1. health and fitness
2. disease

The Management of Refractory Giardiasis
Vincent Marconi, MD
Introduction
Giardiasis is an infection caused by the protozoan parasite Giardia; Giardia lamblia may cause diarrhea,
dyspepsia, and occasionally malabsorption in humans.1 It is recognized as the most common waterborne
enteric parasitic disease in the U.S. and Canada. Worldwide, it causes both endemic and sometimes
epidemic intestinal disease and diarrhea. Giardiasis, known colloquially as “beaver fever”, remains a
nuisance for many but for the immunosuppressed it can be nearly impossible to manage.
Historical Features
First Known Appearance
Cysts of Giardia have been found in fossilized feces, or coprolites, of ancient Peruvians dating
back as far as 3000 BC.2 Cysts have also been identified using immunofluorescence in desiccated
human feces collected from the Big Bone Cave in Van Buren County, Tennessee. Carbon dating places
these coprolites to around 2000 BC.3
Original Descriptions and Nomenclature
The Dutch microscopist, Antoine van Leeuwenhoek, first observed the protozoan in one of his
own diarrheic stools in 1681: “… wherein I have sometimes also seen animalcules a-moving very
prettily … albeit they made a quick motion with their paws, yet for all that they made but slow
progress.4 It was later described by Lambl in 1859 and given the name Cercomonas intestinalis. In
1915, Stiles renamed the organism Giardia lamblia in honor of Professor A. Giard of Paris and Dr. F.
Lambl of Prague, however many consider the protozoan to be named Giardia intestinalis or duodenalis.5
First laboratory culture and Disease Identification
Giardia was first cultured successfully by Karapetyan in 1960 using a mixed culture with
Candida guilliermondi and chick fibroblasts. He was later able to grow the organism in a monoxenic
culture using Saccharomyces cerevisiae from a rabbit isolate. Bingham and Meyer first reported that
treatment of mature cysts with an acidic solution improved excystation of the parasite allowing for
axenic culturing. The current culturing system uses a modified Entamoeba medium that is filtersterilized and bovine bile is added. It was not recognized as a significant human pathogen until the mid
1960s when community outbreaks began to occur and the disease appeared in travelers returning from
endemic areas. Prior to this, it was thought to be more of a nuisance.
Epidemiology
Prevalence of Endemic Disease and Outbreak Identification
Giardia can be found throughout the world. In the U.S., Giardia had been demonstrated to be in
up to seven percent of stool specimens during the period from 1979 to 1991.6 From 1992 to 1997, the
CDC estimated that around 2.5 million cases occurred in the United States annually with around 5000 of
these people requiring hospitalization.7 In one study of 147 pediatric outpatients with acute
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nondysenteric diarrhea, fifteen percent were due to Giardia.8 Forty-three states reported cases of
giardiasis in 1997, which was an increase when compared to 23 states in 1992. New York had the
highest rate of infection with around 20 cases per 100,000 persons compare with the national average of
9.5 per 100,000 persons in 1997. In Ontario, Canada, the mean annual adjusted incidence rate during
the 1990s was 25.8 cases per 100,000 persons. In the developing world, nearly all children become
infected at some time with prevalence rates of 15 to 30% in children under 10 years of age. Because
asymptomatic infections occur much more frequently than symptomatic infections, these persons
become much more effective as carriers and disseminators of the disease. The Giardia carrier can
excrete cysts for up to several years.
Mode of Transmission
Most of the acquisition of the parasite comes from the ingestion of contaminated water. The
parasite survives well in the environment, especially in cold water, and is found in high concentrations
in the mountainous regions of the Northeast, Northwest and Rocky Mountain states of the U.S. and
Canada. The first outbreak occurred in 1965-66 in Aspen, Colorado where it affected nearly 11% of
over 1000 skiers.9 Since that time there have been 131 more outbreaks affecting over 27,000
persons.10,11 One of the more common scenarios occur when hikers or travelers to wilderness areas
ingest surface water, often from mountain streams, that has not been treated or adequately purified by
filtering and chlorination. Peak prevalence occurs in the spring, most likely related to increased outdoor
activity.
