Experimental infection of pregnant sheep with Toxoplasma gondii: pathological and immunological observations on the placenta and foetus.

The pathogenesis of Toxoplasma gondii infection in pregnant sheep was investigated. In one experiment, sheep were infected at 40, 60 and 90 days gestation and examined 30 days later, while in a second experiment, pregnant sheep were infected at either 60 or 90 days gestation and examined 10, 15 and 20 days later. By 10 days post infection (dpi), the parasite had invaded the gravid uterus and caused focal damage in the caruncular septa and, by 15 days, the degenerative changes had spread to the crypt wall and trophoblast. At this time, toxoplasma were isolated from all challenged foetuses. Foetuses did not produce any significant immune response until 20 dpi when increased numbers of IgM positive cells were found in lymphoid tissues. However, at 30 dpi, the numbers were normal while numbers of IgG positive cells and circulating IgG were increased. It was concluded that, while the foetal immune system need not be fully mature to respond to T. gondii infection, its ability to mount a response is limited at 70 days gestation but becomes increasingly competent after this. Furthermore, while it is able to respond more comprehensively with increasing maturity, maturation itself appears to be accelerated by the stimulus of infection.