CHAPTER 80: LEPTOSPIROSIS

Leptospirosis is a globally important zoonotic disease whose apparent reemergence is illustrated by recent outbreaks on virtually all continents. The disease is caused by pathogenic Leptospira species and is characterized by a broad spectrum of clinical manifestations, varying from asymptomatic infection to fulminant, fatal disease. In its mild form, leptospirosis may present as nonspecific symptoms such as fever, headache, and myalgia. Severe leptospirosis, characterized by jaundice, renal dysfunction, and hemorrhagic diathesis, is often referred to as Weil’s syndrome. With or without jaundice, severe pulmonary hemorrhage is increasingly recognized as an important presentation of severe disease.

Leptospira species are spirochetes belonging to the order Spirochaetales and the family Leptospiraceae. Traditionally, the genus Leptospira comprised two species: the pathogenic L. interrogans and the free-living L. biflexa, now designated L. interrogans sensu lato and L. biflexa sensu lato, respectively. Twenty-two Leptospira species with pathogenic (10 species), intermediate (5 species), and nonpathogenic (7 species) status have now been described on the basis of phylogenetic and virulence analyses (Fig. 80-1). Genome sequences of five Leptospira species (L. biflexa, L. interrogans, L. santarosai, L. borgpetersenii, and L. licerasiae) have been published, and the availability of genome sequences of a wide variety of Leptospira strains will undoubtedly lead to a better understanding of the pathogenesis of leptospirosis. However, classification based on serologic differences better serves clinical, diagnostic, and epidemiologic purposes. Pathogenic Leptospira species are divided into serovars according to their antigenic composition. More than 250 serovars make up the 26 serogroups.

Leptospires are coiled, thin, highly motile organisms that have hooked ends and two periplasmic flagella, with polar extrusions from the cytoplasmic membrane that are responsible for motility (Fig. 80-2). These organisms are 6–20 μm long and ~0.1 μm wide; they stain poorly but can be seen microscopically by dark-field examination and after silver impregnation staining of tissues. Leptospires require special media and conditions for growth; it may take weeks to months for cultures to become positive.

Leptospirosis has a worldwide distribution but occurs most commonly in the tropics and subtropics because the climate and occasionally poor hygienic conditions favor the pathogen’s survival and distribution. In most countries, leptospirosis is an underappreciated problem. Most cases occur in men, with a peak incidence during the summer and fall in both the Northern and Southern Hemispheres and during the rainy season in the tropics.

Reliable data on morbidity and mortality from leptospirosis have gradually started to appear. Current information on global human leptospirosis varies but indicates that approximately 1 million severe cases occur per year, with a mean case–fatality rate of nearly 10%.

As a zoonosis, leptospirosis affects almost all mammalian species and represents a significant veterinary burden. Rodents, especially rats, are the most important reservoir, although other wild mammals as well as domestic and farm animals may also harbor these microorganisms. Leptospires establish a symbiotic relationship with their host and can persist in the urogenital tract for years. Some serovars are generally associated with particular animals—e.g., Icterohaemorrhagiae and Copenhageni with rats, Grippotyphosa with voles, Hardjo with cattle, Canicola with dogs, and Pomona with pigs—but may occur in other animals as well.

Leptospirosis presents as both an endemic and an epidemic disease. Transmission of leptospires may follow direct contact with urine, blood, or tissue from an infected animal or, more commonly, exposure to environmental contamination. The dogma that human-to-human transmission is very rare is challenged by recent findings on household clustering, asymptomatic renal colonization, and prolonged excretion of leptospires. (Both of the latter features imply human infection sources that are not recognized.) Because leptospires can survive in a humid environment for many months, water is an important vehicle in their transmission. Epidemics of leptospirosis are not well understood. Outbreaks may result from exposure to flood waters contaminated by urine from infected animals, as has been reported from several countries. However, it is also true that outbreaks may occur without floods, and floods often occur without outbreaks.

The vast majority of infections with Leptospira cause no or only mild disease in humans. A small percentage of infections (~1%) lead to severe, potentially fatal complications. The proportion of leptospirosis cases that are mild is unknown because patients either do not seek or do not have access to medical care or because the nonspecific symptoms are interpreted as an influenza-like illness. Reported cases surely represent a significant underestimation of the total number. Certain occupational groups are at especially high risk, including veterinarians, agricultural workers, sewage workers, slaughterhouse employees, and workers in the fishing industry. Risk factors include direct or indirect contact with animals, including exposure to water and soil contaminated with animal urine. Leptospirosis has also been recognized in deteriorating inner cities and suburban areas where rat populations are expanding.

