Enteric Coccidiosis

Enteric Coccidiosis

J.P. Dubey and Craig E. Greene

Coccidia are obligate intracellular parasites normally found in the intestinal tract. They belong to the phylum Apicomplexa, class Sporozoasida, order Eucoccidiorida, and, depending on the species, family Eimeriidae, Cryptosporidiidae, or Sarcocystidae. Coccidian genera that infect cats and dogs are Isospora (also called Cystoisospora), Hammondia, Besnoitia, Sarcocystis, Caryospora, Toxoplasma, Neospora, Cryptosporidium, and Cyclospora species.13 Most of the genera are covered in this chapter; however, Toxoplasma and Neospora are in Chapter 79 and Cryptosporidium and Cyclospora are in Chapter 81. Another coccidian genus, Eimeria, found commonly in herbivores, birds, lagomorphs, and rodents, is found only in feces of dogs and cats after they ingest intestinal contents or feces from these animals. The oocysts pass unchanged through the feline or canine intestine. Some coccidians of dogs remain unclassified.

Intestinal coccidia discussed in this chapter are host specific (Tables 80-1 and 80-2). Infections of definitive or intermediate hosts generally only occur in cycles established by evolution. Some aberrant cycles exist, such as with Sarcocystis neurona infections in horses. Human health risks from these parasites are considered minimal to nonexistent, even in immunosuppressed humans.

Intestinal Coccidiosis

All coccidians have an asexual and a sexual cycle. In some genera, such as Sarcocystis, the asexual and sexual cycles occur in different hosts, whereas in Isospora both cycles may occur in the same host (see Tables 80-1 and 80-2 and Fig. 80-1). The oocyst is the environmentally resistant stage in the life cycle of all coccidia and is excreted in feces of the definitive host.

A representative coccidian life cycle is best described as follows. Oocysts are passed unsporulated in feces and contain a single nucleated mass called a sporont, which almost fills the oocyst (Fig. 80-2). After exposure to warm (20° C to 37° C [68° F to 98.6° F]) environmental temperatures and moisture, oocysts sporulate, forming two sporocysts. Within each sporocyst are four sporozoites (Fig. 80-3). The sporozoites have a banana shape and are the infective stage (Fig. 80-4). They can survive environmental exposure, protected inside the oocysts for many months. After ingestion of sporulated oocysts by cats or dogs, sporozoites excyst in the intestinal lumen, and the sporozoites initiate the formation of schizonts or meronts (Fig. 80-5). During schizogony or merogony, the sporozoite nucleus divides into two, three, or more nuclei, depending on the parasite and the stage of the cycle. After nuclear division, each nucleus is surrounded by cytoplasm, forming a merozoite. The number of merozoites within a schizont varies from two to several hundred, depending on the stage of the cycle and the species of coccidia. Merozoites are released from the schizont when the host cell ruptures. The number of schizogonic cycles varies with the parasitic species. First-generation merozoites repeat the asexual cycle and form second-generation schizonts or transform into microgamonts (males) and macrogamonts (females). The microgamont divides into many tiny microgametes. A microgamete fertilizes a macrogamete, and an oocyst wall is formed around the zygote. The life cycle is completed when unsporulated oocysts are excreted in feces (see Fig. 80-1).

FIG. 80-3 Sporulated oocysts of Isospora canis (C), Isospora ohioensis (O), and Hammondia heydorni (H) (unstained, ×1700). Compare with Fig. 80-2. (From Dubey JP. 1976. A review of Sarcocystis of domestic animals and of other coccidia of cats and dogs. J Am Vet Med Assoc 169:1061-1078.)

The prevalence rate of coccidial infections in dogs and cats varies with factors such as climate, age, and housing of animals. Geographic prevalence studies have been done in many countries under a variety of housing conditions for dogs or cats.* In one study in the United States, involving more than 600,000 client-owned cats evaluated at veterinary practices, those at increased risk were under 4 years of age, purebred, and living in the East, South, Central, and Mountain regions.12

Isospora Species

Members of the genus Isospora, the most commonly recognized coccidians infecting dogs or cats, are species specific for the definitive host. At least four species—Isospora canis, Isospora ohioensis, Isospora burrowsi, and Isospora neorivolta—infect dogs (see Table 80-1), and two species—Isospora felis and Isospora rivolta—infect cats (see Table 80-2).


