Rocky Mountain spotted fever is an acute febrile disease of the the Americas. In addition to humans, other mammalian hosts include dogs, rodents, and other small animals. The causal organism is Rickettsia rickettsii. The disease is transmitted by ticks that feed on infected animals. Mortality rate varies, averaging about 20%, but as high as 40% in cases where it is not identified and appropriately treated with antibiotics; it is low in children and high in adults.
In the 19th Century, settlers and visitors to the Rocky Mountain region of North America mentioned "mountain fever" as a cause of morbidity and occasional mortality. "Mountain fever" likely was any of a number of febrile illnesses, including typhoid fever, Rocky Mountain spotted fever (RMSF) and Colorado tick fever.
The ticks that generally transmit Rocky Mountain spotted fever are large ticks: American dog tick Dermacentor variabilis, Rocky Mountain wood tick D. andersoni. These are large ticks, in contrast to the small ticks (Deer tick Ixodes scapularis, Pacific Black-Legged tick I. pacificus that generally transmit the spiral-shaped bacterium that causes Lyme disease. However, the larger Lone star tick Amblyomma americanum is capable of transmitting both diseases.
In the USA in 1988, there was a 3 percent fatality rate of the 700 reported cases of Rocky Mountain spotted fever.