06/01/2026
Why Should You Vaccinate Your Dog for Lepto?
What Is Leptospirosis?
Leptospirosis is a bacterial disease caused by Leptospira spp., a group of spiral-shaped bacteria capable of infecting a wide range of mammals — dogs, cats, rodents, livestock, and humans. It is considered a zoonotic disease, meaning it can pass between animals and people, making it a significant public health concern in addition to a veterinary one.
The global burden is substantial. An estimated 1 million human cases occur worldwide each year, with roughly 60,000 deaths. The lead author of the ACVIM statement, Dr. Jane Sykes of UC Davis, believes even those figures are an undercount, because leptospirosis symptoms resemble the flu — or even COVID-19 — making it easy to misdiagnose or overlook entirely.
Who Is at Risk — and Why Everyone Should Care
One of the most significant takeaways from the updated guidance is how broadly risk is now defined. The statement explicitly says:
"All dogs are at risk of leptospirosis, regardless of signalment, geographic location, lifestyle, and the time of year."
This is a meaningful shift. Previously, leptospirosis was often thought of as a disease primarily affecting rural, large-breed, outdoor dogs — hunting dogs, farm dogs, dogs with heavy water exposure. The new guidance dismantles that assumption entirely. The disease has been documented in:
Small breed dogs from urban environments
Puppies as young as 11 weeks old
Geriatric dogs
Indoor-leaning dogs with limited outdoor exposure
Inadequately or previously vaccinated dogs
The epidemiology has clearly shifted, and the updated recommendations reflect that reality.
The Reservoir: Rodents and Beyond
Rats are the single most important reservoir for Leptospira spp. globally. A worldwide prevalence of infection of 30% has been documented in rats, with some regions exceeding 80%. This is alarming given how ubiquitous rats are in both urban and rural environments.
However, rats aren't the only source. Many mammals can carry and shed the bacteria, and the organism itself is remarkably hardy in the environment:
It spreads through urine or body fluids of infected animals
It can survive in water or moist soil for weeks to months
It can survive freezing temperatures
It replicates in waterlogged soil, where it can form biofilms on particulate matter
Dr. Sykes noted that increased rainfall contributes meaningfully to environmental buildup of leptospires, particularly in muddy soil and standing water — which helps explain some of the seasonal and geographic patterns of outbreaks.
Transmission to dogs (and humans) occurs when the bacteria comes into contact with mucous membranes or skin abrasions — through drinking contaminated water, swimming, wading through puddles, or even predating infected rodents.
Geography and Seasonality
Leptospirosis is classically associated with subtropical and tropical climates. Sri Lanka, for example, has the highest number of documented human cases worldwide. But the updated guidance makes clear that the disease is not limited to warm, humid climates.
Outbreaks in dogs have been confirmed across a strikingly diverse set of U.S. locations, including:
Arizona (desert climate)
Los Angeles (Mediterranean climate)
Chicago (cold, continental climate)
Wyoming (semi-arid, alpine)
Seasonality varies by region:
Northeast: Most common in fall
California: Peak in winter or early spring
Southern U.S.: Year-round risk
A newer and concerning trend is the association of outbreaks with boarding facilities, raising the possibility of dog-to-dog transmission or rodent infestation in those settings — a development with real implications for how kennels and doggy daycares manage biosecurity.
Clinical Presentation: A Disease That Hides in Plain Sight
Leptospirosis is a multisystemic disease, which is part of what makes it so diagnostically challenging. While the most prominent manifestations are:
Acute tubulointerstitial nephritis (kidney inflammation)
Liver dysfunction..the disease can present in many other ways depending on which organs are affected, including:
Organ System Manifestation
Kidney Acute kidney injury (AKI)
Liver Elevated liver enzymes, jaundice
Pancreas Pancreatitis
Heart Cardiac arrhythmias
Eyes Uveitis, conjunctivitis, retinal hemorrhage
Lungs Pulmonary hemorrhage
Because the presentation varies so widely, leptospirosis can masquerade as many other conditions. The updated guidance stresses that it should be on the differential list for any dog with acute onset of febrile illness, especially if unvaccinated, and that clinical reassessment and bloodwork are warranted if a dog doesn't improve within 24 hours.
Diagnosis
Diagnosing leptospirosis is complicated by the fact that no single test is perfect. The consensus statement recommends basing diagnosis on a combination of:
Clinical suspicion based on history and presentation
Serologic assays (blood tests looking for antibody response)
Nucleic acid amplification tests (NAATs) — essentially PCR testing to directly detect bacterial DNA
Key practical points on diagnostics:
Both blood and urine should be submitted for NAATs to maximize sensitivity
Samples should ideally be collected before starting antibiotics, as treatment can reduce bacterial load and decrease test sensitivity
Artificial intelligence may soon play a role — a 2022 study in the Journal of Veterinary Diagnostic Investigation found that machine learning prediction models detected leptospirosis with 100% sensitivity, a remarkable result that could prove useful in clinical settings
Treatment
Doxycycline is the antimicrobial of choice. For acutely febrile dogs with a suggestive history (lack of vaccination plus other risk factors), the statement suggests that empiric doxycycline treatment could be considered even before confirmatory testing, given how rapidly leptospirosis can damage kidney function. However, the authors note that more evidence is needed before this becomes a firm recommendation, citing the importance of antimicrobial stewardship.
Beyond antibiotics, supportive care is critical and nuanced:
These dogs are prone to both dehydration and fluid overload simultaneously, requiring very close monitoring and individualized fluid management
Nutritional support is addressed in the statement
For severe cases, extracorporeal kidney support therapies (such as dialysis) may be necessary
Vaccination: The Clearest Recommendation
Given that all dogs are now considered at risk, the expert panel made a clear and sweeping recommendation: all dogs should be vaccinated against leptospirosis annually. This represents a shift from the prior guidance suggesting vaccination every three years and from the historical view that the vaccine was only necessary for high-risk dogs.
Cats, Other Animals, and One Health
Cats present an interesting paradox. There is widespread serologic evidence that cats are exposed to Leptospira spp., and pathogenic leptospires have been detected in the urine of up to 20% of apparently healthy cats via PCR and culture. Yet cats appear largely disease-resistant compared to dogs and other mammals.
The concern is that cats could act as silent reservoir hosts — carrying and shedding the bacteria without showing signs of illness. The statement suggests cats may be an under-recognized source of leptospires in some regions and should be included in One Health investigations.
The One Health framework — which recognizes the interconnection of human, animal, and environmental health — is central to how the ACVIM frames leptospirosis. The bacteria infects an enormous range of mammals, persists in the environment, and circulates between wildlife, domestic animals, and people. Addressing it effectively requires coordination across veterinary medicine, human medicine, and public health.
Human Risk from Infected Dogs
Despite all of this, the risk of a person contracting leptospirosis directly from an infected dog appears to be relatively low. Infected dogs don't shed large quantities of the organism. A CDC investigation following an outbreak in Arizona found no evidence of seroconversion (immune response indicating infection) in people who had been heavily exposed to leptospirosis-positive dogs.
That said, basic precautions are still warranted — particularly for veterinary staff handling infected animals — and the updated statement includes guidance on protecting clinic personnel from exposure.
The overarching message of the updated ACVIM consensus statement is one of broadened awareness: leptospirosis is more widespread, affects a wider range of dogs, and is more environmentally persistent than previously appreciated — and annual vaccination of all dogs is now the recommended standard of care.
References
https://www.avma.org/news/updated-guidance-canine-leptospirosis-reflects-better-understanding-disease
And a special thanks to Meg for putting this all together for us!
