Location: Posterior half of ileum, especially within a few feet forward from the ileo-cecal valve. In heavy infections, the cecum and upper colon may be involved.
Geographic Distribution: North America.
Prevalence: Davis, Boughton and Bowman (1955) found this species in 93% of 102 dairy calves in 6 herds in southeastern United States in a weekly fecal survey; they found it in 24% of 135 animals from which only a single fecal sample was taken; it was present in all of 26 herds from which at least 5 animals were examined. Hasche and Todd (1959) found it in 42% of 355 cattle in Wisconsin.
Morphology: The oocysts have been described by Christensen (1941). They measure 13 to 24 by 11 to 16 u with a mean of 18.9 by 13.4 u. They are typically piriform but may also be subellipsoidal or subcylindrical. The oocyst wall is thin, delicate, homogeneous, transparent, colorless to greyish-lavender to pale brownish yellow, slightly thinner at the narrow end but without a perceptible micropyle. During sporulation there is a parachute-shaped cap at each end of the sporoblasts. Sporulation takes 4 to 5 days. The sporocysts are elongate and gently tapered. Neither oocyst nor sporocyst residua are present. A polar granule is presumably absent.
Life Cycle: Davis, Bowman and Boughton (1957) described the life cycle of E. alabamensis. It is unusual in being an intranuclear parasite, occurring within the nuclei of the epithelial cells at the tips of the villi. Excysted sporozoites were seen in the cytoplasm of the intestinal epithelial cells 2 days after infection. They enter the nuclei and round up, forming schizonts. These are present as early as 2 days after infection and are uncommon by the 8th day. They form 16 to 32 merozoites, which are slender and spindle-shaped while still within the parent schizont but appear short, with bluntly rounded ends in the intracellular spaces and crypts. Davis, Bowman and Boughton (1957) thought that there.is probably more than one generation of schizonts.
Macrogametes and microgametocytes first appear 4 days after infection. Most are found in the lower third of the small intestine, but they may invade the cecum and upper colon in heavy infections. Young oocysts still in the host cell nuclei first appear 6 days after infection. Multiple infections are common, as many as 3 schizonts or microgametocytes and 4 or 5 macrogametes or oocysts having been found in a single host cell nucleus. This crowding may affect the shape of the oocysts in heavy infections, making some of them wedge-shaped or asymmetrical.
The prepatent period in experimentally infected calves was found by Davis, Boughton and Bowman (1955) to range from 6 to 13 days with a mean of 8 to 9 days. The patent period ranged from 1 to 10 days with a mean of 4.6 days in 21 low-grade infections, and from 1 to 13 days with a mean of 7.2 days in 72 heavy infections.
Pathogenesis: Under field conditions, E. alabamensis is considered essentially non-pathogenic. However, Boughton (1943) produced clinical coccidiosis in 5 young calves by feeding them 200 million oocysts. Within 5 days they developed a severe diarrhea, with yellowish feces having a characteristic acrid odor. They become thin, and 1 calf died on the 8th day and another on the 14th. In the first calf the lower half of the small intestine was hyperemic and there was massive tissue involvement with merozoites and macrogametes. In the second calf there was enteritis in only the last 3 feet of the ileum, and only a few parasites remained in the tissues, most of these being within 1 foot of the ileocecal valve.
Davis, Boughton and Bowman (1955) fed two 14-month-old calves 140 million oocysts. One became diarrheic on the fifth day. Its feces were watery, yellowish green, with some bloody mucus and a sharp, acrid odor. The diarrhea gradually subsided. In the second calf the feces were soft toward the end of the prepatent period. A 7-month-old heifer which had previously been exposed to coccidial infection had a slight diarrhea on the 9th and 11th days following similar exposure, and a 2-year-old cow remained normal.
Immunity: Reinfection is considered common in the field. Davis, Boughton and Bowman (1955) reported that in 58 attempts to reinfect calves 2 or more times, there were 39 high-grade infections, 11 low-grade infections and 8 failures. Nine of the low-grade infections and 7 of the failures followed the third or subsequent inoculations. Some animals were reinfected as many as 4 times before reinfection attempts failed.