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BORRELIA SPECIES AND RELAPSING FEVER
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Relapsing fever in epidemic form is caused by Borrelia recurrentis, which is transmitted by the human body louse; it does not occur in the United States. Endemic relapsing fever is caused by borreliae transmitted by ticks of the genus Ornithodoros. The species name of the Borrelia genus is often the same as that of the tick. Borrelia hermsii, the cause of relapsing fever in the western United States, is transmitted by Ornithodoros hermsii.
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Morphology and Identification
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The borreliae form irregular spirals 10–30 μm long and 0.3 μm wide. The distance between turns varies from 2 to 4 μm. The organisms are highly flexible and move both by rotation and by twisting. Borreliae stain readily with bacteriologic dyes as well as with blood stains such as Giemsa stain or Wright stain (Figure 24-2).
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The organism can be cultured in fluid media containing blood, serum, or tissue, but it rapidly loses its pathogenicity for animals when transferred repeatedly in vitro. Multiplication is rapid in chick embryos when blood from patients is inoculated onto the chorioallantoic membrane.
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C. Growth Characteristics
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Little is known of the metabolic requirements or activity of borreliae. At 4°C, the organisms survive for several months in infected blood or in culture. In some ticks (but not in lice), spirochetes are passed from generation to generation.
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The only significant variation of Borrelia species is with respect to its antigenic structure.
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Antibodies develop in high titer after infection with borreliae. The antigenic structure of the organisms changes in the course of a single infection. The antibodies produced initially act as a selective factor that permits the survival only of antigenically distinct variants. The relapsing course of the disease appears to be caused by the multiplication of such antigenic variants, against which the host must then develop new antibodies. Ultimate recovery (after 3–10 relapses) is associated with the presence of antibodies against several antigenic variants.
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Fatal cases show spirochetes in great numbers in the spleen and liver, necrotic foci in other parenchymatous organs, and hemorrhagic lesions in the kidneys and the gastrointestinal tract. Spirochetes have occasionally been demonstrated in the spinal fluid and brain of persons who have had meningitis. In experimental animals (guinea pigs, rats), the brain may serve as a reservoir of borreliae after they have disappeared from the blood.
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Pathogenesis and Clinical Findings
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The incubation period is 3–10 days. The onset is sudden, with chills and an abrupt rise of temperature. During this time, spirochetes abound in the blood. The fever persists for 3–5 days and then declines, leaving the patient weak but not ill. The afebrile period lasts 4–10 days and is followed by a second attack of chills, fever, intense headache, and malaise. There are 3–10 such recurrences, generally of diminishing severity. During the febrile stages (especially when the temperature is rising), organisms are present in the blood; during the afebrile periods, they are absent.
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Antibodies against the spirochetes appear during the febrile stage, and the attack is probably terminated by their agglutinating and lytic effects. These antibodies may select out antigenically distinct variants that multiply and cause a relapse. Several distinct antigenic varieties of borreliae may be isolated from a single patient’s sequential relapses even after experimental inoculation with a single organism.
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Diagnostic Laboratory Tests
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Blood specimens are obtained during the rise in fever for smears and animal inoculation.
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Thin or thick blood smears stained with Wright or Giemsa stain reveal large, loosely coiled spirochetes among the red cells.
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C. Animal Inoculation
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White mice or young rats are inoculated intraperitoneally with blood. Stained films of tail blood are examined for spirochetes 2–4 days later.
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Spirochetes grown in culture can serve as antigens for CF tests, but the preparation of satisfactory antigens is difficult. Patients with epidemic (louse-borne) relapsing fever may develop a positive VDRL test result.
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Immunity following infection is usually of short duration.
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The great variability of the spontaneous remissions of relapsing fever makes evaluation of chemotherapeutic effectiveness difficult. Tetracyclines, erythromycin, and penicillin are all believed to be effective. Treatment for a single day may be sufficient to terminate an individual attack.
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Epidemiology, Prevention, and Control
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Relapsing fever is endemic in many parts of the world. Its main reservoir is the rodent population, which serves as a source of infection for ticks of the genus Ornithodoros. The distribution of endemic foci and the seasonal incidence of the disease are largely determined by the ecology of the ticks in different areas. In the United States, infected ticks are found throughout the West, especially in mountainous areas, but clinical cases are rare. In the tick, Borrelia species may be transmitted transovarially from generation to generation.
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Spirochetes are present in all tissues of the tick and may be transmitted by the bite or by crushing the tick. The tickborne disease is not epidemic. However, when an infected individual harbors lice, the lice become infected by sucking blood; 4–5 days later, they may serve as a source of infection for other individuals. The infection of the lice is not transmitted to the next generation, and the disease is the result of rubbing crushed lice into bite wounds. Severe epidemics may occur in louse-infected populations, and transmission is favored by crowding, malnutrition, and cold climate.
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In endemic areas, human infection may occasionally result from contact with the blood and tissues of infected rodents. The mortality rate of the endemic disease is low, but in epidemics, it may reach 30%.
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Prevention is based on avoidance of exposure to ticks and lice and on delousing (cleanliness, insecticides).
