Phenotypic characterisation of a Neospora caninum temperature-sensitive strain in normal and immunodeficient mice

The in vivo persistence, immunogenicity and pathogenicity of a recently des cribed temperature-sensitive (ts) strain from Neospora caninum, NCts-8, was investigated in normal and immunodeficient mice. Groups of BALB/c and SCID /Bg mice were infected s.c. with 5x10(6) wild-type NC-1, control NCts-8 (pa ss 0) or NCts-8 tachyzoites prepared at four in vitro passage levels (pass 7, 13, 21 and 28). For persistence and immunogenicity studies, BALB/c mice were bled and sacrificed at 4, 6 or 8 weeks p.i. Sera were analysed by IFAT and brain tissues examined for lesions by histology and tested for parasit e presence by PCR. For pathogenicity studies, SCID/Bg mice were monitored b y clinical signs and survival time. Results from parasite persistence exper iments demonstrated microscopic lesions and PCR positive brain tissues in N C-1 infected mice. In contrast, brain tissues from NCts-8-infected groups w ere consistently negative by histology and PCR. Based on IFAT titres, all p arasite strains were immunogenic, although parasite-specific IgG levels wer e lower in the NCts-8 infected groups. Results from pathogenicity studies i n SCID/Bg mice demonstrated a significantly (P < 0.0001) longer mean surviv al time in NCts-8 vs NC-1 infected groups. In addition, there was no signif icant difference in mean survival time between control NCts-8 and experimen tal passage NCts-8 infected mice. Collectively, these studies demonstrate t hat the NCts-8 strain maintains a stable phenotype following multiple passa ges in vitro, and possesses an attenuated, shorter persistence phenotype in vivo compared with the parental wild-type NC-1. (C) 1999 Australian Societ y for Parasitology Inc. Published by Elsevier Science Ltd. All rights reser ved.