Isospora felis Wenyon, 1923

ETYMOLOGY:Isospora (Iso equal; spora spore) and felis for cat.

SYNONYMS:Diplosporabigemina of Wasielewski (1904) in part; Isosporabigemina of Swellengrebel (1914); Isospora rivolta Dobell and O'Connor, 1921; Isospora cati Marotel, 1921; Lucetina felis (Wenyon, 1923) Henry and Leblois, 1926; Isospora felis var. servalis Mackinnon and Dibb, 1938; Levine felis (Wenyon, 1923) Dubey, 1977; Cystoisospora felis (Wenyon, 1923) Frenkel, 1977.

HISTORY: The earliest report of coccidia in cats was probably given by Finck in 1854 (Wenyon, 1923; Shah, 1970a) who described stages in the villi of cats as corpusclesgemines. Wenyon (1923) indicates that these stages were in the lamina propria and not in the enterocytes as were I. felis and I. rivolta. The subepithelial location would indicate that these parasites were a Sarcocystis species. The oocysts of I. felis (large size) and I. rivolta (medium size) found in cats closely resemble the oocysts of I. canis (large size) and I. ohioensis-like (medium size) organisms observed in the feces of dogs (Lindsay and Blagburn, 1991). During the first half of this century dogs and cats were thought to share the same species of coccidia. Nesmeséri (1960) demonstrated that I. felis from cats was not transmissible to dogs and named the canine parasite I. canis. Shah (1970a) latter confirmed these findings. Several researchers were unable to produce patent infections in dogs with I. rivolta oocysts isolated from cats (Pellérdy, 1974; Dubey et al., 1970a, Dubey, 1975a) or in cats with I. rivolta oocysts isolated from dogs (Dubey, 1975a). Based on the results of these studies I. rivolta was retained for the species in cats and the species in dogs was named I. ohioensis (Dubey, 1975a).

In the early 1970's, researchers demonstrated that oocysts of I. felis and I. rivolta would excyst in mice and the sporozoites would invade mesenteric lymph nodes and other extraintestinal sites (Frenkel and Dubey, 1972). These encysted stages are infectious when fed to cats and result in oocyst production.

GEOGRAPHIC DISTRIBUTION:Isospora felis is found worldwide where cats are present.


Feline definitive hosts: Asexual and sexual multiplication occurs in enterocytes primarily in the posterior small intestine. Asexual stages are also observed in extraintestinal tissues.

Paratenic hosts: In these hosts, sporozoites can persist within varioius cells in the lymphatic system of the peritoneal cavity.

PARASITE IDENTIFICATION: Oocysts measure 38-51 by 27-39 μm (mean: 41.6 by 30.5 μm) (Shah, 1970a) ((Fig. 1-1). The length width ratio is 1.3-1.4 (mean: 1.35). The oocysts of Isosporafelis are the largest of the coccidial oocysts observed in cats (Table 3). No micropyle is present. Inclusions (hazy bodies) may be observed between the sporont and oocyst wall in freshly excreted oocysts. The hazy bodies degenerate as the oocysts sporulate. No oocyst residuum is present in sporulated oocysts. Sporulated oocysts contain 2 sporocysts (Fig.1-2). Sporocysts measure 20-26 by 17-22 μm (mean, 22.6 by 18.4 μm), contain a sporocyst residuum, and 4 sporozoites but no Steida body. The sporocyst residuum is granular and may contain refractile globules. Sporozoites are 10-15 μm long, lie lengthwise in the sporocyst, contain a single nucleus and a refractile globule. Occasionally a sporulated I. felis oocyst will be observed that is Caryospora-like having a single sporocyst that contains 8 sporozoites (Shah, 1970a).

Table 3. Comparison of measurements of coccidial oocysts from cats.


Length (mean)

Width (mean)

Isospora felis

38 to 51

27 to 39

Isospora rivolta

18 to 28

16 to 23

Toxoplasma gondii

11 to 13

11 to 13

Hammondia hammondi

10 to 13

10 to 13

Hammondia pardalis

36 to 46 (40.8)

25 to 35 (28.5)

Besnoitia darlingI

11 to 13

11 to 13

Besnoitia wallacei

16 to 19 (17)

10 to 13 (11)

Cryptosporidium parvum

4 to 5

4 to 5

Sarcocystis spp.