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BORRELIA BURGDORFERI AND LYME DISEASE
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Lyme disease is named after the town of Lyme, Connecticut, where clusters of cases in children were identified. Since 1992, three species of Borrelia have been associated with Lyme disease, Borrelia burgdorferi sensu stricto, Borrelia afzelii, and Borrelia garinii. All three species cause disease in Europe, but only B burgdorferi is responsible for disease in North America. The spirochete B burgdorferi is transmitted to humans by the bite of a small Ixodes tick. The disease has early manifestations with a characteristic skin lesion, erythema migrans, along with flulike symptoms, and late manifestations often with arthralgia and arthritis.
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Morphology and Identification
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B burgdorferi is a spiral organism 20–30 μm long and 0.2–0.3 μm wide. The distance between turns varies from 2 to 4 μm. The organisms have variable numbers (7–11) of endoflagella and are highly motile. B burgdorferi stains readily with acid and aniline dyes and by silver impregnation techniques.
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B. Culture and Growth Characteristics
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B burgdorferi grows most readily in a complex liquid medium, Barbour-Stoenner-Kelly medium (BSK II). Rifampin, fosfomycin (phosphonomycin), and amphotericin B can be added to BSK II to reduce the rate of culture contamination by other bacteria and fungi. B burgdorferi has been most easily isolated from erythema migrans skin lesions; isolation of the organism from other sites has been difficult. The organism can also be cultured from ticks. Because culture of the organism is a complex and specialized procedure with a low diagnostic yield, it is seldom used.
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Antigenic Structure and Variation
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B burgdorferi has a morphologic appearance similar to that of other spirochetes. The entire genome of B burgdorferi has been sequenced, allowing prediction of many antigenic structures. There is an unusual linear chromosome of about 950 kb and multiple circular and linear plasmids. There are a large number of sequences for lipoproteins, including outer surface proteins OspA to F. Differential expression of these proteins is thought to help B burgdorferi live in the very different tick and mammalian hosts. OspA and OspB along with lipoprotein 6.6 are expressed primarily in ticks. Other outer surface proteins are upregulated during tick feeding when the organisms migrate from the tick’s midgut to the salivary gland. This may explain why the tick must feed for 24–48 hours before transmission of B burgdorferi occurs.
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Pathogenesis and Clinical Findings
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The transmission of B burgdorferi to humans is by injection of the organism in tick saliva or by regurgitation of the tick’s midgut contents. The organism adheres to proteoglycans on host cells; this is mediated by a borrelial glycosaminoglycan receptor. After injection by the tick, the organism migrates out from the site, producing the characteristic skin lesion. Dissemination occurs by lymphatics or blood to other skin and musculoskeletal sites and to many other organs.
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Lyme disease, similar to other spirochetal diseases, occurs in stages with early and late manifestations. A unique skin lesion that begins 3 days to 4 weeks after a tick bite often marks stage 1. The lesion, erythema migrans, begins as a flat reddened area near the tick bite and slowly expands, with central clearing. With the skin lesion, there is often a flulike illness with fever, chills, myalgia, and headache. Stage 2 occurs weeks to months later and includes arthralgia and arthritis; neurologic manifestations with meningitis, facial nerve palsy, and painful radiculopathy; and cardiac disease with conduction defects and myopericarditis. Stage 3 begins months to years later with chronic skin, nervous system, or joint involvement. Spirochetes have been isolated from all of these sites, and it is likely that some of the late manifestations are caused by deposition of antigen–antibody complexes.
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Diagnostic Laboratory Tests
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In some symptomatic patients, the diagnosis of early Lyme disease can be established clinically by observing the unique skin lesion. When this skin lesion is not present and at later stages of the disease, which must be differentiated from many other diseases, it is necessary to perform diagnostic laboratory tests. There is, however, no one test that is both sensitive and specific.
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Blood is obtained for serologic tests. CSF or joint fluid can be obtained, but culture usually is not recommended. These specimens and others can be used to detect B burgdorferi DNA by PCR.
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B burgdorferi has been found in sections of biopsy specimens, but examination of stained smears is an insensitive method for diagnosis of Lyme disease. B burgdorferi in tissue sections can sometimes be identified using antibodies and immunohistochemical methods.
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Culture is generally not performed because it takes 6–8 weeks to complete and lacks sensitivity.
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D. Nucleic Acid Amplification Methods
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The PCR assay has been applied to detection of B burgdorferi DNA in many body fluids. It is rapid, sensitive, and specific, but it does not differentiate between DNA from live B burgdorferi in active disease and DNA from dead B burgdorferi in treated or inactive disease. It has about 85% sensitivity when applied to synovial fluid samples, but the sensitivity is much lower when it is applied to CSF samples from patients with neuroborreliosis.
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Serology has been the mainstay for the diagnosis of Lyme disease, but 3–5% of normal people and persons with other diseases (eg, rheumatoid arthritis, many infectious diseases) may be seropositive by initial EIA or indirect fluorescent antibody (IFA) assay. When the prevalence of Lyme disease is low as it is in many geographic areas, there is a much greater likelihood that a positive test result is from a person who does not have Lyme disease than from a person who does have the disease (a positive predictive value of <10%). Thus, serology for Lyme disease should only be done when there are highly suggestive clinical findings. A diagnosis of Lyme disease should not be based on a positive EIA or IFA test result in the absence of suggestive clinical findings. A two-stage approach to the serodiagnosis is strongly recommended that includes EIA or IFA followed by an immunoblot assay for reactivity with specific B burgdorferi antigens.
