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Cryptosporidium parvum is a protozoan parasite of the small intestine of dogs, and in large numbers can cause diarrhoea.
The oocyst is ingested by the host. It excysts in the small intestine and the sporozoites (4 per cyst) invade the microvillus border of intestinal epithelial cells. The parasite undergoes two types of schizogony and one round of gametogony. The type I schizont produces zoites that can produce either type I or type II schizonts, thus the parasite can undergo continuous asexual reproduction as type I schizonts. Type II schizonts produce zoites which develop into gametocytes. Sporulated (infective) oocysts leave the cells and most of them pass out with the feces. However, some of the oocysts will excyst in the intestine, invade cells and begin the life cycle over (autoinfection).
The source of infection is oocysts that are fully sporulated and infective when excreted in the feces. Large numbers are excreted during the patent period, resulting in heavy environmental contamination. Transmission may occur directly from dog to dog, indirectly via fomite or human transmission, from contamination in the environment, or by fecal contamination of the feed or water supply.
In dogs, diarrhoea is a consistent symptom, and may vary from intermittent soft stools to bloody diarrhoea. Many other protozoa, viruses and nematodes can cause a diarrhoea of similar nature and thus a differential diagnosis of exclusion must be performed.
Diagnosis is based on detection of oocysts by examination of fecal smears with Ziehl-Neelson stains, by fecal flotation, or by immunologically assisted methods. It has been suggested that if the diarrhea is caused by cryptosporidia, there should be 105 -107 oocysts/mL of feces. The oocysts are small (5-6 mm in diameter) and relatively nonrefractile. They are difficult to detect by normal light microscopy but are readily detected by phase-contrast microscopy.
Symptomatic treatment is required, with supportive fluids and broad-spectrum antimicrobial drugs. Antidiarrhoeal medication is not usually recommended as this may prolong bowel emptying of toxins.
Oocysts are resistant to most disinfectants and can survive for several months in cool and moist conditions. Oocyst infectivity can be destroyed by ammonia, formalin, freeze-drying, and exposure to temperatures <32°F (0°C) or >149°F (65°C). Ammonium hydroxide, hydrogen peroxide, chlorine dioxide, 10% formol saline, and 5% ammonia are effective in destroying oocyst infectivity. Infectivity in calf feces is reduced after 1-4 days of drying.
Concurrent infections with other enteric pathogens, especially rotavirus and coronavirus, are common, and epidemiologic studies suggest that diarrhea is more severe in mixed infections. Immunocompromised animals are more susceptible to clinical disease than immunocompetent animals, but the relationship between disease and failure of passive transfer of colostral immunoglobulins is not clear. Age-related resistance, unrelated to prior exposure, is observed in lambs but not calves. Infection results in the production of parasite-specific antibody, but both cell-mediated and humoral antibody are important in protection, as well as local antibody in the gut of the neonate.