Rocky Mountain spotted fever epidemiology and demographics

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Ilan Dock, B.S.

Overview

Since the 1920's the United States Center for Disease Control and Prevention has deemed Rocky Mountain spotted fever (RMSF) as a reportable disease. RMSF cases were most often reported within the Rocky Mountain region, although recent data reveals that the disease is widespread throughout the United States. Areas that currently harbor the majority of RMSF infections are Oklahoma, Tennessee, and Arkansas. The disease has also been reported throughout the Western Hemisphere. Incidence is highest among children the between the ages of 5-9 years and adults between the ages of 40-64 years. Fatality rates are also higher among these groups, with the highest fatality rate in the elderly at 60 years or more. In terms of demographics, Rocky Mountain spotted fever has been reported at higher rates among males, especially of White and Native American descent.

Epidemiology and Demographics

Incidence

• Rocky Mountain spotted fever has been a reportable disease in the United States since the 1920s.
• In the last 50 years, approximately 250-1200 cases of Rocky Mountain spotted fever have been reported annually, although it is likely that many more cases go unreported.
• CDC compiles the number of cases reported by the state health departments.
• To ensure standardization of reporting across the country, CDC advises that a consistent case definition be used by all states.

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. Case fatality rates vary from year-to-year, but have had an overall decreasing trend from 28% case fatality in 1944 to <1% case fatality beginning in 2001.

Seasonal Distribution of Rocky Mountain spotted fever

• Over 90% of patients with Rocky Mountain spotted fever are infected during April through September.
• This period is the season for increased numbers of adult and nymphal Dermacentor ticks.
• A history of tick bite or exposure to tick-infested habitats is reported in approximately 60% of all cases of Rocky Mountain spotted fever.

Proportion of RMSF cases reported to CDC by month of onset, 1993 through 2010: This figure shows the percent of cases reported from 1993 through 2010 by month of onset to give the seasonality of cases. There are cases reported in each month of the year, however most are reported between May and August. Roughly 20% of cases are reported in both June and July.

Geography of Rocky Mountain spotted fever

• 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 responsible for 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. Through 2009, over 90 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.
• Other incidences have recently been reported in portions of California, the Northwest (Washington, and Oregon), and in portions of the Southwest (Arizona, Texas, and New Mexico.)

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.

Age

• Incidence rate is highest among children between the ages of less than 10 years of age, particularly 5-9, and adults between the ages of 40-64.
• Children ages 0-9 and adults ages 60 or higher are at a higher risk of fatality.

Average annual incidence of Rocky Mountain spotted fever by age-group, 2000 through 2010: This figure shows the average annual incidence of RMSF per million persons by age groups for 2000 through 2010. The graph shows that cases have been reported in every age group with increased incidence as age increases. There is a slightly higher incidence rate in the 5-9 year old age group, which surpasses 4 cases per million persons. The highest incidence rate in observed in age groups 55-59 and 60-64 years old, both of which surpass 8 cases per million persons.

Demographics

• The frequency of reported cases of Rocky Mountain spotted fever is highest among males, particularly of Caucasian and American Indian descent.
• Two-thirds of the Rocky Mountain spotted fever cases occur in children.
• Individuals with frequent exposure to dogs and who reside near wooded areas or areas with high grass may also be at increased risk of infection.

Worldwide Infection

• Incidences of Rocky Mountain spotted fever are most prominent in the Western hemisphere.
• Infections have been documented in portions of Argentina, Brazil, Colombia, Costa Rica, Mexico, and Panama.
• Some synonyms for Rocky Mountain spotted fever in other countries include tick typhus, Tobia fever(Colombia), São Paulo fever, fiebre maculosa (Brazil), and fiebre manchada (Mexico).
• Closely related organisms cause other types of spotted fevers in other parts of the world.