Fomites in infectious disease transmission [electronic resource] : a modeling, laboratory, and field study on microbial transfer between skin and surfaces

This dissertation examines the factors that influence fomite-mediated (e.g., indirect contact) transmission of viral gastrointestinal and respiratory illness. Specifically, the dissertation investigates virus transfer between surfaces and virus recovery from surfaces, models human-fomites interactions to estimate exposure and infection risk, and elucidates causal links between microbial contamination and illness in child care centers. Indirect contact transmission refers to person-to-person transmission of disease via an intermediate fomite (e.g., inanimate object acting as a carrier of infectious disease). The role of indirect contact in disease spread is poorly understood in part because the transmission route of viral pathogens is often difficult to determine. Transmission of respiratory and gastrointestinal viruses can occur through multiple routes (e.g., direct contact, indirect contact, airborne, and common vehicle), and the relative contribution of each route to total disease burden is unclear. The first study in this dissertation examines virus transfer between skin and surfaces, a necessary step in fomite-mediated transmission of viral disease. In the study, transfer of virus between fingerpads and fomites is explored in a laboratory setting. Bacteriophage (fr, MS2, and PHIX174) are used as proxies for pathogenic virus, and over 650 unique transfer events are collected from 20 different volunteers. The study concludes that approximately one quarter (23%) of recoverable virus is readily transferred from a contaminated surface (e.g., a fomite) to an uncontaminated surface (e.g., a finger) on contact. Using the large data set, the direction of transfer (from fingerpads-to-fomite or fomite-to-fingerpad) and virus species are demonstrated to both significantly influence the fraction of virus transferred by approximately 2-5%. To investigate the relative importance of factors contributing to fomite-mediated transmission, a child's risk of illness from exposure to a contaminated fomite is modeled. Specifically, the model estimates a child's exposure to rotavirus using a stochastic-mechanistic framework. Simulations of a child's contacts with the fomite include intermittent fomite-mouth, hand-mouth, and hand-fomite contacts based on activities of a typical child under six years of age. In addition to frequency of contact data, parameters estimated for use in the model include virus concentration on surface; virus inactivation rates on hands and the fomite; virus transfer between hands, fomite, and the child's mouth; and the surface area of objects and hands in contact. From the model, we conclude that a child's median ingested dose from interacting with a rotavirus-contaminated ball ranges from 2 to 1,000 virus over a period of one hour, with a median value of 42 virus. These results were heavily influenced by selected values of model parameters, most notably, the concentration of rotavirus on fomite, frequency of fomite-mouth contacts, frequency of hand-mouth contacts, and virus transferred from fomite to mouth. The model demonstrated that mouthing of fomite is the primary exposure route, with hand mouthing contributions accounting for less than one-fifth of the child's dose over the first 10 minutes of interaction. Based on the findings from the model that concentration of virus on a fomite influences a child's risk of illness, we investigate methods to recover virus from fomites. In a literature review and subsequent meta analysis, we demonstrate that the outcome currently used to describe virus contamination, positivity rate, is biased by the authors' selected sampling methods. We follow up, in the laboratory, with a comparison of the identified methods and demonstrate that polyester-tipped swabs prewetted in 1/4-strength Ringer's solution or saline solution is the most efficient sampling method for virus recovery tested. The recommended method is compatible with plaque assay and quantitative reverse-transcription polymerase chain reaction, two techniques used to quantify virus. The link between hand / fomite contamination and infection risk was explored in a field study at two child care centers over four months. Both respiratory and gastrointestinal disease incidence were tracked daily, while hand and environmental surface contamination were monitored weekly between February 2009 and June 2009. Microbial contamination was determined using quantitative densities of fecal indicator bacteria (e.g. Escherichia coli, enterococci, and fecal coliform) on hands and fomites as well as presence/absence of viral pathogens (e.g. enterovirus and norovirus). Health was monitored daily by childcare staff, who tracked absences, illness-related absences, and symptomatic respiratory and gastrointestinal illness. The resultant data suggests that increases in microbial contamination led to increases in symptomatic respiratory illness four to six days later, in agreement with typical incubation periods for respiratory illness. Similarly, respiratory illness led to increases in microbial contamination on hands during presentation of symptoms, and on fomites in the following three days.

Advisor: Boehm Alexandria primary advisor. Thesis advisor
Canales Robert A. (Robert Anthony) advisor. Thesis advisor
Leckie Jim