Antimicrobial Selection for the Treatment of Clinical Mastitis and the Efficacy of Penicillin Treatment Protocols in Large Estonian Dairy Herds

Clinical mastitis (CM) is the most common microbial disease treated in dairy cows. We analyzed the antimicrobial usage in cows with CM (n = 11,420) in large dairy herds (n = 43) in Estonia. CM treatment data were collected during a 12-month study period. The antimicrobial usage was observed during the 21 days from the initiation of treatment, and the incidence of antimicrobial-treated CM was calculated for each study herd. The effect of intramammary (IMM), systemic, and combined (systemic and IMM) penicillin treatment of CM on the post-treatment somatic cell count (SCC) was analyzed using the treatment records of 2222 cows from 24 herds with a mixed multivariable linear regression model. The median incidence of antimicrobial-treated CM was 35.8 per 100 cow-years. Procaine benzylpenicillin and marbofloxacin were used in 6103 (35.5%, 95% CI 34.8–36.2) and 2839 (16.5%, 95% CI 16.0–17.1) CM treatments, respectively. Post-treatment SCC was higher after IMM penicillin therapy compared to systemic or combination therapy. Treatment of CM usually included first-choice antimicrobials, but different antimicrobial combinations were also widely used. The effect of procaine benzylpenicillin to post-treatment SCC was dependent on the administration route, cow parity, and days in milk. Further studies should evaluate the factors affecting veterinarians’ choice of antimicrobial used in the treatment of CM.

Antimicrobial Usage for the Management of Mastitis in the USA: Impacts on Antimicrobial Resistance and Potential Alternative Approaches

Mastitis is the most frequently diagnosed disease of dairy cattle responsible for the reduction in milk quantity and quality and major economic losses. Dairy farmers use antibiotics for the prevention and treatment of mastitis. Frequent antimicrobial usage (AMU) undeniably increased antimicrobial resistance (AMR) in bacteria from dairy farms. Antimicrobial-resistant bacteria (ARB) from dairy farms can spread to humans directly through contact with carrier animals or indirectly through the consumption of raw milk or undercooked meat from culled dairy cows. Indirect spread from dairy farms to humans can also be through dairy manure fertilized vegetables or run-off waters from dairy farms to the environment. The most frequently used antibiotics in dairy farms are medically important and high-priority classes of antibiotics. As a result, dairy farms are considered one of the potential reservoirs of ARB and antimicrobial resistance genes (ARGs). To mitigate the rise of ARB in dairy farms, reducing AMU by adopting one or more of alternative disease control methods such as good herd health management, selective dry-cow therapy, probiotics, and others is critically important. This chapter is a concise review of the effects of antimicrobials usage to control mastitis in dairy cattle farms and its potential impact on human health.

Estimates of Antibacterial Consumption in Timor-Leste Using Distribution Data and Variation in Municipality Usage Patterns

The association between antimicrobial resistance and antimicrobial usage has become a growing global concern. Many lower-middle income countries including Timor-Leste (TL) have limited information on antimicrobial usage, although recent research suggests increasing resistance rates among human pathogens there. The aim of this study was to use distribution data to estimate antibiotic consumption at both the national and sub-national level in Timor-Leste, stratifying into resistance class and adherence to the national essential medicines list (EML) and WHO AWaRe guidelines. A retrospective review of distribution data from Timor-Leste central medical store (SAMES) was undertaken to give a defined daily dose (DDD)/1000 inhabitants/day using WHO methodology. National antibiotic distribution in the TL EML in 2019 was estimated at 11.1 DDD/1000 inhabitants/day, comparable to consumption rates observed in other lower-middle-income countries using similar methodology. Differences in distribution quantities were noted between municipalities, with 4 of the 13 municipalities notably above the national average and around 32% of listed restricted antimicrobials distributed incongruent with the EML. This study provides insights into estimated antimicrobial consumption in Timor-Leste that has previously been poorly defined. Estimates of consumption can be used to understand emerging resistance in this small island nation, add to the body of knowledge on antimicrobial use to advise policy and guideline development, and help with stewardship activities.

