Three ticks can carry the RMSF bacterium,
Rickettsia rickettsii, in the United States:
(17), (18), (19)
The lone star tick, typically found throughout the southeastern, eastern, and south-central states, has expanded its range north and west over the last few decades. Although it can transmit RMSF, it should be noted that the lone star tick is not as likely to transmit the disease as the dog or wood tick.
RMSF is now reported under the category called Spotted Fever Rickettsiosis (SFR), which includes RMSF, Rickettsia parkeri rickettsiosis, Pacific Coast tick fever, and rickettsialpox. As indicated in the chart on the top right, the annual incidence has gone up and down, but mortality has decreased from 28% in 1940 to less than one percent in 2008. RMSF is the most common rickettsial disease in the U.S., with the number of cases rising in recent years, as displayed in the bottom right chart. Most cases occur from April to September when the tick vectors are active in nature. The number of infected persons increases as patients age, until age 65-70, where it falls slightly. Most cases are seen East of the Rocky Mountains, with most cases reported in North Carolina and in a band across Tennessee, Missouri, Arkansas, and Oklahoma.
