International Health
Volume 1, Issue 1 , Pages 17-25, September 2009

Old and new tick-borne rickettsioses

Unité des Rickettsies, CNRS-IRD UMR6236-198, Université de la Méditerranée, Faculté de Médecine, 27 bd Jean Moulin, 13385 Marseille cedex 5, France

Received 30 January 2009; received in revised form 10 March 2009; accepted 18 March 2009.

Article Outline

Summary 

The field of rickettsiology is rapidly evolving. Rickettsiae are small Gram-negative bacteria that can be transmitted to humans by arthropods. In most cases they are transmitted transovarially in the arthropod; human beings are incidental hosts. In recent years the use of cell culture and molecular biology has profoundly changed our knowledge of rickettsiae and has led to the description of several new species. New rickettsial diseases have been found in three main situations: firstly, in places where no new species have been identified, typical rickettsial symptoms have been observed (Japan, China); secondly, typical rickettsioses have been found to be caused by different organisms – in such cases a new Rickettsia species has been misdiagnosed as a previously identified bacterium (for example, R. parkeri was confused with R. rickettsii); thirdly, atypical clinical symptoms have been found to be caused by rickettsial organisms such as R. slovaca. These findings challenge the old dogma that only one tick-borne rickettsiosis is prevalent in one geographical area. Many Rickettsia spp. have been identified in ticks, but have not yet been implicated in human pathology. These rickettsiae should be considered as potential pathogens. All known or suspected rickettsial diseases should be treated (including in children) with doxycycline.

Keywords: Arthropods, Ticks, Emerging infectious disease, Inoculation, Eschar, Rickettsioses

 

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1. Introduction 

Among the well-known group of diseases named rickettsioses are found both well-characterized diseases1 and emerging pathologies that have only recently been described.2 The latter group represents diseases that have been discovered during the last 15–20 years, concurrently with advances in molecular biology and cell culture techniques. New species in the Rickettsia genus are regularly described,3 and some of them have been shown to play a role in human pathology.2

Rickettsiae are small obligate intracellular bacteria that are strongly associated with eukaryotic cells. They are often found in arthropods (ticks, mites and other insects, including lice, fleas, beetles and homopterans),4 amoebae5 and leeches.6 Only blood-sucking arthropods may transmit the disease to humans, via transdermal inoculation with the arthropod's saliva.4 Rickettsia akari and Orientia tsutsugamushi are transmitted by mites, Candidatus R. felis and R. typhi are transmitted by fleas, and R. prowazekii is transmitted by body lice; the other human rickettsioses, which are reviewed here, are transmitted by ticks.

Tick-borne rickettsioses are diseases of marked endemicity. Their prevalence is strongly linked with vector and natural host distribution. In those regions where both are common, morbidity and seropositivity may be high. Moreover, recent findings provide evidence that warmer weather is linked to an increase in the aggressiveness of the brown dog tick, Rhipicephalus sanguineus.7 Increased aggressiveness would likely lead to an increased incidence of rickettsial diseases. It must be emphasized that if the proper treatment does not begin shortly after infection, the consequences of rickettsiosis can be very serious. Mortality is as high as 5% in cases of Rocky Mountain spotted fever in children, for example. Another important issue is the significant risk to travellers in regions where such diseases are endemic. Analysis of data in the GeoSentinel database showed that 3.1% of febrile travellers have rickettsiosis.8

The economic aspects of tick-borne rickettsioses have yet to be elucidated, but their significance is evident since human contact with the natural environment, as well as the number of emerging rickettsial infections and the morbidity they cause, is increasing continuously.

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2. Bacteriology and taxonomy 

Rickettsiae are strictly intracellular bacteria whose size ranges from 0.3×0.8μm to 0.5×2.0μm. The morphology is that of Gram-negative bacteria and the bacterium is surrounded by a glycocalyx or slime. Gram staining does not visualize rickettsiae, but Gimenez9 and Giemsa stains do.

