Which Leptospira serovar is considered most virulent to humans?

• A. L. interrogans, serovar icterohemorrhagica
• B. L. canicola
• C. L. pomona
• D. L. hardjo

Answer: (A)

Leptospira serovars differ in how severely they affect humans, and one serovar is classically linked with the most severe human disease. The icterohemorrhagiae serovar of Leptospira interrogans is associated with Weil’s disease, a severe form of leptospirosis in people that features jaundice, hepatic hemorrhage, and renal failure. Rats and other rodents are common reservoirs, and transmission occurs through contact with urine or water contaminated with infected urine. The other serovars listed are more commonly tied to specific animal hosts—canicola with dogs, pomona with cattle and swine, hardjo with cattle—and while they can cause human infections, they are not as strongly associated with severe disease in humans. Therefore, the serovar most virulent to humans is icterohemorrhagiae.