Leptospira and Leptospirosis: How It Causes Weil's Disease and How It Is Diagnosed

A rice-field worker develops a sudden high fever, severe muscle pain especially in the calves, headache, and marked conjunctival redness. At first it looks like dengue or influenza. Then, days later, jaundice sets in, urine output falls, and the patient develops pulmonary hemorrhage. This is Weil's disease, the severe form of leptospirosis, caused by a thin, coiled bacterium the person picked up from water contaminated with animal urine: Leptospira.

Leptospirosis is one of the most common infections passed from animals to humans worldwide, and it is easy to miss because its early stage mimics many other fevers. This page is about how Leptospira causes disease, why the illness comes in two distinct phases, and how the laboratory confirms it, a diagnosis that is surprisingly difficult and full of traps.

Introduction

Leptospira is a spirochete, a thin, tightly coiled, corkscrew-shaped bacterium. The pathogenic species (grouped under Leptospira interrogans) cause leptospirosis, whose severe form is called Weil's disease. A separate species, L. biflexa, is a harmless free-living organism. Leptospira is one of the most widespread zoonotic infections in the world, meaning it is passed from animals to humans.

What Leptospira looks like

Leptospira has features that both define it and make it hard to work with.

It is extremely thin and coiled. The cells are long (6 to 12 μm) but only about 0.1 μm wide, so thin they can pass through the filters used to sterilize culture media. They are tightly coiled with hooked ends, which is where the name interrogans comes from: the hooked shape resembles a question mark. They are highly motile, spinning and moving through fluid using internal flagella (endoflagella).

Figure: 1998 Rob Weyant. This scanning electron micrograph (SEM) depicts a number of Leptospira sp. bacteria atop a 0.1. µm polycarbonate filter.

They cannot be seen on an ordinary stain. The name Leptospira means "thin coil" (leptos, thin). Because the cells are so fine, they do not take up ordinary stains and cannot be seen on a routine light microscope. They must be viewed by dark-field or phase-contrast microscopy, or by special silver stains (Fontana) or immunofluorescence.

They are antigenically complex. Leptospira is divided into many serogroups and hundreds of serovars, based on their surface antigens. This diversity is why the main antibody test (the MAT, below) has to use many different strains.

How leptospirosis spreads

Leptospira lives in the kidneys of many animals, rats, dogs, cattle, and pigs are the important sources, and is shed in their urine, often for long periods and even by animals that appear healthy. Humans catch it by contact with water, soil, or food contaminated with that urine, usually through cuts or abrasions in the skin, or through the moist lining of the eyes, nose, or mouth. Swimming, rafting, and other freshwater activities in contaminated water are recognized sources. Person-to-person spread is rare.

It is an occupational and environmental disease: farmers, especially rice-field workers, sewer and slaughterhouse workers, and veterinarians are at higher risk, and outbreaks follow flooding. A useful way to remember the epidemiology is the three R's: Rats, Rainfall, and Rice fields. The heaviest burden is in tropical regions and urban slums.

How Leptospira causes disease: the two phases

Leptospirosis is classically biphasic, it comes in two stages, and understanding why is the key to both the clinical picture and the diagnosis.

First phase: the septicemic (acute) phase. Leptospira enters through skin or mucous membranes, then uses its active corkscrew motility and enzymes to penetrate tissue and reach the bloodstream. It spreads through the blood to many organs. During this phase, roughly the first week, the organism is in the blood and CSF, causing the abrupt fever, severe muscle pain (especially the calves), headache, and red eyes. Because the organism is in the blood now, this is when blood and CSF culture and PCR are most useful.

Second phase: the immune phase. After about a week, the body mounts an antibody response. The antibodies clear Leptospira from the blood (so blood culture becomes negative), but the organism moves to and persists in the kidneys, where it sticks to the tubules and is shed in the urine. The symptoms of this phase, including aseptic meningitis, are driven partly by the immune response itself. Because the organism is now in the urine and antibodies are now present, this is when urine culture and antibody tests (MAT) become useful.

