Epidemiology and Statistics

• Rocky Mountain spotted fever (RMSF) has been a nationally notifiable condition since the 1920s.
• As of January 1, 2010, cases of RMSF are reported under a new category called Spotted Fever Rickettsiosis (SFR).
  • This category captures cases of RMSF, Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsialpox.
  • The change reflects the inability to differentiate between spotted fever group Rickettsia species using commonly available serologic tests.

At a glance

• The number of SFR cases has risen in the last two decades, from 495 cases in 2000, to a peak of 6,248 in 2017. However, cases reported in 2018 and 2019 were slightly lower.
• Because of the inability to differentiate between spotted fever group Rickettsia species using commonly available serologic tests, it is unclear how many of those cases are RMSF, and how many result from other, less severe spotted fevers.

Figure 1–Number of reported cases of spotted fever rickettsiosis –United States, 2000–2019

 

Historical trends

• The number of SFR cases reported to CDC per year have generally increased over time with distinct increases since the mid-1990s.
• Notably, while the number of cases and incidence rose, the case fatality rate (i.e., the proportion of SFR patients that died as a result of infection) has declined since the 1940s when tetracycline antibiotics became available.
• The current case fatality rate for SFRs using surveillance data is still roughly 0.5% of cases.
  • In clinical reviews of RMSF cases, about 5-10% of cases are fatal.
  • Inclusion of less severe spotted fevers, such as R. parkeri rickettsiosis, likely leads to the lower case fatality rate observed in recent decades.

Figure 2–Reported incidence and case fatality of SFR in the United States, 1920–2015

 

Seasonality

• Although SFR cases can occur during any month of the year, most cases reported illness in May–August.
• This period coincides with the season when adult Dermacentor ticks are most active.
• Seasonal trends may vary depending on the area of the country and tick species involved.
  • In Arizona, the majority of SFR cases are spread by the brown dog tick (Rhipicephalus sanguineus sensu lato) and cases occur year-round with peak months of illness onset April through October.

Figure 3–Number of reported cases of spotted fever rickettsiosis, by month of onset–United States, 2000–2019

 

Geography

• SFR cases have been reported throughout the contiguous United States, although five states (Arkansas, Missouri, North Carolina, Tennessee, and Virginia) account for over 50% of SFR cases.
• In Arizona, RMSF cases have recently been identified in an area where the disease had not been previously seen. From 2003 to 2019, nearly 470 cases were reported with a case-fatality rate of about 5%.
  • The tick responsible for transmission of R. rickettsii in Arizona is the brown dog tick, which is found on dogs and in and around people’s homes.
  • Almost all of the cases occurred within communities with large numbers of free-roaming dogs.

Figure 4–Annual incidence (per million population) of reported spotted fever rickettsiosis–United States, 2019. (NN= Not notifiable)

 

People at Risk

• SFR cases are more frequently reported in men than in women.
• People over the age of 40 years account for the highest number of reported cases, however, children under 10 years old represent the highest number of reported deaths.
• Persons with glucose-6-phosphate dehydrogenase (G6PD) deficiency.
• Surveillance data shows higher risk for hospitalization in people with compromised immune systems (e.g., resulting from cancer treatments, advanced HIV infection, prior organ transplants, or some medications).

Figure 5–Average annual incidence (per million population) of reported spotted fever rickettsiosis, by age group–United States, 2019