The health-care environment contains a diverse population of microorganisms, but only a few are significant pathogens for susceptible humans. Microorganisms are present in great numbers in moist, organic environments, but some also can persist under dry conditions. Although pathogenic microorganisms can be detected in air and water and on fomites, assessing their role in causing infection and disease is difficult. Only a few reports clearly delineate a “cause and effect” with respect to the environment and in particular, housekeeping surfaces.
Eight criteria are used to evaluate the strength of evidence for an environmental source or means of transmission of infectious agents. Applying these criteria to disease investigations allows scientists to assess the contribution of the environment to disease transmission. An example of this application is the identification of a pathogen (e.g., vancomycin-resistant enterococci [VRE]) on an environmental surface during an outbreak. The presence of the pathogen does not establish its causal role; its transmission from source to host could be through indirect means (e.g., via hand transferal).
The surface, therefore, would be considered one of a number of potential reservoirs for the pathogen, but not the “de facto” source of exposure. An understanding of how infection occurs after exposure, based on the principles of the “chain of infection,” is also important in evaluating the contribution of the environment to health-care associated disease.13 All of the components of the “chain” must be operational for infection to occur.
Chain of infection components
1. Adequate number of pathogenic organisms (dose)
2. Pathogenic organisms of sufficient virulence
3. A susceptible host
4. An appropriate mode of transmission or transferal of the organism in sufficient number from source to host
5. The correct portal of entry into the host
The presence of the susceptible host is one of these components that underscores the importance of the health-care environment and opportunistic pathogens on fomites and in air and water. As a result of advances in medical technology and therapies (e.g., cytotoxic chemotherapy and transplantation medicine), more patients are becoming immunocompromised in the course of treatment and are therefore at increased risk for acquiring health-care associated opportunistic infections. Trends in health-care delivery (e.g., early discharge of patients from acute care facilities) also are changing the distribution of patient populations and increasing the number of immunocompromised persons in nonacute-care hospitals.
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