Epidemiology of Leptospira borgpetersenii strain Pacifica in primiparous dairy cattle in New Zealand, 2020–2022

This study investigated Leptospira borgpetersenii strain Pacifica (Pacifica) in primiparous dairy cattle from high-risk dairy herds in New Zealand to evaluate its shedding pattern and to isolate strain Pacifica in culture. Herds were sampled across 3 milking seasons; early season in 2020 (n=16), late season in 2021 (6 of 16 herds from early season) and 1 separate herd in peak season in 2022. Blood samples (n=1,324) were tested using the microscopic agglutination test to detect antibodies against the five endemic serovars (Hardjo, Pomona, Ballum, Copenhageni and Tarassovi) where Tarassovi titres were used as a proxy for exposure to strain Pacifica. Urine samples (n=564) were tested using a glmU PCR and positive amplicons were sequenced to determine the Leptospira genotypes. At the herd level, Tarassovi seroprevalence was 69% (95% CI: 24.6-81.9) and urinary shedding for Pacifica was 9.2% (95% CI: 7.2–12.5). Individual cow seroprevalence for Tarassovi was similar at season start (41.9%, 37.3-46.6) and end (48.7%, 43.7-53.6), however geometric mean titre increased by 13% from 316 (95% CI: 259.1-384.2) to 357 (95% CI: 293.9-433.5). In contrast, Pacifica shedding prevalence was seven times higher at season start (27.3%, 95% CI: 19.8-36.3) versus end (3.9%, 95% CI: 2.3-6.5). Tarassovi seropositivity significantly correlated with Pacifica shedding at all sampling stages (p≤0.05). These results demonstrate that peak shedding is at the start of the milking season, and a substantial proportion of high-risk farms maintained active transmission of Pacifica across seasons. Continuous shedding indicates dairy cattle as maintenance hosts for Pacifica.