Synonyms: Isospora cati, Coccidium bigeminum var. cati, Lucetina cati, Lucetina felis.
Hosts: Dog, cat, lion and possibly other carnivores.
Location: Small intestine, sometimes cecum, occasionally colon.
Geographic Distribution: Worldwide.
Prevalence: This species in common in dogs and cats. Gassner (1940) found it in 6% of 320 dogs in Colorado. Catcott (1946) found it in 3.5% of 113 dogs in Ohio. Choquette and Gelinas (1950) found it in 9% of 155 dogs in Montreal. Hitchcock (1953) found it in 75% of 147 kittens in Michigan. Alves da Cruz, de Sousa and Cabral (1952) found it in 10% of 40 stray cats in Lisbon, Portugal.
Morphology: The oocysts are ovoid, 32 to 53 by 26 to 43 u with a mean of 43 by 33 u. The oocyst wall is smooth and colorless, without a micropyle. An oocyst polar granule and residuum are absent. The sporocysts are ellipsoidal, 20 to 27 by 18 to 21 u. A sporocyst residuum is present. The sporozoites are 10 to 15 u long. The sporulation time is 3 days or less.
Life Cycle: The life cycle of I. felis in experimentally infected kittens was described in detail by Hitchcock (1955) and Lickfeld (1959). It is similar in dogs. The parasites are found above or beside the host cell nuclei of the epithelial cells of the villi and sometimes in the subepithelial tissues. There are 2 asexual generations. The first generation schizonts are found in the small intestine and cecum from the second to fourth day after experimental infection. They are ellipsoidal and about 20 u long. They produce 40 to 60 merozoites according to Hitchcock, or 12 or less according to Lickfeld. These merozoites are relatively large, measuring 16 to 18.5 by 5 to 8 u.
The second generation schizonts are found on the 5th and 6th days after infection in the small intestine and less commonly in the large intestine. According to Hitchcock, they produce up to 24 merozoites, but most contain 12 to 16; according to Lickfeld there are 30 to more than 100 of these merozoites, and they measure 7.5 by 2.5 u.
The sexual stages are found on the 7th and 8th days after infection. They occur in the small intestine and less commonly in the cecum. According to Hitchcock (1955), the macrogametes average 25 to 22 u, but other workers have recorded dimensions up to 56 by 48 u. The microgametocytes average 28 by 19 u according to Hitchcock, but other workers have recorded dimensions up to 50 by 30 u, and Lickfeld said that they are 73 u in diameter in life. Well over 2000 spindle-shaped, curved, biflagellate microgametes are formed in each microgametocyte. The oocyst wall is laid down following fertilization while the zygotes are still within the host cells. The young oocysts then break out and are passed in the feces. The prepatent period was found by Hitchcock (1955) to be 7 to 8 days.
According to Walton (1959), the haploid number of chromosomes in I. felis is 2. Lickfeld (1959) described a cryptomitotic type of schizogony, but saw no chromosomes.
Pathogenesis: This species is slightly to moderately pathogenic, depending on the host species, age, degree of infection, etc. It is less serious in cats than in dogs. None of 18 four- to nine-week-old kittens infected by Hitchcock (1955) with 100,000 sporulated oocysts showed signs of disease. Andrews (1926), however, observed enteritis, emaciation, weakness, depression, dysentery and even death in kittens and dogs experimentally infected with I. felis. Hitchcock thought that these signs and deaths in the kittens might well have been due to feline distemper.
The gross pathologic lesions are similar to those caused by L bigemina. There is hemorrhagic enteritis, frequently with ulceration, thickened mucosa and epithelial desquamation.
Immunity: Animals which have recovered from I. felis infections are resistant to reinfection.
Cross Transmission: Lee (1934) infected dogs with I. felis from the cat, and a fox with I. felis from the dog.