Suspected and Confirmed Vector-Borne Rickettsioses of North America Associated with Human Diseases

Tickborne rickettsial diseases continue to cause severe illness and death in otherwise healthy adults and children, despite the availability of low-cost, effective antibacterial therapy. Recognition early in the clinical course is critical because this is the period when antibacterial therapy is most effective. Early signs and symptoms of these illnesses are nonspecific or mimic other illnesses, which can make diagnosis challenging. Previously undescribed tickborne rickettsial diseases continue to be recognized, and since 2004, three additional agents have been described as causes of human disease in the United States: Rickettsia parkeri, Ehrlichia muris-like agent, and Rickettsia species 364D. This report updates the 2006 CDC recommendations on the diagnosis and management of tickborne rickettsial diseases in the United States and includes information on the practical aspects of epidemiology, clinical assessment, treatment, laboratory diagnosis, and prevention of tickborne rickettsial diseases. The CDC Rickettsial Zoonoses Branch, in consultation with external clinical and academic specialists and public health professionals, developed this report to assist health care providers and public health professionals to 1) recognize key epidemiologic features and clinical manifestations of tickborne rickettsial diseases, 2) recognize that doxycycline is the treatment of choice for suspected tickborne rickettsial diseases in adults and children, 3) understand that early empiric antibacterial therapy can prevent severe disease and death, 4) request the appropriate confirmatory diagnostic tests and understand their usefulness and limitations, and 5) report probable and confirmed cases of tickborne rickettsial diseases to public health authorities.

Ticks, including many that bite humans, are hosts to several obligate intracellular bacteria in the spotted fever group (SFG) of the genus Rickettsia. Only Rickettsia rickettsii, the agent of Rocky Mountain spotted fever, has been definitively associated with disease in humans in the United States. Herein we describe disease in a human caused by Rickettsia parkeri, an SFG rickettsia first identified >60 years ago in Gulf Coast ticks (Amblyomma maculatum) collected from the southern United States. Confirmation of the infection was accomplished using serological testing, immunohistochemical staining, cell culture isolation, and molecular methods. Application of specific laboratory assays to clinical specimens obtained from patients with febrile, eschar-associated illnesses following a tick bite may identify additional cases of R. parkeri rickettsiosis and possibly other novel SFG rickettsioses in the United States.

Rocky Mountain spotted fever is a life-threatening, tick-borne disease caused by Rickettsia rickettsii. This disease is rarely reported in Arizona, and the principal vectors, Dermacentor species ticks, are uncommon in the state. From 2002 through 2004, a focus of Rocky Mountain spotted fever was investigated in rural eastern Arizona. We obtained blood and tissue specimens from patients with suspected Rocky Mountain spotted fever and ticks from patients' homesites. Serologic, molecular, immunohistochemical, and culture assays were performed to identify the causative agent. On the basis of specific laboratory criteria, patients were classified as having confirmed or probable Rocky Mountain spotted fever infection. A total of 16 patients with Rocky Mountain spotted fever infection (11 with confirmed and 5 with probable infection) were identified. Of these patients, 13 (81 percent) were children 12 years of age or younger, 15 (94 percent) were hospitalized, and 2 (12 percent) died. Dense populations of Rhipicephalus sanguineus ticks were found on dogs and in the yards of patients' homesites. All patients with confirmed Rocky Mountain spotted fever had contact with tick-infested dogs, and four had a reported history of tick bite preceding the illness. R. rickettsii DNA was detected in nonengorged R. sanguineus ticks collected at one home, and R. rickettsii isolates were cultured from these ticks. This investigation documents the presence of Rocky Mountain spotted fever in eastern Arizona, with common brown dog ticks (R. sanguineus) implicated as a vector of R. rickettsii. The broad distribution of this common tick raises concern about its potential to transmit R. rickettsii in other settings. Copyright 2005 Massachusetts Medical Society.