Improved Case Detection of Fleaborne Typhus by PCR Testing, Los Angeles County, California, USA, 2022-2024

Fleaborne typhus (FBT) is reemerging as a significant cause of febrile illness in southern California, USA. We performed enhanced testing on low-titer Rickettsia typhi antibody-positive specimens from patients with FBT-compatible illness reported in Los Angeles County, California, during 2022-2024. We tested 114 specimens by repeat serology and 113 by real-time PCR; 81 (71%) were PCR-positive, and 95 (83%) had higher antibody titers by repeat serology. Most PCR-positive specimens were collected 8-10 days after illness onset, but some were collected up to 15 days after illness onset. All PCR-positive specimens were R. typhi-specific. After additional testing, 113 (99%) suspected cases were reclassified as probable or confirmed FBT cases. Our findings indicate that many low-titer seropositive results represent true infections and that use of minimum serology thresholds for public health investigation underestimates surveillance case counts. Expanding commercial molecular testing capacity will improve clinical FBT diagnosis and enhance surveillance accuracy.