Eimeria tenella: Parasite‐Specific Incorporation of 3H‐Uracil as a Quantitative Measure of Intracellular Development1

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Abstract
An assay has been developed using parasite‐specific incorporation of 3H‐uracil to assess the intracellular growth of Eimeria tenella in vitro. As shown by both scintillation counts and autoradiography, 3H‐uracil was incorporated specifically into intracellular parasites from the onset of infection and continued throughout development of the first generation schizonts. Mature schizonts and first generation merozoites did not continue to incorporate additional 3H‐uracil, indicating that RNA synthesis had halted in these stages. Based on these findings, a semi‐automated microscale uracil incorporation assay was developed to determine parasite viability. This method should be useful for biochemical studies with intracellular parasites and for screening compounds for anticoccidial activity. The ease, rapidity, and quantitative nature of this assay contrasts favorably with standard morphometric approaches of determining parasite development. In addition, parallel studies using host cell incorporation of 3H‐uridine have been introduced as a method of determining whether antiparasitic activity is direct or indirect in relation to effects on the host cell.