For intracellular bacteria, which express few phenotypic characteristics, molecular techniques have been particularly important for classification. Molecular analysis has significantly altered rickettsial taxonomy, which is a controversial domain that has undergone many changes over time. The Rickettsiaceae family includes only the genera Rickettsia and Orientia (Table 1). For the taxonomy of rickettsiae, the isolated study of 16S rRNA is not useful as this gene is often highly homologous between species (more than 97%). Currently, five rickettsial genes have been proposed to define the genus, the group and the species.10 They are 16S rRNA (rrs), gltA, ompA, ompB and sca4 (gene D). Members of the genus Rickettsia may be classified into the spotted fever group, the typhus group, R. bellii and R. canadensis; the latter two groups lie outside the spotted fever and typhus groups (Table 2).10, 11

Table 1. Taxonomic position of the genus Rickettsia.
GenusRickettsia
FamilyRickettsiaceae (genera included: Rickettsia and Orientia)
OrderRickettsiales
ClassAlpha-proteobacteria
PhylumProteobacteria
DomainBacteria
Table 2. The family Rickettsiaceae, main members of clinical interest.
GroupSpeciesDisease
Typhus groupR. prowazekiiEpidemic typhus
R. typhiMurine typhus
Non-classifiedR. belliiUnknown pathogenicity
R. canadensisUnknown pathogenicity
Spotted fever groupR. akariRickettsialpox
Candidatus R. felisFlea-borne spotted fever or cat flea typhus
R. australisQueensland tick typhus
R. helveticaAneruptive fever
R. asiaticaUnknown pathogenicity
R. tamuraeSuggested spotted fever
R. massiliaeSpotted fever
R. aeschlimanniiSpotted fever
R. montanensisUnknown pathogenicity
R. rhipicephaliUnknown pathogenicity
R. rickettsiiRocky mountain spotted fever
R. sibirica subsp. sibiricaSiberian tick typhus
R. sibirica subsp. mongolitimonaeLymphangitis-associated rickettsiosis
R. slovacaDEBONEL-TIBOLA
R. africaeAfrican tick-bite fever
R. conorii subsp. conoriiMediterranean spotted fever
R. conorii subsp. israelensisIsraeli spotted fever
R. conorii subsp. caspiaAstrakhan fever
R. conorii subsp. indicaIndian tick-bite typhus
R. heilongjiangensisFar Eastern tick-borne rickettsiosis
R. japonicaJapanese or oriental spotted fever
R. parkeriSpotted fever
R. peacockiiUnknown pathogenicity
R. honeiFlinders Island spotted fever
R. raoultiiDEBONEL-TIBOLA
R. amblyommiiSpotted fever
R. monacensisSpotted fever
R. marmioniiAustralian spotted fever
Candidatus R. kellyiSpotted fever
OrientiaO. tsutsugamushiScrub typhus

The tick-borne rickettsioses are underlined. DEBONEL: DErmacentor BOrne Necrosis Erythema Lymphadenopathy; TIBOLA: TIck-BOrne LymphAdenitis.

At this time, there are 25 formally recognized species in the genus Rickettsia. Many other isolates exist, but they are either not recognized or not characterized. Official criteria have been proposed for the creation of subspecies within R. conorii12 and R. sibirica13 based on epidemiological, clinical, serotypical and genotypical differences that were found after multi-spacer typing.

Here, we summarize the clinical findings associated with tick-transmitted rickettsiae. The implications of these findings for human pathology are described (Table 2 and Fig. 1).

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  • Fig. 1. 

    Global distribution of the aetiological agents of the various rickettsioses. Those underlined were described more than 20 years ago. 1: Rickettsia conorii subsp. conorii; 2: R. conorii subsp. israelensis; 3: R. conorii subsp. caspia; 4: R. conorii subsp. indica; 5: R. sibirica subsp. sibirica; 6: R. massiliae; 7: R. heilongjiangensis; 8: R. japonica; 9: R. parkeri; 10: R. aeschlimannii; 11: R. australis; 12: R. honei; 13: R. monacensis; 14: R. rickettsii; 15: R. slovaca; 16: R. raoultii; 17: R. sibirica subsp. mongolitimonae; 18: R. africae; 19: R. helvetica; 20: R. akari; 21: R. amblyommii.

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3. Spotted fever group with inoculation eschar 

Diseases grouped here have some common clinical features, such as fever, rash, regional lymphadenopathy and cutaneous eschar following the tick bite (Fig. 2). The clinical course is usually mild to moderate. Four subspecies of the R. conorii complex were proposed in 2005, based on differences in epidemiology and clinical findings. These subspecies are R. conorii subsp. conorii, israelensis, caspia and indica.12

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  • Fig. 2. 

