BACKGROUND: Neospora caninum is an apicomplexan protozoa, which causes abortion in cattle worldwide. OBJECTIVES: The present study was designed to estimate the rate of transplacental transmission of N.caninum to aborted fetuses in seropositive dams in dairy farms in Mashhad area, Iran. METHODS: Two hundred blood samples of dam with aborted bovine fetuses were collected from dairy farms from 2008 to 2010. First , the maternal sera in each aborted case were tested by using ELISA method, and then the brain samples of aborted fetuses were examined for detecting DNA of N. caninum, using the PCR method. RESULTS: In this study, 38 (19%) of dams\' sera showed seropositive reaction against N.caninum infection, and also 23 (10.5%) aborted brain fetuses showed positive reaction by PCR. A significant association was observed between N.caninum-serapositie dams and infected aborted fetuses (p<0.0001). In the present study, 20 (86%) infected fetuses were aborted from seropositive, and 3 (13%) of the fetuses were aborted from seronegative dams. CONCLUSIONS: Based on the results of molecular and serology examination, the rate of transplacental transmission of N. caninum infection in dairy cattle were calculated as 52

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