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Spatial Clustering by Disease Severity among Reported Rocky Mountain Spotted Fever Cases in the United States, 2001–2005

Rocky Mountain spotted fever (RMSF) occurs throughout much of the United States, ranging in clinical severity from moderate to fatal infection. Yet, little is known about possible differences among severity levels across geographic locations. To identify significant spatial clusters of severe and non-severe disease, RMSF cases reported to Centers for Disease Control and Prevention (CDC) were geocoded by county and classified by severity level. The statistical software program SaTScan was used to detect significant spatial clusters. Of 4,533 RMSF cases reported, 1,089 hospitalizations (168 with complications) and 23 deaths occurred. Significant clusters of 6 deaths (P = 0.05, RR = 11.4) and 19 hospitalizations with complications (P = 0.02, RR = 3.45) were detected in southwestern Tennessee. Two geographic areas were identified in north-central North Carolina with unusually low rates of severity (P = 0.001, RR = 0.62 and P = 0.001, RR = 0.45, respectively). Of all hospitalizations, 20% were clustered in central Oklahoma (P = 0.02, RR = 1.43). Significant geographic differences in severity were observed, suggesting that biologic and/or anthropogenic factors may be impacting RMSF epidemiology in the United States.

Received July 8, 2008. Accepted for publication October 6, 2008.

Acknowledgments: This study would not be possible without the continued support from the states for the RMSF CRF surveillance system, which provides unique, additional data that are vital for better understanding RMSF epidemiology in the United States. In particular, we thank John Dunn, Kristy Bradley, Carl Williams, and Craig Levy for reviewing this manuscript, and the helpful comments received from Robert Massung.

^* Address correspondence to Jennifer Zipser Adjemian, Centers for Disease Control and Prevention, 1600 Clifton Road, NE, MS-G44, Atlanta, GA 30030. E-mail: gdn5{at}cdc.gov

Authors’ addresses: Jennifer Zipser Adjemian, John Krebs, Eric Mandel, and Jennifer McQuiston, Centers for Disease Control and Prevention, 1600 Clifton Road, NE, MS-G44, Atlanta, GA 30030.

The definition of a laboratory confirmed and probable case was changed in 2008 as follows: 1) confirmed case—has serologic evidence of a 4-fold change in immunoglobulin G (IgG)–specific antibody titer reactive with R. rickettsii antigen by IFA between paired serum specimens (one taken in the first week of illness and a second 2–4 weeks later), or detection of R. rickettsii DNA in a clinical specimen via amplification of a specific target by PCR assay, or demonstration of spotted fever group antigen in a biopsy or autopsy specimen by IHC, or isolation of R. rickettsii from a clinical specimen in cell culture; and 2) probable case—has serologic evidence of elevated IgG or IgM antibody reactive with R. rickettsii antigen by IFA, ELISA, dot-ELISA, or latex agglutination.