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Healthcare Outbreaks—Risk Assessment and Mitigation Based on Pathogen, Population, and Environmental Factors: The P2E Concept
Pa Patient Saf Advis 2013 Mar;10(1):27-33.   

James Davis, BSN, RN, CCRN, CIC
Sr. Infection Prevention Analyst
Pennsylvania Patient Safety Authority

Lauren Torso, MPH CDC/CSTE
Applied Epidemiology Fellow
Pennsylvania Department of Health

Julie Paoline, MA
Director, Communicable Disease Control and Prevention
Montgomery County Department of Health 

Corresponding Author
James Davis


Beginning in 2009, Pennsylvania experienced one of the largest and most prolonged outbreaks of invasive group A streptococcus (GAS) within a nursing facility to date. The only known reservoirs for GAS in nature are the skin and mucous membranes of the human host. Therefore, one of the highest-risk patient populations is those who have nonintact skin. In the case example from Pennsylvania, the patient population, at any given time, had several known GAS risk factors (including nonintact skin). Such an observation may indicate that outbreak risk can be assessed, and initiation of proactive intervention may provide opportunities to mitigate risk in order to decrease the probability of an outbreak. Presented herein is a novel framework describing proactive outbreak prevention based on pathogen, population, and environment (P2E) risk assessment. Pennsylvania GAS outbreak facts are used as an example, and the framework is expanded to include carbapenem-resistant Enterobacteriaceae, demonstrating the framework’s applicability to a multitude of outbreak scenarios.


Often, the published literature concerning outbreaks in healthcare settings highlights the event, subsequent investigation, and results. The publication of the outbreak investigation serves to educate healthcare workers and facility management on preventing and controlling these unfortunate events. Investigation of an outbreak is traditionally triggered through surveillance of case reports through a state-specific reporting system such as the Pennsylvania National Electronic Disease Surveillance System (PA-NEDSS). Once an outbreak is identified, investigation focuses on case finding, interventions, and source control. Outbreaks may remain elusive, and mitigation of associated risks to the patient may prove difficult due to the inability to predict those patient populations within a facility that would likely be affected. Herein, the authors use an outbreak of invasive group A streptococcus (GAS) in Pennsylvania as an example for risk assessment. A Morbidity and Mortality Weekly Report (MMWR) article1 relevant to this event will be used as the primary document for the presentation of the facts for this risk assessment. A novel conceptual model for risk assessment and mitigation will be presented in order to provide an example of proactive outbreak prevention, as opposed to the traditional approach of reaction and control.


GAS is defined as “a facultative, gram-positive coccus that grows in chains and causes numerous infections in humans including pharyngitis, tonsillitis, scarlet fever, cellulitis, erysipelas, rheumatic fever, poststreptococcal glomerulonephritis, necrotizing fasciitis, toxic shock syndrome, myonecrosis, and lymphangitis.”2 The pathogenic mechanisms underlying these infections are poorly understood.2 The only known reservoirs for GAS in nature are the skin and mucous membranes of the human host. In order for GAS to become invasive and not solely an established reservoir on human mucous membranes or skin, there needs to be (1) a mode of transmission, (2) a portal of entry into the body, and (3) a host who is susceptible to infection.3 The Centers for Disease Control and Prevention’s (CDC) 2010 Active Bacterial Core surveillance (ABCs) report on GAS-related syndromes in the Emerging Infections Program Network stated that the syndromes that accounted for invasive disease included cellulitis (37%), primary bacteremia (22.5%), pneumonia (15.5%), necrotizing fasciitis (5.7%), and toxic shock (3.3%).4 The 2010 percentages of syndromes presented in the ABCs report were comparable to those reported in 2009.5

In Pennsylvania, healthcare-associated infection outbreak investigations usually begin with the Pennsylvania Department of Health being notified through PA-NEDSS, by a local health department, or by an individual facility because of an increased incidence of patient cases related to a pathogen of interest. Following notification, the local and state departments coordinate investigation and control activities in conjunction with varying levels of support from CDC as warranted by the severity and controllability of the event. Other state agencies may be involved as needed. Once appropriate control measures are in place and case finding is engaged, results of the investigation then yield suggestions for control, as well as interventions to further mitigate risks of continued disease spread.

