) samples, of which 27 (34.6%) and 33 (42.3%) were among those collected from household and market, respectively. The overall mean Staphylococcus aureus count was 4.83 log CFU/mL, with household and market samples accounted for 2.76 and 5.08 log CFU/mL, respectively. Furthermore, 23 (38.3%) of the samples were contaminated with Staphylococcus aureus beyond the recommended level (4-5 log CFU/mL). There was a statistically significant association ( P-value of <.05) between the prevalence of Staphylococcus aureus and other variables such as age of the camel, parity, lactation stages, and sources of milk.

Conclusion:

More than one-third of milk samples were contaminated with Staphylococcus aureus at levels higher than the maximum permitted level. The findings of the current study suggests that there is a potential risk of foodborne infection and intoxication. Therefore, implementation of adequate hygiene and safety practices is very important to prevent the consumption of contaminated fruit juices, which leads to foodborne illness.

Isolation and Identification of Staphylococcus aureus from Milk and Milk Products, Associated Factors for Contamination, and Their Antibiogram in Holeta, Central Ethiopia

Staphylococcus aureus is a pathogenic bacterium-contaminating milk and milk products causing food poisoning primarily due to its enterotoxins. The study aimed at estimating the prevalence of S. aureus in milk and milk products, assessing potential risk factors for contamination, and determining the load and the antimicrobial susceptibility profile of the isolates. A cross-sectional study design was employed to collect a total of 486 samples, comprising 383 raw milk, 47 bulk tank milk, 29 curd milk (Ergo), and 28 Ethiopian cottage cheese (Ayib) samples. Enumeration, isolation, and identification of S. aureus were carried out following standard microbiological techniques. Antibiogram was performed using 12 antimicrobials following the Kirby–Bauer disc diffusion method. Logistic regression analyses were used to assess the association between the occurrence of S. aureus in milk and milk products and potential risk factors. The overall prevalence of S. aureus was 10.69% (52/486) [95% confidence interval (CI):8.09–13.79%]. The prevalence of S. aureus in raw milk, curd milk, bulk tanks at the farm, bulk tanks at milk collection facilities, and cottage cheese was 8.64%, 24.14%, 14.73%, 23.08%, and 14.29%, respectively. The rate of isolation of S. aureus was significantly high in curd milk than in other types of samples (P = 0.010). The study revealed that teat washing (OR: 4.93, 95% CI: 2.06–11.81), use of towel (OR: 12.13, 95% CI: 3.74–39.29), and tick infestations (OR: 4.31, 95% CI: 1.28–14.44) were risk factors associated with the occurrence of S. aureus in milk. About 48.39% of the milk samples assessed had the S. aureus count higher than 10⁵ CFU/ml. The highest rate of resistance was observed to ampicillin (95%), amoxicillin (95%), oxacillin (87.5%), and cefotaxime (80%). All isolates are resistant to at least two classes of antimicrobial drugs, while 65.0% of the isolates were found to be multidrug-resistant. The moderate prevalence, high load, and antimicrobial resistance of S. aureus indicate the higher public health risk due to the widespread consumption of raw milk in the area. Good hygienic practices, regular surveillance of antimicrobial resistance, and prudent use of drugs are suggested.

Comparison of raw cow milk microbiota in two milking systems: a field study

This study investigated the effect of different milking systems on the milk composition and microbial community of raw milk in a commercial dairy farm. Raw milk samples from conventional milking system (CMS) and automatic milking system (AMS) were collected and the microbiota on each was assessed by 16S rRNA gene sequencing. Results showed that the acetone (P = 0.031) and β-hydroxybutyrate (P≤0.001) levels in the raw milk of the AMS group were increased compared with the CMS group. Principal component analysis, unweighted and weighted principal coordinates analysis, and heat map of microbial community composition in the raw milk showed a clear separation between two groups. AMS increased the abundance of the genera Acinetobacter (FDR = 0.004) and Staphylococcus (FDR = 0.004) in the raw milk compared with the CMS group. In contrast, the abundance of the genera Pseudomonas (FDR = 0.028), Lactococcus (FDR = 0.015), Sphingobacterium (FDR = 0.004), Brevundimonas (FDR = 0.005), and Chryseobacterium (FDR = 0.042) in the raw milk was reduced in the AMS group compared with the CMS group. The abundance of the genera Acinetobacter and Staphylococcus in the raw milk was positively correlated with the β-hydroxybutyrate, acetone, free fatty acid, citric acid, and urea nitrogen levels. Furthermore, the abundance of the genus Acinetobacter in the raw milk was negatively correlated with the somatic cell counts. The study demonstrates that the introduction of AMS in the dairy farm can regulate microbiota composition in the raw milk and this modification may exert an effect on reducing the somatic cell counts in the raw milk.

Herd Routines and Veterinary Advice Related to Dry-Cow Therapy and Treatment with Internal Teat Sealants in Dairy Cows

Simple Summary

At the end of lactation, antibiotics (DCT) or internal teat sealants (ITS) can be used to treat or prevent mastitis in dairy cows. Recommendations on how to perform such treatments are available, but little is known about how well these are followed by farmers and veterinarians. To increase this knowledge, questionnaires about farmer routines and veterinary advice were sent to 2472 farmers and 517 veterinarians in Sweden. Fourteen percent of the farmers and 25% of the veterinarians responded. Among the farmers, 81% used DCT to some cows, 3% used DCT to all cows, and 16% did not use DCT at all. Almost all veterinarians prescribed DCT, most only to some cows in a herd while 8% sometimes recommended DCT to all cows in a herd. Most of the farmers did not use ITS and half of the veterinarians never prescribed ITS. Milking system and milk production, and post-graduate training and number of mastitis cases per month were associated with several of the answers by the farmers and veterinarians, respectively. Overall, many farmers and veterinarians followed the recommendations, but it was also clear that more communication is needed as well as an up-date of the recommendations.

Abstract

Dry-cow therapy with antibiotics (DCT) and treatment with internal teat sealants (ITS) are often used to control mastitis in dairy cows. However, the knowledge on farmer and veterinary compliance with recommendations for DCT and ITS is scarce. Thus, the main aim was to collect information on farmer routines and veterinary advice for such treatments. Associations with herd and veterinary variables were also studied. Web-based questionnaires including questions on demographics and the use of DCT and ITS were sent to 2472 farmers and 517 veterinarians in Sweden. The answers were summarized descriptively, and associations with demographics were evaluated using univariable regression models. The response rate was 14% for farmers and 25% for veterinarians. Among the farmers, 81% used selective DCT (SDCT), 3% used blanket DCT (BDCT), and 16% did not use DCT. Almost all (93%) veterinarians prescribed DCT and among those most recommended SDCT while 8% recommended BDCT. Eighty-two percent of the farmers did not use ITS and 45% of the veterinarians never prescribed ITS. Milking system and milk production, and post-graduate training and number of mastitis cases per month were associated with the largest numbers of farmer and veterinary answers, respectively. In conclusion, many farmer routines and veterinary advice complied with the recommendations available at the time, but a clear need for more education was also identified. The results also indicated that an up-date of the national recommendations was warranted.

Annexin A2-Mediated Internalization of Staphylococcus aureus into Bovine Mammary Epithelial Cells Requires Its Interaction with Clumping Factor B

BACKGROUND

Staphylococcus aureus is a leading cause of contagious mastitis in dairy cattle. Internalization of S. aureus by bovine mammary gland epithelial cells is thought to be responsible for persistent and chronic intramammary infection, but the underlying mechanisms are not fully understood.