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The EIA and IFA are the most widely used initial tests for Lyme disease. Multiple variations of these assays using different antigen preparations, techniques, and end points have been marketed. Results of the initial tests are generally reported as positive, negative, or indeterminate.
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The immunoblot assay is generally performed to confirm results obtained by the EIA tests. Recombinant B burgdorferi antigens or antigens from whole-cell lysates are electrophoretically separated, transferred to a nitrocellulose membrane, and reacted with a patient’s serum. Interpretation of the immunoblot is based on the number and molecular size of antibody reactions with the B burgdorferi proteins. Blots can be analyzed for IgG or IgM. The antigen–antibody band patterns on the immunoblots should be interpreted with knowledge of known results from patients at various stages of Lyme borreliosis, and caution should be used to avoid overinterpretation of minimally reactive blots.
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The immunologic response to B burgdorferi develops slowly. Sera obtained in stage 1 are positive in 20–50% of patients. Sera obtained during stage 2 are positive in 70–90% with reactive IgG and IgM; IgG predominates in longstanding infection. In stage 3, nearly 100% of patients have IgG reactive with B burgdorferi. The antibody response can expand from months to years and appears to be directed sequentially against a series of B burgdorferi proteins. Early antimicrobial treatment decreases the antibody response. Antibody titers fall slowly after treatment, but most patients with later manifestation of Lyme disease remain seropositive for years.
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Early infection, either local or disseminated, should be treated with doxycycline, amoxicillin, or cefuroxime axetil for 14–21 days. Treatment relieves early symptoms and promotes resolution of skin lesions. Doxycycline may be more effective than amoxicillin in preventing late manifestations. Established arthritis may respond to prolonged therapy with doxycycline or amoxicillin orally or penicillin G or ceftriaxone intravenously. In refractory cases, ceftriaxone has been effective. Nearly 50% of patients treated with doxycycline or amoxicillin early in the course of Lyme disease develop minor late complications (eg, headache, joint pains).
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Epidemiology, Prevention, and Control
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B burgdorferi is transmitted by a small tick of the genus Ixodes. The vector is Ixodes scapularis (also called Ixodes dammini) in the Northeast and Midwest and Ixodes pacificus on the West Coast of the United States. In Europe, the vector is Ixodes ricinus, and other tick vectors appear to be important in other areas of the world. The Ixodes ticks are quite small and often are not noticed when feeding on the skin. The larvae are about 1 mm; the nymphs about the size of a poppy seed or piece of cracked pepper (~2 mm), and the adult female 3–4 mm. All stages are smaller by one-half or more than comparable stages of the dog tick Dermacentor variabilis. Depending on the developmental stage and the Ixodes species, the ticks must feed for 2–4 days to obtain a blood meal. Transmission of B burgdorferi occurs late in the feeding process. Mice and deer constitute the main animal reservoirs of B burgdorferi, but other rodents and birds may also be infected. In the eastern part of the United States, 10–50% of ticks are infected; in the western states, the infection rate in ticks is much lower, about 2%.
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Most exposures are in May through July, when the nymphal stage of the ticks is most active; however, the larval stage (August and September) and adult stage (spring and fall) also feed on humans and can transmit B burgdorferi.
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Prevention is based on avoidance of exposure to ticks. Wearing long sleeves and long pants tucked into socks is recommended. Careful examination of the skin for ticks after being outdoors can locate ticks for removal before they transmit B burgdorferi.
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Environmental control of ticks using application of insecticides has provided modest success in reducing the number of nymphal ticks for a season.
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A variety of Borrelia species cause disease, usually after the bite of an arthropod or other vector.
B recurrentis, transmitted by the human body louse, causes epidemic relapsing fever; endemic disease is usually transmitted by ticks of the genus Ornithodoros. B hermsii is the cause of relapsing fever in the western United States.
Relapsing fever is characterized by abrupt rise in temperature that persists for 3–5 days. After a brief afebrile hiatus, a second attack occurs, usually related to antigenic variants.
Diagnosis of relapsing fever is best done by obtaining thick and thin blood smears and staining them with Wright or Giemsa stain.
Treatment requires penicillin, tetracycline, or erythromycin.
B burgdorferi is responsible for Lyme disease and is most often transmitted by the nymphal stage of the Ixodes tick.
Lyme disease occurs in stages. The hallmark of stage 1 is erythema migrans at the site of the tick bite. Stages 2 and 3 are characterized by arthritis and cardiac and neurologic manifestations.
Diagnosis rests on a two-stage serologic approach beginning with an EIA or IFA test followed by an immunoblot assay for reactivity to specific antigens if the screening test result is positive.
Treatment depends on the stage of the disease; penicillin, doxycycline, cefuroxime, and parenteral ceftriaxone have all been effective.