Leptospira (2024)

Leptospira

Borrelia

Spirillum

Clinical Manifestations

Clinical manifestations of leptospirosis are associated with a general febriledisease and are not sufficiently characteristic for diagnosis. As a result,leptospirosis often is initially misdiagnosed as meningitis or hepatitis.Typically, the disease is biphasic, which an acute leptospiremic phase followedby the immune leptospiruric phase. The three organ systems most frequentlyinvolved are the central nervous system, kidneys, and liver (Fig. 35-1). After an average incubationperiod of 7 to 14 days, the leptospiremic acute phase is evidenced by abruptonset of fever, severe headache, muscle pain, and nausea; these symptoms persistfor approximately 7 days. Jaundice occurs during this phase in more severeinfections. With the appearance of antileptospiral antibodies, the acute phaseof the disease subsides and leptospires can no longer be isolated from theblood. The immune leptospiruric phase occurs after an asymptomatic period ofseveral days. It is manifested by a fever of shorter duration and centralnervous system involvement (meningitis). Leptospires appear in the urine duringthis phase and are shed for various periods depending on the host. The moresevere form of leptospirosis is frequently associated with infections having theserotype icterohaemorrhagiae and is often referred to as Weil's disease.

Structure, Classification, and Antigenic Types

Leptospira has the general structural characteristics thatdistinguish spirochetes from other bacteria (Fig. 35-2). The cell is encased in a three- to five-layer outermembrane or envelope. Beneath this outer membrane are the flexible, helicalpeptidoglycan layer and the cytoplasmic membrane; these encompass thecytoplasmic contents of the cell. The structures surrounded by the outermembrane are collectively called the protoplasmic cylinder An unusual feature ofthe spirochetes is the location of the flagella, which lie between the outermembrane and the peptidoglycan layer. They are referred to as periplasmicflagella. The periplasmic flagella are attached to the protoplasmic cylindersubterminally at each end and extend toward the center of the cell. The numberof periplasmic flagella per cell varies among the spirochetes. The motility ofbacteria with external flagella is impeded in viscous environments, but that ofspirochetes is enhanced. The slender (0. 1 μm by 8 to 20 μm)leptospires are tightly coiled, flexible cells (Fig. 35-3). In liquid media, one or both ends are usually hooked.Leptospires are too slender to be visualized with the bright-field microscopebut are clearly seen by dark-field or phase microscopy. They do not stain wellwith aniline dyes.

The leptospires have two periplasmic flagella, one originating at each end of thecell. The free ends of the periplasmic flagella extend toward the center of thecell, but do not overlap as they do in other spirochetes. The basal bodies ofLeptospira periplasmic flagella resemble those ofGram-negative bacteria, whereas those of other spirochetes are similar to thebasal bodies of Gram-positive bacteria. Leptospira differs fromother spirochetes in lacking glycolipids and having diaminopimelic acid ratherthan ornithine in its peptidoglycan.

The leptospires are the most readily cultivated of the pathogenic spirochetes.They have relatively simple nutritional requirements; long-chain fatty acids andvitamins B₁ and B₁₂ are the only organic compounds knownto be necessary for growth. When cultivated in media of pH 7.4 at 30°C,their average generation time is about 12 hours. Aeration is required formaximal growth. They can be cultivated in plates containing soft (1 percent)agar medium, in which they form primarily subsurface colonies.

The two species, L interrogans and L biflexa,are further divided into serotypes based on their antigenic composition. Morethan 200 serotypes have been identified in L interrogans. Themost prevalent serotypes in the United States are canicola, grippotyphosa,hardjo, icterohaemorrhagiae, and pomona. Genetic studies have demonstrated thatserologically diverse serotypes may be present in the same genetic group. Atleast seven species of pathogenic leptospires have been identified by nucleotideanalysis.

Pathogenesis

The mucosa and broken skin are the most likely sites of entry for the pathogenicleptospires (Fig. 35-1). A generalizedinfection ensues, but no lesion develops at the site of entry. Bacteremia occursduring the acute, leptospiremic phase of the disease. The host responds byproducing antibodies that, in combination with complement, are leptospiricidal.The leptospires are rapidly eliminated from all host tissues except the brain,eyes, and kidneys. Leptospires surviving in the brain and eyes multiply slowlyif at all; however, in the kidneys they multiply in the convoluted tubules andare shed in the urine (the leptospiruric phase). The leptospires may persist inthe host for weeks to months; in rodents they may be shed in the urine for thelifetime of the animal. Leptospiruric urine is the vehicle of transmission ofthis disease.