11 to 14

7 to 9



LIFE CYCLE: Most members of the cat family Felidae are probably suitable definitive hosts. Levine and Ivens (1981) indicated the following were suitable definitive hosts: European wild cat (Felis silvestris), ocelot (Felis pardalis), serval (Felis serval), tiger (Leotigris), lion (Felisleo), jaguar (Leo onca) and lynx (Lynx lynx). Oocysts are excreted unsporulated. Oocyst sporulate in 40 hr at 20C, 24 hr at 25C, 12 hr at 30C and 8 hr at 38C (Shah, 1970b). Sporulation does not occur at temperatures above 45C.

Several authors have described portions of the endogenous life cycle of I. felis in cats (Wenyon, 1923; Hitchcock, 1955; Shah, 1971; Ferguson et al., 1980a, 1980b, Daly and Markus, 1981). Sporozoites excyst from I. felis oocysts in the small intestine. Developmental stages are located in enterocytes of the distal portions of the villi in the ileum, and rarely the duodenum and jejunum. The first developmental cycle is probably by endodyogeny and at least three structural types of meronts are produced (Shah, 1971). Mature first-generation meronts of I. felis were first observed 4 days PI and produced 16-17 merozoites. Mature second-generation meronts were first observed 5 days PI and produced about 10 merozoites. Mature third-generation meronts were first observed 6 days PI, were in the same host cell as the second-generation meronts and produced 36 to 70 merozoites. Sexual stages were first observed 6 days PI. Oocysts were first observed 7 days PI. The prepatent period is 7-11 days and the patent period is 10-11 days.

Mice (Mus musculus), Norway rats (Rattus norvegicus), golden hamsters (Mesocricetus auratus), cows (Bos taurus), and dogs (Canis familiaris) can serve as paratenic hosts (Dubey, 1975; Dubey and Frenkel, 1972; Fayer and Frenkel, 1979; Frenkel and Dubey, 1972; Wolters et al., 1980). The sporozoite present in the tissues of these hosts are infective to cats if they are ingested.

CLINICAL PRESENTATION AND PATHOGENESIS: Experimental studies indicate that I. felis is moderately pathogenic for 6-week-old to 13-week-old kittens given 1 to 1.5 x 105 oocysts. Soft, mucoid, feces is observed in kittens 8 days after infection but servere disease does not occur. Microscopic lesions in kittens examined early in infection (about 6 days after being given oocysts) are mild and consist of erosion of the superficial epithelial cells. In kittens examined later in the infection (7 to 9 days after infection) congestion, mild neutrophilic infiltration, and hypersecretion of the mucosa are observed (Shah, 1971). Additionally, epithelial hyperplasia was also noted in some kittens. Isospora felis is more pathogenic for younger kittens. Four-week-old kittens may develop severe disease characterized by signs of enteritis, emaciation and death if given 1 x 105 oocysts (Andrews, 1926).

No signs of disease are seen in paratenic hosts.

TREATMENT: Coccidiosis in cats can be treated with various sulfonamides and quinacrine (Table 4)


Table 4. Treatment of intestinal coccidiosis and cryptosporidiosis in cats.

Antiprotozoal agent

Treatment regimen


sulfadimethoxine (SDM)

50 mg/kg for 10 days or 55 mg/kg for 1 day and 27.5 mg/kg until signs disappear

SDM plus ormetoprim (OM)

55 mg/kg SDM plus 11 mg/kg OM for up to 23 days


150 to 200 mg/kg for 5 days

sulfadiazine (SD) & trimethoprim (TRI)

25 to 50 mg/kg SD plus 5 to 10 mg/kg TRI for 6 days for cats over 4 kg

12.5 to 25 mg/kg SD plus 2.5 to 5 mg/kg TRI for 6 days for cats over 4 kg

amprolium HCl (AMP)

300 to 400 mg/kg for 5 days

110 to 220 mg/kg for 7 to 12 days

20 to 40 mg/kg for 10 days (Blagburn)

AMP plus SDM

150 mg/kg AMP plus 25 mg/kg SDM for 14 days


10 mg/kg for 5 days


8 to 20 mg/kg BWT once or twice daily

use ??? this dose if combined with sulfonamides



165 mg/kg every 12 hour for 5 days

EPIZOOTIOLOGY: Cats are very commonly infected with this parasite. It is unclear whether cats are infected more commonly by oocysts or by the ingestion of paratenic hosts.


