Rickettsiae and Other Related Intracellular Bacteria

Structure and Function

Spotted fever, transitional, and typhus group rickettsiae are genetically closely related bacteria that have a thin (0.3–0.5 by 1–2 μm) bacillary morphology and a gram-negative cell wall containing lipopolysaccharide with antigenic components that distinguish the spotted fever and typhus groups. All Rickettsia spp. reside free in the cytosol of the host cell and divide by binary fission. Rapid progress in bioinformatics and phylogenomics have elucidated secretion systems in rickettsiae, known as the Rickettsia secretome ( ). Two Sec-dependent pathways (T5SS and Sec-TolC) are defined by the surface cell antigen (Sca) family and RARP-1 ( Rickettsia ankyrin repeat protein 1), respectively. Two Sec-independent pathways are also conserved across Rickettsia spp.: T1SS and T4SS. An additional secretion system is the Twin-arginine translocation (Tat). The latter three have no known substrates ( ). Rickettsiae attach to the host cell via Sca, a family of autotransporter proteins of which OmpA (Sca0, Sca1, Sca2) and OmpB (Sca5) have been very well characterized ( ; ). OmpA and OmpB are both present in spotted fever group rickettsiae, whereas OmpA is absent in typhus group rickettsiae. OmpB is in fact the ligand of a ubiquitous cell surface receptor known as Ku70 (DNA-dependent protein kinase) ( ; ). Adhesion is followed by internalization, which requires ubiquitination of cholesterol-rich microdomains (lipid rafts) containing Ku70 ( ). Caveolin and clathrin further contribute to the endocytic process. Furthermore, once in the intracellular space, rickettsiae (and other obligate intracellular bacteria such as Chlamydia, Coxiella, Anaplasma, and Ehrlichia ) can manipulate host cholesterol trafficking pathways to access intracellular vesicles in search of nutrients or obtain membrane components for the bacteria or their vacuole ( ). On the other hand, OmpA has been shown to interact with the cell surface protein α2-β1 integrin through a discontinuous RGD motif in OmpA ( ). The roles of Sca1, Sca2, Sca3, Sca4, and Epac are far less well characterized. Disruption of Sca2 inhibits actin-based motility ( ). Furthermore, Sca2 has been shown to nucleate unbranched actin filaments via host profilin (actin-binding protein) ( ). Sca4 has been shown to activate vinculin (another actin-binding protein present in focal adhesions) ( ). Sca3 proteins are only present in typhus-group rickettsiae and R. felis .

Once internalized, rickettsiae escape the phagosome rapidly via phospholipase activity of rickettsial origin ( ; ). Phospholipase D and phospholipase A ₂ are present in rickettsial genomes. Spotted fever group rickettsiae are propelled within cells and during release from the cell by stimulating polymerization of host cell F-actin at one pole ( ; ). Rickettsiae that manifest this activity (e.g., Rickettsia rickettsii ) escape earlier from host cells and spread more quickly to other cells than those lacking this activity (e.g., Rickettsia prowazekii ), which divide intracellularly to massive numbers before the host cell bursts and the organisms are released. According to the molecular phylogeny, Rickettsia spp. that are pathogenic for humans have evolved into three genogroups ( ). The typhus group includes R. prowazekii and R. typhi . The core spotted fever group contains R. rickettsii, R. conorii, R. japonica, R. africae, R. parkeri, R. honei, R. sibirica, R. massiliae , and R. slovaca , among others. A relatively newly recognized transitional clade contains R. akari, R. australis, and R. felis . R. akari and R. australis were traditionally considered to be relatively distant members of the spotted fever group, with which they share lipopolysaccharide antigens. As of 2018, 31 Rickettsia spp. have been validated; 5 more species added since 2013 ( ; )

Rocky Mountain Spotted Fever

The most severe of all the rickettsioses, Rocky Mountain spotted fever has a substantial case fatality rate (5%) even among previously healthy, immunocompetent children and young adults with appropriate antibiotic treatment ( ; ). R. rickettsii normally resides in nature in ticks: Dermacentor variabilis, the American dog tick, in the eastern two-thirds of the United States and California; D. andersoni, the Rocky Mountain wood tick, in the western United States; Rhipicephalus sanguineus, the brown dog tick, in Mexico, Brazil, and Arizona; and Amblyomma cajennense complex and A. aureolatum in South America. These ticks maintain R. rickettsii as they molt from stage to stage (larva, nymph, and adult) and transovarially from generation to generation. Fewer than 1 per 1000 ticks carries virulent R. rickettsii, which is pathogenic for ticks ( ). New lines of ticks become infected by feeding on rickettsemic rodents or cofeeding with infected ticks, replenishing the population of organisms transovarially maintained in ticks ( ).

