A longitudinal study on transmission of Staphylococcus aureus genotype B in Swiss communal dairy herds

District-Wide Herd Sanitation and Eradication of Intramammary Staphylococcus aureus Genotype B Infection in Dairy Herds in Ticino, Switzerland

The present study demonstrates successful herd sanitation and eradication of contagious mastitis caused by Staphylococcus aureus genotype B (S. aureus GTB) in an entire Swiss district (Ticino) including 3,364 dairy cows from 168 farms. Herd sanitation included testing of all cows using a highly GTB specific and sensitive qPCR assay, implementation of related on-farm measures, appropriate antibiotic therapy of GTB-positive cows and culling of therapy-resistant animals, respectively. A treatment index was used as an objective criterion to select GTB-positive cows eligible for culling and replacement payment. 62 herds (37%) were initially GTB-positive with a cow prevalence between 10% and 100% and were submitted to sanitation. Twenty mo after the start of the campaign, all these herds were free from S. aureus GTB, whereby 73% of them were sanitized during the first 7 mo. At the cow level, a total of 343 animals were infected. 50 of them were immediately culled and financially compensated based on their treatment index value. The remaining 293 cows were intramammarily treated with antibiotics either during lactation using the combination of cephalexin-kanamycin or penicillin-gentamicin or at dry-off using cloxacillin. Out of these cows, 275 (93.9%) were treated successfully meaning that their milk was twice GTB-negative by qPCR after therapy. For lactational treatment, control samples were taken ≥10 and ≥20 d after treatment, for dry off treatment ≥14 and ≥24 d after parturition. Neither lactation number nor SCC before treatment of the cow nor the type of therapy were associated with therapeutic cure. Using data of 30 GTB-positive and 71 GTB-negative herds (1855 observations), the impact of GTB sanitation on bulk tank milk SCC (BTSCC) was evaluated applying a linear mixed statistical model. In the year before sanitation, BTSCC was always higher in GTB positive than in GTB negative herds. After the start of the campaign, BTSCC declined rapidly in the herds under GTB sanitation and achieved values that no longer differed statistically from those of GTB-free herds after only 2 mo, remaining very similar for the rest of the campaign. The farmers were very satisfied with the outcome of the campaign as all GTB positive herds could be sanitized rapidly, sanitation was sustainable, and milk quality increased.

Farm-level risk factors for digital dermatitis in dairy cows in mountainous regions

Reduction of risk factors for bovine digital dermatitis (BDD) is crucial in current disease control. However, risk factors that might arise especially in mountainous regions are unknown until now, and an adapted BDD control program is consequently missing. The objective of this observational case-control study was to identify farm-level risk factors for BDD in dairy herds in mountainous regions. To investigate predictors for the occurrence of BDD, 100 farms were visited and information about herd characteristics and management practices, potentially relevant explanatory variables for either introduction or establishment of BDD, were gathered by completing a questionnaire with the farmer or herd manager. Within-herd prevalences of BDD assessed during 3 routine claw trimmings with an interval of 6 mo before the survey were used to define cases (BDD within-herd prevalence of ≥26% during each claw trimming) and controls (no BDD cases in each of the 3 routine claw trimmings before the survey). Data were analyzed using 2 separate binomial generalized linear models according to either establishment or introduction of BDD. After prescreening, 15 of 23 explanatory variables were included in the final analysis, which showed 3 variables related to introduction and establishment, each being significantly associated with the occurrence of BDD within a farm. Results of model 1 (i.e., aspects related to BDD introduction) revealed that access to mountain pastures during the summer season (odds ratio, 95% confidence interval: 0.12, 0.04-0.35), participation in dairy shows (0.32, 0.11-0.94), and the number of new animals introduced into the farm during the last 2 yr (1.28, 1.12-1.52) were significantly associated with the occurrence of BDD. Model 2 (i.e., aspects related to BDD establishment) showed that cows kept in freestalls were at higher risk for BDD compared with those kept in tiestalls (20.65, 1.59-649.37). Furthermore, number of days between diagnosis and treatment of a BDD lesion (10.31, 3.55-81.21) and the amount of concentrate feeding (median 5 kg) per cow and day (7.72, 2.46-6.47) were positively associated with BDD occurrence. In conclusion, the findings of this study provide a set of risk factors that are associated with BDD status within herds in mountainous regions. These results may help in development of adapted control programs for BDD in dairy cows.

