Community acquired MRSA: determinants of carriage, transmissability and virulence

Methicillin-resistant Staphylococcus aureus is increasing all over the world, mainly due to the recent emergence of MRSA in the community. This so called Community Acquired MRSA (CA-MRSA) is also increasing in The Netherlands. As the determinants for carriage are not known, CA-MRSA poses a serious treat to the Dutch MRSA control strategy. To define preventive interventions in the community and to improve the control measures in hospitals more knowledge is needed on the reservoirs and transmission routes in the general population. At present the prevalence of MRSA carriage in the community in The Netherlands is extremely low (<1/1000 inhabitants). Therefore a population based survey is not feasible. A case-control study is proposed to study the determinants of carriage of CA-MRSA. After ascertainment of cases and controls, they are followed prospectively to determine the dynamics of carriage, the associated burden of disease and the socio-economic consequences. The low prevalence of MRSA carriage in The Netherlands is a drawback but also an advantage for this study. New evolutionary trends can be studied most precisely in a low prevalence setting. In other countries with a relatively high background of MRSA with a putative origin in the healthcare setting it is likely that a relatively large proportion of the cases and control will be misclassified. Recruitment of cases and controls is performed in a network of approximately 10 hospitals and their affiliated microbiological laboratories serving a catchment population of more than three million inhabitants. Case-definitions are based on epidemiological characteristics with the objective to select for carriers of MRSA without known risk factors for MRSA carriage, who have most likely acquired their strains from a reservoir in the community (CA-MRSA). Two control groups are selected. One group consists of MRSA-carriers with known risk factors for MRSA carriage and the other consists of individuals who don’t carry MRSA. Risk factors for MRSA carriage for cases, controls and their household members are determined based on a questionnaire taken during a home visit by a public health physician. MRSA strains will be characterised using molecular techniques. The three study groups and their household members are followed for one year to determine the dynamics of carriage, the development of disease and the socio-economic consequences. A screening of contacts in the community, not being household members, will be performed if participants still carry MRSA at the end of follow up. Based on very strict definitions in a low prevalence setting this study will enable the identification of carriers of true CA-MRSA. This will provide a precise estimate of determinants of CA-MRSA carriage. This is important to define targeted and cost-effective control measures, both in the community and in the healthcare setting. Further molecular characterisation may identify strain characteristics that indicate CA-MRSA, enabling early recognition and facilitating future epidemiological studies. Also, the transmissability in the community will be determined as well as the associated burden of disease in comparison with other types of MRSA and with non-carriers. This will provide an estimate of the impact on public health of this emerging pathogen.