Background :Neospora caninum is an important agent of abortion and reproductive loss in dairy cattle. The objective study was to detect N.caninum in bovine aborted fetous by molecular and mouse bioassay examination. Methods: During 2019-2022, 121 bovine aborted fetuses were collected from dairy farms in Khorasan Razavi province. The fetal brain samples were screened for detection of the parasite DNA by polymerase chain reaction assay (PCR). In addition, a portion of PCR-positive brain tissue was homogenized and inoculated in five Balb-c mice. All mice were sacrificed post six-week inoculation and examined by serology, microscopy, and PCR methods. The brains of infected mice were homogenized and the mouse bioassay was repeated two times. Results: The N.caninum DNA was detected in 19.8% (24) brain samples of bovine aborted fetuses. All inoculated mice were seronegative without clinical signs and cysts in the brain after two times bioassays, although three brain samples were PCR-positive. Conclusion: The results indicated that Neospora infection is common in dairy cattle in the Mashhad area and isolated Neospora has low pathogenicity in Balb/C mice.

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