CONDITION
Leptospirosis
Leptospirosis is an infection caused by bacteria of the Leptospira group, which can affect multiple organs in dogs and, more rarely, cats. The bacteria are typically spread through contact with contaminated water or soil, often where infected wildlife has urinated. In many cases the infection produces mild signs or none at all, but in others it can lead to serious illness affecting the kidneys, liver, lungs, or other systems. Owners may notice a range of signs, from lethargy, reduced appetite, and fever in milder cases, to vomiting, increased thirst, jaundice, or difficulty breathing in more severe presentations. The picture varies considerably depending on which organs are involved and how the individual animal responds. Some dogs are diagnosed after routine blood tests show changes in kidney or liver function, even before obvious signs appear. This page explores the signs that may prompt concern, what is happening inside the body during infection, how leptospirosis is investigated through blood tests and other tools, and the approaches used to support affected animals. It also considers prevention, including vaccination, and the broader context of risk in different environments.
Why this matters now
Leptospirosis is a bacterial infection that affects dogs and can spread to humans, making it a zoonotic disease. The bacteria are carried by wildlife, particularly rats, and persist in water and moist environments. Dogs contract infection through contaminated water, soil, or contact with infected animals. Urban and rural dogs alike can be exposed, though risk varies with lifestyle and geography. People can be infected when urine from an infected dog, or water contaminated by rats or other wildlife, reaches the eyes, nose, mouth or broken skin. Human illness ranges from a flu-like fever to severe disease affecting the liver and kidneys, sometimes called Weil's disease. Leptospirosis in dogs is not a notifiable disease in the UK, though human infections are monitored by UK public health agencies.
Following exposure, the bacteria spread through the bloodstream to various organs, particularly the kidneys and liver. Signs usually appear about four to twelve days after exposure, though the interval varies with the strain and with the individual dog. Signs range from subclinical infection to severe, life-threatening illness, and the early picture of lethargy, vomiting and reduced appetite looks the same as far more common stomach upsets. In acute cases the kidneys can fail over a few days, and a small number of dogs become severely ill very quickly. Passing little or no urine, yellowing of the gums or eyes, bleeding or bruising, and fast or laboured breathing mark the more dangerous forms. Bleeding into the lungs carries a high death rate even with intensive care, and dogs whose kidneys stop producing urine have a more guarded outlook. Many dogs that start antibiotics and fluid support early recover, although some are left with lasting kidney damage. Whether the kidneys or liver are already affected cannot be judged from outward signs and is found through blood and urine tests. Less severe infections may cause vague illness that resolves but can lead to chronic kidney changes.
Signals & patterns
Early signals
Sudden fever and lethargy
Lethargy is often marked. Fever may be present early, though many dogs have a normal temperature by the time they are examined, and severely ill dogs can have a low temperature, so a normal temperature does not rule the infection out.
Loss of appetite
Affected dogs typically refuse food.
Muscle pain and stiffness
Dogs may be reluctant to move and appear painful, particularly in the back and legs.
Vomiting and diarrhoea
Gastrointestinal signs are common early in the disease course. They look the same as those of an ordinary stomach upset, so on their own they do not separate leptospirosis from milder causes.
Later signals
Increased thirst and urination, or reduced urination
Kidney involvement affects urine production, either increased or dangerously decreased. Passing little or no urine reflects severe kidney injury and a more guarded outlook.
Jaundice
Yellowing of the skin, gums, and whites of the eyes indicates liver involvement.
Bleeding
Some dogs develop bruising, bloody vomit or stool, or nosebleeds due to clotting abnormalities.
Difficulty breathing
Bleeding into the lungs can cause fast or laboured breathing, coughing, and sometimes blood-tinged froth from the nose or mouth. This form can worsen over hours and carries a high death rate even with intensive care.
Click to read about the biological mechanisms
How this is usually investigated
Diagnosis combines clinical findings with laboratory testing. Because leptospirosis can mimic other conditions and poses zoonotic risk, thorough investigation is valuable.
Blood tests (biochemistry)
Complete blood count
Urinalysis
Leptospira serology (MAT)
PCR testing
Chest radiographs
Options & trade-offs
Treatment involves antibiotic therapy to eliminate the bacteria and supportive care for organ damage. Early treatment improves outcomes.
Antibiotic therapy
Antibiotics active against leptospira, usually started when the disease is suspected rather than after test results return, since confirmation can take one to two weeks.
Trade-offs: A penicillin-type antibiotic by injection is often used while vomiting prevents tablets, followed by doxycycline, typically for two weeks, which also clears the bacteria from the kidneys and ends shedding in urine. Stopping the course early risks continued shedding.