Beginning in 2009, Pennsylvania experienced one of the largest and most prolonged outbreaks of invasive GAS within a nursing facility to date.1 The Figure, which is reprinted from the MMWR article, depicts the extent of illness in individuals fitting the case definition and the timeline related to this event.1 Furthermore, since the MMWR article was published, there were two additional cases of invasive GAS associated with the described outbreak.

Figure. Number of Persons with Culture-Confirmed Group A Streptococcus Infection (N=30)

  Figure. Number of Persons with Culture-Confirmed Group A
  Streptococcus Infection (N = 30), by Infection Type and Month
  of Positive Culture--Nursing Facility, Pennsylvania, 2009-2010


As a result of the prolonged and evolving nature of the outbreak, the investigators decided to conduct a matched case-control study as part of the investigation. The purpose of this approach was to identify associations between potential risk factors and GAS infection, therefore providing clues that may help stop the outbreak.

The case-patient majority (n = 23) in the MMWR report, as compared with control subjects (n = 69), were male (52%); had a length of stay less than or equal to 10 months (83%); resided in a pulmonary unit (83%); received physical therapy (74%) or occupational therapy (61%); had a tracheostomy tube (74%); had an indwelling urinary catheter (83%); had a gastrostomy or jejunostomy (65%); and had at least two underlying conditions (56%), congestive heart failure or myocardial infarction (57%), diabetes (52%), at least one wound (87%), and finally, two or more wounds (70%).1 Of the 23 potential GAS risk factors assessed during the outbreak investigation, the case-patients represented a majority in 13 of those factors. Having a patient population that, at any given time, can express a majority of the known GAS risk factors and having potential at-risk patient demographics like expressed by the MMWR article strongly suggests that this population might be at an increased risk for outbreaks. Therefore, if a facility has residents who are at risk for GAS infection and susceptible to infection, and if mechanisms of transmission are present, those responsible for infection prevention may need to have greater vigilance in enforcing infection prevention measures in order to prevent an outbreak.


Using the aforementioned GAS outbreak event as a working example, comprehending the very high risk to the population represented, and knowing that to date there is no commercially available GAS vaccine,6 an approach for describing proactive outbreak prevention based on assessment of the pathogen, population, and environment (P2E) may be the best method to prevent outbreaks. Factor et al.noted that a large proportion of cases with invasive GAS disease had a cutaneous form and that a large proportion of those cases reported some type of wound prior to the infection, suggesting that nonintact skin is a primary portal of entry into the body.7 As previously mentioned, human skin and mucous membranes are also implicated as the only known reservoirs for GAS in nature.2 Therefore, the highest-risk patient population includes those most likely to have nonintact skin, for example, patients who have a tracheostomy tube, a feeding tube, a urinary catheter, or one or more wounds (likely related to malnutrition and/or the presence of diabetes). Pathogen factors are related to the affinity of GAS to reside in sufficient quantity on human skin and mucous membranes, establishing a reservoir of significant virulence. Environmental factors in this population include patient exposures to the pathogen by way of sick or colonized contacts, through translocation, or possibly (but less likely) from contaminated environmental surfaces (such as therapy mats).

A realistic, population-based, proactive risk mitigation strategy for outbreak prevention may consider a combination of basic infection control interventions,8 a P2E-based risk assessment, situational awareness related to at-risk patients and infected individuals,9 an ongoing educational plan targeting pathogen-specific transmission prevention,10,11 and identification (through the P2E risk assessment) of a related patient population. Table 1 represents an example of a P2E-based risk assessment, with rationale using the facts presented by the MMWR report relevant to the described GAS outbreak event in Pennsylvania.1

Table 1. Example Factor-Based Risk Assessment, with Selected Interventions and Rationale for Group A Streptococcus (GAS)

  Table 1. Example Factor-Based Risk Assessment,
  with Selected Interventions and Rationale for Group A Streptococcus (GAS)


Once P2E risk assessment is performed, it is then possible to review specific recommendations for the prevention of organism-specific outbreaks. Included in the online-only appendix to this article is a list of staged recommendations specific to GAS. The recommendations presented in the appendix were developed during the Pennsylvania GAS outbreak in coordination with participating state and federal agencies.

