Use of Daily Diclazuril Pelleted Top Dress for the Prevention of Sarcocystis neurona Infection in Foals

The daily supplementation of diclazuril pelleted top dress at 0.5 mg/kg body weight has shown a significant reduction in seroconversion against Sarcocystis neurona in foals treated up to 12 months of age when compared to untreated control foals. Authors’ addresses: Department of Medicine and Epidemiology (Pusterla, Mackie), Department of Pathology, Microbiology and Immunology (Packham, Conrad), and Department of Population Health and Reproduction (Kass), School of Veterinary Medicine, University of California, Davis, CA 95616; e-mail: npusterla@ ucdavis.edu.

1. Introduction

Therapeutic treatment strategies for the prevention of S. neurona infection in horses have been empirical. A pelleted top dress 1.56% diclazuril antiprotozoal drug recently introduced to the equine market and labeled for the treatment of EPM has the potential to be used for the prevention of EPM due to its convenient formulation. A low dose of diclazuril given at 0.5 mg/kg body weight has shown to reach plasma and cerebrospinal fluid (CSF) concentrations at steady-state in excess of the minimal concentration known to be inhibitory to S. neurona merozoite production in cell culture. The purpose of this study was to evaluate the temporal serological response against S. neurona in foals receiving daily diclazuril and in untreated herdmates.

2. Materials and Methods

Thirty-three foals from a farm with a high exposure rate to S. neurona were randomly assigned to either an untreated control group or a diclazuril-treated group. Treatment consisted of the administration of 0.5 mg/kg body weight of diclazuril pelleted top dress starting at 4 weeks of age until the foals were 12 months of age. Whole blood was collected from every dam and foal 24 hours post-foaling and monthly thereafter from every study foal for the duration of the study. The blood was tested for IgG against S. neurona using the indirect fluorescent antibody test.

3. Results and Discussion

Following ingestion of colostral antibodies to S. neurona, there was a steady and continuous decline in seroprevalence and antibody titers to S. neurona until the foals from both groups reached weaning age. Thereafter, untreated foals showed an increase in monthly seroprevalence ranging from 53 to 82%. Diclazuril treated foals showed significant lower monthly seroprevalences ranging from 6 to 37%. The difference in temporal seroprevalence can be explained by the successful reduction of S. neurona infection in foals receiving daily diclazuril at a low dose.

Acknowledgments

Conflict of Interest

This study was supported by a grant from The Center for Equine Health, University of California, School of Veterinary Medicine at Davis and Merck Animal Health.