Intravenous fluid therapy
Fluid support, given with care, to correct dehydration and maintain blood flow to the kidneys.
Trade-offs: Supports kidney function. Requires hospitalisation and careful monitoring of fluid balance, as dogs producing little urine can become overloaded with fluid.
Management of kidney failure
Supportive care for acute kidney injury, potentially including dialysis.
Trade-offs: Severe cases may require intensive intervention. Some dogs recover kidney function; others may have permanent damage.
Treatment of bleeding disorders
Plasma transfusions or other interventions for clotting abnormalities.
Trade-offs: May be lifesaving in severely affected dogs. Addresses a serious complication of infection.
Zoonotic precautions
Handling precautions to protect humans caring for infected dogs.
Trade-offs: Gloves, hand hygiene, and care with urine reduce human exposure risk. Particularly important for immunocompromised individuals.
Common misconceptions
"Leptospirosis only affects dogs that swim in ponds"
Dogs can be exposed in urban environments through rat urine contaminating gardens, puddles, or parks. Risk is not limited to rural or water-associated activities.
"Vaccination prevents all leptospirosis infection"
Vaccines protect against specific serovars included in the product. Other serovars may still cause disease. Vaccination significantly reduces risk but is not absolute.
"Leptospirosis is not found in the UK"
Leptospirosis occurs throughout the UK. Certain regions and seasons carry higher risk, but exposure can occur anywhere wildlife is present.
Related conditions
Pyelonephritis
Pyelonephritis, a bacterial infection of the kidneys, can sometimes be difficult to distinguish from leptospirosis in dogs presenting with fever, kidney dysfunction, and signs of systemic illness. Urine culture alongside leptospira testing helps separate them, as the two conditions have different underlying causes and approaches to treatment.
Glomerulonephritis
Leptospirosis mainly damages the kidney tubules and the tissue around them rather than the glomeruli, the kidney's filtering units, but protein in the urine is common during infection and can raise the question of glomerular disease. Urine and blood tests help separate the two, as glomerulonephritis has many other causes and a different course.
Acute Kidney Injury
Acute kidney injury is one of the more serious consequences that can follow leptospirosis infection in dogs, occurring when the bacteria damage kidney tissue directly or through the body's inflammatory response. Other causes of acute kidney injury in UK dogs, including antifreeze and grape or raisin ingestion, can look very similar, and blood and urine tests rather than outward signs are needed to separate them.
Pneumonia
Leptospirosis can cause bleeding into the lungs, which produces coughing and laboured breathing and can look like pneumonia on radiographs. Dogs that are vomiting can also develop aspiration pneumonia. The distinction matters because bleeding into the lungs carries a much higher risk and is managed differently from bacterial pneumonia.
Acute Hepatic Failure
Leptospirosis is one of the infectious causes of sudden liver injury in dogs, producing jaundice, vomiting and bleeding tendencies that overlap with other causes of acute liver failure such as toxins. Blood tests, urine tests and leptospira testing help separate them.
Gastroenteritis in Dogs
The early signs of leptospirosis, including vomiting, diarrhoea, lethargy and reduced appetite, cannot be told apart at home from ordinary gastroenteritis. Changes in thirst or urine output, yellowing, bleeding, breathing difficulty, or blood tests showing kidney or liver changes point away from a simple stomach upset.
Chronic Kidney Disease in Dogs
Some dogs that recover from leptospirosis are left with lasting kidney damage, which can progress to chronic kidney disease. Kidney values and urine tests in the months after recovery show whether function has returned to normal.
Ethylene Glycol Toxicity
Antifreeze poisoning causes acute kidney injury that can resemble the kidney form of leptospirosis. Possible access to antifreeze and early wobbliness resembling drunkenness help separate them, though blood and urine tests are needed.
The early signs of leptospirosis, such as lethargy, vomiting, reduced appetite, reluctance to move or a tender back, are shared with many common and minor illnesses and cannot be told apart at home. Exposure changes how they are read: swimming in or drinking from ponds, rivers, canals or puddles, or contact with rats, in the previous few weeks, particularly in a dog whose vaccination is not current. Kidney and liver damage can be under way before it shows outwardly, and blood and urine tests are how it is found. A dog passing little or no urine, breathing faster or with effort, bleeding or bruising, or developing yellow gums or eyes is showing the forms that change fastest and carry the most risk. Vaccination status, including whether the annual booster is current and which serovars the vaccine covers, is useful context in any conversation about exposure. In a household caring for a dog under investigation or treatment, urine is the main route of spread to people. Gloves, handwashing, and keeping urine away from the eyes, mouth and cuts reduce that risk, and contact with an affected dog is relevant to a person's own doctor if a fever follows.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS