The invention relates to gene NcSAG4, the RNA messenger of said gene, the oligonucleotides designed based on the nucleotide sequence thereof or any of the fragments of same, and to protein NcSAG4 encoded thereby, or any of the recombinant forms of same, expression vectors, and host cells containing same, for the purpose of diagnosis and vaccination for the prevention of neosporosis. The invention also relates to the use thereof as a specific marker of the N. caninum bradyzoite stage, for the analysis of the pathogenesis or the effectiveness of vaccines against the establishment of a chronic infection in the intermediate host.