Background: Neospora caninum is one of the most important causative agent of abortion in cattle. One potential approach to interrupt the diaplacentally dictated transmission mode is to treat congenitally infected off-spring animals. This approach was preliminarily studied in two generations of experimentally infected mice. Toltrazuril, a symmetrical triazinone, is registered under the original name Baycox for the control of coccidiosis in poultry, piglets, calves and lambs. Methods: In our study, C57BL/6-mice were infected with N. caninum tachyzoites during pregnancy, yielding a transplacental infection of developing foetuses. Subsequently, congenitally infected newborn mice were perorally treated with toltrazuril/Baycox 5% suspension (or placebo) 31 mg/kg on days 3, 10, 17 and 24 post partum. Infection status was monitored serologically (ELISA) and post-mortem by molecular analyses (PCR) of brain and other organs. Seropositive animals of all groups (vero- and placebo-treated infected and non-infected animals) were mated thus as to generate a second generation of off-spring. These were parasitologically and serologically identically monitored as the first generations. Results and Conclusions: Both toltrazuril and placebo treatment had no negative effect on newborns, as non-infected treated pups developed normally without differences in mortality and morbidity to matching non-treated control animals. Application of toltrazuril was significantly (p < 0.01) able to delay the outbreak of neosporosis in newborn mice, when compared to placebo-treated infected controls. The number of diseased and Neospora-positive pups was markedly reduced after toltrazuril-treatment when compared to other groups. Treatment also resulted in the highest antibody response. All animals of the second generation that were obtained from congenitally infected first generation mothers demonstrated a seropositive status post partum, but diaplacental transmission did not occur, either in off-spring derived from treated or from placebo-treated mothers. In conclusion, we could show that treatment with toltrazuril had a high impact on the course of infection in the first generation of congenitally N. caninum-infected newborn mice. The murine model, however, appeared not suitable to study second generation effects, as the parasite was not efficiently transmitted congenitally into these animals as shown by negative PCR results in all groups. Long-term studies in calves (several generations) are necessary to clear the questions if successive treatment schedules will lead to parasite clearance in breeding lines.

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