A medication used to treat equine protozoal myeloencephalitis (EPM) also shows promise as a preventive for young horses, according to new research from the University of California—Davis.
Characterized by progressive muscle weakness, incoordination or other neurological signs, EPM is primarily caused by the protozoan Sarcocystis neurona, which is shed in the feces of the definitive host, the opossum, and can then contaminate feed and water. When a horse is exposed to S. neurona, his body produces antibodies that can be detected in the blood---a process called seroconversion---but he will not necessarily become ill. EPM occurs only if S. neurona crosses the blood-brain barrier and infects the central nervous system. In some areas of the country, 90 percent of the equine population tests positive for S. neurona antibodies.
The Davis study involved 33 foals kept on a farm with a historically high exposure rate to the protozoan. “We chose to focus on that group because young performance horses are at higher risk, probably due to a combination of higher susceptibility and exposure rates and immunosuppression due to stress,” says Nicola Pusterla, DVM, PhD.
The researchers divided the study foals into two groups. Every day for 12 months, they gave one group diclazuril (Protazil, Merck Animal Health), a medication used to treat active cases of EPM. The other group of foals were left untreated to serve as controls.
The youngsters in the treatment group received one-half of the standard dose of diclazuril, explains Pusterla, because the goal was not to treat the actual disease. “The label dose reaches a very high concentration of medication in the bloodstream, which is needed to penetrate the central nervous system (CNS). With a preventive dose, we want to control infection at the entrance site [the GI tract] and not in the CNS.” The researchers drew monthly blood samples from the dams and foals until the end of the study period. The blood was tested for antibodies against S. neurona by the indirect fluorescent antibody test (IFAT).
Because all the foals had received colostrum from their dams, who carried antibodies to S. neurona, they had positive blood titers for the organism immediately after birth. However, the maternal antibodies decreased steadily as the youngsters reached weaning age. Thereafter, the untreated group of foals had a monthly increase in antibodies to S. neurona and by the end of the study 83 percent tested seropositive to S. neurona. In contrast, only 6 percent of the treated foals tested seropositive to S. neurona at the end of the study.
The researchers say these findings suggest that diclazuril could be used as a preventive treatment. “The [study] protocol prevents infection [with S. neurona],” says Pusterla. “This was measured indirectly using serology. We have no data showing that the overall attack rate for EPM---the percentage of horses infected with S. neurona that develop EPM---in young performance horses receiving a preventive dose is lower that in untreated horses, [but] you need to remember that the overall attack rate for EPM is relatively low [less than 1 percent].”
He stresses that the half-dose protocol would not be effective on active cases of EPM: “There is no clinical data showing that half the dose has any efficacy in horses diagnosed with EPM.”
Reference: “Use of daily diclazuril pelleted top dress for the prevention of Sarcocystis neurona infectionin foals,” American Association of Equine Practitioners 60th Annual Convention Proceedings, December 2014
This article first appeared in EQUUS issue #451, April 2015.