<I>Neospora hughesi</I> 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 <I>N. hughesi</I>. Only the gamma-IFNKO mice developed severe clinical disease following infection with <I>N. hughesi</I>. The most severe lesions were in the hearts of these mice. Two dogs fed the brains of mice, shown to contain <I>N. hughesi</I> tissue stages by cell culture and g-IFNKO mouse bioassay, did not shed <I>N. hughesi</I> 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 <I>N. hughesi and N. caninum</I>. The newly defined proteins of <I>N. hughesi</I> 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 <I>N. hughesi</I> as a separate species from <I>N. caninum</I> and describes novel methods of distinguishing between the two. / Master of Science