The organism does not grow outside of the animal reservoir so controls designed to restrict the growth of bacteria will be ineffective. There is very little information on the survival of the organism on foods or on how cysts present on foods may be destroyed.
In general the cysts are stable and can last for long periods (months) in the environment. Cool moist conditions favour survival. Cysts have been shown to survive on herbs for 8 days.
Cyst survival in water is better at lower temperatures. Survival was 77 days in distilled water at 8°C, but only 4 days at 37°C. 0.1-25% survived in river water for up to 56 days.
Heating cysts to 60-70°C for 10 min inactivates them. Alternatively heating to boiling for 3 min will achieve the same result. Milk pasteurisation is sufficient to inactivate cysts. Cysts may be inactivated when frozen in water for long periods.
Sanitisers / Disinfectants
Relatively resistant to ozone requiring 0.17 mg-min/l at 25°C or 0.53 mg-min/l at 5°C to reduce viability by 99%. Chlorination at levels required to inactivate E. coli is not sufficient to inactivate Giardia cysts. The protozoan requires <15 mg-min/l at 25°C and 90-170 mg-min/l at 5°C for inactivation. Ct values: chlorine 93 to 121, chloramine 1,470, chlorine dioxide 17, ozone 1.3. Physical removal, e.g. passing through an 8μm pore filter will remove cysts from water. Commercial phenol-based disinfectants are effective at inactivating the organism.
Cysts are resistant to UV radiation. Doses of 42,000 to 63,000 μW-s/cm2 resulted in only a 90% loss of viability. Ct for UV 80 mJ cm-2.
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One to 3 weeks after infection.
Immunocompetent people who are infected may be asymptomatic or experience gastrointestinal disease. In the immunocompromised the results may be more serious (severe diarrhoea, dehydration and loss of weight), and occasionally life threatening. Malabsorption may be quite severe in diarrhoeal cases. Symptoms may last from 4 to 6 weeks, and consist of diarrhoea followed by flatulence, foul-smelling stools and cramps. The mean attack rate is 44%.
Toxins are not formed in food by this organism.
At Risk Groups
Any one may become infected, but the disease is more serious in those who are immunocompromised. Infants and children are more susceptible to infection than adults.
Long Term Effects
Can be fatal in those unable to fight the disease. Lactose intolerance can be induced in 40% of cases for some time after symptoms have ceased.
As few as 10 cysts can cause an infection when ingested.
Most cases are self-limiting, but chemotherapy can be used.
Asymptomatic carriers may excrete the organism for years. The asymptomatic carriage rate has been estimated at 3-20% in the USA.
The organism is found in mammals, birds, reptiles and amphibians. However isolates from these sources are not necessarily pathogenic to humans.
Agricultural products that have directly or indirectly been subjected to faecal contamination may be a source of cysts. Giardia has been isolated from root crops, lettuces, herbs and strawberries irrigated with wastewater.
Cysts are found in waters which have been faecally polluted.
Transmission to humans may be via water, food contaminated by water or a food handler, or person-to-person.
Few well documented outbreaks have been reported, and this may be due to the long incubation period.