    (A) Eschar associated with lymphangitis; (B) Cutaneous eschar and eruption following tick bite. The patients gave informed consent for these photographs to be published.

The aetiological agent of Mediterranean spotted fever (MSF) is R. conorii subsp. conorii. The main vector is the brown dog tick, Rh. sanguineus, which is found throughout the world.1, 4 The agent is found in Europe and Africa. MSF is an urban and peri-urban disease and is endemic in the Mediterranean area, but has also been reported in central Europe, central Africa and southern Africa.1, 4 This disease affects all age groups1, 4 and occurs mainly in summer. Recent findings7 provide evidence that warmer weather is linked to an increase in the aggressiveness of Rh. sanguineus. Typically,1, 4 the incubation period is asymptomatic and lasts approximately 6 days. At the end of the incubation period the infected person rapidly develops a high fever, a maculopapular rash, flu-like symptoms and an inoculation eschar (‘tâche noire’). The eschar is painless and is most often found on the trunk and the limbs. The rash generally develops 2–3 days after the fever and is initially macular, then maculopapular and disseminated; the face is usually spared. Symptoms usually last for 12–20 days and clinical improvement generally occurs following 48h of antibiotic treatment. Severe disease occurs in 5–6% of cases and is associated with disseminated vasculitis, with renal, neurological and cardiovascular complications as well as phlebitis. Mortality may reach up to 2.5% in this group.1, 4

The aetiological agent of Israeli spotted fever is R. conorii subsp. israelensis.14 Although, like R. conorii subsp. conorii, it is transmitted by Rh. sanguineus, molecular studies show differences above the strain level. The bacterium is found in Israel, but new isolations, for example in Italy and Portugal, support the suggestion that the geographical distribution is wider than previously thought.4, 14 An eschar generally does not develop following infection with R. conorii israelensis, which makes the diagnosis more difficult; serious and lethal forms are more common with this agent.15

The aetiological agent of Astrakhan spotted fever is R. conorii subsp. caspia,12 which is transmitted by Rh. pumilio. Acute febrile disease with the characteristic rash was noted in the Astrakhan region of Russia and was provisionally named ‘viral exanthema of unknown aetiology’. It was proved to be a spotted fever group rickettsiosis in 1991.16 Younger men are mostly affected, and most infections take place during the summer.17 Cases are usually described around the Caspian Sea,17 but the geographic zone could be wider than previously thought (Chad, Kosovo).12 Clinically, the disease resembles MSF except for the absence of fatal forms and a lower incidence of a cutaneous eschar.12

The aetiological agent of Indian tick-bite typhus is R. conorii subsp indica, but it has never been isolated in humans.12 Indian tick-bite typhus is prevalent in India, where the clinical findings differ from MSF; there is a frequent purpuric rash and an eschar is rare.18

The aetiological agent of Siberian tick typhus is R. sibirica subsp. sibirica.13 The vectors are Dermacentor nuttalli, D. marginatus, D. silvarum, D. pictus, D. sinicus, D. auratus, Hyalomma wellingtoni, Hy. yeni and Haemaphysalis concinna.13 The disease has been described in southern and eastern Siberia, northern China, Mongolia and Kazakhstan.4 A high fever, an inoculation eschar, regional adenopathy, headache, myalgia, digestive symptoms and a cutaneous rash (possibly purpuric) materializing 2–4 days after the beginning of the symptoms characterize the illness. Siberian typhus is rarely complicated.4

The aetiological agent of Far Eastern tick-borne rickettsiosis is R. heilongjiangensis,10 which is transmitted by D. silvarum, H. concinna and H. japonica douglasi.4 Distributed just to the east of the natural range of R. sibirica, this disease was initially considered to be Siberian tick typhus. Its epidemiology remains unclear, but reported cases have occurred in China and eastern Russia.19 Symptoms include fever, headache, rash, eschar, regional adenopathy and conjunctivitis.19 The main differences between this disease and Siberian tick typhus are an onset in June–July (vs. the end of April) and moderate symptoms in older patients.19