METHODS

In the present study, we evaluated the role of Annexin A2 (AnxA2), a membrane-binding protein, in S. aureus invasion into bovine mammary epithelial cell line (MAC-T). In vitro binding assays were performed to co-immunoprecipitate the binding proteins of AnxA2 in the lysates of S. aureus.

RESULTS

AnxA2 mediated the internalization but not adherence of S. aureus. Engagement of AnxA2 stimulated an integrin-linked protein kinase (ILK)/p38 MAPK cascade to induce S. aureus invasion. One of the AnxA2-precipitated proteins was identified as S. aureus clumping factor B (ClfB) through use of mass spectrometry. Direct binding of ClfB to AnxA2 was further confirmed by using a pull-down assay. Pre-incubation with recombinant ClfB protein enhanced S. aureus internalization, an effect that was specially blocked by anti-AnxA2 antibody.

CONCLUSION

Our results demonstrate that binding of ClfB to AnxA2 has a function in promoting S. aureus internalization. Targeting the interaction of ClfB and AnxA2 may confer protection against S. aureus mastitis.

A Molecular Survey of Bacterial Species in the Guts of Black Soldier Fly Larvae (Hermetia illucens) Reared on Two Urban Organic Waste Streams in Kenya

Globally, the expansion of livestock and fisheries production is severely constrained due to the increasing costs and ecological footprint of feed constituents. The utilization of black soldier fly (BSF) as an alternative protein ingredient to fishmeal and soybean in animal feed has been widely documented. The black soldier fly larvae (BSFL) used are known to voraciously feed and grow in contaminated organic wastes. Thus, several concerns about their safety for inclusion into animal feed remain largely unaddressed. This study evaluated both culture-dependent sequence-based and 16S rDNA amplification analysis to isolate and identify bacterial species associated with BSFL fed on chicken manure (CM) and kitchen waste (KW). The bacteria species from the CM and KW were also isolated and investigated. Results from the culture-dependent isolation strategies revealed that Providencia sp. was the most dominant bacterial species detected from the guts of BSFL reared on CM and KW. Morganella sp. and Brevibacterium sp. were detected in CM, while Staphylococcus sp. and Bordetella sp. were specific to KW. However, metagenomic studies showed that Providencia and Bordetella were the dominant genera observed in BSFL gut and processed waste substrates. Pseudomonas and Comamonas were recorded in the raw waste substrates. The diversity of bacterial genera recorded from the fresh rearing substrates was significantly higher compared to the diversity observed in the gut of the BSFL and BSF frass (leftovers of the rearing substrates). These findings demonstrate that the presence and abundance of microbiota in BSFL and their associated waste vary considerably. However, the presence of clinically pathogenic strains of bacteria in the gut of BSFL fed both substrates highlight the biosafety risk of potential vertical transmission that might occur, if appropriate pre-and-postharvest measures are not enforced.

Bayesian estimation of diagnostic accuracy of somatic cell counts history and on-farm milk culture using Petrifilm® to identify quarters or cows that should be treated with antimicrobials in selective treatment protocols at dry off

Bayesian latent class models were used to estimate the test accuracy (sensitivity (Se), specificity (Sp), and predictive values (NPV and PPV)) of cow-level somatic cell counts (SCC) data, quarter-level Petrifilm® on-farm milk culture, and quarter-level standard milk bacteriology for the identification of quarters that should possibly be treated with antimicrobials at dry off in dairy cows. Data of 282 cows from 9 dairy herds in Québec, Canada, with bulk tank SCC < 250,000 cells/mL were used. Estimated median herd-prevalence of infections that should be treated was 16.2 % (95 % credibility interval (CI): 11.0-22.7). Se and Sp estimates for quarter-milk culture using Petrifilm® were 82.2 % (95 %CI: 74.0-89.5) and 62.0 % (95 %CI: 58.6-65.6), respectively. Se and Sp for quarter-milk standard bacteriology were 67.4 % (95 %CI: 55.8-81.2) and 79.6 % (95 %CI: 76.4-83.0), respectively. Se and Sp of different SCC scenarios and thresholds were estimated. For first parity cows, using only the last Dairy Herd Improvement (DHI) test SCC with a threshold of 100,000 cells/mL appeared quite accurate, with Se, Sp, PPV, NPV and reduction of antimicrobial usage of 85.6 % (95 %CI: 69.6-95.6), 86.0 % (95 %CI: 80.0-91.7), 58.0 % (95 %CI: 42.3-74.2), 96.4 % (95 %CI: 91.3-99.0), and 75.3 % (95 %CI: 70.7-79.3), respectively. For cows of ≥ 2nd parity, using only the last DHI test SCC with a threshold of 200,000 cells/mL resulted in Se, Sp, PPV, NPV and reduction of antimicrobial usage of 75.3 % (95 %CI: 55.8-87.3), 84.0 % (95 %CI: 78.8-89.3), 47.2 % (95 %CI: 32.0-63.7), 94.7 % (95 %CI: 89.0-97.6), and of 77.0 % (95 %CI: 73.3-80.3), respectively. Adding quarter-level milk culture using Petrifilm® to cows identified as unhealthy using cow-level SCC data improved the test accuracy (mainly the PPV) and further reduced the use of antimicrobials. For instance, in ≥ 2nd parity cows, using only the last DHI SCC with a threshold of 200,000 cells/mL, adding a subsequent Petrifilm® test increased the reduction from 77.0 % (95 %CI: 73.3-80.3) to 89.5 % (95 %CI: 86.7-91.8). Considering the availability of SCC data, the easiness of using just the last DHI test, and the high NPV that could be achieved, producers may consider using just the last DHI test as a potential tool to identify cows that should be treated with antimicrobials at dry off. It may be used alone or in combination with quarter-level on-farm Petrifilm® milk culture on high SCC cows to further reduce the use of antimicrobials by identifying quarters that need to be treated.

Effectiveness of dry cow therapy and/or internal teat sealant on existing infections in smallholder dairy farms in Kenya

Background and Aim:

Dry cow therapy (DCT) can be an effective treatment of mastitis that has not responded to conventional treatment during lactation. The aim of this study was to establish the effectiveness of DCT options available in reducing intramammary infections in smallholder dairy farms in Kiambu County, Kenya.

Materials and Methods:

The study targeted smallholder dairy farms which were registered at the local dairy cooperatives and which had cows that were at the point of dry-off. A total of 32 cows with 121 quarters that were California Mastitis Test (CMT) positive were recruited, with the quarters randomly allocated to receive either DCT (DCT – neomycin sulfate, penethamate hydriodide, and procaine benzylpenicillin) and internal teat sealant (ITS) or ITS alone (bismuth nitrate) after aseptically collecting quarter milk samples for bacterial culture. Farm- and animal-level factors were captured through a questionnaire which was administered to the principal farmer or a person who was managing the animals. Post-calving, milk samples were also collected for bacterial culture to establish if the infection was cleared or if there was a new infection.

