After a number of generations of merozoites have been formed, a limit is reached both to the multiplicative capacity of the parasite and to the power of the bird to provide the invader with food. Consequently, resistant forms of the parasite are necessary, and the trophozoites begin to show sexual differentiation instead of forming schizonts, that is, gametogony commences.
Fig. 69.—Eimeria avium. Diagram of life-cycle. For explanation see text. (After Fantham.)
Certain trophozoites store food and become large and granular. These are macrogametocytes (fig. 69, I, ♀). The microgametocytes (fig. 69, I, ♂) are smaller and far less granular. The macrogametocyte continues to grow, and becomes loaded with chromatoid and plastinoid granules (fig. 69, J, ♀), while the microgametocyte has its nucleus divide to form a number of bent, rod-like portions (fig. 69, J, ♂). The macrogametocyte gives rise to a single macrogamete, which forms a cyst wall for itself, leaving a thin spot (micropyle) for the entry of the male (fig 69, K, ♀). The microgametocyte gives rise to numerous small, biflagellate microgametes (fig. 69, K, ♂) around a large, central residual mass, from which they ultimately break free, and swim away. When a macrogamete is reached, the microgamete enters through the micropyle (fig. 69, L)—which then closes, thus excluding the other males—and applies itself to the female nucleus (fig. 69, M). Nuclear fusion occurs, the oöcyst (encysted zygote) being thus produced. Sporogony then ensues. The oöcyst (fig. 69, N) at first has its contents completely filling it. They then concentrate into a central spherical mass (fig. 69, O) which gradually becomes tetranucleate (fig. 69, P). Cytoplasm collects around each nucleus, and four sporoblasts are thus formed (fig. 69, Q). Each sporoblast becomes oval (fig. 69, R) and produces a sporocyst. Ultimately two sporozoites are formed in each sporocyst or spore, at first lying tête-bêche (fig. 69, S), but finally twisting to assume the position most convenient for emergence (fig. 69, T) when they reach a new host. The period of the life-cycle of Eimeria avium (as well as the details of the life-cycle) was determined by Fantham to be from eight to ten days, of which period schizogony occupies four to five days.
The method of infection[175] is contaminative, by way of food or drink. Young birds are especially susceptible to infection. Certain birds, particularly older ones, may act as reservoirs of oöcysts, being continuously infected themselves, without showing any marked ill effects from the parasite, but being highly infectious to other birds. Much moisture retards the development of sporocysts considerably. The duration of vitality of the infective oöcysts has been determined experimentally to extend well over two years, and in certain cases longer. Eimeria avium is the causal agent of “white diarrhœa” or “white scour” in fowls, and of “blackhead” in turkeys.
Eimeria avium of birds and E. stiedæ of rabbits closely resemble one another, but are not the same parasite, for E. avium is not infective to rabbits, nor E. stiedæ to poultry.
Eimeria stiedæ, Lindemann, 1865.
Syn.: Monocystis stiedæ, Lindemann, 1865; Psorospermium cuniculi, Rivolta, 1878; Cytospermium hominis, Rivolta, 1878; Coccidium oviforme, Leuckart, 1879; Coccidium perforans, Leuckart, 1879; Coccidium cuniculi.