Abstract: Background - A leptospirosis outbreak was suspected in west Los Angeles County (April–December, 2021). Hypothesis/Objectives - To describe signalment, clinicopathologic and microbiologic findings, and diagnostic test results in affected dogs. Animals – Fifty-seven dogs with leptospirosis evaluated at two Los Angeles County specialty hospitals. Methods - Signalment variables were compared to the hospital population (n=19,870) (Fisher’s exact or chi-square analysis; significance p ≤ 0.05). Clinicopathologic and microbiologic findings were described for cases. Results - Cases were more likely <= 5 years old, => 15 kg, and male (odds ratio [95% confidence interval] 8.1 [3.8-17.4], 4.2 [2.3-7.4], and 1.8 [1.1-3.2], respectively) than the hospital population. Siberian huskies, Labradoodles, and Golden retrievers were overrepresented (7.9 [2.6-23.9], 5.3 [2.3-12.7], and 3.5 [1.5-8.3], respectively). Exposure to boarding/daycare was reported for 26/57 dogs, 19 between 7/15/21-9/6/21. Forty-five of 47 dogs were unvaccinated for leptospirosis. Laboratory findings included anemia (52%), neutrophilia (49%), thrombocytopenia (33%), increased serum creatinine (88%), increased serum liver enzymes (18%), and glucosuria (37%). Diagnostic testing included WITNESS Lepto (Zoetis), SNAP Lepto (IDEXX), acute microscopic agglutination testing, blood and urine PCR (positive in 19/26, 8/9, 22/28, 15/55, and 47/52, respectively). Isolates from urine (4/4 dogs) were serotyped as Leptospira interrogans serogroup Canicola serovar Canicola and were clonal using whole genome sequencing. Conclusions and Clinical Importance - Findings supported a leptospirosis outbreak in association with boarding/daycare in a predominantly young-adult, unvaccinated dog population. Urine PCR was a sensitive diagnostic test. Diagnosis was optimized using a combination of serologic tests and blood and urine PCR assays.