Person to person transmission is the second most common mode of acquisition, resulting from
inadequate fecal-oral hygiene. Groups at risk include children in daycare centers, men who are sexually
active with other men, and persons in custodial institutions. The prevalence of cysts has been as high as
20% in daycare centers. These children who are often not yet toilet trained can serve as a source for
secondary cases within their families.12 In sexually active gay men, rates have been as high as 20%
especially associated with anal intercourse.13
There have also been reports of transmission occurring in commercial food establishments,
demonstrating a role for food-borne transmission.14 This is uncommon since cooking kills Giardia
cysts, and therefore, the transmission must occur with uncooked food or after the food has been cooked.
Filth flies have also been found to carry viable cysts on their exoskeleton from infected hosts and
deposit them on various foodstuffs.15 Finally, one study showed children living in households with a
higher concentration of peridomicilliary livestock had higher rates of infection, raising the issue of a true
reservoir rather than a marker of poor sanitation.16
Populations at Risk
In addition to the groups at risk of exposure to Giardia based upon hygienic practices,
geographic location and behavioral patterns (i.e. hikers, daycare centers, etc.), various deficiencies and
alterations in the immune system have been identified as predisposing risk factors to developing the
disease once exposed. Common variable immunodeficiency, cystic fibrosis, AIDS, X-linked
agammaglobulinemia and possibly selective IgA deficiency are among the most common underlying
illnesses. Hypochlorhydria has also been identified as a predisposing factor to infection (especially
gastric), placing at risk persons with previous gastric surgery, concurrent H. pylori infection or taking
proton pump inhibitors. These defects can also lead to an inability to clear the organism once the acute
disease occurs, resulting in a chronic relapsing infection or persistent active disease state. It was
previously thought that the severity of the disease was not worse under these circumstances. However,
one case series suggested that significant electrolyte abnormalities and death were associated with
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severe giardiasis in 24 patients with advanced AIDS.17 Children under the age of five and pregnant
women also may have a severe form of the illness requiring hospitalization.
Etiology
Phylogenetics and Taxonomy
Giardia is classified as a eukaryote because of the existence of a nucleus, and because it
possesses various organelles including an endoplasmic reticulum, possibly a Golgi apparatus, and a
lysosome-like peripheral vacuoles. However, due to its simple structure, small subunit ribosomal RNA
sequence and the absence of many organelles including the mitochondria, Giardia was once thought to
be the “missing link” between Prokaryotes and Eukaryotes -- existing prior to their endosymbiotic
merger. To solve this problem, investigators discovered specific nuclear genes (encoding for valyltRNA synthetase, histones, and heat shock proteins) that are functional in both the cytosol and
mitochondria of mitochondriate eukaryotes and are phylogenetically closely linked to certain gramnegative bacteria. The sequence homology of these genes was then used as a measurement of
evolutionary branching. It was shown that Giardia also had very close homology with the gramnegative bacteria and nearly all eukaryotes suggesting that it must have had an endosymbiotic
relationship as well. Thus, these investigators proposed an actual loss of the mitochondria over time
secondary to its parasitic nature18.
Giardia belongs to the kingdom Protista, subkingdom Protozoa, phylum Sarcomastigophora
(having flagella or pseudopodia), subphylum Mastigophora (flagellates); class Zoomastigophorea, which
includes Trypanasoma, Leishmania, Dientamoeba and Trichomonas. It then belongs to the order
Diplomonadida (characterized by the presence of one or two karyomastigonts and either two-fold
rotational or mirror symmetry) and the family Hexamitidae.19 The genus Giardia includes the species
G. agilis and G. muris, which infect amphibians and mice respectively. The distinction between species
was traditionally made by morphology and host range alone, which accounts for the small number of
species. However, newer technology has allowed for differentiation based upon antigenic properties,
lysozyme analysis and genotyping, and has shown the species to be rather heterogeneous made up of
assemblages (A through G), groups and phenotypes.20
Description of the Pathogen
Giardia exists in two states, as a trophozoite and a cyst. The trophozoite, or vegetative form, is 9
to 21 µm long, 5 to 15 µm wide and 2 to 4 µm thick. It is pear-shaped and has a convex dorsal surface
and a flat ventral surface containing a disk. This disk is referred to as the sucking or adhesive disk. It is
composed of a spiral array of microtubules and giardins within microribbons. There are two nuclei with
a prominent central karyosome. Locomotion (and perhaps attachment) is achieved by using four
posteriorly directed flagellae, which project into the cytoplasm (termed axonemes). Tight collections of
microtubules, known as median bodies, transverse the organism and are useful in species differentiation.