Infections occur when and where humans encounter R. rickettsii– infected ticks ( ). Although Rocky Mountain spotted fever has been documented in recent years in nearly every state except Hawaii, Alaska, and Vermont, the highest incidence is in the south Atlantic states from Maryland to Georgia and the south central states of Oklahoma, Missouri, Arkansas, and Tennessee. Most cases occur in late spring and summer but, particularly in the southern latitudes, a few cases may occur even in winter. The highest incidence is in children, adults of retirement age, and others who are exposed to ticks during outdoor activities. Fatality/case ratios are higher for persons older than 30 years of age. Fulminant Rocky Mountain spotted fever (death by the fifth day of illness) occurs in association with moderate hemolysis (e.g., in black males with glucose-6-phosphate dehydrogenase [G6PD] deficiency) ( )

Rickettsiae are injected via the infected tick’s salivary gland secretions into the patient’s dermis after 6 to 10 hours of tick feeding and spread hematogenously throughout the body. The vascular endothelium is the target of intracellular infection, with some invasion into adjacent vascular smooth muscle cells. Infected endothelium is injured by reactive-oxygen-species-induced damage to cell membranes ( ). Damage to the endothelium (directly mediated by rickettsiae or indirectly mediated by cytokines/chemokines) results in increased vascular permeability leading to edema, hypovolemia, and hypotension ( ; ). The life-threatening consequences of vascular injury in the central nervous system (CNS) and lung are rickettsial meningoencephalitis and noncardiogenic pulmonary edema. Early in the course, lesions show endothelial rickettsiae without thrombi or a cellular response. Late in the course, the characteristic lymphohistiocytic perivascular infiltrate appears as interstitial pneumonia, interstitial myocarditis, perivascular glial nodules of the brain, and similar vascular lesions in the dermis, gastrointestinal tract, liver, skeletal muscles, and kidneys. Severe injury may be accompanied by focal hemorrhages but seldom by microinfarcts, except in the white matter of the brain.

The clinical illness usually begins with fever, headache, and myalgia 2 to 14 days after a tick bite ( ). Nausea, vomiting, abdominal pain and tenderness, and diarrhea occur more frequently in the first 3 days of illness. The rash, which usually appears between days 3 and 5, typically begins as macules around the wrists and ankles and, later, on the arms, legs, and trunk. The lesions become maculopapular, and in one-half of cases a central petechia appears in many of the maculopapules. Characteristic involvement of the palms and soles occurs in half of cases as a late manifestation. Renal failure is a feature of severe illness. CNS involvement is ominous; seizures and coma occur in 8% to 10% of cases overall, often preceding a fatal outcome. Thrombocytopenia occurs in half of cases, but disseminated intravascular coagulation is rare ( ).

African Tick Bite Fever, Boutonneuse Fever, Rickettsia Parkeri Infection , and Other Spotted Fevers

R. conorii has been isolated in southern Europe; northern, eastern, and southern Africa; Israel; Turkey; India; Pakistan; Russia; Georgia; and Ukraine. The ecology of R. conorii and the epidemiology of boutonneuse fever are closely tied to ticks, especially R. sanguineus, which maintain the rickettsiae transovarially and transmit the infection to humans while feeding ( ). Imported cases are diagnosed in travelers returning to the United States and northern Europe from the Mediterranean basin. The fatality rate among hospitalized patients is 1.4% to 5.6%, particularly in patients with underlying conditions such as diabetes and alcoholism. A milder disease caused by R. africae occurs with a high frequency in travelers returning from southern Africa ( ). The clinical illness resembles that of the recently associated rickettsioses in the Americas caused by R. parkeri, which is very closely related to R. africae ( , ). Tick bite eschars are often multiple; regional lymphadenopathy is observed frequently; and rash is typically sparse, sometimes vesicular, and often absent. R. sibirica has been isolated in Russia, China, Mongolia, and Pakistan; the distinct strain R. sibirica mongolitimoniae isolated in Asia, Europe, and Africa has been associated in one-half of the cases with lymphangitis extending proximally from the eschar. R. japonica has been documented in Japan, and infection with R. japonica occurs also in Korea, China, and Thailand. Human infections with R. australis occur only in Australia, and infection with R. honei has been documented in Australia and Asia. After an average incubation period of 7 days, these illnesses begin with fever, headache, and myalgias. Frequently, an eschar can be discovered by careful examination of the skin at this time. The pathology of these spotted fevers is well described in the tache noire or eschar at the site of tick bite inoculation of rickettsiae ( ). Mononuclear phagocytes are the initial target cell in eschars of spotted fever rickettsioses.

