Every summer, hundreds of thousands of Americans wade through floodwaters after heavy rains, swim in rivers and creeks, compete in adventure races through mud and streams, kayak through wilderness waterways, and allow their dogs to drink from puddles along hiking trails. Most do not know that any of these activities carry a risk for leptospirosis — a bacterial disease spread through the urine of infected animals that can cause kidney failure, liver failure, meningitis, and death.
Leptospirosis is caused by bacteria in the genus Leptospira, shed in the urine of infected animals — rats, mice, cows, pigs, dogs, horses, and many wildlife species. The bacteria can survive in contaminated soil and water for weeks to months, particularly in warm, humid conditions that peak in summer. Humans are infected when Leptospira enters the body through cuts, abrasions, or mucous membranes after contact with contaminated water or soil. Swimming in or wading through potentially contaminated water — particularly floodwater, which can concentrate animal waste from flooded fields and urban drainage systems — is one of the highest-risk activities.
Globally, leptospirosis causes an estimated 1 million cases and approximately 60,000 deaths annually. In the United States, it is most commonly reported in Hawaii, where warm, wet conditions favor year-round transmission and where multiple historic adventure race outbreaks have occurred. But outbreaks can occur after heavy rainfall or flooding in any area of the country where livestock, rodents, or wildlife are present — and summer 2026's combination of increased rainfall events, record flood activity in multiple regions, and peak outdoor recreation creates conditions for elevated domestic exposure.
What Leptospirosis Does to the Human Body
Most leptospirosis infections cause a relatively mild illness: fever, headache, muscle aches, vomiting, or diarrhea that begins 2 to 30 days after exposure and resembles influenza. Most patients with mild disease recover fully within a week to two weeks with or without antibiotic treatment, though antibiotics are recommended to shorten illness and reduce risk of serious outcomes.
In approximately 10 to 15 percent of cases, the illness progresses to Weil's disease — the severe form of leptospirosis characterized by jaundice (yellowing of skin and eyes from liver involvement), acute kidney injury, and in some cases, hemorrhagic pneumonia and cardiovascular collapse. Weil's disease can be fatal even with intensive care, and survivors may require dialysis during the acute phase. The progression from mild to severe disease can occur suddenly around day 5 to 7 of illness — making it critical that patients with confirmed or suspected leptospirosis who are treated as outpatients be specifically instructed to return immediately if jaundice, decreased urination, severe shortness of breath, or worsening symptoms develop.
The link between leptospirosis and adventure sports has been well documented in landmark outbreak investigations. A famous example is the 2000 Eco-Challenge Sabah race in Malaysian Borneo, in which 42 percent of participants developed leptospirosis after exposure to contaminated river water during the race. Similar outbreaks have been documented in triathlon events in the United States, including a 2005 outbreak among adventure race participants in Florida.
What Outdoor Enthusiasts, Flood Responders, and Pet Owners Must Know
The CDC's updated prevention guidance provides clear direction for summer risk reduction. Avoid wading in or swimming in floodwaters, streams, rivers, or other bodies of water that may be contaminated with animal waste. If water exposure is unavoidable — particularly for those performing flood cleanup or disaster response — wear waterproof protective clothing and boots; cover all cuts and abrasions with waterproof bandages or waterproof tape; do not touch your face while working in contaminated environments; and wash thoroughly with clean water and soap immediately after potential exposure.
For pet owners, dogs are a significant domestic source of leptospirosis and can shed the bacteria in their urine, potentially exposing humans. Veterinarian-recommended vaccination against leptospirosis is available for dogs and is recommended for dogs that spend time outdoors in areas with wildlife or livestock. Cats are not commonly infected but dogs face significant exposure in the same environments that put humans at risk.
Frequently Asked Questions
Q: What is leptospirosis, and how do people get infected?
A: Leptospirosis is a bacterial disease spread through animal urine that contaminates water and soil. Humans are infected when Leptospira enters through cuts, abrasions, or mucous membranes after contact with contaminated floodwater, rivers, streams, or muddy soil.
Q: What are the symptoms of leptospirosis?
A: Fever, headache, severe muscle aches (particularly in the calves), vomiting, and diarrhea beginning 2–30 days after exposure. Serious disease (Weil's disease) includes jaundice, kidney failure, and hemorrhagic pneumonia.
Q: Who is most at risk for leptospirosis in summer 2026?
A: Flood responders and disaster workers, adventure race and mud run participants, swimmers and kayakers in rivers or streams, farmers and livestock workers, and pet owners whose dogs have outdoor exposure to wildlife or water.
Q: Is there a treatment for leptospirosis?
A: Yes. Doxycycline or penicillin-based antibiotics are effective, especially when started early. Doxycycline can also be used as prophylaxis (prevention) for high-risk short-term exposures such as adventure races in endemic areas.
Q: Should dogs be vaccinated against leptospirosis?
A: Yes, for dogs with outdoor access in areas with wildlife or livestock. The leptospirosis vaccine is not part of the core puppy vaccination in some practices, but is recommended for at-risk dogs by the American Veterinary Medical Association.
