Antibody index and specific antibody quotient in horses after intragastric administration of Sarcocystis neurona sporocysts

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. Animalls - 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 LISA 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 1,37 ± 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.