The host defenses that effect killing of intracellular rickettsiae include nitric oxide, reactive oxygen species, and tryptophan limitation induced by cytokines secreted by T lymphocytes and macrophages, which infiltrate around the infected blood vessels and target cell apoptosis triggered by cytotoxic CD8 ⁺ T lymphocytes ( ; ; ). Activation of endothelial cells by cytokines, including γ-interferon and TNF-α, results in intracellular rickettsicidal activity, and ultimate clearance is mediated by cytotoxic T lymphocytes. Disseminated endothelial infection results in maculopapular rash, meningoencephalitis, and vascular lesions in the lungs, kidneys, gastrointestinal tract, and heart ( ). Multifocal hepatocellular necrosis and granuloma-like lesions correlate with moderately increased concentrations of hepatic transaminases ( ).

Other spotted fever rickettsiae that have been occasionally associated with human disease include R. aeshlimannii, R. heilongjianensis, R. massiliae, R. monacensis, Rickettsia strain 364D , R. helvetica, R. raoultii, R. slovaca, and R. tamurae ( ).

Rickettsialpox

R. akari is maintained in nature by transovarial transmission in the gamasid mite, Liponyssoides sanguineus, an ectoparasite of the domestic mouse, Mus musculus. R. akari has been detected only in the United States, Croatia, the Ukraine, Turkey, Mexico, and Korea, perhaps more an indication of the paucity of rickettsial investigations than the actual distribution of this rickettsial species.

A papule develops during the approximately 10-day incubation period at the site of mite bite and progresses to become a 1- to 2.5-cm eschar. Illness begins with chills, fever, malaise, severe headache, and myalgia. Rash, which appears 2 to 6 days later, is initially maculopapular, later papular, and in classic cases pustular and/or vesicular. Some patients also suffer nausea, vomiting, pharyngitis, photophobia, splenomegaly, and nuchal rigidity.

Histopathologic examination of the eschar reveals coagulative necrosis of the epidermis, underlying vascular injury, and a perivascular lymphohistiocytic infiltrate in which mononuclear phagocytes appear to be the main target cell of infection ( ; ; ). Regional lymphadenopathy and cutaneous rash presumably reflect lymphogenous and hematogenous spread, respectively.

Fleaborne Spotted Fever

A widely dispersed organism, R. felis is maintained transovarially in cat fleas ( Ctenocephalides felis ). Human infections have been diagnosed in North America, Europe, Africa, Australia, and Asia only by nucleic acid amplification. R. felis DNA is also detected in blood samples of many healthy persons. There are no human isolates of R. felis, and serologic confirmation of diagnoses is lacking. Thus the role of R. felis in human disease is controversial ( ; ; ; ).

Murine Typhus and Louseborne Typhus

Endemic fleaborne R. typhi infection, murine typhus, is presently the most important typhus group infection in the United States and causes extensive morbidity throughout the warm regions of the world ( ). Most of the cases in the United States are concentrated in Texas and California.

