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Sera from 112 mares from 5 horse-breeding farms was examined for the presence of antibodies to Neospora caninum and Toxoplasma gondii by an indirect fluorescent antibody test (IFAT), as well as from dogs and cattle present on these properties for the presence of antibodies to N. caninum. Among the 112 mares, 35 had a history of reproductive problems in the last breeding season and 77 had no reproductive problems. The rates of seroprevalence of N. caninum in mares with and without a history of reproductive problems were 25.71% and 6.49% and from T. gondii 2.85% and 1.29%, respectively. In dogs and cattle, the rates of seroprevalence of N. caninum were 10.52% and 15.55%, respectively. A positive correlation was found between the presence of antibodies against N. caninum (p=0.010) in mares and the occurrence of reproductive problems using the Fisher's exact test. Significantly higher seroprevalence for N. caninum in mares was observed on the farm that had seropositive dogs (p=0.018). Cattle on this farm were also seropositive. No significant differences in seropositivity were found on farms where dogs were seronegative, or absent. This result suggests, for the first time, the presence of seropositive dogs as a risk factor for N. caninum in mares and the necessity for further investigation of the epidemiology of this parasite in horse-breeding farms with reproductive problems and the presence of cattle and dogs. This is the first report on the occurrence of antibodies against N. caninum in horses from the state of Santa Catarina, Brazil.