Chasing the golden egg: vaccination against poultry coccidiosis.

Eimeria species, of the Phylum Apicomplexa, cause the disease coccidiosis in poultry, resulting in severe economic losses every year. Transmission of the disease is via the faecal-oral route, and is facilitated by intensive rearing conditions in the poultry industry. Additionally, Eimeria has developed drug resistance against most anticoccidials used today, which, along with the public demand for chemical free meat, has lead to the requirement for an effective vaccine strategy. This review focuses on the history and current status of anticoccidial vaccines, and our work in developing the transmission-blocking vaccine, CoxAbic (Netanya, Israel). The vaccine is composed of affinity-purified antigens from the wall-forming bodies of macrogametocytes of Eimeria maxima, which are proteolytically processed and cross-linked via tyrosine residues to form the environmentally resistant oocyst wall. The vaccine is delivered via maternal immunization, where vaccination of laying hens leads to protection of broiler offspring. It has been extensively tested for efficacy and safety in field trials conducted in five countries and involving over 60 million offspring chickens from immunized hens and is currently the only subunit vaccine against any protozoan parasite to reach the marketplace.

The development of the macrogamete and oocyst wall in Eimeria maxima: immuno-light and electron microscopy.

We have identified, and followed the development of three macrogamete organelles involved in the formation of the oocyst wall of Eimeria maxima. The first were small lucent vacuoles that cross-reacted with antibodies to the apple domains of the Toxoplasma gondii microneme protein 4. They appeared early in development and were secreted during macrogamete maturation to form an outer veil and were termed veil forming bodies. The second were the wall forming bodies type 1, large, electron dense vacuoles that stained positively only with antibodies raised to an enriched preparation of the native forms of 56 (gam56), 82 (gam82) and 230 kDa (gam230) gametocyte antigens (termed anti-APGA). The third were the wall forming bodies type 2, which appeared before the wall forming bodies type 1 but remain enclosed within the rough endoplasmic reticulum and stained positively with antibodies raised to recombinant versions of gam56 (anti-gam56), gam82 (anti-gam82) and gam230 (anti-gam230) plus anti-APGA. At the initiation of oocyst wall formation, the anti-T. gondii microneme protein 4 positive outer veil detached from the surface. The outer layer of the oocyst wall was formed by the release of the contents of wall forming bodies type 1 at the surface to form an electron dense, anti-APGA positive layer. The wall forming bodies type 2 appeared, subsequently, to give rise to the electron lucent inner layer. Thus, oocyst wall formation in E. maxima represents a sequential release of the contents of the veil forming bodies, wall forming bodies types 1 and 2 and this may be controlled at the level of the rough endoplasmic reticulum/Golgi body.

Eimeria maxima: identification of gametocyte protein antigens.

The antigenicity of Eimeria maxima gametocyte proteins during the course of an infection and when injected into mice and rabbits was demonstrated using the Western blotting technique. Serum taken from chickens at various times postinfection reacted to a few gametocyte proteins, with the strongest reactivity seen with serum taken 14-days postinfection. Two major antigens having molecular weights of 56,000 and 82,000 were consistently detected by these sera. Using immune rabbit or mouse sera to whole gametocyte detergent extracts, the 56,000 and 82,000 molecular weight proteins were again the immunodominant antigens, despite their representing only a small proportion of the extract which was used to immunize the animals. These results, together with those obtained by Rose (1971) using recovered chicken serum to passively immunize chickens, indicate that these two gametocyte antigens may play a role in protective immunity to E. maxima.