Increasing the MRSA outbreak size (20%–30% absolute increase) in hospitals increased the relative change and number of nursing homes affected. For example, 6 months after outbreak, a 20% outbreak in the largest hospital affected 61 nursing homes, with 18 experiencing ≥4% increase (5.6% average relative increase in prevalence in nursing homes), whereas a 30% outbreak affected 65 nursing homes (7.7% relative MRSA prevalence increase).
Outbreaks in nursing homes tended to most affect the hospital to which they sent the largest number of patients. The outbreak in the nursing home sending the most patients to 1 hospital had the largest effect on that hospital (1.9% relative prevalence increase after 6 mo), whereas it had little effect on the other hospitals (Fig. 2). Similarly, an outbreak in the nursing home sending the second most patients to 1 hospital caused a 4.6% relative increase in prevalence in that hospital within 6 months and 7.0% relative increase in prevalence within 2 years.
Varying the outbreak size varied the number and magnitude of change in MRSA prevalence in OC hospitals. For example, for the second largest nursing home, a 10% absolute MRSA outbreak affected 14 hospitals, whereas a 30% absolute outbreak affected 21 hospitals (relative change in prevalence >0%) after 1 year.
Nursing home MRSA outbreaks did have some effects on other OC nursing homes. Very few experienced relative MRSA prevalence increases of ≥1%. Even among those experiencing increases, effects took longer to manifest (Table 2). Two years after an outbreak in the largest nursing home, 66 others experienced <1% increase and 23 no effect, for an average 0.2% relative increase (Table 2). Four years later, 62 nursing homes showed <1% increase and 20 no effect.
Other OC nursing homes were still affected with smaller MRSA outbreaks in a nursing home; 37 saw a relative prevalence increase >0% when the second largest nursing home had a 10% MRSA outbreak. For a larger outbreak (30% absolute increase), 42 other OC nursing homes had a change >0%.
A 6-month outbreak in the largest hospital showed maximum effects in all other OC facilities 6 months after outbreak for an average relative 2.4% increase in MRSA prevalence (range, no effect to 36.4%); 4 nursing homes experienced a ≥10% prevalence increase. Although effects steadily decreased after the outbreak concluded, MRSA prevalence in affected facilities did not return to preoutbreak levels until 4 years later. A short outbreak in the second largest nursing home resulted in a relative prevalence increase of 0.1% (range, no effect to 4.5%) in all other OC health care facilities 6 month after outbreak (16 hospitals were affected, of which 3 were ≥2%). These affects dwindled within 1 year.
Our study suggests that to fully understand the spread and control of an infectious pathogen such as MRSA, one must consider how all of the inpatient facilities (both hospitals and nursing homes) in a large geographic region are connected by both direct and indirect patient sharing. However, many hospital infection control efforts focus exclusively on hospitals. Even existing multi-institutional collaboratives tend to exclude nursing homes.35–37 Hospital infection control efforts that do include nursing homes tend to only include a few nursing homes that receive or send a substantial number of direct transfers (eg, strong fiscal/administrative ties with the hospital).
Despite their much smaller size and less frequent turnover compared with hospitals, the impact of nursing homes is substantial and reaches across many miles. This may be due in part to the relatively high prevalence of MRSA in nursing homes, averaging 25% in OC,12,38 consistent with published literature.11,12,39–42 MRSA acquisition has substantial sequelae43; MRSA-positive nursing home residents have a 10% risk of MRSA infection within the first month of arrival, with risks as high as 40% within 1 year.41,42,44 These infections are costly and often result in hospital readmission.41,45,46 Of patients hospitalized with MRSA infection, 20%–40% were recently in nursing homes.43,47,48
Nursing homes can influence hospital infection control by several means. First, nursing homes can multiply/magnify the effects of a hospital outbreak on other hospitals. The close quarters, heavy social mixing, and already high prevalence of MRSA in nursing homes can “fuel” a hospital MRSA outbreak by serving a “cauldron” of transmission, multiplying the number of cases, and then sending them to hospitals throughout the county. A nursing home can link 2 hospitals that were not otherwise strongly linked, acting as a bridge for infectious pathogens to spread from facility to facility. Secondly, outbreaks originating in a nursing home can affect multiple hospitals in a region, even those geographically distant. Even if a hospital keeps its own MRSA levels low, it is at risk for an outbreak if nursing homes in the same region do not maintain effective infection control. Third, when an outbreak occurs, determining the original culprit can be challenging. The result of an outbreak in a single hospital or nursing home could seem like multiple outbreaks in many different facilities. Different facilities may rush to control their “outbreaks” without uncovering the true origin, leading to a fruitless chase. Our study did not even consider outbreaks originating concurrently in >1 facility, which could very readily occur in such a large region with so many people (OC has a population of 3.1 million and is the sixth largest US County). Such concurrent outbreaks could produce even more synergistic transmission effects, potentially turning a smaller controllable outbreak into one much more difficult to control. One could envision an outbreak in a community served by multiple facilities leading to such an eventuality.17