In order to perform a P2E risk assessment, the patient population needs to be assessed comprehensively for the pathogens it is likely to be susceptible to. This novel P2E risk assessment approach as presented herein may help guide the infection preventionist as to which pathogens, and in which patient populations, targeted infection prevention measures may be indicated in order to establish an appropriate outbreak prevention strategy. Knowing whether a population is at risk will dictate which prevention strategies to use and how they are applied.

For pathogens that are difficult to eradicate (i.e., hard to kill, unlike GAS), P2E assessment may be a useful strategy for obtaining actionable data for the mitigation of environmental invasion and associated outbreaks of pathogens. Carbapenem-resistant Enterobacteriaceae (CRE), which includes carbapenem-resistant Klebsiella pneumoniae (CRKP), is a grouping of organisms that were rarely encountered in the United States before the 1990s.12 CRE is endemic in certain healthcare patient populations, adding to the likelihood of outbreaks related to inappropriate use of antibiotics.13 Bratu et al. note that “most patients [affected] had been heavily treated with antibiotics before the culture that revealed imipenem-resistant K [Klebsiella] pneumoniae. Although case-cohort studies are necessary to accurately identify risk factors, only 20% of the patients had recent previous therapy with a carbapenem. In contrast, virtually all patients had received previous therapy with a β-lactam or a fluoroquinolone.”14 Looking at Bratu et al.’s observations as a starting point, one may conduct a P2E risk assessment on the patient population at risk for developing an infection with a particular type of resistant organism.

Information like what has been modeled in Table 1 related to GAS can be reproduced for other organisms of interest in order to determine (1) whether a facility and its patients are at risk for an outbreak and (2) if the organism is found to be endemic, how risks should be grouped in order for resources to be assigned to mitigate continued risk. Table 2 represents an example P2E approach using clinical knowledge of risk factors for disease and death, as well as transmission control related to CRE.15-17

Table 2. Example Factor-Based Risk Assessment, with Selected Interventions and Rationale  Table 2. Example Factor-Based Risk Assessment,
  with Selected Interventions and Rationale for
  Carbapenem-Resistant Enterobacteriaceae (CRE) Organisms

Risk assessments that target a pathogen, an at-risk population, and a particular environment can proactively guide mitigation strategies related to outbreaks. The three factors—pathogen, population, and environment—can synergistically combine to produce the circumstances for infection and pathogen transmission. Provided herein is a glimpse into how P2E risk assessment can be conducted and applied to the clinical arena. Solely assessing for the risk of an outbreak for a particular organism in and of itself will not prevent an outbreak. The results of the risk assessment should educate and provide for situational awareness in terms of how ripe a situation is for an outbreak. Information gathered in regard to risk may be used to develop individual and shared situational awareness among staff.

“Individual situational awareness” refers to how the world is seen in individual terms, based on cultural background, education, experiences, and the strengths and limitations of our senses. The mindset that evolves is a result of these inputs and is essentially self-centered.18 “Shared situational awareness obviously differs from individual SA [situational awareness] because it involves a number of persons trying to form a common picture. In any given ‘situation’ these people each have their own ‘individual SA,’ a unique dynamic picture of the situation, which exists in their minds.”18

Schofield and Tauber note the following: 19

A common ground does not mean strong unification; it does not imply that everyone has the same goals, shares the same view of the world, and acts the same way. A common ground allows for certain diversity and individuality, enables shared views and vocabularies, and tolerates sub-communities, sub-disciplines, and the like. However, there is always a core of common concepts and views.

The common ground is dynamic in nature and therefore is often a matter of explicit negotiation and communication. A common ground can fall apart and eventually can get lost; hence, it needs constant maintenance in order to keep the community, culture, and discipline alive.


The P2E risk assessments presented in this article are meant to be examples of how to start developing situational awareness of a facility’s risk given the characteristics of the patients it cares for in combination with the pathogens in its environment. These assessments are meant to be communicated to staff in order for them to further develop individual awareness of risk and adopt behaviors that can mitigate risk to the patient. The lessons gleaned from conducting specialized risk assessment can be incorporated into the education and training of all staff in order to achieve a group situational awareness and common ground related to outbreak prevention. The P2E risk assessment also provides an opportunity to review the effectiveness of the facility’s policies and procedures from the standpoint of how well the policies and procedures address the needs of the population the facility serves.

