Typhus fever in the military: epidemiology of a re-emerging infection

Flea-borne, or endemic, typhus is a re-emerging disease in the U.S. that is only reported by selected states. Due to these limited surveillance efforts, the true burden of the disease has not been adequately assessed, and risk of infection for groups likely to be exposed to the vector, such as military personnel during field training, remains unclear. To better understand the geographic distribution of flea-borne typhus in the U.S. military, Defense Medical Surveillance System (DMSS) data from 1995 through 2024 were retrospectively analyzed, with laboratory data available from 2007 and afterwards. A total of 605 cases of 'typhus fever' were identified among service members of the U.S. military; flea-borne typhus cases were identified through International Classification of Diseases code filtering limited to a specified U.S. geographic area to exclude non-endemic typhus disease. The identification of cases was ultimately limited by reporting definitions stipulated in the Armed Forces Reportable Medical Events Guidelines and Case Definitions, constraints for stored variables within the DMSS, and potential concurrence between flea-based typhus and other rickettsioses. All possible screening was employed, however, to reduce this risk. Most cases occurred in men (82.0%), with a median age of 28 years, a reflection of military demographics. Almost half (46.8%) of cases were in a subset of military occupational categories with elevated field exposures, such as infantry and engineers. Among identified cases, pooled incidence in active duty personnel by person-years (p-yrs) increased to 3.5 from 2.1 cases per 100,000 p-yrs when comparing the 2018-2024 period with 2008-2013. Notable case clusters were observed in Missouri and North Carolina, with cases in active duty members per 100,000 p-yrs in those states exceeding 23.5 and 4.3, respectively. These findings may reveal a growing and under-recognized burden of flea-borne typhus within the U.S. military and highlight the need for enhanced arthropod control as well as reconsideration of national reporting strategies. This study identified an increase in the pooled incidence rate of flea-borne typhus in U.S. military service members when comparing the 2008-2013 and 2018-2024 surveillance periods. Normalizing identified typhus cases against active duty strength in each U.S. state revealed previously unrecognized clusters of elevated disease activity in states without civilian reporting mandates, particularly Missouri and North Carolina, concentrated heavily among personnel in occupations with trainings requiring high field exposure.