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Cryptosporidium parvum

icanis
Contributor(s):

Vetstream Ltd

Synonym(s): C. parvum


Introduction

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Clinical Effects

Epidemiology

Habitat

  • Cryptosporidium parvum infects most species of animal, the predilection site being epithelial cells of the posterior small intestine but occasionally also epithelial cells throughout the gastrointestinal tract and in the respiratory tract.
  • It is intracellular, but extracytoplasmic, lying in a parasitophorus vacuole on the brush border of the villi.
  • Oocysts in the environment (especially water sources).

Lifecycle

  • Schizonts.
  • Merozoites.
  • Gametocytes.
  • Thin-walled oocysts.
  • Thick-walled oocysts.

Transmission

  • Only a very few oocysts, experimentally <10, are required to initiate an infection.
  • The highest numbers of oocysts are excreted in the feces of young animals and immunosuppressed humans but adult animals and dogs may be carriers excreting low numbers of oocysts.
  • Infection may be acquired by direct contact with and ingestion of these.
  • Contamination of surface water with oocysts from the above sources and water-borne transmission are important in the epidemiology of infection in man and probably may be important in companion animals.

Direct lifecycle

  • Feco-oral transmission of thick-walled oocysts in the feces.

Auto-infection

  • Thin-walled oocysts produced in the intestine are able to excyst in the intestine so the sporozoites initiate auto-infection in the same individual.
  • First generation schizogony produces merozoites that can re-initiate first generation schizogony and so autoinfect the same individual.

Pathological effects

  • Protective immunity and decreased susceptibility to infection with age do develop.
  • Immunodeficiency or immaturity of the immune system allows heavy infection to develop, potentially life-threatening in the case of immunodeficiency.
  • Severe pathology has not been described in the dog. In other species, severe infections in the young and immunosuppressed induce pathology.
  • There is loss of microvilli, villous atrophy, villous fusion and crypt hyperplasia with replacement of the epithelium by flattened cells. This occurs particularly in the ileum to result in maldigestion and malabsorption.
  • Inflammatory cell infiltration, particularly neutrophils and monocytes, and edema are apparent in the lamina propria.
  • Hypersecretion of fluid means the intestine becomes filled with gas and a yellow, watery fluid.
  • In these severely affected cases, there is profuse watery diarrhea, dehydration, anorexia and weight loss.
  • Deaths result from the dehydration (only likely if immuno-suppressed).

Other Host Effects

  • Obligate, intracellular parasite lying within a parasitophorus vacuole.

Control

Control via animal

  • No specific, effective therapy for Cryptosporidium has been identified.
  • The dehydration must be treated and, unless the animal is immunosuppressed, the disease usually is self-limiting and immunity to disease develops, although the animal may remain a carrier.

Control via chemotherapies

  • Spiramycin has been used in man and this drug has been tolerated by dogs experimentally at 50-100 mg/kg for 6 days.

Control via environment

  • Strict hygiene with dog feces must be recommended to prevent infection, particularly by the immunosuppressed and young children.
  • Periodic water-borne outbreaks of infection means that, during these, water should be boiled for both the pet and human inhabitants.
  • Dry heat will kill the oocysts in the environment.
  • Ammonia-based disinfectants, eg oocide, are the most effective,

Other countermeasures

  • The severe disease produced by C. parvum in immunosuppressed persons means consideration must be given to a recommendation that they do not have a pet or they must be advised as to the dangers of infection and the strict hygiene required.
  • The dilemma is compounded by the poor sensitivity of diagnostic tests when they are applied to carrier animals or humans and oocysts can be excreted for prolonged periods.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Casemore D P (1991) The epidemiology of human cryptosporidiosis and the water route of infection. Water Science and Technology 24 (2), 157-164 IWAP.

Other sources of information

  • Lloyd S & Smith J (1997) Pattern of Cryptosporidium parvum oocyst excretion by experimentally infected dogs. Int J Parasitol. In press.

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