What is Giardia?

Giardia is a single-celled, flagellate, protozoan parasite belonging to the order Diplomonadida. The cells are unusual in having two nuclei. The species important in human illness is Giardia intestinalis (previously referred to as G. lamblia, or G. duodenalis). G. intestinalis is also found in a number of domestic and wild animals, including cattle, cats and dogs.

G. intestinalis is an obligate parasite and requires a host in order to multiply. It was first discovered in 1859, but was not confirmed as a human pathogen until the late 1970’s. It is a cause of gastrointestinal infection (giardiasis) in humans and some other animals, and is found worldwide. G. intestinalis has a two-stage life cycle, and exists in two forms. Pear-shaped flagellated trophozoites exist and multiply within the gastrointestinal tract of the host. Some of these form spore-like resistant cysts within the small intestine. Both forms may be excreted in the host’s faeces, but the trophozoites die quickly and the transmissible stage in the cycle is the resistant, thick-walled cyst.

What foods can be contaminated?

Giardia intestinalis is mainly associated with surface water that has been polluted by human or animal faeces, but cysts have also been found in a number of unprocessed foods, including root crops, lettuce, herbs and strawberries. Any food that may come into contact with contaminated water during production, and where there is no subsequent process that will destroy cysts, may be at risk from G. intestinalis contamination. The parasite cannot grow in foods or in water and does not multiply in the environment outside of a suitable host.

Cysts are destroyed by heat and G. intestinalis is not normally associated with cooked and processed foods. However, food is not considered to be a major vehicle for the transmission of the parasite. The water-borne and person-to-person transmission routes are thought to be much more common and animal-to-human (zoonotic) transmission may also occur.

How does it affect human health?

G. intestinalis can cause an acute gastrointestinal infection in humans, and children are especially vulnerable to infection. The mechanism by which it causes disease is unclear. The trophozoites attach to the cells lining the gut, but do not seem to invade them. They may produce a toxin in the small intestine, but this has not been confirmed. The incubation time for the infection is usually between 1-3 weeks from ingestion of cysts. The main symptom is diarrhoea, often accompanied by abdominal pain. Flatulence, fever and loss of appetite may also occur. In healthy adults, symptoms typically last for 1-2 weeks, but may last for up to 6 weeks in some cases. The infection is generally self-limiting, but drug treatment is sometimes required. However, in immuno-compromised individuals, infection can be more serious and long lasting, requiring hospital treatment, and occasional deaths have been recorded. Complications of chronic giardiasis may include severe weight loss and the development of lactose intolerance.

The infective dose is thought to be very low and ingestion of as few as 10 cysts (trophozoites are virtually non-infective) may be enough to cause giardiasis. Infected individuals shed very large numbers of infectious cysts in their faeces, and this may continue for months after symptoms have subsided. Asymptomatic cases of infection are quite common and asymptomatic carriers have been reported to continue shedding cysts for years.

How common is illness?

G. intestinalis is probably the most commonly reported intestinal parasite in the developed world. In England and Wales between 1986 and 1996, the number of reported cases each year generally ranged from 5,000 to 7,000, but from 1996 to 2006 the number of confirmed cases fell and now averages around 3,000 cases each year.

The most recent data for the EU refers to 2008 and shows a total of 167,414 reported cases of giardiasis from 22 countries. However, there are large differences between surveillance systems in different European countries and there is likely to be significant under-reporting. The data also show that children aged 0-4 years were most commonly infected and that there are seasonal peaks of infection in spring and autumn. Giardiasis is a notifiable disease in much of the EU and in the USA.

There were 19,140 reported cases of giardiasis in the USA in 2008. This figure has been relatively stable in recent years. Most cases were reported from the Northern states and there was a peak in the summer and early autumn.

There is little or no information about the proportion of reported cases that are foodborne, but it is thought likely that the majority are caused by contact with contaminated water, infected people, and occasionally animals.


Most reported outbreaks of giardiasis are associated with contaminated surface water, or person-to-person transmission. Most of the documented outbreaks have been recorded in the USA, and outbreaks in Europe appear to be rare. Foodborne outbreaks have also been recorded in the USA, usually caused by an infected food handler, or by faecal contamination, either direct or through polluted water. Outbreaks have been linked to salad, lettuce and tomatoes, noodle salad, canned salmon, cheese dip, sandwiches, fruit salad and ice.

Where does it come from?

G. intestinalis is an obligate parasite and thus originates from the host. The primary source of G. intestinalis is therefore the faeces of infected humans and animals, which may contain up to a billion cysts in a single day. The cysts are extremely infectious and may be transferred to food via an infected food handler, or through polluted water used for crop irrigation or processing.

G. intestinalis cysts are larger than those of Cryptosporidium (9-12 μm diameter) and are not so difficult to remove from water using modern water treatment methods. They are also less resistant to chlorine, but are not inactivated by the concentrations normally used to treat water. They are much less likely to pass through water treatment plants into the public water supply system.

How is it affected by environmental factors?

G. intestinalis cysts are generally resistant to environmental factors. Cysts can persist for months in cool moist conditions and have been shown to survive for 8 days on the leaves of herbs. However, there is little information on their survival and inactivation in foods.


G. intestinalis cysts are not especially heat resistant and are destroyed by conventional milk pasteurisation. A temperature of 60-70oC for 10 minutes is reported to inactivate cysts completely.

Oocysts can survive for significant periods at temperatures below 0oC, especially in water, but frozen storage is reported to cause inactivation.


There is little information on the effect of pH, but it has been reported that cysts are resistant to low pH values down to about 3.0.


The cysts are relatively resistant to some sanitisers and disinfectants, notably chlorine and ozone, but are reported to be inactivated by phenolic disinfectants.

How can it be controlled?

Control measures for G. intestinalis in food processing focus largely on the control of contamination in water and the management of infected food handlers.

For food processors and caterers

Care should be taken to ensure that raw food ingredients and products that do not undergo further processing do not come into contact with contaminated surface water. Fresh produce should be obtained from suppliers practicing Good Agricultural Practice. Fresh produce and other raw foods should only be washed/processed using potable quality water.

Heat processing is an effective control against G. intestinalis cysts in food. Normal milk pasteurisation processes are effective, as are recommended ‘Listeria cook’ processes for meat products (70oC for at least 2 mins). Reheating cooked foods to at least 74oC will destroy cysts immediately. Freezing foods for at least 7 days is also an effective control.

Infected food handlers are also a major G. intestinalis contamination risk for foods that do not undergo any further processing, such as sandwiches and salads. Good personal hygiene practice, especially hand washing, is an essential control and any staff suffering from gastroenteritis should be excluded from processing areas.

Are there rules and regulations?

G. intestinalis is generally considered to be a water-borne pathogen rather than foodborne. It may therefore be covered in drinking water regulations, as is the case in the UK, but is not usually mentioned specifically in food safety and hygiene law.

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