Environmental Investigations of Travel-Associated Legionnaires’ Disease Cases: Timeliness, Outcomes, and Source Attribution

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Abstract
In Europe, travel-associated Legionnaires’ disease (TALD) cases require timely environmental investigations to support risk assessment, rapid control measures, and prompt reporting of investigation findings to the European Legionnaires’ Disease Surveillance Network (ELDSNet). This study evaluated TALD-related environmental investigations conducted during 2025 and early 2026 in Crete, Greece, following notifications through ELDSNet. Overall, 30 notifications corresponded to 24 unique confirmed TALD cases with illness onset in 2025 and 24 implicated hotels, with some cases involving stays in multiple hotels and Regional Units and clusters identified in some implicated hotels. The investigation framework combined microbiological, physicochemical, and operational data, focusing on delays from symptom onset, notification, sampling, and laboratory reporting. Overall, 516 environmental samples were collected, of which 503 yielded valid analytical results. Among the 503 analyzed samples, Legionella spp. was detected at ≥50 CFU/L in 127 samples (25.25%). This included 123 samples positive for L. pneumophila (24.45%), of which 31 were serogroup 1 (6.16%). Concentrations exceeding the >1000 CFU/L threshold were recorded in 53 samples (10.54%). Operational indicators varied, with median values of 31.0 days for reporting delay (RD), 14.5 days from notification to first sampling (TTF), 47.5 days from symptom onset to first sampling (TDS), and 67.0 days from symptom onset to first laboratory result (OELR). These findings support documenting response delays, strengthening inspector capacity and cross-regional coordination, and integrating microbiological results with operational indicators for timelier source attribution, faster reporting, and TALD public health action.

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