Studies on the Stage of Action of Ionophorous Antibiotics against Eimeria

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Abstract
Sporozoites of E. tenella were treated with different anticoccidial drugs in vitro and their subsequent viability was tested by inoculating them into chicken embryos. Monensin, salinomycin, lasalocid and arprinocid, at concentrations between 0.01-1.0 .mu.g/ml, greatly reduced sporozoite viability as judged by mortality, hemorrhage and specific lesions in the embryo chorioallantois. Monensin was also effective in reducing the viability of sporozoites of E. mivati and E. tenella as judged by oocyst production occurring in embryos; activity of monensin was greater against E. tenella than against E. mivati. Monensin (0.1 mg) inoculated into embryos inhibited development of E. tenella. Oocytes which were produced in the presence of the drug sporulated normally and sporozoites obtained from them were fully infective. Initiating treatment of chickens with monensin at different times in relation to infection, showed that the drug exerts its anticoccidial effect on the primary invasive stage and on the gametogonous stage of E. tenella and E. necatrix. The effect on gametogony was tested by initiating infections with 2nd generation merozoites of E. tenella. Significant reduction in oocyst production occurred in 3 of 4 strains of E. tenella tested. Medication with monensin initiated before merozoite inoculation was effective in inhibiting oocyst production, but medication starting 5 h after merozoite inoculation was not. This differed from the effects of arprinocid and sulfaquinoxaline which were expressed both before and 5 h after merozoite inoculation. Results show that the ionophorous anticoccidial drugs exert their anticoccidial action primarily against the invasive stage of Eimeria.