Characteristics of the organism and its antigens

The complex life cycle is monoxenous and sporo-zoan. Within the host, a period of asexual replication (schizogony) is followed by a sexual phase (gametogony). The number of cycles of schizogony, usually fixed, varies with the species but is commonly three or four, the last being followed by the formation of gametes (Figure 1). After fertilization, the zygote, protected within a resistant shell (oocyst), is voided in the feces. A period of maturation outside the host (sporogony), including a meiotic nuclear division, is necessary for the oocyst to become infective. Transmission is fecal-oral.

Infection causes enteritis of varying severity, depending upon the number and the biology of the

Figure 1 Life cycle of Eimeria species. (Reproduced with permission of Springer-Verlag GmbH & Co. KG from Rose ME (1985) The Eimeria. In: Parkhouse RME (ed) Parasite antigens in protection, diagnosis and escape. Current Topics in Microbiology and Immunology 120: 7-17.

Eimeria spp. ingested. Those that develop in crypt epithelium, resulting in hemorrhage, are highly pathogenic and may cause serious mortality. Anorexia and stunted growth are common consequences of infection.

The various developmental stages of the parasite are morphologically distinct and possess both stage-specific and common antigens. Some of these have been purified and characterized, mostly those from species parasitizing the domestic fowl, such as E. tenella and E. maxima. The emphasis has been on the initial infective stage, the easily accessible sporo-zoite; this is a major target for immune inhibition and, once development has been initiated, is capable of inducing immunity.

Sporozoite antigens of current interest are those associated with some of the organelles, i.e. the refractile globule and those forming the apical complex (rhoptries, dense granules and micronemes) and presumably involved in invasion of the host cell. Treatment with a refractile body antigen of 26-28 kDa has been reported to protect chickens against challenge infections with four different Eimeria spp.; similar antigens are present in sporozoites of E. bovis (infecting cattle), becoming distributed within the cytoplasm of the invaded host cell. Rhoptry polypeptides migrate in the 45-65 kDa range and differ anti-

genically between species, paralleling the species-specificity of immunity induced by infection. A mero-zoite protein from E. maxima of 230 kDa has been reported to induce maternally transmissible immunity (to homologous and heterologous challenge), as have gametocyte-derived 56, 82 and 230 kDa antigens of the same species.

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