Dogs get leptospirosis by coming into contact with the urine of infected animals or with water and soil contaminated by that urine. The bacteria enter through mucous membranes (eyes, nose, mouth), through broken skin, or through bite wounds. It’s one of the most common ways dogs pick up a serious bacterial infection from the environment, and the risk climbs after heavy rain or flooding.
How the Bacteria Get Into Your Dog’s Body
Leptospira bacteria are shed in the urine of infected animals. When that urine ends up in puddles, ponds, streams, or damp soil, it creates a contaminated zone your dog can walk through, drink from, or splash in. The bacteria can survive in wet environments for weeks to months, which means the infected animal doesn’t need to be anywhere nearby for your dog to be exposed.
Once your dog makes contact, the bacteria enter through soft tissue: the lining of the mouth, the eyes, the nose, or any cut or scrape on the skin. Dogs that swim in stagnant water, drink from natural water sources, or dig in moist soil are at higher risk. Bite wounds from wildlife or other animals are another direct entry point. Dogs can also become infected by eating tissue from an infected animal, which is relevant for dogs that hunt or scavenge.
Which Animals Spread It
Rodents are the most significant carriers. Rats and mice can shed leptospira in their urine for their entire lives without showing symptoms, steadily contaminating the areas where they live. But the list of potential sources is long: raccoons, skunks, opossums, deer, feral pigs, farm animals like cattle and pigs, and even other dogs. An infected dog that hasn’t been fully treated can continue shedding bacteria in its urine for up to three months.
This means your dog doesn’t need to encounter a wild animal face-to-face. A raccoon urinating near a backyard water bowl, a rat passing through a yard overnight, or livestock upstream from a creek your dog drinks from can all set up an exposure. Urban, suburban, and rural dogs are all at risk, though the specific wildlife sources vary by setting.
Environmental Conditions That Raise Risk
Leptospira thrive in warm, wet conditions. Risk spikes after hurricanes, heavy rain, or flooding because animal urine on the ground gets carried into floodwater, puddles, and streams. Stagnant water is a particularly good reservoir for the bacteria. If you live in a region with warm, humid weather or frequent flooding, your dog faces higher baseline exposure than dogs in dry, cold climates.
Seasonal patterns matter too. Late summer and fall tend to see more cases in temperate regions, when rainfall is high and wildlife activity overlaps with dogs spending more time outdoors.
What Happens After Exposure
The incubation period is 4 to 20 days. During this window, the bacteria enter the bloodstream and spread to multiple organs, particularly the liver and kidneys, where they replicate for about 7 to 10 days. This phase of the infection produces the first visible symptoms.
Dogs with acute leptospirosis typically develop fever, loss of appetite, vomiting, lethargy, and muscle pain. As the infection progresses, it can cause kidney damage, liver damage, or both. Some dogs develop jaundice (yellowing of the gums and eyes), increased thirst and urination, or bleeding problems. Respiratory disease and inflammation of the eyes are also possible. The severity ranges widely: some dogs have mild illness, while others deteriorate rapidly and can die without treatment.
How Veterinarians Confirm It
The primary diagnostic tool is a blood test called the microscopic agglutination test (MAT), which detects antibodies against the bacteria. A single test can be suggestive, but veterinarians get the most reliable results by testing two blood samples drawn 2 to 6 weeks apart. A fourfold rise in antibody levels between the two samples, or a high initial reading, confirms active infection. PCR testing on blood, urine, or eye fluid can also detect the bacteria’s genetic material directly, which is especially useful early in the illness or when the timeline of exposure isn’t clear.
Vaccination and Prevention
Vaccines are the most effective preventive measure. Modern canine leptospirosis vaccines cover four bacterial strains (called serovars) that cause most infections in dogs. A systematic review of commercially available vaccines found they provide roughly 84% protection against clinical disease and 88% protection against the dog becoming a long-term carrier that sheds bacteria in its urine. Vaccinated dogs that do get infected tend to have milder illness.
Protection isn’t absolute, and some bacterial strains are covered more reliably than others. The vaccine requires an initial two-dose series followed by annual boosters to maintain immunity. Dogs that swim in natural water, live in flood-prone areas, or have regular contact with wildlife or livestock benefit most from vaccination, but many veterinarians now recommend it broadly because urban wildlife like rats and raccoons have expanded the risk zone well beyond rural settings.
Beyond vaccination, practical steps include preventing your dog from drinking standing water outdoors, limiting contact with wildlife, and controlling rodent populations around your home. If your dog is diagnosed with leptospirosis, keep in mind that this is a zoonotic disease. The bacteria can pass from infected dogs to humans through contact with urine or body fluids. A 2023 CDC report documented a human case linked directly to occupational exposure to sick dogs during a canine leptospirosis outbreak in Wyoming. Basic hygiene measures like wearing gloves when cleaning up after a sick dog and washing your hands thoroughly reduce the risk of transmission.