P-1116. Another One Bites the Flush: Metagenomic Next-Generation Sequencing (mNGS) Signal leads to Rapid Water Safety Overhaul on a Bone Marrow Transplant (BMT) Unit

Background Hospital-onset legionellosis is difficult to control as Legionella thrives inside pluming biofilms, evades routine culturing methods and disproportionately affects immunocompromised patients. A water management (WM) program is a regulatory requirement. Turning genomic signals into immediate, unit-wide controls remains under-reported. Methods Two neutropenic patients in a BMT unit of a 1400-bed tertiary hospital were diagnosed with L. dumoffii pneumonia via cell-free mNGS within a week of each other. A multidisciplinary incident command activated the WM plan that included: closure of rooms that housed the two patients during the index hospitalization and patient relocation, bottled water for consumption and oral care, suspension of showering, and unit-wide vigorous flushing of every fixture. Environmental sampling included all fixtures in the rooms that housed the patients, other selected outlets on the unit, and all ice machines. Positive outlets received 10 % chlorine disinfection. Daily flushing with audit logs, monthly fixture disinfection, enhanced water temperature and residual oxidant audits, removal of ice machines, a 3-ft sink clutter-free zone, and an ANC-based shower restriction were added. Results Of 22 water samples, three fixtures in the index room yielded Legionella species which were further speciated by 16S tNGS as L. dumoffii. The whole genome sequencing and ribosomal multilocus sequence type (rMLST) of these isolates, performed at the hospital’s laboratory, demonstrated related strains NY-23 and rMLST 39551 on all three isolates. Water temperature and residual oxidant values were within expected parameters and daily flushing compliance reached 100%. Water flushing declinations were resolved. No additional Legionella cases were detected during 90-day enhanced surveillance. Both patients fully recovered. Conclusion Rapid mNGS diagnosis, followed by a policy-driven, multidisciplinary response, rapidly contained a L. dumoffii cluster and prevented further cases. This genomics-to-policy approach, integrating advanced diagnostics, engineering controls, and patient-specific precautions can safeguard immunocompromised populations. Disclosures All Authors: No reported disclosures