An ongoing global outbreak of Mycobacterium chimaera infections is associated with contamination of a widely used surgical device, the 3T heater-cooler unit, researchers report. The outbreak began in Switzerland in 2011, and since then the Swiss Chimaera Taskforce estimates an annual incidence of 156-282 cases for the Ten major valve replacement markets, with 51-80 cases in the U.S. alone.

In a paper online in Clinical Infectious Diseases, Dr. Shannon H. Kasperbauer and Dr. Charles L. Daley from National Jewish Health, Denver, and the University of Colorado, Aurora, review diagnostic methods and treatment options to guide clinicians in the management of these complicated infections.

M. chimaera is a non-tuberculous mycobacterium within the M. avium complex that is closely related to and commonly misidentified as M. intracellulare. It is easily recovered from household water and biofilms.

The outbreak has been characterized by long latency periods (more than six years from surgery to the presentation of symptoms in one case) and high mortality rates (46%-63%). Most cases of disseminated infection have had prosthetic material in place, including prosthetic valves, vascular grafts and left ventricular assist devices.

Diagnosis of NTM infection

Common presenting symptoms include fever, malaise, weight loss, cough and dyspnea, and important distinguishing features include splenomegaly and chorioretinitis. The Centers for Disease Control and Prevention case definition requires positive cultures obtained from an invasive sample, careful clinical assessment and a history of surgery requiring cardiopulmonary bypass prior to diagnosis of NTM infection.

Current guidelines from the American Thoracic Society and Infectious Diseases Society of America recommend treatment with a three- to four-drug regimen, but the authors favor a multidrug regimen that includes four to five antibiotics, depending on the severity of disease and the underlying comorbidities. The regimen should include a macrolide, rifamycin, ethambutol, and another oral agent, such as moxifloxacin or clofazimine, as well as a parenteral agent, such as amikacin, added to the oral regimen initially.

Optimal duration 

The optimal duration of therapy is unknown, but commonly extends to 12 months and may exceed 24 months. Once a diagnosis of disseminated M. chimaera infection is made, plans for foreign material removal or exchange should be initiated, although the optimal timing of this intervention remains unclear.

"Unfortunately, given the delayed presentation and infection rate of 1/100 to 1/1000, we can expect to see more cases in the future," the authors conclude. "Hopefully, with improved case finding and enhanced risk mitigation, prognosis will improve and the number of cases will decline."