View source for Giardiasis

{{Short description|Parasitic disease that results in diarrhea}}
{{Infobox medical condition (new)
| name            = Giardiasis
| synonyms        = Beaver fever, giardia
| image           = Giardia lamblia SEM 8698 lores.jpg
| image_size      = 300px
| caption         = ''Giardia'' cell viewed with [[scanning electron microscope]]
| field           = [[Infectious disease (medical specialty)|Infectious disease]], [[gastroenterology]]
| symptoms        = [[Diarrhoea]], [[abdominal pain]], weight loss, nausea<ref name=BMJ2016/>
| onset           = 1 to 3 weeks after exposure<ref name=CDC2015>{{cite web|title=Giardia. General information |url= https://www.cdc.gov/parasites/giardia/general-info.html |website=CDC|access-date=17 November 2016|date=21 July 2015|url-status=live|archive-url=https://web.archive.org/web/20161117211242/https://www.cdc.gov/parasites/giardia/general-info.html|archive-date=17 November 2016}}</ref>
| duration        =
| causes          = ''[[Giardia duodenalis]]'' spread mainly through contaminated food or water<ref name=BMJ2016/>
| risks           = [[Hypogammaglobulinemia]]
| diagnosis       = Stool testing<ref name=BMJ2016/>
| differential    = [[Irritable bowel syndrome]]<ref name=BMJ2016/>
| prevention      = Improved [[sanitation]]<ref name=BMJ2016/>
| treatment       = [[Antiprotozoal]] medications
| medication      = [[Tinidazole]], [[metronidazole]]<ref name=BMJ2016/>
| frequency       = Up to 7% (developed world), up to 30% (developing world)<ref name=BMJ2016/>
| deaths          =
|alt              =
}}

'''Giardiasis''' is a [[parasitic disease]] caused by the [[protist]] enteropathogen ''[[Giardia duodenalis]]'' (also known as ''G. lamblia'' and ''G. intestinalis''), especially common in children and travelers.<ref name="Esch2013">{{cite journal | vauthors = Esch KJ, Petersen CA | title = Transmission and epidemiology of zoonotic protozoal diseases of companion animals | journal = Clinical Microbiology Reviews | volume = 26 | issue = 1 | pages = 58–85 | date = January 2013 | pmid = 23297259 | pmc = 3553666 | doi = 10.1128/CMR.00067-12 }}</ref><ref name=":12">{{Cite journal |last=Buret |first=A.G. |date=September 2008 |title=Pathophysiology of enteric infections with Giardia duodenalis |url=http://www.parasite-journal.org/10.1051/parasite/2008153261 |journal=Parasite |volume=15 |issue=3 |pages=261–265 |doi=10.1051/parasite/2008153261 |pmid=18814692 |issn=1252-607X}}</ref> Infected individuals experience [[steatorrhea]], a type of [[diarrhea]] with fatty sticky stool; [[abdominal pain]], [[weight loss]], and weakness due to [[dehydration]] and [[malabsorption]].<ref name=BMJ2016/> Less common symptoms include skin rash, [[hives]] and [[Joint effusion|joint swelling]].<ref name=BMJ2016/> Symptoms usually begin one to three weeks after exposure and, without treatment, may last two to six weeks or longer.<ref name=CDCSymptoms/> Some infected individuals experience mild or no symptoms and remain symptom-free even if the infection persists for a long time.<ref>{{Cite web |title=Giardiasis fact sheet |url=https://www.health.nsw.gov.au/Infectious/factsheets/Pages/giardiasis.aspx |access-date=2025-06-03 |website=www.health.nsw.gov.au |language=en}}</ref> 

Giardiasis spreads via the [[Fecal–oral route|fecal-oral]] route, when ''Giardia'' cysts excreted with feces contaminate food or water that is later consumed orally.<ref name=BMJ2016/> The disease can also spread between people and between people and animals, mainly via pets.<ref name=BMJ2016/><ref name="Heyworth2016" /><ref>{{Cite journal |last1=Cacciò |first1=Simone M. |last2=Lalle |first2=Marco |last3=Svärd |first3=Staffan G. |date=December 2018 |title=Host specificity in the Giardia duodenalis species complex |url=https://linkinghub.elsevier.com/retrieve/pii/S1567134817304185 |journal=Infection, Genetics and Evolution |language=en |volume=66 |pages=335–345 |doi=10.1016/j.meegid.2017.12.001|pmid=29225147 |bibcode=2018InfGE..66..335C |url-access=subscription }}</ref> Cysts may survive for nearly three months in cold water.<ref name=BMJ2016/> 

The microscopic identification of Giardia and its cysts in fecal samples is considered the gold standard method for diagnosing giardiasis.<ref name=":11">{{Cite journal |last1=Vicente |first1=Bruno |last2=Freitas |first2=Anna De |last3=Freitas |first3=Marcus |last4=Midlej |first4=Victor |date=2024-02-07 |title=Systematic Review of Diagnostic Approaches for Human Giardiasis: Unveiling Optimal Strategies |journal=Diagnostics |language=en |volume=14 |issue=4 |page=364 |doi=10.3390/diagnostics14040364 |doi-access=free |issn=2075-4418 |pmc=10887752 |pmid=38396402}}</ref> Immunoassays, such as [[ELISA]] and [[PCR amplification|PCR]] for giardia gene loci, are also available as diagnostic tools, although are not widely used due to methods complexity and costs.<ref name=":11" />

Prevention may be improved through proper personal [[hygiene]] practices and by cooking and sanitizing food.<ref name="BMJ2016" /> Asymptomatic cases often do not need treatment. When symptoms are present, treatment is typically provided with either [[tinidazole]] or [[metronidazole]].<ref name="BMJ2016" /> Other drugs, such as [[nitazoxanide]], [[albendazole]], [[Mepacrine|quinacrine]], [[chloroquine]], [[paromomycin]], and other drug combinations are also used in clinics.<ref>{{Cite journal |last1=Lalle |first1=Marco |last2=Hanevik |first2=Kurt |date=2018-10-24 |title=Treatment-refractory giardiasis: challenges and solutions |journal=Infection and Drug Resistance |language=English |volume=11 |pages=1921–1933 |doi=10.2147/IDR.S141468 |doi-access=free |pmc=6207226 |pmid=30498364}}</ref> Refractory giardiasis and resistant strains are reported more and more often.<ref>{{Cite journal |last1=Nabarro |first1=L. E. B. |last2=Lever |first2=R. A. |last3=Armstrong |first3=M. |last4=Chiodini |first4=P. L. |date=2015-08-01 |title=Increased incidence of nitroimidazole-refractory giardiasis at the Hospital for Tropical Diseases, London: 2008–2013 |url=https://www.clinicalmicrobiologyandinfection.com/article/S1198-743X(15)00432-2/fulltext |journal=Clinical Microbiology and Infection |language=English |volume=21 |issue=8 |pages=791–796 |doi=10.1016/j.cmi.2015.04.019 |pmid=25975511 |issn=1198-743X|doi-access=free }}</ref> Infection may cause a person to become [[lactose intolerant]], so it is recommended to temporarily avoid [[lactose]] following an infection or use [[lactase]] supplements.<ref name="BMJ2016" />  

