Cryptosporidium

Introduction

• Cryptosporidium species are coccidian-like protozoan parasites that infect a wide variety of vertebrate hosts.
• Most species and strains are highly adapted to specific hosts.
  • When other hosts are exposed to these strains, they are unlikely to be infected unless they are immunocompromised.
  • Humans are more likely to become infected by C. hominis and certain genotypes of C. parvum than by either a canine or feline species.
• However, some species are thought to be shared among dogs, cats, and humans.

Stages

• Sporulated oocysts—resistant stages for environmental transmission—are infective when shed by a host.

• Schizonts (asexual stages)

• Gametes (sexual stages)

Oocysts of Cryptosporidium from fecal flotation

Photomicrograph of developmental stages of Cryptosporidium from the small intestine

Scanning electron microscopy showing Cryptosporidium stages in the microvillus border of epithelial cells

Disease

• Cryptosporidiosis is a self-limited, small bowel infection characterized by secretory diarrhea. Fluid loss may be severe in clinical cases.

• The disease may be more severe, prolonged, and sometimes life-threatening in immunocompromised hosts.

Prevalence

• Seroprevalence studies suggest that infection with Cryptosporidium is common in dogs and cats.

• Confirmation of current infection by fecal examination is infrequent; commonly seen in only 1% to 5% of cases. Seroprevalence (antibody) studies, on the other hand, suggest higher rates of resolved infection; antibody titers are measurable in 20% to 50% of cases.

Transmission

• Infection occurs following ingestion of sporulated oocysts from fecal-contaminated environments, food, articles, or water.

Host Associations and Transmission Between Hosts

• Cryptosporidium exists as several distinct species with different degrees of infectivity for animals and humans:
  • C. canis - dogs, rarely humans
  • C. felis - cats, rarely humans
  • C. hominis - only in humans
  • C. parvum - a species in ruminants and other hosts that also will readily infect people. This species does not appear to occur in dogs and cats.

• There have been very few reports of people infected with C. canis and C. felis, and most of these infections occurred in individuals who were immunocompromised. In the United States, all human cases in which C. canis (1 case) or C. felis (6 cases) were identified occurred in immunocompromised individuals (Lucio-Forster et al., 2010)1. See section on 'Zoonotic Potential of Valid Species of Cryptospodium' for further information.

1Lucio-Forster A, et al, 2010. Minimal zoonotic risk of cryptosporidiosis from pet dogs and cats. Trends Parasitol., 6:4:174.

Prepatent Period and Environmental Factors

• Oocysts are generally shed in the feces 3 to 6 days after infection.

• Oocysts are immediately infectious when passed and are capable of surviving in the environment for extended periods.

• Oocysts are resistant to most disinfectants, including routine chlorine concentrations in drinking water and swimming pools.

• Oocysts are susceptible to commercial formulations of ammonia and heat over 70°C.

Site of Infection and Pathogenesis

• Cryptosporidium spp. infect the microvillar border of cells lining the small intestine and other organs. The developmental cycle is similar to Isospora spp.
• In immmunocompromised humans, infections may also occur in the large intestine, respiratory tract, biliary tract, pancreatic duct, and other sites.
• Most Cryptosporidium infections in dogs and cats are subclinical.
  • Diarrhea and dehydration are the primary clinical signs
  • In immunocompetent and healthy dogs and cats, the infections are usually self-limiting.
• Persistent infections usually denote an underlying cause such as canine distemper.

Diagnosis

• Diagnosis can be difficult. Specimens should be sent to an academic or commercial testing laboratory familiar with the necessary diagnostic techniques.
• Specific species identification requires molecular methods (e.g., PCR) that are currently only performed in only a few laboratories.
• Oocysts are typically isolated using sucrose flotation.
  • The oocysts are spheroid and small, 4-6 µm in diameter. In sucrose preparations, the oocysts float just under the coverslip on the slide.
  • These are some of the smallest parasites seen in fecal samples and require skill and practice for accurate diagnosis.
• Oocysts in fecal smears are red when acid-fast stains are used.
• Fluorescent antibody tests can be performed on fecal smears.
• Fecal antigen is detected with enzyme-linked immunosorbent assays (ELISAs).

Treatment

• Few drugs are consistently effective against Cryptosporidium.
• The following drugs have been used with some success in cases where animals have persistent diarrhea with oocyst shedding:
  • Paromomycin: 150 mg/kg SID for 5 days (dogs and cats)
  • Tylosin: 10 to 15 mg/kg TID for 14 to 21 days (cats)
  • Azithromycin: 5 to 10 mg/kg BID for 5 to 7 days (dogs); 7 to 15 mg/kg for 5 to 7 days (cats)
  • Until recently nitazoxanide (Alinia™)2 was approved only for use in humans. A regimen has been approved to treat adult animals: 100 mg BID for 5 days in animals 24 to 47 months old and 200 mg BID for 5 days in animals 4 to 11 years old. The efficacy of nitazoxanide in dogs and cats is not known.

2 Alinia is a trademark of Romark Laboratories.

Zoonotic Potential of Valid Species of Cryptosporidium

C. hominis

• Primary Host: Humans
• Location: Small intestine
• Zoonotic Potential: No

C. andersoni

• Primary Host: Cattle
• Location: Abomasum
• Zoonotic Potential: No

C. baileyi

• Primary Host: Avians
• Location: Bursa of Fabricius, cloaca
• Zoonotic Potential: ??

C. canis

• Primary Host: Dog
• Location: Small intestine
• Zoonotic Potential: Yes, rare in immunocompromised humans

C. felis

• Primary Host: Cat
• Location: Small intestine
• Zoonotic Potential: Yes, rare in immunocompromised humans

C. galli

• Primary Host: Avians

C. meleagridis

• Primary Host: Avians
• Location: Small intestine

C. muris

• Primary Host: Mice
• Location: Stomach
• Zoonotic Potential: ??

C. parvum*

• Primary Host: Many mammals
• Location: Small intestine (other locations in immunocompromised hosts)
• Zoonotic Potential: Yes (some genotypes)

C. suis

• Primary Host: Pigs, cattle
• Location: Small intestine, large intestine
• Zoonotic Potential: ??

C. saurophilum

• Primary Host: Snakes, lizards
• Location: Stomach, small intestine
• Zoonotic Potential: No

C. serpentis

• Primary Host: Snakes, lizards
• Location: Stomach
• Zoonotic Potential: No

C. molnari

• Primary Host: Fish
• Location: Stomach, small intestine
• Zoonotic Potential: No

C. wrairi

• Primary Host: Guinea pigs
• Location: Small intestine
• Zoonotic Potential: No

*Several genotypes are recognized (see Host Ranges of Genotypes of Cryptosporidium parvum)

?? = Some evidence exists for human infections.

Host Ranges of Genotypes of Cryptosporidium parvum

Genotypes of C. parvum and Host Range

• Cattle - Humans and many other mammals
• Mouse - Mice, Bats
• Marsupial - Koala, Kangaroo
• Ferret - Ferret