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Superbug claims two lives as all drugs fail
We had never seen anything like this’
A nosocomial outbreak of a novel strain of strikingly resistant Acinetobacter baumanii led to two patient deaths before it was eradicated through strict isolation and environmental decontamination, an infection control professional reports.
"What is so concerning is that it was almost untreatable," says Hillary Cooper, RN, MS, CIC, director of infection control at Hahnemann University Hospital in Philadelphia. "It was not susceptible to commercial antibiotics."
A. baumanii is often resistant to multiple classes of antibiotics including penicillins (PCN), carbapenems (CP), aminoglycosides (AG), and quinolones (Q), Cooper and colleagues remind.1 Infections due to these strains frequently are treated with colistin or polymyxin B. However, the outbreak strain in this case also developed resistance to colistin, an old drug that has proven a worthy weapon against pan-resistant strains of A. baumanii.
"We had never seen anything like this in the hospital," she says. "Colistin resistance is very unusual in this organism."
Investigators identified a cluster of three patients: two who were infected and one who was colonized. All patients stayed on the same hospital floor. The strain was not susceptible to PCN, CP, AG, Q, and colistin (MIC_16 _g/ml). The strain had intermediate susceptibilities to amikacin (MIC_16 to 64 _g/ml) and ceftazidime (MIC_32 _g/ml). Patient A, the likely index case, had sacral osteomyelitis caused by a similar strain, but colistin resistance was yet to appear. As has been seen with Staphylococcus aureus and vancomycin, the A. baumanii strain developed resistance to colistin after prolonged antibiotic therapy with the drug.
"What we think actually happened was that patient A had an osteo [bone infection] that he came in with that was actually caused by acinetobacter," Cooper says.
"The acinetobacter was resistant to all antibiotics except colistin and amikacin. So he came in with this very resistant bug. He received colistin for at least six weeks and then a culture was detected in his urine that was resistant to everything," she adds. "We sent cultures out to test for relatedness. We believe that both of these strains that he had — the more susceptible one and the resistant one — are related. They look almost identical. It occurred after he received six weeks of colistin."
Patient B had a ventilator-associated pneumonia due to the resistant strain. Patient C had bacteremia. "The patient [C] that survived was actually colonized in his urinary tract and did not receive any treatment."
The precise mode of spread was not determined, but hospital staff and patient care equipment may have transmitted the bug. Multiple environmental cultures from patient rooms and patient care devices were positive.
"The extensive cultures and the epidemiology suggested that hospital staff or patient care equipment may have been important," Cooper says. "We did find it in the environment, in immediate vicinity of the patients who were colonized or infected. We also found it on some ancillary equipment that we cultured."
Strict isolation of the patients and rigorous cleaning of the patient care environment were instituted. No new cases have been identified as of four months after the outbreak.
"We detected the outbreak rather rapidly, and as soon as we detected it, we realized that this was very unusual," Cooper says. "We geared up and cultured the environment, cultured the patients, rapidly implemented very strict contact isolation, and of course, educated all personnel. We did rigorous environmental cleaning, and we were able to effectively control the outbreak very quickly. It did not spread beyond the three initial patients who cultured positive."
Though the colistin resistance developed after six weeks of therapy with the drug, the clinicians had few therapeutic alternatives given the pathogen’s resistance to other antibiotics.
"There really wasn’t anything else to treat the patient with for his serious osteo," she says. "We didn’t really have much of a choice because there was nothing else available, and it was so very resistant."
Given the nature of resistance encountered, Hospital Infection Control solicited the opinion of veteran epidemiologist William Schaffner, MD, chairman of the department of preventive medicine at the Vanderbilt University Medical Center in Nashville, TN. Having dealt with A. baumanii outbreaks before, Schaffner says the level of resistance and the presence of the pathogen in the environment are both noteworthy findings.
"When we looked at our environment, we were hard-pressed to find it, and that is true of many other acinetobacter outbreaks," he explains. "It’s in the patients, and the notion is that it goes from patient to patient on [health care worker] hands. Usually, it takes a combination of all things [to quell an outbreak]: a degree of antibiotic surveillance and control, along with early identification and isolation, and then rigorous attention to hand washing as well as environmental hygiene. But the last one doesn’t usually play quite as big a role, because you invariably can’t find the organism in the environment. This is obviously one circumstance where it did happen."
1. Cooper H, Frye-Arrhighy B, Fraimow H, et al. A nosocomial outbreak due to Acinetobacter baumanii not susceptible to antibiotics. Abstract 576. Presented at the Infectious Disease Society of America. San Diego; October 2003.