The life cycles of Isospora infecting dogs and cats is similar to the basic coccidian intestinal cycle, except an asexual cycle can also occur in the definitive or intermediate host. On ingestion by definitive or suitable paratenic (intermediate) hosts, oocysts excyst in the presence of bile, and free sporozoites invade the intestine. Some sporozoites penetrate the intestinal wall and enter mesenteric lymph nodes or other extraintestinal tissues (e.g., spleen and liver), where they form enlarging unicellular cysts (Fig. 80-6). If no replication occurs, the host is called a paratenic host rather than an intermediate host. Monozoic cysts of Isospora may remain in extraintestinal tissues of definitive and paratenic hosts for the life of the host. In cell culture, these monozoic cysts have been shown to contain a single sporozoite.59 In dogs and cats, these cysts may serve as a source of intestinal reinfection and relapse of enteric coccidiosis. Ingestion of monozoic cysts in paratenic hosts leads to intestinal infection in the definitive dog and cat host. The life cycle after the ingestion of paratenic host is the same as after the ingestion of sporulated oocysts from feces.

Clinical Findings

Diarrhea with coccidiosis in immunocompetent animals probably represents incidental or concurrent infections with coccidia and other infectious agents because coccidial infection can be present in the absence of clinical illness. Enzootic infections are frequently found in catteries or kennels where animals congregate. Clinical signs are most apparent in neonates. Experimental studies have shown that clinical signs of intestinal disease are uncommon unless large numbers of oocysts are fed to very young (younger than 1 month) or immunosuppressed animals.4 Infections with I. canis produce longer prepatent periods, longer excretion times, and higher concentrations of oocysts as compared to infections with I. ohioensis.4 However, diarrhea did develop in 8-week-old pups experimentally infected with I. canis, even in the absence of immunosuppression or concurrent infections.59 Clinically, severe diarrhea has been associated with naturally occurring coccidiosis in immunosuppressed dogs and cats or in those with other co-infections or co-infestations.45,50 In some cases, the enteric coccidial infections are incidental to other infecting organisms or causes of diarrhea.67 German shepherd dogs may have an increased susceptibility to clinical infection.40,66 Diarrhea with weight loss and dehydration and, although rare, hemorrhage is the primary sign attributed to coccidiosis in dogs and cats. Anorexia, vomiting, mental depression, and ultimately death may be seen in severely affected animals. Severely immunosuppressed dogs and cats may have extraintestinal stages in macrophages of the lymphocyte-depleted mesenteric lymph nodes or extraintestinal tissues.

Intestinal coccidiosis may be manifest clinically when dogs or cats are shipped or weaned or experience a change in ownership. Diarrhea might result from the extraintestinal stages of Isospora returning to the intestines. Monozoic cysts do not cause clinical disease in paratenic hosts.


Intestinal coccidial infection in dogs and cats is diagnosed by identification of the oocysts with any of the fecal flotation methods commonly used to diagnose parasitic infections (see Examination of Feces, Chapter 70). Shedding of oocysts by some animals may be erratic; therefore, repeated examinations are recommended. In dogs, only I. canis can be identified with certainty by oocyst size and shape (see Fig. 80-2). The two species of Isospora found in cats can be readily distinguished by oocyst size (see Fig. 79-14). Oocysts of I. felis in cats and I. canis in dogs are large and easily distinguished from small oocysts, whereas it is almost impossible to distinguish I. rivolta, I. burrowsi, and I. ohioensis morphologically (Fig. 80-7; see also Figs. 80-2 and 80-3). Oocysts of Hammondia heydorni and Neospora caninum are very difficult to distinguish from each other (see Chapter 79).24,76 Although I. felis-, I. rivolta-, I. canis-, and I. ohioensis-like oocysts are passed unsporulated in freshly excreted feces, they sporulate partially by the time a fecal examination is made. Partially sporulated oocysts contain two sporocysts without sporozoites. Isospora spp. may sporulate within 8 hours of excretion, and these Isospora are highly infectious. In severe cases, merozoites and schizonts from the small intestine maybe found in smears of diarrheic feces (see Fig. 80-5).


The presence of underlying disease or host immunosuppression should be suspected when coccidial infections persist for extended periods in older animals or when associated with chronic diarrhea. Treatment is often indicated in bitches and their newborn puppies because of the severity of clinical signs at this age. If diarrhea or dehydration is severe, parenteral fluid therapy must be considered as a supportive measure. Blood transfusion may be required when severe intestinal hemorrhage results in anemia.

Specific therapy involves the use of drugs that are coccidiostatic rather than curative (Table 80-3). However, as with many protozoal diseases, the presence of low-level infection may lead to premunition. This is a state of chronic low-level persistent infection, which leads to resistance to further infection or reinfection, thereby preventing clinical illness. The drugs shorten the prepatent period and may shorten the course of the disease.

Aug 6, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Enteric Coccidiosis

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