Recreational exposure and domestic-animal contact are prominent sources of leptospirosis. Recreational freshwater activities, such as canoeing, windsurfing, swimming, and waterskiing, place persons at risk for infection. Several outbreaks have followed sporting events. For example, an outbreak took place in 1998 among athletes after a triathlon in Springfield, Illinois. Ingestion of one or more swallows of lake water during the swimming leg of the triathlon was a prominent risk factor for illness. Heavy rains that preceded the triathlon, with consequent agricultural runoff, are likely to have increased the level of leptospiral contamination in the lake water. In another outbreak, 42% of participants contracted leptospirosis during the 2000 Eco-Challenge-Sabah multisport endurance race in Malaysian Borneo. Swimming in the Segama River was shown to be an independent risk factor.

In addition, leptospirosis is a traveler’s disease. Large proportions of patients acquire the infection while traveling in tropical countries, usually during adventurous activities such as whitewater rafting, jungle trekking, and caving. Transmission via laboratory accidents has been reported but is rare. New data indicate that leptospirosis may develop after unanticipated immersion in contaminated water (e.g., in an automobile accident) more frequently than has generally been thought and can also result from an animal bite.

Transmission occurs through cuts, abraded skin, or mucous membranes, especially the conjunctival and oral mucosa. After entry, the organisms proliferate, cross tissue barriers, and disseminate hematogenously to all organs (leptospiremic phase). During this initial incubation period, leptospires can be isolated from the bloodstream (Fig. 80-3). The organisms are able to survive in the nonimmune host: they evade complement-mediated killing by binding factor H, a strong inhibitor of the complement system, on their surface. Moreover, pathogenic leptospires resist ingestion and killing by neutrophils, monocytes, and macrophages. During the immune phase, the appearance of antibodies coincides with the disappearance of leptospires from the blood. However, the bacteria persist in various organs, including liver, lung, kidney, heart, and brain. Autopsy findings illustrate the involvement of multiple organ systems in severe disease. Renal pathology shows both acute tubular damage and interstitial nephritis. Acute tubular lesions progress in time to interstitial edema and acute tubular necrosis. Severe nephritis is observed in patients who survive long enough to develop it and seems to be a secondary response to acute epithelial damage. The reported deregulation of the expression of several transporters along the nephron, including the proximal sodium-hydrogen exchanger 3 (NHE3), aquaporins 1 and 2 (AQP1 and AQP2), Na+-K+ ATPase, and the Na-K-2Cl cotransporter NKCC2, contributes to tubular potassium wasting, hypokalemia, and polyuria. Histopathology of the liver shows focal necrosis, foci of inflammation, and plugging of bile canaliculi. Widespread hepatocellular necrosis is not found. Petechiae and hemorrhages are observed in the heart, lungs (Fig. 80-4), kidneys (and adrenals), pancreas, liver, gastrointestinal tract (including retroperitoneal fat, mesentery, and omentum), muscles, prostate, testis, and brain (subarachnoid bleeding). Several studies show an association between hemorrhage and thrombocytopenia. Although the underlying mechanisms of thrombocytopenia have not been elucidated, it seems likely that platelet consumption plays an important role. A consumptive coagulopathy may occur, with elevated markers of coagulation activation (thrombin–antithrombin complexes, prothrombin fragments 1 and 2, d-dimer), diminished anticoagulant markers (antithrombin, protein C), and deregulated fibrinolytic activity. Overt disseminated intravascular coagulation (DIC) has been documented in patients from Thailand and Indonesia. Elevated plasma levels of soluble E-selectin and von Willebrand factor in patients with leptospirosis reflect endothelial cell activation. Experimental models show that pathogenic leptospires or leptospiral proteins are able to activate endothelial cells in vitro and to disrupt endothelial-cell barrier function, promoting dissemination. Platelets have been shown to aggregate on activated endothelium in the human lung, whereas histology reveals swelling of activated endothelial cells but no evident vasculitis or necrosis. Immunoglobulin and complement deposition have been demonstrated in lung tissue involved in pulmonary hemorrhage.

