PHILADELPHIA — The misidentification of spotted fever group Rickettsia infections may be responsible for the increased incidence and reduced severity of Rocky Mountain spotted fever cases reported in recent years, according to study data.
“The case fatality rate for [Rocky Mountain spotted fever (RMSF)], which was 1% to 10% in the United States during 1950 to 2000, dropped precipitously in 2001, and has remained under 1% for 12 consecutive years,” F. Scott Dahlgren, MSPH, of the CDC’s Rickettsial Zoonoses Branch within the Division of Vector-Borne Diseases, and colleagues wrote. “Accompanying this trend was a sizable increase in reported incidence, with the reported annual incidence rate topping historical highs of six cases per million persons.
F. Scott Dahlgren
“Determining whether this trend observed in passive surveillance is a result of changes in the epidemiology of [spotted fever group (SFG)] rickettsiosis or an artifact of surveillance is a matter of public health importance.”
Dahlgren and colleagues examined CDC case report form (CRF) data on hospitalizations and case fatalities related to SFG rickettsiosis collected from 1981 to 2013, as well as incidence rate data collected from the CDC’s Nationally Notifiable Disease Surveillance System (NNDSS) from 1993 to 2013. These were modeled against county-level distribution data of the “Lone Star tick” Amblyomma americanum, a frequent carrier of the unexamined SFG parasite Candidatus Rickettsii. The researchers hypothesized that a correlation between disease incidence and A. americanum could explain recent trends in RMSF surveillance.
There were 31,234 cases of SFG rickettsiosis reported through the NNDSS, and 21,483 cases reported through CRF. Incidence ratios estimated using NNDSS cases was highest in a region ranging from Arkansas and Missouri to Virginia; however, hospitalizations and case fatalities reported through CRF decreased with time and the presence of A. americanum. Overall, modeling of incidence rates, hospitalization rates and case fatalities rates were well matched with populations of A. americanum (P < .0001).
“We have demonstrated that the expansion of the Lone Star tick is associated with increasing incidence and decreasing severity of reported [spotted fever group] rickettsiosis,” Dahlgren and colleagues wrote. “Enhanced surveillance for tick-borne rickettsial diseases at endemic sites may provide the ecological, clinical, epidemiological, and laboratory data needed to further elucidate the complex, dynamic natural history of [spotted fever group] rickettsiosis in the United States.”
Dahlgren told Infectious Disease News that other factors, such as greater awareness and increased use of diagnostic testing, also may be partially responsible for the surge in reported RMSF cases. He also urged clinicians to continue empirical prescription of doxycycline for patients with suspected spotted fever group rickettsiosis, despite these findings. – by Dave Muoio
Dahlgren FS, et al. Am J Trop Med Hyg. 2015;doi:10.4269/ajtmh.15-0580.
Paddock C. Session 11. Presented at: American Society of Tropical Medicine and Hygiene Annual Meeting; Oct. 25-29, 2015; Philadelphia.
Disclosure: The researchers report no relevant financial disclosures.