GIARDIA
drawing of trophozoite form of Giardia
from U. C.
Publications in Zoology, 1952
Who is Giardia?
Does Giardia have any other names?
Who does Giardia make
friends with?
How do people become infected with Giardia?
How can someone recognize Giardia?
How does someone know that Giardia
is there?
How does one get rid of Giardia?
How does one keep Giardia
from returning?
Who is Giardia?
Giardia is a protozoan from the phyla
Mastigophora. Its form that primarily
affects humans goes by the name Giardia lamblia. It is the most common flagellate found in
the human digestive tract and it is very contagious, though usually not fatal
(McGill University).
The Giardia microorganism was originally discovered by
Antony van Leeuwenhoek. He described
the Giardia trophozoite from a sample of his own stool in 1681:
“animalcules a-moving very prettily. . .Their bodies were somewhat longer than broad, and their belly, which was flatlike, furnisht with sundry little paws. . . and albeit they made a quick motion with their paws, yet for all that they made but slow progress” (Katz, Despommier, and Gwadz, p. 129-30).
Does Giardia have any other names?
Giardiasis, the symptomology caused
by Giardia, has been called “Beaver Feaver”, after an outbreak at
Canadian Banff National Park, which
was attributed to beavers infecting stream water which hikers drank (McGill
University).
Who does Giardia
affect?
Everyone
is at risk for Giardia. In the
United States, Giardia most commonly affects those who come into
frequent contact with young children, such as their family members or workers
at day care centers. It also occurs in
people who drink improperly treated surface water (lakes, streams, etc.) and
foreign travelers. Giardia is closely associated with recreational water
use (i.e., swimming pools and water parks, where epidemics have occurred).
Fecal-oral exposure during sex is also a risk factor for Giardia.
The CDC has reported that the
two groups in the United States with the highest risk of becoming infected with
Giardia are:
-children
younger than 5 years of age
-women
of childbearing age
Its overall prevalence in the
United States ranges from 1.5 to 20 percent.
It is estimated that
approximately 200 million people worldwide are currently infected with Giardia
(Markell, John, Krotoski, p. 16).
Who does Giardia
make friends with?
Giardia is a zoonosis that is found in more than 40 animal
species, and five strains of the bacteria have been described. Rodents, birds, and reptiles are affected by
Giardia muris, Giardia intestinalis affects other mammals, and Giardia
agilis affects amphibians (WHO).
It can be transmitted across
species (e.g., dogs to beavers, beavers to humans).
What is Giardia’s life like?
Giardia exists in active (trophozoite) and dormant (cyst)
stages.
Its active, motile, but noninfective, trophozoite stage
takes place after the Giardia cyst has entered the host.
Each
cyst undergoes a process called excystation, by which it produces 2 to 4 trophozoites.
The trophozoite has a length
from 9 to 21 μm and width of 5 to 15 μm.
Each trophozoite possesses
two nuclei and four pairs of flagella.
The anterior side of the
trophozoite’s body consists of a sucking disk, which it uses to attach to
the villi of the small
intestine.
The posterior side of the
sucking disk contains two curved rod-shaped structures of unknown function
called median bodies.
Once they have reached the
small intestine, the trophozoites can multiply via binary fission.
Giardia trophozoites in the small intestine
From Markell & Voge’s Medical Parasitology
Giardia assumes its dormant cyst state to survive in the
environment.
The
harsh conditions in the colon cause Giardia to assume the cyst
state.
This
process is called encystation and protects Giardia for when it enters
the environment.
The Giardia
cyst is very hardy and able to survive in the environment for long periods of
time.
The cysts are oval-shaped and are 8 to 14 μm by 7 to 10
μm.
Each cyst contains four nuclei, four median bodies and 8
flagella, all contained within the cyst’s outer wall.
An infective dose consists of 10 to 100 cysts (Erlandsen, p.
334).
How do people become infected with Giardia?
Humans acquire Giardia
through ingestion of infective Giardia cysts in contaminated drinking
water (or anything else contaminated with infected fecal matter). Excystment occurs when the cyst reaches the
small intestine. After traveling to the large intestine Giardia can
encyst. The organism can leave the body
with the feces in either the cyst or trophozoite form.
from McGill
University
What does Giardia eat?