Leptospira species have a typical double-membrane cell wall structure harboring a variety of membrane-associated proteins, including an unusually high number of lipoproteins. The peptidoglycan layer is located close to the cytoplasmic membrane. The lipopolysaccharide (LPS) in the outer membrane has an unusual structure with a relatively low endotoxic potency. Pathogenic leptospires contain a variety of genes coding for proteins involved in motility and in cell and tissue adhesion and invasion that represent potential virulence factors. Many of these are surface-exposed outer-membrane proteins (OMPs). To date, the only leptospiral virulence factor shown to satisfy Koch’s molecular postulates is loa22 encoding a surface-exposed protein with an unknown function. However, the gene is not confined to pathogenic Leptospira species.

Immunity to Leptospira depends on the production of circulating antibodies to serovar-specific LPS. It is unclear whether other antigens play a significant role in protective humoral immunity. Moreover, immunity may not be confined to antibody responses; involvement of the innate-immune Toll-like receptor 2 (TLR2) and TLR4 activation pathways in controlling infection has been demonstrated, whereas in vaccinated cattle a cell-mediated immune response is correlated with protection.

It is likely that several surface-exposed proteins mediate leptospire–host cell interactions, and these proteins may represent candidate vaccine components. Although animal-model studies have shown various degrees of vaccine efficacy for various putative virulence-associated OMPs, it is not yet clear whether such proteins elicit acceptable levels of sterilizing immunity.

Although leptospirosis is a potentially fatal disease with bleeding and multiorgan failure as its clinical hallmarks, the majority of cases are thought to be relatively mild, presenting as the sudden onset of a febrile illness. The incubation period is usually 1–2 weeks but ranges from 1 to 30 days. (Figure 80-3 indicates a slightly different range, but an incubation period of up to 1 month has now been documented.) Leptospirosis is classically described as biphasic. The acute leptospiremic phase is characterized by fever of 3–10 days’ duration, during which time the organism can be cultured from blood. During the immune phase, resolution of symptoms may coincide with the appearance of antibodies, and leptospires can be cultured from the urine. The distinction between the first and second phases is not always clear: milder cases do not always include the second phase, and severe disease may be monophasic and fulminant. The idea that distinct clinical syndromes are associated with specific serogroups has been refuted, although some serovars tend to cause more severe disease than others.

Mild leptospirosis

Most patients are asymptomatic or only mildly ill and do not seek medical attention. Serologic evidence of past inapparent infection is frequently found in persons who have been exposed but have not become ill. Mild symptomatic leptospirosis usually presents as a flu-like illness of sudden onset, with fever, chills, headache, nausea, vomiting, abdominal pain, conjunctival suffusion (redness without exudate), and myalgia. Muscle pain is intense and especially affects the calves, back, and abdomen. The headache is intense, localized to the frontal or retroorbital region (resembling that occurring in dengue), and sometimes accompanied by photophobia. Aseptic meningitis may be present and is more common among children than among adults. Although Leptospira can be cultured from the cerebrospinal fluid (CSF) in the early phase, the majority of cases follow a benign course with regard to the central nervous system; symptoms disappear within a few days but may persist for weeks.

Physical examination may include any of the following findings, none of which is pathognomonic for leptospirosis: fever, conjunctival suffusion, pharyngeal injection, muscle tenderness, lymphadenopathy, rash, meningismus, hepatomegaly, and splenomegaly. If present, the rash is often transient; may be macular, maculopapular, erythematous, or hemorrhagic (petechial or ecchymotic); and may be misdiagnosed as due to scrub typhus or viral infection. Lung auscultation may reveal crackles, and mild jaundice may be present.

The natural course of mild leptospirosis usually involves spontaneous resolution within 7–10 days, but persistent symptoms have been documented. In the absence of a clinical diagnosis and antimicrobial therapy, the mortality rate in mild leptospirosis is low.

Severe leptospirosis

Although the onset of severe leptospirosis may be no different from that of mild leptospirosis, severe disease is often rapidly progressive and is associated with a case–fatality rate ranging from 1 to 50%. Higher mortality rates are associated with an age >40, altered mental status, acute renal failure, respiratory insufficiency, hypotension, and arrhythmias. The classic presentation, often referred to as Weil's syndrome, encompasses the triad of hemorrhage, jaundice, and acute kidney injury.

