Methicillin-resistant Staphylococcus aureus outbreak in a Dutch equine referral clinic.

The research investigates outbreaks of Methicillin-resistant Staphylococcus aureus (MRSA), a type of bacterial infection, in a Dutch equine clinic. Using Nanopore whole-genome sequencing, researchers identified three different MRSA strains and tracked their transmission within the clinic environment, staff, and horse population.

Approach to Research

• The research team focused on nine cases of MRSA infections in horses that were identified in a Dutch horse referral clinic during the years 2020 and 2022.
• A total of 16 MRSA isolates from horses, the clinic environment, and clinic staff members were retrospectively analyzed using Nanopore whole-genome sequencing. This technique exposes the comprehensive genetic information of the bacteria, allowing the investigators to classify the infections and track their transmission.

Finding of the Research

• The MRSA isolates were identified as strains ST398 and ST612. These labels refer to genetic “sequence types” that differentiate variants of MRSA.
• Three distinct clusters of ST398 were identified from the 2020 and 2022 samples. The first cluster was suspected to have spread within the clinic due to similar isolate identification in five horses and one staff member.
• The second cluster from 2022 showed a higher level of genetic variation and was found in two horses and two staff members. One additional ST398 sample from a staff member was determined to be unrelated to the other clusters.
• In 2022, ST612 isolates were identified in two horses and three environmental samples. The low level of genetic variation between these samples suggests the transmission of MRSA ST612 within the clinic during this year.

Characteristics of the Isolates

• The research team performed a comprehensive molecular characterization of the MRSA samples, revealing a considerable number of virulence genes and plasmids in the ST612 isolates compared to the ST398 isolates. These elements are associated with the bacteria’s ability to spread and cause disease.
• Antimicrobial susceptibility tests demonstrated differences between the two MRSA strains. These findings were consistent with their respective genetic characteristics.

Significance and Implications

• The research team observed that ST612, a dominant MRSA strain in African hospitals, had not been reported in Europe before. However, it has been identified in both horses and people interacting with horses in Australia.
• The investigation results underscore the importance of continuous surveillance not only for the prevention and containment of MRSA outbreaks within equine clinics, but also for tracking the global distribution of these bacteria strains.