Isospora spp.

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Isospora spp.
Kingdom Protista
Phylum Apicomplexa
Class Coccidiasina
Family Eimeriidae
Genus Isospora
Species There are many e.g. Isospora suis.


Isospora suis from a pig - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona
Isospora - Joaquim Castellà Veterinary Parasitology Universitat Autònoma de Barcelona

There are many different species of Isospora, all of which are host specific. The most commonly seen of all the Isospora species is Isospora suis in the pig.


Each oocyst measures 20-50μm. The eggs are subspherical, and the wall is colourless and thin. When sporulated each one contains two sporocyts each with four sporozoites.

Life Cycle

There are essentially three stages in the Isospora life cycle. The first is called sporogony and is the asexual stage of the parasite development. It occurs exogenously, and leads to the development of sporozoites in the oocysts. After this occurs, the oocysts are now deemed infective.

The host ingests the infectious oocyst and the digestive enzymes break down the oocyst wall causing the release of infective sporozoites. The sporozoites then go on to penetrate the intestinal villus epithelium, namely the jejunum and the ileum. Each sporulated oocyst contains 2 sporocysts each with 4 sporozoites. This stage will occur relatively quickly under optimal conditions of high humidity and temperatures between 20 and 400C.

The next step is schizogony. This is an asexual process which occurs endogenously. After the sporozoites invade the epithelia, they then form trophozoites. These trophozoites then form merozoites, which is known as merogony.

Gametogony, which is sexual division occurs endogenously, namely in the intestinal cells. Merozoites then form either microgamonts (male) or macrogamonts (female). Invasion of macrogametocytes containing cells by microgametocytes leads to fertilization, and the cycle continues.

Clinical Signs

Clinical signs are due to destruction of the intestinal epithelium and sometimes the underlying connective tissue of the mucosa.

There may be haemorrhage into the lumen of the intestine, catarrhal inflammation and diarrhoea. There may be dysentry, tenesmus and dehydration. Anaemia is only seen in severely affected animals.


Sugar or salt flotation methods enable oocysts to be observed in the faeces.

Diarrhoea may precede a heavy output of oocysts, and may continue after the output has finished, therefore multiple faecal samples may be necessary to identify oocysts.

The number of oocysts in a sample can vary and must be related to clinical signs and lesions, and the species observed must be found to be pathogenic to that host. Other causes of diarrhoea should be ruled out before Isospora infection is diagnosed.

Many nonpathogenic species can be found during episodes of diarrhoea, which will not allow a diagnosis of coccidiosis in that species.


The life cycle of Isospora is self limiting and infection should resolve spontaneously within a few weeks unless reinfection occurs. Medication can shorten the length of clinical signs and lessen the likelihood of complications and death.

Sick animals should be isolated and treated individually if possible.

Soluble sulfonamides such as sulfaquinoxaline are effective in most species. Amprolium can be used in large animals, and can be given as a preventative treatment to healthy in contact animals.

Isospora in Cats and Dogs

Isospora species in cats are I. felis and I. rivolta which can be identified by size and shape. Four species infect the dog: I. canis, I. ohioensis, I. burrowsi and I. neorivolta.

Clinical illness is uncommon but heavy infections have been reported in kittens and puppies. In kittens, infection is usually seen at weaning.

Clinical signs include: bloody diarrhoea, weight loss and dehydration.

The illness can be associated with other infections and immunosuppression.

Cats usually spontaneously eliminate the infection, but if they are clinically ill, trimethoprim-sulfa can be given.

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Isospora spp. publications


Bowman, D. (2002) Feline Clinical Parasitology Wiley-Blackwell

Merck and Co (2008) Merck Veterinary Manual Merial

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