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Neospora hughesi is a recently described cause of equine protozoal myeloencephalitis (EPM). In the present study, we examined the susceptibility of BALB/c gamma-interferon gene knockout (gamma-INFKO), BALB/c, CD-1, and C57BL/6 strains of mice and gerbils to infection with tachyzoites of the Nh-A1 strain of N. hughesi. Only the gamma-IFNKO mice developed severe clinical disease following infection with N. hughesi. The most severe lesions were in the hearts of these mice. Two dogs fed the brains of mice, shown to contain N. hughesi tissue stages by cell culture and g-IFNKO mouse bioassay, did not shed N. hughesi oocysts over a 23 day observation period. We report important differences between the nucleotide and deduced amino acid sequences of the dense granule proteins GRA6 and GRA7 of N. hughesi and N. caninum. The newly defined proteins of N. hughesi are referred to as NhGRA6 and NhGRA7. From analysis of the sequences we found that there is a 14.8% difference in deduced amino acid sequence between NhGRA7 and NcGRA7, and a 4% difference between NhGRA6 and NcGRA6 in areas that could be compared. This thesis supports the identification of N. hughesi as a separate species from N. caninum and describes novel methods of distinguishing between the two.