Dose-dependent effects of experimental infection with the virulent Neospora caninum Nc-Spain7 isolate in a pregnant mouse model

- Several doses from 2 × 106 to 102 Nc-Spain7 tachyzoites were subcutaneously inoculated to BALB/c mice.
- No differences were observed after reduction of tachyzoite numbers from 2 × 106 to 105.
- Infection with only 100 tachyzoites could induce high mortality in both dams and offspring.
- A lowered dose of 105 tachyzoites could be used in further proof-of-concept experiments.

Pregnant BALB/c mice have been widely used as an in vivo model to study Neospora caninum infection biology and to provide proof-of-concept for assessments of drugs and vaccines against neosporosis. The fact that this model has been used with different isolates of variable virulence, varying infection routes and differing methods to prepare the parasites for infection, has rendered the comparison of results from different laboratories impossible. In most studies, mice were infected with similar number of parasites (2 × 106) as employed in ruminant models (107 for cows and 106 for sheep), which seems inappropriate considering the enormous differences in the weight of these species. Thus, for achieving meaningful results in vaccination and drug efficacy experiments, a refinement and standardization of this experimental model is necessary. Thus, 2 × 106, 105, 104, 103 and 102 tachyzoites of the highly virulent and well-characterised Nc-Spain7 isolate were subcutaneously inoculated into mice at day 7 of pregnancy, and clinical outcome, vertical transmission, parasite burden and antibody responses were compared. Dams from all infected groups presented nervous signs and the percentage of surviving pups at day 30 postpartum was surprisingly low (24%) in mice infected with only 102 tachyzoites. Importantly, infection with 105 tachyzoites resulted in antibody levels, cerebral parasite burden in dams and 100% mortality rate in pups, which was identical to infection with 2 × 106 tachyzoites. Considering these results, it is reasonable to lower the challenge dose to 105 tachyzoites in further experiments when assessing drugs or vaccine candidates.