The aetiological agent of Japanese (or Oriental) spotted fever is R. japonica, which is transmitted by H. longicornis, H. flava, D. taiwanensis and Ixodes ovatus.4 The disease occurs from April to October.4 The places at risk are bamboo plantations, crop fields, coastal hills and forests.4 Although this disease is usually found in Japan, one case has recently been described in Thailand,20 and Haemaphysalis ticks positive for R. japonica have been found in Korea.21 The onset is sudden, with the development of a high fever, headache, chills, an inoculation eschar and a maculopapular rash that appears after 2–3 days of evolution and can become petechial. Severe forms of this disease have been observed.4

Rickettsia parkeri is transmitted by Amblyomma maculatum (in the US)22 and A. triste (in South America).23 It may be responsible for the milder forms of tick-borne rickettsiosis in those areas that are not endemic for Rocky Mountain spotted fever (RMSF). Spotted fever related to R. parkeri has been described in the USA22, 24 and in South America.23 The main difficulty in identifying this disease is its differentiation from the other US spotted fevers [i.e. Rocky Mountain spotted fever (RMSF) and rickettsialpox].25 Rickettsia parkeri rickettsiosis and RMSF are both characterized by fever, myalgia, malaise, headache and a maculopapular eruption, but the R. parkeri infection differs with respect to the occurrence of an eschar, a vesicular or pustular rash and the relative absence of digestive symptoms. It has previously been confused with rickettsialpox, but R. parkeri rickettsiosis has been described in large metropolitan centres in the north-eastern states. Western blot techniques have been shown to differentiate between the two diseases.26

The aetiological agent of Queensland tick typhus is R. australis, which is transmitted by I. holocyclus, I. tasmani and I. cornuatus.27 The disease occurs in eastern Australia from June to November.27 The onset is sudden and is associated with fever, headache, myalgia, maculopapular or vesicular rash, an inoculation eschar and adenopathy. Severe forms have been described (renal failure, purpura fulminans, severe pneumonia).27

The aetiological agent of Flinders Island spotted fever is R. honei.28 This bacterium is transmitted by the ticks I. tasmani, Aponomma hydrosauri and I. granulates, which generally feed on reptiles.4 Cases occur mostly in December and January. The disease was originally described on Flinders Island to the south of Australia, and was later found on the continent and in Thailand.4, 28 The onset is sudden, with fever, headache, arthromyalgia and a moderate cough. An erythematous rash, which can be purpuric, follows these symptoms. By contrast with Queensland tick typhus there is no vesicular rash in Flinders Island spotted fever.4

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4. Rocky Mountain spotted fever 

RMSF stands well apart from other tick-borne rickettsioses when characterized by the severity of symptoms. It was the first spotted fever to have its aetiology revealed; the aetiological agent is R. rickettsii transmitted by D. variabilis, D. andersoni, A. cajennense and Rh. sanguineus.29 RMSF has been described in the western hemisphere,29, 30 in both rural and urban zones, from April to September.4 The incubation period lasts approximately 7 days.29 There is usually no eschar at the inoculation site. During the first 3 days of illness the disease is characterized by a sudden onset of fever, significant malaise and severe headache, which can be accompanied by other non-specific symptoms such as myalgia, nausea, vomiting, anorexia, abdominal pain, diarrhoea and photophobia. During this phase RMSF can be misdiagnosed as a viral illness.29 The rash appears only on the third day of fever. It is erythematous then erythemato-papular, and spreads inwards from the wrists and ankles to the arms, legs and trunk. In 60–80% of patients, disseminated petechial lesions develop on the palms and soles.4 Various clinical manifestations can occur, including neurological and pulmonary pathologies, renal failure, myocarditis, and necrosis of the extremities. The risk factors for severity are old age, postponed antibiotic treatment (more than 5 days after onset), incorrect antibiotic choice (antibiotic other than a tetracycline or chloramphenicol) and glucose-6-phosphate dehydrogenase deficiency.29 Although patients may present with moderate or severe illness, this is the most severe tick-borne rickettsiosis and it is fatal in up to 5% of patients.30, 31 The non-specific presentation of this disease in its early course indicates the necessity for early empirical treatment.29

A striking apparent decline in the US case-fatality rate over the past 25 years, as well as an increase in the number of reported cases and winter cases has been noted.29 These changes are probably related to the inappropriate grouping of other rickettsial diseases (as described above) under the generic name of RMSF.29 Other proven or proposed rickettsial pathogens in the USA have been recently reported, including R. parkeri,26 Candidatus R. felis,26 R. rhipicephali,26 R. massiliae32 and R. amblyommii.33 Further investigations concerning rickettsial diseases in the Americas are required to better understand each of them.

