MRSA of Unknown Origin: Risk Assessment and Adjustment of Risk Classification

The sources of MRSA have changed over the last years. Where MRSA initially presented as a hospital acquired pathogen, now the US and many European countries report high prevalences of community acquired (CA)-MRSA causing invasive infections and skin and soft tissue infections. In The Netherlands, the majority of these CA-MRSA are MRSA of unknown origin (MUO), i.e. the persons carrying these strains do not have any of the known risk factors as described in the current national guideline by the WIP. In The Netherlands, at least 350-400 MUO are detected yearly. These numbers result from the routine questionnaire accompanying the strain sent to the RIVM for confirmation and molecular typing. However, in 2007, 33% of all strains sent to the RIVM, no epidemiological data are given or these data are incomplete. Therefore, up until now, we do not know to what extent MUO is present in our country and what the source and transmission routes are. MUO can be spreading from the hospital to the community or they can be new MRSA strains spontaneously emerging in the community e.g. from zoonotic sources. In the recent past, pig farmers are found to have high risk of carrying MRSA and are therefore included in the WIP guideline in 2006. Since that time, these persons do not belong to the MUO-group, as they are already identified. Once risk factors of MUO have been elucidated these can and should be included in risk classification schemes of the national guidelines. Only after knowing risk factors for MUO, we can prevent these strains from dissemination in the community. The latter had happened in the USA where CA-MRSA is now invading in the hospitals and replacing the old hospital acquired strains [1], and, more importantly, they reported that CA-MRSA is now the most common identifiable source cause of skin and soft tissue infections in patients presenting to emerging departments in the USA [2]. In this and other countries, CA-MRSA and thus MUO are seen as a threatening infectious diseases, for which we do not have an answer or prevention program yet. Our study will focus on MUO. We aim 1. to resolve the sources and 2. mode(s) of transmission and 3. identify determinants of acquisition of MUO found in patients in The Netherlands. Finally, the identified risk factors and determinants will be translated into an algorithm to be included in the Search and Destroy program if feasible, and thus be used to timely identify persons at risk and prevent the MUO strains from spreading in the community and health care centers. The multicenter study group consist of important players in the field of prevention of MRSA colonization and infection. We will answer two study questions by means of a case-control study. Before we can perform the case control study, we first will identify as much as MUO as possible, by actively ask the laboratory for epidemiological data in case of incomplete questionnaire. Then, we will contact part of the identified MUO population to learn the common epidemiological determinants by using trawling questionnaires. After this enquiry, we can define controls and perform the prospective case control study. Cases are those detected with MRSA but without any known risk factor as described by the current WIP for acquisition of MRSA. Controls are defined by the study group after analysis of the retrospective MUO cases. Data acquisition will be by questionnaires, by mail or online. Questions will address e.g. occupation, work, school, leisure activities, having lived abroad for themselves or their close contacts, and other contacts with foreigners or visits abroad. We intend to include 500 cases and 1000 controls. We will genetically characterise MRSA strains defined as MUO, in comparison with those found among MRSA carriers with known risk factors for acquisition and Methicillin sensitive strains (MSSA). Finally, we will construct an algorithm or risk classification system for the early detection of MUO. If we will not be able to define the determinants of the emergence of MUO soon, there is a chance that we will end up with much higher endemicity of MRSA. To prevent increasing community diseases from MRSA, we have to identify the risk factors and subsequently prevent spreading of these MUO strains.