Giardiasis occurs worldwide.<ref name=CDC_Giardiasis/> It is one of the most common [[List of parasites of humans|parasitic human diseases]].<ref name=Esch2013/> Infection rates are as high as 7% in the [[developed world]] and 30% in the [[developing world]].<ref name=BMJ2016/> In 2013, there were approximately 280 million people worldwide with symptomatic cases of giardiasis.<ref name=Esch2013/> The World Health Organization classifies giardiasis as a [[neglected disease]].<ref name=BMJ2016>{{cite journal | vauthors = Minetti C, Chalmers RM, Beeching NJ, Probert C, Lamden K | s2cid = 220092781 | title = Giardiasis | journal = BMJ | volume = 355 | article-number = i5369 | date = October 2016 | pmid = 27789441 | doi = 10.1136/bmj.i5369 | url = https://archive.lstmed.ac.uk/6353/1/BMJ_355_i5369_Giardiasis.pdf | archive-date = 2019-11-06 | access-date = 2019-11-06 | archive-url = https://web.archive.org/web/20191106212001/https://archive.lstmed.ac.uk/6353/1/BMJ_355_i5369_Giardiasis.pdf | url-status = dead }}</ref> It is popularly known as '''beaver fever'''<ref name=NYSDOH>{{cite web |url= http://www.health.ny.gov/diseases/communicable/giardiasis/fact_sheet.htm |title=Giardiasis (beaver fever) |access-date=27 March 2022 |publisher=New York State Department of Health |date=January 2022 |url-status=live |archive-url= https://web.archive.org/web/20220120123149/https://www.health.ny.gov/diseases/communicable/giardiasis/fact_sheet.htm |archive-date=20 January 2022}}</ref> in North America.

==Signs and symptoms==
The signs and symptoms vary from none to severe diarrhoea with poor nutrient absorption.<ref name=CDC_Giardiasis/> 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.<ref name=Cot2011>{{cite journal | vauthors = Cotton JA, Beatty JK, Buret AG | title = Host parasite interactions and pathophysiology in Giardia infections | journal = International Journal for Parasitology | volume = 41 | issue = 9 | pages = 925–33 | date = August 2011 | pmid = 21683702 | doi = 10.1016/j.ijpara.2011.05.002 }}</ref><ref name=":4">{{cite journal | vauthors = Einarsson E, Ma'ayeh S, Svärd SG | title = An up-date on Giardia and giardiasis | journal = Current Opinion in Microbiology | volume = 34 | pages = 47–52 | date = December 2016 | pmid = 27501461 | doi = 10.1016/j.mib.2016.07.019 }}</ref> Diarrhoea is less likely to occur in people from developing countries.<ref name=Cot2011 />

Symptoms typically develop 9–15&nbsp;days after exposure,<ref name="Barry2013" /> but may occur as early as one day.<ref name="CDC_Giardiasis">{{cite web |title=Giardiasis |url=https://www.cdc.gov/dpdx/giardiasis/|website=cdc.gov |date=December 9, 2017 |access-date=27 March 2022 |url-status=live|archive-url= https://web.archive.org/web/20210414044832/https://www.cdc.gov/dpdx/giardiasis/ |archive-date=April 14, 2021}}</ref> The most common and prominent symptom is chronic diarrhoea, which can occur for weeks or months if untreated.<ref name=PD6>{{cite book |chapter-url=http://www.parasiteswithoutborders.com/parasitic-diseases-6th-edition/ |access-date=11 July 2018 |title=Parasitic Diseases |edition=6 |vauthors=Despommier DD, Griffin DO, Gwadz RW, Hotez PJ, Knirsch CA |publisher=Parasites Without Borders |location=NY |chapter=III. Eukaryotic Parasites |pages=11–17 |archive-date=7 March 2020 |archive-url=https://web.archive.org/web/20200307144527/http://www.parasiteswithoutborders.com/parasitic-diseases-6th-edition }}</ref><ref name=Robertson2010>{{cite journal | vauthors = Robertson LJ, Hanevik K, Escobedo AA, Mørch K, Langeland N | title = Giardiasis--why do the symptoms sometimes never stop? | journal = Trends in Parasitology | volume = 26 | issue = 2 | pages = 75–82 | date = February 2010 | pmid = 20056486 | doi = 10.1016/j.pt.2009.11.010 }}</ref> Diarrhoea is often greasy and foul-smelling, with a tendency to float.<ref name=PD6/><ref name=CDCSymptoms>{{cite web |url=https://www.cdc.gov/parasites/giardia/illness.html |title=Giardia. Illness and symptoms |website=CDC |date=26 February 2021 |access-date=27 March 2022}}</ref> This characteristic diarrhoea (steatorrhea) is often accompanied by several other symptoms, including [[flatulence|gas]], abdominal cramps, and nausea or vomiting.<ref name="PD6" /><ref name=CDCSymptoms/> 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.<ref name=CDCSymptoms/> Fever occurs in only about 15% of infected people,<ref>{{Cite book |title=Tropical infectious diseases: principles, pathogens and practice |url=https://archive.org/details/tropicalinfectio00mdri_338 |url-access=limited|date=2011|publisher=Saunders/Elsevier|author1=Guerrant, Richard L. |author2=Walker, David H. |author3=Weller, Peter F. |isbn=978-1-4377-3777-6|edition=3rd|location=Edinburgh|pages=[https://archive.org/details/tropicalinfectio00mdri_338/page/n647 623]|oclc=722800379}}</ref> despite the nickname "beaver fever".<ref name=NYSDOH/>

Prolonged disease is often characterised by chronic diarrhoea and [[Malabsorption|malabsorption of nutrients]] in the intestine.<ref name=PD6/><ref name=":12" /> This malabsorption causes [[steatorrhea|fatty stools]], substantial weight loss, and [[fatigue]].<ref name=PD6/> Additionally, those with giardiasis often have difficulty absorbing [[lactose]], [[vitamin A]], [[folate]], and [[Vitamin B12 deficiency|vitamin B<sub>12</sub>]].<ref name=Robertson2010/><ref name=CDCSymptoms/> In children, prolonged giardiasis can cause [[failure to thrive]] and may impair mental development.<ref name="PD6" /><ref name=Robertson2010/> Symptomatic infections are well recognised as causing [[lactose intolerance]],<ref name="pmid2614615">{{cite journal | vauthors = Pettoello Mantovani M, Guandalini S, Ecuba P, Corvino C, di Martino L | s2cid = 32254397 | title = Lactose malabsorption in children with symptomatic Giardia lamblia infection: feasibility of yogurt supplementation | journal = Journal of Pediatric Gastroenterology and Nutrition | volume = 9 | issue = 3 | pages = 295–300 | date = October 1989 | pmid = 2614615 | doi = 10.1097/00005176-198910000-00006 | doi-access = free }}</ref> which, though usually temporary, may become permanent.<ref name="pmid1174208">{{cite journal | vauthors = Wolfe MS | title = Giardiasis | journal = JAMA | volume = 233 | issue = 13 | pages = 1362–5 | date = September 1975 | pmid = 1174208 | doi = 10.1001/jama.233.13.1362 }}</ref><ref name="pmid3430245">{{cite journal | vauthors = Vega-Franco L, Meza C, Romero JL, Alanis SE, Meijerink J | s2cid = 20733304 | title = Breath hydrogen test in children with giardiasis | journal = Journal of Pediatric Gastroenterology and Nutrition | volume = 6 | issue = 3 | pages = 365–8 | year = 1987 | pmid = 3430245 | doi = 10.1097/00005176-198705000-00010 | doi-access = free }}</ref>

==Cause==
{{multiple image
| width             = 150
| image1            = Infectiology - Giarda intestinalis - Cyst -- Smart-Servier.png
| alt1              = Cyst of Giarda intestinalis
| image2            = Infectiology - Giarda intestinalis - Trophozoite -- Smart-Servier.png
| alt2              = Trophozoite of Giarda intestinalis
| footer            = Cyst (left) and [[trophozoite]] (right) of ''Giarda intestinalis''
}}
Giardiasis is caused by the [[protozoan]] ''[[Giardia lamblia|Giardia duodenalis]]''.<ref name=Auer2012/> The infection occurs in many animals, including pets, beavers, other rodents, cows, and sheep.<ref name=Auer2012/> Animals are believed to play a role in keeping infections present in an environment.<ref name=Auer2012/>

