A Hidden Superbug Cluster in a Chicago Ventilator Nursing Facility, 2016–2018

Pseudomonas aeruginosa that resists carbapenem antibiotics by making the enzyme Verona integron-encoded metallo-beta-lactamase (VIM) is rare in the United States: a CDC pilot at five U.S. sites found only two carbapenemase-producing strains among 129 carbapenem-resistant Pseudomonas tested. The first U.S. outbreak of this organism, known as VIM-CRPA, was in a Chicago hospital in 2003. Later outbreaks had been described, but none tied to a single skilled nursing facility — until this one.

How it was found

On November 1, 2016, a regional survey looking for carbapenemase-producing organisms in Chicago-area health care facilities screened a skilled nursing facility for ventilated residents (called vSNF A), a licensed 312-bed facility. It found 20 residents carrying VIM-CRPA — colonized, not necessarily ill.

The Chicago Department of Public Health investigated. Between November 2016 and March 2018 it ran 10 more one-day screening surveys, taking rectal swabs from every resident and swabs from the tracheostomy site of those who had one, on two floors: a standard skilled nursing floor and a floor for residents with tracheostomies or on ventilators.

What the screening showed

• 903 swabs from 209 residents identified 38 carrying VIM-CRPA; a 39th was found when an admission swab was taken at an acute care hospital.
• 35 (90%) of the 39 lived on the ventilator floor, 4 (10%) on the standard floor.
• 30 (77%) were positive by rectal swab only, 4 (10%) by tracheostomy swab only, and 5 (13%) by both.

Of the 18 new cases found after the first survey, 17 (94%) had earlier screened negative at the facility — so they probably acquired it there. No new cases appeared in the last two surveys on the standard floor or the last four on the ventilator floor.

A regional strain

Surveys from November 2016 to July 2017 at six other ventilator nursing facilities and six long-term acute care hospitals in the region found 12 more patients with VIM-CRPA, at five of the nursing facilities and one hospital, with facility prevalence of 1% to 4%.

Whole genome sequencing of 26 isolates from five facilities (19 from vSNF A) and two historical isolates from 2003 showed:

• every one carried the VIM-2 variant
• 25 belonged to sequence type ST233, found across all five facilities and clustering apart from unrelated ST233 samples in CDC’s reference collection (the rest were ST277, ST708 and ST875)
• tight clusters of near-identical isolates — 1 to 10 genetic differences — fit transmission inside vSNF A

The conclusion: a common VIM-CRPA strain has been present in the Chicago region for a long time, with recent spread in this one facility.

Stopping the spread

The health department assessed infection control on site throughout. The facility improved hand hygiene, compliance with isolation precautions, grouping of colonized residents, bathing practices and environmental cleaning — and stopped rinsing respiratory equipment with tap water in residents’ room sinks.

Why it matters

This is the largest health care–associated cluster of VIM-CRPA colonizing residents reported in the United States. It shows how health care facilities are linked by the patients they share, and how prolonged, undetected transmission can carry a resistant germ across a region. CDC’s containment guidance — intensive on-site help and colonization screening — limited transmission at the facility even though prevalence stayed high. Early detection depends on laboratories being able to test for resistance mechanisms and to screen, services now offered through CDC’s Antibiotic Resistance Laboratory Network.

Sources

Based on Clegg WJ, Pacilli M, Kemble SK, et al., "Notes from the Field: Large Cluster of Verona Integron-Encoded Metallo-Beta-Lactamase–Producing Carbapenem-Resistant Pseudomonas aeruginosa Isolates Colonizing Residents at a Skilled Nursing Facility — Chicago, Illinois, November 2016–March 2018," MMWR Vol. 67, No. 40, CDC; a work of the United States government in the public domain.