Selection and development of a Spanish precocious strain of Eimeria necatrix.
A precocious line of Eimeria necatrix (PEN E-281/20) with an abbreviated life cycle was derived from a Spanish field strain (E-281) by repeated passages of the first shed oocysts recovered from the caecal contents of previously infected chickens. After 20 passages, the 'useful' prepatent period (time from infection to obtaining sufficient oocysts to repassage) of the parasite was reduced by 30 h (from 148 to 118 h). The earliest oocysts found in the caecal content were 114 h postinfection (hpi), on the 19th passage. The pathogenicity of the parasite was reduced in comparison with the parent strain, its immunogenicity against homologous and heterologous strains was maintained and its reproductive capacity was similar to or higher than that of the parent strain. Compared with the parent strain, the second generation of schizonts was reduced in size (reduced pathogenicity), third generation schizonts were bigger and with more merozoites (maintenance of the reproductive index) and the life cycle progressed faster from the second generation of schizonts (reduction of prepatent period). Complete second schizogony, from trophozoites to mature schizonts was observed frequently in the caeca of birds infected with both parent and precocious lines.