''G. duodenalis'' has been sub-classified into eight genetic assemblages (designated A–H).<ref name="Heyworth2016" /> Genotyping of ''G. duodenalis'' isolated from various hosts has shown that assemblages A and B infect the largest range of host species, and appear to be the main and possibly only ''G. duodenalis'' assemblages that infect humans.<ref name="Heyworth2016">{{cite journal | vauthors = Heyworth MF | title = Giardia duodenalis genetic assemblages and hosts | journal = Parasite | volume = 23 | page = 13 | year = 2016 | pmid = 26984116 | pmc = 4794627 | doi = 10.1051/parasite/2016013 | url = https://www.parasite-journal.org/articles/parasite/full_html/2016/01/parasite150104/parasite150104.html| url-status = live | archive-url = https://web.archive.org/web/20170910163601/https://www.parasite-journal.org/articles/parasite/full_html/2016/01/parasite150104/parasite150104.html | archive-date = 2017-09-10 }} {{open access}}</ref><ref>{{cite journal | vauthors = Lalle M, Hanevik K | title = Treatment-refractory giardiasis: challenges and solutions | language = en | journal = Infection and Drug Resistance | volume = 11 | pages = 1921–1933 | date = 2018-10-24 | pmid = 30498364 | pmc = 6207226 | doi = 10.2147/idr.s141468 | doi-access = free }}</ref>

===Risk factors===
According to the United States [[Centers for Disease Control and Prevention]] (CDC), people at greatest risk of infection are:<ref name="cdc.gov">{{cite web|author=CDC|date=July 15, 2015|title=Sources of Infection & Risk Factors|url=https://www.cdc.gov/parasites/giardia/infection-sources.html|url-status=live|archive-url=https://web.archive.org/web/20170907165106/https://www.cdc.gov/parasites/giardia/infection-sources.html|archive-date=September 7, 2017|website=Parasites – Giardia}}</ref>
* People in childcare settings
* People who are in close contact with someone who has the disease
* Travellers within areas that have poor sanitation
* People who have contact with faeces, such as during sexual activity
* Backpackers or campers who drink untreated water from springs, lakes, or rivers
* Swimmers who swallow water from swimming pools, hot tubs, interactive fountains, or untreated recreational water from springs, lakes, or rivers
* People who get their household water from a shallow well
* People with weakened immune systems
* People who have contact with infected animals or animal environments contaminated with faeces

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]].<ref name=BMJ2016/> However, 75% of infections in the United Kingdom are acquired in the UK, not through travel elsewhere.<ref name=BMJ2016/> 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.<ref name=Auer2012/>

===Transmission===
Giardiasis is transmitted via the [[fecal-oral route|faecal-oral route]] with the ingestion of [[Microbial cyst|cyst]]s.<ref name="Barry2013">{{cite journal | vauthors = Barry MA, Weatherhead JE, Hotez PJ, Woc-Colburn L | title = Childhood parasitic infections endemic to the United States | journal = Pediatric Clinics of North America | volume = 60 | issue = 2 | pages = 471–85 | date = April 2013 | pmid = 23481112 | doi = 10.1016/j.pcl.2012.12.011 }}</ref> Primary routes are personal contact and contaminated water and food.<ref name="Barry2013"/> The cysts can stay infectious for up to three months in cold water.<ref name=Auer2012/> Both symptomatic and [[Asymptomatic carrier|asymptomatic carriers]] excrete cysts and thus spread the disease.<ref name="Gard2001">{{cite journal |vauthors=Gardner TB, Hill DR |date=January 2001 |title=Treatment of giardiasis |journal=Clinical Microbiology Reviews |volume=14 |issue=1 |pages=114–28 |doi=10.1128/CMR.14.1.114-128.2001 |pmc=88965 |pmid=11148005}}</ref>

==Pathophysiology==
[[File:Giardia LifeCycle.gif|thumb|upright=1.4|Life cycle of ''Giardia'']]
The life cycle of ''Giardia'' consists of a cyst form and a trophozoite form.<ref name=":4" /> The cyst form is infectious and once it has found a host, it transforms into the trophozoite form.<ref name=":4" /> This trophozoite attaches to the intestinal wall and replicates within the gut.<ref name=":4" /> As trophozoites continue along the gastrointestinal tract, they convert back to their cyst form, which is then excreted with faeces.<ref name="BMJ2016" /> Ingestion of only a few of these cysts is needed to generate infection in another host.<ref name=":6" />

Infection with ''Giardia'' results in decreased expression of [[brush border]] enzymes, morphological changes to the [[microvillus]], increased intestinal permeability, and [[apoptosis|programmed cell death]] of small intestinal [[Epithelium|epithelial]] cells.<ref name=":5">{{cite journal | vauthors = Buret AG | title = Pathophysiology of enteric infections with Giardia duodenalis | journal = Parasite | volume = 15 | issue = 3 | pages = 261–5 | date = September 2008 | pmid = 18814692 | doi = 10.1051/parasite/2008153261 | doi-access = free }}</ref> Both trophozoites and cysts are contained within the gastrointestinal tract and do not invade beyond it.<ref>{{cite journal | vauthors = Bartelt LA, Sartor RB | title = Advances in understanding Giardia: determinants and mechanisms of chronic sequelae | journal = F1000Prime Reports | volume = 7 | issue = 62 | page = 62 | date = 2015-05-26 | pmid = 26097735 | pmc = 4447054 | doi = 10.12703/P7-62 | doi-access = free }}</ref>

The attachment of [[trophozoites]] causes villous flattening and inhibition of [[disaccharidase|enzymes that break down disaccharide sugars]] in the intestines.<ref name=Cot2011 /><ref name=":5" /> Ultimately, the [[Microbiome|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 [[Intestinal water absorption|water absorption]] from the intestine, resulting in diarrhoea, one of the predominant symptoms.<ref name=":5" /> In the case of asymptomatic giardiasis, there can be malabsorption with or without histological changes to the small intestine. The degree to which malabsorption occurs in symptomatic and asymptomatic cases varies greatly.{{citation needed|date=May 2021}}

The species ''Giardia intestinalis'' uses [[Protease|enzymes that break down proteins]] to attack the villi of the brush border and appears to increase crypt cell proliferation and crypt length of crypt cells existing on the sides of the villi. On an immunological level, activated host [[T cell|T lymphocytes]] attack endothelial cells that have been injured to remove the cells.<ref name=Cot2011 /> This occurs after the disruption of proteins that [[Tight junction|connect brush border endothelial cells to one another]].<ref name=":5" /> The result is increased intestinal permeability.<ref name=":5" />

There appears to be a further increase in programmed enterocyte cell death by ''Giardia intestinalis'', which further damages the intestinal barrier and increases permeability.<ref name=":5" /> There is significant upregulation of the programmed cell death cascade by the parasite, and substantial downregulation of the anti-apoptotic protein [[Bcl-2]] and upregulation of the proapoptotic protein [[Bcl-2-associated X protein|Bax]].<ref name=Cot2011 /> These connections suggest a role of [[caspase|caspase-dependent apoptosis]] in the pathogenesis of giardiasis.<ref name=Cot2011 />

''Giardia'' protects its growth by reducing the formation of the gas [[nitric oxide]] by consuming all local [[arginine]], which is the [[amino acid]] necessary to make nitric oxide.<ref name=Cot2011 /> Arginine starvation is known to be a cause of programmed cell death, and local removal is a strong apoptotic agent.<ref>{{cite web|last=Muhkerjee|first=Sandeep|title=Giardiasis|url=http://emedicine.medscape.com/article/176718-overview#a0104|publisher=Medscape Reference|access-date=21 November 2012|url-status=live|archive-url=https://web.archive.org/web/20121117033849/http://emedicine.medscape.com/article/176718-overview#a0104|archive-date=17 November 2012}}</ref>

