Rocky Mountain spotted fever (RMSF) is a life-threatening, tick-borne disease of humans and dogs caused by Rickettsia rickettsii. Despite its name, RMSF has been prevalent in humans throughout the United States since the 1930s, but was not recognized in dogs until the 1970s.

The tick vector and reservoir of RMSF is usually the American dog tick (Dermacentor variabilis) in the eastern United States, or the Rocky Mountain wood tick (Dermacentor andersoni) in the western United States. However, a 2002-2004 outbreak in humans in Arizona was traced to the common brown dog tick (Rhipicephalus sanguineus), a tick common worldwide and not previously considered to be an important vector of disease. An estimated 4% of the American dog ticks are infected with Rickettsia species, but the vast majority of these organisms are nonpathogenic Rickettsia. Therefore, the chance of an individual tick harboring Rickettsia rickettsii is slight, and prophylactic treatment for a tick bite is not routinely indicated.

Ticks can become infected by feeding on parasitemic small mammals, such as chipmunks and squirrels, that then serve as reservoirs for the organism. Dogs and humans are incidental hosts and are the only species that display clinical signs of disease. Infected ticks can spread RMSF to other ticks through the transfer of bodily fluids during mating or from the pregnant female to her offspring, but transovarial infection is the primary means by which R. rickettsii propagates in nature. Once infected, ticks then transmit RMSF to vertebrates (including dogs and humans) when taking blood meals. An infected tick must be attached to its host from 5-20 h for transmission to occur.

Once a human or dog is bitten by an infected tick, the RMSF organism undergoes a 2-14 d incubation period before it enters the host's circulation. The organism then invades endothelial cells of the microcirculation, causing vasculitis. Replication of R. rickettsii and vasculitis can result in edema, hemorrhage, shock, and vascular collapse. The organs most often affected are the brain, skin, heart and kidneys. Vascular leakage also activates the animal's platelets and coagulation system.

Humans From 2002-2004, 16 human patients were infected in eastern Arizona (11 confirmed and 5 probable cases); 13 were children under 12 years of age and 2 patients died because infection was either not suspected or treatment started too late in the course of illness. All had contact with tick-infested dogs, and 4 reported being bitten by a tick. Three additional patients were retrospectively identified as having RMSF in 2001; 2 of these patients were young children and died before the diagnosis was established. Large populations of tick-infested dogs that roamed freely were present in the affected communities, and serum from 4 dogs owned by infected patients had high titer antibodies to R. rickettsii. Dogs thus served as transport hosts by carrying infected ticks close to their owners.

The infection is classically characterized by fever, myalgias, headache, and a petechial rash, and is the most common fatal tick-borne human disease in the United States. Mortality rates in humans are about 4% currently, or as high as 20% in untreated patients. A significant portion of this persistent mortality likely is due to delay in diagnosis and treatment.

Dogs Most cases of RMSF in dogs are reported between the months of March and October, coinciding with the prevalence of ticks in the environment. Most infected dogs are less than 3 yrs old and have had a recent history of tick exposure. A red rash develops about 12 days after the initial tick bite. However, the most consistent clinical finding is fever of 102.5–105ºF about 5 days after the tick bite. Also seen are petechial and ecchymotic hemorrhages on mucosal surfaces from platelet destruction and vasculitis. Vasculitis may cause edema and necrosis in the extremities including the scrotum, prepuce, and ears of affected dogs. Other signs include joint swelling, myalgia, dyspnea, and neurological abnormalities such as vestibular ataxia due to meningoencephalitis.

Ocular lesions are often seen with RMSF as a result of the vasculitis and hemorrhage. Conjunctival hyperemia, hyphema, retinal hemorrhage, and anterior uveitis have been reported. While the disease in dogs is often self-limiting within approximately 2 weeks duration, untreated or severely affected dogs may succumb.