Leptospirosis: Pathophysiology and Clinical Features

The Pathogen

• Lipopolysaccharide (LPS) - antigenic, but with unusual structure that poorly activates innate immunity
• Cytotoxic glycolipoprotein (GLP) - directly toxic to tubular Na+/K+-ATPase
• Lipoproteins (especially LipL32, the most abundant outer membrane protein) - highly immunogenic, binds TLR-2, triggers NF-κB activation

Transmission

• Broken skin or mucous membranes (conjunctiva, nasal/oral mucosa) exposed to water or soil contaminated with urine from infected reservoir animals
• Ingestion through the upper alimentary tract mucosa
• The hooked ends and periplasmic flagella drive active burrowing through tissues

Pathophysiology

Step 1 - Entry and Bacteremia (Leptospiremic Phase, Days 1-7)

Step 2 - Organ Injury Mechanisms

• Leptospires in hepatic sinusoids cause direct hepatocyte disruption
• Bile leak → conjugated (direct) hyperbilirubinemia and jaundice
• Aminotransferase elevation is typically modest (contrasting with viral hepatitis)

• Direct tubular toxicity: LipL32 binds TLR-2 → NF-κB activation → iNOS, MCP-1, TNF release → inflammatory cell recruitment → tubular cell apoptosis and necrosis
• Na+/K+-ATPase inhibition by GLP: impairs proximal tubular sodium and water reabsorption
• Result: non-oliguric renal failure with urinary Na+/K+ wasting → paradoxical hypokalemia (a hallmark)
• Tubular functional defects precede GFR reduction, explaining early hypokalemia
• Histopathology: tubulointerstitial nephritis with cellular infiltration and interstitial edema (glomeruli are typically spared)

• Disruption of endothelial junctions + thrombocytopenia + coagulopathy → diffuse alveolar hemorrhage
• Can progress to ARDS
• CT: diffuse ground-glass opacities and airspace nodules

• Thrombocytopenia
• Coagulopathy (elevated INR, aPTT, D-dimer)
• Endothelial disruption

• Direct leptospiral invasion of myocytes
• Elevated CK and aldolase
• Rhabdomyolysis can contribute to AKI

Step 3 - Immune Phase (Days 7-14+)

• Clearance of bacteremia coincides with appearance of agglutinating IgM antibodies (typically week 1-2)
• However, the immune response itself drives disease - generalized immune activation may trigger a sepsis-like syndrome with multiorgan failure
• Organisms persist in the renal tubules despite bloodstream clearance (explaining urinary shedding into week 2-3)
• Aseptic meningitis during this phase is immune-mediated (organisms rarely recovered from CSF despite meningeal signs) - Sherris & Ryan's Medical Microbiology, 8th Ed., p. 1424

Clinical Features

Biphasic Course

Phase 1: Leptospiremic (days 1-7)    →    Phase 2: Immune (days 7-14+)
[Bacteremia, initial flu-like illness]      [Antibody appearance, organ complications]

Frequency of Presenting Signs and Symptoms

Phase 1 - Anicteric (Mild) Leptospirosis (~90% of cases)

Phase 2 - Weil Disease (Icteric/Severe Leptospirosis, ~10% of cases)

WHO Case Definitions (Table 298-1)

Key Laboratory Findings

Key Distinguishing Features from Differential Diagnoses

• Conjunctival suffusion (not conjunctivitis - no exudate)
• Calf/lower limb myalgia (often dominant complaint)
• Hypokalemia with renal insufficiency (paradoxical)
• Modest transaminase elevation with deep jaundice (unlike viral hepatitis)
• Elevated CRP/procalcitonin helps distinguish from dengue
• Epidemiologic exposure (flooding, rodent contact, water sports)

• Goldman-Cecil Medicine, International Edition, Chapter 298 (DAVID A. HAAKE), pp. 3242-3245
• Comprehensive Clinical Nephrology, 7th Edition, Chapter 71, pp. 976-977
• Sherris & Ryan's Medical Microbiology, 8th Edition, Chapter 5, pp. 1424-1425
• Andrews' Diseases of the Skin, Chapter on Leptospirosis, pp. 1399-1407
• Recent reviews: Nature Reviews Disease Primers 2025 (PMID 40316520); Clinical Medicine 2022 (PMID 35078790)

