The US Centers for Disease Control and Prevention (CDC) today announced the country's fourth human detection of the concerning MCR-1 resistance gene, in a toddler who may have been exposed during travel, as well as follow-up investigation details of the first case, which hints that the transmission risk to others may be low.
The pair of reports appeared today in an early online edition of Morbidity and Mortality Weekly Report (MMWR). Both involved Escherichia coli that carried the MCR-1 gene, which confers resistance against the last-line antibiotic colistin.
Case involved travel, possibly foodborne, exposure
The latest MCR-1 case was identified on Jun 16 in an E coli O157:H7 isolate from the stool of a Connecticut child with diarrhea. Four days earlier the child got sick with fever and bloody diarrhea while visiting friends and relatives in the Caribbean. The child is a 2-year-old girl, and the detection was an incidental finding, the Washington Post reported, citing CDC officials.
The child was seen by a primary care provider and had a brief emergency department visit but wasn't hospitalized. At symptom onset the child received the aminoglycoside antibiotic paromomycin.
Tests showed the isolates carried the blaCMY-2 gene, which confers resistance to third-generation cephalosporins, and were susceptible to carbapenems. Follow-up tests on samples on Jun 18 and Jun 23 were positive for MCR-1, but on Jun 24 and Jul 1 were negative.
A review of possible risks found that the girl had been healthy with no hospitalizations. While traveling in the Caribbean she ate chicken and goat meat from a live-animal market, but did not visit the location. During travel the child stayed in a home that had a dog and cat, but there was no animal contact in the United States.
Perirectal sampling of six of the child's household contacts in mid July were negative for MCR-1, as was a swab of a soiled diaper from the girl, suggesting that none were colonized with bacteria that harbored the MCR-1 gene. Environmental samples were also negative.
Investigators concluded that the MCR-1 gene was potentially acquired through travel, with no transmission to contacts or lingering environmental contamination.
Probe of first case found no further spread
In the second MMWR report, CDC and Pennsylvania officials spelled out the follow-up investigation of the nation's first MCR-1 case, involving a Pennsylvania woman who had sought care for a urinary tract infection in May.
Federal and state investigators conducted extensive contact tracing and perirectal screening for bacteria carrying the MCR-1 gene in the patient's household and in two hospitals where the woman had frequent, extensive, and prolonged interaction with health workers. All 20 of the patient's high-risk contacts agreed to screening, as did 78 of 98 lower-risk contacts and 7 patients from one of the hospitals that agreed to offer testing.
No bacteria carrying the MCR-1 gene turned up in samples from any of the 105 people tested. Also, testing of extended-spectrum beta lactamase (EBSL)-producing isolates from the two hospitals over a 30-day period found no colistin-resistant organisms. Taken together, the findings suggest that the transmission risk—even among high-risk contacts—from a colonized patient may be relatively low, the group wrote.
Follow-up swabs on the patient were positive for MCR-1 on samples collected on May 31 and Jun 26, but negative on those collected on Aug 1. The authors noted that the patient didn't receive any antibiotics during that time frame.
Health officials still don't know how the woman became colonized, but the authors suggest that careful screening of contacts at highest transmission risk is useful for identifying and preventing the spread to others, including hospital patients and workers.
Sep 9 MMWR report on fourth US MCR-1 case
Sep 9 Washington Post story
May 26 CIDRAP News story "Highly resistant MCR-1 'superbug' found in US for the first time"