== Host defence ==
Host defence against ''Giardia'' consists of natural barriers, production of nitric oxide, and activation of the innate and adaptive immune systems.{{citation needed|date=January 2023}}

=== Natural barriers ===
Natural barriers defend against the parasite entering the host's body. Natural barriers consist of [[mucus]] layers, bile salt, [[protease]]s, and [[lipase]]s. Additionally, [[peristalsis]] and the renewal of [[enterocyte]]s provide further protection against parasites.<ref>{{Cite journal|last=Eckmann|first=Lars|date=May 2003|title=Mucosal defences against Giardia|journal=Parasite Immunology|volume=25|issue=5|pages=259–270|doi=10.1046/j.1365-3024.2003.00634.x|pmid=12969444|s2cid=7155680|issn=0141-9838}}</ref><ref>{{Cite journal|last=Thompson|first=R. C. Andrew|date=July 2011|title=Giardia infections|journal=Oxford Medicine Online|doi=10.1093/med/9780198570028.003.0052|isbn=978-0-19-969782-3}}</ref>

=== Nitric oxide production ===
Nitric oxide does not kill the parasite, but it inhibits the growth of trophozoites as well as excystation and encystation.<ref>{{Cite journal|last1=Eckmann|first1=Lars|last2=Laurent|first2=Fabrice|last3=Langford|first3=T. Dianne|last4=Hetsko|first4=Michael L.|last5=Smith|first5=Jennifer R.|last6=Kagnoff|first6=Martin F.|last7=Gillin|first7=Frances D.|date=2000-02-01|title=Nitric Oxide Production by Human Intestinal Epithelial Cells and Competition for Arginine as Potential Determinants of Host Defense Against the Lumen-Dwelling PathogenGiardia lamblia|journal=The Journal of Immunology|volume=164|issue=3|pages=1478–1487|doi=10.4049/jimmunol.164.3.1478|pmid=10640765|issn=0022-1767|doi-access=free}}</ref><ref>{{Cite journal|last1=Li|first1=Erqiu|last2=Zhou|first2=Ping|last3=Singer|first3=Steven M.|date=2005-12-19|title=Neuronal Nitric Oxide Synthase Is Necessary for Elimination of Giardia lamblia Infections in Mice|journal=The Journal of Immunology|volume=176|issue=1|pages=516–521|doi=10.4049/jimmunol.176.1.516|pmid=16365445|pmc=2585514|issn=0022-1767|doi-access=free}}</ref>

=== Innate immune system ===

==== Lectin pathway of complement ====
The lectin pathway of complement is activated by mannose-binding lectin (MBL), which binds to N-acetylglucosamine. N-acetylglucosamine is a ligand for MBL and is present on the surface of ''Giardia''.<ref>{{Cite journal|last1=Evans-Osses|first1=Ingrid|last2=Ansa-Addo|first2=Ephraim A.|last3=Inal|first3=Jameel M.|last4=Ramirez|first4=Marcel I.|date=May 2010|title=Involvement of lectin pathway activation in the complement killing of Giardia intestinalis|journal=Biochemical and Biophysical Research Communications|volume=395|issue=3|pages=382–386|doi=10.1016/j.bbrc.2010.04.025|pmid=20382117|bibcode=2010BBRC..395..382E |issn=0006-291X|doi-access=free}}</ref>

==== The classical pathway of complement ====
The classical pathway of complement is activated by antibodies specific against ''Giardia''.{{citation needed|date=January 2023}}

=== Adaptive immune system ===

==== Antibodies ====
Antibodies inhibit parasite replication and also induce parasite death via the classical pathway of complement.{{citation needed|date=June 2022}}

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. IgA is the most abundant isotype in intestinal secretions, and it is also the dominant isotype in a mother's milk. Antibodies in a mother's milk protect children against giardiasis (passive immunisation).<ref>{{Cite journal|last1=Tellez|first1=Aleyda|last2=Winiecka-krusnell|first2=Jadwiga|last3=Paniagua|first3=Margarita|last4=Linder|first4=Ewert|date=May 2003|title=Antibodies in Mother's Milk Protect Children Against Giardiasis|journal=Scandinavian Journal of Infectious Diseases|volume=35|issue=5|pages=322–325|doi=10.1080/00365540310008041|pmid=12875519|s2cid=12621232|issn=0036-5548}}</ref>

==== T-cells ====
The major aspect of adaptive immune responses is the [[T-cell]] response. ''Giardia'' is an extracellular pathogen. Therefore, CD4+ helper T-cells are primarily responsible for this protective effect.<ref name=":10">{{Cite journal|last1=Singer|first1=Steven M.|last2=Nash|first2=Theodore E.|date=2000-01-01|title=T-Cell-Dependent Control of Acute Giardia lamblia Infections in Mice|journal=Infection and Immunity|volume=68|issue=1|pages=170–175|doi=10.1128/iai.68.1.170-175.2000|pmid=10603384|pmc=97117|issn=1098-5522|doi-access=free}}</ref>

One role of helper T-cells is to promote antibody production and isotype switching. Other roles include cytokine production (IL-4, IL-9) to help recruit other effector cells of the immune response.<ref name=":10" /><ref>{{Cite journal|last1=Scott|first1=Kevin G.-E.|last2=Yu|first2=Linda C. H.|last3=Buret|first3=André G.|date=June 2004|title=Role of CD8+ and CD4+ T Lymphocytes in Jejunal Mucosal Injury during Murine Giardiasis|journal=Infection and Immunity|volume=72|issue=6|pages=3536–3542|doi=10.1128/iai.72.6.3536-3542.2004|pmid=15155662|pmc=415705|issn=0019-9567|doi-access=free}}</ref>

==Diagnosis==
[[File:Histopathology of Giardia lamblia in duodenum.png|thumb|A duodenal biopsy may incidentally detect ''Giardia'' organisms, as in this [[H&E stain]]ed sample.<ref>{{cite journal |author1=Kalas MA |author2=Alduaij A |author3=Alkhatib AA| title=Incidental Diagnosis of Duodenal Giardiasis. | journal=Cureus | year= 2021 | volume= 13 | issue= 6 | article-number= e15499 | pmid=34268030 | doi=10.7759/cureus.15499 |doi-access=free | pmc=8262110 }}<br />- "This is an open access article distributed under the terms of the Creative Commons Attribution License CC-BY 4.0."</ref>]]
* According to the CDC, the detection of antigens on the surface of organisms in stool specimens is the current test of choice for the diagnosis of giardiasis and provides increased sensitivity over more common microscopy techniques.<ref>{{cite web |url=https://www.cdc.gov/dpdx/diagnosticProcedures/stool/antigendetection.html |title=Stool Specimens - Detection of Parasite Antigens |website=CDC |access-date=2017-09-09 |url-status=live |archive-url=https://web.archive.org/web/20170617214619/https://www.cdc.gov/dpdx/diagnosticprocedures/stool/antigendetection.html |archive-date=2017-06-17 }}</ref>
* A trichrome stain of preserved stool is another method used to detect ''Giardia''.<ref>{{cite web |url=http://ltd.aruplab.com/Tests/Pub/2002272 |title=Ova and Parasite Exam, Fecal (Immunocompromised or Travel History) |access-date=2014-10-29 |url-status=live |archive-url=https://web.archive.org/web/20141029104806/http://ltd.aruplab.com/Tests/Pub/2002272 |archive-date=2014-10-29 }}</ref>
* Microscopic examination of the stool can be performed for diagnosis.<ref name="BMJ2016" /> This method is not preferred, however, due to inconsistent shedding of trophozoites and cysts in infected hosts.<ref name="BMJ2016" />  Multiple samples over some time, typically one week, must be examined.<ref name="BMJ2016" />
* The [[Entero-Test]] uses a gelatin capsule with an attached thread. One end is attached to the inner aspect of the host's cheek, and the capsule is swallowed. Later, the thread is withdrawn and shaken in saline to release trophozoites, which can be detected with a microscope. The sensitivity of this test is low, however, and is not routinely used for diagnosis.<ref>{{cite journal | vauthors = Hooshyar H, Rostamkhani P, Arbabi M, Delavari M | title = Giardia lamblia infection: review of current diagnostic strategies | journal = Gastroenterology and Hepatology from Bed to Bench | volume = 12 | issue = 1 | pages = 3–12 | date = 2019 | pmid = 30949313 | pmc = 6441489 }}</ref>
* Immunologic enzyme-linked immunosorbent assay ([[ELISA]]) testing may be used for diagnosis.<ref name=":7" /> These tests are capable of a 90% detection rate or more.<ref name=":7">{{cite journal | vauthors = Rosenblatt JE, Sloan LM, Schneider SK | title = Evaluation of an enzyme-linked immunosorbent assay for the detection of Giardia lamblia in stool specimens | journal = Diagnostic Microbiology and Infectious Disease | volume = 16 | issue = 4 | pages = 337–41 | date = May–June 1993 | pmid = 8495591 | doi = 10.1016/0732-8893(93)90086-M }}</ref>

