Leptospira
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Direct answer
After the monsoon recedes, fever with severe calf myalgia, conjunctival suffusion and a rising creatinine in a man who waded through floodwater is leptospirosis until excluded. Leptospira interrogans is a finely coiled spirochaete with hooked ends, carried in the renal tubules of rats, cattle, dogs and pigs, shed in urine, and acquired through abraded skin and mucosa in contaminated water — paddy farmers, sewer workers and urban flood-waders are the classic Indian risk groups. Illness is biphasic: a septicaemic week of fever, myalgia and conjunctival injection, then an immune phase ranging from self-limited aseptic meningitis to Weil disease — jaundice, renal failure and haemorrhage, with pulmonary haemorrhage the feared killer. Diagnosis rests on IgM ELISA or rapid tests early, with the microscopic agglutination test on paired sera as the reference standard; severe disease is treated with crystalline penicillin or ceftriaxone.
What you must remember
- Morphology and naming: fine, hook-ended spirochaetes too thin for routine light microscopy; darkfield (pitfall-prone) or silver staining; the hooked interrogans shape is the exam image.
- Serovar-reservoir pairs: icterohaemorrhagiae-rat (classic Weil disease), canicola-dog, pomona-pig, hardjo-cattle, and autumnalis plus australis among common Indian serovars; over 250 serovars complicate vaccine design.
- Route and season: animal urine contaminates water and wet soil; entry through cuts, macerated skin, conjunctiva; monsoon flooding in Mumbai, Chennai and Kerala concentrates outbreaks into post-monsoon weeks.
- Biphasic course: leptospiraemic phase (days 4-7: abrupt fever, severe myalgia especially calves, headache, conjunctival suffusion) then immune phase (aseptic meningitis, uveitis, or the Weil triad — jaundice, renal failure, haemorrhage).
- The jaundice clue: bilirubin rises disproportionately with only mild transaminase elevation — hepatocellular dysfunction without massive necrosis — separating Weil disease from acute viral hepatitis A and E co-circulating in the same season.
- Diagnosis ladder: IgM ELISA or rapid tests from about day five; the microscopic agglutination test on paired sera (four-fold rise) is the reference method; culture on EMJH or Fletcher media takes weeks at 28-30 degrees; PCR detects early bacteraemia.
- Treatment: mild — doxycycline, amoxicillin or azithromycin; severe — crystalline penicillin G or ceftriaxone; watch for Jarisch-Herxheimer; dialysis and ventilation drive survival in Weil disease.
- Prophylaxis: weekly doxycycline 200 milligrams during outbreak season for sewer workers, paddy farmers and rescue workers — used in Indian post-flood responses; rodent control and protective footwear complete prevention.
Two phases, two tests
Time the investigation to the biology. In the first week, the spirochaete is in the blood: PCR (if accessible) and, with less yield, culture capture it, while serology may still be negative — a negative IgM rapid test on day three excludes nothing. From the second week, antibody rises and urine shedding begins: IgM ELISA turns positive and persists for months, so a single titre proves exposure but not timing; the microscopic agglutination test, performed in a handful of Indian reference laboratories, assigns serogroup and, when a convalescent sample shows a four-fold rise, confirms current infection. Therapy follows the same clock: antibiotics started in the first week shorten illness; by Weil disease, penicillin or ceftriaxone still clears the spirochaete, but the renal failure, jaundice and pulmonary haemorrhage are immune-and-toxin-mediated damage needing intensive supportive care — the reason case fatality tracks ICU access more than drug choice.
The public-health layer: after floods, endemic districts issue advisories — avoid wading, protect cuts, doxycycline prophylaxis for high-risk groups — because the leptospirosis curve follows the flood curve by one to two weeks.
Where the exam sets traps
The conjunctival suffusion sign is the discriminator examiners reward: present from the early days in leptospirosis and absent in dengue, scrub typhus and enteric fever — a bedside sign worth more than an early negative serology. Second trap: the hepatitis mimic — monsoon season brings hepatitis A and E, and the jaundiced febrile patient is often written off; disproportionate bilirubin, mild transaminases, leucocytosis (viral hepatitis is leucopenic) and conjunctival injection reorient the diagnosis. Third, students order darkfield microscopy of blood as definitive — the technique is insensitive and artefact-ridden (fibrin strands masquerade as spirochaetes), so it belongs in textbooks, not in answers about modern practice. Finally, know the antibiotic logic in pregnancy and children: penicillin and ceftriaxone are safe at any age, where doxycycline is not the first choice — the prescribing detail that separates a complete answer from a memory answer.
Frequently asked questions
What is Weil disease?
The severe immune-phase syndrome of leptospirosis — jaundice, renal failure and haemorrhagic manifestations, often with pulmonary haemorrhage — classically associated with the icterohaemorrhagiae serovar and rat urine exposure.
Why is conjunctival suffusion important in diagnosis?
It appears early, is characteristic of leptospirosis, and is absent in the common monsoon mimics dengue, scrub typhus and enteric fever, making it a high-value bedside discriminator.
Which serological test is the reference standard for leptospirosis?
The microscopic agglutination test on paired sera (four-fold rise or single high titre); IgM ELISA and rapid tests serve as screening.
When is doxycycline prophylaxis used?
As weekly 200-milligram prophylaxis for defined high-risk groups — sewer workers, paddy farmers, flood rescue personnel — during outbreak seasons in endemic districts.
Why does leptospirotic jaundice differ from viral hepatitis?
Bilirubin rises disproportionately to mildly elevated transaminases because the injury is hepatocellular dysfunction and cholestasis rather than widespread hepatocyte necrosis, and leucocytosis accompanies it.