Thursday, February 04, 2016

NEJM: Probable Person-to-Person Transmission Of Legionnaires’ Disease

Legionella Bacteria - Photo Credit CDC PHIL

Conventional wisdom just isn't what it used to be.

For nearly forty years - since it famously sparked a major pneumonia outbreak at Philadelphia’s Bellevue Stratford Hotel during an American Legion convention in 1976 - Legionella pneumonia has been considered strictly an environmental illness.

The mantra - oft repeated in this blog - is that it cannot be transmitted from person to person. Because . . . well . . . it had never been shown to do so.

While still likely 99.99% true, we have an outlier to look at - published today in the NEJM - that strongly suggests that while exceedingly rare, person-to-person transmission of Legionnaires Disease may have occurred in 2014 during an outbreak in Portugal.

The outbreak, which I blogged on a couple of times in late 2014 (see ECDC: Rapid Risk Assessment On Portugal’s Legionnaire’s Disease Outbreak), was one of the largest outbreaks of Legionella that we’d seen in years; with more than 300 cases reported in a week’s time.  At the time I wrote:

Based on the size and rapid growth of the cluster, the WHO called it a `major public health emergency’.  But Legionnaire’s is not a communicable disease, and so the threat is limited to those who are directly exposed to the environmental source of the bacteria.

Here's where it gets interesting.  

One of the earliest victims of this outbreak was a maintenance worker (Pt #1 M, 48) who worked on one of the cooling towers in Vila Franca de Xira that was later found to be contaminated with the Legionella bacteria.
His first respiratory symptoms began to appear on October 14th.

He returned to his home in Porto - 300 km distant from the outbreak - which he shared with his elderly mother (Pt #2, F, 74) on Oct. 11th, and again on Oct. 19th. By the 19th, his respiratory symptoms had become severe, and his mother cared for him through the night (approx. 8 hours), until he was admitted to a local hospital the following morning. 

Three days later Pt #1 was transferred to another hospital to be placed on ECMO, and he died on January 7th, 2015.

Meanwhile, the patient's mother - who had never visited the site of the outbreak - began to experience respiratory symptoms on Oct. 27th. She was admitted to the local hospital on Nov. 3rd with septic shock and pneumonia, and died on December 1st.

The following NEJM correspondence provides additional details, and rationale for their conclusions.

Probable Person-to-Person Transmission of Legionnaires’ Disease

N Engl J Med 2016; 374:497-498 February 4, 2016 
DOI: 10.1056/NEJMc1505356

From a practical standpoint, this probably doesn't change much. This report notwithstanding - H-2-H transmission of Legionella is probably extraordinarily rare.

Still, it is a fascinating report.

And a reminder why epidemiologists never like to to say `never'.

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