The Summer the Clinics Couldn't Keep Up

Diane Ferraro had dropped her Vizsla, Copper, off at his usual West Los Angeles dog daycare on a Monday morning in July 2021, the same way she had every week for two years. By Wednesday night, Copper was at a specialty animal hospital with a fever, vomiting, and kidneys that were failing fast. The attending veterinarian told Diane she had seen three other dogs from the same facility that week with an identical presentation. By the end of that month, she would see many more.

"I had never even heard of leptospirosis," Diane said. "I didn't know it was something I was supposed to ask about."

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She was far from alone. What unfolded in Los Angeles County in the summer and fall of 2021 was, according to the scientists who later studied it, possibly the largest recognized leptospirosis outbreak in dogs ever recorded. And a study published on May 26, 2026 in the Journal of Clinical Microbiology finally laid out what went wrong — and why.

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What Leptospirosis Is

Leptospirosis is a bacterial disease caused by Leptospira — a spirochete that spreads through the urine of infected animals, most commonly rodents. Dogs contract it through contact with contaminated water, soil, or surfaces. Once inside a dog's body, the bacteria target the kidneys and liver, causing acute kidney injury that can progress to organ failure and death within days if not treated aggressively. Survivors often require weeks of hospitalization and may face permanent kidney damage.

The disease is also zoonotic: humans can contract leptospirosis through contact with infected animal urine or contaminated water. In people, it typically presents as a flu-like illness but can progress to a severe syndrome affecting the kidneys, liver, and lungs. Human cases in the United States are often underrecognized because symptoms overlap with many other illnesses and clinicians don't always think to test for it.

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A vaccine against leptospirosis has existed for decades. It covers four of the most common serovars — strains — that infect dogs in the United States, including Leptospira interrogans serovar Canicola, the strain laboratory testing would later identify as the cause of the Los Angeles outbreak.

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Why Most Dogs Weren't Vaccinated

The answer to that question is partly geographic and partly historical, and the UC Davis study, led by Dr. Jane Sykes, a professor of small animal internal medicine, documents it clearly. At the time of the outbreak, Los Angeles area veterinarians rarely offered leptospirosis vaccinations because the bacteria were thought to thrive primarily in water from heavy rainfall, and Los Angeles is an arid climate. The disease was considered a rural or Midwestern problem — something for dogs who swam in farm ponds or lived near livestock, not urban pets going to daycare facilities in Santa Monica.

"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, the outbreak proved, was wrong. The outbreak peaked in July and August 2021 — months when rainfall in Los Angeles County was not above average. Whatever ignited it, rain was not the explanation.

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What the Study Found

The UC Davis researchers investigated 59 confirmed cases of leptospirosis across two specialty animal hospitals during the outbreak period. They compared those cases to data on more than 15,500 control dogs seen at the same facilities. The statistical association between dog daycare attendance and leptospirosis diagnosis was strong and consistent.

Scientists traced the outbreak to dog daycare environments where close contact among animals could have accelerated dog-to-dog transmission, an unusual feature of leptospirosis spread that the research team said was worth particular attention. Typical leptospirosis transmission involves an environmental reservoir — rodents contaminating soil or water — and a dog picking up the bacteria from that contaminated surface. What Los Angeles appeared to show was something different: dogs passing the bacteria to one another directly in crowded indoor facilities, or sharing exposure to a rodent problem inside a specific building, or both.

During the peak of the outbreak, some veterinary clinics were seeing more than one case of leptospirosis a day coming from dogs that had recently been at dog daycare facilities on the west side of Los Angeles County.

Dr. Sykes called it what it was: "The outbreak was massive. It might have been the biggest outbreak of leptospirosis in dogs that's ever been recognized."

Genetic evidence from the four dogs whose samples underwent whole-genome sequencing pointed toward multiple independent sources of infection rather than a single origin point. Researchers could not pin the outbreak to one facility or one moment. The bacteria appeared to have arrived through several pathways simultaneously.

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The Broader Warning

The study's authors are explicit about what the Los Angeles outbreak means beyond Southern California. The combination of post-pandemic crowding in dog daycare facilities, increased urban rodent activity, and the widespread assumption that leptospirosis is only a risk in wet, rural environments created the conditions for an outbreak in a city where most dogs weren't vaccinated and most veterinarians weren't routinely offering the vaccine.

UC Davis researchers are now investigating cases of leptospirosis in dogs living in homeless encampments in the San Francisco Bay Area and providing vaccinations to those animals — a recognition that dense, high-rodent urban environments present a real and underappreciated transmission risk, regardless of rainfall.

The study's conclusions are direct: its findings support the need for vaccination of dogs to reduce the risk to dog and human health, a high index of suspicion for the disease in unvaccinated dogs, the combined use of molecular and serologic tests to optimize diagnosis, and early treatment.

Copper, Diane Ferraro's Vizsla, survived. He spent eleven days in the hospital and came home with ongoing kidney monitoring scheduled for the following six months. His kidneys stabilized. He still goes to daycare — a different facility now, with a stated vaccination policy for all enrolled dogs. His leptospirosis booster is current.

"The vet said it was very preventable," Diane said. "That part was hard to hear."