Introduction
Tick-borne diseases remain a major challenge for beef cattle in tropical regions. The enzootic stability of Theileria- and Babesia-caused infections depends on exposure levels to pathogens like Anaplasma marginale, Babesia bovis, and Babesia bigemina, balanced against tick burdens. This study investigates whether strategic use of fluralaner (Exzolt® 5%) affects enzootic stability in crossbred beef calves under real-world farm conditions in Rio Verde, Goiás, Brazil.
Experimental setting and animals
The trial ran from October 2023 to May 2024 on a commercial cattle farm with about 4,500 head. The region experiences a tropical climate with a distinct rainy season. A total of 100 calves (3/4 European × 1/4 Nelore) aged about 25 days were enrolled and divided into two groups of 50: FLU and FIFLUA. Both groups remained in separate paddocks and received identical nutrition and management, including maternal milk and grazing with similar grass cover.
Treatments and strategic tick control
Two pour-on acaricides were used:
– FIFLUA: fipronil 1.25 mg/kg + fluazuron 2.5 mg/kg (TickGard®) – farm-standard treatment.
– FLU: fluralaner 2.5 mg/kg (Exzolt® 5%) – applied as a strategic, preventive approach.
Calves were treated at 25 days of age and re-treated according to a herd-based criterion: re-treatment when more than 30% of the batch had np < 4 mm ticks. Re-treatment intervals varied, with the FLU group sometimes extending up to 105 days between applications, reflecting the high efficacy of fluralaner in reducing tick burdens.
Tick burden and parasite exposure
Tick counts focused on engorged female Rhipicephalus (Boophilus) microplus on the left flank. Tick burdens differed between groups over time. The FLU group generally showed lower tick counts at key time points, indicating effective strategic control. However, under some intervals, FIFLUA calves had lower burdens, highlighting the dynamics of tick populations under tropical conditions where multiple generations per year can occur.
Enzootic stability assessment
Enzootic stability for three TF pathogens (A. marginale, B. bovis, B. bigemina) was assessed through:
- Indirect ELISA (iELISA) to monitor antibody prevalence, with stability defined as more than 75% antibody-positive animals.
- Quantitative PCR (qPCR) to detect DNA from the pathogens in blood samples, indicating exposure rather than disease alone.
Serology and qPCR were conducted at multiple ages: approximately 25, 60, 135, 188, and 241 days. The thresholds and interpretation followed established criteria for enzootic stability in tropical cattle herds.
Key findings on enzootic stability
– B. bovis: Enzootic stability was reached earlier in FLU (133 days) than FIFLUA (188 days).
– B. bigemina: Stability appeared earlier in FLU (60 days) compared with FIFLUA (188 days).
– A. marginale: FLU calves achieved stability by 188 days, while FIFLUA reached stability at 241 days.
Overall, despite the strong tick control achieved with fluralaner, the FLU group did not compromise enzootic stability. In fact, stability for all pathogens tended to occur earlier in the FLU group according to serology, with qPCR data supporting ongoing exposure patterns consistent with enzootic infection cycles in a tropical setting.
Clinical observations and welfare
Clinical cases of A. marginale bacteremia were more frequent in the FIFLUA group, with some calves requiring salvation treatment. By contrast, the FLU-treated group had fewer clinical events, suggesting improved welfare and disease resilience when strategic fluralaner use reduces tick exposure without eradicating pathogen circulation.
Practical implications for producers
The study supports the idea that strategic tick control with fluralaner can maintain enzootic stability for TF pathogens in crossbred calves raised on pasture in tropical climates. This approach may reduce disease risk, improve welfare, and allow longer intervals between treatments, potentially lowering labor and chemical use while maintaining herd health.
Limitations and regional considerations
Results are context-specific. Variations in tick pressure, climate, cattle genetics, and management across farms may influence outcomes. Regions with different rainfall patterns or tick dynamics could observe different enzootic stability trajectories under fluralaner-based strategies.
Conclusion
Strategic use of fluralaner (Exzolt® 5%) in tropical beef calves did not disrupt, and may even support, enzootic stability for TF pathogens in a real-world Brazilian farm setting. This finding aligns with prior dairy-focused work in tropical climates and offers practical guidance for veterinarians and producers aiming to balance effective tick control with maintaining herd-pathogen dynamics.
