Rocky Mountain spotted fever epidemiology and demographics

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. In Arizona, where transmission is associated with the brown dog tick, peak months of illness onset are August and September.



Geography of Rocky Mountain Spotted Fever
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. 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.



Age factor
The frequency of reported cases of Rocky Mountain spotted fever is highest among males, American Indians, and people aged 50-69. 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 ages 0-9 and American Indians have an increased risk of fatal outcome from RMSF.



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.



Demographics
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.

Infection with Rickettsia rickettsii has also been documented in Argentina, Brazil, Colombia, Costa Rica, Mexico, and Panama. Closely related organisms cause other types of spotted fevers in other parts of the world.