A 2021 outbreak of leptospirosis that sickened more than 200 dogs in Los Angeles County reveals critical gaps in vaccination practices and raises broader concerns about the spread of the disease between animals and people, according to new research led by the University of California, Davis.
Leptospirosis is a bacterial disease that can cause severe illness in dogs, including acute kidney injury. In severe cases, dogs can die. Humans can also contract the disease through contact with contaminated animal urine (especially from rodents or livestock) or contaminated water. Human cases often result in flu-like symptoms that can be treated with antibiotics.
In the new study, scientists traced the outbreak to dog daycare environments where close contact among animals could have accelerated dog-to-dog transmission, an atypical occurrence.
“We know that the boarding itself was a risk factor,” said lead author Jane Sykes , professor of small animal internal medicine at the UC Davis Weill School of Veterinary Medicine. “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.”
The study was published in Journal of Clinical Microbiology, an American Society for Microbiology journal.
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 in the westside of Los Angeles County.
“The outbreak was massive,” Sykes said. “It might have been the biggest outbreak of leptospirosis in dogs that’s ever been recognized.”
Researchers analyzed 59 confirmed cases from two specialty veterinary centers and compared them with more than 15,000 control patients. The study confirmed the infections were caused by Leptospira interrogans serovar Canicola, one of the four strains the dog leptospirosis vaccine protects against.
While this strain is known to infect dogs, transmission typically occurs through contaminated environments, not direct contact between dogs. But conditions inside crowded daycare facilities after the COVID pandemic could have changed that dynamic.
The findings underscore the importance of vaccinations and vigilance in settings where dogs congregate.
“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.”
As vaccination rates increased and some dog daycares temporarily closed, the outbreak subsided. Major veterinary organizations now recommend annual leptospirosis vaccination for all dogs.
While the westside of Los Angeles outbreak was centered in high-end dog day care facilities, researchers are now investigating cases in homeless encampments in Berkeley and Oakland, where dogs, wildlife and environmental conditions are creating new pathways for transmission.
“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.”
Early findings from investigations in the Bay Area show high infection rates in local rat populations. Rats are the most likely hosts for leptospirosis.
Dogs in encampments often roam between locations and may end up in shelters, increasing the risk of exposure to other animals and people. Although no human cases were linked in the Los Angeles outbreak, experts say the disease is likely underdiagnosed in people, and in recent years, leptospirosis has been recognized in people in large U.S. cities, albeit rarely.
“It’s probably the tip of the iceberg,” Sykes said. “There are probably more unrecognized cases than we know about.”
Leptospirosis is also expected to become more common as climate change brings more flooding.
Researchers emphasize that vaccination is the most effective way to protect dogs and reduce the risk of transmission to people.
“This is a really important One Health problem,” Sykes said. “It affects dogs and it affects people.”
Other UC Davis authors include Max W. Randolph and Krystle Reagan. No funding was received for this study.
Journal of Clinical Microbiology
Animals
Clinical and molecular characterization of an outbreak of leptospirosis in dogs from Los Angeles County, California, USA, 2021
26-May-2026
The authors declare no conflicts of interest.