Genome sequencing and comparative genomic analysis of bovine mastitis-associated Staphylococcus aureus strains from India

BACKGROUND

Bovine mastitis accounts for significant economic losses to the dairy industry worldwide. Staphylococcus aureus is the most common causative agent of bovine mastitis. Investigating the prevalence of virulence factors and antimicrobial resistance would provide insight into the molecular epidemiology of mastitis-associated S. aureus strains. The present study is focused on the whole genome sequencing and comparative genomic analysis of 41 mastitis-associated S. aureus strains isolated from India.

RESULTS

The results elucidate explicit knowledge of 15 diverse sequence types (STs) and five clonal complexes (CCs). The clonal complexes CC8 and CC97 were found to be the predominant genotypes comprising 21 and 10 isolates, respectively. The mean genome size was 2.7 Mbp with a 32.7% average GC content. The pan-genome of the Indian strains of mastitis-associated S. aureus is almost closed. The genome-wide SNP-based phylogenetic analysis differentiated 41 strains into six major clades. Sixteen different spa types were identified, and eight isolates were untypeable. The cgMLST analysis of all S. aureus genome sequences reported from India revealed that S. aureus strain MUF256, isolated from wound fluids of a diabetic patient, was the common ancestor. Further, we observed that all the Indian mastitis-associated S. aureus isolates belonging to the CC97 are mastitis-associated. We identified 17 different antimicrobial resistance (AMR) genes among these isolates, and all the isolates used in this study were susceptible to methicillin. We also identified 108 virulence-associated genes and discuss their associations with different genotypes.

CONCLUSION

This is the first study presenting a comprehensive whole genome analysis of bovine mastitis-associated S. aureus isolates from India. Comparative genomic analysis revealed the genome diversity, major genotypes, antimicrobial resistome, and virulome of clinical and subclinical mastitis-associated S. aureus strains.

Bovine Staphylococcus aureus: a European study of contagiousness and antimicrobial resistance

In dairy herds managements, mastitis is the leading cause of economic losses. One of the most important pathogens responsible for intra-mammary infections is Staphylococcus aureus. The genetic properties of S. aureus have a strong influence on its pathogenicity and contagiousness. In this study, we aimed to obtain a comprehensive overview of the key bovine S. aureus clinical properties, such as contagiousness and antimicrobial resistance, present in European strains. For this, 211 bovine S. aureus strains from ten European countries that were used in a previous study were used in this study. Contagiousness was assessed using qPCR for the detection of the marker gene adlb. Antimicrobial resistance was evaluated using a broth microdilution assay and mPCR for the detection of genes involved in penicillin resistance (blaI, blaR1, and blaZ). It was found that adlb was present in CC8/CLB strains; however, in Germany, it was found in CC97/CLI and in an unknown CC/CLR strains. CC705/CLC strains from all countries were found to be susceptible to all tested antibiotics. Major resistance to penicillin/ampicillin, chloramphenicol, clindamycin and tetracycline was detected. Resistance to oxacillin, trimethoprim/sulfamethoxazole and cephalosporins was rarely observed. In addition, contagiousness and antibiotic resistance seem to correlate with different CCs and genotypic clusters. Hence, it is recommended that multilocus sequence typing or genotyping be utilized as a clinical instrument to identify the most appropriate antibiotic to use in mastitis treatment. Actualization of the breakpoints of veterinary strains is necessary to address the existing antibiotic resistance of the bacteria involved in veterinary mastitis.

Optimal reduced-mixing for an SIS infectious-disease model

Which reduced-mixing strategy maximizes economic output during a disease outbreak? To answer this question, we formulate an optimal-control problem that maximizes the difference between revenue, due to healthy individuals, and medical costs, associated with infective individuals, for SIS disease dynamics. The control variable is the level of mixing in the population, which influences both revenue and the spread of the disease. Using Pontryagin's maximum principle, we find a closed-form solution for our problem. We explore an example of our problem with parameters for the transmission of Staphylococcus aureus in dairy cows, and we perform sensitivity analyses to determine how model parameters affect optimal strategies. We find that less mixing is preferable when the transmission rate is high, the per-capita recovery rate is low, or when the revenue parameter is much smaller than the cost parameter.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): antimicrobial-resistant Staphylococcus aureus in cattle and horses

Staphylococcus aureus (S. aureus) was identified among the most relevant antimicrobial-resistant (AMR) bacteria in the EU for cattle and horses in previous scientific opinions. Thus, it has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as in Article 9, and Article 8 for listing animal species related to the bacterium. The assessment has been performed following a methodology previously published. The outcome is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound ≥ 66%) or not (upper bound ≤ 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with uncertain outcome. According to the assessment here performed, it is uncertain whether AMR S. aureus can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (60-90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that the bacterium does not meet the criteria in Sections 1, 2 and 4 (Categories A, B and D; 1-5%, 5-10% and 10-33% probability of meeting the criteria, respectively) and the AHAW Panel was uncertain whether it meets the criteria in Sections 3 and 5 (Categories C and E, 33-90% and 60-90% probability of meeting the criteria, respectively). The animal species to be listed for AMR S. aureus according to Article 8 criteria include mainly mammals, birds, reptiles and fish.

