Rickettsiosis Prevalence in the United States
The overall prevalence of spotted fever group (SFG) rickettsiosis in the United States is 8.9 cases per million persons annually, with substantial geographic variation ranging from <5 to >60 cases per million depending on location. 1
National Prevalence Data
During 2008-2012, the estimated average annual incidence of SFG rickettsiosis was 8.9 cases per million persons in the United States based on passive surveillance. 1
The reported annual incidence has increased substantially over the past 2 decades, suggesting either true increases in disease burden or improved recognition and reporting. 1
Cases have been reported from all 48 contiguous states and the District of Columbia, though the disease is not uniformly distributed. 1
Geographic Variation in Prevalence
The highest disease burden is concentrated in specific states, with 63% of all reported cases originating from just five states. 1
High-Prevalence States
Arkansas, Missouri, North Carolina, Oklahoma, and Tennessee account for 63% of all reported SFG rickettsiosis cases during 2008-2012. 1
These states correspond to regions where the primary tick vectors (Dermacentor variabilis and Dermacentor andersoni) are most abundant. 1
Exceptionally High-Risk Areas
American Indian reservations in Arizona represent the highest prevalence areas in the United States, with rates approximately 150 times the national average. 1
On the three most affected Arizona reservations, the average annual incidence rate for 2009-2012 was approximately 1,360 cases per million persons. 1
This represents a dramatic emergence, with approximately 300 cases and 20 deaths reported during 2003-2013 compared to only three cases in the previous decade. 1
The case-fatality rate in these communities is 7-10%, the highest of any region in the United States, primarily due to delayed recognition and treatment. 1
This emergence is associated with a different tick vector (Rhipicephalus sanguineus, the brown dog tick) compared to other U.S. regions. 1
Age-Specific Prevalence Patterns
The highest incidence occurs in persons aged 60-69 years. 1
However, the highest case-fatality rate is among children aged <10 years, making early recognition critical in pediatric populations. 1
Illness occurs across all age groups, so clinicians should maintain suspicion regardless of patient age. 1
Seasonal Variation
Most cases (majority) are reported during April-September, coinciding with peak tick host-seeking activity. 1
Cases can occur in any month of the year, particularly in geographic regions with warmer climates where tick activity is year-round. 1
Regional Rickettsiosis Variants
Beyond Rocky Mountain spotted fever, other rickettsial infections have distinct geographic distributions:
R. parkeri rickettsiosis: Most commonly reported along the Gulf Coast and eastern seaboard (first described 2004). 1
Rickettsia species 364D: All reported cases are from California (first described 2010). 1
International Context
For comparison, prevalence data from other regions:
Mexico: Pooled prevalence of R. rickettsii in ticks is 9.89% (95% CI: 2.03-22.74), with human seroprevalence in Baja California of 2.9%. 2, 3
Colombia: Highest pooled prevalence in ticks at 17.00% (95% CI: 7.01-30.24). 2
Brazil: Pooled prevalence in ticks of 2.03% (95% CI: 0.66-4.14). 2
Critical Clinical Caveats
The true prevalence is likely underestimated due to several factors:
Passive surveillance systems may not capture all cases, particularly milder infections or those misdiagnosed as viral illnesses. 1
The surveillance category for SFG rickettsiosis may not differentiate between RMSF and other SFG rickettsioses due to limitations of submitted diagnostic evidence. 1
Only 55-60% of confirmed RMSF cases report a recognized tick bite, meaning many patients and clinicians may not consider the diagnosis. 1
In endemic areas, actual disease incidence is likely underrepresented in passive surveillance estimates. 1
Clinicians should maintain high suspicion in endemic areas regardless of whether patients report tick exposure, as absence of recognized tick bite should never dissuade consideration of rickettsiosis. 1