Differentiation of Neospora hughesi from Neospora caninum based on their immunodominant surface antigen, SAG1 and SRS2

Neospora hughesi is a newly recognised parasite that is closely related to Neospora caninum, and is a cause of equine protozoal myeloencephalitis. We have characterised two N. hughesi immunodominant tachyzoite antigens which exhibit antigenic and molecular differences from the homologous tachyzoite antigens on N. caninum. These antigens on N. hughesi are referred to as NhS AG1 and NhSRS2, using the same mnemonics as used for the N. caninum antigen s (NcSAG1 and NcSRS2), and are homologous to Toxoplasma gondii surface anti gen 1 (SAG1) and SAG1-related sequence 2 (SRS2). The NcSAG1 and NcSRS2 were antigenically conserved in six different N. caninum isolates from cattle a nd dogs. The two equine-derived Neospora isolates, one designated as N. hug hesi, were similar to each other but different from N. caninum. There was 6 % difference in amino acid identity between NcSAG1 and NhSAG1, whereas ther e was a 9% difference when NcSRS2 and NhSRS2 were compared. The polymorphis m of these genes and their corresponding proteins provide additional marker s which can be used to distinguish N, caninum from N. hughesi. (C) 1999 Aus tralian Society for Parasitology Inc. Published by Elsevier Science Ltd. Al l rights reserved.