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Properties of Rickettsiae
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Rickettsiae are pleomorphic coccobacilli, appearing either as short rods (0.3 × 1–2 μm) or as cocci (0.3 μm in diameter). They do not stain well with Gram stain but are readily visible under the light microscope when stained with Giemsa stain, Gimenez stain, acridine orange, or other stains. However, immunohistochemical or immunofluorescence stains performed at a laboratory skilled in rickettsial diagnostics are the most useful methods for confirming a diagnosis of rickettsial infections.
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Rickettsiae grow readily in yolk sacs of embryonated eggs. Pure preparations of rickettsiae for use in laboratory testing can be obtained by differential centrifugation of yolk sac suspensions. Many strains of rickettsiae also grow in cell culture, where the generation time is 8–10 hours at 34°C. Cell culture has replaced animal inoculation (except for Orientia species) and yolk sac cultivation for isolation of these organisms. For reasons of biosafety, isolation of rickettsiae should be done only in reference laboratories.
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Rickettsiae have gram-negative cell wall structures that include peptidoglycan-containing muramic acid and diaminopimelic acid. The genus is divided into several groups. The typhus group, the spotted fever group, and the transitional group have species that are pathogenic to humans. Rickettsiae contain lipopolysaccharide and the cell wall proteins include the surface proteins OmpA and OmpB. These surface proteins are important in adherence to host cells and in the humoral immune response and also provide the basis for serotyping.
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Rickettsiae grow in different parts of the cell. Those of the typhus group are usually found in the cytoplasm, and those of the spotted fever group are usually found in the nucleus. Rickettsial growth is enhanced in the presence of sulfonamides, and rickettsial diseases are made more severe by these drugs. Tetracyclines and chloramphenicol inhibit the growth of rickettsiae and can be therapeutically effective.
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Most rickettsiae survive only for short times outside of the vector or host. Rickettsiae are quickly destroyed by heat, drying, and bactericidal chemicals. Dried feces of infected lice may contain infectious Rickettsia prowazekii for months at room temperature.
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Rickettsial Antigens and Serology
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The direct immunofluorescent antibody test can be used to detect rickettsiae in ticks and sections of tissues. The test has been most useful to detect Rickettsia rickettsii in skin biopsy specimens to aid in the diagnosis of Rocky Mountain spotted fever (RMSF); however, the test is performed in only a few reference laboratories.
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Serologic evidence of infection occurs no earlier than the second week of illness for any of the rickettsial diseases. Thus, serologic tests are useful only to confirm the diagnosis, which is based on clinical findings (eg, fever, headache, rash) and epidemiologic information (eg, tick bite). Therapy for potentially severe diseases, such as RMSF and typhus, should be instituted before seroconversion occurs.
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A variety of serologic tests have been used to diagnose rickettsial diseases. Most of these tests are performed only in reference laboratories. Antigens for the indirect immunofluorescence, latex agglutination, indirect immunoperoxidase tests, and enzyme immunoassay for RMSF are commercially available. Reagents for other tests are prepared only in public health or other reference laboratories. The indirect fluorescent antibody technique may be the most widely used method because of the availability of reagents and the ease with which it can be performed. The test is relatively sensitive, requires little antigen, and can be used to detect immunoglobulin M (IgM) and IgG. Rickettsiae partially purified from infected yolk sac material are tested with dilutions of a patient’s serum. Reactive antibody is detected with a fluorescein-labeled antihuman globulin. The results indicate the presence of partly species-specific antibodies, but cross-reactions are observed.
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Rickettsiae multiply in endothelial cells of small blood vessels and produce vasculitis characterized by lymphocytes that surround the blood vessels. The cells become swollen and necrotic; there is thrombosis of the vessel, leading to rupture and necrosis. Vascular lesions are prominent in the skin, but vasculitis occurs in many organs and appears to be the basis of hemostatic disturbances. Disseminated intravascular coagulation and vascular occlusion may develop. In the brain, aggregations of lymphocytes, polymorphonuclear leukocytes, and macrophages are associated with the blood vessels of the gray matter; these are called “typhus nodules.” The heart shows similar lesions of the small blood vessels. Other organs may also be involved.
