Persistence of Theileria orientalis Chitose genotype on a closed dairy farm: possible vertical transmission and limited clearance after buparvaquone treatment

Vollständiger Abstract

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Abstract Theileria orientalis comprises tick‑borne protozoan parasites that can cause substantial economic losses in cattle. We previously reported an outbreak of oriental theileriosis associated with mixed T. orientalis Ikeda and Chitose infection on a closed dairy farm. However, the predominant genotype and the mechanisms underlying long-term persistence under closed-herd management remain unclear. In December 2024, conventional PCR detected T. orientalis in 41 of 147 periparturient cows; sequencing of MPSP amplicons from 18 positive samples detected Chitose in all 18 and co-detected Ikeda in four. Chitose‑specific PCR was subsequently developed from the MPSP sequences. In 528 cross-sectional samples collected from April to September 2025, Chitose-specific PCR positivity ranged from 38.00% to 79.27%. Among 35 calves born to PCR‑positive dams, 15 (42.86%) were positive at one or more sampling times; two of the five calves sampled at 2 days postpartum were already positive. These findings are compatible with vertical transmission but not exclude peripartum contamination or early postnatal transmission. Ninety-seven PCR-positive periparturient cows received buparvaquone (2.5 mg/kg in two- or three-dose regimens). At 30 days after treatment, 56.76–72.00% remained positive by Chitose-specific PCR. Because the study lacked an untreated control group, quantitative parasitemia measurements, and clinical outcomes, these data indicate persistence of PCR-detectable DNA rather than absence of therapeutic benefit. Chitose was the most frequently detected genotype among the sequenced samples, possible vertical transmission may contribute to persistence, and the test buparvaquone regimens did not eliminate PCR-detectable infection within 30 days. Therefore, future research should focus on more effective drugs or optimized combination therapies to clear subclinical infections in cattle.