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5. TIBOLA (TIck-Borne LymphAdenitis) or DEBONEL (DErmacentor BOrne Necrosis Erythema Lymphadenopathy) 

The aetiological agents of DEBONEL-TIBOLA are R. slovaca34 and R. raoultii,3 which are transmitted by D. marginatus and D. reticulatus. These ticks host R. raoultii in 20–40% cases and R. slovaca in 5–10% cases. The R. slovaca-raoultii model is of particular interest because Dermacentor ticks infected by R. raoultii cause infection five times more rarely than those infected by R. slovaca.35 DEBONEL-TIBOLA occurs in Europe, and women and children are most often affected. The disease is seasonal and is common in cold periods. The incubation period lasts for a few days, and the disease presents with an inoculation eschar (mostly on the scalp), loco-regional adenopathy and headaches. Sequelae, such as alopecia at the bite point and chronic asthenia have been observed.34 Rickettsia raoultii can exhibit rifampicin resistance, but no clinical cases of resistance have been described.3

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6. Lymphangitis-associated rickettsioses (LAR) 

The aetiological agent of LAR is R. sibirica mongolitimonae (formerly R. mongolotimonae), which is transmitted by Hy. excavatum,36 Hy. asiaticum37 and Hy. truncatum.37 The strains isolated and the cases described were located in Inner Mongolia, France, Greece, Portugal, South Africa and sub-Saharan Africa.37 The distribution of this subspecies is worldwide and differs completely from that of R. sibirica sibirica because of differences in vectors. Cases are mostly seen from March to July in Europe. The incubation period lasts for approximately 5 days. Unique or, commonly, multiple eschars may be associated with rope-like lymphangitis, painful regional adenopathy, fever, headache, myalgia and maculopapular rash. This is a rare and benign disease.37 An ocular form has recently been reported.38

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7. African tick-bite fever (ATBF) 

The aetiological agent of ATBF is R. africae, which is transmitted by Amblyomma ticks39 and Rhipicephalus (Boophilus) decoloratus.40 This is the most common rickettsial disease in sub-Saharan Africa and is also present in the West Indies.39 The most affected persons are adult men. The incubation period lasts for 5–7 days. The disease is mild and presents with a sudden onset of fever, multiple eschars, regional adenopathy, headache, cervical myalgia, nausea and asthenia. A cutaneous rash is observed in 50% of cases and is vesicular in 50% of these cases.39 Clustered cases and the appearance of multiple eschars are specific to this disease and are related to the aggressiveness of the ticks. Despite a high seroprevalence in autochthonous people, reported cases concern almost only European and American travellers.39 Early contact of local inhabitants with R. africae may be responsible for this finding. ATBF should be considered after a diagnosis of fever when a patient has returned from an endemic zone. It must be noted that the elevation of specific antibody titres occurs late compared with other rickettsioses.17

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8. Infrequently reported tick-borne rickettsial diseases 

Rickettsia helvetica is mainly found in Ixodes ticks2 and has been recovered in Europe, northern Africa and Asia.2 Clinically, an aneruptive fever is generally observed.2 Clinical cases involving this bacterium as the aetiological agent commonly have moderate symptoms of headache and myalgia; a rash or an eschar is reported less frequently.17 A different clinical picture was recently described, which included acute febrile illness, macular rash, severe headache, subjective neck stiffness, photophobia and long-lasting symptoms. In this case, the presence of R. helvetica in the blood was verified by PCR together with serological evidence of infection.2

Rickettsia monacensis has recently been implicated in one human case of illness.41 It is transmitted by I. ricinus and has been found in Europe,41 northern Africa42 and the USA.43 The single reported case developed a fever, asthenia, a maculopapular rash and headache.41 Its epidemiology remains unknown.