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In cell cultures of macrophages, rickettsiae are phagocytosed and replicate intracellularly even in the presence of antibody. The addition of lymphocytes from immune animals stops this multiplication in vitro. Infection in humans is followed by partial immunity to reinfection from external sources, but relapses occur (see the discussion of Brill-Zinsser disease).
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Rickettsial infections are characterized by fever, headache, malaise, prostration, skin rash, and enlargement of the spleen and liver.
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1. Epidemic typhus (R prowazekii)—Disease is transmitted by the body louse in a human–louse cycle. In epidemic typhus, systemic infection and prostration are severe, and fever lasts for about 2 weeks. The disease is more severe and more often fatal in patients older than 40 years of age. During epidemics, the case fatality rate has been 6–30%.
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2. Endemic typhus or murine typhus (Rickettsia typhi)—Infected flea feces rubbed into the bite wound is the method of transmission. The clinical picture of endemic typhus has many features in common with that of epidemic typhus, but the disease is milder and is rarely fatal except in elderly patients.
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B. Spotted Fever Group
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The spotted fever group resembles typhus clinically; however, unlike the rash in other rickettsial diseases, the rash of the spotted fever group usually appears after 3–5 days of illness, first on the extremities, then moves centripetally, and involves the palms and soles. Some, such as Brazilian spotted fever and RMSF, may produce severe infections possibly due to infection of endothelial cells that leads to vascular permeability and consequently complications such as pulmonary edema and hemorrhages; others, such as Mediterranean spotted fever, are mild. The case fatality rate varies greatly. RMSF is life-threatening for all age groups but mortality is usually much greater in elderly persons (up to 50%) than in young adults or children.
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C. Transitional Group
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Rickettsialpox (Rickettsia akari) is a mild disease with a vesicular rash resembling that of varicella. About 1 week before onset of fever, a firm red papule appears at the site of the mite bite and develops into a deep-seated vesicle that in turn forms a black eschar (see later discussion).
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Scrub typhus (Orientia tsutsugamushi)—This disease resembles epidemic typhus clinically. One feature is the eschar, the punched-out ulcer covered with a blackened scab that indicates the location of the mite bite. Generalized lymphadenopathy and lymphocytosis are common. Illness can be severe with associated cardiac and cerebral involvement leading to death in about 30% of patients.
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Isolation of rickettsiae is technically difficult and is of only limited usefulness in diagnosis. It is also hazardous and must be performed in a biosafety level 3 laboratory. Animal inoculation has been replaced by cell culture methods for cultivation of most of the rickettsiae. Appropriate specimens include heparinized plasma, buffy coat, and skin lesions. Organisms can be detected in cell cultures by molecular methods or by immunofluorescence staining.
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In RMSF, some other rickettsial infections, and scrub typhus, skin biopsies taken from patients between the fourth and eighth days of illness may reveal rickettsiae by immunohistochemical stains that are available in a specialized laboratory at the Centers for Disease Control and Prevention.
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The polymerase chain reaction (PCR) has been used to help diagnose RMSF, other diseases of the spotted fever group, murine typhus, and scrub typhus. Real-time PCR methods have enhanced sensitivity and allow for diagnosis before a serologic response. Appropriate specimens include tissues, plasma, peripheral blood, and buffy coat specimens. Molecular techniques have also been applied to detection of rickettsiae in the vectors as well. These assays are available on a limited basis primarily in reference laboratories.
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Serology is the main method available to clinical laboratories for the diagnosis of rickettsial infections. The most widely used serologic tests are indirect immunofluorescence and enzyme immunoassays (see earlier discussion). Complement fixation is no longer used in most laboratories. An antibody rise should be demonstrated during the course of the illness. In RMSF, the antibody response may not occur until after the second week of illness.