Although [[hydrogen breath test]]s indicate poorer rates of carbohydrate absorption in those asymptomatically infected, such tests are not diagnostic of infection.<ref name="pmid12139217">{{cite journal | vauthors = Moya-Camarena SY, Sotelo N, Valencia ME | title = Effects of asymptomatic Giardia intestinalis infection on carbohydrate absorption in well-nourished Mexican children | journal = The American Journal of Tropical Medicine and Hygiene | volume = 66 | issue = 3 | pages = 255–9 | date = March 2002 | pmid = 12139217 | doi = 10.4269/ajtmh.2002.66.255 | doi-access = free }}</ref> Serological tests are not helpful in diagnosis.<ref name="BMJ2016" />

==Prevention==
The CDC recommends hand-washing and avoiding potentially contaminated food and untreated water.<ref>{{cite web|title=Parasites - Giardia, Prevention & Control|url=https://www.cdc.gov/parasites/giardia/prevent.html|website=Centers for Disease Control and Prevention|publisher=CDC|access-date=26 April 2015|url-status=live|archive-url=https://web.archive.org/web/20150430022503/http://www.cdc.gov/parasites/giardia/prevent.html|archive-date=30 April 2015}}</ref>

Boiling water contaminated with ''Giardia'' effectively kills infectious cysts.<ref>{{cite web|url=http://water.epa.gov/drink/emerprep/emergencydisinfection.cfm|title=Emergency Disinfection of Drinking Water|publisher=United States Environment Protection Agency|access-date=21 June 2015|archive-url=https://web.archive.org/web/20150623065142/http://water.epa.gov/drink/emerprep/emergencydisinfection.cfm|archive-date=23 June 2015|date=2013-02-20}} Retrieved 24 February 2011</ref> Chemical disinfectants or filters may be used.<ref>{{cite journal | vauthors = Betancourt WQ, Rose JB | title = Drinking water treatment processes for removal of Cryptosporidium and Giardia | journal = Veterinary Parasitology | volume = 126 | issue = 1–2 | pages = 219–34 | date = December 2004 | pmid = 15567586 | doi = 10.1016/j.vetpar.2004.09.002 }}</ref><ref>{{cite journal | vauthors = Exner M, Gornik V | title = [Parasitic zoonoses transmitted by drinking water. Giardiasis and cryptosporidiosis] | journal = Bundesgesundheitsblatt - Gesundheitsforschung - Gesundheitsschutz | volume = 47 | issue = 7 | pages = 698–704 | date = July 2004 | pmid = 15254826 | doi = 10.1007/s00103-004-0863-y }}</ref> Iodine-based disinfectants are preferred over chlorination as the latter is ineffective at destroying cysts.<ref>{{Citation|last1=Dunn|first1=Noel|title=Giardiasis|date=2019|url=https://www.ncbi.nlm.nih.gov/books/NBK513239/|work=StatPearls|publisher=StatPearls Publishing|pmid=30020611|access-date=2019-10-31|last2=Juergens|first2=Andrew L.}}</ref><ref>{{cite journal | vauthors = Ongerth JE, Johnson RL, Macdonald SC, Frost F, Stibbs HH | title = Back-country water treatment to prevent giardiasis | journal = American Journal of Public Health | volume = 79 | issue = 12 | pages = 1633–7 | date = December 1989 | pmid = 2817191 | pmc = 1349767 | doi = 10.2105/ajph.79.12.1633 }}</ref>

Although the evidence linking the drinking of water in the North American wilderness and giardiasis has been questioned, several studies raise concerns.<ref>{{cite journal|author1=Julia E. Painter, PhD |author2=Julia W. Gargano, PhD |author3=Sarah A. Collier, MPH |author4=Jonathan S. Yoder, MPH|title=Giardiasis Surveillance—United States, 2011–2012|journal= MMWR Supplements|date=2015|volume=64|issue=Suppl 3|pages=15–25|url=https://www.cdc.gov/mmwr/preview/mmwrhtml/ss6403a2.htm|access-date=31 March 2018 |pmid=25928582}}</ref> Most if not all CDC verified backcountry giardiasis outbreaks have been attributed to water. Surveillance data (for 2013 and 2014) reports six outbreaks (96 cases) of waterborne giardiasis contracted from rivers, streams or springs<ref>{{cite journal | vauthors = McClung RP, Roth DM, Vigar M, Roberts VA, Kahler AM, Cooley LA, Hilborn ED, Wade TJ, Fullerton KE, Yoder JS, Hill VR | display-authors = 6 | title = Waterborne Disease Outbreaks Associated With Environmental and Undetermined Exposures to Water - United States, 2013-2014 | language = en-us | journal = MMWR. Morbidity and Mortality Weekly Report | volume = 66 | issue = 44 | pages = 1222–1225 | date = November 2017 | pmid = 29120997 | pmc = 5679586 | doi = 10.15585/mmwr.mm6644a4 }}</ref> and less than 1% of reported giardiasis cases are associated with outbreaks.<ref>{{cite journal|title=Giardiasis Surveillance — United States, 2009–2010|website=www.cdc.gov|url=https://www.cdc.gov/mmwr/preview/mmwrhtml/ss6105a2.htm|language=en}}</ref>

Person-to-person transmission accounts for the majority of ''Giardia'' infections and is usually associated with poor hygiene and sanitation. ''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 [[feces|faeces]].<ref>{{cite web |title=Giardia |date=March 2011 |work=Parasites |publisher=[[Centers for Disease Control and Prevention]] |url=https://www.cdc.gov/parasites/giardia/}}</ref> Water-borne [[Transmission (medicine)|transmission]] is associated with the ingestion of contaminated water. In the U.S., outbreaks typically occur in small water systems using inadequately treated surface water. [[Sexually transmitted disease|Venereal]] transmission happens through faecal-oral contamination. Additionally, nappy/diaper changing and inadequate handwashing are risk factors for transmission from infected children. Lastly, food-borne epidemics of ''Giardia'' have developed through the contamination of food by infected food handlers.<ref name="pennardt_g">{{EMedicine|article|176718|Giardiasis}}</ref>