A stochastic modelling approach to determine the effect of diverse Staphylococcus aureus strains on the economic and epidemiological outcomes of mastitis intervention strategies in dairy cattle

Staphylococcus aureus (S. aureus) strains with considerable genetic and phenotypic differences have previously been identified. The economic and epidemiologic impact of S. aureus mastitis has been investigated, but none of these studies took differences between strains into account. Here we aimed to investigate how differences between S. aureus strains affect the economic and epidemiologic outcome of various intervention strategies against clinical and subclinical intramammary infections. Five S. aureus strains were modelled using a stochastic bio-economic model simulating a dairy herd of 200 cows using single-day time steps. The strain characteristics of the five simulated S. aureus strains (general, contagious, spill-over, clinical and persistent) were based on divergent phenotypes as described in literature. Outcomes of the model included incidence (both clinical and subclinical), number of antibiotic treatment days, number of culled cows, and net income. Intervention strategies against clinical and subclinical intramammary infections were based on (variations of) intramammary antibiotic treatment, testing, and culling. Both single and multiple pathogen (intramammary infection caused by S. aureus, Escherichia coli, and non-aureus staphylococci) scenarios were simulated to determine the effect of the five S. aureus strains on the impact of 19 different intervention strategies. The results showed that the incidence (both clinical and subclinical), number of treatment days, number of culled cows, and net income varied considerably for the different S. aureus strains. Comparison of the model outcomes within and between strains showed that for most intervention strategies the relative impact differed per strain. However, the intervention strategy with the best outcome for most variables and strains was the culling of cows with a recovery probability lower than 50%. This shows that the relative economic and epidemiologic impact of most of the modelled intervention strategies were strain-dependent, while some intervention strategies were not strain-dependent. From this, we conclude that, depending on the intervention strategy applied on a farm, it could be advantageous to type S. aureus to determine whether it would be economically and epidemiologically beneficial for the existing intervention strategy to be changed.

Transmission dynamics of Staphylococcus aureus and Streptococcus agalactiae in a Dutch dairy herd using an automatic milking system

Staphylococcus aureus and Streptococcus agalactiae are important contagious mastitis pathogens and are considered to mainly transmit between cows through the milking machine. Controlling contagious mastitis on dairy farms requires a reduction of the transmission rate or the duration of intramammary infections (IMI), or both. These parameters may differ in dairy herds milked with an automatic milking system (AMS) as compared to those milked with a conventional milking system (CMS). The aims of this prospective longitudinal study were to estimate the transmission rate, the median duration of IMI and the basic reproduction number (R₀) of Staph. aureus and Strep. agalactiae in a Dutch AMS herd. Bacteriological cultures of quarter milk samples were collected every 2 wks. Using 3 different definitions of IMI, we estimated the transmission rate for Staph. aureus to be within the range of 0.002 (95 % CI: 0-0.005) quarter-day^-1 to 0.019 (95 % CI: 0.010-0.032) quarter-day^-1, and for Strep. agalactiae of 0.007 (95 % CI: 0.005-0.010) quarter-day^-1 to 0.019 (95 % CI: 0.011-0.032) quarter-day^-1, the median duration of chronic IMI at 95 (95 % CI: 72-125) days for Staph. aureus and at 86 (95 % CI: 67-111) days for Strep. agalactiae, and the R₀ between 0.16 (95 % CI: 0.05-0.27) and 0.34 (95 % CI: 0.20-0.48) for Staph. aureus, and between 0.64 (95 % CI: 0.41-0.87) and 0.68 (95 % CI: 0.48-0.88) for Strep. agalactiae. Transmission of these two contagious pathogens in this herd was limited and theoretically the IMI would not sustain, given that R₀ of both pathogens was lower than 1. The estimated transmission rate of Staph. aureus in this AMS herd was found to be comparable to those described for CMS herds, while for Strep. agalactiae, it was slightly higher than in CMS herds. The duration of Staph. aureus IMI was in line with results from CMS farms, while the duration of Strep. agalactiae was lower than what has been described in CMS herds. The R₀ of these contagious pathogens was found to be lower than the estimates in CMS herds. Our study suggests that the transmission rate of these two contagious pathogens in this AMS herd were comparable to what has been reported about well-performing CMS herds that have a low rate of transmission.