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Tetracyclines, preferably doxycycline, are effective, provided treatment is started early. Doxycycline is given daily orally and continued for 3–4 days after defervescence. In severely ill patients, the initial doses can be given intravenously. Chloramphenicol also can be effective.
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Sulfonamides enhance the disease and are contraindicated. There is limited clinical experience with the fluoroquinolones, although they have been shown to have in vitro activity.
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A variety of arthropods, especially ticks and mites, harbor rickettsia-like organisms in the cells that line the alimentary tract. Many such organisms are not evidently pathogenic for humans.
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The life cycles of different rickettsiae vary. R prowazekii has a life cycle in humans and the human louse (Pediculus humanus corporis and Pediculus humanus capitis). The louse obtains the organism by biting infected human beings and transmits the agent by fecal excretion on the surface of the skin of another person. Whenever a louse bites, it defecates at the same time. Scratching the area of the bite allows the rickettsiae excreted in the feces to penetrate the skin. As a result of the infection, the louse dies, but the organisms remain viable for some time in its dried feces. Rickettsiae are not transmitted from one generation of lice to another. Delousing large proportions of the population with insecticides has controlled typhus epidemics.
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Brill-Zinsser disease is a recrudescence of an old typhus infection. The rickettsiae can persist for many years in the lymph nodes of an individual without any symptoms being manifest. The rickettsiae isolated from such cases behave like classic R prowazekii; this suggests that humans themselves are the reservoir of the rickettsiae of epidemic typhus. Typhus epidemics have been associated with war and the lowering of standards of personal hygiene, which in turn have increased the opportunities for human lice to flourish. If this occurs at the time of recrudescence of an old typhus infection, an epidemic may be set off. Brill-Zinsser disease occurs in local populations of typhus areas as well as in persons who migrate from such areas to places where the disease does not exist. Serologic characteristics readily distinguish Brill disease from primary epidemic typhus. Antibodies arise earlier and are IgG rather than the IgM detected after primary infection. They reach a maximum by the 10th day of disease. This early IgG antibody response and the mild course of the disease suggest that partial immunity is still present from the primary infection.
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In the United States, R prowazekii has an extrahuman reservoir in the southern flying squirrel, Glaucomys volans. In areas where southern flying squirrels are indigenous (southern Maine to Florida to the center of the United States), human infections have occurred after bites by ectoparasites of this rodent. Infection between humans occurs by the human body louse Pediculus humanus corporis. Recent reports indicate that epidemic typhus may be increasing in some areas; R prowazekii is considered a biothreat agent.
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R typhi has its reservoir in the rat, in which the infection is inapparent and long lasting. Rat fleas carry the rickettsiae from rat to rat and sometimes from rats to humans, who develop endemic typhus. Cat fleas can serve as vectors. In endemic typhus, the flea cannot transmit the rickettsiae transovarially.
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O tsutsugamushi has its true reservoir in the mites that infest rodents. Rickettsiae can persist in rats for more than 1 year after infection. Mites transmit the infection transovarially. Occasionally, infected mites or rat fleas bite humans, and scrub typhus results. The rickettsiae persist in the mite–rat–mite cycle in the scrub or secondary jungle vegetation that has replaced virgin jungle in areas of partial cultivation. Such areas may become infested with rats and trombiculid mites.
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R rickettsii may be found in healthy wood ticks (Dermacentor andersoni) and is passed transovarially. Infected ticks in the western United States occasionally bite vertebrates such as rodents, deer, and humans. To be infectious, the tick carrying the rickettsiae must be engorged with blood because this increases the number of rickettsiae in the tick. Thus, there is a delay of 45–90 minutes between the time of the attachment of the tick and its becoming infective. In the eastern United States, the dog tick Dermacentor variabilis and Rhipicephalus sanguineus ticks transmit RMSF. Dogs are hosts to these ticks and may serve as a reservoir for tick infection. Small rodents are another reservoir. Most cases of RMSF in the United States now occur in the eastern and southeastern regions.
