A-Z Index × Submit A-Z Index × Submit A-Z Index Search Dropdown × Submit Facebook Twitter LinkedIn Syndicate Emerging Infectious Disease journal ISSN: 1080-6059 Disclaimer: Early release articles are not considered as final versions. Any changes will be reflected in the online version in the month the article is officially released.
Fleaborne typhus (FBT) is reemerging as a major cause of febrile illness in 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.
Fleaborne typhus (FBT), also referred to as murine or endemic typhus, is a febrile illness spread by fleas infected with the bacterium Rickettsia typhi . Infected fleas are often found on mammalian hosts, including rodents, opossums, dogs, and free-roaming cats ( 1 , 2 ). FBT became a major public health threat in the United States from the 1930s through the early 1940s; thousands of cases were reported annually. Improvements in sanitation, alongside the use of novel rodenticides and synthetic insecticides like dichlorodiphenyltrichloroethane, led to a sharp decline in cases by the 1950s ( 3 , 4 ). Today, most cases are reported in Hawaii, Texas, and California; in California, where the disease is reemerging as a major cause of febrile illness ( 5 – 7 ). In Los Angeles County (LAC), FBT has increased since 2010; an average of 169 cases were documented per year during 2021–2025, and an all-time high of 220 cases were reported in 2025.
FBT ranges from a mild, self-limited infection to severe or fatal illness. Cases typically manifest with nonspecific symptoms such as fever, myalgia, headache, nausea, vomiting, and, less frequently, a rash ( 8 ). Severe clinical manifestations are rare but might include complications such as meningoencephalitis, myocarditis, and hemophagocytic lymphohistiocytosis ( 9 ). Doxycycline is the treatment of choice for FBT and is highly effective ( 5 , 10 ). Although the overall case-fatality rate for FBT is low (<1% with appropriate doxycycline treatment), fatal outcomes can still occur, especially in patients with delayed care or underlying health conditions ( 11 , 12 ). In 2022, LAC recorded 3 FBT-associated deaths involving hemophagocytic lymphohistiocytosis, myocarditis, and septic shock in patients with underlying conditions ( 11 ).
Because FBT manifests with undifferentiated influenza-like symptoms that resemble other illnesses, cases are often overlooked or misdiagnosed by healthcare providers ( 13 ). Delayed or incorrect diagnosis might postpone necessary treatment, which can increase the risk for complications, prolonged illness, or,…