From 1997 to 2009, the Tick-Borne Disease Laboratory of the U.S. Army Public Health Command (USAPHC) (formerly the U.S. Army Center for Health Promotion and Preventive Medicine) screened 5286 Dermacentor variabilis ticks removed from Department of Defense (DOD) personnel, their dependents, and DOD civilian personnel for spotted fever group rickettsiae using polymerase chain reaction and restriction fragment length polymorphism analysis. Rickettsia montanensis (171/5286=3.2%) and Rickettsia amblyommii (7/5286=0.1%) were detected in a small number of samples, but no ticks were found positive for Rickettsia rickettsii, the agent of Rocky Mountain spotted fever (RMSF) until May 2009, when it was detected in one D. variabilis male removed from a child in Maryland. This result was confirmed by nucleotide sequence analysis of the rickettsial isolate and of the positive control used in the polymerase chain reaction, which was different from the isolate. Lethal effects of rickettsiostatic proteins of D. variabilis onR. rickettsii and lethal effects of R. rickettsii infection on tick hosts may account for this extremely low prevalence. Recent reports of R. rickettsii in species Rhipicephalus sanguineus and Amblyomma americanum ticks suggest their involvement in transmission of RMSF, and other pathogenic rickettsiae have been detected in Amblyomma maculatum. The areas of the U.S. endemic for RMSF are also those where D. variabilisexist in sympatry with populations of A. americanum and A. maculatum. Interactions among the sympatric species of ticks may be involved in the development of a focus of RMSF transmission. On the other hand, the overlap of foci of RMSF cases and areas of A. americanum and A. maculatum populations might indicate the misdiagnosis as RMSF of diseases actually caused by other rickettsiae vectored by these ticks. Further studies on tick vectors are needed to elucidate the etiology of RMSF.

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Dogs and people are exposed to and susceptible to infection by
many of the same tick-borne bacterial pathogens in the order
Rickettsiales, including Anaplasma phagocytophilum, Ehrlichia
canis, E. chaffeensis, E. ewingii, Rickettsia rickettsii, R.
conorii, and other spotted fever group rickettsiae. Recent
findings include descriptions of novel Ehrlichia and Rickettsia
species, recognition of the occurrence and clinical significance
of co-infection, and increasing awareness of Rhipicephalus
sanguineus-associated diseases. Newer molecular assays are
available, although renewed efforts to encourage their use are
needed. This review highlights the ecology and epidemiology of
these diseases, and proposes avenues for future investigation.
Published by Elsevier Ltd.