Parasites and animals.

Eimeria tenella (XJ strain) and E. maxima (BJ strain) were maintained by passaging in coccidian-free, 2- to 5-week-old Arbor Acres (AA) broilers. The procedures for collection, purification, and sporulation were carried out as previously described (40).

Toxoplasma gondii (RH strain) was maintained by serial passages in African green monkey kidney (Vero) cells (Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences) in Dulbecco modified Eagle medium (DMEM) supplemented with fetal bovine serum (FBS) (10%, vol/vol), penicillin (200 U ml⁻¹), and streptomycin (20 mg ml⁻¹) in a humidified atmosphere of 5% CO₂ at 37°C.

One-week-old specific-pathogen-free (SPF) chickens were purchased from Merial Animal Health Co., Ltd. (Beijing, China) and were fed a pathogen-free diet and water ad libitum.