Location: Middle and posterior small intestine.
Geographic Distribution: Worldwide.
Morphology: The oocysts are ovoid, smooth or somewhat roughened, yellowish, 21 to 42 by 16 to 30 u with a mean of 29 by 23 u. A micropyle is absent. An oocyst polar granule is present. An oocyst residuum is absent. The sporocysts are elongate ovoid, 15 to 19 by 8 to 9 u, with a Stieda body. A sporocyst residuum is absent. The sporozoites are 19 by 4 u, with a conspicuous refractile globule (Long, 1959). The sporulation time is 2 days. Edgar (1955) and Long (1959) found some infective oocysts as early as 30 hours at 28° C.
Life Cycle: The life cycle of this species has been studied by Tyzzer (1929), Scholtyseck (1959) and Long (1959), among others. The schizonts are found above the host cell nuclei or occasionally beside them in the epithelial cells of the tips of the villi of the duodenum and upper ileum. There are 2 generations of schizonts, both of which are relatively small, measuring about 10 by 8 u; they produce only about 8 to 16 merozoites each. Schizonts may be present thru the 5th day. The second generation merozoites enter new epithelial cells, where they round up and enter the sexual phase of the life cycle.
The sexual stages are found beneath the host cell nuclei. As they become larger, the host cells are displaced toward the center of the villus and come to lie in its interior. The mature microgametocytes measure 30 to 39 by 22 to 33 u and form a large number of biflagellate microgametes. The macrogametes are somewhat smaller, averaging 19 by 15 u (Long, 1959). After fertilization, they lay down an oocyst wall, break out of the villus and are passed in the feces. The prepatent period is 5 to 6 days, and the patent period is only a few days.
Brackett and Bliznick (1950, 1952) reported that the maximum number of oocysts produced per oocyst fed in their experiments was 12,000. In a series of 3-week-old chicks, they found that 11,500, 2250 and 940 to 2900 oocysts were produced per oocyst fed when the inoculating doses were 200, 2000 and 10,000 oocysts, respectively.
Long (1959) found that the number of oocysts produced per oocyst fed varied with the age of the birds and the inoculum. With an inoculum of 10,000 oocysts it averaged 128, 33, 176, 448, 1049 and 3294, respectively, in chicks 3, 7, 14, 21, 28 and 42 days old, while with an inoculum of 80,000 oocysts it was 9, 31 and 169, respectively in chicks 7, 14 and 21 days old.
Pathogenesis: E. maxima is slightly to moderately pathogenic. Tyzzer (1929), Brackett and Bliznick (1950), Scholtyseck (1959) and Long (1959) studied its effects on chickens. The asexual stages cause relatively little damage, the most serious effects being due to the sexual stages. Brackett and Bliznick (1950) observed a mortality of 35% in one group of young chicks infected with 500,000 oocysts each, but there were no deaths in another group. The survivors lost some weight and then gained less than the controls for a time, but infection with 100,000 oocysts had no significant effect on weight gains. Long (1959) observed no deaths in a group of 6-week-old chicks infected with 500,000 oocysts each or in three 17-day-old chicks infected with 1 million oocysts each, altho diarrhea was present and the infected birds gained less than the controls. Immunity is quickly produced.
Berg, Hamilton and Bearse (1951) found that inoculation of White Leghorn laying pullets with 8000 oocysts each produced a mild infection and temporary cessation of egg-laying.
The principal lesions are hemorrhages in the small intestine. The intestinal muscles lose their tone, and the intestine becomes flaccid and dilated, with a somewhat thickened wall. Short, fine, hairlike hemorrhages in the intestinal wall are sometimes present. There is a catarrhal enteritis and the intestinal contents are viscid and mucoid, greyish, brownish, orange or pinkish, occasionally but not usually with flecks of blood. Birds which recover soon return to normal.