=== Vaccine ===
There are no vaccines for humans; however, several vaccine candidates are in development. They are targeting: recombinant proteins, DNA vaccines, variant-specific surface proteins (VSP), cyst wall proteins (CWP), giadins, and enzymes.<ref>{{Cite journal|last1=Davids|first1=Barbara J.|last2=Liu|first2=Ching M.|last3=Hanson|first3=Elaine M.|last4=Le|first4=Christine H. Y.|last5=Ang|first5=Jonathan|last6=Hanevik|first6=Kurt|last7=Fischer|first7=Marvin|last8=Radunovic|first8=Matej|last9=Langeland|first9=Nina|last10=Ferella|first10=Marcela|last11=Svärd|first11=Staffan G.|date=2019-04-08|title=Identification of Conserved Candidate Vaccine Antigens in the Surface Proteome of Giardia lamblia|journal=Infection and Immunity|volume=87|issue=6|article-number=e00219-19 |doi=10.1128/iai.00219-19|pmid=30962402|pmc=6529650|issn=0019-9567|doi-access=free}}</ref> Researchers at [[National Scientific and Technical Research Council|CONICET]] have produced an oral vaccine after engineering customised proteins mimicking those expressed on the surface of Giardia trophozoites. The vaccine has proven effective in mice.<ref>{{Cite journal |last1=Serradell |first1=Marianela C. |last2=Rupil |first2=Lucía L. |last3=Martino |first3=Román A. |last4=Prucca |first4=César G. |last5=Carranza |first5=Pedro G. |last6=Saura |first6=Alicia |last7=Fernández |first7=Elmer A. |last8=Gargantini |first8=Pablo R. |last9=Tenaglia |first9=Albano H. |last10=Petiti |first10=Juan P. |last11=Tonelli |first11=Renata R. |last12=Reinoso-Vizcaino |first12=Nicolás |last13=Echenique |first13=José |last14=Berod |first14=Luciana |last15=Piaggio |first15=Eliane |date=2019-01-21 |title=Efficient oral vaccination by bioengineering virus-like particles with protozoan surface proteins |journal=Nature Communications |language=en |volume=10 |issue=1 |page=361 |doi=10.1038/s41467-018-08265-9 |pmid=30664644 |issn=2041-1723|pmc=6341118 |bibcode=2019NatCo..10..361S }}</ref><ref>{{Cite journal |last1=Rupil |first1=Lucía Lara |last2=Serradell |first2=Marianela Del Carmen |last3=Luján |first3=Hugo Daniel |date=January 2020 |title=Using Protozoan Surface Proteins for Effective Oral Vaccination |journal=Trends in Parasitology |volume=36 |issue=1 |pages=7–10 |doi=10.1016/j.pt.2019.07.004 |issn=1471-5007 |pmid=31362858|s2cid=198999660 }}</ref>

One commercially available vaccine exists – GiardiaVax, made from ''G. lamblia'' whole trophozoite lysate. It is a vaccine for veterinary use only in dogs and cats. GiardiaVax should promote the production of specific antibodies.<ref>{{cite journal |last1=Palatnik-de-Sousa |first1=CB |last2=Nico |first2=D |title=The Delay in the Licensing of Protozoal Vaccines: A Comparative History. |journal=Frontiers in Immunology |date=2020 |volume=11 |article-number=204 |doi=10.3389/fimmu.2020.00204 |pmid=32210953 |pmc=7068796 |doi-access=free}}</ref>

==Treatment==
Treatment is not always necessary as the infection usually resolves on its own.<ref name=Cot2011 /> However, if the illness is acute or symptoms persist and medications are needed to treat it, a [[nitroimidazole]] medication is used such as [[metronidazole]], [[tinidazole]], [[secnidazole]] or [[ornidazole]].<ref name="Barry2013"/>

The [[World Health Organization|World Health Organisation]] and [[Infectious Diseases Society of America|Infectious Disease Society of America]] recommend metronidazole as first-line therapy.<ref>{{cite journal | vauthors = Guerrant RL, Van Gilder T, Steiner TS, Thielman NM, Slutsker L, Tauxe RV, Hennessy T, Griffin PM, DuPont H, Sack RB, Tarr P, Neill M, Nachamkin I, Reller LB, Osterholm MT, Bennish ML, Pickering LK | display-authors = 6 | title = Practice guidelines for the management of infectious diarrhea | journal = Clinical Infectious Diseases | volume = 32 | issue = 3 | pages = 331–51 | date = February 2001 | pmid = 11170940 | doi = 10.1086/318514 | url = http://www.idsociety.org/uploadedFiles/IDSA/Guidelines-Patient_Care/PDF_Library/Diarrhea.pdf | url-status = live | archive-url = https://web.archive.org/web/20160210051836/http://www.idsociety.org/uploadedFiles/IDSA/Guidelines-Patient_Care/PDF_Library/Diarrhea.pdf | archive-date = February 10, 2016 | doi-access = free }}</ref><ref>{{Cite web|title = Chapter 7.5.4 Continuing Diarrhoea {{!}} ICHRC|url = http://www.ichrc.org/chapter-754-continuing-diarrhoea|website = www.ichrc.org|access-date = 2016-01-09|url-status = live|archive-url = https://web.archive.org/web/20160129235636/http://www.ichrc.org/chapter-754-continuing-diarrhoea|archive-date = 2016-01-29}}</ref> The [[US CDC]] lists metronidazole, tinidazole, and [[nitazoxanide]] as effective first-line therapies;<ref>{{cite web|title=Giardia: Treatment|url=https://www.cdc.gov/parasites/giardia/treatment.html|publisher=United States Centers for Disease Control and Prevention|access-date=10 January 2016|date=21 July 2015|quote=Several drugs can be used to treat Giardia infection. Effective treatments include metronidazole, tinidazole, and nitazoxanide<sup>1</sup>. Alternatives to these medications include paromomycin, quinacrine, and furazolidone<sup>1,2</sup>.|url-status=live|archive-url=https://web.archive.org/web/20151224232327/http://www.cdc.gov/parasites/giardia/treatment.html|archive-date=24 December 2015}}</ref> of these three, only nitazoxanide and tinidazole are approved for the treatment of giardiasis by the US [[Food and Drug Administration|FDA]].<ref name="FDA Nitazoxanide">{{cite web|title=Nitazoxanide Prescribing Information|url=http://www.alinia.com/images/Alinia-Prescribing-Information.pdf|publisher=Romark Pharmaceuticals|access-date=3 January 2016|pages=1–5|date=August 2013|archive-url=https://web.archive.org/web/20160116192246/http://alinia.com/images/Alinia-Prescribing-Information.pdf|archive-date=16 January 2016}}</ref><ref>{{cite web|title=Metronidazole Prescribing Information|url=https://www.accessdata.fda.gov/drugsatfda_docs/label/2015/012623s066lbl.pdf|website=United States Food and Drug Administration|publisher=Pfizer|access-date=10 January 2016|pages=6–7|date=June 2015|url-status=live|archive-url=https://web.archive.org/web/20160304085137/http://www.accessdata.fda.gov/drugsatfda_docs/label/2015/012623s066lbl.pdf|archive-date=4 March 2016}}</ref><ref>{{cite web|title=Tinidazole Prescribing Informatiuon|url=http://www.accessdata.fda.gov/drugsatfda_docs/label/2007/021618s003lbl.pdf|website=United States Food and Drug Administration|publisher=Mission Pharma|access-date=10 January 2016|page=1|date=May 2007|archive-url=https://web.archive.org/web/20160304061029/http://www.accessdata.fda.gov/drugsatfda_docs/label/2007/021618s003lbl.pdf|archive-date=4 March 2016}}</ref> A meta-analysis published by the [[Cochrane (organisation)|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.<ref name=":0">{{cite journal | vauthors = Granados CE, Reveiz L, Uribe LG, Criollo CP | title = Drugs for treating giardiasis | journal = The Cochrane Database of Systematic Reviews | volume = 12 | article-number = CD007787 | date = December 2012 | issue = 12 | pmid = 23235648 | pmc = 6532677 | doi = 10.1002/14651858.cd007787.pub2 }}</ref> Other meta-analyses have reached similar conclusions.<ref name="Solaymani 2010">{{cite journal | vauthors = Solaymani-Mohammadi S, Genkinger JM, Loffredo CA, Singer SM | title = A meta-analysis of the effectiveness of albendazole compared with metronidazole as treatments for infections with Giardia duodenalis | journal = PLOS Neglected Tropical Diseases | volume = 4 | issue = 5 | article-number = e682 | date = May 2010 | pmid = 20485492 | pmc = 2867942 | doi = 10.1371/journal.pntd.0000682 | editor1-last = Keiser | editor1-first = Jennifer | doi-access = free }}</ref> Both medications need a five to ten-day-long course; albendazole is taken once a day, while metronidazole needs to be taken three times a day. The evidence for comparing metronidazole to other alternatives such as mebendazole, tinidazole, or nitazoxanide was felt to be of very low quality.<ref name=":0" /> While tinidazole has side effects and efficacy similar to those of metronidazole, it is administered with a single dose.<ref name="Gard2001" />