The small intestine provides a source
of nourishment for Giardia trophozoites, which use their sucking disks
to attach themselves to the columnar epithelial cells. Their main food source is glucose. Giardia obtains glucose from the
lumen of the small intestine by means of diffusion or pinocytosis.
How can someone recognize Giardia?
The best way to diagnose Giardia
is through microscopic examination of the stool. Immunoassays such as ELISA (Enzyme-linked Immunosorbent Assay)
and DFA (Direct Fluorescence Assay) can also be used.
Diagnosis by examination of
the stool can be affected by the fact that the organism presents in the stool
in three patterns:
Low : small numbers of
parasites present in 40 percent of stool specimens
Mixed: 1 to 3 weeks of a high
excretion pattern, that alternates with a short period of low excretion
High: parasites present in almost all stools
(Markell, John, Krotoski, p. 59)
How does someone know that Giardia is there?
The incubation period is about eight days.
Many people infected with
Giardia are asymptomatic.
The symptoms that Giardia
causes are lumped under the term giardiasis, which can consist of diarrhea,
dehydration, abdominal pain, flatulence, anorexia, steatorrhea, weight loss,
and in extreme cases, malabsorption syndrome.
Severe cases of Giardia
can cause changes in the intestinal villi that are associated with
malabsorption syndrome. Atrophied, or
flattened villi is the result:
From Katz, Despommier, Gwadz
Giardiasis can last from 2 to 6 weeks. (CDC)
Lactose intolerance can
persist after the eradication of Giardia from the digestive tract (Markell,
John, Krotoski, p. 59).
How does one get rid of Giardia?
The
drug of choice for treating Giardia is the antibiotic Metronidazole
(Flagyl). Other antibiotics, such as
quinacrine HCl and bacitracin, can also be used. Like many other bacterial infections Giardia has developed
resistance to many of drugs used to treat it.
The
world will probably not be rid of Giardia anytime soon—the probability
of eradication is low due to its global prevalence and multitude of animal
reservoirs.
How does one keep Giardia from returning?
The best way to prevent Giardia
is by frequent handwashing.
Giardia is able to survive water filtration and
chlorination.
It follows that the best way
to prevent giardiasis is to protect water sources from contamination.
Public Health efforts have
focused mainly on water filtration and disinfection. E. coliform tests that are routinely done on water supplies are
sometimes not able to detect the presence of Giardia, and outbreaks,
such as one in Las Vegas, have occurred in which water tests have been negative
for Giardia.
Iodine is effective in
purifying small supplies of drinking water.
A saturated solution must be used double strength with a 20-minute
exposure at 20 degrees Celsius (Markell, John, Krotoski, p.62).
Links
You can learn more
about water filters useful for hiking and backpacking here.
The
CDC has more information and further discussion of ways to protect yourself
from Giardia here.
The
WHO has detailed information about Giardia and other intestinal parasites here.
References:
Erlandsen, Stanley L., ed.
Giardia and Giardiasis: Biology, Pathogenesis, and Epidemiology. Plenum Press: New York, 1984.
Filice, Francis Patrick.
“
Studies on the cytology and life history of a Giardia from the laboratory rat.”
U.
C. Publications in Zoology. v. 57, no.
2. University of California Press: Berkeley, 1952.
“Giardia”. McGill
University website. http://martin.parasitology.mcgill.ca/jimspage/biol/giardia.htm
“Giardiasis Infection Fact Sheet.” Centers for Disease Control and Prevention. http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/factsht_giardia.htm
Katz, Michael, Dickson D. Despommier and Robert W.
Gwadz. Parasitic Diseases.
Second edition. Springer-Verlag: New York, 1989.
Markell, Edward K., David T. John and Wojciech A.
Krotoski. Markell and Voge’s Medical
Parasitology. Eighth Edition. W. B. Saunders: Philadelphia, 1999.
“Protozoan parasites (Cryptosporidium, Giardia, Cyclospora).”
World Health Organization. http://www.who.int/water_sanitation_health/GDWQ/Microbiology/Microbioladd/GDWQMicroAdd4.pdf
by Michelle LaCour, Stanford University,
2003.