Historically, epidemic louseborne R. prowazekii infections have had a major impact on the outcome of military campaigns as well as scourging general populations disrupted by war, famine, and natural disasters ( ; ). R. prowazekii continues to cause disease in some poverty-stricken areas of the world and reappears in situations such as the civil war in Burundi, the extreme poverty of indigenous populations of the Andes, and other unsettled social and economic conditions ( ). An outbreak in Russia was also described in the 1990s ( ). Recrudescence of latent R. prowazekii infections can occur years after the primary infection in immigrants from typhus-afflicted areas (Brill-Zinsser disease). These cases are responsible for initiating outbreaks in populations with a high incidence of body louse infestation. Endemic transmission of R. prowazekii from a natural infectious cycle of flying squirrels (Glaucoma volans volans) and their ectoparasites occurs in the United States ( ; ). The incubation period ranges from 10 to 14 days, and the disease is more severe than its counterpart endemic typhus. Rash occurs in a variable proportion of cases depending largely on light skin pigmentation for its recognition. Neurologic manifestations can occur in as many as 80% of cases. Respiratory manifestations occur in 40% to 70% of cases. Case fatality rates can be as high as 60% in the absence of antibiotic therapy and appropriate nursing care, and 15% in patients with previous good general health and good supportive care, and 4% when treated with appropriate antibiotics ( ).

Murine typhus occurs particularly in tropical and subtropical coastal areas where Rattus rattus, R. norvegicus, and the Oriental rat flea abound ( ). The fleas imbibe rickettsiae in the blood of infected rats and maintain the infection for their normal life span. Transovarian transmission occurs only at low levels; thus horizontal transmission to other rats is a key factor in maintenance of R. typhi in nature. Other mammal-arthropod cycles maintain the rickettsiae and result in transmission of infections to humans (e.g., the cat flea, C. felis, and the opossum in Texas and California [Los Angeles County and a few other southern counties]) ( ). Hawaii also experienced an outbreak of endemic typhus in 2002 across five islands (Maui had the majority of cases), and the animal reservoir has not been identified. The disease remains endemic in Maui, Kauai, and Oahu, and serologic studies have revealed the presence of R. typhi in R. rattus, R. norvegicus, R. exulans (Polynesian rat), and Mus musculus (house mice) (Centers for Disease Control and Prevention ).

Humans are believed to become infected by intradermal inoculation of infected flea feces into skin excoriated by scratching. However, inhalation of a rickettsial aerosol from dried infected flea feces or inoculation by flea bite may account for transmission in some cases. Flea bites are recalled in 0% to 40% of cases. After an incubation period of 1 to 2 weeks, illness begins with fever accompanied in some cases by severe headache, chills, myalgias, and nausea. A macular or maculopapular rash, most prominent on the trunk, is visualized on day 5 or 6 in 80% of patients with fair skin and in 20% with darkly pigmented skin. A small proportion of patients have cough and pulmonary infiltrates. Severely ill patients may also suffer coma, seizures, and other neurologic signs. Approximately 10% of hospitalized patients require admission to the intensive care unit, and 1% to 2% of murine typhus patients die ( ). CNS complications occur in 2% to 10% of cases and appear late in the acute phase of the disease (10 days to 3 weeks).

The pathologic lesions of murine typhus include endothelial swelling and perivascular lymphohistiocytic infiltrates involving the blood vessels in the dermis, CNS, lungs, heart, gastrointestinal tract, and kidneys ( ). The most serious consequences are meningoencephalomyelitis and diffuse alveolar injury.

Rickettsiae as Agents of Bioterrorism

R. prowazekii is a select agent, the possession of which is restricted by law to registered scientists in approved institutions where rigorous security and safety regulations are applied to the laboratories. This organism exists in nature, can be recovered and propagated, and is infectious via a stable aerosol with infectivity of as little as a single bacterium. R. rickettsii was formerly considered a select agent, but the CDC decided to remove it from the select agent list in 2012.

Case fatality rates of 15% to 25% in previously healthy persons would occur without prompt diagnosis and treatment. The potential for genetically engineered resistance to the effective antibiotics, tetracycline, and chloramphenicol would render these cases of epidemic typhus untreatable ( ). Although the case fatality rates would be lower, bioterrorist-dispersed R. typhi or R. conorii could also create terror and overwhelm the medical and public health systems.

Laboratory Diagnosis

Unlike most infectious diseases for which precise diagnosis is sought during the acute phase of illness, when critical therapeutic decisions are made, rickettsial diseases are usually diagnosed acutely purely on clinicoepidemiologic suspicion and are treated empirically on a presumptive basis ( ; ). Serologic diagnosis, which is often mistakenly sought early in the course of illness, provides the majority of laboratory confirmed diagnoses by demonstration of a fourfold or greater rise in titer only during convalescence. Even with the most sensitive serologic methods, fewer than 20% of patients have detectable specific antibodies to rickettsiae when presenting to the physician for medical attention. These antibodies may have been stimulated at an earlier time by organisms of low pathogenicity such as R. amblyommatis . Other approaches to diagnosis at the time of presentation include immunohistologic demonstration of rickettsiae in cutaneous lesions, immunocytologic identification of rickettsiae in circulating detached endothelial cells, detection of rickettsial DNA in blood and tissue specimens by polymerase chain reaction (PCR) ( ; ; ; ; ), and cultivation of rickettsiae from blood or tissue specimens; but these tests are not available in most clinical laboratories.

