P-2224. Clinical and Public Health Implications of Tularemia Diagnoses by Microbial Cell-Free DNA Testing
Authors: Marx Grace E , Dietrich Elizabeth A , Respicio-Kingry Laurel , Rich Shannan , Ahmad Sharjeel , Das KP Nitin , Jabra Elio , Haider Mobeen , Khan Amir , Austin Constance C , AlSalman Ahmad , Coughlin Julie , Plate Markus , Ackelsberg Joel , White Jennifer , Damodaran Thara , Nichols Matthew , McMillion Benjamin , Seibert Allan M , Rettler Hannah , Petersen Jeannine , Park Sarah Y 
Published: January 2025

Abstract

Background
Tularemia is a nationally notifiable disease and Category A potential bioterrorist threat caused by Francisella tularensis. Microbial cell-free DNA (mcfDNA) sequencing is a pathogen-agnostic diagnostic method using human plasma. mcfDNA detections of F. tularensis are not currently included in public health surveillance criteria for a tularemia case and are not automatically reported to public health jurisdictions. We reviewed recent F. tularensis mcfDNA detections to better understand their clinical and public health implications.Table 1.Francisella tularensis diagnostic test results and reported exposure histories among 15 patients with F. tularensis detected by mcfDNA sequencing — United States, 2018–2023.

Methods
Available residual patient samples, in which F. tularensis mcfDNA were detected by the Karius CLIA certified/CAP accredited laboratory, were sent to the Centers for Disease Control and Prevention (CDC) for F. tularensis testing by PCR. Treating clinicians and local/state public health jurisdictions where these patients lived at the time of testing were invited to complete a survey regarding clinical details and public health reporting.Table 2.Demographic and clinical characteristics of patients with Francisella tularensis detected by microbial cell-free DNA (mcfDNA) testing, as reported by the ordering clinician — United States, 2018–2023.

Results
During 2018–2023, mcfDNA sequencing detected F. tularensis in 15 patient samples in 6 states. CDC detected F. tularensis by PCR in 4 (31%) of 13 residual samples, all in samples with higher DNA quantities. All invited jurisdictions (n=6) and most clinicians (8/13, 62%) participated in the survey. Most cases (12/15, 80%) were reported to public health; 3 of 12 (25%) were determined not to meet the tularemia surveillance case definition, and only 1 case reported to CDC mentioned mcfDNA results. Among 11 cases with clinical information, tularemia was not suspected before receipt of mcfDNA results in 10 (91%), despite all patients reporting outdoor activities that could increase F. tularensis exposure risk. Other diagnostic tests for tularemia were positive in 7 of 11 (64%) cases; blood cultures were negative in all cases. mcfDNA results changed clinical management in all cases, resulting in antibiotic changes.Figure 1.Cross-sectional computed tomography of the lungs in a patient diagnosed with pneumonic tularemia by microbial cell-free DNA (mcfDNA) testing, showing bilateral moderate pleural effusion and right upper and middle lobe pulmonary nodules. This patient also exhibited nonspecific mediastinal and bilateral axillary lymphadenopathy and typhoidal symptoms. In this case, mcfDNA testing identified Francisella tularensis nearly 50 hours before any conventional testing confirmed the diagnosis and prompted changes in antibiotic therapy and infection prevention efforts to begin.

Conclusion
mcfDNA appears to be clinically impactful and highly sensitive for diagnosis of tularemia, particularly in febrile patients with consistent exposure history. New diagnostic methods may pose a challenge to public health surveillance and require changes to case definitions and reporting practices to ensure robustness of public health utility and timely prevention efforts.Figure 2.Left medial knee with 3 cm nodular ulcer with purulence and adjacent erythema at location of suspected tick bite in a patient diagnosed with ulceroglandular tularemia by microbial cell-free DNA (mcfDNA) testing.

Disclosures
Sarah Y. Park, MD, FAAP, Karius, Inc.: employee