Transmissibility of Livestock-associated Methicillin-Resistant Staphylococcus aureus

David J. Hetem

, Martin C.J. Bootsma, Annet Troelstra, and Marc J.M. Bonten

Author affiliations: University Medical Center Utrecht, Utrecht, the Netherlands (D.J. Hetem, M.C.J. Bootsma, A. Troelstra, M.J.M. Bonten); University of Utrecht, Utrecht (M.C.J. Bootsma)

Abstract

Previous findings have suggested that the nosocomial transmission capacity of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) is lower than that of other MRSA genotypes. We therefore performed a 6-month (June 1–November 30, 2011) nationwide study to quantify the single-admission reproduction number, R_A, for LA-MRSA in 62 hospitals in the Netherlands and to compare this transmission capacity to previous estimates. We used spa typing for genotyping. Quantification of R_A was based on a mathematical model incorporating outbreak sizes, detection rates, and length of hospital stay. There were 141 index cases, 40 (28%) of which were LA-MRSA. Contact screening of 2,101 patients and 7,260 health care workers identified 18 outbreaks (2 LA-MRSA) and 47 secondary cases (3 LA-MRSA). R_A values indicated that transmissibility of LA-MRSA is 4.4 times lower than that of other MRSA (not associated with livestock).

Methicillin-resistant Staphylococcus aureus (MRSA) is one of the leading causes of nosocomial infections and leads to considerable illness, death, and health care costs (1,2). The worldwide epidemiology of MRSA has changed as MRSA originating in the community has increased. These community-associated MRSA (CA-MRSA) strains are replacing their hospital-associated counterparts in hospitals in the United States; the major dominant clone is MRSA strain USA300 (3). In recent years, another MRSA clone, which originated in the community and is associated with exposure to livestock, has emerged in different countries worldwide, including the United States (4,5). Even more worrying, countries with a historically low prevalence of MRSA, like the Netherlands and Denmark, have seen an increase in livestock-associated MRSA (LA-MRSA), belonging to clonal complex 398 (5). In the Netherlands, LA-MRSA accounted for 39% of all new MRSA isolated in 2011 (6). Yet almost all isolates have been detected through screening, and in 2009, nine infections were caused by MRSA sequence type 398 (7). Invasive infections caused by LA-MRSA include endocarditis, osteomyelitis, and ventilator-associated pneumonia (8,9).