Experimental induction of equine protozoan myeloencephalitis (EPM) in the horse: effect of Sarcocystis neurona sporocyst inoculation dose on the development of clinical neurologic disease.

This research examined the impact of varying doses of Sarcocystis neurona sporocysts on the development of neurological disease in horses. It found that the severity of clinical signs, seroconversion times and presence of microscopic lesions varied according to the dose.

Study Design

• Twenty-four seronegative weanling horses underwent stress from transportation and were then divided into six treatment groups.
• Each group was given a different dose of S. neurona sporocysts, ranging from 10(2) to 10(6), via a nasogastric tube.
• A placebo group was also included for comparison.
• The horses were housed indoors for the duration of the study.

Observations and Measurements

• Weekly neurologic examinations were conducted by an observer who was unaware of the treatment each horse had received.
• Blood was collected weekly for antibody analysis.
• Cerebrospinal fluid was taken before the inoculation and before euthanasia to evaluate antibodies for S. neurona.

Findings and Conclusions

• Varied results were observed among the groups based on the dose of S. neurona sporocysts administered.
• Seroconversion, or the time it took for the horses’ bodies to produce antibodies to the infection, differed among the groups and was as quick as 14 days in the group given the highest dose of sporocysts.
• All groups displayed mild to moderate neurological symptoms, with the most consistent symptoms displayed in the highest dose group.
• Microscopic lesions indicative of S. neurona infection were found in four of the twenty horses that had been fed sporocysts.
• Parasites were not detected in equine tissues using light microscopy, immunohistochemistry, or bioassay in gamma-interferon gene knockout mice.
• Horses in the control group remained seronegative for the duration of the study and showed no histological signs of protozoal infection.

The study thus shows that the dose of S. neurona sporocysts has a direct impact on the development and severity of neurological disease in horses, which also raises questions about the potential for varying results in other species.