MRSA – How to Prevent and Deal with it in Real Life

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is a bacterium that can be identified in the nasal cavity of almost 10% of horses admitted to equine hospitals [1]. Methicillin resistance is mediated by the production of an altered penicillin-binding protein (PBP). The gene that encodes this altered PBP is called mecA and is located on a mobile genetic element, facilitating easy spread of antimicrobial resistance between bacteria. This gene causes resistance against all beta-lactam antibiotics, including cephalosporins. Often, MRSA isolates are also resistant to many other antimicrobials. When MRSA causes an infection, it can therefore be very difficult to treat.

MRSA in skin disease

Staphylococcal cutaneous infection is often secondary, resulting from an impaired skin barrier due to trauma, ectoparasites or allergies. All dermatological conditions caused by infections with Staphylococcus aureus, such as (pastern) folliculitis/ furunculosis, cellulitis, subcutaneous abscesses and pyoderma, can also be caused by MRSA. However, the majority are caused by Staphylococcus aureus isolates that are not methicillin- resistant. MRSA is more frequently isolated from infected wounds.

MRSA in healthcare-associated infections (HAIs)

MRSA is associated with several HAIs, such as surgical site infections and catheter site infections (causing thrombophlebitis). In some instances, MRSA can enter the bloodstream (sepsis) and disseminate to other organs, such as the lungs to cause pneumonia. This can occur in foals as well as adults and is often life-threatening.

Diagnosis of MRSA infections

Cytology, bacteriological culturing and/or biopsy can all be used to confirm a diagnosis of an MRSA infection. Bacteriological culturing has the advantage that antimicrobial susceptibility can be determined to aid the clinician in selecting antimicrobial drugs for treatment. This is important, as antimicrobial susceptibility patterns of MRSA isolates can be unpredictable.

Treatment of MRSA infections

If MRSA infections are suspected to be secondary, addressing the primary underlying cause/disease is essential. Clipping of hairs and repeated wound cleaning are indicated to remove matted hairs and debris. If possible, drainage of purulent material should be established. Superficial infections can be managed by topical treatment with chlorhexidine shampoo. Severe (widespread superficial or deep) infections also require antimicrobial treatment. MRSA isolates often are susceptible to TMS, making this a good choice for initial treatment while awaiting bacteriological culture and susceptibility testing results. Deep infections generally require a long duration of treatment (minimum of 2–3 weeks). Patients with exudative MRSA infections should be isolated to prevent spread. Also, people with conditions potentially compromising their immune system (young, old, pregnant, immunosuppressed persons) should not handle infected horses as MRSA has zoonotic potential.

MRSA prevention and hospital hygiene

Hospital hygiene is of utmost importance to prevent MRSA infections in patients. Up to 20% of veterinarians and staff in equine hospitals have been reported to be colonised by MRSA in their nasal cavity [2]. Unintentional transfer of micro- organisms during surgery or treatment may therefore occur. Hand hygiene (washing and disinfecting hands regularly and wearing gloves while handling patients) is one of the most important steps to take to prevent (spread of) MRSA within the hospital. Implementation of a standardised protocol for presurgical hand asepsis can aid in prevention of surgical site infections [3]. Isolation of patients with exudative (surgical site) wound infections can aid in preventing spread of MRSA within the hospital. Regular cleaning and disinfection of stables and equipment (especially twitches!) can prevent the establishment of MRSA reservoirs and spread of MRSA in the hospital. Office surfaces, such as keyboards, desks and tables, can also serve as reservoirs for MRSA and should therefore also be cleaned and disinfected on a regular basis. Routine collection and culturing of environmental samples in the hospital can help identify and monitor local reservoirs. Whole genome sequencing can be used as a molecular typing tool for MRSA outbreak investigation. Data from routine screening and outbreak investigation can be used to design and implement an infection prevention and control protocol (IPC) which can significantly reduce MRSA prevalence and the occurrence of HAIs.

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