The dynamics of methicillin-resistant Staphylococcus aureusexposure in a hospital model and the potential for environmental intervention

BACKGROUND: Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of healthcare-associated infections. An important control strategy is hand hygiene; however, non-compliance has been a major problem in healthcare settings. Furthermore, modeling studies have suggested that the law of diminishing return applies to hand hygiene. Other additional control strategies such as environmental cleaning may be warranted, given that MRSA-positive individuals constantly shed contaminated desquamated skin particles to the environment. METHODS: We constructed and analyzed a deterministic environmental compartmental model of MRSA fate, transport, and exposure between two hypothetical hospital rooms: one with a colonized patient, shedding MRSA; another with an uncolonized patient, susceptible to exposure. Healthcare workers (HCWs), acting solely as vectors, spread MRSA from one patient room to the other. RESULTS: Although porous surfaces became highly contaminated, their low transfer efficiency limited the exposure dose to HCWs and the uncolonized patient. Conversely, the high transfer efficiency of nonporous surfaces allows greater MRSA transfer when touched. In the colonized patient’s room, HCW exposure occurred more predominantly through the indirect (patient to surfaces to HCW) mode compared to the direct (patient to HCW) mode. In contrast, in the uncolonized patient’s room, patient exposure was more predominant in the direct (HCW to patient) mode compared to the indirect (HCW to surfaces to patient) mode. Surface wiping decreased MRSA exposure to the uncolonized patient more than daily surface decontamination. This was because wiping allowed higher cleaning frequency and cleaned more total surface area per day. CONCLUSIONS: Environmental cleaning should be considered as an integral component of MRSA infection control in hospitals. Given the previously under-appreciated role of surface contamination in MRSA transmission, this intervention mode can contribute to an effective multiple barrier approach in concert with hand hygiene

The dynamics of methicillin-resistant Staphylococcus aureus exposure in a hospital model and the potential for environmental intervention

Abstract Background Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of healthcare-associated infections. An important control strategy is hand hygiene; however, non-compliance has been a major problem in healthcare settings. Furthermore, modeling studies have suggested that the law of diminishing return applies to hand hygiene. Other additional control strategies such as environmental cleaning may be warranted, given that MRSA-positive individuals constantly shed contaminated desquamated skin particles to the environment. Methods We constructed and analyzed a deterministic environmental compartmental model of MRSA fate, transport, and exposure between two hypothetical hospital rooms: one with a colonized patient, shedding MRSA; another with an uncolonized patient, susceptible to exposure. Healthcare workers (HCWs), acting solely as vectors, spread MRSA from one patient room to the other. Results Although porous surfaces became highly contaminated, their low transfer efficiency limited the exposure dose to HCWs and the uncolonized patient. Conversely, the high transfer efficiency of nonporous surfaces allows greater MRSA transfer when touched. In the colonized patient’s room, HCW exposure occurred more predominantly through the indirect (patient to surfaces to HCW) mode compared to the direct (patient to HCW) mode. In contrast, in the uncolonized patient’s room, patient exposure was more predominant in the direct (HCW to patient) mode compared to the indirect (HCW to surfaces to patient) mode. Surface wiping decreased MRSA exposure to the uncolonized patient more than daily surface decontamination. This was because wiping allowed higher cleaning frequency and cleaned more total surface area per day. Conclusions Environmental cleaning should be considered as an integral component of MRSA infection control in hospitals. Given the previously under-appreciated role of surface contamination in MRSA transmission, this intervention mode can contribute to an effective multiple barrier approach in concert with hand hygiene.http://deepblue.lib.umich.edu/bitstream/2027.42/112924/1/12879_2013_Article_2936.pd

Fomite-mediated transmission as a sufficient pathway: a comparative analysis across three viral pathogens

