Outbreak in Tucumán increases to 22 cases and 6 deaths likely due to Legionella pneumophila with negative urine antigen test for serogroup 1

On Sept. 6, the Ministry of Public Health of Tucumán in Argentina reported that the outbreak of pneumonia attributed to Legionella pneumophila, based on initial reference laboratory testing, had increased to 22 people, with six deaths and 10 ambulatory outpatients.

On Sept. 5, this same ministry reported new inclusion criteria for the detection of the Legionella outbreak. Specifically, they quite understandably expanded the case definition to include “any health personnel, patient or family giver who has been in the month of August or has received a surgical intervention … in this private sanatorium and has some compatible symptom such as fever or respiratory symptoms, not necessarily bilateral pneumonia.”

On Sept. 3, the ministry reported that four patients of the 11 total at that time had been diagnosed as having Legionella most likely due to L. pneumophilia (three of the patients had respiratory samples tested, and the fourth patient had a biopsy of the lung).

Also on Sept. 3, the Pan American Health Organization referenced diagnostic results from the Argentinian reference laboratory (ANLIS):

“the ANLIS reported that amplification products of the 16S ribosomal gene for Legionella were sequenced from two bronchoalveolar lavage (BAL) samples by massive sequencing and by four bioinformatic analysis methods, with results compatible with Legionella pneumophila. In the preliminary results of two BAL samples analyzed by total DNA sequencing (metagenomics), readings are compatible with Legionella spp. Confirmation of these results is pending of the sequencing processes. In addition, culture and seroconversion analysis are ongoing, which are complementary tests for Legionella infection.”

In my opinion, this rapid and transparent outbreak investigation has been well coordinated and well communicated locally, nationally and internationally.

That the urine antigen test for Legionella was negative in multiple patients, including the two patients diagnosed by BAL above, is most likely explained by the fact that this urine antigen test only detects L. pneumophila serogroup 1. Although serogroup 1 accounts for approximately 80% of L. pneumophila infections, serogroups 2, 3, 4, 5, 6, 7, 8 and 13 were reported to cause disease in a large multi-hospital study by Yu et al. in JID in 2002.

A less likely explanation could include that because this urine antigen test has an overall sensitivity of 70% to 100%, it may have given a false-negative result. An even less likely hypothesis is that a mutation has occurred in the part of the Legionella antigen that is detected in this urine assay and thereby gave a false-negative result.