Results:

DCT with ITS significantly reduced the proportion of quarters infected with Staphylococcus aureus from 64.0% at dry-off to 44.0% post-calving (35% reduction). In the control group, ITS alone, there was a small reduction in proportions of S. aureus from 46.8% to 40.4%. Proportions of quarter infections by coagulase-negative Staphylococcus in the treatment group reduced from 16.0% at dry-off to 2.0% post-calving, with a significant reduction in the control group too from 19.1% to 4.3%, which could be due to self-cure. Actinomyces species, Escherichia coli, Streptococcus species, and Pseudomonas species proportions slightly increased in the treatment group, as did E. coli and Pseudomonas species proportions in the control group.

Conclusion:

In smallholder dairy farms with subclinical mastitis, DCT of CMT-positive cows leads to a significant decrease of S. aureus infections at calving.

Genome-Wide DNA Methylation Analysis of Mammary Gland Tissues From Chinese Holstein Cows With Staphylococcus aureus Induced Mastitis

Staphylococcus aureus intramammary infection is one of the most common causes of chronic mastitis in dairy cows, whose development may be associated with epigenetic changes in the expression of important host defense genes. This study aimed to construct a genome-wide DNA methylation profile of the mammary gland of Chinese Holstein cows (n = 3) following experimentally induced S. aureus mastitis, and to explore the potential gene regulatory mechanisms affected by DNA methylation during S. aureus mastitis. DNA was extracted from S. aureus-positive (n = 3) and S. aureus-negative (n = 3) mammary gland quarters and subjected to methylation-dependent restriction-site associated DNA sequencing (Methyl-RAD Seq). Results showed that CmCGG/CmCWGG DNA methylation sites were unevenly distributed and concentrated on chromosomes 5, 11, and 19, and within intergenic regions and intron regions of genes. Compared with healthy control quarters, 9,181 significantly differentially methylated (DM) CmCGG sites and 1,790 DM CmCWGG sites were found in the S. aureus-positive quarters (P < 0.05, |log2FC| > 1). Furthermore, 363 CmCGG differently methylated genes (DMGs) and 301 CmCWGG DMGs (adjusted P < 0.05, |log2FC| > 1) were identified. Gene ontology and KEGG enrichment analysis indicated that CmCGG DMGs are involved in immune response pathways, while the CmCWGG DMGs were mainly enriched in gene ontology terms related to metabolism. The mRNAs of 526 differentially methylated CmCGG genes and 124 differentially methylated CmCWGG genes were also significantly differentially expressed (RNA-Seq data) in the same samples, herein denoted differentially methylated and expressed genes (DMEGs) (P < 0.05). Functional enrichment analysis of DMEGs revealed roles related to biological processes, especially the regulation of immune response to diseases. CmCGG DMEGs like IL6R, TNF, BTK, IL1R2, and TNFSF8 enriched in several immune-related GO terms and pathways indicated their important roles in host immune response and their potential as candidate genes for S. aureus mastitis. These results suggest potential regulatory roles for DNA methylation in bovine mammary gland processes during S. aureus mastitis and serves as a reference for future epigenetic regulation and mechanistic studies.

The occurrence and distribution of livestock-associated methicillin-resistant Staphylococcus aureus ST398 on German dairy farms

The objective of this study was to investigate the occurrence and distribution of methicillin-resistant Staphylococcus aureus (MRSA) on 20 German dairy farms. Farms were selected based on previous MRSA reports from phenotypic susceptibility testing of mastitis pathogens. Samples were collected from predefined groups of cows, young stock, farm personnel, and the environment. A high MRSA-positive test rate was detected in swab samples from milk-fed calves (22.7%; 46/203). In postweaning calves, the MRSA-positive test rate was 9.1% (17/187). From prefresh heifers, both nasal swabs and udder cleft swabs were collected if possible. Including both sample types, the MRSA-positive test rate in prefresh heifers was 13.0% (26/200). The positive test rate was 8.9% (17/191) in nasal swabs and 6.5% (11/170) in udder cleft swabs. In quarter milk samples (QMS), the MRSA-positive test rate was 2.9% (67/2347), and on cow level, 7.9% (47/597) of the dairy cows were affected. Among all cows included in this study, the geometric mean of somatic cell counts was higher in QMS that carried MRSA (345,000 cells/mL) in comparison to all QMS (114,000 cells/mL). No differences in parity or the affected mammary quarter position on the udder were observed among the 47 infected cows. Methicillin-resistant S. aureus was also detected in boot swab samples (dust), teat liners, and in suckers from automatic calf feeders. All isolates belonged to livestock-associated sequence type 398 and most common staphylococcal protein A (spa)-types were t011 and t034. Most isolates harbored the staphylococcal cassette chromosome mec (SCCmec)-type V, with the exception of some isolates with SCCmec-type IVa on 1 farm. Similar MRSA genotypes in samples from humans and dairy cows underline the possible zoonotic and reverse-zoonotic transmission of livestock-associated MRSA strains from dairy farms. Similar MRSA genotypes in pig and cattle barns were detected on only 1 of 5 farms that kept both cattle and pigs. Similar MRSA spa-types were detected in samples from different sources (dairy cows, young stock, environment, and humans), suggesting a possible contagious transmission on some of the farms. Sporadically, up to 3 different MRSA spa-types were detected in QMS from the respective farms. On MRSA-affected farms, improper milking hygiene procedures and elevated bulk-tank milk somatic cell counts (>250,000 cells/mL) were observed. The occurrence of livestock-associated MRSA ST398 in different samples from dairy farms, and especially in young calves, should be considered for future MRSA-monitoring programs and biosecurity guidelines.

A Randomized Controlled Trial of Teat-Sealant and Antibiotic Dry-Cow Treatments for Mastitis Prevention Shows Similar Effect on the Healthy Milk Microbiome

Lactating cows are routinely treated at dry-off with antibiotic infusions in each quarter for the cure and prevention of pathogenic intramammary infection, which remains the most common disease in dairy herds. This approach is known as blanket dry-cow therapy, usually effective for the prevention and cure of infections, but has been shown to potentially contribute to the emergence and spreading of antibiotic resistant bacterial strains. Exploring the use of non-antibiotic treatments coupled with selective dry-cow therapy is necessary to reduce the risk of antibiotic resistance and potential interference with milk microbiome balance. The impact of selective dry-cow therapy on the physiological milk microbiome needs to be carefully evaluated. In this small-scale trial, five healthy (no mastits, SCC <200,000 cells mL-1) second-parity cows from dry-off to 5 days after calving were sampled. For every cow, each quarter received a different treatment: (i) bismuth salnitrate (internal teat sealant, OrbSeal®, Zoetis, Italy), front right quarter; (ii) cephalonium dihydrate (Cepravin®, MSD, Italy), rear right quarter; (iii) benzathine cloxacillin (Cloxalene dry, Ati, Italy), rear left quarter. No treatment was applied to the remaining quarter (front left) which served as experimental control. For 16S rRNA gene sequencing, bacterial DNA was extracted from 5 ml of milk samples, amplified using the primers for the V3-V4 hypervariable regions and sequenced in one MiSeq (Illumina) run with 2 × 250-base paired-end reads. Bacteriological results confirmed that the quarters were all healthy. The phyla Proteobacteria, Firmicutes, and Actinobacteria were the most abundant for all treatments and controls at all three timepoints, accounting for over 80% of the entire milk microbiota composition. No significant differences were found between treatments and controls in terms of the major alpha and beta diversity indexes, revealing that antibiotic, and non-antibiotic treatments for selective dry-cow therapy did not alter significantly the milk microbiome of dairy cows. The milk microbiota composition showed a clear evolution over the lactation cycle, and the overall changes in the milk microbiota diversity over the lactation cycle were mainly independent of treatments.