Clinical Features of Leptospirosis

Overview

Biphasic Course at a Glance

Phase 1: Leptospiremic Phase         Phase 2: Immune Phase
(Days 1-7)                           (Days 7-30)
─────────────────────────            ──────────────────────────────
Bacteremia present                   Antibody-mediated clearance
Flu-like illness                     Organ complications develop
Fever, myalgia, headache             Jaundice, AKI, hemorrhage
↓ (brief 1-3 day remission)          Aseptic meningitis, uveitis

Phase 1 - Leptospiremic Phase (Anicteric Leptospirosis)

Constitutional

• Abrupt-onset high fever with chills and rigors
• Prostration and anorexia

Headache

• Severe, bitemporal or frontal, throbbing in character
• Accompanied by retro-orbital pain and photophobia
• One of the most prominent complaints

Musculoskeletal - Hallmark Feature

• Myalgia - characteristically involves the calves and lower back
• Calf tenderness on palpation is a characteristic and diagnostically important sign
• Severity can mimic a surgical emergency or deep vein thrombosis

Ocular - Pathognomonic Sign

• Conjunctival suffusion - bilateral conjunctival hyperemia/injection without exudate (distinguishes it from conjunctivitis)
• Subconjunctival hemorrhage
• Retro-orbital pain and photophobia

Respiratory

• Nonproductive cough in up to ~50% of cases
• Chest pain may occur

Gastrointestinal

• Nausea, vomiting, diarrhea
• Abdominal pain (can suggest pancreatitis)
• Poor oral intake worsens dehydration, which can precipitate oliguric renal failure

Skin

• Maculopapular erythematous rash, most marked on the shins (pretibial)
• In anicteric disease associated with serovar autumnalis - called "Fort Bragg fever" or pretibial fever
• Rash consists of 1-5 cm erythematous patches/plaques; resolves in 4-7 days spontaneously
• Petechiae and purpura in severe cases

Early Renal Involvement

• Non-oliguric renal insufficiency (early, often subtle)
• Hypokalemia - paradoxical and characteristic, due to renal tubular K+ wasting (Na+/K+-ATPase inhibition by GLP)
• Proteinuria, hematuria, pyuria

CNS

• Aseptic meningitis - headache, neck stiffness
• CSF: lymphocytic pleocytosis, elevated protein, normal glucose
• Organisms are rarely recovered from CSF despite meningeal signs (immune-mediated)

Frequency of Symptoms (Across 9 Large Case Series)

Phase 2 - Immune Phase: Weil Disease (Icteric/Severe Leptospirosis, ~10%)

Jaundice + Acute Renal Failure + Bleeding

Hepatic Features

• Deep jaundice - predominantly conjugated (direct) hyperbilirubinemia
• Mild-to-moderate aminotransferase elevation only (unlike viral hepatitis where transaminases are very high)
• This dissociation - deep jaundice with modest transaminases - is a clue to leptospirosis
• Hepatomegaly and tenderness

Renal Features

• AKI develops in 10-80% depending on severity
• Initially non-oliguric with hypokalemia (tubular dysfunction)
• Can progress to oliguric AKI requiring dialysis
• Urinary findings: proteinuria, hematuria, pyuria, casts
• Long-term sequelae: CKD and persistent tubular dysfunction reported on follow-up
• Hypokalemia persists and may be profound

Pulmonary Features (Most Common Cause of Death)

• Pulmonary hemorrhage - hemoptysis, frank alveolar bleeding
• ARDS - diffuse alveolar hemorrhage
• Chest CT: bilateral ground-glass opacities and airspace nodules
• Rapidly fatal if not recognized; can occur without jaundice

Hemorrhagic Features

• Petechiae and ecchymoses on skin and mucous membranes
• Severe GI hemorrhage
• Pulmonary hemorrhage (hemoptysis)
• Driven by: thrombocytopenia + coagulopathy + endothelial injury

Cardiac Features

• Arrhythmias (bradycardia, heart block, atrial fibrillation)
• Myocarditis
• Cardiac arrhythmia is an independent predictor of severity

Hematologic Features

• Leukocytosis (WBC 15,000-30,000/mm³, >80% neutrophils)
• Thrombocytopenia (<100,000/µL)
• Coagulopathy: elevated INR, aPTT, D-dimer
• Hemolytic anemia - particularly in G6PD-deficient patients; contributes to jaundice

Ocular

• Uveitis (anterior or panuveitis) - may develop weeks to months after acute illness; important to recognize as a delayed manifestation

Neurological

• Aseptic meningitis (continued from phase 1)
• Encephalitis in severe cases
• Peripheral neuropathy (rare)

Pancreatic

• Elevated serum amylase and lipase
• Frank pancreatitis (elevated amylase may also reflect reduced renal clearance)

WHO Clinical Case Definitions

Key Distinguishing Clinical Clues

Complications Summary

• Goldman-Cecil Medicine, International Edition, Chapter 298, pp. 3242-3245
• Andrews' Diseases of the Skin, pp. 1399-1407
• Sherris & Ryan's Medical Microbiology, 8th Ed., pp. 1423-1425
• Comprehensive Clinical Nephrology, 7th Ed., pp. 976-977

Why Does Leptospirosis Produce Two Different Phases of Clinical Features?

