1160 infection control & hospital epidemiology october 2015, vol. 36, no. 10
an indwelling device. Our data show 15% of this high-risk population are colonized with MDR A. baumannii and nearly 50% of colonizations recur over time. Compared with con- trols, cases are more likely to be colonized with P. mirabilis, have diabetes, and have more severe functional disability. Those with colonization were also more likely to have been followed up in our study for at least 2 months and to have had longer NH stays before enrollment. Prior or concurrent colonization with other AROs and co-colonization with other R-GNBs is common. Long-term care facilities may serve as important reservoirs
for MDR A. baumannii.24–26 In a study of a single NH also providing subacute care, 20% of residents surveyed were colonized with A. baumannii; among these isolates, most were MDR and a third were carbapenem resistant.26 Our study focusing on high-risk patients from 4 NH facilities shows similar rates of colonization and increased rates of drug resistance, including 70% resistance to carbapenems among isolates. Cases were rarely colonized with a single ARO. Our study,
one of the few from multiple NHs, shows that 88% of all residents with indwelling devices who were colonized with MDR A. baumannii were colonized with another ARO and 64% exhibited R-GNB co-colonization; P. mirabilis was particularly common. This is consistent with studies in acute care facilities that identified co-colonization with carbapenem- resistant Enterobacteriaceae and either A. baumannii or Pseudomonas aeruginosa as an independent predictor of mortality.39,40 In one of these studies, co-colonized residents were also older, had a higher acuity of illness, and were more likely to have come from a long-term care and/or long-term acute care facility.39 By contrast, a study in an intensive care unit found that younger age, major trauma, and length of stay were positively associated with co-colonization with carbapenemase-producing Klebsiella pneumoniae and A. baumannii.41 Whether the high risk of co-colonization of other organisms with A. baumannii represents a commonality of risk factors or a biologic relationship among the organisms deserves further study. Consistent with previous findings of our group,42–44 func-
tional disability was an important predictor of A. baumannii colonization in this study. Using active surveillance cultures in a cross-sectional study design, our group recently showed that there is a dose-response relationship between functional disability burden (in activities of daily living) and colonization with R-GNB.42 Subsequently, a prospective study by our group identified functional disability as the only independent risk factor for new acquisition of an MDRO in NHs.43 Contact- intense activities of daily living, such as bathing and toileting, increase this risk.44 Within acute care settings, prior exposure to antibiotics is an
important predictor of MDR A. baumannii.6,12,16,28 All our study subjects had indwelling devices with a high antibiotic utilization and there was not enough statistical power to detect differences in antibiotic exposure between cases and controls.
Consistent with prior studies, diabetes presents as a significant risk factor in colonization with A. baumannii.45,46 This study has several limitations. First, all residents already
had an indwelling device at baseline and thus were at high risk of colonization with MDROs. By focusing on NH resi- dents with an indwelling device, we likely missed important information from nonenrolled residents or residents enrolled but discharged from the study. Future studies on MDR A. baumannii colonization in NH residents should focus on functionally disabled older adults with longer lengths of stay in a larger NH cohort. Next, one NH in our study contributed a large number of cases relative to its size, suggesting the possibility of environmental reservoirs. Environmental con- tamination was outside the scope of this study but is part of an ongoing follow-up study. The large number of cases could also be attributable to colonization pressure at referring hospitals; however, no informationwas collected on colonization rates at referring hospitals. The number and heterogeneity of the facilities involved also limit this study. More facilities would allow better models to be estimated, which could give a cleaner picture of some of the marginal effects such as diabetes. Additional studies are needed to further evaluate clinical, microbiologic, and molecular epidemiology of these organisms in a larger consortium of NHs. Molecular characterization of MDR A. baumannii isolates will assist in further defining transmission dynamics in this setting and will be evaluated in a follow-up study. Limitations notwithstanding, our study also has a number
of strengths. The information on residents was collected longitudinally. Ours is one of very few studies that investigate MDR A. baumannii in long-term care settings. Whereas many studies of A. baumannii focus on outbreak investigations and clinical infections, this study explored the epidemiology of asymptomatic colonization in high-riskNHresidents that may have implications for future infection prevention practices. Further studies are needed to better define the epidemiology of MDR A. baumannii among high-risk NH residents because this growing population may have a pivotal role in the global strategy for controlling the spread of A. baumannii and other MDROs.
acknowledgments
We thank the leadership and healthcare personnel at all participating NH facilities; Carol Young and Irina Sanches in the University of Michigan Clinical Laboratory for their assistance in the identification of samples; and the mem- bers of the TIP Study Team, including Kay Cherian, MSc; Jay Fisch, MSc; James T. Fitzgerald, PhD; Andrzej Galecki, MD; Mohammed Kabeto, MS; Carol A. Kauffman, MD; Evonne Koo, MS, MPH; Sarah L. Krein, PhD; Bonnie Lansing, LPN; Lillian Min, MD; Ana Montoya, MD; Tisha Moore, BA; Russell Olmsted, MPH; Ruth Anne Rye, BS; Sanjay Saint, MD; Kathleen Symons, BA; and Linda Wang, BS. Financial support. Veterans Affairs Healthcare System Geriatric Research Education and Clinical Care Center (to Mody), National Institute on Aging Pepper Center (grant P30AG024824 to Mody), and National Institute on Aging (grants R01AG032298 and R01AG041780 to Mody).
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