Rickettsia amblyommii is transmitted by A. americanum, A. coelebs and A. cajennense,4 and is found in the Americas. Although the pathogenicity of this Rickettsia species has long been unknown, it appears that it could be associated with a mild febrile illness with or without a macular rash.33 It may be the cause of some cases of rickettsiosis reported as RMSF; however, this hypothesis requires further study.33

Rickettsia massiliae has been recovered in southern Europe,44 Africa44 and the USA.32 The known vectors include Rh. sanguineus, Rh. turanicus, Rh. muhsamae, Rh. lunulatus and Rh. sulcatus. The first reported case related to this species44 presented with a fever, eschar and maculopapular rash. The second case7 presented with a fever, night sweats, headache, two necrotic skin lesions, a maculopapular rash and bilateral chorioretinitis. Rickettsia massiliae is naturally resistant to rifampicin.45 Rifampicin must therefore not be used as a treatment in geographical zones where R. massiliae is endemic, because the disease cannot be differentiated clinically or serologically from the MSF diseases.44

Rickettsia aeschlimannii is transmitted by Hy. marginatum marginatum17 and Hy. marginatum rufipes.46 It has been found in Africa,46 southern Europe and Kazakhstan.4, 47 For a long time it was not considered to be a pathogenic species; however, infected persons have recently presented with an inoculation eschar, fever, maculopapular rash and purpuric lesions.4, 46 The probability of being bitten by several infected Hyalomma ticks is high and could lead to several eschars in a patient.4 Based on molecular and laboratory findings, this species is considered to be resistant to rifampicin.48

Candidatus R. kellyi has been linked with a fever associated with a maculopapular rash in a 1-year-old boy in India.49 Culture and phenotypic description of the isolate was not possible. Further studies are needed to isolate the strain from humans or arthropods and to establish this as a new rickettsial strain.

Rickettsia tamurae was fully described in 2006.50 It could be a pathogen because it is closely related to R. monacensis and has been isolated in Amblyomma ticks,51 which have a high propensity to bite humans. A serological survey of acutely ill patients in Laos showed that one patient had probable rickettsial infection due to R. tamurae.52

Rickettsia asiatica was fully described in 2006; it is closely related to R. helvetica and could be a pathogen.51, 53

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9. Diagnostic tools 

Several biological tools can be used to confirm a diagnosis of rickettsiosis. They are of different significance and complexity. Elevated erythrocyte sedimentation rate and C-reactive protein levels, thrombocytopenia, high liver enzymes and an abnormal white blood cell count are often observed.

Serum is the easiest and quickest biological sample to collect. Serology can even be performed on blotting paper.17 It is the first diagnostic test that should be performed when a rickettsiosis is suspected. If possible, the microbiologist should test an early and a late (separated by 2–3 weeks) sample. If antibody titres remain negative in these two samples a later sample (4–6 weeks) must be obtained,17 because seroconversion for some rickettsioses is only observed after 2–3 weeks.

An indirect immunofluorescence assay (IFA) is the reference method.54 Sensitivity and specificity vary depending on the species involved and the cut-off levels determined by each laboratory. Titres of ≥1/128 for IgG and ≥1/32 for IgM in acute phase serum specimens and/or evidence of seroconversion with a four fold increase in IgG titre are considered to be evidence of a recent infection with a Rickettsia species.55 The main problem with this technique is cross-reactivity between species and, in some cases, with other intracellular bacteria. IFA is therefore useful for confirming a diagnosis of rickettsiosis and it can sometimes direct a clinician to a certain cluster of species according to the serological profile, but cannot precisely determine the Rickettsia species involved.

Western blot analysis (Fig. 3) is more sensitive and allows an earlier diagnosis than IFA. It is particularly useful for determining the involved Rickettsia species, either by the detection of specific antigens or, more frequently, by cross-adsorption. Cross-adsorption54 is performed by mixing the patient's sample with several antigens that correspond to the different species that are suspected of being the aetiological agent. Briefly, homologous and heterologous antibodies are adsorbed and thus removed by incubation with the causative bacteria, whereas only homologous antibodies are removed by adsorption with cross-reacting bacteria (Fig. 3).54 The specific choice of bacteria to be tested is based on the patient's anamnesis and the epidemiological data.

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  • Fig. 3. 

    Western blot before and after cross-adsorption with Rickettsia conorii (1), R. massiliae (2), Candidatus R. felis (3) and R. aeschlimanii (4). When cross-adsorption is performed with R. felis, the specific antigen-corresponding line disappears. This implicates R. felis as the causative microorganism.

