Tick-borne rickettsioses around the world: emerging diseases challenging old concepts

Abstract

During most of the 20th century, the epidemiology of tick-borne rickettsioses could be summarized as the occurrence of a single pathogenic rickettsia on each continent. An element of this paradigm suggested that the many other characterized and noncharacterized rickettsiae isolated from ticks were not pathogenic to humans. In this context, it was considered that relatively few tick-borne rickettsiae caused human disease. This concept was modified extensively from 1984 through 2005 by the identification of at least 11 additional rickettsial species or subspecies that cause tick-borne rickettsioses around the world. Of these agents, seven were initially isolated from ticks, often years or decades before a definitive association with human disease was established. We present here the tick-borne rickettsioses described through 2005 and focus on the epidemiological circumstances that have played a role in the emergence of the newly recognized diseases.

FIG. 1.

Phylogenetic organization of tick-transmitted rickettsiae based on the comparison of gltA, ompA, ompB, and gene D sequences by using the parsimony method.

FIG. 2.

Dermacentor variabilis, the primary vector of Rocky Mountain spotted fever in most of the United States. From left to right, male, female, nymph, and larva. Bar scale, 1 cm.

FIG. 3.

Inoculation eschar (top panel) and maculopapular rash (bottom panel) on a patient with Mediterranean spotted fever.

FIG. 4.

Multiple tick bites on a patient with African tick bite fever caused by R. africae.

FIG. 5.

Multiples eschars on a patient infected with R. sibirica subsp. mongolitimonae.

FIG. 6.

Inoculation eschar of the scalp (top panel) and enlarged cervical lymph nodes (bottom panel) of a patient with tick-borne lymphadenopathy (R. slovaca infection).

FIG. 7.

Tick-borne rickettsiae in Africa. Colored symbols indicate pathogenic rickettsiae. White symbols indicate rickettsiae of possible pathogenicity and rickettsiae of unknown pathogenicity.

FIG. 8.

Tick-borne rickettsiae in the Americas. Colored symbols indicate pathogenic rickettsiae. White symbols indicate rickettsiae of possible pathogenicity and rickettsiae of unknown pathogenicity.

FIG. 9.

Tick-borne rickettsiae in Asia and Australia. Colored symbols indicate pathogenic rickettsiae. White symbols indicate rickettsiae of possible pathogenicity and rickettsiae of unknown pathogenicity.

FIG. 10.

Tick-borne rickettsiae in Europe. Colored symbols indicate pathogenic rickettsiae. White symbols indicate rickettsiae of possible pathogenicity and rickettsiae of unknown pathogenicity.

FIG. 11.

Western blot assay of an acute MIF-positive serum showing reactivity with the 135- and 115-kDa specific protein antigens of Rickettsia conorii and Rickettsia africae, respectively. Columns 1, 3, and 5, R. conorii antigens. Columns 2, 4, and 6, R. africae antigens. Columns 1 and 2, untreated sera. Columns 3 and 4, sera absorbed with R. conorii antigens. Columns 5 and 6, sera absorbed with R. africae antigens. MW, molecular weight marker. The interpretation is that when absorption is performed with R. africae, it results in the disappearance of homologous and heterologous antibodies, but when it is performed with R. conorii, only homologous antibodies disappeared. This indicates that antibodies are specific for R. africae. Molecular masses are indicated on the left.

FIG. 12.

Immunohistochemical detection of Rickettsia sibirica subsp. mongolotimonae (arrows, rickettsiae staining red) in a skin biopsy specimen of an eschar of the patient presented in Fig. 5. Note the abundant inflammatory infiltrate with necrotic features and vascular injury in the dermis (polyclonal rabbit anti-R. sibirica subsp. mongolitimonae antibody used at a dilution of 1/2,000 with hemalin counterstain; original magnification, ×400).