Combination of cell culture and qPCR to assess the efficacy of different anticoccidials on Eimeria tenella sporozoites.

Three in vitro studies were designed to develop an assay for anticoccidial efficacy by use of laboratory (Houghton) and field (T-376) Eimeria tenella strains. In study (1), minimum inhibitory concentrations (MICs) of monensin (Mon), maduramicin (Mad), salinomycin (Sal), and lasalocid (Las) were determined that are able to inhibit more than 50% of sporozoites in host cell (Madin-Darby bovine kidney (MDBK)) penetration and more than 95% of Houghton sporozoites development to mature merozoites (treatment time 24 h) using quantitative real-time PCR (qPCR). MICs were 0.5, 2.5, 1, and 0.5 μg/ml for Mon, Mad, Sal, and Las, respectively. Applying the previous MIC on T-376 strain revealed a different sensitivity profile. Mad reduced T-376 gene copies by only 89.3% after 96 h of infection. In study (2), Houghton strain sporozoites were incubated with MIC of the different tested ionophores for 2 and 4 h, respectively; afterwards, their ability to invade MDBK cells was determined using phase-contrast microscopy and qPCR. Treatment of sporozoites with ionophores for 4 h resulted in significant inhibition of invasion compared with non-treated parasites as assessed both by microscopy as well as qPCR. Inhibition rates for Mon, Mad, Sal, and Las were 90.2, 75.0, 88.3, and 82.6% using phase-contrast microscopy and 83.9, 81.4, 85.8, and 75.4% using qPCR, respectively. T-376 sporozoite invasion into MDBK cells was reduced to 48.9% by Mad. Study (3) was conducted to determine inhibition exerted by toltrazuril (Tol). Tol at 5 μg/ml reduced reproduction of Houghton strain by 95%, whereas T-376 was only reduced by 86.5%. The presented experiments indicate that infectivity inhibition of sporozoites incubated for 4 h with anticoccidials and development inhibition after 96 h of infection by qPCR are suitable means to assess sensitivity of E. tenella strains to anticoccidials.