Abstract Background Fomite mediated transmission can be an important pathway causing significant disease transmission in number of settings such as schools, daycare centers, and long-term care facilities. The importance of these pathways relative to other transmission pathways such as direct person-person or airborne will depend on the characteristics of the particular pathogen and the venue in which transmission occurs. Here we analyze fomite mediated transmission through a comparative analysis across multiple pathogens and venues. Methods We developed and analyzed a compartmental model that explicitly accounts for fomite transmission by including pathogen transfer between hands and surfaces. We consider two sub-types of fomite-mediated transmission: direct fomite (e.g., shedding onto fomites) and hand-fomite (e.g., shedding onto hands and then contacting fomites). We use this model to examine three pathogens with distinct environmental characteristics (influenza, rhinovirus, and norovirus) in four venue types. To parameterize the model for each pathogen we conducted a thorough literature search. Results Based on parameter estimates from the literature the reproductive number ( R 0 $\mathcal {R}_{0}$ ) for the fomite route for rhinovirus and norovirus is greater than 1 in nearly all venues considered, suggesting that this route can sustain transmission. For influenza, on the other hand, R 0 $\mathcal {R}_{0}$ for the fomite route is smaller suggesting many conditions in which the pathway may not sustain transmission. Additionally, the direct fomite route is more relevant than the hand-fomite route for influenza and rhinovirus, compared to norovirus. The relative importance of the hand-fomite vs. direct fomite route for norovirus is strongly dependent on the fraction of pathogens initially shed to hands. Sensitivity analysis stresses the need for accurate measurements of environmental inactivation rates, transfer efficiencies, and pathogen shedding. Conclusions Fomite-mediated transmission is an important pathway for the three pathogens examined. The effectiveness of environmental interventions differs significantly both by pathogen and venue. While fomite-based interventions may be able to lower R 0 $\mathcal {R}_{0}$ for fomites below 1 and interrupt transmission, rhinovirus and norovirus are so infectious ( R 0 > > 1 $\mathcal {R}_{0}>>1$ ) that single environmental interventions are unlikely to interrupt fomite transmission for these pathogens.https://deepblue.lib.umich.edu/bitstream/2027.42/146145/1/12879_2018_Article_3425.pd

Enviromental Infection Transmission: Routes of Transmission for Influenza and Other Agents

Most infections that are assumed to be $directly$ transmitted have some environmental component between excretion and exposure, in which pathogens exists in the environment before exposure, potentially initiating infection in a susceptible. Modeling a potentially infection transmitting contact involves implicit pathogen transfer from a contagious person, animal, or other entity, to a susceptible person. Traditional transmission models use an abstract and usually poorly defined concept of $contact$ to transmit infection. This dissertation models detailed processes leading to transmission that more closely resemble processes that do so in reality by modeling the ex-homo intermediate environmental stage of pathogen existence. This allows us to investigate unique issues that conventional transmission models are unable to address, such as examining factors that alter transmission mode strength, as well as assessing the effect of environment-based interventions such as hand hygiene or surface decontamination. We use both an individual based as well as a deterministic compartmental modeling framework to model transmission through the environment, either via aerosol, direct droplet-spray, or contact-mediated transmission whereby pathogens are excreted to the fomite environment, later picked up by susceptibles, and eventually self-inoculated to potentially cause infection. We found that either aerosol, direct droplet-spray, or contact mediated transmission routes may cause high transmission either each on their own or in combination with one another, given realistic parameter values. We also found that increasingly non-random touching may either increase or decrease contact transmission, depending on the degree of shedding to one's own hands; hand hygiene became more effective as touching became more specified to specific objects in the environment, while when touching is quite random, broad surface decontamination is most effective. Finally, we found that norovirus and S. aureus were much more transmissible via the contact mediated route compared to influenza and rhinovirus, but that they were also much more amenable to hand hygiene or surface decontamination intervention.Ph.D.Epidemiological ScienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/84440/1/ispickna_1.pd

Informing Optimal Environmental Influenza Interventions: How the Host, Agent, and Environment Alter Dominant Routes of Transmission

Influenza can be transmitted through respirable (small airborne particles), inspirable (intermediate size), direct-dropletspray, and contact modes. How these modes are affected by features of the virus strain (infectivity, survivability, transferability, or shedding profiles), host population (behavior, susceptibility, or shedding profiles), and environment (host density, surface area to volume ratios, or host movement patterns) have only recently come under investigation. A discreteevent, continuous-time, stochastic transmission model was constructed to analyze the environmental processes through which a virus passes from one person to another via different transmission modes, and explore which factors increase or decrease different modes of transmission. With the exception of the inspiratory route, each route on its own can cause high transmission in isolation of other modes. Mode-specific transmission was highly sensitive to parameter values. For example, droplet and respirable transmission usually required high host density, while the contact route had no such requirement. Depending on the specific context, one or more modes may be sufficient to cause high transmission, while in other contexts no transmission may result. Because of this, when making intervention decisions that involve blocking environmental pathways, generic recommendations applied indiscriminately may be ineffective; instead intervention choice should b