Antimicrobial Potential of Essential Oils from Cerrado Plants against Multidrug-Resistant Foodborne Microorganisms

Foodborne pathogens are a real public health concern in an escalating antimicrobial resistance scenario. Natural products represent a promising source of bioactive molecules, and essential oils have attracted much attention due to their myriad of biological properties, including antibacterial activities. In this context, essential oils obtained from the leaves of Chromolaena squalida, Campomanesia sessiliflora, Myrsine guianensis, Matayba guianensis, Siparuna guianensis, Ocotea minarum and Endlicheria paniculata—species from the Cerrado biome of Midwest Brazil—were extracted and evaluated for their antibacterial activity against a panel of four standard and three clinical multidrug−resistant bacterial strains. All tested oils showed moderate to good activity against at least four bacterial strains, including Salmonella Typhi and oxacillin−resistant Staphylococcus. The essential oils from C. squalida, C. sessiliflora, My. guianensis and Ma. guianensis showed strong inhibition of clinical Staphylococcus strains, which cause bovine mastitis and are related to milk−borne diseases. Their chemical profiles were investigated by gas chromatography coupled to mass spectrometry (GC/MS), which revealed a predominance of mono− and sesquiterpene hydrocarbons, some of which with well−known antimicrobial properties. The essential oil from Cerrado plants proved active against resistant Gram−positive and Gram−negative bacteria, revealing their potentialities for the development of new alternative agents to prevent the spreading of resistant bacterial contamination.

Cathelicidins Mitigate Staphylococcus aureus Mastitis and Reduce Bacterial Invasion in Murine Mammary Epithelium

Staphylococcus aureus, an important cause of mastitis in mammals, is becoming increasingly problematic due to the development of resistance to conventional antibiotics. The ability of S. aureus to invade host cells is key to its propensity to evade immune defense and antibiotics. This study focuses on the functions of cathelicidins, small cationic peptides secreted by epithelial cells and leukocytes, in the pathogenesis of S. aureus mastitis in mice. We determined that endogenous murine cathelicidin (CRAMP; Camp) was important in controlling S. aureus infection, as cathelicidin knockout mice (Camp^-/- ) intramammarily challenged with S. aureus had higher bacterial burdens and more severe mastitis than did wild-type mice. The exogenous administration of both a synthetic human cathelicidin (LL-37) and a synthetic murine cathelicidin (CRAMP) (8 μM) reduced the invasion of S. aureus into the murine mammary epithelium. Additionally, this exogenous LL-37 was internalized into cultured mammary epithelial cells and impaired S. aureus growth in vitro We conclude that cathelicidins may be potential therapeutic agents against mastitis; both endogenous and exogenous cathelicidins conferred protection against S. aureus infection by reducing bacterial internalization and potentially by directly killing this pathogen.

Antimicrobial Activity of a Phage Mixture and a Lactic Acid Bacterium against Staphylococcus aureus from Bovine Mastitis

The antimicrobial activity of a phage mixture and a lactic acid bacterium against Staphylococcus aureus isolates from bovine origin was investigated in vitro with regard to possible applications in the therapy of udder inflammation (mastitis) caused by bacterial infections. The S. aureus isolates used for inoculation derived from quarter foremilk samples of mastitis cases. For the examination of the antimicrobial activity, the reduction of the S. aureus germ density was determined [log10 cfu/mL]. The phage mixture consisted of the three obligatory lytic and S. aureus-specific phages STA1.ST29, EB1.ST11 and EB1.ST27 (1:1:1). The selected Lactobacillus plantarum strain with proven antimicrobial properties and the phage mixture were tested against S. aureus in milk, both alone and in combination. The application of the lactic acid bacterium showed only a low reduction ability for a 24 h incubation period. The bacteriophage mixture as well as its combination with the lactic acid bacterium showed high antimicrobial activity against S. aureus for a 24 h incubation period at 37 °C, with only the phage mixture showing significance.

Staphylococcus aureus facilitates its survival in bovine macrophages by blocking autophagic flux

Staphylococcus aureus is a pathogen that is the causative agent of several human and veterinary infections and plays a critical role in the clinical and subclinical mastitis of cattle. Autophagy is a conserved pathogen defence mechanism in eukaryotes. Studies have reported that S aureus can subvert autophagy and survive in cells. Staphylococcus aureus survival in cells is an important cause of chronic persistent mastitis infection. However, it is unclear whether S aureus can escape autophagy in innate immune cells. In this study, initiation of autophagy due to the presence of S aureus was detected in bovine macrophages. We observed autophagic vacuoles increased after S aureus infection of bovine macrophages by transmission electron microscopy (TEM). It was also found that S aureus‐infected bovine macrophages increased the expression of LC3 at different times(0, 0.5, 1, 1.5, 2, 2.5, 3 and 4 hours). Data also showed the accumulation of p62 induced by S aureus infection. Application of autophagy regulatory agents showed that the degradation of p62 was blocked in S aureus induced bovine macrophages. In addition, we also found that the accumulation of autophagosomes promotes S aureus to survive in macrophage cells. In conclusion, this study indicates that autophagy occurs in S aureus‐infected bovine macrophages but is blocked at a later stage of autophagy. The accumulation of autophagosomes facilitates the survival of S aureus in bovine macrophages. These findings provide new insights into the interaction of S aureus with autophagy in bovine macrophages.

Boas práticas agropecuárias na produção leiteira: diagnóstico e ajuste de não conformidades

RESUMO Objetivou-se com este estudo desenvolver e validar uma ferramenta semiológica para diagnóstico do nível de adoção e conformidade das boas práticas agropecuárias em fazendas de produção de leite, segundo requisitos preconizados pela Food and Agriculture Organization of the United Nations (FAO) e pela International Dairy Federation (IDF). A ferramenta foi testada em 62 fazendas de produção de leite, em seis diferentes regiões do estado do Rio Grande do Sul, como parte das ações do Projeto Protambo - "Transferência de tecnologias para o desenvolvimento da atividade leiteira no RS com base nas boas práticas agropecuárias" - da Empresa Brasileira de Pesquisa Agropecuária, Embrapa, entre janeiro de 2014 e março de 2017. A validação da ferramenta não apontou divergências estatisticamente significativas entre avaliadores, satisfazendo o parâmetro de exatidão para métodos de medições exigidos pela ISO/IEC 17025. Foi evidenciado um espaço de variação que mostrou desenvolvimento de métrica, em que se obteve consistência (coerência) de medição. O teste t-Student aproximado para a comparação de médias de não conformidades nas BPA mostrou melhora significativa no grupo tratamento (diagnóstico seguido de plano de ajuste) quando comparado ao controle (diagnóstico sem plano de ajuste subsequente). Essa nova abordagem semiológica contribui para a adoção das BPA em fazendas leiteiras, para a melhoria da qualidade do leite e da segurança na cadeia produtiva de lácteos.

