Chicken coccidiosis is one of the most feared parasite infections in the poultry industry, and it can cause the death of about 50% of the affected individuals within a few days after infection. In chronic infection, diarrhea, emaciation, anastasia are noted, causing an enormous damage to the breeding of broilers in the poultry industry. In particular, chicks in the stage of young chicks and middle chicks, important periods for meat chickens, are vulnerable to the disease, and once infection has occurred, it is very difficult to root coccidium out. Currently, though antibiotics and chemical synthetic agents play important roles as preventive measures against this disease, the need to administer them prior to the onset of the disease and in succession leads to higher cost in production and side effects, due to successive use are concerned.
Furthermore, in order to solve these problems, various live vaccines have been developed. Some of them are coccidia with moderate pathogenicity, which per se form significant lesions. Recently, a vaccine that uses precocious strains and that is effective to some extent was developed. However, as there is residual pathogenicity in this vaccine too, there remains a concern over the mutation of the vaccine strain to the wild type strain. For the inoculation of a vaccine strain, the inoculation schedule together with other vaccines must be taken into account and, as the use of antibacterial agents is limited, it is feared that other infections may occure.
Concerning about side effects by these drugs and live vaccines, persistent efforts have been made to search for protective antigens for coccidiosis in order to develop safer component vaccines and recombinant vector vaccines, but there are no reports that indicate the achievement of sufficient effects.
Currently, many of the coccidium antigens have been analyzed to the level of sequence analysis, and the antigens have been localized. A lot of genes of antigen proteins have been found such as Ta4, an oocyst antigen of E. tenella (U.S. Pat. No. 5,028,694) ns GX3262 (U.S. Pat. No. 5,122,471), Mzp5-7 (U.S. Pat. No. 5,403,581), EtMIC5 (Mol. Biochem. Parasitol., 15:91-102, 2000), EtMIC2 (Mol. Biochem. Parasitol., 79:195-206, 1996), cMZ8 of E. acervulina (Infect. Immun., 57:2434-2440, 1989), and EAMZ 250 (Immunology, 71:127-132, 1990). Though some of these antigen genes have been integrated into fowlpox virus and vaccinia virus (U.S. Pat. No. 5,814,320, U.S. Pat. No. 5,403,581), they have not been confirmed to give effective vaccines.
This results from the fact that antibody alone cannot protect against protozoan infections, in addition to the difference in genome size of virus and protozoa or the difference in the morphology in the infected organism.