Model Analysis of Fomite Mediated Influenza Transmission

<div><p>Fomites involved in influenza transmission are either hand- or droplet-contaminated. We evaluated the interactions of fomite characteristics and human behaviors affecting these routes using an Environmental Infection Transmission System (EITS) model by comparing the basic reproduction numbers (<em>R</em>₀) for different fomite mediated transmission pathways. Fomites classified as large versus small surface sizes (reflecting high versus low droplet contamination levels) and high versus low touching frequency have important differences. For example, 1) the highly touched large surface fomite (public tables) has the highest transmission potential and generally strongest control measure effects; 2) transmission from droplet-contaminated routes exceed those from hand-contaminated routes except for highly touched small surface fomites such as door knob handles; and 3) covering a cough using the upper arm or using tissues effectively removes virus from the system and thus decreases total fomite transmission. Because covering a cough by hands diverts pathogens from the droplet-fomite route to the hand-fomite route, this has the potential to increase total fomite transmission for highly touched small surface fomites. An improved understanding and more refined data related to fomite mediated transmission routes will help inform intervention strategies for influenza and other pathogens that are mediated through the environment.</p> </div

Fomites Classification Parameters and Derived Parameter Values.

a<p>highly touched small surface.</p>b<p>rarely touched small surface.</p>c<p>highly touched large surface.</p>d<p>rarely touched large surface.</p>e<p>The sum of <i>ρ_ptr</i> for the four fomite types is 0.75, which is the same value as that in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone.0051984-Spicknall1" target="_blank">[4]</a>.</p>f<p>Within the venue we assume that there are many realizations of the same type of fomite, and that there are 250 fomites of each type and a population size of 1,000. For a small surface, we assume the ratio of finger area to one of the small fomite surface area is 0.3, therefore the ratio of finger area to the total surface area of the small fomite is 0.0012. The ratio of finger area to the total surface area of the large fomite is 0.0012 divided by 29.</p

The total fomite transmission potentials.

<p>Total fomite transmission potential (Reproduction number) as a function of A) shedding amount <i>α/γ</i>, B) dose response of virus on mucosa <i>π_F</i>, C) fraction of virus shed on hands <i>φ_H</i>, D) inoculation rate <i>ρ_inoc</i>, E) virus inactivation rate on hands <i>µ</i><i>_H</i>, F) virus inactivation rate on fomites <i>µ</i><i>_F</i>, G) population size <i>N</i>, H) fraction of pathogen transferred from surface to hands <i>ρ_tfsh</i>, and I) fraction of pathogen transferred from hands to surface <i>ρ_tfhs</i>, stratefied by fomite types. Stars on the lines are values of <i>R_{0_F}</i> when the parameters take on values in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t001" target="_blank">Tables 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t002" target="_blank">2</a>. Across all variation considered, the highly touched large surface fomite type has the highest transmission potential. The slope of each line, which can be interpreted as strength of each hypothetical control, is largest for the highly touched large surface fomite type.</p

Total transmission potential and the percentage of transmission through hand-contaminated-fomite route.

<p>A) total transmission potential (Reproduction number); B) the percentage of transmission through hand-contaminated-fomite route (<i>R_{0_hF}/R_0F</i>) as functions of the proportion of virus that settle on fomites (<i>λ</i>) and the personal touching rate (<i>ρ_ptr</i>). Stars in the figures are the cases when the parameters take on values in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t001" target="_blank">Tables 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t002" target="_blank">2</a>. Changes in <i>ρ_ptr</i> have more impact on the fomite transmission when <i>ρ_ptr</i> is low than when <i>ρ_ptr</i> is high.</p

The proportion of all transmission mediated through the hand-contaminated-fomite route.

<p>The proportion of all transmission mediated through the hand-contaminated-fomite route as a function of A) fraction of virus shed on hands <i>φ_H</i>, B) virus inactivation rate on hands <i>µ</i><i>_H</i>, C) fraction of pathogen transferred from hands to surface <i>ρ_tfhs</i> and D) inoculation rate <i>ρ_inoc</i>, stratified by fomite type. Stars on the lines are values of <i>R_{0_F}</i> when the parameters take on values in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t001" target="_blank">Tables 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0051984#pone-0051984-t002" target="_blank">2</a>. The percentage of hand-contaminated-fomite route does not exceed 50% except when the fraction shed to hand is larger than 42% or when the virus inactivation rate on hands is less than 0.2/min or when the virus transfer fraction from hand to surface is larger than 48%.</p