Resistance has been observed clinically to both nitroimidazoles and albendazole, but not nitazoxanide, though nitazoxanide resistance has been induced in research laboratories.<ref name=":6" /><ref name=":1">{{cite journal | vauthors = Leitsch D | title = Giardia lamblia | journal = Current Tropical Medicine Reports | volume = 2 | issue = 3 | pages = 128–135 | date = 2015-07-07 | pmid = 26258002 | pmc = 4523694 | doi = 10.1007/s40475-015-0051-1 }}</ref> The exact mechanism of resistance to all of these medications is not well understood.<ref name=":1" /> 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]].<ref name="Gard2001"/> [[Mepacrine]] may also be used for refractory cases.<ref name=":6" />

Probiotics, when given in combination with the standard treatment, have been shown to assist with clearance of ''Giardia''.<ref>{{Cite journal|last1=Lalle|first1=Marco|last2=Hanevik|first2=Kurt|date=2018-10-24|title=Treatment-refractory giardiasis: challenges and solutions|journal=Infection and Drug Resistance|volume=11|pages=1921–1933|doi=10.2147/IDR.S141468|issn=1178-6973|pmc=6207226|pmid=30498364 |doi-access=free }}</ref>

During pregnancy, paromomycin is the preferred treatment drug because of its poor intestinal absorption, resulting in less exposure to the foetus.<ref>{{cite journal | vauthors = Farthing MJ | s2cid = 19657328 | title = Treatment options for the eradication of intestinal protozoa | journal = Nature Clinical Practice. Gastroenterology & Hepatology | volume = 3 | issue = 8 | pages = 436–45 | date = August 2006 | pmid = 16883348 | doi = 10.1038/ncpgasthep0557 }}</ref> Alternatively, metronidazole can be used after the first trimester as there has been wide experience in its use for [[trichomonas]] in pregnancy.<ref name="Gard2001"/><ref>{{cite journal | vauthors = Workowski KA, Bolan GA | title = Sexually transmitted diseases treatment guidelines, 2015 | journal = MMWR. Recommendations and Reports | volume = 64 | issue = RR-03 | pages = 1–137 | date = June 2015 | pmid = 26042815 | pmc = 5885289 }}</ref>

==Prognosis==
In people with a properly functioning immune system, infection may resolve without medication.<ref name=Cot2011 /> A small portion, however, develop a chronic infection.<ref name=Cot2011 /> People with an impaired immune system are at higher risk of chronic infection.<ref name=Cot2011 /> Medication is an effective cure for nearly all people, although there is growing drug resistance.<ref name="BMJ2016" /><ref name=":3">{{Cite book|title=Harrison's principles of internal medicine.|others=Kasper, Dennis L.,, Fauci, Anthony S., 1940-, Hauser, Stephen L.,, Longo, Dan L. (Dan Louis), 1949-, Jameson, J. Larry,, Loscalzo, Joseph|isbn=978-0-07-180215-4|edition=19th|location=New York|oclc=893557976|last1 = Kasper|first1 = Dennis L.|last2 = Larry Jameson|first2 = J.|last3 = Hauser|first3 = Stephen L.|last4 = Loscalzo|first4 = Joseph|last5 = Fauci|first5 = Anthony S.|last6 = Longo|first6 = Dan L.|date = 2015-04-08}}</ref><ref name=":6">{{cite journal | vauthors = Carter ER, Nabarro LE, Hedley L, Chiodini PL | title = Nitroimidazole-refractory giardiasis: a growing problem requiring rational solutions | journal = Clinical Microbiology and Infection | volume = 24 | issue = 1 | pages = 37–42 | date = January 2018 | pmid = 28624613 | doi = 10.1016/j.cmi.2017.05.028 | doi-access = free }}</ref>

Children with chronic giardiasis are at risk for failure to thrive as well as more long-lasting sequelae such as growth stunting.<ref>{{cite journal | vauthors = Donowitz JR, Alam M, Kabir M, Ma JZ, Nazib F, Platts-Mills JA, Bartelt LA, Haque R, Petri WA | display-authors = 6 | title = A Prospective Longitudinal Cohort to Investigate the Effects of Early Life Giardiasis on Growth and All-Cause Diarrhea | journal = Clinical Infectious Diseases | volume = 63 | issue = 6 | pages = 792–7 | date = September 2016 | pmid = 27313261 | pmc = 4996141 | doi = 10.1093/cid/ciw391 }}</ref> Up to half of infected people develop a temporary [[lactose intolerance]] leading to symptoms that may mimic a chronic infection.<ref name="BMJ2016" /> Some people experience post-infectious [[irritable bowel syndrome]] after the infection has cleared.<ref name=Cot2011 /> Giardiasis has also been implicated in the development of [[food allergies]].<ref name=Cot2011 /> This is thought to be due to its effect on intestinal permeability.<ref name=Cot2011 />

==Epidemiology==
[[File:US giardiasis incidence 2005.gif|upright=1.3|thumb|Rates of giardiasis in 2005 in the United States]]
In some [[developing countries]] ''Giardia'' is present in 30% of the population.<ref name=Auer2012>{{cite book|last=Auerbach|first=Paul S.|title=Wilderness medicine|date=2012|publisher=Elsevier/Mosby|location=Philadelphia, PA|isbn=978-1-4377-1678-8|pages=Chapter 68|edition= 6th}}</ref> In the United States it is estimated that it is present in 3–7% of the population.<ref name=Auer2012/> Giardiasis is associated with impaired growth and development in children, particularly influencing a country's economic growth by affecting Disability Adjusted Life Year (DALY) rates.<ref>{{Cite journal |last=Adam |first=Rodney D. |title=Biology of Giardia lamblia |journal=Clinical Microbiology Reviews |date=2001 |language=en |volume=14 |issue=3 |pages=447–475 |doi=10.1128/CMR.14.3.447-475.2001 |issn=0893-8512 |pmc=88984 |pmid=11432808}}</ref><ref>{{Cite journal |last1=Dormond |first1=Megan |last2=Gutierrez |first2=Ramiro L. |last3=Porter |first3=Chad K. |date=December 2016 |title=Giardia lamblia infection increases risk of chronic gastrointestinal disorders |journal=Tropical Diseases, Travel Medicine and Vaccines |language=en |volume=2 |issue=1 |page=17 |doi=10.1186/s40794-016-0030-0 |issn=2055-0936|doi-access=free |pmid=28883961 |pmc=5530925 }}</ref>