The mechanism by which leptospires cause disease remains unresolved, as neitherendotoxin nor exotoxins have been associated with them. The marked contrastbetween the extent of functional impairment in leptospirosis and the scarcity ofhistologic lesions suggests that most damage occurs at the subcellular level.Damage to the endothelial lining of the capillaries and subsequent interferencewith blood flow appear responsible for the lesions associated withleptospirosis. The most notable feature of severe leptospirosis is theprogressive impairment of hepatic and renal function. Renal failure is the mostcommon cause of death. The lack of substantial cell destruction in leptospirosisis reflected in the complete recovery of hepatic and renal function insurvivors. Although spontaneous abortion is common in infected cattle and swine,only recently has a human case of fatal congenital leptospirosis beendocumented.

The host's immunologic response to leptospirosis is thought to be responsible forlesions associated with the late phase of this disease; this helps to explainthe ineffectiveness of antibiotics once symptoms of the disease have beenpresent for 4 days or more.

Host Defenses

Nonspecific host defenses appear ineffective against the virulent leptospires,which are rapidly killed in vitro by the antibody-complement system; virulentstrains are more resistant to this leptospiricidal activity than are avirulentstrains. Immunity to leptospirosis is primarily humoral; cell-mediated immunitydoes not appear to be important, but may be responsible for some of the latemanifestations of the disease. Immunity to leptospirosis is serotype specificand may persist for years. Immune serum has been used to treat humanleptospirosis and passively protects experimental animals from the disease. Thesurvival of leptospires within the convoluted tubules of the kidneys may berelated to the ineffectiveness of the antibody-complement system at this site.Previously infected animals can become seronegative and continue to shedleptospires in their urine, possibly because of the lack of antigenicstimulation by leptospires in the kidneys.

Epidemiology

Leptospirosis is a worldwide zoonosis with a broad spectrum of animal hosts. Theprimary reservoir hosts are wild animals such as rodents, which can shedleptospires throughout their lifetimes. Domestic animals are also an importantsource of human infections. Leptospires have been isolated from approximately160 mammalian species in the temperate zone. The disease is more widespread intropical countries, where the infectious agent may be one of many serotypescarried by a large variety of hosts.

Direct or indirect contact with urine containing virulent leptospires is themajor means by which leptospirosis is transmitted. As mentioned above,leptospires from urine-contaminated environments, such as water and soil, enterthe host through the mucous membranes and through small breaks in the skin.Moist environments with a neutral pH provide suitable conditions for survival ofleptospires outside the host. Urine-contaminated soil can remain infective foras long as 14 days. In humans, leptospirosis has occurred in an infant beingbreast-fed by a mother with the disease. The cellular structure of leptospirescauses them to be susceptible to killing by adverse conditions such asdehydration, exposure to detergents, and temperatures above 50°C. Mostcases of leptospirosis occur during summer and fall.

Diagnosis

Because clinical manifestations of leptospirosis are too variable and nonspecificto be diagnostically useful, microscopic demonstration of the organisms,serologic tests, or both are used in diagnosis. The microscopic agglutinationtest is most frequently used for serodiagnosis. The organisms can be isolatedfrom blood or urine on commercially available media, but the test must berequested specifically because special media are needed. Isolation of theorganisms confirms the diagnosis.

Control

Human leptospirosis can be controlled by reducing its prevalence in wild anddomestic animals. Although little can be done about controlling the disease inwild animals, leptospirosis in domestic animals can be controlled throughvaccination with inactivated whole cells or an outer membrane preparation. Ifvaccines do not contain a sufficient immunogenic mass, the resulting immuneresponse protects the host against clinical disease but not against developmentof the renal shedder state. Because a multiplicity of serotypes may exist in agiven geographic region and the protection afforded by the inactivated vaccinesis serotype specific, the use of polyvalent vaccines is recommended. Vaccinesfor human use are not available in the United States.

Although the leptospires are susceptible to antibiotics such as penicillin andtetracycline in vitro, use of these drugs in the treatment of leptospirosis issomewhat controversial. Treatment is most effective if initiated within a weekof disease onset. At later times, immunologic damage may already have begun,rendering antimicrobial therapy less effective. Doxycycline has been usedsuccessfully as a chemoprophylactic agent for military personnel training intropical areas.

Clinical Manifestations

Once the relapsing-fever borreliae have entered the host, they cause ageneralized infection, apparent after an incubation period of approximately 1week (Fig. 35-4). The onset of thedisease, which is associated with numerous spirochetes in the blood is abrupt,with fever, headache, and muscle pain that persists for 4 to 10 days, followedby an afebrile period of 5 to 6 days correlated with the absence ofspirochetemia. Usually, a single relapse occurs in louse-borne relapsingfever.

The clinical features of relapsing fever, other than its recurrent pattern, arenot diagnostic. The mortality in untreated epidemic relapsing fever can behigher than 40 percent, and myocarditis probably is the most common cause ofdeath. The tick-borne relapsing fever is similar to the louse-borne disease, butis less severe (mortality, 0 to 8 percent), and several relapses of decreasingintensity are commonly experienced.

