Leptospirosis is a bacterial infection that damages a dog’s kidneys and liver, sometimes fatally. It’s caused by corkscrew-shaped bacteria called Leptospira that spread through the urine of infected wildlife and can survive in soil and water for weeks to months. The disease is treatable when caught early, and roughly 87% of dogs who receive hospital care recover fully.
How Dogs Get Infected
The bacteria live in the kidneys of infected animals, primarily rodents, raccoons, skunks, deer, and feral pigs. These animals shed the bacteria in their urine, which then contaminates puddles, ponds, rivers, streams, and damp soil. Your dog doesn’t need to drink the water to become infected. The bacteria can enter through cuts, scrapes, or the mucous membranes of the eyes, nose, and mouth. Even splashing through a contaminated puddle on a walk can be enough.
The most common ways dogs pick up leptospirosis include drinking from or wading in slow-moving or stagnant water, direct contact with infected animal urine, bite wounds from wildlife or rodents, and eating infected animal tissue. Dogs that spend time outdoors in rural or suburban areas, those that encounter wildlife, and those that drink from natural water sources face the highest risk. But urban dogs aren’t immune. Rats are major carriers, and any city park or alley with a rat population is a potential exposure site.
What the Bacteria Do Inside the Body
Once Leptospira enters the bloodstream, it targets two organs in particular: the kidneys and the liver. In the kidneys, the bacteria trigger inflammation of the tissue surrounding the tiny filtering tubes, a condition called tubulointerstitial nephritis. This is typically the first organ damage to appear. The bacteria attach to host cells using a surface protein that helps them evade the immune system and establish infection deep in the tissue.
In the liver, the infection causes cell death and inflammation that can lead to bile backing up inside the organ. If the infection goes untreated or progresses, the initial kidney inflammation can advance to scarring (fibrosis), which permanently reduces kidney function. The bacteria also damage blood vessel walls throughout the body, which contributes to the muscle pain, organ swelling, and general sickness dogs experience.
Signs to Watch For
Leptospirosis symptoms can look like many other illnesses, which makes it tricky to recognize at home. The earliest signs are often vague: your dog may seem tired, eat less, or develop a fever. As the infection progresses and organ damage sets in, more distinctive symptoms emerge.
Muscle pain, stiffness, weakness, trembling, or reluctance to move are common. These can result from the blood vessel inflammation, muscle inflammation, or kidney damage the bacteria cause. Dogs with kidney involvement may start drinking and urinating far more than usual, even before blood tests show obvious kidney failure. This happens because the damaged kidney tubes lose their ability to concentrate urine properly.
Signs that the liver is affected include yellowing of the gums, eyes, or skin (jaundice), vomiting, and diarrhea. Some dogs develop bleeding tendencies, bruising easily or producing bloody urine. In severe cases, dogs can go into kidney or liver failure rapidly. Because these symptoms overlap with so many other conditions, any combination of fever, muscle pain, increased thirst, vomiting, and lethargy in a dog with potential exposure warrants prompt veterinary attention.
How Leptospirosis Is Diagnosed
Diagnosing leptospirosis is harder than you might expect. The gold-standard blood test, called the microscopic agglutination test (MAT), detects antibodies the immune system produces against the bacteria. The problem is that antibodies take time to develop. In the early stage of illness, when your dog is actively sick and needs treatment most, the MAT can come back negative. A second blood test taken two to four weeks later is often needed to confirm the diagnosis by showing a rise in antibody levels.
The MAT has another significant limitation: it can’t distinguish between antibodies from a natural infection and antibodies from vaccination. So if your dog has been vaccinated, interpreting the results gets more complicated. Growing the bacteria in a lab culture provides a definitive answer, but it takes weeks, far too slow to guide treatment decisions.
PCR testing, which detects the bacteria’s genetic material directly, offers a faster and more sensitive option during acute illness. It can identify the organism in blood or urine before antibodies appear. Some dogs become chronic carriers that shed bacteria in their urine without showing antibodies on blood tests at all, making PCR especially valuable in those cases. In practice, vets often start treatment based on symptoms and exposure history while waiting for test results to come back.
Treatment and Recovery
Antibiotics are the cornerstone of leptospirosis treatment. Treatment typically happens in two phases. The first phase uses antibiotics that rapidly clear the bacteria from the bloodstream and reduce active organ damage. The second phase targets bacteria hiding in the kidneys to prevent the dog from becoming a chronic carrier that sheds the organism in urine for months.
Beyond antibiotics, most dogs need supportive care. Dogs with kidney damage often require aggressive intravenous fluids to maintain hydration and support kidney function. Those with severe vomiting need anti-nausea medications. Dogs in full kidney failure may need dialysis to survive until the kidneys recover, though this is available only at specialty hospitals. Liver support and treatment for bleeding disorders may also be necessary depending on the severity.
The outlook for treated dogs is encouraging. In one study of hospitalized dogs, 87.5% made a full recovery. The remaining 12.5% did not survive, typically due to severe organ failure at the time of diagnosis. Early treatment makes a significant difference. Dogs whose kidneys sustain permanent scarring may have reduced kidney function long-term and need monitoring, but many dogs return to normal lives after treatment.
It Can Spread to People
Leptospirosis is zoonotic, meaning it can pass from animals to humans. The bacteria spread through contact with an infected dog’s urine. If your dog is diagnosed or suspected of having leptospirosis, you should take precautions: wear gloves when cleaning up urine, wash your hands thoroughly after handling your dog, and avoid letting urine contact your eyes or any open wounds. Clean any urine-stained surfaces or items with a 10% bleach solution (1 part household bleach to 9 parts warm water).
Person-to-person transmission is rare. The primary risk is during the period when your dog is actively shedding bacteria in urine, which can continue even after symptoms improve until the full course of antibiotics is completed. In humans, leptospirosis causes flu-like symptoms that can progress to liver and kidney problems similar to those seen in dogs.
Vaccination and Prevention
The American Animal Hospital Association now classifies the leptospirosis vaccine as a core vaccine, meaning it’s recommended for all dogs regardless of lifestyle. This is a change from earlier guidelines that considered it optional. The current vaccine is a killed, four-serovar product that protects against the most common disease-causing strains in North America: Pomona, Hardjo, Icterohaemorrhagiae, Grippotyphosa, and Canicola.
The vaccination schedule is straightforward. Puppies can start at 12 weeks of age and need two doses given 2 to 4 weeks apart. Adult dogs receiving the vaccine for the first time also need two initial doses spaced 2 to 4 weeks apart. After the initial series, a single booster is given annually. The vaccine doesn’t guarantee complete protection against every strain of Leptospira (there are over 250 serovars worldwide), but it covers the strains most commonly responsible for disease in dogs.
Beyond vaccination, reducing your dog’s exposure helps. Limit access to stagnant water sources, especially after heavy rains when bacteria wash into standing water. Control rodent populations around your property. If you live in an area with heavy wildlife traffic, keep your yard maintained and food sources secured. These steps won’t eliminate risk entirely, but they meaningfully reduce it alongside regular vaccination.