Neospora caninum experimental infection in pregnant sheep as a model for exogenous transplacental transmission for ruminant neosporosis

Abstract

The availability of well-defined ruminant models of infection is one key factor for studying the pathogenesis and to evaluate vaccine candidates and therapeutics in bovine neosporosis. Although laboratory (mice) models have been broadly employed, due to the evident differences between mice and ruminants, results obtained could not be always extrapolated to cattle. Sheep has several advantages over cattle as an experimental animal model including size, length of gestation and cost. Furthermore, recently the awareness and importance of Neospora caninum as an abortifacient in small ruminants has increased. The aim of this work was to investigate experimental infection by N. caninum in pregnant sheep as a valid model for exogenous transplacental transmission for ruminant neosporosis. Thus, pregnant ewes were intravenously inoculated at days 40, 90 and 120 of gestation, with 106 tachyzoites of Nc-Spain7 isolate. Infection during first and second third of gestation resulted in abortion of all foetuses, although it happened earlier at day 40 (19-21 days post-infection) than at day 90 (34-48 days post-infection). Lambs born from ewes infected during the third gestation period were all viable, although some of them were very weak. Placentas and foetuses from all infected ewes showed histological lesions characteristic of neosporosis. In the foetuses, lesions were found mainly in liver and lung while they become more frequent in brain and skeletal muscle as the pregnancy advanced. In general, lesions were mainly necrotic in foetal samples from ewes infected in the first trimester and showed more conspicuous infiltration of inflammatory cells on those infected in the second and last trimester. The results obtained so far show that the infection had a similar outcome in experimentally inoculated sheep to that described in natural bovine or ovine neosporosis.