Rickettsiae were originally demonstrated in tissues of patients with Rocky Mountain spotted fever and epidemic louseborne typhus by using Giemsa stain during and shortly after World War I. This method, essentially a lost art, requires careful attention to details of fixation and staining of rickettsiae and is not performed successfully in this manner in contemporary histology laboratories. A modified Brown-Hopps method stains a small fraction of organisms, which appear as thin bacilli within endothelial cells. A more sensitive and specific approach to visualization of rickettsiae in tissue sections is immunohistology, either immunofluorescence or immunoenzyme staining, using antibodies specific for the spotted fever or typhus group ( ; , , ; ). Staining of skin biopsies from patients with Rocky Mountain spotted fever by immunohistochemistry has a sensitivity of 60% to 80% and a specificity close to 100%. Patients with boutonneuse fever, African tick bite fever, murine typhus, and rickettsialpox have also been diagnosed by immunohistologic detection of rickettsiae in rash and eschar lesions. A monoclonal antibody to a spotted-fever-group–specific epitope on the cell wall lipopolysaccharide demonstrates R. rickettsii, R. parkeri, R. conorii, R. akari, R. japonica, R. australis, R. africae, R. honei, and R. sibirica in formalin-fixed, paraffin-embedded tissues, and a typhus group lipopolysaccharide-specific monoclonal antibody is similarly useful for detecting R. typhi and R. prowazekii ( ). Currently, reagents for diagnostic immunohistology of rickettsioses are not commercially available, but it is feasible that kits could be developed for rickettsial-group–specific diagnosis using antibodies produced in research laboratories.

A unique diagnostic approach is the immunocytologic demonstration of R. conorii in detached, circulating endothelial cells captured from patient blood samples by immunomagnetic beads coated with a monoclonal antibody to a surface antigen of human endothelial cells ( ; ). In boutonneuse fever patients, this method has a sensitivity of 58% for examination of a single blood sample and may be used in patients prior to the onset of rash, which must be present for selection of the site of skin biopsy for immunohistologic diagnosis.

Nucleic acid amplification (NAA) tests used for spotted fever group and typhus group rickettsioses include PCR and its variants (nested, real-time, conventional) and loop-mediated isothermal amplification (LAMP). Abundant literature documents the use of several PCR targets, but the most commonly used genes include the 17 kDa lipoprotein gene, gltA, rrs, groEL, ompA, ompB, RC0338, and real-time PCR panrickettsia assays. Analytic sensitivity for PCR ranges from 10 ³ to 10 ⁵ genome equivalents/mL blood DNA for nested PCR (sensitivity is much lower for conventional PCR). For real-time PCR the sensitivity ranges from 100 to 5000 genome equivalents/mL of blood DNA. Reverse transcriptase real-time PCR targeting ribosomal RNA can increase analytic sensitivity of nucleic acid detection as much as 100X compared to regular PCR (as low as 9 copies per reaction) ( ). LAMP assays for SFG and TG rickettsioses are not well evaluated. However, one study revealed low sensitivity in patients with endemic typhus, most likely due to low levels of bacteremia ( ). In fact, detection of TG rickettsiae from blood samples using PCR is far lower when compared to SFG rickettsioses (2–10% vs 24–56%) ( ).

Samples for NAA include biopsy of eschar or rash, blood, buffy coat, necropsy tissue, and even arthropod vectors removed from patients. NAA tests may fail to detect rickettsial nucleic acids early in the course or after development of immunity or effective antimicrobial treatment ( ; ; ; ; ; ; ; ; ; ; ; ; ). Isolation of rickettsiae is achieved frequently in antibiotic-free, centrifugation-enhanced shell vial cell culture in reference and research laboratories with biosafety level-3 containment and specialized expertise.

