[4] Giardiasis usually spreads when Giardia duodenalis cysts within faeces contaminate food or water that is later consumed orally.

[5] The cause of this wide range in severity of symptoms is not fully known but the intestinal flora of the infected host may play a role.

[10][4] This characteristic diarrhoea is often accompanied by several other symptoms, including gas, abdominal cramps, and nausea or vomiting.

[10][4] Some people also experience symptoms outside of the gastrointestinal tract, such as itchy skin, hives, and swelling of the eyes and joints, although these are less common.

[11][4] In children, prolonged giardiasis can cause failure to thrive and may impair mental development.

[17][18] According to the United States Centers for Disease Control and Prevention (CDC), people at greatest risk of infection are:[19] Factors that increase infection risk for people from developed countries include changing nappies/diapers, consuming raw food, owning a dog, and travelling in the developing world.

[1] In the United States, giardiasis occurs more often in summer, which is believed to be due to a greater amount of time spent on outdoor activities and travelling in the wilderness.

[8] As trophozoites continue along the gastrointestinal tract, they convert back to their cyst form which is then excreted with faeces.

[23] The attachment of trophozoites causes villous flattening and inhibition of enzymes that break down disaccharide sugars in the intestines.

[7][22] Ultimately, the community of microorganisms that lives in the intestine may overgrow and may be the cause of further symptoms, though this idea has not been fully investigated.

The alteration of the villi leads to an inability of nutrient and water absorption from the intestine, resulting in diarrhoea, one of the predominant symptoms.

[7] Arginine starvation is known to be a cause of programmed cell death, and local removal is a strong apoptotic agent.

[24] Host defence against Giardia consists of natural barriers, production of nitric oxide, and activation of the innate and adaptive immune systems.

[citation needed] Infection with Giardia typically results in a strong antibody response against the parasite.

While IgG is made in significant amounts, IgA is believed to be more important in parasite control.

Other roles include cytokine production (Il-4, IL-9) to help recruit other effector cells of the immune response.

[43][44] Although the evidence linking the drinking of water in the North American wilderness and giardiasis has been questioned, a number of studies raise concern.

Giardia is often found on the surface of the ground, in the soil, in undercooked foods, and in water, and on hands that have not been properly cleaned after handling infected faeces.

Lastly, food-borne epidemics of Giardia have developed through the contamination of food by infected food-handlers.

[50] Researchers at CONICET have produced an oral vaccine after engineering customised proteins mimicking those expressed on the surface of Giardia trophozoites.

[9] The World Health Organisation and Infectious Disease Society of America recommend metronidazole as first-line therapy.

[57][58][59] A meta-analysis published by the Cochrane Collaboration in 2012 found that compared to the standard of metronidazole, albendazole had equivalent efficacy while having fewer side effects, such as gastrointestinal or neurologic issues.

The evidence for comparing metronidazole to other alternatives such as mebendazole, tinidazole, or nitazoxanide was felt to be of very low quality.

[62] In the case of nitroimidazole-resistant strains of Giardia, other drugs are available which have shown efficacy in treatment including quinacrine, nitazoxanide, bacitracin zinc, furazolidone and paromomycin.

[63] During pregnancy, paromomycin is the preferred treatment drug because of its poor intestinal absorption, resulting in less exposure to the foetus.

[1][66][21] Children with chronic giardiasis are at risk for failure to thrive as well as more long-lasting sequelae such as growth stunting.

[70] The states of Illinois, Kentucky, Mississippi, North Carolina, Oklahoma, Tennessee, Texas, and Vermont did not notify the Centers for Disease Control and Prevention regarding cases in 2018.

[71] July, August, and September are the months with the highest incidence of giardiasis in the United States.

[73] Some intestinal parasitic infections may play a role in irritable bowel syndrome[74] and other long-term sequelae such as chronic fatigue.

[75][76] The mechanism of transformation from cyst to trophozoites has not been characterised[8] but may help develop drug targets for treatment-resistant Giardia.