Association between teat-end hyperkeratosis and mastitis in dairy cows: A systematic review

A systematic review was conducted to elucidate the role of teat-end hyperkeratosis (THK) as a risk factor for clinical mastitis (CM) or subclinical mastitis (SCM). Scientific papers on the subject were identified by means of a database search. All types of peer-reviewed analytical studies, observational or experimental and published in English, could be included in the review, regardless of publication year. Of 152 identified records, 18 articles were selected, of which 8 were prospective cohort studies, 9 were cross-sectional, and 1 was a hybrid case-control study. Internal validity of studies was assessed using a score system ranging from 0 to 6, based on design, risk of bias, and statistical methods. The most frequent study limitation was improper use of statistical methods to avoid confounding of associations between THK and CM or SCM. The 3 studies that used CM as outcome (all with high validity scores) showed positive associations with THK (especially severe), although the magnitude and statistical significance of the estimates differed among them. Most studies that used SCM as the primary outcome (based on microbiological examination of milk) reported that only severe THK was associated with SCM. Two studies with high validity scores reported moderate to strong associations between severe THK and incidence or prevalence of Staphylococcus aureus intramammary infection. Two studies with high validity scores reported that only severe THK was associated with the risk of somatic cell count (SCC) ≥200,000 cells/mL and increased mean SCC, respectively. Although 4 cross-sectional studies reported positive associations between THK and SCC, these associations were possibly spurious because confounding factors, such as parity, were not considered in the analyses. Results of the reviewed studies suggest that severe THK is a risk factor for both CM and SCM, as defined by microbiological examination of milk, SCC thresholds, or mean SCC. The effect of severe THK on both contagious (especially Staphylococcus aureus) and environmental CM or SCM emphasizes the importance of teat health for mastitis control. Four studies demonstrated that quarters with mild THK had lower prevalence of intramammary infection or lower mean SCC than quarters with no THK, suggesting that development of mild THK, as a physiological response to milking, can have a protective effect. Dairy consultants should focus on monitoring and minimizing occurrence of severe THK to prevent CM and SCM.

Molecular typing of Staphylococcus aureus from different sources by RAPD-PCR analysis

Staphylococcus aureus is an opportunistic bacterium which is carried as a normal flora organism but has a major role in the epidemiology and pathogenesis of different staphylococcal infections in humans and animals. Fifty S. aureus isolated from banknotes, foods, human infections and bovine mastitis were subjected to DNA fingerprinting by randomly amplified polymorphic DNA (RAPD) analysis to examine their genotypic polymorphism and investigate the amount of genetic relatedness among these various isolates. At 100% RAPD profile similarity level, isolates were classified into four, five and seven groups of the same clone, according to the RAPDPCR with OLP6, OLP11 and OLP13 primers, respectively. Amplification of the isolates resulted in several polymorphic bands ranged from >50 to >1500 bp in size. Maximum number of bands was obtained by primer OLP13 which produced seven bands in bovine mastitis isolates. Most polymorphisms were observed in isolates of bovine mastitis and the lowest were associated with human infections isolates. There was no relationship between the RAPD patterns and the sources of isolates, except the three clusters which showed host specificity and only included the strains from the same sources. The results confirm the wide genotypic diversity of the studied S. aureus strains. RAPD-PCR technique can be a valuable tool for assessing the genetic relationship, detection of polymorphism in S. aureus and tracing the sources and management of S. aureus infections.

A qualitative study of Ontario dairy farmer attitudes and perceptions toward implementing recommended milking practices

Recommended milking practices (RMP) are protective against mastitis. However, many producers do not adopt, or only partially adopt, these measures. This study aimed to explore the attitudes and perceptions of Ontario dairy farmers toward barriers to implementation of RMP and to investigate what motivates behavior change in relation to milking hygiene. Four focus groups with Ontario dairy producers were conducted, and verbatim transcripts were analyzed thematically. The main barriers to adoption of RMP were identified and categorized into 2 groups: intrinsic barriers and physical barriers. Intrinsic barriers included personal habits and convenience, not perceiving udder health as a priority on their farm, and lack of information. Physical barriers included employee training and compliance, convenience of implementing RMP, and time, money, and labor barriers. Producers used their bulk tank somatic cell count (SCC) as a measure of perceived severity of udder health problems on farm. Those with lower SCC were less likely to prioritize udder health compared with peers experiencing elevations in SCC. Lack of udder health problems translated for some producers into non-adoption of certain RMP, as they felt these practices were not needed unless a problem arose. Others felt motivated to implement more practices and work toward better udder health if such efforts translated into rewards for better-quality milk. Some producers perceived RMP as not meaningful or useful, seemingly due to a lack of education about the reasons behind RMP implementation. Understanding the importance of these practices is one key to implementing them. To overcome some of the intrinsic barriers, increased efforts in knowledge translation are needed, including efforts in retraining current practices, as well as in establishing best practices.

Effect of microbial sanitizers for reducing biofilm formation of Staphylococcus aureus and Pseudomonas aeruginosa on stainless steel by cultivation with UHT milk

Biofilm is a serious issue in the dairy factory due to it increases the opportunity for microbial contamination. Staphylococcus aureus and Pseudomonas aeruginosa are the bacteria capable to construct the biofilm on materials and equipments. Therefore, the bacterial growth and efficiency of sanitizing agents to solve the problems were evaluated. These bacteria grew well in UHT milk when they were cultivated at 37 °C, especially S. aureus. The exponential growth phase and biofilm on stainless steel were discovered by short contact time at 2 h. The mature stage of biofilm cycle was found at 4 h during bacteria growth and it was continuously constructed until 48 h. The 10, 24, and 48 h-old biofilm adhering on stainless steel were established with oxisan and chlorine used as microbial sanitizers. The 4% of sanitizing agents was the efficiency concentration to reduce biofilm on stainless steel up to 82% when these bacteria grew in UHT milk.

Prevalence and Characterization of Staphylococcus aureus Cultured From Raw Milk Taken From Dairy Cows With Mastitis in Beijing, China

The colonization of dairy herds and subsequent contamination of raw milk by Staphylococcus aureus (S. aureus), especially those expressing a multi-drug resistance (MDR), biofilm and toxins producing ability, remains an important issue for both the dairy producer and public health. In this study, we investigated the prevalence, antimicrobial resistance, virulence, and genetic diversity of S. aureus in raw milk taken from 2 dairy farms in Beijing, China. Ninety (46.2%, 90/195) samples were positive for S. aureus. Resistant to penicillin (PEN) (31.3%), ciprofloxacin (18.8%) and enrofloxacin (15.6%) were the most often observed. Isolates cultured from farm B showed significantly higher resistance to penicillin (73.9%), ciprofloxacin (34.8%), enrofloxacin (34.8%), tilmicosin (17.4%), and erythromycin (17.4%) than those from farm A (p < 0.05). Totally, 94.8% S. aureus harbored at least one virulence gene and the pvl (93.8%), sec (65.6%), and sea (60.4%) genes were the most frequently detected. The pvl and sec genes were more often detected in isolates from farm A (97.3% and 84.9% respectively) than those from farm B (p < 0.05). Of all 77 staphylococcus enterotoxin (SE)-positive isolates, more than 90% could produce enterotoxins and 70.1% could produce two types. Biofilm related genes (icaA/D, clf/B, can, and fnbA) were detected in all96 isolates. All 96 isolates could produce biofilm with 8.3, 70.8, and 18.8% of the isolates demonstrating weak, moderate and strong biofilm formation, respectively. A total of 5 STs, 7 spa types (1 novel spa type t17182), 3agr types (no agrII), and 14 SmaI-pulso-types were found in this study. PFGE cluster II-CC1-ST1-t127-agr III was the most prevalent clone (56.3%). Isolates of agr III (PFGE Cluster I/II-CC1-ST1-t127/2279) had higher detection of virulence genes than those of agr I and agr IV. TheMSSA-ST398-t1456-agr I clone expressed the greatest MDRbut with no virulence genes and weakly biofilm formation. Our finding indicated a relatively high prevalence of S. aureus with less antimicrobial resistance but often positive for enterotoxigenicity and biofilm formation. This study could help identify predominant clones and provide surveillance measures to eliminate and decrease the contamination of S. aureus in raw milk of dairy cows with mastitis.

