Effects of Experimental Sarcocystis neurona-Induced Infection on Immunity in an Equine Model.

Sarcocystis neurona is the most common cause of Equine Protozoal Myeloencephalitis (EPM), affecting 0.5-1% horses in the United States during their lifetimes. The objective of this study was to evaluate the equine immune responses in an experimentally induced Sarcocystis neurona infection model. Neurologic parameters were recorded prior to and throughout the 70-day study by blinded investigators. Recombinant SnSAG1 ELISA for serum and CSF were used to confirm and track disease progression. All experimentally infected horses displayed neurologic signs after infection. Neutrophils, monocytes, and lymphocytes from infected horses displayed significantly delayed apoptosis at some time points. Cell proliferation was significantly increased in S. neurona-infected horses when stimulated nonspecifically with PMA/I but significantly decreased when stimulated with S. neurona compared to controls. Collectively, our results suggest that horses experimentally infected with S. neurona manifest impaired antigen specific response to S. neurona, which could be a function of altered antigen presentation, lack of antigen recognition, or both.

Efficacy of decoquinate against Sarcocystis neurona in cell cultures.

Decoquinate is a quinolone anticoccidial approved for use in the prevention of intestinal coccidiosis in farm animals. This compound has good activity against Toxoplasma gondii and Neospora caninum in cell cultures. The drug acts on the parasites' mitochondria. The activity of decoquinate against developing merozoites of 2 isolates of Sarcocystis neurona was examined in cell culture. Merozoite production at 10 days was completely inhibited when decoquinate was used at 20 or 240 nM. The IC50 of decoquinate was 0.5 ± 0.09nM for the Sn6 isolate of S. neurona from a horse and 1.1 ± 0.6 nM for the SnOP15 isolate of S. neurona from an opossum. Levamisole was toxic at 5 µg/ml and no synergism was observed when decoquinate was combined with levamisole and tested against the Sn3YFP isolate of S. neurona. Decoquinate was cidal for developing schizonts of S. neurona at 240 nM.

Molecular characterisation of a major 29 kDa surface antigen of Sarcocystis neurona.

A gene encoding a major 29 kDa surface antigen from Sarcocystis neurona, the primary causative agent of equine protozoal myeloencephalitis (EPM), was cloned, sequenced, and expressed as a recombinant protein. A cDNA library was prepared in the expression vector lambda ZAP from polyA+mRNA isolated from S. neurona merozoites cultivated in vitro. Random sequencing of 96 clones identified a clone of an abundant transcript having a translated amino acid sequence with 30% identity to the 31-kDa surface antigen of Sarcocystis muris cyst merozoites. Southern blot analysis indicated that the corresponding gene exists in low copy number within the S. neurona genome, but RNA blot analysis and other data indicated that the gene transcript is highly abundant. The sequence of the cDNA clone encoded an open reading frame specifying a polypeptide of 276 amino acids with a predicted size of 28.7 kDa. The deduced amino acid sequence displayed a hypothetical N-terminal signal peptide sequence followed by a polypeptide containing 12 cysteines. The coding region of the cDNA insert was subcloned into the expression vector pET14b, and a fusion protein expressed. The recombinant polypeptide was recognised by mAb 2A7 and mAb 1631, directed against a 29 kDa native protein found on the surface of cultured merozoites. Antibodies in serum and cerebrospinal fluid from a horse with EPM recognised a 29 kDa native protein of S. neurona merozoites and the 29 kDa recombinant protein. This S. neurona surface antigen is named SnSAG1.