Patients die of septic shock with multiorgan failure and/or severe bleeding complications that most commonly involve the lungs (pulmonary hemorrhage), gastrointestinal tract (melena, hemoptysis), urogenital tract (hematuria), and skin (petechiae, ecchymosis, and bleeding from venipuncture sites). Pulmonary hemorrhage (with or without jaundice) is now recognized as a widespread public health problem, presenting with cough, chest pain, respiratory distress, and hemoptysis that may not be apparent until patients are intubated.

Jaundice occurs in 5–10% of all patients with leptospirosis; it can be profound and give an orange cast to the skin but usually is not associated with fulminant hepatic necrosis. Physical examination may reveal an enlarged and tender liver.

Acute kidney injury is common in severe disease, presenting after several days of illness, and can be either nonoliguric or oliguric. Typical electrolyte abnormalities include hypokalemia and hyponatremia. Loss of magnesium in the urine is uniquely associated with leptospiral nephropathy. Hypotension is associated with acute tubular necrosis, oliguria, or anuria, requiring fluid resuscitation and sometimes vasopressor therapy. Hemodialysis can be life-saving, with renal function typically returning to normal in survivors.

Other syndromes include (necrotizing) pancreatitis, cholecystitis, skeletal muscle involvement, rhabdomyolysis (with moderately elevated serum creatine kinase levels), and neurologic manifestations including aseptic meningitis. Cardiac involvement is commonly reflected on the electrocardiogram as nonspecific ST- and T-wave changes. Repolarization abnormalities and arrhythmias are considered poor prognostic factors. Myocarditis has been described. Rare hematologic complications include hemolysis, thrombotic thrombocytopenic purpura, and hemolytic-uremic syndrome.

Long-term symptoms following severe leptospirosis include fatigue, myalgia, malaise, and headache and may persist for years. Autoimmune-associated uveitis, a potentially chronic condition, is a recognized sequela of leptospirosis.

The clinical diagnosis of leptospirosis should be based on an appropriate exposure history combined with any of the protean manifestations of the disease. Returning travelers from endemic areas usually have a history of recreational freshwater activities or other mucosal or percutaneous contact with contaminated surface waters or soil. For nontravelers, recreational water contact and occupational hazards that involve direct or indirect animal contact should be explored (see “Epidemiology,” above).

Although biochemical, hematologic, and urinalysis findings in acute leptospirosis are nonspecific, certain patterns may suggest the diagnosis. Laboratory results usually show signs of a bacterial infection, including leukocytosis with a left shift and elevated markers of inflammation (C-reactive protein level and erythrocyte sedimentation rate). Thrombocytopenia (platelet count ≤100 × 10⁹/L) is common and is associated with bleeding and renal failure. In severe disease, signs of coagulation activation may be present, varying from borderline abnormalities to a serious derangement compatible with DIC as defined by international criteria. The kidneys are invariably involved in leptospirosis. Related findings range from urinary sediment changes (leukocytes, erythrocytes, and hyaline or granular casts) and mild proteinuria in mild disease to renal failure and azotemia in severe leptospirosis. Nonoliguric hypokalemic renal insufficiency (see “Clinical Manifestations,” above) is characteristic of early leptospirosis. Serum bilirubin levels may be high, whereas rises in aminotransferase and alkaline phosphatase levels are usually moderate. Although clinical symptoms of pancreatitis are not a common finding, amylase levels are often elevated. When symptoms of aseptic meningitis develop, examination of the CSF shows pleocytosis that can range from a few cells to >1000 cells/μL, with a polymorphonuclear cell predominance. The protein concentration in the CSF may be elevated; CSF glucose levels are normal.

In severe leptospirosis, pulmonary radiographic abnormalities are more common than would be expected on the basis of physical examination (Fig. 80-4). The most common radiographic finding is a patchy bilateral alveolar pattern that corresponds to scattered alveolar hemorrhage. These abnormalities predominantly affect the lower lobes. Other findings include pleura-based densities (representing areas of hemorrhage) and diffuse ground-glass attenuation typical of acute respiratory distress syndrome (ARDS).

