Giardia duodenalis
Laura Rojas-López ,1
Rafael C. Marques ,1
and Staffan G. Svärd 1,2,
*
1
Department of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden
2
SciLifeLab, Uppsala University, Sweden
TrendsTrendsininParasitologyParasitology
Giardia duodenalis (syn. Giardia lamblia and Giardia intestinalis) is a major cause of parasite-induced diarrheal
disease. It is found worldwide but the prevalence is higher in low-income countries. Transmission occurs via the
fecal–oral route, and sources of G. duodenalis infection include contaminated water or food, or direct contact
with infected people or animals. Cysts are immediately infectious upon excretion in feces. After ingestion of the
cysts, motile trophozoites emerge (excystation) and attach to intestinal epithelial cells of the upper small intestine
where they replicate extracellularly. Giardia completes its life cycle by forming cysts (encystation) in the lower intestinal
region. Replication of the parasite can result in profuse, fatty diarrhea in infected individuals, but asymptomatic
infections are also common. The major drug used for treatment is metronidazole, but resistance is an emerging
problem.
Intestinal lumenIntestinal lumen
Lamina propriaLamina propria
2
5
3
4
9
ADI Chemokines
Cysteine Proteases
NO Nutrients
IgA
10
Cl Cl
Cl
7
8
L-Arginine
Cysteine proteases
Muc2
Villin
2
1
1
3
5
4 8
7
6
9Arginine starvation and reduced nitric oxide (NO)
Microvilli shortening, destruction and malabsorption
Chemokine upregulation and recruitment of immune
cells
6
Increased intestinal permeability; bacterial and
antigen translocation
Induction of apoptosis
Chemokine degradation
Disruption of apical junction complex
Disruption of mucin integrity
10
Alteration in microbiota
Increased chloride secretion
Immune cells
Antigenic variation
11
11 Antigenic variation
OCT
Variable surface proteins
(V SPs)
Cytosol
Cell membrane
production
TrendsTrendsininParasitologyParasitology
KEY FACTS:
G. duodenalis is grouped into eight
different assemblages (genotypes A–H)
based on host specificity and genetic
differences. Assemblages A and B infect
humans, and certain subgenotypes have
zoonotic potential.
A small genome (12.6 Mbp with 4963
protein-encoding genes), divided on five
different chromosomes in two nuclei
(tetraploid organism). The highly reduced
mitochondria-like organelles
(mitosomes) lack DNA.
Recent advances: complete reference
genomes, tractable life cycle in vitro,
genetic engineering producing
knockouts (Cre/LoxP, CRISPR/Cas9)
and knockdowns (morpholinos and
CRISPRi), culture with organoids and
mouse and gerbil models to study
immunity and pathogenesis.
DISEASE FACTS:
Giardia infections can result in a wide
range of symptoms, including profuse,
fatty diarrhea, abdominal cramps,
nausea, and wasting; however,
infections can also be subclinical.
Chronic infection may cause a disturbed
intestinal balance with changes in the
microbiota, villus blunting, leaky gut,
nutrient malabsorption, and stunted
growth.
Postinfectious problems include food
allergies, irritable bowel syndrome, and
chronic fatigue syndrome.
Giardia infection induces robust innate
and adaptive immune responses, but in
contrast to most other intestinal
infections, it causes low level or no
inflammation.
TAXONOMY AND CLASSIFICATION:
PHYLUM: Metamonada
CLASS: Fornicata
ORDER: Diplomonadida
FAMILY: Hexamitidae
GENUS: Giardia
SPECIES: G. duodenalis
*Correspondence:
Staffan.svard@icm.uu.se (S.G. Svärd).
Trends in Parasitology, July 2022, Vol. 38, No. 7 © 2022 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1016/j.pt.2022.01.001 605
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Acknowledgment
Our work on Giardia is supported by grants from the Swedish Research Council to S.G.S. and a Consejo Nacional de Ciencia y Tecnología (CONACyT) scholarship to L.R.L.
Declaration of interests
The authors declare no competing interests.
Resources
www.cdc.gov/parasites/giardia/index.html
https://giardiadb.org/giardiadb/
Literature
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settings: longitudinal analysis of results from the MAL-ED cohort study. Lancet Glob. Health 6, e1319–e1328
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Med. 12, eaay7019
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6. Jex, A.R. et al. (2020) Recent advances in functional research in Giardia intestinalis. Adv. Parasitol. 107, 97–137
7. Holthaus, D. et al. (2021) Dissection of barrier dysfunction in organoid-derived human intestinal epithelia induced by Giardia duodenalis. Gastroenterology Published
online November 22, 2021. https://doi.org/10.1053/j.gastro.2021.11.022
8. Xu, F. et al. (2020) A chromosome-scale reference genome for Giardia intestinalis WB. Sci. Data 7, 38
9. McInally, S.G. et al. (2019) Robust and stable transcriptional repression in Giardia using CRISPRi. Mol. Biol. Cell 30, 119–130
10. Horáčková, V. et al. (2021) Cas9-mediated gene disruption in tetraploid Giardia intestinalis. bioRxiv Published online May 8, 2021. https://doi.org/10.1101/2021.04.21.440745
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606 Trends in Parasitology, July 2022, Vol. 38, No. 7 © 2022 The Authors. Published by Elsevier Ltd.
This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
https://doi.org/10.1016/j.pt.2022.01.001