Leptospirosis
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Direct answer
Leptospirosis is a zoonosis caused by Leptospira species, thin hook-ended spirochaetes that enter through broken skin or mucous membranes exposed to rodent urine. The illness ranges from mild febrile disease to Weil disease, with jaundice, renal failure and haemorrhage. Diagnosis uses early PCR or culture and paired-serum microscopic agglutination testing, and penicillin or doxycycline is the main treatment.
What you must remember
- Organism: Leptospira interrogans is the commonest pathogenic species, and it is best seen by dark-field microscopy or silver staining rather than Gram stain.
- Physiology: an obligate aerobe that grows slowly in specialised liquid or semi-solid media.
- Reservoir: rodents, especially rats, excrete the organism in urine, and domestic animals can also carry it.
- Transmission: through contaminated water, mud and soil, with occupational risk for farmers, sewage workers and people exposed to floodwater.
- Clinical picture: a biphasic illness with fever, myalgia, calf tenderness and conjunctival suffusion, followed in some cases by an immune phase with meningitis.
- Weil disease: the severe icterohaemorrhagic form with jaundice, acute kidney injury, thrombocytopenia and haemorrhage, and pulmonary haemorrhage carries a high mortality.
- Diagnosis: PCR or culture of blood early in illness, urine culture later, and the microscopic agglutination test on paired sera as the reference serology.
- Treatment: intravenous penicillin G for severe disease, doxycycline for mild disease, and ceftriaxone as an alternative in severe cases.
From flood exposure to diagnosis
Leptospirosis often follows monsoon flooding in India, so a febrile patient with a flood history should be considered for it. The steps below follow the course of infection from exposure to a confirmed diagnosis.
- Exposure: contaminated floodwater, paddy fields or rat-infested drains enter the body through a cut, abrasion or the conjunctiva.
- Septicaemic phase: the organism reaches the bloodstream within days, producing fever, headache, myalgia and red eyes without discharge.
- Immune phase: antibodies appear after about a week, and the fever may settle before a second phase brings meningitis, uveitis or renal involvement.
- Test early: blood PCR or culture is most useful in the first week, and urine culture is used later in the illness.
- Confirm serologically: the microscopic agglutination test on paired acute and convalescent sera is the reference test, and a fourfold rise in titre confirms infection.
- Treat and support: penicillin for severe disease or doxycycline for mild disease, with fluid balance, dialysis or ventilation as the organ failure requires.
Where the exam tries to catch you
Leptospirosis questions test the clinical mimics, the staining method and the serological test of choice.
- Trap: dismissing fever with jaundice after floods as ordinary viral hepatitis Correct: leptospirosis causes jaundice with renal failure and is a classic mimic, so flood exposure should prompt testing.
- Trap: using Gram stain to identify Leptospira Correct: the organism is too thin to see well on Gram stain, so dark-field microscopy or silver staining is used.
- Trap: treating one negative MAT as excluding the disease Correct: paired samples are needed, because a rising titre is what confirms infection.
Frequently asked questions
What is Weil disease in leptospirosis?
Weil disease is the severe icterohaemorrhagic form of leptospirosis, with jaundice, acute kidney injury, thrombocytopenia and bleeding. Pulmonary haemorrhage in Weil disease carries a high mortality and needs intensive support.
Why is the microscopic agglutination test used for leptospirosis?
The microscopic agglutination test is the reference serological test for leptospirosis, detecting antibodies against a panel of live Leptospira serovars. It is run on paired acute and convalescent sera, because a fourfold rise in titre confirms recent infection.
Which antibiotics treat leptospirosis?
Penicillin G is used intravenously for severe leptospirosis, and doxycycline is used for mild disease. Ceftriaxone is an alternative in severe cases, and treatment should start early because antibiotics work best in the first week.
Why is leptospirosis mistaken for viral hepatitis?
Leptospirosis can cause fever, jaundice and raised transaminases that closely resemble viral hepatitis, so the two are often confused in early assessment. Leptospirosis is suggested by renal involvement, calf tenderness, conjunctival suffusion and a history of water exposure.
Who is at highest risk of leptospirosis?
People with occupational or recreational contact with contaminated water, such as farmers, sewage workers and people wading through floodwater, are at highest risk of leptospirosis. Doxycycline prophylaxis may be considered for high-risk exposures, per current guidance.