The development of immune responses in Balb/c mice following inoculation with attenuated or virulent Neospora caninum tachyzoites

Balb/c mice were inoculated intraperitoneally (i.p.) with either 5 × 10⁶ live virulent (group 1) or 5 × 10⁶ live attenuated (group 2) tachyzoites, or Vero cells (group 3). Animals were killed at 0, 14, 28 and 42 days post-inoculation (p.i.), with the remaining mice receiving a lethal challenge on day 48 p.i. Serum, spleen and brain samples were collected post-mortem to examine humoral and cell-mediated immune responses as well as pathological lesions and to quantify parasite loads. On day 14 p.i. group 2 (attenuated) demonstrated statistically significant ( P  < 0·001) lower levels of mean morbidity and weight loss, while also showing significantly ( P  = 0·01) higher levels of splenocyte proliferation and IFN-γ production ( P  = 0·003), compared to group 1 (virulent). Histology of brain samples showed milder lesions and a lower incidence of positive immunohistochemistry, demonstrating tachyzoites and tissue cysts, and statistically significant ( P  = 0·03) lower mean burdens of parasite DNA in group 2 (attenuated) compared to group 1 (virulent). All mice in group 2 were protected following challenge on day 48 p.i. whereas naïve control mice succumbed to the challenge. No mice from group 1 (virulent) survived beyond day 24 p.i. so they were not included in the challenge.