Clinical and molecular characterization of an outbreak of leptospirosis in dogs from Los Angeles County, California, USA, 2021

In 2021, the Los Angeles County (LAC) Department of Public Health suspected a leptospirosis outbreak in LAC affecting over 200 client-owned dogs. We aimed to characterize the outbreak and describe microbiologic findings, risk factors, diagnostic test performance, and outcomes in dogs diagnosed with leptospirosis at two specialty practices. Leptospira culture isolates from four cases were subjected to serotyping and whole-genome sequencing (WGS); WGS was also performed on one enriched genome isolate. After the outbreak, data were gathered on 59 cases and compared to the background hospital population (controls, n = 15,536). All isolates were Leptospira interrogans serovar Canicola, but each was distinct based on WGS. Cases clustered in space and in time. Cases evaluated during the outbreak peak had increased odds of exposure to indoor congregate facilities (ICFs). None of the 47 dogs with known leptospirosis vaccination history were completely vaccinated. Leptospira real-time PCR on blood and urine was positive in 15/56 (27%) and 49/54 (91%) of dogs, respectively. Initial serologic testing using the microscopic agglutination test and point-of-care tests was positive in 22/29 (76%) and 27/35 (77%) of dogs, respectively. Fifty-four (92%) of 59 dogs survived to discharge; some remained azotemic. No associated human cases were identified. In conclusion, L. interrogans serovar Canicola was associated with a leptospirosis outbreak in unvaccinated dogs, which had public health implications given widespread dog ownership rates. Data analysis suggested multiple infection sources, including ICFs. Urine PCR was the most sensitive diagnostic test. Such outbreaks might be prevented through more widespread vaccination. IMPORTANCE Leptospirosis is a zoonotic bacterial disease transmitted through the urine of infected animals. We characterized an outbreak of leptospirosis in unvaccinated dogs in Los Angeles County and showed that cases were associated with housing in dog daycare and boarding facilities. Culture of several Leptospira isolates from the post-peak period identified Leptospira interrogans serovar Canicola, supporting the potential for dogs to act both as reservoirs and incident hosts for this serovar. Across the entire study period, multiple independent infection sources seemed likely based on molecular and spatial epidemiologic analysis, which might have included exposure to other dogs in indoor congregate facilities, rodents, or rodent urine. Our 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 to optimize outcomes.