Isospora

Classification: Taxonomic ranks under review (cf. Illustrated Guide to Protozoa, 2000. Allen Press)

Protista (unicellular eukaryotes)
Apicomplexa (cells with cluster of organelles known as apical complex)
Coccidea (gamonts small and intracellular, form small resistant spores called oocysts)
Eimeriida (gametes develop independently without syzygy; known as coccidian parasites)

Family: Eimeriidae
These protozoa are known as the enteric coccidia; monoxenous (one-host) parasites in the digestive tracts of herbivores or carnivores causing diarrhoeal disease (known as coccidiosis). Parasites form environmentally-resistant oocysts which undergo faecal-oral transmission between hosts. There are three sequential stages in the parasite life-cycle: endogenous multiplication by asexual merogony (variously known as schizogony) followed by sexual gamogony (♂ microgametes fertilize ♀ macrogametes producing oocysts) which are excreted and undergo asexual sporogony (forming sporocysts containing infective sporozoites). Many genera are recognized on the basis of oocyst configuration (the number of sporocysts per oocyst, and the number of sporozoites per sporocyst).

Isospora spp. [these species cause coccidiosis in vertebrates, especially carnivores]

Parasite morphology: Coccidian parasites form three developmental stages: schizonts, gamonts and oocysts. Schizonts first appear as small basophilic rounded cells (mother meronts) located intracellularly within host cells. The meronts form numerous daughter merozoites by endogenous division of the nucleus followed by cytokinesis. Mature schizonts range in diameter from 10-50µm and appear as membrane-bound clusters of small basophilic bodies (similar to bunches of grapes). Gamonts exhibit sexual differentiation, with microgamonts (♂) apparent as multinucleate basophilic stages ultimately shedding small biflagellated microgametes; and macrogamonts (♀) evident as uninucleate eosinophilic cells with a single ovoid nucleus. Developing oocysts contain numerous eosinophilic wall-forming bodies which give rise to the tough outer oocyst walls. Unsporulated oocysts contain a developing sporoblast which eventually undergoes sporulation forming sporocysts which contain the infective sporozoites. Isospora oocysts exhibit a characteristic 1:2:4 configuration, that is, each oocyst contains 2 sporocysts each containing 4 sporozoites. Oocysts are generally ovoid to ellipsoid in shape, range from 10-40µm in length by 10-30µm in width, and may contain specialized structures, such as polar caps, micropyles, residual and crystalline bodies.

Host range: Infections have been detected throughout the world, mainly in carnivores (particularly canids and felids) as well as in some omnivores (humans, pigs, lizards) and birds (especially passerines). Most coccidian species are considered to be highly host-specific and only parasitize single host species (oioxenous), although some species in birds and reptiles may parasitize closely-related hosts (stenoxenous) or even unrelated hosts (euryxenous). Many hosts also harbour multiple species of coccidia which may vary considerably in morphology, developmental cycle, site of infection and pathogenicity. Three Isospora spp. have been described from dogs, 2 species from cats, one from pigs and one from humans. Other small coccidian species found in some of these hosts include Sarcocystis, Frenkelia, Hammondia, Besnoitia spp. and Toxoplasma gondii.

Site of infection: Parasites undergo merogony and gamogony in the small intestinal mucosa, located intracellularly within epithelial cells. They undergo several cycles of schizogony, each culminating in host cell lysis releasing merozoites. Ultimately, gamonts are formed which mature to produce micro- and macro-gametes that undergo fertilization forming a non-motile zygote (oocyst) which is excreted with host faeces.

Pathogenesis: Most species are only mildly pathogenic but can cause transient diarrhoea, colic, weight loss and fever. When mature, endogenous developmental stages of the parasite lyse their host epithelial cells lining small intestinal villi, producing villous atrophy, crypt hypertrophy, inflammation, malabsorption and sometimes petechial haemorrhages. There is substantial epidemiological evidence that the severity of infections may be exacerbated by concomitant viral disease or other immunosuppressive agents. Young animals are most susceptible to disease but develop a strong specific protective immunity thereafter.

Mode of transmission: Infections are passed between hosts by the faecal-oral transmission of infective oocysts contaminating the external environment, including food and water supplies. Following ingestion by susceptible hosts, the oocysts and sporocysts excyst in the intestines releasing their contained sporozoites which invade host cells.

Differential diagnosis: Clinical signs generally coincide with parasite patency (period during which oocysts are produced). Infections are usually diagnosed by the coprological examination of host faeces for coccidial oocysts (concentrated using various sedimentation-flotation techniques). Faeces from carnivores can also be pretreated with ether/chloroform to remove fatty material. Unstained oocysts are best observed by light microscopy using suboptimal transmitted illumination (condenser wound down to introduce diffraction), phase-contrast or interference-contrast optics. Alternatively, oocysts can be stained with Giemsa or acid-fast stains of dried smears or with fluorescence dyes (auramine-rhodamine) in wet preparations. Fresh faecal samples may only contain unsporulated oocysts so differential specific diagnosis may sometime require short-term storage to facilitate sporulation (2% potassium dichromate is often used to suppress microflora during storage, and refrigeration can slow the process down if so required for field samples).

Treatment and control: Coccidiostatic drugs, particularly sulfonamides (trimethoprim-sulfamethoxazole), are effective for therapeutic use, acting against endogenous developmental stages to limit infections. Control measures include good sanitation, proper effluent disposal, isolation of infected individuals and avoiding crowding, particularly in intensive husbandry situations, breeding establishments, kennels and rescue centres. Conventional disinfectants are ineffective against coccidian oocysts, although some ammonia-based products have been shown to kill infective oocysts.