Cell culture can be performed only in biosafety level 3 laboratories by qualified staff. The most frequently received samples are heparinized blood, skin biopsy samples and ticks. Inoculation of the sample into a shell vial containing a monolayer of a specific cell line (HEL, L929, XTC and others) may be of particular interest.56 Detection is performed after 3, 6 and 14 days. Isolation of rickettsial strains is often difficult, and samples should be inoculated as soon as possible in order to increase the sensitivity of the procedure. Shell vial culture remains the best tool for the isolation of intracellular bacteria.56

Molecular tools are sensitive and rapid. The samples that can be tested include blood, cutaneous biopsy samples, paraffin-embedded tissues17 and arthropods. Several genes have been studied and proved to be useful for a rickettsial molecular diagnosis. These genes include: gltA (encoding citrate synthase), ompA (outer membrane protein A), sca4 (formerly gene D) and sca5 (or ompB).10, 54, 57, 58 Several molecular techniques have been proposed: nested PCR enhances sensitivity, but has a high risk of contamination;59 real-time PCR has been recently developed and is used with increasing frequency;60 suicide PCR has been described, but is of restricted use.61 This is a nested PCR technique in which single-use primers target a sequence that has not been previously amplified in the laboratory. This technique is sensitive and specific and avoids the possibility of contamination by previous experiments.

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10. Main therapeutic approaches 

The antibiotic susceptibility profile of rickettsiae is complicated, but empirical treatment is quite simple. Rickettsiae are strictly intracellular bacteria and conventional microbiological techniques cannot be applied to them. Cell culture methods allow testing of the in vitro sensitivity of rickettsiae and determination of minimal inhibitory concentrations (MIC).45 Resistance to rifampicin has been described in R. massiliae, R. raoultii, R. aeschlimannii and several non-pathogenic species.48 This resistance is natural and strongly related to phylogenetic grouping within the genus Rickettsia. Effective antibiotics include doxycycline, chloramphenicol, fluoroquinolones and certain macrolides (excluding erythromycin).45 Ineffective antibiotics include β-lactams, aminoglycosides and cotrimoxazole.45

When rickettsiosis is suspected, empirical treatment must be prescribed immediately, without waiting for laboratory confirmation. On the other hand, a tick bite must not be treated if it remains asymptomatic, especially in areas where Lyme disease is not endemic and where antibiotic prophylaxis may be recommended.62 Doxycycline is the reference antibiotic for treating tick-borne rickettsioses for adults (200mg/day) and children4 (5mg/kg/day). The alternative is chloramphenicol (50–75mg/kg/day).4 For pregnant women, josamycin (3g/day) treatment over 7 days can be used;4 in children, josamycin can be used at 100mg/kg/day for 5 days.63 The precise duration of treatment is linked to the clinical presentation and should continue for at least 3 days after apyrexia.4 For severe forms of MSF and RMSF intravenous doxycycline (200mg) can be prescribed, followed by oral doxycycline (200mg/day) treatment for 10 days.1, 29, 63

The prevention of tick-borne rickettsioses should be based on avoiding tick bites. Here, knowledge of the natural spatiotemporal risk patterns for tick exposure and personal protection measures play a major role. Also, strategies for the suppression of host-seeking ticks and pathogen-infected ticks could be applied as was performed for borreliosis.64 Antibiotic prophylaxis for rickettsiosis following a tick bite is not advised. Vaccination is theoretically possible, but a vaccine is not yet available, probably because of potential cost-ineffectiveness. An important issue may be the transfer of information to the medical community and the general public.

A growing pool of rickettsioses is joining the classical, previously described and emerging diseases. New species are regularly added to this evolving genus and new implications for human pathology are often reported. Advances in biological tools may allow for a better understanding of this very interesting field. Rickettsiology is both a vast and homogeneous domain. Interest in these diseases is increasing and their study should not be restricted to specialists, as emerging rickettsioses are very important public health issues.

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Authors’ contributions 

AR, OM and DR undertook all the duties of authorship. DR is guarantor of the paper.

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Funding 

None.

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Conflicts of interest 

None declared.

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Ethical approval 

Not required.

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References 

  1. Rovery C, Brouqui P, Raoult D. Questions on Mediterranean spotted fever a century after its discovery. Emerg Infect Dis. 2008;14:1360–1367
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PII: S1876-3413(09)00013-8

doi:10.1016/j.inhe.2009.03.003

International Health
Volume 1, Issue 1 , Pages 17-25, September 2009

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