Person-to-Person Transmission of Legionnaire's Disease Reported

Diana Phillips

February 03, 2016

An apparent instance of person-to-person transmission of Legionnaires' disease linked to a community outbreak of the respiratory infection has experts questioning the disease's exposure mechanism.

A woman who cared for her adult son with Legionnaires' disease, one of hundreds of confirmed cases in a cluster that occurred in Vila Franca de Xira, Portugal, in late November 2014, was infected with the same strain of Legionella pneumophila despite having no geographic link to the cluster epicenter, write Ana M. Correia, MD, from the Northern Regional Health Administration in Porto, Portugal, and colleagues in a letter to the editor published in the February 4 issue of the New England Journal of Medicine.

The authors report that the woman shared a home approximately 300 km from Vila Franca de Xira with her son, a maintenance worker at an industrial cooling tower complex that was found to be contaminated with the L pneumophila isolate linked to the outbreak.

Collected water samples from the small, nonventilated house and a swab of the shower drain were negative for legionella, leading officials to suspect that person-to-person transmission "probably occurred" when the woman cared for her severely ill son. Both mother and son died from complications of the infection.

The mother, Patient 2, had never traveled to the site of the outbreak. In addition, genetic analysis showed that both patients were infected with an identical strain of bacteria, further supporting the possibility of person-to-person transmission.

"Factors that suggest person-to-person transmission are the severity of the respiratory symptoms in Patient 1, the very close contact that occurred during the 8 consecutive hours when Patient 2 took care of Patient 1, and the small area of the nonventilated room where this contact took place," the authors write. In addition, the timeline of the events was consistent with the typical incubation period for the disease.

The possible person-to-person transmission is notable, in that studies to date suggest that person-to-person spread of the disease does not occur, according to the Centers for Disease Control and Prevention. The infection is usually contracted through inhalation of a water aerosol containing the bacteria.

The authors have disclosed no relevant financial relationships.

N Engl J Med. 2016;374;5:497-498.

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