Leptospirosis

01Pathophysiology

Leptospirosis is caused by pathogenic spirochetes of the genus Leptospira, most commonly Leptospira interrogans. The organism enters the body through breaks in the skin (cuts, abrasions) or through intact mucous membranes (conjunctivae, oropharynx) after contact with contaminated water, soil, or animal urine. Rats are the most important reservoir because they are chronic asymptomatic carriers that shed the organism in their urine for their entire lifespan.

Once the spirochete penetrates the epithelial barrier, it enters the bloodstream and disseminates hematogenously to virtually every organ. The hallmark pathological process is endothelial damage and vasculitis. Leptospires directly injure capillary endothelial cells, producing increased vascular permeability and capillary leak. This endothelial injury explains the wide range of organ involvement: hemorrhage in the lungs, petechiae in the skin, and vascular congestion in the liver and kidneys.

The clinical course is classically biphasic. The first phase (septicemic/leptospiremic phase) lasts approximately 3 to 7 days. During this phase, spirochetes circulate freely in the blood, and the patient develops abrupt onset of high fever, rigors, and severe myalgia. After a brief period of defervescence (1 to 3 days), the second phase (immune phase) begins. This phase corresponds to the appearance of IgM antibodies and immune complex deposition. During the immune phase, the organism clears from the blood but can be found in the urine. Meningeal symptoms, uveitis, and rash are features driven by immune-mediated mechanisms rather than direct bacterial invasion.

In the kidneys, leptospires cause acute interstitial nephritis and tubular dysfunction. A critical teaching point is that renal failure in leptospirosis is characteristically nonoliguric and hypokalemic, which distinguishes it from most other causes of acute kidney injury. The tubular damage impairs sodium and potassium reabsorption, leading to potassium wasting.

In the liver, the pattern of injury is centrilobular dysfunction without significant hepatocyte necrosis. This produces a disproportionately elevated conjugated (direct) bilirubin relative to the transaminase levels, which typically remain below 200 U/L. This is an important discriminator: in viral hepatitis, transaminase levels are usually in the thousands, whereas in leptospirosis, jaundice is prominent but transaminases are only mildly elevated.

In severe cases (Weil disease), pulmonary hemorrhage from diffuse alveolar damage is the leading cause of death. This occurs due to widespread endothelial injury in the pulmonary vasculature, resulting in hemoptysis and acute respiratory distress.

02Classification and Clinical Manifestation

Phase-Based Manifestation:

03Diagnostic Workup

When a patient presents with acute febrile illness, myalgia, conjunctival suffusion, and a history of freshwater or animal exposure in a tropical setting, leptospirosis should be high on the differential.

The best initial test is the IgM ELISA or a rapid immunochromatographic IgM test. These become positive around day 5 to 7 of illness and serve as the primary screening tool. They are widely available and offer results within hours. However, a negative IgM in the first few days does not rule out the diagnosis, as seroconversion may not have occurred yet.

The gold standard is the Microscopic Agglutination Test (MAT). This test requires paired sera: an acute sample and a convalescent sample drawn 2 to 4 weeks later. A diagnostic fourfold rise in titer between the two samples confirms the diagnosis. A single titer $\ge \backslash geq$ 1:800 in a patient with compatible clinical features in an endemic area is also considered diagnostic. The MAT is highly serovar-dependent and is only performed in reference laboratories, which limits its usefulness in acute clinical decision-making. For this reason, treatment should never be delayed while waiting for MAT results.

PCR on blood (early) or urine (later) is increasingly recognized as a reliable rapid confirmatory test. It has high sensitivity in the first 10 days of illness and provides results within hours. In exam settings, if the question stem emphasizes rapid confirmation in the acute phase, PCR is frequently the best answer.

Blood cultures on Ellinghausen-McCullough-Johnson-Harris (EMJH) medium can isolate the organism, but growth is extremely slow (up to 4 weeks) and sensitivity is low. This makes culture impractical for guiding acute treatment decisions.

Supporting laboratory findings include: thrombocytopenia, elevated conjugated bilirubin with only mildly elevated transaminases (AST/ALT usually < 200 U/L), elevated creatinine with hypokalemia (reflecting nonoliguric renal failure with potassium wasting), and an elevated ESR/CRP. The combination of jaundice + renal failure + thrombocytopenia in a patient with exposure history is a classic vignette pattern for Weil disease.