As one paper notes, “To get the members of the group to develop a shared awareness of the situation requires that they build individual situational awareness within the framework of the mission to be accomplished; share their individual situational awareness, which requires being ‘aware’ of relevant actions and functions of other team members; [and] develop the group ‘shared situational awareness.’”18 A critical element in building a shared situational awareness is establishing common ground, which must be achieved through training and experience.18

The authors believe that focused training and development of relevant clinical experience is only obtainable through P2E exercises that serve to provide risk prediction and mitigation interventions that can be used as a basis for staff education, situational awareness, and appropriate policy and procedure development in order to prevent probable outbreaks.


  1. Centers for Disease Control and Prevention. Invasive group A streptococcus in a skilled nursing facility—Pennsylvania, 2009-2010. MMWR Morb Mortal Wkly Rep 2011 Oct 28;60(42):1445-9. Also available at
  2. Stevens DL, Bryant A. Group A streptococcus: virulence factors and pathogenic mechanisms [online]. Up To Date 2012 Aug 1 [cited 2012 Oct 6].
  3. Centers for Disease Control and Prevention (CDC). Principles of epidemiology in public health practice. 3rd ed. Atlanta: CDC; 2006. Also available at
  4. Centers for Disease Control and Prevention. Active Bacterial Core surveillance (ABCs) report: Emerging Infections Program Network: group A Streptococcus, 2010 [online]. 2011 Oct [cited 2012 Oct 7].
  5. Centers for Disease Control and Prevention. Active Bacterial Core surveillance (ABCs) report: Emerging Infections Program Network: group A Streptococcus, 2009 [online]. 2010 Oct [cited 2012 Oct 7].
  6. World Health Organization (WHO). Group A streptococcal vaccine development: current status and issues of relevance to less developed countries [online]. 2005 [cited 2012 Oct 12].
  7. Factor SH, Levine OS, Schwartz B, et al. Invasive group A streptococcal disease: risk factors for adults. Emerg Infect Dis 2003 Aug;9(8):970-7.
  8. Siegel JD, Rhinehart E, Jackson M, et al. 2007 guideline for isolation precautions: preventing transmission of infectious agents in healthcare settings [online]. [cited 2012 Oct 7].
  9. Steer JA, Lamagni T, Healy B, et al. Guidelines for prevention and control of group A streptococcal infection in acute healthcare and maternity settings in the UK. J Infect 2012 Jan;64(1):1-18.
  10. Controlling the annual threat of norovirus gastroenteritis outbreaks. Pa Patient Saf Advis [online] 2010 Dec [cited 2012 Oct 7].
  11. Rainbow J, Jewell B, Danila RN, et al. Invasive group A streptococcal disease in nursing homes, Minnesota, 1995-2006. Emerg Infect Dis 2008 May;14(5):772-7.
  12. Centers for Disease Control and Prevention. Guidance for control of carbapenem-resistant Enterobacteriaceae (CRE) [online]. 2012 Jun 19 [cited 2012 Oct 17].
  13. Gupta N, Limbago BM, Patel JB, et al. Carbapenem-resistant Enterobacteriaceae: epidemiology and prevention. Clin Infect Dis 2011 Jul 1;53(1):60-7.
  14. Bratu S, Landman D, Haag R, et al. Rapid spread of carbapenem-resistant Klebsiella pneumoniae in New York City: a new threat to our antibiotic armamentarium. Arch Intern Med 2005 Jun 27;165(12):1430-5.
  15. Falagas ME, Rafailidis PI, Kofteridis D, et al. Risk factors of carbapenem-resistant Klebsiella pneumoniae infections: a matched case-control study. J Antimicrob Chemother 2007 Nov;60(5):1124-30.
  16. Schwaber MJ, Klarfeld-Lidji S, Navon-Venezia S, et al. Predictors of carbapenem-resistant Klebsiella pneumoniae acquisition among hospitalized adults and effect of acquisition on mortality. Antimicrob Agents Chemother 2008 Mar;52(3):1028-33.
  17. Ben-David D, Maor Y, Keller N, et al. Potential role of active surveillance in the control of a hospital-wide outbreak of carbapenem-resistant Klebsiella pneumoniae infection. Infect Control Hosp Epidemiol 2010 Jun;31(6):620-6.
  18. Nofi AA; Center for Naval Analyses. Defining and measuring shared situational awareness [online]. 2000 Nov [cited 2012 Oct 20].
  19. Schofield K, Tauber MJ. Common ground [online]. 1996 Apr [cited 2012 Oct 22].