Structure, Classification, and Antigenic Types

Borrelia has morphologic characteristics similar to those ofLeptospira, except that cells average 0.2 to 0.5 μmby 4 to 18 μm and have fewer coils (Fig.35-5). Seven to twenty periplasmic flagella originate at each end andoverlap at the center of the cell. In contrast to theLeptospira peptidoglycan, that of Borreliacontains ornithine rather than diaminopimelic acid. Basal bodies of periplasmicflagella of borreliae resemble those in Gram-positive bacteria. Because of theirlarger diameter, borreliae are more readily stained with aniline dyes than areother spirochetes. Their lipid components are unusual in that they includecholesterol; this substance has been found in only one other bacterial genus,Mycoplasma. The nutritional requirements of the borreliaeare more complex than those of leptospires. Glucose, amino acids, long-chainfatty acids, N-acetylglucosamine, and several vitamins are some of theirrequired organic nutrients. The borreliae are microaerophilic organisms.Borrelia hermsii has a generation time of 12 hours whencultivated in artificial media at 35°C compared with only 6 to 10 hoursin the mouse.

Pathogenesis

In most cases, borreliae must rely on an insect vector to transmit the organismsthrough the epidermis (Fig. 35-4). Thesite of entry is usually not prominent as the organisms are not clinicallyrecognized until they enter the blood. The mechanisms by which they reach thebloodstream are unknown. The relapses are due to the ability of borreliae toundergo multiple cyclic antigenic variations (Fig. 35-6). As antibodies for the predominant antigenic typemultiplying within the host appear, these organisms“disappear” from the peripheral blood and are replaced by adifferent antigenic variant within a few days. This process may occur severaltimes in an untreated host, depending on the infecting Borreliastrain.

The mechanism by which borreliae cause Lyme disease has not been elucidated.Early Lyme disease is characterized by an expanding annular red rash, erythemamigrans, in approximately 70 percent of patients, which is frequentlyaccompanied by fever, fatigue, headache, and muscle and joint pain. Arthritis,neuritis, and carditis may also be present during early Lyme disease. Persistentneurologic and arthritic infections (lasting for months to years) may occur insome patients. In contrast to the relapsing fevers, there are very fewspirochetes in Lyme disease, but viable B burgdorferi organismsare necessary for the disease to manifest itself. Although the disease has thesame general features in the United States and Europe, arthritis occurs morefrequently in the former and neurologic disease in the latter.

Host Defenses

Borrelia appears to be resistant to nonspecific host defensemechanisms and elicits an inflammatory response consisting of mononuclear cells.These spirochetes are rapidly killed in vitro by the antibody-complement system.Immunity to the borrelioses is primarily humoral, and immune serum passivelyprotects experimental animals from infection. Several genospecies of Bburgdorferi have been reported.

Epidemiology

Borrelia is transmitted to humans by the body louse or ticks.Borrelia recurrentis, the cause of louse-borne (epidemic)relapsing fever, is carried by the human louse Pediculushumanus. The louse ingests the bacterium while feeding on aborrelemic host. The organisms multiply in the hemolymph and central ganglion ofthe louse. Because other organs are not invaded, transovarial transmission doesnot occur in the louse. In addition, Borrelia organisms arereleased when the louse is injured by host activities such as scratching.Accordingly, one louse can infect only a single host and is infective only forits life-span of around 1 month. Therefore, humans rather than the louse are thereservoir of this disease. The louse-borne disease is called epidemic relapsingfever because it can be rapidly disseminated under conditions of overcrowdingand poor personal hygiene, such as during wars and natural disasters. Relapsingfever, which depends on the aforementioned conditions favoring themultiplication and transfer of the human louse, has disappeared from the UnitedStates except for imported cases. Ethiopia appears to have the highest incidenceof this disease.

The tick-borne relapsing fever is called endemic relapsing fever because itoccurs whenever humans are exposed to infected ticks. The soft ticksOrnithodoros hermsi and O turicata mostfrequently transmit the disease in the United States. These ticks often obtaintheir blood meal at night, and because the tick bite is usually painless andfeedings are short (5 to 20 minutes), people may not be aware of having beenbitten. The designation of species of these borreliae is based on their vector(e.g., B hermsii is associated with O hermsi).Genetic studies have shown that this basis is incorrect. The two North Americanspecies, B hermsii and B turicatae, actuallyrepresent a single species. The ticks usually become infected by feeding onborrelemic rodents. In contrast to the louse, all tissues of the tick areinvaded, resulting in transovarial transmission and the presence of borreliae insalivary and coxal (basal segment of appendage) secretions. These spirochetes inthe salivary and coxal secretions enter the host through the bite wound whilethe tick is feeding (less than 1 minute may be required for transmission). Thelargest outbreak of tick-borne relapsing fever in the western hemisphereoccurred in 1973 on the north rim of the Grand Canyon, where 62 persons stayingin log cabins developed the disease.

