A major outbreak of leptospirosis among dogs in Los Angeles County during 2021 has provided valuable insights into how this potentially serious bacterial disease spreads, how it can be diagnosed, and why vaccination remains one of the most effective preventive measures.
Researchers recently analyzed dozens of canine cases linked to the outbreak and discovered important trends involving vaccination status, boarding facilities, diagnostic testing, and treatment outcomes. Their findings offer important lessons for dog owners, veterinarians, and public health officials.
Leptospirosis is a bacterial disease caused by pathogenic species of the genus Leptospira. It affects both animals and humans, making it a zoonotic disease. Dogs can become infected when they come into contact with contaminated water, soil, or urine from infected animals, particularly rodents.
The disease can range from mild illness to severe organ damage. In dogs, leptospirosis commonly affects the kidneys and liver, potentially causing life threatening complications if left untreated.
Common symptoms include:
Because many of these symptoms are not unique to leptospirosis, accurate diagnostic testing is essential.
In 2021, public health authorities in Los Angeles County identified a significant increase in leptospirosis cases among pet dogs. More than 200 dogs were suspected to have been affected across the region.
A detailed investigation focused on 59 confirmed cases treated at two major veterinary specialty hospitals in West Los Angeles. Researchers examined clinical records, laboratory results, vaccination histories, and environmental exposures to better understand the outbreak.
The study confirmed that the outbreak was associated with Leptospira interrogans serovar Canicola, a strain historically linked to dogs.
Interestingly, genetic analysis showed that infected dogs were not all carrying identical bacterial strains. This suggested that multiple sources of infection were likely involved rather than a single transmission chain.
One of the most notable findings involved indoor congregate facilities such as dog boarding centers and daycare establishments.
More than half of the affected dogs had recently visited these environments. During the peak months of the outbreak, dogs exposed to such facilities had significantly higher odds of developing leptospirosis.
Researchers believe these locations may have increased opportunities for exposure to contaminated urine, infected animals, or environmental contamination.
However, the evidence also indicated that dog daycare facilities were probably not the only source. Other potential contributors included:
The outbreak likely resulted from several overlapping transmission pathways.
Perhaps the most striking discovery involved vaccination history.
Among dogs with known vaccination records, nearly 90% had never received a leptospirosis vaccine.
Only a small number had received any vaccine protection, and none were considered fully vaccinated at the time illness developed.
This finding reinforces growing veterinary recommendations that leptospirosis vaccination should not be limited to traditionally high risk dogs.
Historically, vaccination was often reserved for dogs living in rural environments or those frequently exposed to standing water. Modern outbreaks, however, increasingly affect urban dogs, including pets living in major metropolitan areas.
Researchers concluded that broader vaccine coverage could significantly reduce disease risk and help prevent future outbreaks.
The study identified several demographic factors associated with increased risk.
Affected dogs were more likely to be:
Certain breeds appeared more frequently among cases, including Golden Retrievers and Siberian Huskies.
However, researchers noted that these associations may partly reflect lifestyle factors, particularly attendance at boarding and daycare facilities.
An important goal of the investigation was evaluating the performance of different diagnostic methods.
The results showed significant differences between testing approaches.
Polymerase chain reaction (PCR) testing performed on urine samples detected infection in approximately 91% of affected dogs.
This made urine PCR the most sensitive diagnostic tool used during the outbreak.
Blood PCR testing identified only about 27% of cases.
Researchers believe this occurs because the bacteria circulate in the bloodstream for a relatively short period before concentrating in the kidneys and urine.
Antibody based tests also played an important role.
Many dogs tested positive through:
However, dogs tested very early in the disease process sometimes produced negative antibody results because their immune systems had not yet generated detectable responses.
For this reason, experts recommend combining molecular testing and serology whenever possible.
Despite the severity of leptospirosis, treatment outcomes were encouraging.
Of the 59 dogs included in the analysis:
Treatment commonly included:
A small number of dogs required dialysis due to severe kidney injury.
Researchers suggested that heightened awareness during the outbreak may have contributed to improved outcomes because owners sought veterinary care sooner and veterinarians maintained a higher index of suspicion.
Leptospirosis remains a concern not only for animal health but also for human health.
Because infected animals can shed bacteria in their urine, there is potential for transmission to people under certain circumstances.
Fortunately, no confirmed human infections linked to this outbreak were identified.
Nevertheless, researchers emphasized that preventing infection in dogs can also help reduce public health risks.
Vaccination, environmental hygiene, rodent control, and prompt diagnosis all play important roles in reducing transmission opportunities.
The Los Angeles leptospirosis outbreak serves as a reminder that this disease can affect dogs living in urban environments, not just those in rural settings.
Dog owners should consider the following preventive measures:
Early diagnosis and treatment can dramatically improve outcomes.
The 2021 Los Angeles County leptospirosis outbreak provided one of the most comprehensive modern investigations of canine leptospirosis in the United States. Researchers found strong evidence linking the outbreak to Leptospira interrogans serovar Canicola and identified important risk factors, including lack of vaccination and exposure to indoor dog gathering facilities.
Perhaps the most important message is that leptospirosis remains a preventable disease. Wider vaccine adoption, improved awareness, and early testing can help protect both dogs and the people who care for them.
Randolph MW, Nally JE, Yoshimoto SK, Chow B, Wagner DM, Stone NE, Sahl JW, Sykes JE, et al. Clinical and Molecular Characterization of an Outbreak of Leptospirosis in Dogs from Los Angeles County, California, USA, 2021. Journal of Clinical Microbiology. Published May 26, 2026. DOI: 10.1128/jcm.00240-26.
This article is intended for educational and informational purposes only. It summarizes findings from a published scientific study and should not be considered veterinary or medical advice. Pet owners should consult a licensed veterinarian regarding vaccination schedules, diagnosis, treatment, or concerns about leptospirosis and other infectious diseases.
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