Short communication: Drying-off practices and use of dry cow therapy in Finnish dairy herds

The objective of this study was to survey drying-off practices and use of dry cow therapy (DCT) in Finland through an online questionnaire. The questionnaire was accessible to all dairy farmers of the Finnish dairy herd recording system in 2016 (approximately 5,400 farms). In total, 715 dairy producers across the country, representative of the Finnish dairy industry, participated in the survey. Cows were dried off gradually in most of the farms. Most farms (78%) reported using selective DCT, whereas 9% of farms did not use any DCT, and 13% of farms applied blanket DCT. A significant trend was observed with increasing herd size and proportion of farms using blanket DCT. Percentage of farms using blanket DCT was also higher in farms with automatic milking system. Farmer's own experience was the most commonly reported reason for choosing a particular approach to DCT. Microbiological testing of milk samples at dry-off was the preferred method of selecting cows for DCT; 82 and 64% of farms using selective and blanket DCT approach, respectively, reported testing milk samples before treatment. The second most common criteria for using antibiotic DCT were clinical mastitis history and high somatic cell count. A high number of farms using selective DCT reported treating only up to one-fourth of their cows at dry-off. Information acquired on drying-off practices in Finland allows for future monitoring of prudent antimicrobial usage at dry-off.

Proactive udder health management in South Africa and monitoring of antibiotic resistance of Staphylococcus aureus; in dairy herds from 2001 to 2010

Antibiotic resistance of strains of Staphylococcus aureus isolated from bovine milk is of concern internationally. The objective of this study was to investigate trends of resistance of S. aureus to antibiotics administered to dairy cows in 19 South African and one Zambian dairy herds (participating in the South African proactive udder health management programme) and to identify possible contributing factors. The resistance of S. aureus strains to eight commonly used antibiotics in South Africa from 2001 to 2010 was evaluated. Staphylococcus aureus isolates (n = 2532) were selected from cows with subclinical mastitis in 20 herds routinely sampled as part of the proactive udder health management programme. The isolates were selected from milk samples that had somatic cell counts more than 400 000 cells/mL and were tested for antibiotic resistance using a standard Kirby–Bauer test with published clinical breakpoints. The prevalence of antibiotic resistance was evaluated as a percentage of S. aureus isolates susceptible out of the total numbers for each antibiotic selected per year. Staphylococcus aureus showed a significant increase in percentage of susceptible isolates over time for all antibiotics tested except for ampicillin. The overall prevalence of mastitis did not change during the study period. However, the prevalence of mastitis caused by S. aureus (mostly subclinical cases) in the selected herds decreased numerically but not significantly. Reduction in the incidence of antibiotic resistance shown by S. aureus was presumed to be a result of the application of the proactive udder health management programme. The fact that the overall prevalence of mastitis was kept stable was possibly because of the influence of the management programme in conjunction with the return of infections caused by non-resistant strains.

Canadian National Dairy Study: Herd-level milk quality

The objective of this study was to estimate Canadian national milk quality parameters and estimate the bulk tank milk (BTM) prevalence of 4 mastitis pathogens, Staphylococcus aureus, Streptococcus agalactiae, Mycoplasma bovis, and Prototheca spp., on Canadian dairy farms. A questionnaire was sent to all Canadian dairy producers. Of the 1,062 producers who completed the questionnaire, 374 producers from across the country were visited and milking hygiene was assessed. Farm-level milk quality data for all Canadian dairy producers was collected from the provincial marketing boards and combined with the questionnaire and farm visit data. In addition, a BTM sample was collected either during the farm visit or by the marketing board in November of 2015 and was tested for 4 major mastitis pathogens using the PathoProof Mastitis Major 4 PCR Assay (Thermo Fisher Scientific Inc., Waltham, MA). Apparent herd-level prevalence was 46% for S. aureus, 6% for Prototheca spp., 0% for M. bovis, and 0% for Strep. agalactiae. Due to the low prevalence of M. bovis and Strep. agalactiae and a lack of significant factors associated with farms testing positive for Prototheca spp., an association analysis could only be carried out for Staph. aureus-positive farms. Factors associated with Staph. aureus-positive farms were not fore-stripping cows before milking (odds ratio = 1.87), milking with a pipeline system (odds ratio = 2.21), and stall bases made of a rubberized surface (mats and mattresses), whereas protective factors were using blanket dry cow therapy (odds ratio = 0.49) and applying a tag or visible mark on cows known to have chronic mastitis infections (odds ratio = 0.45). The Canadian national production-weighted geometric mean somatic cell count was determined to be 208,000 cells/mL. This is the first national dairy study conducted in Canada. Participating farms had higher milk yield; were more likely to have a loose housing system, parlor, or automated milking system; and had lower weighted mean BTM somatic cell count than the national level. Sampling larger farms with better milk quality means the apparent prevalence of the 4 mastitis pathogens likely underestimates the true levels.

Diagnosing intramammary infection: Controlling misclassification bias in longitudinal udder health studies

Using imperfect tests may lead to biased estimates of disease frequency and of associations between risk factors and disease. For instance in longitudinal udder health studies, both quarters at risk and incident intramammary infections (IMI) can be wrongly identified, resulting in selection and misclassification bias, respectively. Diagnostic accuracy can possibly be improved by using duplicate or triplicate samples for identifying quarters at risk and, subsequently, incident IMI. The objectives of this study were to evaluate the relative impact of selection and misclassification biases resulting from IMI misclassification on measures of disease frequency (incidence) and of association with hypothetical exposures. The effect of improving the sampling strategy by collecting duplicate or triplicate samples at first or second sampling was also assessed. Data sets from a hypothetical cohort study were simulated and analyzed based on a separate scenario for two common mastitis pathogens representing two distinct prevailing patterns. Staphylococcus aureus, a relatively uncommon pathogen with a low incidence, is identified with excellent sensitivity and almost perfect specificity. Coagulase negative staphylococci (CNS) are more prevalent, with a high incidence, and with milk bacteriological culture having fair Se but excellent Sp. The generated data sets for each scenario were emulating a longitudinal cohort study with two milk samples collected one month apart from each quarter of a random sample of 30 cows/herd, from 100 herds, with a herd-level exposure having a known strength of association. Incidence of IMI and measure of association with exposure (odds ratio; OR) were estimated using Markov Chain Monte Carlo (MCMC) for each data set and using different sampling strategies (single, duplicate, triplicate samples with series or parallel interpretation) for identifying quarters at risk and incident IMI. For S. aureus biases were small with an observed incidence of 0.29 versus a true incidence of 0.25IMI/100 quarter-month. In the CNS scenario, diagnostic errors in the two samples led to important selection (40IMI/100 quarter-month) and misclassification (23IMI/100 quarter-month) biases for estimation of IMI incidence, respectively. These biases were in opposite direction and therefore the incidence measure obtained using single sampling on both the first and second test (29IMI/100 quarter-month) was exactly the true value. In the S. aureus scenario the OR for association with exposure showed little bias (observed OR of 3.1 versus true OR of 3.2). The CNS scenario revealed the presence of a large misclassification bias moving the association towards the null value (OR of 1.7 versus true OR of 2.6). Little improvement could be brought using different sampling strategies aiming at improving Se and/or Sp on first and/or second sampling or using a two out of three interpretation for IMI definition. Increasing number of samples or tests can prevent bias in some situations but efforts can be spared by holding to a single sampling approach in others. When designing longitudinal studies, evaluating potential biases and best sampling strategy is as critical as the choice of test.