Borrelia burgdorferi, the agent of Lyme disease, is transmittedby members of the Ixodes ricinus complex (hard ticks). In thenortheastern and midwestern United States the spirochete is transmitted by thedeer tick, I scapularis, whereas the western black-legged tick,I pacificus, is the primary vector in the western UnitedStates. However, only one Borrelia species, Bburgdorferi, appears responsible for the disease in North America,and it does not undergo the antigenic changes of the relapsing-fever borreliae.Transovarial transmission of the spirochete is infrequent, with fewer than 1percent of the larvae infected. The pinhead-sized nymph form of the tick is theprimary source of human infections, but the disease can also be transmitted bythe large adult female tick. Feedings are lengthy and require several days. Thespirochetes are present in the midgut of the tick, and 12 to 24 hours isrequired before the spirochetes are transmitted. Within endemic areas of Lymedisease, 20 to 60 percent of I scapularis may be carriers ofB burgdorferi and a similar percentage of white-footedmice, a major reservoir host, are infected. The white-tailed deer, although nota reservoir host for the spirochete, plays a critical role in the life cycle ofthe tick, and Lyme disease occurs in areas in which deer are present. Dogs andbirds are important in the dispersal of infected ticks to new locations. Thenumber of cases of Lyme disease reported in the United States is about 10,000per year.

Diagnosis

Clinical features of the relapsing fevers other than their recurring pattern arenot diagnostic. Diagnosis is based primarily on demonstration of the spirochetesin blood during febrile episodes by dark-field examination, use of stained bloodsmears, or mouse inoculation. Antibody detection by indirect immunofluorescenceassay is available. There is strong cross-reactivity with antibodies toB burgdorferi and weaker reactivity with those toTreponema pallidum.

The characteristic expanding red skin lesion, erythema migrans, is diagnostic forLyme disease. However, 30 percent of patients do not develop this rash. Theusual symptoms of early disease (fever, fatigue, headache, and muscle and jointpain) are too nonspecific to be diagnostic. Although Bburgdorferi has been isolated from blood, skin, and cerebrospinalfluid, this is a low yield procedure and is not recommended. Serologic tests areused most commonly for diagnosis of Lyme disease.

Unfortunately, most of the currently available tests are unable to detect earlyantibody responses to B burgdoferi (which may take up to twomonths to develop), thus the diagnosis of a patient without erythema migrans whois suspected of having early Lyme disease can be difficult. Moreover, falsepositive results may occur due to cross-reacting antibodies against otherbacteria, especially T pallidum or other spirochetes, andpatients with connective tissue disorders may demonstrate false serologicresults. Despite these limitations, however, if patients being tested arecarefully selected for symptoms compatible with Lyme disease and epidemiologicfactors are included in consideration of the diagnosis, the positive predictivevalue of serologic tests can be quite acceptable, especially if positiveserologic results are confirmed by a Western blotting procedure.

Many of the problems with serological testing for Lyme disease are based oninterlaboratory differences in the substrate antigens, in antigen preparation,and in interpretive criteria. To improve the sensitivity and specificity ofserodiagnosis of Lyme disease, evaluating serum samples from patients withsymptoms of Lyme disease by a two-step process which includes a sensitivescreening test such as immunofluorescence assay or enzyme immunoassay isrecommended. Samples judged equivocal or positive by the screening should befurther tested by Western blot. Both IgM and IgG Western blot should beperformed for persons with suspected Lyme disease who present within the firstfour weeks of disease onset. Only IgG Western should be performed for patientspresenting later because interpretation of IgM band patterns after that time isless reliable. Serologic testing of a convalescent serum sample two to fourweeks after the first sample is recommended for patients who have symptoms ofearly Lyme disease and negative screening test results.

Control

Relapsing fevers and Lyme disease are prevented by avoiding the vectors. It isimportant to be aware of endemic areas and to take proper precautions. When inpotential tick habitats, one should wear clothing that covers as much of theskin as possible and use tick repellents. Periodic skin inspection and tickremoval prevent Lyme disease. A Lyme disease vaccine may be available in thenear future. The relapsing-fever and Lyme disease borreliae have similarantibiotic susceptibilities.

Early Lyme disease can be effectively treated with oral tetracyclines andsemisynthetic penicillins. Arthritic and neurologic disorders are treated withhigh-dose intravenous penicillin G or ceftriaxone. Patients who have failed torespond to penicillin or tetracycline therapy have been effectively treated withceftriaxone.