The trophozoite wall is covered in cysteine-rich surface proteins known as variant specific surface
protein (VSPs). These proteins appear to provide one mechanism for the parasite to infect multiple
different hosts. The cyst, or infectious form, is 8 to 12 µm long and 7 to 10 µm wide, very hardy and
can survive for several months in very cold water. It is smooth, oval in shape and has a thin wall.
Whenever the trophozoite experiences significant physical stress, it undergoes a process of
encystation. Such stressors include cholesterol starvation and an alkaline pH followed by an excess of
bile salts. This initiates a signal transduction pathway, which results in the downregulation of
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trophozoite genes and induces the expression of early encystation-specific genes. These genes encode
various enzymes, including a calcium-sequestering molecule21, glucosamine-6-phosphate isomerase22,
and extracellular signal-regulated protein kinases 1 and 223. These enzymes assist in the production of
the various components contained within specific encystment vesicles, which are secreted to form the
cyst wall proteins or CWPs (mostly fibrous material made of N-acetylgalactosamine and
leucine/cysteine-rich proteins). During the process of encystation, the VSPs are internalized within
vacuoles effectively removing them from the surface of the parasite.
Excystation occurs in an acidic environment and is much less understood. It is a more rapid
process and therefore cell signaling (involving protein kinase A and calmodulin) appears to be the
predominant mechanism rather than gene expression.24 A cysteine protease in the cathepsin enzyme
family is released from vesicles within the organism just prior to excystation and appears to facilitate
this process.25 Pancreatic enzymes also have a role in cyst wall digestion. During excystation, the
individual isolates undergo antigenic variation and reexpression of the VSPs. This antigenic switch may
allow the parasite to evade host responses26.
Metabolism and Molecular Biology
As demonstrated by its predilection to colonize the upper small bowel, the growth of Giardia is
enhanced by the presence of biliary lipids and intestinal mucous and epithelial cells. It can absorb
conjugated bile salts by both active and passive means facilitating encystation. Giardia also prefers a
low oxygen tension and is an aerotolerant anaerobe. It metabolizes glucose to ethanol, acetate, and
carbon dioxide. It can generate ATP via the arginine dihyrolase pathway and scavenges fatty acids,
phospholipids, cholesterol and purines and pyrimidines from the environment, but it requires exogenous
cysteine, alanine, arginine and aspartate.
Not yet cloned in its entirety, the total haploid genome is 1.2 x 107 base pairs with a GC content
of 46% on five chromosomes in each of two nuclei.27 It divides by binary fission longitudinally and has
a doubling time of 9 to 12 hours in culture. The trophozoite cycles between cellular ploides of 4N and
8N (2N and 4N in each nucleus respectively). During encystation, nuclear division and subsequent
DNA replication occur within the cyst (without a cell division event) forming a ploidy of 16N. After
excystment, two rounds of binary fission with one intervening step of DNA replication occur, and four
trophozoites are formed with a ploidy of 4N28.
Life Cycle and Pathogenesis
Hosts/Life Cycle
Humans are the primary host for Giardia. Although several species of Giardia have been found
to infect various mammalian (cats, dogs, beavers, and cattle) and amphibian species, there has not been
conclusive evidence to support an animal reservoir for Giardia lamblia. However, an outbreak of
Giardiasis occurred in Canada involving beaver exposure implicating a zoonotic transfer.29
Cysts are obtained from the environment, either in contaminated water, food or the fecal oral
route (hands or fomites). They can survive in the environment for prolonged periods if kept moist. The
infectious dose is 10 to 25 cysts. After the cysts are ingested, excystation occurs in the small bowel after
exposure to gastric acid and pancreatic enzymes. The trophozoites are then released into the small
intestine. Multiple rounds of binary fission occur resulting in a large burden of trophozoites. The
trophozoites then remain either free in the lumen of the intestine or attach to the epithelium by the
sucking disk. Sometimes trophozoites can be found in the gallbladder and common bile duct. They
may also exist within epithelial cells, but invasion is rare. As the parasite travels toward the colon,
4
encystation occurs facilitated by conjugated bile salts and an alkaline pH. The cysts are passed in the
feces, become immediately infectious, and can remain infectious for a long period of time.
