Abstract:The 2-week-old chicks were randomly divided into 3 groups. The chicks of experimental group were injected subcutaneously with 0.1 mL inoculum suspension containing 107 nonreplicating Toxoplasma uracil auxotrophs (NRTUAs) per milliliter, and the chickens of the infected control group (RH group) and the blank control group were injected with equal doses of the virulent Toxoplasma gondii RH strains and PBS respectively. On the 4th day of infection, the heart, liver, spleen, lung, kidney, testis, bursa of Fabricius, thymus, brain, pectoral muscle and leg muscle of chicks in each group were collected, from which tissue DNA was extracted. The parasite load in each organ was detected by absolute fluorescence quantitative PCR. RNA was extracted from the remaining spleen of each group, and the transcription levels of IL-1β, IL-6, TNF-α, CCL26, STAT1, IRF1, IFN-γ and IL-10 in spleens were detected by relative fluorescent quantitative PCR. The results showed that on the 4th day of Toxoplasma infection, NRTUAs were not completely cleared from all organs, and the parasite load of NRTUAs group was lower than that of RH group except for liver, testis, leg muscle and brain. Among them, the parasite load of heart in NRTUAs group accounted for 1.14% of that in the RH group, the parasite load of pectoralis in the NRTUAs group accounted for 0.63% of that in the RH group, the parasite load of spleen in the NRTUAs group accounted for 10.01% of that in the RH group, the parasite load of lung in the NRTUAs group accounted for 0.18% of that in the RH group, the parasite load of kidney in the NRTUAs group accounted for 2.86% of that in the RH group, the parasite load of thymus in the NRTUAs group accounted for 1.33% of that in the RH group, and the parasite load of bursa of Fabricius in NRTUAs group accounted for 0.75% of that in the RH group. The measured immune-related cytokines were not significantly up-regulated or down-regulated in the spleens with NRTUAs stimulation, the expression levels of which were similar to those in the PBS group. Therefore, NRTUAs was a low pathogenic strain, which did not stimulate strong innate immune response or excessive production of pro-inflammatory and anti-inflammatory factors in chicks spleens.