A POLYGEN (TM)-adjuvanted killed Neospora caninum tachyzoite preparation failed to prevent foetal infection in pregnant cattle following i.v./i.m. experimental tachyzoite challenge

Cattle immunised with a POLYGEN(TM)-adjuvanted killed Neospora caninum tach yzoite preparation were previously shown to produce interferon (IFN)-gamma at levels similar to those of tachyzoite-infected cattle. In view of the cr itical role of IFN-gamma in resistance of mice to N. caninum infection, the se results prompted us to test the POLYGEN(TM)-adjuvanted preparation in pr egnant cattle to determine whether it will be able to prevent foetal infect ion following an experimental tachyzoite challenge. Seven heifers were immu nised at 35 and 63 days of gestation with the POLYGEN(TM)-adjuvanted prepar ation, while five heifers were inoculated with POLYGEN alone at the same da ys of gestation. Four weeks later, all heifers were challenged with a combi ned i.v./im. inoculation of tachyzoites. The same challenge was given to se ven unimmunised heifers at the same stage of gestation. An additional unimm unised heifer was inoculated with uninfected monolayer cell culture materia l. All challenged heifers, immunised and unimmunised, had infected foetuses . Immunised heifers developed both parasite-specific humoral and cellular i mmune responses, characterised by increased IFAT titres, a predominant IgG1 response, elevated lymphoproliferative response and IFN-gamma production. Following tachyzoite challenge, they developed an anamnestic humoral respon se and produced similar amounts of IgG1 and IgG2 antibodies, but did not ha ve an anamnestic cellular immune response. The lack of anamnestic cellular immune response and/or the large i.v/i.m tachyzoite inoculum may have contr ibuted to the failure of the preparation. (C) 2000 Australian Society for P arasitology Inc. Published by Elsevier Science Ltd. All rights reserved.