PASSAGES OF OOCYSTS COLLECTED AFTER PEAK OOCYST PRODUCTION

A line of Eimeria necatrix was selected by repeated passages of oocysts that were collected after peak oocyst production from feces or cecal contents of previously infected chickens. When compared with the parent strain, the new line of E. necatrix after 16 successive passages had the following characteristics: (1) the peak of oocyst production was delayed by 2 days; (2) the sizes of 9 endogenous development stages became larger; and (3) the reproductive capacity and the immunogenicity were both enhanced. This new line of E. necatrix may be used for the development of new coccidiosis vaccines. Immunization with attenuated vaccines is increasingly play- ing an important role in the control of coccidiosis in the poultry industry because of resistance and residue issues associated with chemotherapy (Williams, 1994). The attenuated live anti- coccidial vaccines, Paracox and Livacox, are widely used. The former was obtained from precocious lines, while the latter was acquired from precocious and egg-passaged lines. The preco- cious line of Eimeria tenella was obtained by repeated passages of oocysts first collected from feces of previously infected chickens (Jeffers, 1975). Compared with the parent strains, the precocious lines had a reduced prepatent period, markedly less pathogenicity, and much lower reproductive potential, but re- tained immunogenicity (Jeffers, 1975; McDonald et al., 1982; McDonald and Ballingall, 1983; Shirley and Bellatti, 1984; Shirley et al., 1984, 1986; Johnson et al., 1986; Kawaguchi et al., 1988; Long and Johnson, 1988; Montes et al., 1998). Suo et al. (1996) selected a line of E. tenella by repeated passages of oocysts collected after peak production from the cecal contents of previously infected chickens. After 14 suc- cessive passages, the peak of oocyst production was delayed by 1 day in comparison with the parent strain. Moreover, oocyst production at 312-352 hr postinoculation (PI) was 9-20 times higher than that of the parent strain. In addition, the immuno- genicity of this line of E. tenella in chickens was greater than that of the parent strain or a precocious line. Less pathogenic than E. tenella^ Eimeria necatrix is the sec- ond most pathogenic species of coccidia in chickens. Eimeria necatrix develops first in the small intestine (the first and second generation schizonts) and later in the cecum (the third genera- tion schizonts, gamonts, gametes, and oocysts) (Levine, 1985). The objective of the present study was to obtain a new line of E. necatrix via repeated inoculation of oocysts produced after peak oocyst production and to observe the changes in biological characteristics such as pathogenicity, reproductive ability, and immunogenicity, with emphasis on determining the potential for the development of a more effective vaccine.