MOLECULAR INVESTIGATION OF LEGIONNAIRES` DISEASE CASES IN ISRAEL, 2013-2017

Legionnaires` disease (LD) caused by Legionella spp is a significant public health threat. The identification and typing of L.pneumophila (Lp), crucial for control and prevention of Legionnaires’ disease (LD), is greatly limited by the reliance on urinary antigen testing (UAT) and rare recovery of Lp in respiratory samples via culture.

During 2013-2017, clinical specimens of suspected and confirmed LD cases and related environmental isolates were submitted to the MOH molecular reference laboratory of Legionella. Microbiological investigation aimed at establishing the epidemiological link and identifying the source of infection included mip sequencing for species identification, Sequence-Based Typing (SBT) of cultured isolates, and direct/Nested SBT, applied directly to respiratory samples.

Of 227 reported LD cases, 47% were diagnosed by UAT, while 53% corresponded to respiratory samples, including 25% and 24%, culture and PCR-positive, respectively. Of 110 cases with positive respiratory samples, the Sequence Type (ST) could be determined in 88 cases (80%), revealing Lp sg1-ST1 as the most prevalent subtype (49, 52%), followed by sg1-ST42 and sg3-ST464 (5, 5% both). In two cases, non-pneumophila strains (L.bozemanii and L.longbeachae) were identified as the etiologic agent. Notably, 4 (4.5%) cases were caused by novel Lp STs. In 42 cases (19%) with environmental isolates available, the source of infection could be determined. Of those, sg1-ST1 accounted for half of the cases. As ST1 is the most dominant environmental strain in Israel, higher degree of resolution is required to improve the source tracking capability.

Molecular analysis of legionellosis cases greatly enhanced public health tools for LD control in Israel. We are currently testing the Next Generation Sequencing approach for differentiation between ST1 strains.