Barriers to Infection
Once exposure to the parasite occurs through ingestion of cysts, there are several barriers to help
prevent infection from occurring. There are both epidemiological evidence and experimental
observations in animal models, which have demonstrated that previous infection provides protection
against reinfection. In developing countries, the prevalence is higher in children than adults, and in
endemic areas residents have lower rates of infection than visitors to the area. It appears that preformed
secretory IgA plays the most important role for humoral immunity in averting infection. It acts by
binding to the trophozoites in the intestinal lumen preventing the critical adherence step. However,
Giardia can produce IgA proteases weakening this response.
Human milk has also been found to play a role in protecting the host against infection. The fatty
acids released in the gut are toxic to the trophozoite. This milk has also been found to contain antiGiardia antibodies. Nitric oxide released by the enterocyte also has an anti-parasitic effect; however
Giardia can inhibit nitric oxide production by consuming arginine, a necessary metabolite. Paneth cells
and enterocytes also produce defensins (alpha and beta respectively), which have been shown to create
pores in the parasite membrane ultimately resulting in cell lysis.30 The mucous layer of the bowel lumen
itself consisting of mucin and water may provide a physical barrier trapping the trophozoite enabling
evacuation by peristalsis.
Pathogenesis/Tropism/Virulence
The attachment of trophozoites to the brush border enterocytes may involve more than just a
suction or clasping mechanism by the disk: there appears to be more specific receptor-ligand
interactions. This adherence allows the parasite to avoid peristalsis by becoming trapped in the mucosa
and between villi. Other mechanisms of virulence have been postulated but not effectively proven,
including the elaboration of an enterotoxin and effective mucosal invasion. The previously mentioned
assemblages have been found to have differing virulences. Assemblage A has been associated with
intermittent diarrhea and assemblage B with persistent diarrhea.31 In contrast, a recent prospective study
found children less than five years old that had assemblage A to be 26 times more likely to have diarrhea
than those with assemblage B.32
Host Response to Infection
After an infection has been established, the host responds with a vigorous inflammatory reaction
in an attempt to diminish the degree of pathology. The first step occurs when the nonspecific immune
system responds to the presence of Giardia in the brush border of the intestine. The trophozoites first
become phagocytosed by macrophages. The specific cellular immune system response is then activated
when antigens are presented to CD4+ helper T cells by these macrophages. It has been shown that
athymic mice are unable to clear an infection with Giardia muris unless reconstituted with lymphoid
cells. The helper T cells then stimulate IgA production as well as the production of secretory IgM and
IgG. These antibodies appear to have activity against heat shock proteins and VSPs found on the
surface of the parasite. It is unclear whether the parasite’s ability to alter its specific surface proteins
allows for immune system evasion during the infection. The development of these antibodies correlates
temporally with the clearance of the parasite. Persons unable to produce an effective IgA response often
develop chronic giardiasis. The latter classes of antibodies act by engaging in specific anti-Giardia
toxicity in conjunction with the complement cascade although a controversial mechanism. There has
5
also been murine data showing B cell-dependent but immunoglobulin-independent mechanisms to be
critical for host defense.33 Although the immune response is critical to clearing the infection, it
contributes to the pathology as well.
Pathology and Pathophysiology
Histopathology
Most of the pathology in giardiasis exists, if at all, in the small intestine. Nearly 96 percent of
patients examined in one study had normal mucosa on duodenal biopsy.34 Sprue-like lesions are often
seen but not required for infection. These lesions appear on light microscopy as mild or moderate partial
villous atrophy and in some severe cases subtotal villous atrophy. Crypt depth may also be increased
(crypt hyperplasia). Even if no abnormality is seen by light microcopy, the electron microscope can
reveal disruption of the brush border and a shortening of the microvilli. There also appears to be more
frequent simultaneous bowel overgrowth of Enterobacteriaceae and yeast. Gallbladder irritation can
also be seen.
Organ System Dysfunction
There are several mechanisms that investigators have postulated to cause diarrhea in giardiasis.
