Isospora Bigemina

Synonyms: Coccidium bigeminum, Lucetina bigemina.

Hosts: Dog, cat, fox, polecat (Putorius foetidus), mink (Mustela vison), man (?).

Location: Thruout small intestine.

Geographic Distribution: Worldwide.

Prevalence: This species is quite common in dogs and cats. Gassner (1940) found it in 74% of 320 dogs in Colorado. Catcott (1946) found it in 3% of 113 dogs in Ohio. Choquette and Gelinas (1950) found it in 2% of 155 dogs in Montreal. Ehrenford (1953) found it in 0.7% of 377 dogs in Indiana and other midwestern states. Hitchcock (1953) found it in 1% of 147 kittens in Michigan. Levine (1948) reviewed reports of this species in Mustelidae.

Morphology: The oocysts are very thin-walled, spherical to ellipsoidal when unsporulated, but with the wall stretched around the sporocysts and usually constricted somewhat between them when sporulated. The oocyst wall is smooth, colorless, and composed of a single layer. Two sizes of oocyst have been reported. The larger ones measure 18 to 20 by 14 to 16 u, and the smaller, more common ones 10 to 14 by 7 to 9 u. Micropyle, oocyst polar granule and oocyst residuum are absent. The sporocysts are ellipsoidal, 7.5 to 9 by 5 to 7 u, without a Stieda body. A sporocyst residuum is present. The oocysts are sporulated when passed. The oocyst wall is often ruptured so that the sporocysts are found free in the feces. In acute infections, the oocysts may be unsporulated when passed; their sporulation time is about 4 days.

Life Cycle: The life cycle of this species has been studied by Wenyon (1926a) and Wenyon and Sheather (1925). The endogenous stages occur thruout the small intestine. Altho the course of infection has not been followed consecutively in a series of experimentally infected animals, it appears that the epithelial cells are invaded first, followed later on by the subepithelial cells. At any rate, Wenyon and Sheather (1925) found coccidia only in the epithelial cells of a dog killed during the acute phase of the infection. The schizonts of this stage contain 8 merozoites. Later on, the coccidia are found in the subepithelial cells and cores of the villi. The schizonts here contain about 12 merozoites. Sexual stages appear to be produced in both locations. The oocysts produced in the epithelial cells during the acute phase are unsporulated when passed in the feces. They appear 6 to 7 days after infection. The oocysts produced in the subepithelial cells are sporulated when passed. A number of unanswered questions are raised by this account, and the whole life cycle deserves re-investigation.

Pathogenesis: This species is markedly pathogenic for both cats and dogs. Its effects on the dog, cat and fox were studied by Lee (1934). Puppies and kittens are most seriously affected, while adults are usually carriers, having developed an immunity following earlier infection.

The first signs usually begin 4 to 6 days after infection. Their severity depends on the degree of infection. In severe cases, catarrhal or bloody diarrhea, rapid emaciation and anemia occur. Affected animals are weak, depressed and lose their appetite. There may be a rise in temperature or muscular tremors of the hind legs. If the animal survives the acute phase, the dysentery is replaced by mucous stools for 2 to 4 days and the other signs subside, disappearing 7 to 10 days after their onset. Recovered animals may continue to shed oocysts for a time.

In severe cases, hemorrhagic enteritis is present thruout the small intestine; it is most severe in the lower ileum and becomes progressively less so anteriorly. Petechiae are present in light infections, and diffuse hemorrhages in more severe ones. There may be ulcers in addition. The mucosa is thickened, and there may be extensive desquamation. A circulating eosinophilia may be present, and the parasitized region is infiltrated with eosinophiles.

Cross-Transmission: Lee (1934) transmitted I. bigemina from the dog to the cat and fox, but failed to infect rabbits or guinea pigs with it.

Remarks: The oocysts of I. hominis of man are apparently indistinguishable from those of I. bigemina, and a number of investigators believe that they are the same species (Elsdon-Dew and Freedman, 1953; Routh, McCroan and Hames, 1955; Becker, 1956). Cross-transmission experiments are needed to determine whether they are.