04Management and Treatment

Acute Stabilization is the first priority in any patient presenting with severe leptospirosis. This includes aggressive IV fluid resuscitation to correct hypovolemia from capillary leak, electrolyte replacement (particularly potassium, given the characteristic renal potassium wasting), and hemodynamic monitoring. Patients with pulmonary hemorrhage may require mechanical ventilation with lung-protective strategies. Renal replacement therapy (hemodialysis) should be initiated for patients with refractory hyperkalemia (rare in leptospirosis, but possible late), severe metabolic acidosis, volume overload, or uremic complications.

Antibiotic therapy should be started as early as possible. For mild disease, oral doxycycline 100 mg twice daily for 7 days is the standard of care. In patients who cannot tolerate doxycycline (e.g., pregnant women, children under 8 years), amoxicillin 500 mg three times daily for 7 days is the recommended alternative. Azithromycin is another option with favorable evidence.

For severe disease (Weil disease), IV antibiotics are required. Penicillin G 1.5 million units IV every 6 hours for 7 days is the classic first-line regimen. Ceftriaxone 1 g IV once daily for 7 days is an equally effective and more practical alternative, particularly in resource-limited settings due to its once-daily dosing. There is no significant clinical difference between penicillin and ceftriaxone in outcomes for severe leptospirosis.

A Jarisch-Herxheimer reaction can occur within the first few hours of initiating antibiotic therapy. This is a transient worsening of fever, rigors, hypotension, and tachycardia caused by the release of endotoxin-like substances from dying spirochetes. It is important to recognize this reaction and not mistake it for treatment failure or septic shock. Management is supportive (fluids, antipyretics). This reaction is a frequently tested concept because it also occurs in the treatment of syphilis and other spirochetal infections.

Chemoprophylaxis with doxycycline 200 mg orally once weekly is recommended for individuals with anticipated high-risk exposure (military personnel deployed to endemic flood zones, adventure travelers). Prophylaxis should begin before exposure and continue throughout the exposure period.

Contraindications and special populations:

• Pregnancy: Doxycycline is contraindicated. Use amoxicillin for mild cases; penicillin G or ceftriaxone for severe cases.

• Children < 8 years: Avoid doxycycline due to risk of dental staining. Amoxicillin or ampicillin are alternatives.

• Penicillin allergy: Use ceftriaxone (if the allergy is not anaphylactic) or doxycycline.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students get leptospirosis questions wrong is by confusing it with dengue fever or viral hepatitis. All three can present with fever, thrombocytopenia, and jaundice in a tropical setting. The discriminating clue in the vignette is the exposure history (flooding, freshwater contact, occupational animal exposure) combined with conjunctival suffusion, which is nearly pathognomonic for leptospirosis. If a question stem describes a patient with red, injected eyes but explicitly states "no purulent discharge," this is pointing directly at conjunctival suffusion and therefore at leptospirosis.

Another common trap involves the renal failure pattern. Students who see "acute kidney injury" may default to thinking of oliguric renal failure. In leptospirosis, renal failure is characteristically nonoliguric with hypokalemia. If the vignette shows a patient with elevated creatinine, normal or increased urine output, and low serum potassium, this pattern should immediately raise suspicion for leptospirosis.

The liver enzyme pattern is equally testable. Students may see jaundice and assume severe hepatocellular damage. However, in leptospirosis, the bilirubin is disproportionately elevated (often > 15 mg/dL) while transaminases remain below 200 U/L. If a question shows "total bilirubin 18 mg/dL, AST 120 U/L, ALT 95 U/L," this mismatch is a strong signal for leptospirosis rather than viral hepatitis, where transaminases would be expected to reach into the thousands.

The Jarisch-Herxheimer reaction is another frequent testing point. If a patient with diagnosed leptospirosis develops worsening fever and hypotension shortly after starting penicillin, the correct answer is the Jarisch-Herxheimer reaction, not drug allergy or treatment failure. The management is supportive care, not switching antibiotics.

Finally, remember the prophylaxis angle: doxycycline 200 mg once weekly is the standard chemoprophylactic regimen for anticipated exposure. This is a commonly tested "prevention" question for military or disaster-relief scenarios.

The core competency being tested with leptospirosis questions is the ability to integrate an exposure history with a characteristic constellation of findings (conjunctival suffusion, jaundice with mild transaminase elevation, nonoliguric hypokalemic renal failure) and to correctly sequence the diagnostic workup from screening IgM to confirmatory MAT, while initiating empiric antibiotics without delay.