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

Authors: RANDOLPH, MAX - University Of California, Davis; Nally, Jarlath; YOSHIMITO, SEAN - Vca All Pets Animal Hospital; CHOW, BETTY - Vca All Pets Animal Hospital; WAGNER, DAVID - Northern Arizona University; STONE, NATHAN - Northern Arizona University; SAHL, JASON - Northern Arizona University; HAMOND, CAMILA - Oak Ridge Institute For Science And Education (ORISE); LECOUNT, KAREN - Diagnostic Virology Laboratory/ National Veterinary Services Laboratories; STUBER, TOD - Diagnostic Virology Laboratory/ National Veterinary Services Laboratories; VAN DER LINDEN, HANS - University Of Amsterdam; REAGAN, KRYSTLE - University Of California; SCHREIBER, ALEXANDER - Vca All Pets Animal Hospital; SEBASTIAN, JAMIE - University Of California, Davis; SYKES, JANE - University Of California, Davis

Submitted to: Journal of Clinical Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/8/2026
Publication Date: 5/26/2026
Citation: Randolph, M.W., Nally, J.E., Yoshimito, S.K., Chow, B., Wagner, D.M., Stone, N.E., Sahl, J.W., Hamond, C., LeCount, K., Stuber, T., Van Der Linden, H., Reagan, K., Schreiber, A., Sebastian, J., Sykes, J.E. 2026. Clinical and molecular characterization of an outbreak of leptospirosis in dogs from Los Angeles County, California, USA, 2021. Journal of Clinical Microbiology. https://doi.org/10.1128/jcm.00240-26.
DOI: https://doi.org/10.1128/jcm.00240-26

Interpretive Summary: Leptospirosis is a zoonotic bacterial disease that is endemic in the United States and infects domestic livestock and companion animals. In 2021, an outbreak occurred in at least 201 dogs in Los Angeles County, CA. Investigation of this outbreak determined that urine PCR was more sensitive than initial serologic testing or blood PCR for diagnosis. More than 70% of clustered cases had exposure to boarding facilities and risk of infection was higher in dogs that were male, < 6 years of age, and = 15 kg. The causative agent was identified as Leptospira interrogans serovar Canicola.

Technical Abstract: Background: Between April and December 2021, a leptospirosis outbreak occurred in at least 201 dogs in Los Angeles County. Objective: To describe microbiologic findings, risk factors, diagnostic test performance, and outcomes in dogs diagnosed with leptospirosis at two Los Angeles veterinary specialty practices during the outbreak. Methods: Leptospira isolates from 5 affected dogs were subjected to serotyping and whole genome sequencing (WGS). After the outbreak, medical record review was used to gather data on history, diagnostic test performance, and treatment outcome for 59 dogs diagnosed with leptospirosis. Results: Although all isolates were Leptospira interrogans serovar Canicola, each was distinct based on WGS (differing by 12-224 core genome SNPs). Compared to the hospital population, cases had a higher odds of being male, < 6 years of age, and = 15 kg. Cases clustered in zip codes southwest of the clinics and in time; 73% of clustered cases had exposure to boarding facilities. None of 47 dogs with known leptospirosis vaccination history were completely vaccinated. Leptospira PCR was positive on blood or urine in 14/56 (25%) and 48/53 (91%) tested dogs, respectively. Antibodies were detected using initial MAT and point-of-care tests in 23/30 (77%) and 27/35 (77%) dogs, respectively. Fifty-four of 59 (91%) dogs survived to discharge, but some remained azotemic. Conclusions and Clinical Importance: Leptospira interrogans serovar Canicola was associated with a leptospirosis outbreak in unvaccinated dogs from west Los Angeles. Urine PCR was more sensitive than initial serologic testing or blood PCR. Such outbreaks might be prevented through more widespread vaccination.