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Cryptosporidium

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From the same stable as malaria and toxoplasmosis – a parasite on the increase…

Cryptosporidium parvum and Cryptosporidium hominis are intracellular protozoan parasites that can produce gastrointestinal symptoms when ingested by humans. The number of cases of cryptosporidiosis reported in the UK in 2012 was 6,612. This is almost double the number reported in 2011, and the increase was observed across England, Wales and Scotland. The increase is likely due to the year-on-year variation which is observed in Cryptosporidium cases, exacerbated by a large outbreak of over 300 cases associated with the consumption of mixed salad leaves. 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. 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 for this reason care should be taken when reviewing pre-2002 literature. 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).

Growth and Survival Inactivation (CCPs and Hurdles) Clinical Notes Reservoirs / Sources
The organism does not replicate outside the intestines of hosts. Note that viability refers to whether the organism is still alive, infectivity relates to the organism’s ability to infect a host.

Temperature

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
pH

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).

Temperature

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

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.

Radiation

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

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

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%).
Don’t forget to read the disclaimer!

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.

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.

At Risk Groups

Children aged under 1 year and immunocompromised people are more susceptible.

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 but some antibiotics (e.g. spiramycin) are effective.

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. 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.

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