The disruption of the brush border may lead to disaccharidase (e.g. lactase) deficiencies seen frequently
in Giardiasis. This deficiency can cause an osmotic diarrhea in a manner similar to that seen in lactose
intolerance. Also the replication of the trophozoite leads to a physical barrier between the enterocyte
and the lumen of the small intestine causing a malabsorptive diarrhea.35 The bacterial and yeast
overgrowth may also lead to malabsorption by the deconjugation of bile salts. There have also been
cytopathic substances secreted by the parasite such as lectins, proteinases and glycoproteins which may
also lead to enterocyte damage and malabsorption.36 The mere presence of the glycoproteins in the
lumen has been shown to induce an osmotic diarrhea. Another source of mucosal injury is due to the
inflammatory response described earlier wherein a cytokine release results in a secretory diarrhea. This
may also stimulate epithelial cell turnover in the crypt region again altering the gut absorptive capacity.
Certain strains have been shown to induce apoptosis and alter epithelial permeability in a T-cell
independent fashion.37,38 There has even been evidence in gerbil models of the disease that Giardia can
increase gut transit time and increase smooth muscle contractility.
Clinical Presentation
There are three ways that infection with Giardia can manifest itself. One can have asymptomatic
passage of cysts, acute self-limited diarrhea or a chronic process consisting of diarrhea, weight loss and
malabsorption. The spectrum of clinical disease may be due to a combination of factors including the
virulence of the giardial isolate, the parasite load, the host immune response and perhaps other host
determinants.
Anywhere from 35 to 70% of patients will have no evidence of infection at all after exposure.
Between 5 and 15% of exposed individuals will have asymptomatic passage of cysts. This can last for
as long as six months. Acute giardiasis occurs in fewer than 50 percent of exposed individuals (range of
25 to 50%). Symptoms include a sudden onset diarrhea (initially watery), malaise, stools that are foul
smelling and steatorrheic, abdominal discomfort (cramping and bloating) and flatulence, nausea,
vomiting and weight loss. Fever occurs in a small percentage of patients and may not be related to the
disease. Blood, mucus and pus are usually not present in the stool. The average incubation period is
6
between one to two weeks (with a range of 1 to 45 days); however, there is often a lag between the onset
of symptoms and cyst passage. The symptoms can last for long periods of time, often at least two to
four weeks.
Symptoms of Giardiasis
Percentage
Range
Diarrhea
89
64-100
Malaise
84
72-97
Flatulence
74
35-97
Foul-smelling, greasy stools
72
57-79
Abdominal cramps
70
44-85
Bloating
69
42-97
Nausea
68
59-79
Anorexia
64
41-82
Weight loss
64
56-73
Vomiting
27
17-36
Fever
13
0-21
Urticaria
9
4-14
Constipation
9
0-17
Adapted from Principles and Practices in Infectious Diseases39
Chronic giardiasis can be a sequelae of an acute episode of giardiasis or may develop in the
absence of an antecedent illness. One study showed 16% of patients experimentally infected with
Giardia went on to develop a chronic infection40. As many as 30 to 50% of patients who experience an
acute episode of giardiasis can develop chronic giardiasis. This illness is characterized by loose stools
but not usually watery diarrhea, steatorrhea, profound weight loss, protein malabsorption, folate and fat
soluble vitamin deficiencies, malaise, fatigue and depression. There can also be periodic abdominal
cramping, boborygmi, flatulence and sulfuric belching. These symptoms can wax and wane over many
months. Additionally nearly 20 to 40% of patients can acquire lactose intolerance as a result of this
illness further worsening these symptoms. This can often last for many weeks after the infection has
cleared.
The weight loss in chronic giardiasis can be significant (10 to 20 percent of body weight) and in
developing countries where children are already at a nutritional disadvantage, this can result in
devastating effects on growth and development41 as well as on cognition scores42. However some
studies have failed to show a correlation with the presence of Giardia in stools and impaired growth.
This may be the result of a reporter bias.43 Occasionally a hypersensitivity reaction can occur, with such
phenomena as rash, urticaria, apthous ulcers and a reactive arthritis or synovitis. Also very rarely can
7
infection spread to the biliary tree and pancreatic ducts leading to cholecystis, cholangitis,
granulomatous hepatitis or impaired pancreatic exocrine function.
