Attenuation of Eimeria necatrix and E. tenella of U.S. origin by serial embryo passage

Two lines of Eimeria tenella (PS and FS65) and one line of E. necatrix (FS144) of U.S. origin were passaged in chicken embryos. The embryo-passaged line of E. tenella PS was significantly less pathogenic than the parent line when tested in chickens after 33 embryo passages. After 29 embryo passages, E. tenella FS65 was just as pathogenic to chickens as the parent line. A comparison of the immunogenicity of the embryo-adapted lines and the parent lines in chickens showed that embryo-passaged E. tenella PS and E. tenella FS65 were as immunogenic as parent lines. The embryo-passaged line of E. necatrix FS144 was significantly less pathogenic when tested in chickens after 30 embryo passages. Although there was also some loss of immunogenicity in the embryo-passaged line of E. necatrix, immunity to this parasite could be achieved by increasing the dose of oocysts.