## Epidemiology of Leptospira Serovars in India **Key Point:** Serovar Icterohaemorrhagiae is the most common cause of severe leptospirosis (Weil's disease) globally and in India, typically transmitted via infected rat urine. ### Serovar Distribution and Clinical Severity | Serovar | Primary Animal Host | Geographic Distribution | Clinical Severity | Frequency in India | |---------|-------------------|------------------------|------------------|-------------------| | Icterohaemorrhagiae | Rats (Norway rat) | Worldwide, especially urban | Severe (Weil's disease) | Most common severe form | | Hardjo | Cattle, pigs | Worldwide, rural areas | Mild to moderate | Common in agricultural workers | | Canicola | Dogs | Worldwide | Mild to moderate | Less common | | Pomona | Pigs, cattle | Worldwide | Mild to moderate | Uncommon in India | **High-Yield:** Icterohaemorrhagiae is associated with the biphasic illness pattern: leptospiremic phase (fever, myalgia, headache) followed by immune phase (jaundice, renal failure, hemorrhage). **Clinical Pearl:** In India, occupational exposure (farmers, sewage workers, slaughterhouse workers) to rat-contaminated water during monsoon is the classic epidemiological setting for Icterohaemorrhagiae transmission. ### Why Icterohaemorrhagiae Causes Weil's Disease 1. **Virulence factors:** Lipopolysaccharide (LPS) endotoxin triggers severe inflammatory response 2. **Rat reservoir:** Chronically infected rats shed organism in urine for life 3. **Urban/periurban transmission:** Flooded areas, sewage, contaminated water bodies 4. **Immune-mediated phase:** Antibody formation leads to immune complex deposition in kidneys and liver **Mnemonic:** **RAT = Icterohaemorrhagiae** — Rats are the primary reservoir, causing the most severe form (Weil's disease). [cite:Park 26e Ch 18]
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