Effect of enhanced hygiene on transmission of Staphylococcus aureus, Streptococcus agalactiae, and Streptococcus dysgalactiae in dairy herds with automatic milking systems

The objective of the study was to evaluate the effect of hygiene measures in automatic milking units on the transmission of 3 mastitis pathogens considered to be mainly or partly transmitted from cow to cow during milking events. Two studies were conducted as within-herd experimental trials in 2 Danish commercial dairy herds (A and B) with automatic milking systems. Interventions to enhance hygiene were implemented on the automatic milking units. The 2 studies evaluated separate interventions. In herd A, the hygiene interventions were manual wash with the Lely foam unit and adjustments on the brush-mediated teat cleaning procedure. In herd B, the hygiene intervention included automatic disinfection spray on the upper surface of the brush motor and daily change of brushes. Composite milk samples were collected longitudinally at 3- or 4-wk intervals from all lactating cows. Additional milk samples were taken from cows entering or leaving the study groups. Milk samples were analyzed with quantitative PCR. A hidden Markov model implemented within a Bayesian framework was used to estimate the transmission probability. For analysis, 701 samples from 156 cows were used for herd A, and 1,349 samples from 390 cows were used for herd B. In the intervention group in herd B, transmission of Streptococcus agalactiae was reduced to 19% (95% posterior credibility interval: 0.00-64%) of the transmission in the control group, whereas transmission of Streptococcus dysgalactiae was reduced to 17% (95% posterior credibility interval: 0.00-85%) of transmission in the control group. This suggests that automatic spray on the upper surface of the brush motor with disinfectant along with daily change of brushes collectively reduced transmission of Strep. agalactiae and Strep. dysgalactiae. Results on Staphylococcus aureus in herd B and results on manual foam cleaning and brush-mediated teat cleaning adjustments in herd A were inconclusive.

Dynamics of somatic cell count (SCC) and differential SCC during and following intramammary infections

Somatic cell count is frequently used as an indicator of intramammary infections (IMI) in dairy cattle worldwide. The newly introduced differential SCC (DSCC) can potentially contribute to detection of IMI. The purpose of this study was to investigate the dynamics of SCC and DSCC after IMI. We used a data set with monthly samples from 2 Danish dairy herds through 1 yr, using bacterial culture to identify IMI. The dynamics of SCC and DSCC with regard to IMI were assessed at quarter level following new IMI with each of 3 defined pathogen groups, major, minor, or "other" pathogens, using general additive models. Both SCC and DSCC increased after IMI, with a more pronounced increase if major or other pathogens were detected compared with minor pathogens. We found that DSCC increased after IMI with other pathogens in both herds and, in herd 2, after IMI caused by major and minor pathogens. We also estimated the duration of increased SCC and DSCC when they exceeded a threshold, done separately for each pathogen group. Major pathogens had the longest-lasting effect in both herds for both SCC and DSCC. We conclude that the magnitude and duration of response of SCC and DSCC to IMI differs between herds and causative pathogens.

Genomic analysis of European bovine Staphylococcus aureus from clinical versus subclinical mastitis

Intramammary infections (IMI) with Staphylococcus aureus are a common cause of bovine mastitis and can result in both clinical (CM) or subclinical mastitis (SCM). Although bacterial isolates of S. aureus differ in their virulence potential it is largely unclear which bacterial virulence factors are responsible for increased clinical severity. We performed a genome wide association study and used a generalized linear mixed model to investigate the correlation between gene carriage, lineage and clinical outcome of IMI in a collection of S. aureus isolates from cattle with CM (n = 125) and SCM (n = 151) from 11 European countries. An additional aim was to describe the genetic variation of bovine S. aureus in Europa. The dominant lineages in our collection were clonal complex (CC) 151 (81/276, 29.3%), CC97 (54/276, 19.6%), CC479 (32/276, 11.6%) and CC398 (19/276, 6.9%). Virulence and antimicrobial resistance (AMR) gene carriage was highly associated with CC. Among a selection of nine virulence and AMR genes, CC151, CC479 and CC133 carried more virulence genes than other CCs, and CC398 was associated with AMR gene carriage. Whereas CC151, CC97 were widespread in Europe, CC479, CC398 and CC8 were only found in specific countries. Compared to CC151, CC479 was associated with CM rather than SCM (OR 3.62; 95% CI 1.38-9.50) and the other CCs were not. Multiple genes were associated with CM, but due to the clustering within CC of carriage of these genes, it was not possible to differentiate between the effect of gene carriage and CC on clinical outcome of IMI. Nevertheless, this study demonstrates that characterization of S. aureus CC and virulence genes helps to predict the likelihood of the occurrence of CM following S. aureus IMI and highlights the potential benefit of diagnostics tools to identify S. aureus CC during bovine mastitis.