Previously published models may not fully capture these effects. Hospital-only models17–20 miss key nursing home reservoirs. A literature search found 2 models that included both hospitals and nursing homes. Barnes et al22 constructed a theoretical mathematical model comprised of a single generic hospital and 2 connected nursing homes. Their model suggested that hospitals can affect MRSA prevalence in a nursing home, but transferring patients from a nursing home to a hospital would have a negligible effect on MRSA prevalence in that hospital unless patients are consistently transferred to the same unit in that hospital. Lesosky et al23 constructed a stochastic discrete-time Monte Carlo simulation comprised of teaching hospitals, nonteaching hospitals, and nursing homes. Their model only focused on MRSA acquisition rate, but suggested transfer patterns and rate changes do not affect MRSA transmission. Our study, which includes many more nursing homes and hospitals and their complex connections, suggests otherwise: nursing homes do not have to transfer patients to the same unit of hospital to affect the hospital (in our model patients went from nursing homes to many different wards/units in many different hospitals) and transfer patterns and rate changes may be key drivers of MRSA transmission (an outbreak in a nursing home has heterogenous effects on hospitals). The differing findings likely emerge from 3 considerable differences from our model. First, the Barnes and Lesosky models are theoretical and simplified; one assumes uniform characteristics for each of the facility types and the other does not model actual facilities. In contrast, our model uses extensive and detailed real-world data, (eg, parameterized with facility-specific admission volume, ICU volume, bed capacity, LOS, and transfer probabilities MRSA carriage, and transmission). Second, our model included a much larger geographic region and all of their inpatient facilities. Third, our model accounted for both direct and indirect patient sharing among facilities. Our previous work showed that excluding indirect patient sharing (ie, patient movement from facility to facility with an intervening stay in the community) neglects the majority of patient sharing1,2 and in turn MRSA transmission routes.
The difference between prior models and our models can serve a lesson for infection control. Not considering nursing homes, true connections based on real-world data among hospitals and nursing homes, all of the facilities in a region, and indirect patient sharing could limit the understanding and implementation of infection control. Even if adequate control is maintained in all hospitals, a single nursing home with poor MRSA control can affect multiple facilities. Understanding drivers and mitigators of pathogen spread in nursing homes is urgently needed as the Department of Health and Human Services has named nursing homes the focus of Phase 3 of its Action Plan to reduce health care–associated infections.
Our model may underestimate the amount of MRSA in nursing homes as we assumed that MRSA-positive nursing home residents lost MRSA carriage at the same rate as hospitalized patients (ie, 6 mo half-life), where in fact carriage may persist for several years in these settings.32,49 In addition, homogenous mixing may potentially overestimate the actual contact rate.
By definition, all models are simplifications of real life and as such cannot represent every possible outcome or event.50 Our model does not include comorbidities that may affect MRSA transmission, but its admission and readmission rates in nursing homes and hospitals reflect the health status of this California region. We did not include emergency departments, which could have a potential impact, as inpatients were the scope of this study. In addition, our model does not include pediatric hospitalizations or account for the effect on health care facilities outside of OC. Nevertheless, as pediatric patients uncommonly mix with adult patients during hospitalizations or nursing home care and as the vast majority of hospitalized adults remain within OC facilities (≥83.4% for all types of patient transfers), this model is a reasonable representation for adult transmission of MRSA. OC may not be representative of all counties or regions. However, similar findings may apply regions similar to OC, that is, metropolitan counties with multiple facilities and health care systems.
Nursing homes may play an important role in the spread and control of infectious pathogens such as MRSA. Nursing homes may multiply the effects of a hospital outbreak, originate outbreaks that in turn affect multiple hospitals, and make it even more difficult to trace the source of an outbreak. Even if hospitals maintain effective infection control, even a single nursing home with poor infection control can lead to hospital outbreaks. These findings have several implications for hospital infection control. Hospitals should consider and even include nursing homes in their infection control measures. Hospitals should better understand the true (based on real-world data) connections among other hospitals and nursing homes in their county/region through patient sharing. These connections should include both direct and indirect patient sharing. There may be benefit in applying the same rigor in infection control seen in many hospitals to nursing homes. Ultimately, controlling MRSA and other MDROs may necessitate close collaboration among hospitals and nursing homes across financial and administrative relationships.
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