Primary and secondary responses of the ovine lymph node to Toxoplasma gondii: cell output in efferent lymph and parasite detection.

Efferent lymphatic cannulation was used to study the dissemination of strain S48 of Toxoplasma gondii and the cell output from the prefemoral lymph node, after infection of both "naive" and vaccinated sheep. In the former the mean cell output decreased for 3 days before reaching a peak at 11 and 12 days, but in vaccinated ewes a similar drop in cell output and subsequent peak occurred significantly earlier, at 24 h and 5 days, respectively. The cellular response in both types of sheep was largely due to a marked increase in blast cells. The detection of live toxoplasms and parasite DNA by mouse inoculation and the polymerase chain reaction, respectively, gave similar results; the parasite was demonstrated in lymph from days 3 to 12 during a primary infection but with a sharp cut-off after day 9 coinciding with the peak blast cell response. Very little evidence of T. gondii was found in lymph of vaccinated sheep after challenge. Immunity, which is thought to be largely T-cell mediated and is sustained without continuous antigenic stimulation, suppresses dissemination of the parasite in the lymph and therefore to other sites, which might include the gravid uterus.