HAZARDS TO HUMANS: It is possible that humans could serve as paratenic hosts. No recorded cases of human infection exist.


Andrews JM. 1926. Coccidiosis in mammals. Am. J. Hyg. 6:784-798.

Daly TJM, Markus MB. 1981. Enteric multiplication of Isospora felis by endodyogeny. Elect Micro Soc South Afr 11:99-100.

Dubey JP. 1975a. Isospora ohioensis sp. n. proposed for I. rivolta of the dog. J Parasitol 61:462-465.

Dubey JP. 1975b. Experimental Isosporacanis and Isospora felis infection in mice, cats, and dogs. J Protozool 22:416-417.

Dubey JP, Frenkel JK. 1972. Extra-intestinal stages of Isospora felis and I. rivolta (Protozoa: Eimeriidae) in cats. J. Protozool. 19:89-92.

Dubey JP, Streitel RH. 1976. Isospora felis and I. rivolta infections in cats induced by mouse tissue or oocysts. Br. Vet. J. 132:649-651.

Dubey JP, Miller NL, Frenkel JK. 1970a. The Toxoplasma gondii oocyst from cat feces. JExp Med 132:636-662.

Elwasila, M. 1983. A fine-structural comparison of the sporozoites of Grellia (Eucoccidium) dinophili in Dinophilus gyrociliatus and Isospora felis in the mouse. Z. Parasitenkd. 69:583-589.

Fayer R, Frenkel JK. 1979. Comparative infectivity for calves of oocysts of feline coccidia: Besnoitia, Hammondia, Cystoisospora, Sarcocystis, and Toxoplasma. J. Parasitol. 65: 756-762.

Ferguson DJP, Birch-Anderson A, Hutchinson WM, Siim JC. 1980a. Ultrastructural observations showing enteric multiplication of Cystoisospora (Isospora) felis by endodyogeny. Z Parasitenkd 63:289-291.

Ferguson DJP, Birch-Anderson A, Hutchinson WM, and Siim JC. 1980b. Ultrastructural observations on microgametogenesis and the structure of the microgamete of Isosporafelis. Acta Path Microbiol Scand B 88:151-159.

Frenkel JK, Dubey JP. 1972. Rodents as vectors for the feline coccidia, Isospora felis and Isospora rivolta. J. Infect. Dis. 125:69-72.

Hitchcock DJ. 1955. The life cycle of Isospora felis in the kitten. J Parasitol 41:383-397.

Levine ND, Ivens V. 1981. The Coccidian parasites (Protozoa, Apicomplexa) of Carnivores. Illinois Biological Monographs 51, University of Illinois Press, Urbana, Illinois. pp. 248.

Lindsay, DS, Blagburn BL. .1991. Coccidial parasites of cats and dogs. Comp. Contin. Ed. Pract. Vet. 13:759-765.

Mehlhorn, H. 1976. Electron microscopy of stages of Isospora felis of the cat in the mesenteric lymph node of the mouse. Z. Parasitenk. 51:15-24.

Nemeséri L. 1960. Beiträge zur Ätiologie der Coccidiose der Hunde I. Isospora canis sp. n. Acta Vet Hung 10:95-99.

Pellérdy L. 1974. Studies on the coccidia of the domestic cat. Isospora cati sp. n. Acta Vet

Hung 24:127-131.

Shah HL. 1970a. Isospora species of the cat and attempted transmission of I. felis Wenyon, 1923 from the cat to the dog. J Protozool 17:6003-609.

Shah HL. 1970b. Sporogony of the oocysts of Isospora felis Wenyon, 1923 from the cat. J Protozool 17:609-614.

Shah HL. 1971. The life cycle of Isospora felis Wenyon, 1923, a coccidium of the cat. J Protozool 18:3-17.

Wenyon CM. 1923. Coccidiosis of cats and dogs and the status of the Isospora of Man. Trop Med Parasitol 17: 231-288.

Wolters E, Heydorn AO, Laudahn C. 1980. Das Rind als Zwischenwirt von Cystoisospora felis. Berl Münch Tierärztl Wschr 93: 207-210.

Figure 1-1. Isospora felis. Oocysts passed in the feces of a naturally infected cat.


Figure 1-2. Isospora felis. Sporulated oocyst.