Pennsylvania Department of Health Recommendations Specific to Invasive Group A Streptococcus (GAS) Outbreak Prevention and Control

(  Available in PDF)

Foundational Layer (Basic Infection Control Strategies for All Environments)

  1. Survey for and document patient and employee infections.
  2. Achieve 100% compliance with best practices related to hand hygiene policies and procedures.
  3. Include the World Health Organization’s opportunities for hand hygiene as part of infection control orientation and annual in-service training.
  4. Conduct random, periodic (e.g., quarterly) hand hygiene compliance audits; document and provide feedback of results to frontline staff.
  5. Ensure alcohol-based hand sanitizer and gloves are present in patient rooms and common areas.
  6. Develop a protocol for restocking gloves and refilling hand hygiene product.
  7. Ensure patients perform hand hygiene if they congregate in common areas.
  8. Achieve 100% compliance with best practices related to contact-based precaution policies and procedures.
  9. Provide a mechanism for timely communication between the infection control department and staff for implementation of contact precautions.
  10. Disinfect bathtubs, showers, negative-pressure therapy devices, and other equipment and devices in between each patient use, according to manufacturer recommendations.
  11. Achieve 100% compliance with best practices related to respiratory care.
  12. Achieve 100% compliance with best practices related to wound care.
  13. Dispose of unused gauze and other wound care supplies that have been exposed to the patient environment; furthermore, gauze and other wound care supplies that have been opened and exposed to one patient’s environment should not be used for any other patient regardless of exposure time.
  14. Ensure adherence to policies related to exclusions from work for ill staff.
  15. Maintain policies to encourage visitors who feel ill to postpone contact with patients to reduce the likelihood of transmission. In situations in which this is not possible, minimize contact with patients during the visit (e.g., ensuring visitors utilize personal protective equipment).
  16. Consider placing additional, basic signage in patient rooms and in common areas to remind staff and visitors to practice hand hygiene.

Recommendations Following Identification of One Case of Invasive GAS in a Long-Term Care Facility

  1. Report the case to the state public health agency.
  2. Save the isolate for further testing.
  3. Review medical records to identify any residents with invasive GAS.
  4. Actively monitor all staff and residents for any symptoms of GAS (continue for four months following index case).
  5. Culture any symptomatic staff member or resident.
  6. Culture close contacts of index case (e.g., roommate) to identify any GAS carriers.
  7. Treat residents with invasive GAS according to clinical recommendations; contact the department of health for antibiotic recommendations for carriers.
  8. Review infection control practices, including hand hygiene and cleaning procedures.

Additional Recommendations Following Identification of Two Invasive GAS Cases (or One Invasive GAS Case and One Noninvasive GAS Case)

Two cases must occur within three months of each other.

  1. Follow single invasive GAS case recommendations 1 through 3.
  2. Culture all residents of the institution to identify any additional cases or carriers; contact the department of health for screening recommendations.
  3. Treat residents with invasive GAS according to clinical recommendations; contact the department of health for antibiotic recommendations for carriers.
  4. Review infection control practices, including hand hygiene, cleaning procedures, wound care, and respiratory care.
  5. Initiate epidemiologic investigation to understand potential links between cases.
  6. Send isolates to a state public health laboratory for additional processing.

Additional Recommendations Following Identification of Three or More Invasive GAS Cases (or Two Invasive GAS Cases and One Noninvasive GAS Case)

Three or more cases must occur within three months of each other.

  1. Follow single invasive GAS case recommendations 1 through 3.
  2. Culture all residents and staff members of the institution to identify any additional cases or carriers.
  3. Treat residents with invasive GAS according to clinical recommendations; contact the department of health for antibiotic recommendations for carriers.
  4. Review infection control practices, including hand hygiene, cleaning procedures, wound care, and respiratory care.
  5. Consider restricting new admissions and visitors and cohorting culture-positive residents to decrease transmission.
  6. Expand epidemiologic investigation to identify breaches in infection control.
  7. Send isolates to a state public health laboratory for additional processing.
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