Changing epidemiology of methicillin-resistant Staphylococcus aureus in a low endemicity area-new challenges for MRSA control

Abstract

The incidence of methicillin-resistant Staphylococcus aureus (MRSA) has increased sharply in Hospital District of Southwest Finland (HD). To understand reasons behind this, a retrospective, population-based study covering 10 years was conducted. All new 983 MRSA cases in HD from January 2007 to December 2016 were analysed. Several data sources were used to gather background information on the cases. MRSA cases were classified as healthcare-associated (HA-MRSA), community-associated (CA-MRSA), and livestock contact was determined (livestock-associated MRSA, LA-MRSA). Spa typing was performed to all available strains. The incidence of MRSA doubled from 12.4 to 24.9 cases/100000 persons/year. The proportion of clinical infections increased from 25 to 32% in the 5-year periods, respectively, (p < 0.05). The median age decreased from 61 years in 2007 to 30 years in 2016. HA-MRSA accounted for 68% of all cases, of which 32% associated with 26 healthcare outbreaks. The proportion of CA-MRSA cases increased from 13% in 2007 to 43% in 2016. Of CA-MRSA cases, 43% were among family clusters, 32% in immigrants and 4% were LA-MRSA. The Gini-Simpson diversity index for spa types increased from 0.86 to 0.95 from the first to the second 5-year period. The proportion of a predominant strain t172 decreased from 43% in 2009 to 7% in 2016. The rise in the proportion of CA-MRSA, the switch to younger age groups, the complexity of possible transmission routes and the growing spa-type diversity characterize our current MRSA landscape. This creates challenges for targeted infection control measures, demanding further studies.

Keywords: Community-associated; Epidemiology; Family cluster; Infection control; MRSA; spa type.

Fig. 1

Incidence (per 100,000 inhabitants) of methicillin-resistant Staphylococcus aureus (MRSA) in Hospital District of Southwest Finland and in the whole country in 1996–2016. Source: Finnish Institute for Health and Welfare

Fig. 2

Age groups and median age of MRSA cases in 2007–2016 in Hospital District of Southwest Finland. The columns show yearly proportions (%) of age groups (age in years), and the line indicates the yearly median age of MRSA cases

Fig. 3

MRSA cases and MRSA screening in Hospital District of Southwest Finland in 2007–2016. The graph columns show the number of MRSA cases by year divided in community-associated MRSA (CA-MRSA), solitary healthcare-associated MRSA (HA-MRSA) and outbreak-associated HA-MRSA cases. The line indicates the number of persons screened yearly with its scale on the right side of the graph. The timeline below the graph shows changes in the indications for MRSA screening in the Hospital District of Southwest Finland

Fig. 4

The spa types of altogether 976 MRSA strains in Hospital District of Southwest Finland in 2007–2011 and in 2012–2016. Each stripe indicates one spa type, and the height of the stripe represents the proportion of the type in each time period. The fifteen most common spa types (identified in at least 10 cases) are listed by type name, following by the number of cases. NT = non-typeable