Felis ISSN 2398-2950

Isospora felis

Contributor(s): Stephen Barr, Maggie Fisher, Grace Mulcahy




  • Class: Coccidia.
  • Family: Eimeriidae.
  • Genus: Isospora.

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Clinical Effects



  • Asexual stages in the epithelial cells (surface and crypt) of the small intestine and cecum.
  • Cystozoites occur in the parenteral tissues, probably mesenteric lymph nodes and abdominal viscera, of rats, mice, probably other herbivores and also the tissues of the cat.


Direct and facultative indirect life cycle

  • Intestinal development, merogony
  • Oocysts Isospora oocyte .
  • Cystozoites in paratenic hosts.
  • Cystozoites in tissues of cat from where reactivation and renewed intestinal infection is a likely scenario.


Direct life-cycle

  • Feco-oral transmission in catteries.
  • Development of the oocysts is very rapid, the prepatent period of infection is short and, as the biotic potential of the Isospora is high, very large numbers of oocysts can build up rapidly to pathogenic levels.

Indirect life-cycle

  • Infection is derived by ingestion of cystozoites in the tissues of prey.
  • Numbers are unlikely to be sufficient to cause disease but this source of infection could maintain the life cycle.

Reactivation of cystozoites in the cat

  • Histories of heavily infected animals continue to suggest that reactivation of cystozoites in immunosuppressed or stressed kittens may then initiate intestinal mutiplication and so could be an important factor in causing disease.

Pathological effects

  • Some protective immunity and decreased susceptibility to infection with age do seem to occur in regard to the intestinal stages.
  • The development of clinical disease seems related to some form of immunosuppression as it is seen at times of stress, eg weaning, transport, malnutrition, or concurrently with bacterial or viral disease.
  • Remains controversial but high oocyst counts have been associated with disease particularly in young kittens that have been stressed at weaning and by transport.
  • Clinical signs include bloody or mucoid diarrhea, abdominal pain, dehydration, anemia, anorexia, weight loss, neurologic and respiratory signs.

Other Host Effects

  • Obligate, intracellular parasite lying within a parasitophorous vacuole.


Control via animal

  • In view of the controversy surrounding the pathogenicity of Isospora, concurrent exacerbating infections should be sought and treated and any sources of stress corrected.
  • Remove animal from surroundings and so from source of infection.
  • There is no clear evidence that common anti-coccidials have real efficacy but they may shorten the course of disease so they and supportive therapy (fluids, nursing, as required) should be administered.

Control via chemotherapies

  • No products are licensed.
  • Toltrazuril and diclazuril seem the most likely candidates with their proven efficacy against Isospora suis in pigs but have been used only in very few dogs. 5 mg/kg clazuril was very effective for the treatment of some cats.
  • Trimethoprin/sulfadiazine Silver sulfadiazine, 30 mg/kg (half this for <4 kg cats) administered daily for 6 days.
  • Sulfamethoxine (50mg/kg PO SID until 2 days after signs resolve).
  • Some toxicity has been reported.

Control via environment

  • The oocysts sporulate very rapidly and are very resistant to disinfectants.
  • Parasite control can be difficult due to the presence of both oocyst and rodent sources of infection and the rapid multiplication of the parasite.
  • It may be impossible to rid a cattery of infection.
  • Remove feces frequently.
  • Wash pens well to remove oocysts via the drains.
  • Desiccation will kill oocysts over several days.
  • Ammonia disinfectants, ie oocide, will be the most effective.
  • Steam cleaning is effective.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • McAllister M M, Jolley W R, Wills R A et al (1998) Oral inoculation of cats with tissue cysts of Neospora caninum. Am J Vet Res 59 (4), 441-444 PubMed.
  • Lindsay D S & Blagburn B L (1991) Coccidial parasites of dogs and cats. Compendium of Continuing Education for the Practicing Veterinarian 13 (5), 759-765 VetMedResource.