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R akari has its vector in bloodsucking mites of the species Liponyssoides sanguineus. These mites may be found on the mice (Mus musculus) trapped in apartment houses in the United States where rickettsialpox has occurred. Transovarial transmission of the rickettsiae occurs in the mite. Thus, the mite may act as a true reservoir as well as a vector. R akari has also been isolated in eastern Europe, South Africa, and Korea.
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Geographic Occurrence
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This potentially worldwide infection has disappeared from the United States, Britain, and Scandinavia. It is still present in the Balkans, Asia, Africa, Mexico, and the Andes mountains of South America. In view of its long duration in humans as a latent infection (Brill-Zinsser disease), it can emerge and flourish quickly under proper environmental conditions, as it did in Europe during World War II because of the deterioration of community hygiene.
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B. Endemic Murine Typhus
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Disease exists worldwide, especially in areas of high rat infestation. It may exist in the same areas as—and may be confused with—epidemic typhus or scrub typhus.
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Infection is seen in the Far East, especially Myanmar (Burma), India, Sri Lanka, New Guinea, Japan, western Australia, eastern Russia, China, and Taiwan. The larval stage (chigger) of various trombiculid mites serves both as a reservoir, through transovarian transmission, and as a vector for infecting humans and rodents.
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D. Spotted Fever Group
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These infections occur around the globe, exhibiting as a rule some epidemiologic and immunologic differences in different areas. Transmission by a tick of the Ixodidae family is common to the group. The diseases that are grouped together include RMSF and Colombian, Brazilian, and Mexican spotted fevers; Mediterranean (boutonneuse), South African tick, and Kenya fevers; North Queensland tick typhus; and North Asian tickborne rickettsioses.
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The human disease has been found among inhabitants of apartment houses in the northern United States. However, the infection also occurs in Russia, Africa, and Korea.
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Epidemic typhus is more common in cool climates, reaching its peak in winter and waning in the spring. This is probably a reflection of crowding, lack of fuel, and low standards of personal hygiene, which favor louse infestation.
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Rickettsial infections that must be transmitted to the human host by vector reach their peak incidence at the time the vector is most prevalent—the summer and fall months.
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Control must rely on breaking the infection chain, treating patients with antibiotics, and immunizing when possible. Patients with rickettsial disease who are free from ectoparasites are not contagious and do not transmit the infection.
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A. Prevention of Transmission by Breaking the Chain of Infection
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1. Epidemic typhus—Delousing with insecticide
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2. Murine typhus—Rat proofing buildings and using rat poisons
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3. Scrub typhus—Clearing from campsites the secondary jungle vegetation in which rats and mites live
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4. Spotted fever—Similar measures for the spotted fevers may be used, including clearing of infested land, personal prophylaxis in the form of protective clothing such as high boots and socks worn over trousers, tick repellents, and frequent removal of attached ticks.
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5. Rickettsialpox—Elimination of rodents and their parasites from human domiciles
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Rickettsia are pleomorphic coccobacilli that are obligate intracellular pathogens similar to gram-negative bacteria but they do not stain with Gram stain.
Rickettsia can be cultivated in cell culture lines and yolk sacs, but immunohistochemical or immunofluorescent stains, serology, or molecular methods are usually used for their detection in clinical material.
The hallmark of infection with Rickettsia is vasculitis.
Rickettsia can be divided into the typhus, spotted fever, and transitional groups; O tsutsugamushi causes scrub typhus. Vectors, clinical manifestations, and geographic distributions vary by group.
Disease may be mild as in the case of Rickettsialpox or severe as in RMSF.
Doxycycline is the drug of choice.