Mathematical studies on nosocomial spread of antibiotic-resistant bacteria

Abstract

Infections with antibiotic-resistant bacteria are a worldwide problem in hospitals and their rates remain high in many countries despite efforts to reduce the rates. Infection prevention is complicated by asymptomatic carriers. Using mathematical modelling, different intervention strategies were considered in this study. The costs and effects of four admission screening strategies followed by 100%, 50%, 25% and 10% effective isolation were estimated. As expected, screening of all patients at hospital admission is the most expensive but also most effective strategy to reduce methicillin-resistant Staphylococcus aureus (MRSA). Two other strategies, i) screening all patients at admission to intensive care units (ICUs) together with screening those patients with a history of MRSA colonization, and ii) screening only patients with a history of MRSA colonization are much cheaper, but still reduce the prevalence of MRSA significantly. These two strategies become cost-saving within 10 years, even when the efficacy of isolation is low, they result in the shortest time till return of investments, and are, therefore attractive candidates to control MRSA. Further, the contribution of persistently colonized health care workers (HCWs) to the transmission process in endemic settings was investigated. It is shown that, in most endemic settings, screening of patients, followed by isolation and/or decolonization is a more effective strategy than screening of HCWs followed by decolonization and/or replacement of identified carriers. The only exception is the situation when a low amount of persistently colonized HCWs is responsible for large amount of transmissions. The key parameters for the effectiveness of the interventions targeted at HCWs were determined: the fraction of colonized HCWs and the proportion of the MRSA acquisitions they are responsible for. Earlier, the study of Jain et al showed a reduction of almost 70% in infections with MRSA. This phenomenal reduction was ascribed to screening and isolation of patients, while the intervention consisted of complex bundle of measures. To prevent possible inappropriate expectations from single measures by policy makers, the maximal impact of screening and isolation of patients on MRSA infection rates was determined. Using a mathematical model, it was shown that only a small fraction of the remarkable reduction in infection rate described by Jain et al. could be the result of transmission prevention. Transmission of Enterobacteriaceae that are resistant to third-generation cephalosporins from patient to patient is only one of possible routes of spreading resistant genes, but it can be controlled by standard measures. Transmission capacities of E. coli and non-E. coli (mainly Klebsiella) Enterobacteriaceae were estimated. It was shown, that non-E. coli bacteria are 3.7 times more transmissible than E. coli in the same conditions. The estimations were based on the data from 24 months extensive microbiological surveillance in 13 ICUs. Still the per admission reproduction numbers were below one for both bacteria, which means the transmission of the bacteria in ICUs is already low. In such cases additional measures targeted on prevention of transmission within hospitals are unlikely to be very effective.