Data below detection limits, left-censored data, are common in environmental microbiology, and decisions in handling censored data may have implications for quantitative microbial risk assessment (QMRA). In this paper, we utilize simulated data sets informed by real-world enterovirus water data to evaluate methods for handling left-censored data. Data sets were simulated with four censoring degrees (low (10%), medium (35%), high (65%), and severe (90%)) and one real life censoring example (97%) and were informed by enterovirus data assuming a lognormal distribution with a 2.3 genome copies/L limit of detection (LOD). For each data set, 5 methods for handling left-censored data were applied: 1) Substitution with LOD/√2, 2) Lognormal maximum likelihood estimation (MLE) to estimate mean and standard deviation, 3) Kaplan-Meier estimates (KM), 4) Imputation method using MLE to estimate distribution parameters (MI Method 1), 5) Imputing from a uniform distribution (MI Method 2). Each data set mean was used to estimate enterovirus dose and infection risk. Root mean square error (RMSE) and bias were used to compare estimated and known doses and infection risks. MI Method 1 resulted in lowest dose and infection risk RMSE and bias ranges for most censoring degrees, predicting infection risks at most 1.17 x 10^-2 from known values under 97% censoring. MI Method 2 was the next overall best method. For medium to severe censoring, MI Method 1 may result in the least error. If unsure of the distribution, MI Method 2 may be a preferred method to avoid distribution misspecification. IMPORTANCE This study evaluates methods for handling low (10%) to severe (90%) left-censored data within an environmental microbiology context, and demonstrates that some of these methods may be appropriate when using data containing concentrations below a limit of detection to estimate infection risks. Additionally, this study uses a skewed data set, which is an issue typically faced by environmental microbiologists.

Norovirus accounts for a large portion of the gastroenteritis disease burden, and outbreaks have occurred in a wide variety of environments. Understanding the role of fomites in norovirus transmission will inform behavioral interventions, such as hand washing and surface disinfection. The purpose of this study was to estimate the contribution of fomite-mediated exposures to infection and illness risks in outbreaks. A simulation model in discrete time that accounted for hand-to-porous surfaces, hand-to-nonporous surfaces, hand-to-mouth, -eyes, -nose, and hand washing events was used to predict 17 hr of simulated human behavior. Norovirus concentrations originated from monitoring contamination levels on surfaces during an outbreak on houseboats. To predict infection risk, two dose-response models (fractional Poisson and ₂F₁ hypergeometric) were used to capture a range of infection risks. A triangular distribution describing the conditional probability of illness given an infection was multiplied by modeled infection risks to estimate illness risks. Infection risks ranged from 70.22% to 72.20% and illness risks ranged from 21.29% to 70.36%. A sensitivity analysis revealed that the number of hand-to-mouth contacts and the number of hand washing events had strong relationships with model-predicted doses. Predicted infection risks ranged from 21.29% to 70.36%, overlapping with leisure setting and environmental attack rates reported in the literature. In the outbreak associated with the viral concentrations used in this study, attack rates ranged from 50% to 86%. This model suggests that fomites may have accounted for 25% to 82% of illnesses in this outbreak. Fomite-mediated exposures may contribute to a large portion of total attack rates in outbreaks involving multiple transmission modes. The findings of this study reinforce the importance of frequent fomite cleaning and hand washing, especially when ill persons are present.

Bacterial transmission from contaminated surfaces via hand contact plays a critical role in disease spread. However, the fomite‐to‐finger transfer efficiency of microorganisms during multiple sequential surface contacts with and without gloves has not been formerly investigated. We measured the quantity of Escherichia coli on fingertips of participants after one‐to‐eight sequential contacts with inoculated plastic coupons with and without nitrile gloves. A Bayesian approach was used to develop a mechanistic model of pathogen accretion to examine finger loading as a function of the difference between E. coli on surfaces and fingers. We used the model to determine the coefficient of transfer efficiency (λ), and influence of swabbing efficiency and finger area. Results showed λ for bare skin was higher (49%, 95%CI=32‐72%) than for gloved hands (30%, CI=17‐49%). Microbial load tended towards a dynamic equilibrium after four and six contacts for gloved hands and bare skin, respectively. Individual differences between volunteers’ hands had a negligible effect compared with use of gloves (p<0.01). Gloves reduced loading by 4.7% (CI=‐12%‐21%) over bare skin contacts whilst 20% of participants accrued more microorganisms on gloved hands. This was due to poor fitting, which created a larger finger surface area than bare hands.

Hand-to-face contacts are important for estimating chemical and microbial exposures. Few studies describe children’s hand-to-eye or -nose contacts or adults’ hand-to-face contacts. The study objective was to characterize hand-to-head (mouth, eyes, nose, and other) contacts for children in a daycare and adults in multiple locations. Macro-activities and sequences of hand-to-face contacts were recorded for 263 people observed for 30 min each. Statistically significant differences between locations, males and females, adults and children, and during eating and non-eating macro-activities were evaluated. Discrete Markov chains were fit to observed contact sequences and compared among adults and children during eating and non-eating macro-activities. No significant differences in contact frequency were observed between males and females with the exception of hand-to-nose contacts. Children tended to touch the mouth, eyes, and nose more frequently than adults during non-eating macro-activities. Significant differences in contact frequency were observed between locations. Transitional probabilities indicated that children make repetitive mouth, eye, and nose contacts while adults frequently transition to contacts of the head other than the mouth, eyes, or nose. More data are needed to evaluate the effect of age on adults’ contact frequencies and to confirm lack of statistically significant differences between adults and children during eating macro-activities.

It is known that the use of EPA-registered surface disinfectant sprays can reduce infection risk if used according to the manufacturer's instructions. However, there are currently no standards for health care environments related to contamination levels on surfaces. The significance of this research is in quantifying needed reductions to meet various risk targets using realistic viral concentrations on surfaces for health care environments. This research informs the design of cleaning protocols by demonstrating that multiple applications may be needed to reduce risk and by highlighting a need for more models exploring the relationship among microbial contamination of surfaces, patient and health care worker behaviors, and infection risks.