Epidemiology

The global prevalence of MRSA infection was increasing until around 2006.[4][5][6]​​ Since then, surveillance programmes have reported a steady decrease in the incidence of MRSA infections in community and healthcare settings.[7] Hospital-onset MRSA bloodstream infections in the US declined by around 17% per year from 2005-2012, but this decline slowed between 2013-2016.[8]​ A similar pattern has been seen in the UK and the EU thought to be linked to national infection control programmes introduced around the early 2000s.[9][10]​​​

In Europe, the percentage of MRSA isolates reported during the period 2017-2021 decreased from 18.4% to 15.8%.[10]​ However, MRSA is still an important pathogen in Europe, with percentages remaining high in several countries; in 2021, 30% of countries in the WHO European Region reported MRSA percentages equal to or above 25%.[10]​ There are large intercountry variations for MRSA, with generally higher antimicrobial resistance percentages reported from southern and eastern Europe than northern Europe.[10]

There is some uncertainty with regards to these figures as studies based on the European antimicrobial resistance surveillance network data for the period 2005-2018 highlighted the fact that the decrease in the percentage of MRSA among S aureus bloodstream infections was mainly due to the increasing number of methicillin-susceptible S aureus bloodstream infections and the quoted figures relate to MRSA as a percentage of S aureus infections. This highlights the need to improve surveillance to include data on the number and incidence of infections as well as MRSA percentages of S aureus infections.[11]​​ Of note, in the US, hospital-onset methicillin-susceptible Staphylococcus aureus (MSSA) bloodstream infections have not changed and community-onset MSSA infections increased from 2012-2017.[8]

MRSA was once only associated with hospital settings. Since the 1980s however, community-acquired MRSA infections have been rising in frequency and reservoirs exist in both settings. The indiscriminate use of antimicrobial agents in agriculture has largely contributed to the wide distribution of MRSA among livestock. Humans in contact with livestock may be at high risk of becoming colonised and infected with livestock-associated MRSA.[12]​​

Prevalence of carriers of MRSA in the US is estimated to be between 1% and 2% of the population. Factors associated with MRSA carriage include recent hospitalisation, recent outpatient visit, nursing-home exposure, chronic disease, illicit intravenous drug use, or contact with a person with known risk factors. One surveillance study of 6 sites in the US observed that the risk of developing an invasive MRSA infection was 16.3 times higher among people who inject drugs.[13] For those without healthcare-related risk factors, prevalence of colonisation in the US is estimated to be 1.25%.[14] Children and younger adults (median age 22 years) are more commonly afflicted with community-associated MRSA, while hospital-associated MRSA is more common in older age groups (median age 64 years).[4][15][16] Colonisation is believed to precede infection and is, therefore, considered a risk factor for invasive MRSA infection. The primary site of MRSA colonisation is the anterior nares, although non-nasal sites (e.g., throat, axilla, inguinal area, and peri-rectal area) can also be colonised.

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