Rocky Mountain spotted fever.

R ocky Mountain spotted fever (RMSF), a classic metazoonosis that involves both vertebrate and non-vertebrate reservoir hosts, is a seasonal disease of dogs and humans in the Americas. The clinical illness was first described among Native Americans, soldiers, and settlers in the Bitterroot River and Snake River valleys of Montana and Idaho during the late 1890s, but remained unrecognized in dogs until the late 1970s. The causative organism, Rickettsia rickettsii, was first described by Howard T. Ricketts in 1909 and is maintained in nature by ixodid (hard-bodied) ticks via transmission to-and-from various rodent reservoirs. As primary reservoir hosts, the ticks vector R rickettsii to larger mammals; however, dogs and humans are the only ones that display clinically recognizable illnesses. Rickettsia rickettsii are not naturally transmitted dog-to-dog, dog-to-human, or human-to-human. Public health surveillance, since the 1930s, has revealed RMSF to be the most frequently reported and most severe human rickettsial illness in the United States and, probably, in the Western Hemisphere. Reducing exposure to ticks, prompt removal of ticks, and early diagnosis with appropriate antibiotic treatment are the most important factors in reducing morbidity and mortality from RMSF. The Causative Organism Rickettsia rickettsii are small (0.2 X 0.5 µm to 0.3 X 2.0 µm) coccobacillary, gram-negative, obligate intra-cellular parasites in the family Rickettsiaceae. Like most other spotted fever group (SFG) rickettsiae, R rickettsii are conveyed to vertebrate hosts by tick vectors. 1,2 Once they infect dogs or humans, R rickettsii multiply within vascular endothelium and vascular smooth muscle, inducing vasculitis and thrombosis in many organs, notably those with an abundant endarterial circulation or immunohistochemical) are required to demonstrate R rickettsii in histologic sections. 1,2,4 Although other SFG rickettsiae are transmitted by arthropods and cause various illnesses worldwide, R rickettsii is the only one known to be pathogenic for both animals and humans in the Western Hemisphere. Cycle of the Organism in Nature and the Vectors Rickettsia rickettsii are maintained in nature by transstadial passage within, and transovarial (vertical) transmission between, generations of ixodid ticks. These ticks also vector R rickettsii to and from various rodent reservoirs and other small mammals. Naïve larval and nymphal ticks become infected while feeding on small rodents (eg, mice, voles, squirrels, or chipmunks) with acute rickettsemia. 2,5 To enable transovarial transmission, female ticks need to ingest numerous rickettsiae or be infected transstadially. Male ticks can transfer R rickettsii to females during the mating process via spermatozoa …