The surface contamination network and its application in fomite infection studies

Abstract

Pathogen-contaminated surfaces and objects, also called fomites, are known to transmit infections via touching, and thus surface/hand hygiene is an important intervention for infection prevention. However, unlike airborne and close contact routes, our understanding of fomite route infection transmission remains at the empirical level. In this study, we first developed two approaches, i.e., a mathematical model and computer simulations, to explore the speed at which surfaces become contaminated. Both methods revealed the logistic growth of the number of contaminated surfaces when there was no or low-frequency hand/surface hygiene, indicating that infections could be transmitted quickly and effectively via the fomite route. Our work also suggested that infections transmitted to a remote location were not a unique feature of the airborne route as commonly believed. We also explored how hand/surface hygiene frequency could be used to control the growth of contaminated surfaces, and found a critical condition for controlling the growth of contaminated surfaces and hands. To verify our model, we simulated two inflight norovirus outbreaks, and the transmission of the norovirus in cabin air and the spatial infection pattern in the two outbreaks were well explained by the simulation results. Furthermore, it is commonly believed that some respiratory and enteric infections are transmitted through multiple routes, including airborne, large droplet and fomite routes. Knowledge about the relative importance of different transmission route(s) is fundamental to developing effective intervention strategies for infectious diseases. We therefore developed a multi-route infection transmission model in a cabin air environment, and a surface contamination network was constructed for the first time to study the fomite route transmission. A comparative analysis of inflight outbreaks of the norovirus, influenza A (H1N1) and severe acute respiratory syndrome coronavirus (SARS CoV) was done. We found that different transmission routes of infections led to different spatial patterns of secondary cases. We determined that the dominant transmission routes in the cabin air were close contact (large droplet) for influenza, the fomite route for the norovirus and all three routes for SARS CoV. Our comparative analysis may become a new approach for disease outbreak analyses in the future. We subsequently developed a fomite route methicillin-resistant Staphylococcus aureus (MRSA) transmission model in hospitals to explore effective surface cleaning strategies to prevent fomite route MRSA transmission. The roles of different surfaces were explored for the first time. Patients close to high-touch surfaces were found to play the most important role in fomite route MRSA transmission in a hospital. Based on the different roles of different surfaces, we found that not all surfaces needed to be cleaned equally. Under a constant cleaning workload, the optimal distribution of the cleaning workload on different surfaces with different contact rates by individuals should be proportional to the frequency with which they are touched, which agrees with the current Healthcare Infection Control Practices Advisory Committee recommendations. Lastly, from the epidemic perspective, we explored the effectiveness of contact precaution (CPs) on preventing contact transmission of MRSA in an intensive care unit (ICU) under different situations. We built a MRSA transmission dynamics model to estimate the effectiveness of CPs, and found that CPs were more effective when the MRSA prevalence was relatively high.