Statistics and Epidemiology

Rocky Mountain spotted fever (RMSF) has been a reportable disease in the United States since the 1920’s. CDC compiles the number of cases reported by state health departments. The incidence of RMSF (the number of RMSF cases for every million persons) has increased during the last decade, from less than 2 cases per million persons in 2000 to over 6 cases per million in 2010. During the same time period, the proportion of RMSF cases resulting in death (case fatality) has declined to a low of less than 0.5%.

Epidemiology Figure 1 – Reported incidence and case fatality of RMSF in the United States, 1920–2010

Incidence and case fatality from 1920-2010 Cases of RMSF have been recorded from the 1920s through present day. Trends in RMSF incidence can be observed as ebbs and flows of several years at a time. Periods of increased incidence can be seen between 1930 and 1950 and 1968 through 1990. More recently there has been a more dramatic increase in incidence of RMSF increasing from 1.9 cases per million persons in 2000 to an all-time high of 8.4 cases per million persons in 2008. Case fatality rate was first reported in 1940. Fatality rates vary from year-to-year, but have had an overall decreasing trend from 28% case fatality in 1944 to

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Epidemiology Figure 2 – Number of U.S. RMSF cases* reported to CDC, 1993–2010. *Numbers presented here may differ from numbers presented in the MMWR Annual Summary of Notifiable Diseases. Data presented here are based on year of illness onset rather than reporting year.

Number of annual RMSF cases 1993-2010 The graph displays the number of human cases of RMSF cases reported to CDC annually from 1993 through 2010. The number of cases of RMSF reported to CDC have generally increased annually from 345 cases in 1993, to 2553 cases reported in 2008. Cases decreased significantly in 2009 to 1791 cases and increased slightly in 2010.

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Geography

Although RMSF cases have been reported throughout most of the contiguous United States, five states (North Carolina, Oklahoma, Arkansas, Tennessee, and Missouri) account for over 60% of RMSF cases. The primary tick that transmits R. rickettsii in these states is the American dog tick (Dermacentor variabilis Dermacentor andersoni).

In eastern Arizona, RMSF cases have recently been identified in an area where the disease had not been previously seen. Between 2003 and 2010, roughly 140 cases had been reported, and approximately 10% of the people diagnosed with the disease in this part of the state have died. The tick responsible for transmission of R. rickettii in Arizona is the brown dog tick (Rhipicephalus sanguineus), which is found on dogs and around people’s homes. Almost all of the cases occurred within communities with a large number of free-roaming dogs.

Epidemiology Figure 3 – Annual reported incidence (per million population) for RMSF in the United States for 2010. (NN= Not notifiable)

Geographic distribution of RMSF incidence in 2010 This figure shows the annual reported incidence of RMSF cases by state in 2010 per million persons. RMSF was not notifiable in Alaska and Hawaii in 2010. The incidence rate was zero for Connecticut, Kansas, Massachusetts, Nevada, South Dakota, Vermont and West Virginia. Incidence ranged between 0.2 to 1.5 cases per million persons for California, Colorado, Florida, Kentucky, Louisiana, Michigan, Minnesota, New Hampshire, New Mexico, North Dakota, Ohio, Oregon, Pennsylvania, Texas, Utah, Washington and Wisconsin. Annual incidence ranged from 1.5 to 19 cases per million persons in Alabama, Arizona, the District of Columbia, Georgia, Idaho, Illinois, Indiana, Iowa, Maine, Maryland, Mississippi, Montana, Nebraska, New Jersey, New York, Rhode Island, South Carolina, Virginia and Wyoming. The highest incidence rates, ranging from 19 to 63 cases per million persons were found in Arkansas, Delaware, Missouri, North Carolina, Oklahoma, and Tennessee.

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Seasonality

Although cases of RMSF can occur during any month of the year, the majority of cases reported to the CDC have an illness onset during the summer months and a peak in cases typically occurs in the months of June and July. This seasonality varies somewhat for different regions of the country due to the climate and the tick vectors involved. In Arizona, where transmission is associated with the brown dog tick, peak months of illness onset are April through October.

Epidemiology Figure 4 – Proportion of RMSF cases reported to CDC by month of onset 1993 through 2010.

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Persons at Risk

The frequency of reported cases of Rocky Mountain spotted fever is highest among males, American Indians, and people at least 40 years old. Individuals with frequent exposure to dogs and who reside near wooded areas or areas with high grass may be at increased risk of infection. Children under 10 years old, American Indians, people with a compromised immune system, and people with delayed treatment are at an increased risk of fatal outcome from RMSF.

Epidemiology Figure 5 – Average annual incidence of Rocky Mountain spotted fever by age group, 2000 through 2010.

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Reporting and Surveillance

As of January 1, 2010, cases of RMSF are reported under a new category called Spotted Fever Rickettsiosis (including Rocky Mountain spotted fever). This change was made to better reflect the scope of cases being reported under the previous heading of RMSF, as many of those cases were not identified as being specifically caused by R. rickettsii. The Council of State and Territorial Epidemiologists released a Position Statement detailing the new category. For more information on how to report cases of Rocky Mountain spotted fever and other tick-borne Rickettsial diseases, visit the tick portal for State Health Departments.

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Rocky Mountain Spotted Fever (RMSF)