Effect of Scalding Temperature on Growth of Staphylococcus aureus and Formation of Staphylococcal Enterotoxin during the Production of Alpine Cheese in a Laboratory Cheesemaking Model

ABSTRACT

To reduce the number of cheese with potential Staphylococcus aureus contamination reaching consumers, European legislation has stipulated that all cheese must be tested for coagulase-positive staphylococci (CPS) at the point in production when numbers are expected to be highest. When CPS counts exceed 105 CFU/mL, staphylococcal enterotoxin (SE) tests must be conducted. When SE tests are positive, the cheese must be destroyed. Manufacturers of Swiss Alpine cheese are exempt from this legislation because SE formation in hard cheese is expected to be very unlikely because of the high scalding temperatures used for cheeses during production, which inactive CPS in the curd. However, this assumption has not been scientifically tested. A laboratory-scale cheese production experiment was performed in which the conditions corresponded to certain limitations in practical cheesemaking conditions such as temperature and time exposure as for production of Gruyere or Tete de Moine Swiss type cheeses. Raw milk aliquots (200 mL) were inoculated with five strains of CPS, and scalding temperatures of 46 to 56°C were used during cheese production. The temperatures applied after the curd was pressed were meant to reproduce the temperature curve in the peripheral zone of a real cheese wheel. Contrary to expectations, SE formation occurred and differed according to the scalding temperature (52 to 56°C). The differences in SE formation were more associated with strain type rather than temperature. These results indicate that the mechanisms of SE formation in cheese require further study.

HIGHLIGHTS

Transmission dynamics of Staphylococcus aureus within two Danish dairy cattle herds

Staphylococcus aureus is a major pathogen causing intramammary infections (IMI) in dairy cattle herds worldwide. Simulation models can be used to investigate the epidemiologic and economic outcomes of different control strategies against IMI. The transmission rate parameter is one of the most influential parameters on the outcomes of these simulation models. Very few studies have estimated the transmission rate parameter and investigated the transmission dynamics of Staph. aureus IMI in dairy cattle herds. The objective of our study was therefore to analyze the transmission dynamics of Staph. aureus in 2 Danish dairy herds participating in a longitudinal study. The 2 herds had 180 and 360 milking cows, and animals were tested at quarter level once per month over a period of 1 yr. We estimated the quarter-level prevalence to be 34% for herd 1 and 2.57% for herd 2. The daily quarter-level transmission rate was estimated to be 0.0132 and 0.0077 cases/quarter-day for herds 1 and 2, respectively, and the median duration of infection was estimated to be 91 and 64 d for herds 1 and 2, respectively. We also estimated the reproductive ratio at 1.21 for herd 1 and 0.52 for herd 2. The results can provide valuable information for simulation models to aid decision-making in terms of the prevention and control of Staph. aureus IMI in dairy cattle herds.

Development of a new real-time quantitative PCR assay for the detection of Staphylococcus aureus genotype B in cow milk, targeting the new gene adlb

The specific and reliable diagnosis of mastitis pathogens is essential for successful sanitation programs. The aim of the present study was to develop and evaluate a new real-time quantitative PCR (qPCR) assay for the very sensitive and specific detection of Staphylococcus aureus genotype B in cow milk samples. This mastitis pathogen is contagious and particularly prevalent in Switzerland and other central European countries. The new test is based on a rapid preparation of bacteria, followed by DNA isolation and qPCR for a unique target gene coding for the adhesion-like bovine protein (adlb). The inclusivity of the new target gene was 97% and the exclusivity 98%, meaning that other genotypes and bacterial species could be excluded with high reliability. The limit of detection of the new assay was 235 staphylococcal cell equivalents/mL of culture. The new test shows high intra- and interassay repeatability. Results are available within 2 d after sampling, allowing farmers and veterinarians to apply sanitation measures immediately. Based on the results of a preliminary field study, the diagnostic sensitivity and specificity of the new qPCR assay are 99 and 100%, respectively. The new analytical procedure is straightforward and can be applied for routine diagnostics.