Introduction
Tick fever (TF), caused by pathogens such as Anaplasma marginale, Babesia bigemina, and Babesia bovis, remains a major health and economic concern for cattle in tropical regions. Enzootic stability—a situation where most animals have subclinical exposure and herd-level immunity—guards against severe outbreaks. A common question for producers is whether introducing a highly effective acaricide like fluralaner (Exzolt® 5%) for strategic tick control could disrupt this stability. A field study conducted on a commercial cattle farm in Rio Verde, Goiás, Brazil, provides practical insights into this issue.
Study design in a tropical setting
Researchers followed 100 crossbred calves (3/4 European × 1/4 Nelore) aged about 25 days, naturally infested with Rhipicephalus microplus. The calves were split into two groups of 50: FLU, treated with fluralaner pour-on (2.5 mg/kg) at 25 days and re-treated when >30% of the herd had small ticks; and FIFLUA, the farm’s ongoing control using fipronil + fluazuron (TickGard®). Calves remained with their dams in separate paddocks to preserve consistent exposure to tick challenge and TF pathogens. The trial ran until weaning at about 241 days.
Assessment methods for enzootic stability
Enzootic stability was evaluated using multiple, complementary approaches:
- Tick counts on a sub-sample of animals across five ages (25, 60, 135, 188, 241 days).
- Serology (iELISA) measuring IgG against A. marginale, B. bovis, and B. bigemina on subsets of 15 animals per group.
- qPCR to detect pathogen DNA (A. marginale, B. bovis, B. bigemina) in blood samples, to quantify exposure frequency.
- Blood smears for parasite/bacteremia but used only as supplementary data due to limited sensitivity.
Key findings: tick burden and pathogen exposure
Tick burden differed between groups at several time points. The FLU group (fluralaner) showed lower tick counts at 60 and 241 days but higher counts at 135 days, a pattern reflecting the timing of treatments and tick re-infestation dynamics. Importantly, A. marginale bacteremia was lower in the FLU group at 60 days, suggesting an early protective effect against bacteremia when tick challenge begins to rise.
Serology revealed different timelines for enzootic stability. In the FLU group, seropositivity indicating stability for B. bovis, B. bigemina, and A. marginale was achieved earlier (133 days for B. bovis, 60 days for B. bigemina, and 188 days for A. marginale) compared with the FIFLUA group (188 days for B. bovis, 188 days for B. bigemina, and 241 days for A. marginale). Across the entire period, mean antibody titres for the three TF pathogens were higher in the FLU group, highlighting a robust immune exposure under fluralaner strategy.
qPCR data showed distinct exposure frequencies. In FIFLUA calves, B. bovis and B. bigemina infections increased with age, and A. marginale DNA rose steadily, reaching 100% by 241 days. The FLU group also showed substantial exposure, but the patterns suggested that strategic fluralaner did not prevent pathogen presence and did not suppress the development of serological immunity needed for enzootic stability.
Interpretation: does fluralaner disrupt enzootic stability?
The results indicate that strategic use of fluralaner can suppress tick burden effectively without compromising enzootic stability for TF pathogens in crossbred calves raised on tropical pasture. In fact, earlier seroconversion for several pathogens in the FLU group suggests that exposure and immune training occur under controlled tick challenge, maintaining herd-level preparedness against outbreaks. This aligns with prior findings in dairy cattle in tropical climates, where fluralaner-based strategies did not disturb enzootic stability while reducing clinical risk.
Practical implications for tropical beef production
Key takeaways for veterinarians and producers include:
- Fluralaner can provide strong and sustained tick control, potentially reducing A. marginale bacteremia at critical ages.
- Strategic re-treatment criteria, based on tick burden rather than fixed calendar intervals, may optimize efficacy and minimize tick-borne pathogen exposure.
- Enzootic stability for TF pathogens can be maintained in crossbred beef calves under tropical conditions with appropriate tick management and monitoring.
Limitations and considerations
Results are region-specific. Tropical climates with high R. microplus generation turnover may yield different dynamics under other husbandry practices or tick resistance patterns. The study emphasizes careful interpretation and cautions against broad extrapolation to subtropical regions or pure breeds without local validation.
Conclusion
In this tropical Brazilian setting, strategic fluralaner use for tick control did not disrupt, and may even promote earlier enzootic stability for bovine TF pathogens in crossbred calves. The approach offered practical benefits: effective tick suppression, maintained pathogen exposure enabling herd immunity, and potential reductions in disease risk during the vulnerable early life stage.