Evaluation
Prolonged diarrhea, malabsorption and weight loss should trigger an investigation which
includes giardiasis as a cause. Giardiasis accounts for 10-30% of cases of chronic diarrhea where
symptoms have persisted for more than two weeks. A search of epidemiologic risk factors might reveal
a recent trip to the mountains or a small child at home who attends a daycare center. The most specific
clinical findings which are helpful in making the diagnosis includes a duration of illness lasting seven or
more days and at least two of the following symptoms: diarrhea, flatulence, foul-smelling stools,
nausea, abdominal cramps and excessive fatigue.44 The differential diagnosis includes other protozoans
such as Cryptosporidium and Cyclospora, along with noninvasive bacteria and viruses. Other possible
causes to consider are sprue (either celiac or tropical), Whipple’s disease, abetalipoproteinemia, small
bowel lymphoma, tuberculosis and lymphangiectasia. Leukocytosis and eosinophilia are usually not
present. Fecal fat excretion and other laboratory tests demonstrating malabsorption will be abnormal.
Barium studies and computed tomographic imaging shows nonspecific findings of small bowel
thickening and in fact may make stool examination more difficult because of the contrast dye.
Many laboratories still rely on the traditional method of stool examination for trophozoites or
cysts. This is the gold standard by which other tests have been compared. Stool must be examined fresh
and after preservation. White blood cells and red blood cells are usually not found in stool specimens
owing to the absence of invasion by the parasite. Trophozoites are not seen in stool unless the patient
has been given a saline cathartic or has persistent diarrhea. These can be seen using a wet mount of
fresh liquid stool. Living trophozoites on a direct smear have a characteristic flutter, “falling-leaf” type
of movement. Cysts are usually seen in non-diarrheal or semiformed stools. To see cysts, the slides
need to be stained with iodine. Alternatively, the stool can be fixed in formalin or polyvinyl alcohol and
stained with trichrome or iron hematoxylin. When stained, two, four or occasionally more nuclei can be
seen with the fibrils. To increase the yield, zinc sulfate flotation or formalin-ether concentration
techniques can be used. Using these techniques, Giardia can be identified in 50-70% of the time
examining one stool. The yield can increase with each subsequent stool examination approaching 95%
after three stools.
There are several antigen tests, probes and amplification techniques available which are gaining
in popularity and are the test of choice in most centers. These tests have a sensitivity in the range or 8598% and specificity of 90-100% with a cost comparable to stool examination. They are useful in
outbreaks and to assess the success of a course of treatment. The antigen tests include an ELISA used to
detect a 65-kD cyst wall glycoprotein (ProSpecT, Alexon, Inc) which has a sensitivity of 94-97% and
specificity of 99-100%. There is also a direct immunofluorescence assay which can be used to detect
Giardia using a fluorescein-tagged monoclonal antibody (Merifluor DFA, Meridian Bioscience, Inc). It
has a sensitivity and specificity of 96-100%45. Recently a non-enzymatic solid-phase qualitative
immunochromatographic assay that can detect and distinguish Giardia from Cryptocococcus in aqueous
extracts of human fecal specimens (ImmunoCard STAT!, Meridian Bioscience, Inc). It had a sensitivity
of 93.5%, specificity and positive predictive value of 100% with a negative predictive value of 95.5%.
The yield decreases with a low parasite number. When comparing the three assays, the DFA was
found to have the highest sensitivity.46 Polymerase chain reaction (PCR) diagnostic techniques are
currently only experimental. PCR has shown the ability to detect 10 parasites per 100 microliters of
stool and had a 100 percent correlation with microscopy and ELISA in patients with suggestive
8
symptoms. This tool may be useful in screening water supplies for contamination. These test, although
shown to be faster and have a similar cost, they lack the ability to identify other pathogens, which might
be found on routine microscopy.
In most circumstances, antigen assays or stool examinations are all that is needed. However, in
difficult cases, more aggressive diagnostic procedures are required including duodenal aspiration and
biopsy (especially helpful if the patient has HIV). The string test (or Entero-Test) is of historical interest
only. It involves the ingestion of a string which when removed contains bile-stained mucus from the
duodenum for microscopic examination. Duodenal aspiration has the advantage of identifying small
bowel bacterial overgrowth. Duodenal biopsy, although more invasive, has the advantage of identifying
other histologic abnormalities that might be the cause.
