Giardiasis is a zoonotic parasitic disease caused by the flagellate protozoan Giardia
lamblia.
The giardia organism inhabits the digestive tract of a wide variety of domestic and wild
animal species, as well as humans.
It is the most common pathogenic parasitic infection in humans worldwide; in 2013, there
were about 280 million people worldwide with symptomatic giardiasis.
Signs and symptoms Symptoms include loss of appetite, diarrhea,
loose or watery stools, stomach cramps, upset stomach, projectile vomiting, bloating, excessive
gas, and burping.
The incubation period for giardiasis is 9–15 days.
Symptoms are caused by Giardia organisms infecting the cells of the duodenum and jejunum of the
small intestine and blocking nutrient absorption.
Most people are asymptomatic; only about a third of infected people exhibit symptoms.
If the infection is not treated, these symptoms may last for six weeks or more.
Symptomatic infections are well recognized as causing lactose intolerance, which, while
usually temporary, may become permanent.
Although hydrogen breath tests indicate poorer rates of carbohydrate absorption in those
asymptomatically infected, such tests are not diagnostic of infection.
It has been suggested that these observations are explained by symptomatic giardia infection
allowing for the overgrowth of other bacteria.
Some studies have shown giardiasis should be considered as a cause of vitamin B12 deficiency
as a result of the problems caused within the intestinal absorption system.
Cause Giardiasis is caused by the protozoan Giardia
lamblia.
The infection occurs in many animals including beavers, cows, rodents, and sheep.
Animals are believed to play a role in keeping infections present in an environment.
Risk factors It occurs more often during the summer in
the United States.
This is believed to be due to a greater amount of time spent on outdoor activities.
Wilderness travel within the United States is believed to be a risk factor with poorly
treated or untreated water playing a role.
Transmission Giardiasis is transmitted via the fecal-oral
route with the ingestion of cysts.
Primary routes are personal contact and contaminated water and food.
The cysts can stay infectious for up to three months in cold water.
According to the CDC, those at greatest risk are travelers to countries where giardiasis
is common, people in child care settings, those who are in close contact with someone
who has the disease, people who swallow contaminated drinking water, backpackers or campers who
drink untreated water from lakes or rivers, people who have contact with animals who have
the disease, and men who have sex with men.
Not all Giardia infections are symptomatic, and many people can unknowingly serve as carriers
of the parasite.
Pathophysiology Giardia are flagellated protozoans that cause
decreased expression of brush border enzymes, morphological changes to the microvillus,
and programmed cell death of small intestinal epithelial cells.
There is no invasion of giardia trophozoites, and small intestinal morphology may appear
normal in light microscopy.
The attachment of trophozoites causes villus flattening and inhibition of enzymes that
break down dissacharide sugars in the intestines.
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 diarrhea, one of the predominant symptoms.
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 is highly
varied.
The species Giardia intestinalis uses 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 lymphocytes attack endothelial cells that
have been injured in order to remove the cell.
This occurs after the disruption of proteins that connect brush border endothelial cells
to one another.
The result is heavily increased intestinal permeability.
There appears to be a further increase in programmed cell death by Giardia intestinalis,
which further damages the intestinal barrier and increases permeability.
There is significant upregulation of the programmed cell death cascade by the parasite, and, furthermore,
substantial downregulation of the anti-apoptotic protein Bcl-2 and upregulation of the proapoptotic
protein Bax.
These connections suggest a role of caspase-dependent apoptosis in the pathogenesis of giardiasis.
Giardia protects its own 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.
Arginine starvation is known to be a cause of programmed cell death, and local removal
is a strong apoptotic agent.
Diagnosis The mainstay of diagnosis of giardiasis is
microscopic examination of the stool.
This can be for motile trophozoites or for the distinctive oval G.lamblia cysts.
The entero-test uses a gelatin capsule with an attached thread.
One end is attached to the inner aspect of the patient'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.
Immunologic enzyme-linked immunosorbent assay testing is now available.
These tests are capable of a 90% detection rate or more.
Because Giardia lamblia is difficult to detect, this often leads to a delay in diagnosis or
misdiagnosis; several tests should be conducted over a one-week period.
Prevention Boiling suspect water for one minute is the
surest method to make water safe to drink and kill disease-causing microorganisms such
as Giardia lamblia if in doubt about whether water is infected.
Chemical disinfectants or filters may be used.
The CDC recommends the washing of hands before handling food.
According to a review of the literature from 2000, there is little evidence linking the
drinking of water in the N. American wilderness and Giardia.
The researcher notes that treatment of drinking water for Giardia may not be as important
as recommended hand-washing in wilderness regions in North America.
CDC surveillance data reports one outbreak of waterborne giardiasis contracted from drinking
wilderness river water in Colorado.
However, less than 1% of reported giardiasis cases are associated with outbreaks.
Treatment Treatment is not always necessary as the infection
usually resolves on its own.
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.
In the US and other countries, the medication of choice used to treat giardiasis is metronidazole.
In the EU a single dose of tinidazole is considered to be a first-line agent.
While tinidazole has similar side effects and efficacy to metronidasole, it is administered
with a single dose.
Current research evidence suggests albendazole is probably as effective as metronidazole,
but has fewer gastrointestinal and neurological side effects and is more convenient to take
or administer.
Both medications need a five to 10 day long course; albendazole is taken once a day, while
metronidazole needs to be taken three times a day.
A comparison of multiple drugs also favoured albendazole in preference to metronidazole,
but did not draw any conclusions on the effectiveness of the other drugs due to a lack of reliable
evidence.
In the case of nitroimidazole-resistant strains of Giardia, other drugs are available which
have showed efficacy in treatment: quinacrine, nitazoxanide, bacitracin zinc, furazolidone
and paromomycin.
Epidemiology In some developing countries Giardia is present
in 30% of the population.
In the United States it is estimated that it is present in 3-7% of the population.
Research Some intestinal parasitic infections may play
a role in irritable bowel syndrome.
References
Further reading Kathleen Meyer, How to Shit in the Woods:
An Environmentally Sound Approach to a Lost Art, Ten Speed Press.
A practical guide to wilderness waste disposal to prevent giardiasis.
External links Giardiasis Fact Sheet
eMedicine Health Giardiasis