Rocky Mountain spotted fever (RMSF) is a disease of people and dogs caused by Rickettsia rickettsii. R rickettsii and closely related members of the spotted fever group of rickettsiae are in parts of North, South, and Central America. These pathogens are transmitted primarily through the bites of infected ticks. Among the numerous related Rickettsia species, R rickettsii's ability to infect dogs is best-documented. The ability of genetically similar rickettsial organisms, such as R parkeri, to cause clinically similar disease in dogs is unknown. Because of their susceptibility to R rickettsii and relatively higher rates of tick exposure, dogs may serve as excellent sentinels of risk for R rickettsii infection in people. Clusters of disease are frequently reported in defined geographic areas, and temporally associated infections may be seen both in dogs and their owners.
In the USA, Dermacentor variabilis (the American dog tick) and D andersoni (the Rocky Mountain wood tick) have been considered the primary vectors for R rickettsii, which is most common in North America is southeastern states in the USA. In South America, several Amblyomma spp of ticks have been implicated in transmission. Rhipicephalus sanguineus ticks (the brown dog tick) are the primary vector in some focal areas of Arizona, particularly on native American tribal lands. R sanguineus ticks are also associated with transmission of R rickettsii in Central America and with large, city-based outbreaks in Mexico.
The pathogen is acquired by larval and nymph stages of ticks while feeding on infected vertebrate hosts and is also passed from female ticks to progeny through transovarial transmission. Typically, very few individual ticks (eg, <1% of Dermacentor spp ticks) carry R rickettsii, even in areas considered highly endemic. However, in highly enzootic regions of Arizona where R rickettsii is transmitted by the brown dog tick, as many as 5% of ticks may be infected.
Seroprevalence in dogs from endemic areas ranges from 4.3%–77%, but these values do not accurately reflect infection rates antirickettsial antibodies may cross-react among genetically similar rickettsiae. RMSF transmission through blood transfusion has been documented in a human case and should be considered when selecting canine blood donors. Direct transmission from dogs to people has not been reported, although human infection may occur after contact of abraded skin or conjunctiva with tick hemolymph or excreta during removal of engorged ticks from pets.
Dogs are highly susceptible to clinical infection with R rickettsii; in contrast, it is rarely diagnosed in cats. Early signs in dogs may include:
Petechial hemorrhages of the conjunctiva and oral mucosa may be seen in severe cases. Focal retinal hemorrhage may be seen during the early course of disease. Neurologic manifestations such as altered mental states, vestibular dysfunction, and paraspinal hyperesthesia may occur.
Thrombocytopenia is common. Leukopenia develops during the early stages of infection and, in untreated cases, is followed by progressive leukocytosis. Serum biochemical abnormalities may include hypoproteinemia, hypoalbuminemia, azotemia, hyponatremia, hypocalcemia, and increased liver enzyme activities. Case fatality rates of ~1%–10% are expected.
Vascular endothelial damage is due to direct cytopathic effects of the rickettsiae, which reproduce within endothelial cells. Vascular endothelial damage and thrombocytopenia contribute to development of petechiae and ecchymoses. Necrosis of the extremities (acryl gangrene) or disseminated intravascular coagulation can develop in severely affected dogs.
Dogs presenting with characteristic clinical signs of Rocky Mountain spotted fever, particularly fever or neurologic abnormalities, should be assessed for RMSF, and it is crucial to maintain an index of suspicion in areas where the tick vectors may be found. Differential diagnoses include other causes of fever of unknown origin; in areas where the brown dog tick is the main vector, most febrile dogs will likely be infected with Ehrlichia canis, which is more common than RMSF. Very early in infection, PCR of circulating blood may be positive, although many severely affected animals become PCR-negative as the bacteria become resident in endothelium. PCR of a tick-bite eschar can be useful when such a lesion is present. Suspect cases should be treated with appropriate antimicrobials without waiting for test confirmation, and the therapeutic response is usually dramatic, as it is in other canine rickettsial diseases.
Indirect fluorescent antibody titer (IFA) and ELISA are available for serologic testing. However, there is a high incidence of cross-reacting antibodies to a variety of non- and less-pathogenic spotted fever group rickettsiae, so a single seropositive test does not confirm RMSF as a cause of clinical disease. Dogs that have been sick for a week or more may develop very high titers, which can support a diagnosis, although antibodies may persist for months after acute RMSF infection. Demonstration of a 4-fold rise in titer over 2–4 weeks can retrospectively support that a dog has been infected with some type of Rickettsia.
Antibiotic treatment for Rocky Mountain spotter fever should be administered based on clinical suspicion without waiting for results of serologic tests, because delayed administration of antibiotics may result in higher rates of severe or fatal outcome. Doxycycline is the treatment of choice, regardless of age of the dog, and should be administered at a dosage of 5–10 mg/kg/day, PO or IV, for 10–21 days. Tetracycline at 22 mg/kg, PO, three times a day for 2 weeks is also effective. Chloramphenicol has been used to treat RMSF in the past, but its use is associated with higher rates of fatal outcome in people, and it is not recommended. Other broad-spectrum antibiotics are ineffective against R rickettsii infection, and there is some evidence in human cases that use of fluoroquinolones may actually worsen infection.
Early seronegative tests should not be considered a reason to stop therapy, because antibodies may take ≥1 week to develop in acute cases. Supportive care for dehydration and hemorrhagic diathesis may be necessary. Because of alterations in vascular integrity, conservative rates of fluid administration are advised. Animals with neurologic dysfunction may have residual deficits. Immunity appears to be lifelong after natural infection; therefore, recurrent episodes should not be attributed to RMSF.
Precautions should be taken for the safe removal and control of ticks. In settings in which R rickettsii transmission from R sanguineus is suspected, medications and products with proven efficacy against this tick species are important to use. Because R sanguineus infestations can be problematic in kennels and around homes, and longterm tick control is needed for outbreak control, use of effective long-acting tick collars on all susceptible dogs might be considered; collars containing propoxur, amitraz, or flumethrin have proven activity against R sanguineus.
R rickettsii is considered a zoonotic pathogen. The potential for household clustering and large urban outbreaks, particularly in areas with transmission by brown dog ticks, makes RMSF a disease of significant public health concern. Although clinical disease occurs both in animals and people, the involvement of a required intermediate tick vector for transmission means dogs and other infected animals do not pose a direct transmission risk in normal circumstances. Nevertheless, dogs are the main host for brown dog ticks, and overly abundant and stray dogs can increase overall risk for other dogs and people. Particularly in areas where transmission occurs via R sanguineus, close cooperation between veterinary, medical, and public health officials is important to achieve control.
Although there are multiple related Rickettsia species that can be transmitted to people, the most common in dogs and the most severe in people and dogs is Rickettsia rickettsii, the agent of Rocky Mountain spotted fever.
Infected dogs serve to increase risk for people and dogs where the vector is the brown dog tick, and in any region, the presence of infected dogs is a valuable sentinel for human risk.
RMSF causes disease primarily attributable to vasculitis and can be difficult to confirm as a diagnosis, requiring that clinicians know about the local prevalence, have an index of suspicion, and treat suspected cases as quickly as possible.