The disease that swept 200 LA dogs, and the vaccine that would have stopped it
Doges Editorial · 2026-05-29 · 5 min read
A new UC Davis study reveals how a 2021 leptospirosis outbreak sickened over 200 dogs in Los Angeles, traced back to dog daycares and a quiet gap in local vaccination practice.
In the fall of 2021, something was making dogs in Los Angeles very sick. Veterinary clinics on the city's westside started comparing notes. Some of them were seeing more than one case a day — dogs coming in from daycare with fever, lethargy, kidney damage, the specific symptoms of leptospirosis, a bacterial infection that had not been expected to move through Los Angeles in significant numbers. LA is an arid city. The disease was associated with rain, with muddy trails, with rural conditions. It was not supposed to be a dog daycare problem.
By the time the outbreak subsided, more than 200 dogs had been sickened across the county. A new study published this week in the Journal of Clinical Microbiology — led by the University of California, Davis and released May 26, 2026 — documents what happened, and why.
The Outbreak Nobody Was Expecting
Leptospirosis is caused by Leptospira bacteria, carried primarily by rodents and shed through their urine into water, soil, and any surface a dog might investigate with its nose. The disease has traditionally been associated with specific environments: heavy rainfall, flooding, standing water, the kind of conditions that put dogs in contact with contaminated ground. Hiking trails. Muddy parks. Streams running through areas with high rodent populations. Urban dog daycares were not in the established risk profile.
The UC Davis study analyzed 59 confirmed cases from two specialty veterinary centers and compared them against more than 15,000 control patients. Daycare attendance was a significant risk factor. The mechanism may have involved rodents in the facilities themselves — leptospirosis moves readily from rat urine into any surface a dog contacts. Or it may have involved the kind of close-quarters dog-to-dog transmission that typically doesn't drive outbreaks, but did here, in the density of a busy urban daycare.
The outbreak was massive. It might have been the biggest outbreak of leptospirosis in dogs that's ever been recognized.
— Jane Sykes, professor of veterinary internal medicine, UC Davis
What Leptospirosis Does
The disease moves quickly in dogs. Leptospirosis causes acute kidney injury; in severe cases, it is fatal. Dogs that survive often require intensive hospitalization — IV fluids, antibiotics, sometimes dialysis-equivalent care. The disease is also zoonotic, meaning it can cross from animals to people. Humans contract it primarily through contact with contaminated water or animal urine. In people, it's often misdiagnosed as flu or dismissed as a minor viral illness.
Jane Sykes, the UC Davis professor who led the study, believes the 2021 LA outbreak produced human cases that went largely unrecognized. There was no surveillance system connecting what dogs at daycare were experiencing with what their owners or daycare workers might have been exposed to.
This disease — there's no boundaries for it.
— Jane Sykes, UC Davis
The Vaccine Gap
Here is the part of this story that is hardest to look at cleanly: the strain that drove the 2021 outbreak — Leptospira interrogans serovar Canicola — is covered by the standard four-way dog leptospirosis vaccine. The vaccine has existed for years. It works. The dogs who got sick in Los Angeles largely hadn't received it.
The reason had a certain logic to it, even if that logic turned out to be wrong. At the time, Los Angeles veterinarians rarely offered leptospirosis vaccinations because the bacteria had historically been associated with water-heavy environments, and LA is famously arid. The local risk calculus, based on everything that had come before, said: this is not a leptospirosis city. The bacteria found a way to make the category irrelevant.
As vaccination rates increased in the LA area following the outbreak, new cases dropped and the outbreak subsided. The counter to the disease had always been available. The conversation just hadn't happened.
Why Dog Daycares Changed the Equation
Leptospirosis transmission has traditionally been understood as a dog-environment problem — the dog contacts contaminated water or soil and becomes infected. Dog-to-dog transmission was considered unusual, not a meaningful driver of outbreaks. The 2021 LA case suggests the category may be more complex than that. In a daycare setting, where dozens of dogs are in close contact for hours at a stretch, the conditions for transmission expand.
"We know that the boarding itself was a risk factor," Sykes told UC Davis. Whether the primary mechanism was rodent contamination of shared facilities or something happening between dogs in close quarters, the daycare environment was where the outbreak ran. An urban setting, with urban density, produced conditions that rural-framed disease models didn't anticipate.
What the Study Means for the Drop-Off This Morning
Most dogs that attend daycare are fine. The overwhelming majority of drop-offs end with a tired, happy dog and an owner who got some uninterrupted hours. The 2021 Los Angeles outbreak was unusual — the convergence of an under-vaccinated population, an unexpected transmission setting, and a bacterial strain that LA veterinary practice hadn't been routinely protecting against.
But the UC Davis paper, published this week, arrives as a straightforward prompt: if your dog attends daycare or boarding, and if you're not sure whether its leptospirosis vaccination is current, this is worth asking your vet about at the next visit. Not as an alarm — as a question. The four-way vaccine is routine. The conversation takes about thirty seconds.
It's probably the tip of the iceberg.
— Jane Sykes, on unrecognized human leptospirosis cases connected to the outbreak
The Disease That Has No Borders
Major veterinary organizations now recommend annual leptospirosis vaccination for all dogs — not just dogs in high-rainfall areas, not just rural dogs, not just dogs who swim or hike. The 2021 Los Angeles outbreak is part of why that recommendation exists. A city can be arid and still have rodents. An urban daycare can be well-run and still become a vector. The traditional map of where the disease lives turned out to be a map of where we'd been looking.
Every morning, dogs are dropped at kennels and daycares by owners running a quick calculation about their dog's happiness and their own schedule. Those drop-offs are, overwhelmingly, fine. The 2021 outbreak is a reminder that one conversation with a vet — about a single vaccine, once a year — is one of the more practical ways to make sure it stays that way.