Rocky Mountain Spotted Fever : Epidemiology

by CDC

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Major Tick Vectors in the United States

There are two major vectors of R. rickettsii in the United States, the American dog tick and the Rocky Mountain wood tick. However, other ticks can acquire R. rickettsii in nature and some may serve as experimental vectors of R. rickettsii in the laboratory.

American Dog Tick

American dog tick (Dermacentor variabilis) is widely distributed east of the Rocky Mountains and also occurs in limited areas on the Pacific Coast. Dogs and medium-sized mammals are the preferred hosts of adult D. variabilis, although it feeds readily on other large mammals, including humans. This tick is the most commonly identified species responsible for transmitting R. rickettsii to humans.

Rocky Mountain Wood Tick

Rocky Mountain wood tick (Dermacentor andersoni) is found in the Rocky Mountain states and in southwestern Canada. The life cycle of this tick may require up to 2 to 3 years for completion. Adult ticks feed primarily on large mammals. Larvae and nymphs feed on small rodents.

Other Tick Species

Other tick species like Rhipicephalus sanguineus and Amblyomma cajennense have been shown to be naturally infected with R. rickettsii or serve as experimental vectors in the laboratory. While these species appear to play only a minor role in the ecology and transmission of R. rickettsii in the United States, they can cause infections in visitors to Central and South America.

Epidemiology

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.

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.

Geography of Rocky Mountain Spotted Fever

Over half of Rocky Mountain spotted fever infections are reported from the south-Atlantic region of the United States (Delaware, Maryland, Washington D.C., Virginia, West Virginia, North Carolina, South Carolina, Georgia, and Florida). Infection also occurs in other parts of the United States, namely the Pacific region (Washington, Oregon, and California) and west south-central (Arkansas, Louisiana, Oklahoma, and Texas) region.

The states with the highest incidences of Rocky Mountain spotted fever are North Carolina and Oklahoma; these two states combined accounted for 35% of the total number of U.S. cases reported to CDC during 1993 through 1996. Although Rocky Mountain spotted fever was first identified in the Rocky Mountain states, less than 3% of the U.S. cases were reported from that area during the same interval (1993-1996).

Persons at Risk for Infection

The frequency of reported cases of Rocky Mountain spotted fever is highest among males, Caucasians, and children. Two-thirds of the Rocky Mountain spotted fever cases occur in children under the age of 15 years, with the peak age being 5 to 9 years old. 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

Infection with Rickettsia rickettsii has also been documented in 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 and fiebre maculosa (Brazil), and fiebre manchada (Mexico). Closely related organisms cause other types of spotted fevers in other parts of the world.

About the Author

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The Centers for Disease Control and Prevention (CDC) is one of the 13 major operating components of the Department of Health and Human Services (HHS), which is the principal agency in the United States government for protecting the health and safety of all Americans and for providing essential human services, especially for those people who are least able to help themselves.