RAI BugWiki


Cryptosporidium parvum and Cryptosporidium hominis are intracellular protozoan parasites that may produce gastrointestinal symptoms when ingested by humans. Up until 2002 C. parvum was named C. parvum genotype 2 (cattle genotype) and C. hominis was named C. parvum genotype 1 (the human genotype). They are now recognised as different species based on genetic distinctions, but care should be taken when reviewing pre-2002 literature for this reason. There are a further thirteen recognised species of the genus Cryptosporidium. C. parvum and C. hominis are the most widespread and most frequently associated with human infections (C. parvum infects cattle too; other spp infect other mammals and birds but not humans.

C. hominis and C. parvum are transmitted person-to-person. C. parvum is also transmitted animal-to-animal or animal-to-human (zoonotically). Both organisms move between hosts in an environmentally-resistant oocyst form that is excreted with faeces. An oocyst is the resting stage of the protozoan, similar to a bacterial spore, and may facilitate transmission of the organism via contaminated water or food.

[important]Growth and Control[/important]

N.B. Viability refers to whether the organism is still alive, infectivity relates to the organism’s ability to infect a host.


The organism does not replicate outside the intestines of hosts.



Oocysts remain infective in water at:

67.5°C for 1 minute

-5°C for up to 8 weeks

-10°C for up to 7 days

-15°C for up to 24 hours

-20°C for up to 5 hours.


Viable C. parvum oocysts were detected after 14 days, in media based on citric acid (pH 4.6, 3.6 and 2.6), acetic acid (pH 3.6) or lactic acid (pH 4.6) held at 4°C or 22°C.

Water Activity

Oocysts survive on wet stainless steel (93.1% viable after 30 minutes).



Oocysts become non-infective when:

Pasteurised (71.7°C, 15 seconds)

Held in water at 64.2°C for 2 minutes

Frozen in de-ionised water (-20°C for 24 hours and -70°C for 1 hour)

Frozen in liquid nitrogen (-196°C)

Frozen in ice-cream (-20°C for 24 hours)


pH <4 or >11 results in minor viability loss. Ammonia can enhance inactivation at low and high pH values. Fruit juice organic acids inhibit oocyst infectivity.

Water Activity

Oocysts are sensitive to air drying (e.g. at ambient temperatures). Viability on stainless steel after 10 minutes = 81%, 1 hour = 69%, 2 hours = 45%, 4 hours = 5%. At aw 0.85, 99.9% are non-infective after 24 hours at 28°C, or 1 week at 7°C. At aw 0.95, 99.99% are non-infective after 1 week at 28°C, or 2 weeks at 7°C.


Sensitive to UV from sunlight and UV lamps. Under strong sunlight for 8 and 12 hours, oocyst viability reduced from 98% to 11.7% and 0.3%, respectively. A synergistic effect occurs at temperatures greater than 45°C.

Disinfectants / Sanitisers

See here for guidance.

Oocysts are resistant to chlorine and monochloramine. Swimming pool transmission has been problematic for this reason.Few disinfectants tested were effective with short exposure. These were:

Ammonia 5% for 120 minutes or 50% for 30 minutes

Formalin 10% for 120 minutes

Hydrogen peroxide 10 vol; 3% for 30 minutes

Ozone (25°C, 1ppm) renders oocysts non-infective after 3 minutes (90% decrease), 5 minutes (99%) and 10 minutes (99.9%).

[important]Clinical Notes[/important]

[warning]Don’t forget to read the disclaimer![/warning]

Incubation: Time from ingestion to illness usually 3-5 days, up to 2 weeks.

Symptoms: Copious watery non-bloody diarrhoea, vomiting, anorexia, fever, malaise, abdominal cramping and weight loss.

Usually self-limiting. Symptoms typically last 2-4 days but can last up to two weeks. Infectious oocysts usually excreted for 6-9 days, but can be excreted by asymptomatic carriers for up to 2 months or longer following infection. The respiratory system may also be adversely affected by the illness.

Condition: Cryptosporidiosis.

Dose: Healthy human volunteer studies have shown that the infectious dose varies depending on the isolate used. The dose needed to infect 50% of the population ranged from 9-1042 oocysts (Chappell et al., 2006; Okhuysen et al., 1999).

At Risk Groups: Children aged under 1 year and immunocompromised people are more susceptible (Carey et al., 2004).

Long Term Effects: Immunocompromised patients (e.g. AIDS patients or those receiving immunosuppressant drugs) develop severe symptoms that can last months or years and the condition can be fatal. The infection may also spread to other parts of the body in immunocompromised patients, such as the respiratory tract.

Treatment: There is no specific treatment for cryptosporidiosis currently available, but some antibiotics (e.g. spiramycin) are effective.

[important]Reservoirs / Sources[/important]

Human: Person-to-person transmission occurs with C. parvum and C. hominis. More than 108-1010 oocysts can be excreted daily in the faeces of infected hosts.

Animal: C. parvum has been identified in 155 species of mammal including domestic animals. Pre-weaned ruminants, especially calves, are especially vulnerable to infection. Shedding of oocysts increases in calves to a peak at day 12 (excreting approximately 4×1010 oocysts). Flies are potential vehicles of transmission as oocysts can pass unaltered though their gastro-intestinal tract and be deposited in fly faeces, but flies have never been linked to an outbreak.

Around 80% of oocysts remain in an infected host’s gut and can autoinfect the host. The remaining 20% develop thick (trilaminar) cell walls and are released in the faeces of the infected host into the environment. This can lead to subsequent infection of a new host.

Food: Oocysts have been detected in raw milk, raw meat, raw fruits and vegetables. Foods suspected as acting as vehicles of disease include raw milk, green onions, apple cider, sausage, fruit/vegetables and frozen tripe.

Environment: Oocysts in the environment are particularly resilient, especially under cool, moist conditions. Infectivity can be retained for months, especially in low water temperatures (<5°C but above freezing) (Fayer, 2004). The long term viability of oocysts in animal faeces is unclear. A study of a variety of slurries, soil types, pH ranges and temperatures found that acid to neutral soils at 4°C were most conducive to oocyst survival (still viable after 162 days).

Transmission Routes: Three main routes of transmission are person-to-person, zoonotic, and via faecally-contaminated water or food.