The association between subclinical mastitis around calving and reproductive performance in grazing dairy cows

The objective of this study was to evaluate the effect of subclinical mastitis (SCM) on calving-to-first-service interval (CFS), calving-to-conception interval (CC), and on the number of services per conception (S/C) in grazing Holstein and Normande cows. Primiparous (n=43) and multiparous (n=165) cows were selected from five dairy herds. Two composite milk samples were aseptically collected from each cow at drying-off, and then every week during the first postpartum month. One sample was used for somatic cell count (SCC), and the other one for bacteriological analysis. Cows were followed up to 300 d after calving. Non-parametric and parametric survival models, and negative binomial regression were used to assess the association between SCM, evaluated by SCC and milk culture, and reproductive indices. Staphylococcus aureus, CNS, and Streptococcus uberis were the most frequent isolated pathogens. Subclinical mastitis in the first month of lactation was not associated with CFS; however, the CC interval was longer in cows with SCM compared to healthy cows, the former also had a higher number of S/C.

DNA carryover in milk samples from routine milk recording used for PCR-based diagnosis of bovine Staphylococcus aureus mastitis

Real-time PCR techniques are increasingly used to detect udder pathogens from milk samples collected non-aseptically at routine milk recording. The objectives of this study were (1) to estimate the statistical associations between cycle threshold (Ct) values for Staphylococcus aureus in non-aseptically collected composite samples taken at routine milk recording from cows milked consecutively with the same milking unit and milk meter; and (2) to formulate practical and plausible guidelines for understanding the diagnostic implications of PCR testing for Staph. aureus intramammary infection at routine milk recording. The study included 4 herds with conventional milking parlors and repeatedly low Ct-values for Staph. aureus (representing a high DNA load) in bulk tank milk. Composite milk samples were collected from all cows at all milking units during routine milk recording using the Tru-Test electronic milk meter (Tru-Test Group, Auckland, New Zealand) and analyzed using the PathoProof PCR (Thermo Fisher Scientific, Vantaa, Finland) assay. Milking clock times were retrieved at each milk meter to establish the milking order of the cows at each unit. A multinomial logistic regression was applied to estimate the association between Ct-values from cows milked consecutively with the same milking unit and milk meter. The following groups were selected based on Ct-values: (1) 0-31.3, (2) 31.4-33.9, (3) 34.0-37, (4) 37.1-39.9, and (5) 40 (negative result). The association between groups from cows milked consecutively with the same milking unit and milk meter was statistically significant. Approximately 60% of cows were in Ct group 5 if the antecedent cow was also in Ct group 5, but only 20% of cows were in Ct group 5 if the antecedent cow was in Ct group 1. The probability of cows being in Ct group 1 was not markedly influenced by the group of the antecedent cow. Statistical relationships in the intermediate range gave a plausible indication of a dose-response relationship. Carryover of bacterial DNA via the milking unit and milk meter is very likely to affect PCR results for Staph. aureus. Therefore, information about milking order must be considered in mastitis control efforts. We suggest a practical interpretation of PCR results: cows with a Ct-value <32 can be labeled "very likely to be infected with Staph. aureus," but cows with Ct-values of >37 and 32-37 can be labeled "very likely to be negative for Staph. aureus" and "uncertain Staph. aureus status," respectively.

Effect of different scenarios for selective dry-cow therapy on udder health, antimicrobial usage, and economics

The goal of dry-cow therapy (DCT) is to reduce the prevalence of intramammary infections (IMI) by eliminating existing IMI at drying off and preventing new IMI from occurring during the dry period. Due to public health concerns, however, preventive use of antimicrobials has become questionable. In this study, we evaluated the effects of 8 scenarios for selecting animals for DCT, taking into account variation in parity and cow-level somatic cell count (SCC) at drying off. The aim of this study was to evaluate udder health, antimicrobial usage, and economics at the herd level when using different scenarios for selecting cows for DCT. To enable calculation and comparison of the effects of different scenarios to select cows for DCT in an "average" herd, we created an example herd, with a virtual herd size of 100 dairy cows to be calving during a year. Udder health, antimicrobial usage, and economics were evaluated during the dry period and the first 100 d in lactation, the period during which the greatest effect of DCT is expected. This leads to an estimated 13,551 cow-days at risk during a year in a 100-cow dairy herd. In addition to a blanket DCT (BDCT) scenario, we developed 7 scenarios to select cows for DCT based on SCC. The scenarios covered a range of possible approaches to select low-SCC cows for DCT, all based on cow-level SCC thresholds on the last milk recording before drying off. The incidence rate of clinical mastitis in the example herd varied from 11.6 to 14.5 cases of clinical mastitis per 10,000 cow-days at risk in the different scenarios, and the prevalence of subclinical mastitis varied from 38.8% in scenario 1 (BDCT) to 48.3% in scenario 8. Total antimicrobial usage for DCT and clinical mastitis treatment varied over the scenarios from 1.27 (scenario 8) to 3.15 animal daily dosages (BDCT), leading to a maximum reduction in antimicrobial usage of 60% for scenario 8 compared with BDCT. The total costs for each of the scenarios showed little variation, varying from €4,893 for scenario 5 to €5,383 for scenario 8. The effect of selective DCT compared with BDCT on udder health, antimicrobial usage, and economics is influenced by the SCC criteria used to select cows for DCT. Scenario 2 resulted in the lowest increases in clinical and subclinical mastitis compared with BDCT. The greatest reduction in antimicrobial usage was achieved under scenario 8. From an economic perspective, lowest costs were achieved with scenario 5. Drying off dairy cows with antimicrobials has an effect on udder health, antimicrobial usage, and economics.

