Flourescent antibody test (FAT) was applied to determine the cross-reactivities of
monoclonal (mAb), polyclonal (pAb) antibodies to Neospora, Toxoplasma and Cryptosporidium
and antisera from cattle naturally infected with Neospora canium against antigens from a number
of sources. Both mAb and pAb to Neospora reacted strongly (FAT titre up to 2560) with the
homologous antigens and demonstrated weak titre (80) or no reaction with both Toxoplasma and
Cryptosporidium antigens. Also mAb and pAb to Toxoplasma gondii reacted at titres of 80 - 640
with homologous antigens and at titres of 10-40 with N. caninum. No cross-reactions with either
mAb or pAb antibodies to N. caninum and T. gondii were observed with Cryptosporidium
parvum. The same results were observed with C. parvum mAb when tested with both N. caninum
and T. gondii antigens. Sera from cattle naturally infected with N. caninum had titres ranging
from 80- 640 with N. caninum antigens, and 10- 40 with T. gondii and C. parvum antigens. At low
dilutions, the complete surfaces of Neospora and Toxoplasma parasites were fluorescent, while
in higher dilutions only dotted fluorescence appeared on the apical complex. These results
indicated the presence of cross-reactivity between Neospora and Toxoplasma but not with
Cryptosporidium. Accordingly the recommended cut-off antibody titre for diagnosis of neosporosis
is 80.