The single most important consequence: which test works depends on which phase the patient is in. Blood early, urine and antibodies later. Testing the wrong specimen at the wrong time is a common reason leptospirosis is missed.

The spectrum of illness

• Mild (anicteric) leptospirosis occurs in about 90% of cases: a biphasic flu-like fever that usually resolves. It is easily mistaken for dengue, influenza, or other fevers.
• Weil's disease is the severe (icteric) form, about 10% of cases: jaundice and liver dysfunction, kidney failure, and bleeding, including dangerous pulmonary hemorrhage. It carries a significant risk of death. The classic triad is jaundice, kidney failure, and bleeding.

Because the mild form mimics so many other fevers (dengue especially), leptospirosis is frequently misdiagnosed, and diagnosis rests on laboratory testing rather than symptoms alone.

Diagnosing leptospirosis: match the test to the phase

No single test is reliable on its own, and the right test depends on the phase of illness. This is the part students most need to reason through.

Figure: (a) Growth from blood in EMJH semisolid media Leptospirosis forming subsurface colonies in the tube on the left and no growth in the tube on the right; (b) dark field microscopy image of the culture showing spirochetes with morphology compatible with Leptospira; (c) Conventional PCR targeting lipL32 gene  (Image source: Christopher Ryan Larson)

Dark-field microscopy (direct visualization). Because Leptospira is too thin for ordinary light microscopy, dark-field microscopy can show the thin, coiled, rapidly moving organisms directly in blood, CSF, or urine. In practice it is unreliable: serum proteins, fibrin strands, and cell debris look like leptospires and cause false positives, and the organism is often too sparse (especially in urine) to see. It needs real expertise and is easily misread. Treat a dark-field result with caution.

Culture (definitive but slow and insensitive). Leptospira can be grown in special serum-containing media (Fletcher's or EMJH), but growth is very slow (a doubling time of hours and incubation for weeks, up to three months), so culture rarely helps with an acute decision. Timing follows the phases: culture blood or CSF in the first week (septicemic phase), and urine later (immune phase), when the organism has moved to the kidney. Culture confirms but is too slow to guide early treatment.

Antibody detection, the microscopic agglutination test (MAT).

Figure: Leptospiral microscopic agglutination test with live antigen using dark field microscopy

The MAT is the reference-standard antibody test. It detects serovar-specific antibodies by mixing the patient's serum with live Leptospira strains and looking for agglutination under dark-field. Its strengths and limits both matter:

• Antibodies appear only after about 5 to 7 days, so the MAT is often negative early, in the phase when the patient first presents. A single result can miss early disease.
• Diagnosis usually needs paired sera: a fourfold rise in titer between an early (acute) and a later (convalescent) sample confirms it. This means confirmation is often retrospective.
• In endemic areas, many healthy people have background antibodies, so a single raised titer can mislead.
• It is technically demanding (it requires keeping live strains) and is available mainly in reference laboratories.

Faster antibody tests. IgM ELISA and IgM dipstick (dot-ELISA) tests are more widely available and useful earlier than the MAT, though IgM can be negative in the first days and can persist for a long time after infection.

Molecular testing (PCR). PCR detects Leptospira DNA in blood, CSF, or urine and is most useful early, in the first phase, before antibodies appear, which is exactly when serology fails. Its limits: it is expensive, and it identifies the organism but not the specific serovar.

The practical rule

Early (first week, septicemic phase): the organism is in the blood, so PCR and blood culture are the tests that can catch it; antibodies are usually still negative. Later (immune phase): antibodies have risen and the organism is in the urine, so MAT (paired sera) and urine culture become useful. Matching the test to the phase is the whole skill.