Occurrence of methicillin-resistant Staphylococcus aureus in dairy cattle herds, related swine farms, and humans in contact with herds

In this study we investigated the circulation of methicillin-resistant Staphylococcus aureus (MRSA) in 2 dairy cattle farms (farm A and B), previously identified as MRSA-positive in bulk tank milk samples, and epidemiologically related to swine farms. Collected specimens included quarter milk samples and nasal swabs from dairy cows, pig nasal swabs collected at both the farm and slaughterhouse level, environmental dust samples, and human nasal swabs from the farms' owners and workers. The prevalence of MRSA was estimated at the herd level by testing quarter milk samples. The prevalence of MRSA was 4.8% (3/63; 95% confidence interval=0-10.2%) and 60% (33/55; 95% confidence interval=47.05-72.95) in farm A and B, respectively. In farm A, MRSA was also isolated from humans, pigs sampled at both farm and slaughterhouse level, and from environmental samples collected at the pig facilities. The dairy cattle facilities of farm A tested negative for MRSA. In farm B, MRSA was isolated from environmental dust samples in both the cattle and pig facilities, whereas nasal swabs collected from cows and from humans tested negative. Sixty-three selected MRSA isolates obtained from different sources in farm A and B were genetically characterized by multilocus sequence typing, spa-typing, ribosomal spacer-PCR, and also tested for the presence of specific virulence genes and for their phenotypical antimicrobial susceptibility by broth microdilution method. Different clonal complex (CC) and spa-types were identified, including CC398, CC97, and CC1, CC already reported in livestock animals in Italy. The MRSA isolates from quarter milk of farm A and B mostly belonged to CC97 and CC398, respectively. Both lineages were also identified in humans in farm A. The CC97 and CC398 quarter milk isolates were also identified as genotype GTBE and GTAF by ribosomal spacer-PCR respectively, belonging to distinct clusters with specific virulence and resistance patterns. The GTBE and GTAF clusters also included swine, environmental, and human isolates from both farms. A high heterogeneity in the genetic and phenotypic profiles was observed in environmental isolates, in particular from farm B. These results demonstrate the possibility of a dynamic sharing and exchange of MRSA lineages or genotypes between different species and farm compartments in mixed-species farms. The risk of transmission between swine and related dairy cattle herds should be considered. Our findings also confirm the zoonotic potential of livestock-associated MRSA and underline the importance of applying biosecurity measures and good hygiene practices to prevent MRSA spread at the farm level and throughout the food production chain.

Staphylococcus aureus Entrance into the Dairy Chain: Tracking S. aureus from Dairy Cow to Cheese

Staphylococcus aureus is one of the most important contagious mastitis pathogens in dairy cattle. Due to its zoonotic potential, control of S. aureus is not only of great economic importance in the dairy industry but also a significant public health concern. The aim of this study was to decipher the potential of bovine udder associated S. aureus as reservoir for S. aureus contamination in dairy production and processing. From 18 farms, delivering their milk to an alpine dairy plant for the production of smeared semi-hard and hard cheese. one thousand hundred seventy six one thousand hundred seventy six quarter milk (QM) samples of all cows in lactation (n = 294) and representative samples form bulk tank milk (BTM) of all farms were surveyed for coagulase positive (CPS) and coagulase negative Staphylococci (CNS). Furthermore, samples from different steps of the cheese manufacturing process were tested for CPS and CNS. As revealed by chemometric-assisted FTIR spectroscopy and molecular subtyping (spa typing and multi locus sequence typing), dairy cattle represent indeed an important, yet underreported, entrance point of S. aureus into the dairy chain. Our data clearly show that certain S. aureus subtypes are present in primary production as well as in the cheese processing at the dairy plant. However, although a considerable diversity of S. aureus subtypes was observed in QM and BTM at the farms, only certain S. aureus subtypes were able to enter and persist in the cheese manufacturing at the dairy plant and could be isolated from cheese until day 14 of ripening. Farm strains belonging to the FTIR cluster B1 and B3, which show genetic characteristics (t2953, ST8, enterotoxin profile: sea/sed/sej) of the recently described S. aureus genotype B, most successfully contaminated the cheese production at the dairy plant. Thus, our study fosters the hypothesis that genotype B S. aureus represent a specific challenge in control of S. aureus in the dairy chain that requires effective clearance strategies and hygienic measures already in primary production to avoid a potential transfer of enterotoxic strains or enterotoxins into the dairy processing and the final retail product.

Antimicrobial Susceptibility Patterns of Environmental Streptococci Recovered from Bovine Milk Samples in the Maritime Provinces of Canada

Determination of antimicrobial susceptibility of bovine mastitis pathogens is important for guiding antimicrobial treatment decisions and for the detection of emerging resistance. Environmental streptococci are ubiquitous in the farm environment and are a frequent cause of mastitis in dairy cows. The aim of the study was to determine patterns of antimicrobial susceptibility among species of environmental streptococci isolated from dairy cows in the Maritime Provinces of Canada. The collection consisted of 192 isolates identified in milk samples collected from 177 cows originating from 18 dairy herds. Results were aggregated into: (1) Streptococcus uberis (n = 70), (2) Streptococcus dysgalactiae (n = 28), (3) other Streptococci spp. (n = 35), (4), Lactococcus spp. (n = 32), and (5) Enterococcus spp. (n = 27). Minimum inhibitory concentrations (MICs) were determined using the Sensititre microdilution system and mastitis plate format. Multilevel logistic regression models were used to analyze the data, with antimicrobial susceptibility as the outcome. The proportion of susceptible S. uberis ranged from 23% (for penicillin) to 99% (for penicillin/novobiocin), with a median of 82%. All S. dysgalactiae were susceptible to all antimicrobials except for penicillin (93% susceptible) and tetracycline (18% susceptible). The range of susceptibility for other Streptococcus spp. was 43% (for tetracycline) to 100%, with a median percent susceptibility of 92%. Lactococcus spp. isolates displayed percent susceptibilities ranging from 0% (for penicillin) to 97% (for erythromycin), median 75%. For the antimicrobials tested, the minimum inhibitory concentrations were higher for Enterococcus spp. than for the other species. According to the multilevel models, there was a significant interaction between antimicrobial and bacterial species, indicating that susceptibility against a particular antimicrobial varied among the species of environmental streptococci and vice versa. Generally, susceptibility decreased with increasing within-herd average somatic cell count, isolates recovered in mid-lactation were more susceptible than isolates recovered in early lactation, and isolates recovered in samples collected post-clinical mastitis were more susceptible than isolates recovered from non-clinical lactating quarters. The results of this research support continued susceptibility of environmental streptococci to beta-lactam antimicrobials. A departure from the expected susceptibility to beta-lactams was the apparent reduced susceptibility of S. uberis to penicillin.

Combined analysis of DNA methylome and transcriptome reveal novel candidate genes with susceptibility to bovine Staphylococcus aureus subclinical mastitis

Subclinical mastitis is a widely spread disease of lactating cows. Its major pathogen is Staphylococcus aureus (S. aureus). In this study, we performed genome-wide integrative analysis of DNA methylation and transcriptional expression to identify candidate genes and pathways relevant to bovine S. aureus subclinical mastitis. The genome-scale DNA methylation profiles of peripheral blood lymphocytes in cows with S. aureus subclinical mastitis (SA group) and healthy controls (CK) were generated by methylated DNA immunoprecipitation combined with microarrays. We identified 1078 differentially methylated genes in SA cows compared with the controls. By integrating DNA methylation and transcriptome data, 58 differentially methylated genes were shared with differently expressed genes, in which 20.7% distinctly hypermethylated genes showed down-regulated expression in SA versus CK, whereas 14.3% dramatically hypomethylated genes showed up-regulated expression. Integrated pathway analysis suggested that these genes were related to inflammation, ErbB signalling pathway and mismatch repair. Further functional analysis revealed that three genes, NRG1, MST1 and NAT9, were strongly correlated with the progression of S. aureus subclinical mastitis and could be used as powerful biomarkers for the improvement of bovine mastitis resistance. Our studies lay the groundwork for epigenetic modification and mechanistic studies on susceptibility of bovine mastitis.