Investigation of the Bi-Weekly Administration of Diclazuril on the Antibody Kinetics to Sarcocystis Neurona in Healthy Horses.
Authors: Pusterla Nicola, James Kaitlyn, Bain Fairfield, Barnett D Craig, Chappell Duane, Gaughan Earl, Craig Bryant, Schneider Chrissie, Vaala Wendy, Papich Mark
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Diclazuril and Sarcocystis neurona Antibody Response in Exposed Horses Sarcocystis neurona, the causative agent of equine protozoal myeloencephalitis, remains a significant threat to horse health in endemic areas, yet the optimal prophylactic strategy for seronegative animals moving into high-exposure environments remains unclear. Pusterla and colleagues investigated whether twice-weekly administration of diclazuril at half the conventional label dose (0.5 mg/kg) could suppress seroconversion in 12 healthy horses relocated to a farm with high S. neurona prevalence, monitoring antibody titres monthly and plasma drug levels bi-monthly over 12 months. Whilst seroprevalence inevitably reached 100% in both treatment and control groups by day 90—indicating that half-dose prophylaxis cannot prevent exposure-induced seroconversion—treated horses maintained significantly lower antibody titres throughout the study period despite comparable exposure, and all treated animals sustained plasma diclazuril concentrations above the in vitro inhibitory threshold (1.0 ng/mL). For practitioners managing horses in high-risk areas, these findings suggest that even sub-label diclazuril dosing achieves drug exposure sufficient to suppress parasite-specific immune responses, which may reduce clinical disease risk if breakthrough infection occurs, though further investigation into whether titre suppression translates to reduced EPM incidence is warranted.
Read the full abstract on PubMed
Practical Takeaways
- •Bi-weekly diclazuril at reduced dose maintains suppressed antibody titers in healthy horses with natural S. neurona exposure, potentially reducing clinical disease risk even if seroconversion cannot be prevented
- •This dosing protocol achieves adequate plasma concentrations known to inhibit the parasite in vitro, offering a potentially more economical prevention strategy than label dosing
- •While bi-weekly reduced-dose treatment cannot prevent eventual seroconversion on high-exposure farms, the suppression of titer magnitude may be clinically significant in preventing overt EPM signs
Key Findings
- •Bi-weekly diclazuril at half label dose (0.5 mg/kg) significantly reduced antibody titers to S. neurona throughout the 12-month treatment period compared to controls (P < 0.05)
- •Seroprevalence eventually reached 100% in both treatment and control groups by day 90, despite treatment reducing initial seroconversion to 50% at day 30
- •All treated horses maintained detectable plasma trough diclazuril levels exceeding the 1.0 ng/mL in vitro inhibitory concentration at all 6 measurement timepoints
- •All 12 horses remained clinically healthy throughout the entire study despite natural exposure to S. neurona on a high-prevalence farm