Protective efficacy of the NcGRA7-deficient parasite as a live attenuated vaccine against <i>Neospora caninum</i> infection in mice

Live vaccination is the most protective method against bovine neosporosis, which is the major cause of bovine abortion globally. Here, the Neospora caninum parenteral strain Nc1 and NcGRA7-deficient N. caninum (NcGRA7KO), which is less virulent in mice, were evaluated as potential live vaccines. BALB/c mice were subcutaneously inoculated with high (1 × 10⁵) or low (1 × 10⁴) doses of tachyzoites. At high doses, Nc1-inoculated female mice presented decreased body weight gain and increased clinical signs and died before challenge infection with green fluorescent protein (GFP)-expressing Nc1 (Nc1-GFP), whereas NcGRA7KO-inoculated animals exhibited increased survival before and after challenge infection. Although inoculation of female mice with Nc1 or NcGRA7KO resulted in a lower brain parasite number of challenged Nc1-GFP than in noninoculated animals, the total brain parasite burden in NcGRA7KO-infected mice decreased compared with that in Nc1-infetced animals. At low dose of NcGRA7KO, increased survival rates of mice and lower total brain parasite number were observed compared with high dose of NcGRA7KO. In male mice, a significant lower brain parasite burden of Nc1-GFP was observed in both high and low doses of NcGRA7-inoculated mice, and the total parasite number in the brains of low dose of NcGRA7KO-inoculated animals was lower than that in the brains of high dose of NcGRA7KO-inoculated or noninoculated animals. In conclusion, these results suggest that NcGRA7KO parasites have potential for use as a live vaccine against N. caninum infection.