Abstract Objective—To investigate the use of a specific antibody index (AI) that relates Sarcocystis neurona–specific IgG quotient (QSN) to total IgG quotient (QIgG) for the detection of the anti–S neurona antibody fraction of CNS origin in CSF samples obtained from horses after intragastric administration of S neurona sporocysts. Animals—18 adult horses. Procedures—14 horses underwent intragastric inoculation (day 0) with S neurona sporocysts, and 4 horses remained unchallenged; blood and CSF samples were collected on days – 1 and 84. For purposes of another study, some challenged horses received intermittent administration of ponazuril (20 mg/kg, PO). Sarcocystis neurona–specific IgG concentrations in CSF (SNCSF) and plasma (SNplasma) were measured via a direct ELISA involving merozoite lysate antigen and reported as ELISA units (EUs; arbitrary units based on a nominal titer for undiluted immune plasma of 100,000 EUs/mL). Total IgG concentrations in CSF (IgGCSF) and plasma (IgGplasma) were quantified via a sandwich ELISA and a radial immunodiffusion assay, respectively; QSN, QIgG, and AI were calculated. Results—Following sporocyst challenge, mean ± SEM SNCSF and SNplasma increased significantly (from 8.8 ± 1.0 EUs/mL to 270.0 ± 112.7 EUs/mL and from 1,737 ± 245 EUs/mL to 43,169 ± 13,770 EUs/mL, respectively). Challenge did not affect total IgG concentration, QSN, QIgG, or AI. Conclusions and Clinical Relevance—S neurona–specific IgG detected in CSF samples from sporocyst-challenged horses appeared to be extraneural in origin; thus, this experimental challenge may not reliably result in CNS infection. Calculation of a specific AI may have application to the diagnosis of S neurona–associated myeloencephalitis in horses.