Modifications in virulence of Neospora caninum after long term maintenance in murine macrophage

The aim of the present study was to attenuate Neospora caninum tachyzoites through continuous passage in murine macrophage and examine the virulence of attenuated tachyzoites in the mouse. The NC-1 reference isolate was 120 times passaged in the J774 cell line to produce attenuated strain. The virulence of tachyzoites was evaluated in a mouse model. Thirty-five female BALB/c mice randomly divided into seven groups. The first group, as control, was inoculated with culture media by subcutaneous injection. The second, third, and fourth groups received various dilutions (10×106, 5×106, 20×106) of acute NC-1 tachyzoites. Other groups were inoculated by attenuated tachyzoites with the same dilutions. The skin test and ELISA assay were performed to assess the cellular and humoral immunity responses. The mortality and clinical changes were noted daily. Notably, the significant difference was observed in the mortality rate of mice inoculated by acute and attenuated tachyzoites. Besides, serum antibody titers for N.caninum were higher in mice that received attenuated N.caninum. Also, mice that received live attenuated NC-1 showed significant footpad swelling. In conclusion, the present findings indicated that the culture of N.caninum in the J774 cells led to decrease of virulence of NC-1 significantly, which is confirmed here by the response of humoral and cellular immunity. This attenuated tachyzoites might be used as a candidate for further research to develop live vaccines against N.caninum.