The Genetic Relatedness and Antimicrobial Resistance Patterns of Mastitis-Causing Staphylococcus aureus Strains Isolated from New Zealand Dairy Cattle

Staphylococcus aureus is one of the leading causes of bovine mastitis worldwide and is a common indication for use of antimicrobials on dairy farms. This study aims to investigate the association between on-farm antimicrobial usage and the antimicrobial resistance (AMR) profiles of mastitis-causing S. aureus. Whole-genome sequencing was performed on 57 S. aureus isolates derived from cows with either clinical or subclinical mastitis from 17 dairy herds in New Zealand. The genetic relatedness between isolates was examined using the core single nucleotide polymorphism alignment whilst AMR and virulence genes were identified in-silico. The association between gene presence-absence and sequence type (ST), antimicrobial susceptibility and dry cow therapy treatment was investigated using Scoary. Altogether, eight STs were identified with 61.4% (35/57) belonging to ST-1. Furthermore, 14 AMR-associated genes and 76 virulence-associated genes were identified, with little genetic diversity between isolates belonging to the same ST. Several genes including merR1 which is thought to play a role in ciprofloxacin-resistance were found to be significantly overrepresented in isolates sampled from herds using ampicillin/cloxacillin dry cow therapy. Overall, the presence of resistance genes remains low and current antimicrobial usage patterns do not appear to be driving AMR in S. aureus associated with bovine mastitis.

Milk microbiome in dairy cattle and the challenges of low microbial biomass and exogenous contamination

Background The blanket usage of antimicrobials at the end of lactation (or “drying off”) in dairy cattle is under increasing scrutiny due to concerns about antimicrobial resistance. To lower antimicrobial usage in dairy farming, farmers are now encouraged to use “selective dry cow therapy” whereby only cows viewed as at high risk of mastitis are administered antimicrobial agents. It is important to gain a better understanding of how this practice affects the udder-associated microbiota and the potential knock-on effects on antimicrobial-resistant bacterial populations circulating on the farm. However, there are challenges associated with studying low biomass environments such as milk, due to known contamination effects on microbiome datasets. Here, we obtained milk samples from cattle at drying off and at calving to measure potential shifts in bacterial load and microbiota composition, with a critical assessment of contamination effects. Results Several samples had no detectable 16S rRNA gene copies and crucially, exogenous contamination was detected in the initial microbiome dataset. The affected samples were removed from the final microbiome analysis, which compromised the experimental design and statistical analysis. There was no significant difference in bacterial load between treatments (P > 0.05), but load was lower at calving than at drying off (P = 0.039). Escherichia coli counts by both sequence and culture data increased significantly in the presence of reduced bacterial load and a decreasing trend of microbiome richness and diversity. The milk samples revealed diverse microbiomes not reflecting a typical infection profile and were largely comprised of gut- and skin-associated taxa, with the former decreasing somewhat after prolonged sealing of the teats. Conclusions The drying off period had a key influence on microbiota composition and bacterial load, which appeared to be independent of antimicrobial usage. The interactions between drying off treatment protocol and milk microbiome dynamics are clearly complex, and our evaluations of these interactions were restricted by low biomass samples and contamination effects. Therefore, our analysis will inform the design of future studies to establish whether different selection protocols could be implemented to further minimise antimicrobial usage.

