Dogs were showing up at West Los Angeles veterinary hospitals with the same alarming pattern: fever, vomiting, lethargy, damaged kidneys, and, in some cases, a fast slide toward death. Many had recently spent time at dog daycare.
A new investigation led by the University of California, Davis, now suggests that a 2021 leptospirosis outbreak that sickened at least 201 dogs across Los Angeles County was fueled by conditions that many owners would have considered routine, crowded boarding and daycare settings where vaccination against the disease was uncommon.
The study, published in the Journal of Clinical Microbiology, traced 59 confirmed cases seen at two specialty hospitals and compared them with more than 15,000 other canine patients. The analysis points to a major weakness in local prevention practices at the time: most infected dogs with known vaccine histories had never received a leptospirosis vaccine.
“The outbreak was massive,” said lead author Jane Sykes, professor of small animal internal medicine at the UC Davis Weill School of Veterinary Medicine. “It might have been the biggest outbreak of leptospirosis in dogs that’s ever been recognized.”
Leptospirosis is a bacterial disease caused by Leptospira spirochetes, which are shed in urine and often spread through contaminated water, soil, or surfaces. Rodents are especially important hosts. In dogs, severe illness can look a lot like Weil’s disease in humans, with acute kidney injury, liver problems, vomiting, diarrhea, fever, and extreme fatigue.
What made the Los Angeles outbreak unusual was not just its size, but its setting. The researchers found that 31 of the 59 dogs, about 53%, had exposure to indoor congregate facilities such as boarding or daycare, and those exposures were especially concentrated during the outbreak’s peak. Cases seen during that peak had significantly higher odds of recent boarding or daycare exposure than cases seen outside it.
“We know that the boarding itself was a risk factor,” Sykes said. “It might have been rodent problems in those facilities, or it might have just been really overcrowded facilities with lots of dogs in close contact with one another.”
That last possibility mattered because the culprit was Leptospira interrogans serovar Canicola, one of the four strains covered by standard canine leptospirosis vaccines. Canicola is known to infect dogs, but transmission usually happens through contaminated environments rather than obvious dog-to-dog spread. In the crowded conditions of daycare facilities, that boundary may have blurred.
Of the 47 dogs with known vaccination histories, 41, or 87.2%, had never received a leptospirosis vaccine. The six dogs that had been vaccinated were not fully protected at the time illness began. Four had received only the first dose of an initial two-dose series, one got the second dose five days before symptoms started, and another received a second dose after illness had already begun.
![Maps of the Los Angeles area used for spatial analysis (zip code regions within a 15 km radius of two veterinary specialty clinics [purple circles]).](https://www.thebrighterside.news/uploads/2026/05/dogs-5-e1779819967673.jpg)
“At the time, Los Angeles area veterinarians rarely offered leptospirosis vaccinations because the bacteria thrive in water from heavy rainfall and L.A. is an arid climate,” Sykes said. “It was considered a low risk.”
That assumption now looks shaky. The outbreak peaked in July and August 2021, even though rainfall in the county from January through July was not above average. The study raises several possible explanations, including post-pandemic crowding in dog daycare facilities, increased rodent exposure in large cities during the COVID period, and, for some later cases, environmental exposure near Santa Monica. But the researchers could not pin the outbreak to a single source.
Genetic evidence suggests they should not have expected to. Whole-genome sequencing of samples from five dogs showed the infections shared a common ancestry but were not identical, a sign that multiple infection sources were likely involved rather than one simple chain of transmission.
The most common signs were lethargy, reduced appetite, vomiting, increased thirst or urination, anorexia, and diarrhea. Most dogs had evidence of kidney injury. Some also showed signs of liver involvement, and one dog developed findings consistent with leptospiral pulmonary hemorrhage syndrome, a severe complication.
The diagnostic results carried a practical lesson for veterinarians. PCR testing on urine was positive in 49 of 54 dogs, or 91%, while blood PCR was positive in only 15 of 56, or 27%. Serologic tests were useful too, but early in illness they often missed cases. All seven dogs with negative microscopic agglutination test results had been tested within the first week of illness.
That helps explain why the researchers argue for combining molecular and serologic testing instead of relying on one test alone. The bacteria appear in blood early, then in urine later, and antibody responses may lag behind symptoms.
Treatment outcomes were better than the disease’s reputation might suggest, at least in this specialty-care group. Seventy-three percent of dogs were hospitalized, some receiving intravenous antibiotics, fluids, and, in two severe cases, hemodialysis. Fifty-four of the 59 dogs, about 92%, survived to discharge.
Still, the authors caution that these numbers may not tell the whole story. The study looked back at two referral hospitals, which means some dogs in the broader outbreak may have died without a diagnosis or never reached specialty care.
The Los Angeles cluster may have ended as vaccination efforts rose and some daycare facilities temporarily closed, but the authors argue that the larger concern remains. Leptospirosis is zoonotic, meaning it can pass between animals and people, and human infections are likely underrecognized.
“It’s probably the tip of the iceberg,” Sykes said. “There are probably more unrecognized cases than we know about.”
No human cases were linked to the Los Angeles outbreak. But the same paper places the event in a wider public health frame. The researchers note that the disease is now being investigated in homeless encampments in Berkeley and Oakland, where dogs, rodents, wildlife, and unstable living conditions can create new opportunities for exposure.
“This disease — there’s no boundaries for it,” Sykes said. “We’re talking about dogs with this disease owned by wealthy people in L.A. and dogs that are in homeless encampments on the streets of Berkeley dying with this disease because of rodent exposure.”
That broad reach is one reason the researchers call leptospirosis a “One Health” issue, one that links veterinary care, human health, sanitation, rodent control, and climate. Flooding is expected to make the disease more common in some settings, and urban outbreaks do not need a tropical climate to take hold.
“This is a really important One Health problem,” Sykes said. “It affects dogs and it affects people.”
The clearest takeaway is simple: annual leptospirosis vaccination should not be treated as optional based on old regional assumptions. This outbreak involved a vaccine-covered strain, and it spread in a dry major city where many veterinarians had seen the disease as uncommon.
The study also shows that crowded dog daycare and boarding facilities need closer attention to hygiene, rodent control, and animal density.
For clinicians, the findings support testing urine and blood with PCR while also using serology, especially early in illness when no single test is reliable enough on its own.
For owners, the message is less abstract. A dog that suddenly becomes lethargic, vomits, drinks excessively, or urinates more than usual after boarding or daycare may need rapid veterinary care, not watchful waiting.
Research findings are available online in the Journal of Clinical Microbiology.
The original story “Los Angeles dog outbreak reveals how a preventable bacterial disease spread in plain sight” is published in The Brighter Side of News.
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