Cryptosporidiosis

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 (see Tables 1 and 2).

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 (see images on right)

Sporulated oocysts—resistant stages for environmental transmission—are infective when shed by a host.
Schizonts (asexual stages)
Gametes (sexual stages)

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 Table 1 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. (Slide 2: Link to PDF Images 190K)
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 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. (Slide 1: Link to PDF Images 190K)
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.

²Alinia is a trademark of Romark Laboratories.

Tables

Table 1. Zoonotic Potential of Valid Species of Cryptosporidium

Species	Primary Host	Location	Zoonotic Potential?
C. hominis	Humans	Small intestine	No
C. andersoni	Cattle	Abomasum	No
C. baileyi	Avians	Bursa of Fabricius, cloaca	??
C. canis	Dog	Small intestine	Yes
C. felis	Cat	Small intestine	Yes
C. galli	Avians
C. meleagridis	Avians	Small intestine	Yes
C. muris	Mice	Stomach	??
C. parvum*	Many mammals	Small intestine (other locations in immunocompromised hosts)	Yes (some genotypes)
C. suis	Pigs, cattle	Small intestine, large intestine	??
C. saurophilum	Snakes, lizards	Stomach, small intestine	No
C. serpentis	Snakes, lizards	Stomach	No
C. molnari	Fish	Stomach, small intestine	No
C. wrairi	Guinea Pigs	Small intestine	No

*Several genotypes are recognized (see Table 2).
?? = Some evidence exists for human infections.

Table 2. Host Ranges of Genotypes of Cryptosporidium parvum.

Genotypes of C. parvum	*Host* *Range*
Cattle	Humans and many other mammals
Mouse	Mice, Bats
Marsupial	Koala, Kangaroo
Ferret	Ferret

The Companion Animal Parasite Council