592. Antimicrobial Utilization in Solid Organ Transplant Recipients 12-Months Post-Transplantation

Background Antimicrobials are widely used in solid organ transplant recipients (SOTr). Yet, antimicrobial utilization in the transplant (TP) population is not well characterized. National Healthcare Safety Network antimicrobial use (NHSN-AU) does not provide data specific to SOTr. This study sought to describe inpatient antibiotic use among SOTr up to 1-year post-TP. Methods A cross-sectional study was performed of all SOTr who received a TP between January 2015 to December 2016. Demographics, TP type, antibiotic use variables, hospital days, and Clostridioides difficile infection (CDI) are described. Inpatient antibiotic administration was measured for 365 days starting from date of TP surgery. Automated data generated for NHSN-AU reporting was utilized, and SOTr data was abstracted by cross-matching with the transplant database. Transplant-patient days was used as the denominator for metrics. Variables included duration of therapy (DOT), DOT/1000 patient days, antimicrobial free days (inpatient days no antimicrobials were administered), and NHSN-AU reporting targets of anti-methicillin resistant S. aureus (MRSA), broad spectrum, and high-risk CDI agents. Data was analyzed using descriptive statistics via Microsoft Excel®. Results A total of 530 SOTr were analyzed. Baseline characteristics are shown in Table 1. Median age was 61, male gender 64%, median Charlson Comorbidity Index was 5. Kidney TP (43%), liver TP (32%), lung (9%) and heart (8%) were most common TP types. Among these four TP types: Lung TP had the highest median DOT (13 days), DOT/1000 patient days (6.6) and ratio of DOT/total patient (1.9) (Table 2). Liver TP had the most antimicrobial free days (34%). Proportionally, anti-MRSA agents use was highest in thoracic TP (lung/heart), broad-spectrum agent use was common in all but kidney TPs, and high-risk CDI agents use was highest among kidney TP (Table 3). A total of 34 SOTr had CDI, 76% in kidney/liver TPs. Table 1. Antimicrobial usage and SOT - ID Week 2021 Table 2. Antimicrobial usage and SOT - ID Week 2021 Table 3. Antimicrobial usage and SOT - ID Week 2021 Conclusion Our study provides preliminary and important data of inpatient antibiotic utilization specifically in SOTr, generated using automated NHSN-AU data cross-matched to transplant database. These metrics can be utilized to promote antimicrobial stewardship efforts directed to specific TP types. Disclosures Rachel Kenney, PharmD, Medtronic, Inc. (Other Financial or Material Support, spouse is an employee and shareholder)

A genomic epidemiological study shows that prevalence of antimicrobial resistance in Enterobacterales is associated with the livestock host, as well as antimicrobial usage

Enterobacterales from livestock are potentially important reservoirs for antimicrobial resistance (AMR) to pass through the food chain to humans, thereby increasing the AMR burden and affecting our ability to tackle infections. In this study 168 isolates from four genera of the order Enterobacterales , primarily Escherichia coli , were purified from livestock (cattle, pigs and sheep) faeces from 14 farms in the United Kingdom. Their genomes were resolved using long- and short-read sequencing to analyse AMR genes and their genetic context, as well as to explore the relationship between AMR burden and on-farm antimicrobial usage (AMU), in the three months prior to sampling. Although E. coli isolates were genomically diverse, phylogenetic analysis using a core-genome SNP tree indicated pig isolates to generally be distinct from sheep isolates, with cattle isolates being intermediates. Approximately 28 % of isolates harboured AMR genes, with the greatest proportion detected in pigs, followed by cattle then sheep; pig isolates also harboured the highest number of AMR genes per isolate. Although 90 % of sequenced isolates harboured diverse plasmids, only 11 % of plasmids (n=58 out of 522) identified contained AMR genes, with 91 % of AMR plasmids being from pig, 9 % from cattle and none from sheep isolates; these results indicated that pigs were a principle reservoir of AMR genes harboured by plasmids and likely to be involved in their horizontal transfer. Significant associations were observed between AMU (mg kg−1) and AMR. As both the total and the numbers of different antimicrobial classes used on-farm increased, the risk of multi-drug resistance (MDR) in isolates rose. However, even when AMU on pig farms was comparatively low, pig isolates had increased likelihood of being MDR; harbouring relatively more resistances than those from other livestock species. Therefore, our results indicate that AMR prevalence in livestock is not only influenced by recent AMU on-farm but also livestock-related factors, which can influence the AMR burden in these reservoirs and its plasmid mediated transmission.