The number of reported cases in the United States in 2018 was 15,584.<ref name=":2">{{Cite web|url=https://wonder.cdc.gov/nndss/static/2018/annual/2018-table2f.html|title=Table 2f. Annual reported cases of notifiable diseases, by region and reporting area - - United States and U.S. Territories, 2018|website=wonder.cdc.gov|access-date=2019-11-13|archive-date=2019-11-13|archive-url=https://web.archive.org/web/20191113013708/https://wonder.cdc.gov/nndss/static/2018/annual/2018-table2f.html|url-status=dead}}</ref> All states that classify giardiasis as a notifiable disease had cases of giardiasis.<ref name=":2" /> 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.<ref name=":2" /> There are seasonal trends associated with giardiasis.<ref>{{Cite journal|last1=Painter|first1=Julia E.|last2=Gargano|first2=Julia W.|last3=Collier|first3=Sarah A.|last4=Yoder|first4=Jonathan S. |date=2015-05-01|title=Giardiasis surveillance -- United States, 2011-2012|journal=MMWR Supplements|volume=64|issue=3|pages=15–25|issn=2380-8942|pmid=25928582}}</ref> July, August, and September are the months with the highest incidence of giardiasis in the United States.<ref>{{cite journal|author1=Jonathan S. Yoder, MPH |author2=Julia W. Gargano, PhD |author3=Ryan M. Wallace, DVM |author4=Michael J. Beach, PhD|title=Giardiasis Surveillance-United States, 2009-2010|journal=Morbidity and Mortality Weekly Report. Surveillance Summaries |volume=61|issue=5|pages=13–23|url=https://www.cdc.gov/mmwr/preview/mmwrhtml/ss6105a2.htm|publisher=Centers of Disease Control and Prevention|access-date=30 November 2017|pmid=22951494|year=2012}}</ref>

In the ECDC's (European Centre for Disease Prevention and Control) annual epidemiological report containing 2014 data, 17,278 confirmed giardiasis cases were reported by 23 of the 31 countries that are members of the EU/EEA.<ref name=":8" /> Germany reported the highest number at 4,011 cases.<ref name=":8" /> Following Germany, the UK reported 3,628 confirmed giardiasis cases. Together, this accounts for 44% of total reported cases.<ref name=":8">{{cite web|title=Giardiasis- Annual Epidemiological Report 2016|url=https://ecdc.europa.eu/en/publications-data/giardiasis-annual-epidemiological-report-2016-2014-data|website=European Centre for Disease Prevention and Control|access-date=30 November 2017|date=2017-01-30}}</ref>

==Research==
Some intestinal parasitic infections may play a role in [[irritable bowel syndrome]]<ref>{{cite journal | vauthors = Stark D, van Hal S, Marriott D, Ellis J, Harkness J | title = Irritable bowel syndrome: a review on the role of intestinal protozoa and the importance of their detection and diagnosis | journal = International Journal for Parasitology | volume = 37 | issue = 1 | pages = 11–20 | date = January 2007 | pmid = 17070814 | doi = 10.1016/j.ijpara.2006.09.009 }}</ref> and other long-term sequelae such as chronic fatigue.<ref>{{cite journal | vauthors = Hanevik K, Wensaas KA, Rortveit G, Eide GE, Mørch K, Langeland N | title = Irritable bowel syndrome and chronic fatigue 6 years after giardia infection: a controlled prospective cohort study | journal = Clinical Infectious Diseases | volume = 59 | issue = 10 | pages = 1394–400 | date = November 2014 | pmid = 25115874 | pmc = 4207419 | doi = 10.1093/cid/ciu629 }}</ref><ref>Quote: "for unclear reasons, chronic sequelae, including post-infectious irritable bowel syndrome, chronic fatigue [..], malnutrition [..], cognitive impairment [..], and extra-intestinal manifestations (such as food allergy, urticaria, reactive arthritis, and inflammatory ocular manifestations), can develop and possibly persist beyond detectable parasite shedding". Quoted from: {{cite journal | vauthors = Bartelt LA, Sartor RB | title = Advances in understanding Giardia: determinants and mechanisms of chronic sequelae | journal = F1000Prime Reports | volume = 7 | page = 62 | year = 2015 | pmid = 26097735 | pmc = 4447054 | doi = 10.12703/P7-62 | type = Review | doi-access = free }}</ref> The mechanism of transformation from cyst to trophozoites has not been characterised<ref name=":4" /> but may help develop drug targets for treatment-resistant ''Giardia''. The interaction between ''Giardia'' and host immunity, internal flora, and other pathogens is not well understood.<ref name=Cot2011 />In vitro cell cultures have been widely used to study host-parasite interactions, and human enteroids are now being used as non-transformed intestinal epithelial cell infection models for G. intestinalis and other pathogens.<ref>{{Cite journal |last1=Grüttner |first1=Jana |last2=Rijn |first2=Jorik M. van |last3=Geiser |first3=Petra |last4=Florbrant |first4=Alexandra |last5=Webb |first5=Dominic-Luc |last6=Hellström |first6=Per M. |last7=Sundbom |first7=Magnus |last8=Sellin |first8=Mikael E. |last9=Svärd |first9=Staffan G. |date=2023-05-04 |title=Trophozoite fitness dictates the intestinal epithelial cell response to Giardia intestinalis infection |journal=PLOS Pathogens |language=en |volume=19 |issue=5 |article-number=e1011372 |doi=10.1371/journal.ppat.1011372 |issn=1553-7374 |pmc=10187934 |pmid=37141303 |doi-access=free }}</ref>

The main congress about giardiasis is the "International Giardia and Cryptosporidium Conference" (IGCC). A summary of results presented at the most recent edition (2019, in [[Rouen]], France) is available.<ref name="BuretCacciò2020">{{cite journal|last1=Buret|first1=André G.|last2=Cacciò|first2=Simone M.|last3=Favennec|first3=Loïc|last4=Svärd|first4=Staffan|title=Update on Giardia: Highlights from the seventh International ''Giardia'' and ''Cryptosporidium'' Conference|journal=Parasite|volume=27|year=2020|page=49|issn=1776-1042|doi=10.1051/parasite/2020047|url= |pmid=32788035|pmc=7425178}} {{open access}}</ref>

==Other animals==
In both cats and dogs, giardiasis usually responds to metronidazole and fenbendazole. Metronidazole in pregnant cats can cause developmental malformations.<ref name=":9" /> Many cats dislike the taste of fenbendazole.<ref name=":9">{{Cite book|title=Cat Owner's Home Veterinary Handbook|last=Eldredge|first=Debra M.|publisher=Howell Book House|year=2008|page=67}}</ref> Giardiasis has been shown to decrease weight in livestock.<ref name=Cot2011 />

== References ==
{{Reflist}}

== External links ==
* [https://www.cdc.gov/parasites/giardia/index.html Giardiasis Fact Sheet]
{{Medical condition classification and resources | DiseasesDB     = 5213
 | ICD11          =  {{ICD11|1A31}}
 | ICD10          =  {{ICD10|A|07|1|a|00}}
 | ICD9           = {{ICD9|007.1}}
 | ICDO           =
 | OMIM           =
 | MedlinePlus    = 000288
 | eMedicineSubj  = emerg
 | eMedicineTopic = 215
 | MeshID         = D005873
}}

{{Protozoal diseases}}
{{Authority control}}

[[Category:Protozoal diseases]]
[[Category:Waterborne diseases]]
[[Category:Animal diseases]]
[[Category:Tropical diseases]]
[[Category:Zoonoses]]
[[Category:Feces]]
[[Category:Wikipedia medicine articles ready to translate]]
[[Category:Wikipedia infectious disease articles ready to translate]]