Introduction

• Parasites are widely spread in the environment and are responsible for causing diseases associated with diarrheal illness.
• Food-borne and water-borne parasites are mainly associated with the disease and are the greatest concern in food production.
• Parasitic protozoans cause the greatest risk to human health and economic loss to the nation mainly in developing nations.
• Some of the threat-causing parasitic protozoans to humans are Cryptosporidium, Cyclospora, Giardia, and Toxoplasma.
• Giardia is the primary known parasite associated with human disease and many epidemiological cases have been reported every year.
• Giardiasis causing species that infect humans and other mammals are Giardia duodenalis, Giardia lamblia, Giardia intestinalis and Giardia muris.
• Many food-borne parasitic cases occur each year but very few cases are reported which may be due to the limitation of surveillance and detection methods.

Sources of contamination of Foodborne Giardiasis

• The main source of contamination is the feces of pets, livestock, and wild animals and is commonly transmitted through the fecal-oral route.
• Giardia cysts are also found in sewage effluent and shallow spring water as well as the cysts are very infectious and can remain for a long period in a cool and damp place.
• Giardia is resistant to chlorine and can survive some common water treatments creating a great risk to water supplies.
• They can be removed by filtration process but Cryptosporidium cannot be removed by filtration because of their smaller size.

Characteristics of Giardia

• Flagellated protists 
• Tumbling motility
• Diplomonads i.e, two bodies having individual nuclei
• Mitochondria replaced by mitosome
• Metabolizes by anaerobic glycolysis
• The life cycle completes at two stages: a reproductive trophozoite and a cyst stage.
• Reproduces asexually
• The reproductive trophozoite is 9 – 21 µm long and cysts are 9 – 12 µm long
• Giardia cysts are susceptible to disinfectants like chlorine, iodine, and chlorine dioxide.

Clinical manifestations of Foodborne Giardiasis

• The infectious dose of Giardia is 10 to 100 cysts and the symptoms usually begin within 1 to 2 weeks of infection.
• Symptom starts with diarrhea, nausea, bloating of the stomach, abdominal cramps, malabsorption, and flatulence.
• In severe conditions, dehydration and weight loss might lead to chronic infection and even death.
• Some cases may occur where the patients are asymptomatic carriers but the disease only lasts for 5 days or a week.

Pathogenesis of Giardia species

• The life cycle of Giardia completes in two-stage, first the cysts that initiate infection, and the second vegetative trophozoite which replicates and causes disease.
• The cysts are environmentally resistant especially at low temperatures because of their outer filamentous layer made of N-acetyl-galactosamine and three proteins namely CWPs 1 – 3 which allow them to survive for a long time.
• About 10 to 100 cysts are capable of causing infection when it passes through the stomach acid and reaches the duodenum.
• After entering the duodenum, two trophozoites are formed from each cyst and colonize in the small intestine by absorbing essential nutrition and causing illness to the host.
• Giardia reproduces asexually, therefore, trophozoites replicate by the mode of binary fission and are tetraploid i.e, contain four sets of a chromosome.
• The above cycle is followed by excystation when the cyst passes through the stomach’s acidic environment to the duodenum and in the small intestine, and encystation takes place.
• In response to the trophozoite, T lymphocytes are activated by the body’s immune system which disrupts the epithelial microvilli and shortens them.
• It also disrupts the epithelial tight junctions that lead to an increase in permeability and diarrhea.
• The cyst also goes through bile salt and cholesterol starvation, starting its replication and getting divided into two trophozoites and infecting the other host.
• It is also evident that the host acquires resistance to the infection from an effective immune response.
• The organism is reported to have adapted to the different intestinal environments according to their antigenic variation.
• Although the organism does not produce any toxins or virulence factors, some organisms with variant-specific proteins (VSPs) build resistance to antibodies.

Epidemiology of Foodborne Giardiasis

• The protozoan parasite Giardia causes approximately 4 to 20 cases of outbreak per 100,000 population every year and most are reported to be caused by the fecal-oral transmission route.
• Giardia is reported at the 11^th position out of 24 foodborne parasites according to the Food and Agriculture Organisation (FAO) and WHO.
• The epidemiology of giardiasis is more prevalent in developing countries and is also associated with environmental and climatic conditions.
• Children under 5 years are at high risk as compared to adults and about 60-80% of infections are asymptomatic, however, chronic infection is less than 4%.
• In Peru, 95% of children aged under 2 years were infected but after treatment, the infection reoccurred within 6 months.
• The illness is usually asymptomatic and leads to stunting growth in children at an early stage of growth.
• Recently, a large outbreak has been reported in Bergen, Norway infecting more than 3000 people which was due to the consumption of contaminated drinking water.
• It was reported to be a waterborne outbreak as the excretion dose is higher than the infective dose per day.
• An infected person excretes up to 10⁹ cysts per day but the infective dose required to cause infection is 10 to 100 cysts at a time.
• The possible rate of contamination is high in developing countries where people consume tap water, shallow wells, and surface water.
• These water sources are rich in zoonotic parasites and when consumed without purification lead to human infection.
• The giardiasis infection usually peaks in the late summer as the consumption of water is high at this time and most children are affected in daycare centers.
• An outbreak that occurred in Minnesota was from the consumption of home-canned salmon infecting 60 employees at a school.
• But the outbreak was not reported as giardiasis as the transmission was asymptomatic carriers of Giardia.

Detection Methods of Giardia species

• The Giardia cysts in feces samples can be identified by the use of a microscope but this traditional method lacks specificity and sensitivity.
• Immunoassay methods such as enzymatic immunoassays (EIAs) and immunochromatographic tests are also used but they also lack specificity due to the cross-reaction between antibodies.
• The most widely used method by the US Environmental Protection Agency (EPA) is the separation of cysts by an immunomagnetic method.
• After separation, the cysts are visualized under a microscope by staining them with fluorescent dye and differential interference contrast (DIC).
• The PCR-based method offers improved specificity and sensitivity as compared to other detection methods as it identifies the pathogens at the species and genotype level.

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Prevention and Control measures of Foodborne Giardiasis

• Symptomatic infections are easy to treat but asymptomatic infection is hard to detect and are observed once it becomes chronic.
• The first choice for treatment is metronidazole, nitroimidazole and tinidazole.
• Tinidazole is most effective as it has high efficacy in a single dose as compared to metronidazole which requires 5-10 days to reach the effective dose.
• For pregnant women, paromomycin is recommended as it is less effective than nitroimidazole and nonabsorbed aminoglycoside.
• Human giardiasis can be prevented by breaking the life cycle of Giardia from transmitting which can be done by providing the treated municipal water supplies. 
• Municipal water supplies process the water by coagulation, clarification, filtration and chlorination which is highly effective at removing cysts and inactivating other pathogens.
• Giardia cysts are also inactivated by heat treatment at 55°C and above for 5 mins and the use of household portable filters of size less than 1µm is also effective in filtering cysts.
• Washing hands and use of protective gloves by food handlers while preparing food also prevents the risk of asymptomatic contamination.