Serologic assays do not have a specific role in individual diagnosis but are helpful in prevalence
studies. The IgG antibodies persist making them less useful during outbreaks, however the IgM
antibodies can be used in this setting. Culturing also does not prove useful in the clinical setting, as
these organisms are rather fastidious and not reliably isolated from patient samples.
Management and Outcomes
Due to the difficulty in reliable culturing of clinical isolates, susceptibility testing of Giardia has
not been standardized nor used in any routine management strategy. As a result, the response or lack of
response to therapy has not been shown specifically to be the result of drug resistance. Resistance can
be induced in vitro, however studies have not been done to corroborate this finding clinically.
Furthermore, isolates that have been tested in vitro and shown to be resistant still respond to this therapy
in vivo. The opposite has also been demonstrated.
Medical Therapy
Metronidazole, a nitroimidazole, although never approved by the Food and Drug Administration
for giardiasis is the drug of choice for this disease. Physicians appear to be most comfortable with its
use because of its favorable side-effect profile and ease of availability. This compound serves as a
terminal electron acceptor, which binds covalently to DNA macromolecules and results in DNA damage
as well as inhibiting trophozoite respiration. Due to poor penetration, it has decreased activity against
cysts. It is given for five to seven days in divided doses (250 mg po tid) with an efficacy of 80-95%.
Several dose, frequency and duration comparison trials have been performed to establish this
recommendation. There is complete absorption orally and the drug is metabolized mainly in the liver.
Side effects include a metallic taste in the mouth, nausea, vertigo, headache and rarely reversible
neutropenia. Rarely seen side effects include dark urine and paresthesias. Most troublesome for some
patients is a disulfiram-like effect, which occurs when taken with alcohol. Some prefer a higher dose,
short course regimen but is generally less well tolerated and subsequently has lower rate of efficacy.
Refractory cases may require higher doses and a longer course (i.e. 750 mg tid for 10 days). Isolates
resistant to metronidazole have been described and the mechanism is due to a decreased activity of the
parasite pyruvate:ferredoxin oxidoreductase required for the reduction of all nitroimidazoles.
Susceptibility testing has been done using various methods including growth inhibition, serum killing,
and inhibition of adherence, however a valid standardization has not been established preventing
widespread acceptance of such results.
Tinidazole is another nitroimidazole not available in the United States but has excellent efficacy
when given in a single 2 g dose, approaching 90% with few associated side effects. In one study, 63
expatriates and travelers from Bangladesh with symptomatic and asymptomatic giardiasis were
9
randomized to therapy and followed after 4 weeks to determine if the stools were free from parasite.
The first group received either metronidazole 2 g as a single dose versus tinidazole 2 g as a single dose.
The metronidazole group had 56% eradication versus 94% for tinidazole. In a second group, one arm
received metronidazole 2 g over 3 d versus tinidazole 2 g as a single dose. Metronidazole had 93%
success versus 100% for tinidazole. Symptoms were discussed but not included in the analysis.47
Ornidazole been in use overseas since the mid 1970s and appears to have excellent efficacy over several
days and efficacy similar to tinidazole as a single dose. Recently a dose comparison study in Turkish
children showed a significant benefit with all doses of ornidazole over metronidazole (94-97% versus
89%, p<0.05).48 Secnidazole is a long-acting nitroimidazole (half-life 17 to 29 hours) with efficacy of
over 85% with a single daily dose of 2g. It did not, however, show clinical superiority when compared
with albendazole in a RCT of pediatric patients.49 Adverse effects are similar but not as common as
metronidazole.
Quinacrine (Atabrine) production was discontinued in 1992 in the United States. The
mechanism of action is postulated to involve DNA intercalation. It is usually given for 5 to7 days at 100
mg tid with an efficacy also close to 90%. In vitro data is conflicting but most studies show it to be
inferior to metronidazole and furazolidone. It is rapidly absorbed and widely distributed in the tissues.
Side effects include a bitter taste, headache, dizziness, and vomiting which likely limits adherence. An
exfoliative dermatitis, psychosis and a retinopathy are rare side effects but more commonly patients may
see a yellow discoloration of the skin, urine and sclerae.