The Core Concept

Phase 1 (Days 1-7)                    Phase 2 (Days 7-30)
─────────────────────────             ──────────────────────────────
CAUSE = Living bacteria               CAUSE = Immune system activation
in the bloodstream                    + bacteria hiding in organs

Bacteria → directly damage            Antibodies appear → clear blood
tissues and trigger                   But bacteria persist in kidneys,
non-specific inflammation             liver, lungs, brain →
                                      Immune response attacks tissues

Phase 1 in Detail: Direct Bacterial Damage (Leptospiremic Phase)

1. Direct tissue invasion

• The hooked ends and periplasmic flagella physically burrow through tissues
• Leptospires invade liver sinusoids, renal tubules, muscle, meninges, and lungs
• Disruption of endothelial cell junctions increases vascular permeability

2. Bacterial toxins directly injure cells

• LipL32 (major outer membrane lipoprotein) binds TLR-2 on cells → triggers NF-κB → release of inflammatory cytokines (TNF, iNOS, MCP-1)
• Cytotoxic glycolipoprotein (GLP) directly inhibits Na+/K+-ATPase on renal tubular cells → explains the hypokalemia seen even in early disease

3. Non-specific febrile response

• Bacteremia itself activates innate immunity - fever, rigors, myalgia, headache, prostration
• This is similar to the leptospiremic phase of any spirochetal infection

The Turning Point: Why Does the Immune System Then Cause More Damage?

Step 1 - Bacteria escape the immune system (Phase 1)

Step 2 - Antibodies appear (start of Phase 2)

• Clear leptospires from the bloodstream effectively
• But cannot reach bacteria hiding in kidney tubules, liver, lungs, brain, and aqueous humor of the eye
• This is why blood cultures and PCR on blood become negative in Phase 2

Step 3 - The immune response itself becomes the problem

A. Consumptive Coagulopathy / DIC

• Elevated thrombin-antithrombin complexes, prothrombin fragments 1+2, D-dimer
• Diminished anticoagulant markers (antithrombin, protein C)
• Deregulated fibrinolysis
• Thrombocytopenia (platelet consumption on activated endothelium)
• Result: bleeding - petechiae, pulmonary hemorrhage, GI hemorrhage

B. Endothelial Activation

• Elevated soluble E-selectin and von Willebrand factor in blood = markers of endothelial cell activation
• These correlate directly with disease severity
• Activated endothelium → platelets aggregate on lung vasculature → pulmonary hemorrhage
• Increased vascular permeability → edema, ARDS
• Harrison's: "Experimental models show that pathogenic leptospires or leptospiral proteins are able to activate endothelial cells in vitro and to disrupt endothelial-cell barrier function, thus increasing permeability."

C. Persistent Organ Damage from Bacteria in Tissues

• Kidney: Acute tubular damage → interstitial edema → interstitial nephritis → AKI
• Liver: Focal necrosis, hepatocyte apoptosis, bile canaliculi disruption → jaundice
• Lungs: Hemorrhage and alveolar destruction
• "Autopsy findings illustrate the involvement of multiple organ systems in severe disease." - Harrison's, p. 1485

D. Why Does Antibiotic Treatment Fail in Phase 2?

• Antibiotics given during Phase 2 do not stop the immune cascade
• The sepsis-like syndrome continues even after bacteria are killed
• This parallels other post-infectious immune syndromes (e.g., rheumatic fever, reactive arthritis)
• The meningitis of Phase 2 is also immune-mediated - organisms are rarely recovered from CSF despite signs of meningeal irritation

Why Does Only ~10% Progress to Weil Disease?

Summary: Why Two Phases in One Disease?

• Harrison's Principles of Internal Medicine, 22nd Ed., Chapter 189, pp. 1484-1485
• Goldman-Cecil Medicine, International Edition, Chapter 298, pp. 3242-3243
• Sherris & Ryan's Medical Microbiology, 8th Ed., pp. 1424-1425
• Comprehensive Clinical Nephrology, 7th Ed., pp. 976-977