A definitive diagnosis of leptospirosis is based on isolation of the organism from the patient, on a positive result in the polymerase chain reaction (PCR), or on seroconversion or a rise in antibody titer. In cases with strong clinical evidence of infection, a single antibody titer of 1:200–1:800 (depending on whether the case occurs in a low- or high-endemic area) in the microscopic agglutination test (MAT) is required. Preferably, a fourfold or greater rise in titer is detected between acute- and convalescent-phase serum specimens. Antibodies generally do not reach detectable levels until the second week of illness. The antibody response can be affected by early treatment with antibiotics.

The MAT, which uses a battery of live leptospiral strains, and the enzyme-linked immunosorbent assay (ELISA), which uses a broadly reacting antigen, are the standard serologic procedures. The MAT usually is available only in specialized laboratories and is used for determination of the antibody titer and for tentative identification of the involved leptospiral serogroup—and, when epidemiologic background information is available, the putative serovar. This point underscores the importance of testing antigens representative of the serovars prevalent in the particular geographic area. However, cross-reactions occur frequently, and thus definitive identification of the infecting serovar or serogroup is not possible without isolation of the causative organism. Because serologic testing lacks sensitivity in the early acute phase of the disease (up to day 5), it cannot be used as the basis for a timely decision about whether to start treatment.

In addition to the MAT and the ELISA, various rapid tests with diagnostic value have been developed, and some of these are commercially available. These rapid tests mainly apply lateral flow, (latex) agglutination, or ELISA methodology and are reasonably sensitive and specific, although results reported in the literature vary, probably as a consequence of differences in test interpretation, (re)exposure risks, serovar distribution, and the use of biased serum panels. These methods do not require culture or MAT facilities and are useful in settings that lack a strong medical infrastructure. PCR methodologies, notably real-time PCR, have become increasingly widely implemented. Compared with serology, PCR offers a great advantage: the capacity to confirm the diagnosis of leptospirosis with a high degree of accuracy during the first 5 days of illness.

The differential diagnosis of leptospirosis is broad, reflecting the diverse clinical presentations of the disease. Although leptospirosis transmission is more common in tropical and subtropical regions, the absence of a travel history does not exclude the diagnosis. When fever, headache, and myalgia predominate, influenza and other common and less common (e.g., dengue and chikungunya) viral infections should be considered. Malaria, typhoid fever, ehrlichiosis, viral hepatitis, and acute HIV infection may mimic the early stages of leptospirosis and are important to recognize. Rickettsial diseases, hantavirus infections (hemorrhagic fever with renal syndrome or hantavirus cardiopulmonary syndrome), and dengue share epidemiologic and clinical features with leptospirosis. Dual infections have been reported. In this light, it is advisable to conduct serologic testing for hantavirus, rickettsiae, and dengue virus when leptospirosis is suspected. When bleeding is detected, dengue hemorrhagic fever and other viral hemorrhagic fevers, including hantavirus infection, yellow fever, Rift Valley fever, filovirus infections, and Lassa fever, should be considered.

Most patients with leptospirosis recover. However, post-leptospirosis symptoms, mainly of a depression-like nature, may occur and persist for years after the acute disease. Mortality rates are highest among patients who are elderly and those who have severe disease (pulmonary hemorrhage, Weil’s syndrome). Leptospirosis during pregnancy is associated with high fetal mortality rates. Long-term follow-up of patients with renal failure and hepatic dysfunction has documented good recovery of renal and hepatic function.

Individuals who may be exposed to Leptospira through their occupations or their involvement in recreational freshwater activities should be informed about the risks. Measures for controlling leptospirosis include avoidance of exposure to urine and tissues from infected animals through proper eyewear, footwear, and other protective equipment. Targeted rodent control strategies could be considered.

Vaccines for agricultural and companion animals are generally available, and their use should be encouraged. The veterinary vaccine used in a given area should contain the serovars known to be present in that area. Unfortunately, some ­vaccinated animals still excrete leptospires in their urine. Vaccination of humans against a specific serovar prevalent in an area has been undertaken in some European and Asian countries and has proved effective. Although a large-scale trial of vaccine in humans has been reported from Cuba, no conclusions can be drawn about efficacy and adverse reactions because of insufficient details on study design. The efficacy of chemoprophylaxis with doxycycline (200 mg once a week) or azithromycin (in pregnant women and children) is being disputed, but focused pre- and postexposure administration is indicated in instances of well-defined short-term exposure (Table 80-1).