Treatment

Leptospirosis is treated with antibiotics, and, importantly, treatment works best when started early, before the results of slow confirmatory tests are back, so in a suspected case with a compatible exposure, treatment is often begun on clinical grounds. Mild disease is treated with an oral agent such as doxycycline; severe disease (Weil's) is treated with an intravenous agent such as penicillin or ceftriaxone, alongside supportive care for the kidney, liver, and lungs.

Two points worth knowing. Doxycycline is also used as prophylaxis for short high-risk exposures (for example, during outbreaks or for some travelers), a rare instance where antibiotic prophylaxis is genuinely used. And a Jarisch-Herxheimer reaction (a brief worsening of fever and symptoms as the spirochetes are killed) can occur when treatment starts, as with other spirochete infections. Specific doses and durations are clinical decisions and are not covered here.

How to remember

Question-mark shape, question-mark name. Leptospira interrogans has hooked ends that look like a question mark, and interrogans means "questioning." The shape is built into the name. And leptos means thin, so Leptospira = "thin coil," too thin for an ordinary stain, which is why you need dark-field microscopy.

The three R's: Rats, Rainfall, Rice fields. The epidemiology in three words. Animal urine (rats) contaminates water (rainfall, floods) where people are exposed (rice fields). If you remember the three R's, you remember who gets it and how.

Two phases, two specimens. Blood early (septicemic phase, organism in the blood), urine late (immune phase, organism moved to the kidney). Antibodies rise late too, so serology is for the second phase. Match the test to the phase.

Weil's triad: yellow, no urine, bleeding. Weil's disease is jaundice (liver), kidney failure (little urine), and bleeding (including the lungs). Three organs, one severe syndrome.

Weil's disease is NOT the Weil-Felix test. A crucial coincidence to untangle: Weil's disease is severe leptospirosis. The Weil-Felix test is a Proteus-based test for rickettsial infection (typhus). Same name "Weil," completely unrelated. Do not confuse them.

Key exam facts in one table

Where students get confused

Weil's disease vs the Weil-Felix test. The single most common confusion. Weil's disease is severe leptospirosis. The Weil-Felix test uses Proteus antigens to detect antibodies against Rickettsia (typhus). They share only the name "Weil" and are otherwise unrelated.

Why you can't just stain it. Leptospira is too thin to take up ordinary stains or be seen on a routine microscope. This is why diagnosis relies on dark-field microscopy, serology, or PCR, not a Gram stain. Students expecting a Gram result are stuck.

Which specimen, when. Blood and CSF in the first (septicemic) week; urine in the later (immune) phase, once the organism has moved to the kidney. Testing urine too early or blood too late misses it.

MAT is often negative early. Antibodies take 5 to 7 days to appear, so a single early MAT can be falsely negative in a genuinely infected patient. Paired sera showing a rising titer are usually needed, which makes confirmation retrospective. Do not rule out leptospirosis on one early negative.

It mimics dengue and flu. The common mild form looks like many other fevers, so it is under-diagnosed. In the right setting (exposure, calf pain, red eyes, tropical/flood context), think of it even when the picture is nonspecific.

Dark-field "positives" are often artifacts. Fibrin and cell debris resemble leptospires, so an inexperienced reader can call a false positive. Dark-field microscopy is much less reliable than it sounds.

References

1. Budihal, S. V., & Perwez, K. (2014). Leptospirosis diagnosis: competency of various laboratory tests. Journal of Clinical and Diagnostic Research, 8(1), 199–202. https://doi.org/10.7860/JCDR/2014/6593.3950
2. Ahmad, S. N., Shah, S., & Ahmad, F. M. (2005). Laboratory diagnosis of leptospirosis. Journal of Postgraduate Medicine, 51(3), 195–200.
3. World Health Organization. (2003). Human leptospirosis: guidance for diagnosis, surveillance and control. WHO.
4. Centers for Disease Control and Prevention. (2023). Leptospirosis. https://www.cdc.gov/leptospirosis/index.html
5. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.