Serodiagnosis still remains the gold standard for rickettsioses ( ; ). Test formats include immunofluorescent antibody (IFA), microimmunofluorescence (MIF) assay, and enzyme-linked immunosorbent assay (ELISA). MIF is very similar to IFA but allows simultaneous testing of several rickettsial antigens spotted in the testing wells. The indirect immunoperoxidase antibody test yields similar results. For spotted fever- and typhus-group rickettsial infections in the United States, IFA titers of 1:64 or greater are considered to indicate a probable diagnosis in a compatible clinicoepidemiologic situation. In countries where there is a high prevalence of persons with antibodies to these rickettsiae, due hypothetically to stimulation by nonpathogenic rickettsiae or subclinical or undiagnosed infection, higher titers are required to establish the diagnosis. In any event, a fourfold rise in IFA antibody titer to at least a titer of 1:64, but usually 1:256 or higher, is diagnostic. The sensitivity of the IFA for Rocky Mountain spotted fever is 94% to 100%, and the specificity is 100%. The sensitivity of IFA for spotted fever rickettsioses in general is 85% to 100%, and the specificity is 99% to 100% when detecting IgG antibodies. These values decrease slightly when using IgM. ELISA IgG testing has a sensitivity of 83% and specificity of 87% ( ; ). Both sensitivity and specificity increase as the number of illness days also increase. By day 30, virtually all cases can be diagnosed serologically. With a cutoff titer for IgG of 1:128 and for IgM of 1:32, the indirect immunoperoxidase test yields similar results and has the advantage of requiring only a light microscope instead of an ultraviolet microscope.

Commercially available serologic tests include indirect immunofluorescence, latex agglutination, and standard solid-phase enzyme immunoassay ( ). Latex agglutination and solid-phase enzyme immunoassay provide diagnostically useful information, require less expensive equipment to perform, but generally are not considered as reliable as the IFA. The greatly increased reliance of reference laboratories on enzyme immunoassays has occurred in parallel with anomalous changes in public health reports of Rocky Mountain spotted fever. It has been suggested that a substantial portion of the tick-exposed population has standing titers of antibodies stimulated by spotted fever group rickettsiae of low pathogenicity ( ). The Weil-Felix tests, which measure agglutination of Proteus vulgaris strains OX-19 and OX-2 ( ), are insensitive and nonspecific and should not be used except in developing countries in which no other method can be performed. Serology is seldom useful in assisting therapeutic decisions because antibodies appear later in the course.

A promising technology to detect low levels of rickettsia-specific circulating antibodies is by using protein microarrays produced from recombinant protein expression using ORF libraries, followed by fluorescent detection ( ).

Treatment

Well-controlled, double-blind randomized clinical trials are lacking for rickettsioses in general. However several in vitro susceptibility studies have been published mostly for the main pathogenic rickettsioses, although standardization is lacking, and therefore minimum inhibitory concentrations (MICs) are difficult to interpret. Studies in humans are mostly retrospective.

In general, rickettsiae are sensitive to tetracyclines, fluoroquinolones, chloramphenicol, and rifamycins. Less effective antibiotics are the macrolides ( ; ). All Rickettsiales are inherently resistant to aminoglycosides, β-lactams, cephalosporins, and sulfa-derived medications. Sulfas are in fact contraindicated in any infection caused by Rickettsiales due to greater severity of disease and fatal outcomes. The presence of a toxin-antitoxin condition led to reconsideration of the use of fluoroquinolones. The gold standard treatment for all rickettsioses is doxycycline, even in patients younger than 8 years of age since the occurrence of discolored teeth and bone accumulation is minimal. When compared with other antimicrobial agents, doxycycline is associated with earlier defervescence and better protection against progression to severe forms of the disease. Alternatives include tetracycline, chloramphenicol, and selected macrolides and fluoroquinolones. Duration of treatment depends on the severity of the disease and rickettsial species. In general, Rocky Mountain spotted fever and epidemic typhus cases should be treated for several days compared with less severe rickettsioses, which can be treated even with single doses of doxycycline ( ). Relapses are very common when patients with Rocky Mountain spotted fever or epidemic typhus are treated with a single dose. Reports of greater morbidity and mortality in patients with Rocky Mountain spotted fever treated with chloramphenicol compared with doxycycline have been published ( ). Alternatives in children and pregnant women include macrolides (azithromycin and clarithromycin) and rifampin for the treatment of less severe rickettsioses.