Furazolidone (Furoxone), a nitrofuran, is a preferred alternative in the pediatric age group
because of a liquid suspension formulation. It also utilizes a reduced form of the compound to induce
DNA damage. Efficacy is around 80% for 100 mg qid using the 7 to 10 day course but much lower
when given in only 5 days. In vitro testing shows it to be comparable to metronidazole and superior to
quinacrine; however clinical trials show this drug to be slightly less efficacious when compared to
metronidazole and quinacrine. It has complete and rapid absorption and resistance is likely from
decreased entry of the drug in the parasite or from toxic radical scavengers that can prevent its activity.
It can cause gastrointestinal side effects, turn urine brown and cause mild hemolysis in G6PD deficient
individuals.
The benzimidazoles have gained interest in the last few years as an alternative to traditional
therapy. These compounds exert their effect by inhibiting cytoskeleton polymerization and impair
glucose uptake. In comparison to nitroimidazoles or quinacrine, these drugs have superior activity in
vitro. Although mebendazole has been less than impressive in its efficacy in this setting, albendazole in
a single daily dose of 400 mg or 22.5 mg/kg for 5 days has been rather successful. One study of
children in Bangladesh showed an efficacy of 97 percent.50 Decreased efficacy was shown with a 3-day
schedule. An additional benefit with this class is the antihelminthic properties make this option ideal in
countries where concomitant parasitic infections exist. There is poor oral absorption that can be
improved with the coingestion of a fatty meal. Very few side effects occur with such a short course;
prolonged courses with helminthes have shown reversible neutropenia and transaminase elevation.
The FDA has recently approved nitazoxanide for the treatment of giardiasis in children. It is
given at a dose of 100 to 500 mg twice daily for three days. Efficacy has ranged from 81 to 85% and
was found to be equivalent to five days of metronidazole in one study seven days after initiation of
therapy.51 Other compounds under investigation for this use include ivermectin, disulfuram and
rifampin.
Other agents such as bacitracin and bacitracin-zinc have also demonstrated impressive efficacy,
approaching cure rates of 87.5% and 94.7% respectively. These compounds act by interfering with a
10
dephosphorylation step. Both are given at a dose of 120,000 U twice daily for 10 days. Gastrointestinal
upset appears to be the major side effect; however, prolonged use can lead to renal failure.
Various luminal agents have also been useful in eradication when the disease has become
refractory. Paromomycin, an oral nonabsorbable aminoglycoside that interferes with ribosomal
subunits, has an efficacy rate of 60-70%. It is given in divided doses for 5 to 10 days. It has minimal
absorption with minimal side effects.
Several complimentary methods of therapy and dietary recommendations have been proposed by
various investigators in the treatment of giardiasis. Probiotics have had variable success for other
gastrointestinal diseases and investigation is ongoing with respect to giardiasis.52 Among the dietary
recommendations are specific changes such as increasing fiber intake and reducing fat, lactose and
complex carbohydrates. Garlic, flavonoids, Indian long pepper, Pippali rasayana, Geranium nivem
extracts and berberine-containing herbs have been found to be beneficial as well. In fact, two clinical
trials have looked at berberine-containing regimens and found some evidence to support its use.
Berberine is an isoquinolone alkaloid found in medicinal plants and has been shown to have anti-giardial
trophozoite activity in vitro. One such trial was a RCT of forty patients comparing metronidazole versus
a berberine-containing herb versus placebo. The metronidazole arm was 100% parasite free, while the
berberine regimen was around 62%. However, the berberine group had significantly less symptoms
overall than the other two arms. The authors proposed a higher dose regimen would be necessary to
achieve significant results in parasite clearance.53 A second study involved a dose escalation of the
berberine compound with a maximum dose of 10 mg/kg/day when compared with metronidazole and
furazolidone. Cure rates in the highest-dose group were similar to the metronidazole group; however a
subset of the berberine arm relapsed after one month.54
The Approach
The initial approach to treatment should involve documentation of cysts or trophozoites in the
stool. Thereafter the regimens are limited to those available in the United States. Metronidazole is the
treatment of choice although not FDA approved for this indication. Quinacrine has been discontinued
(but is available through Panorama); albendazole and nitazoxanide are promising alternatives.
Furazolidone is effective but has a long course with frequent dosing possibly affecting adherence. It can
be expected that the parasites will be cleared in 3 to 5 days with symptoms resolving in 5 to 7 days. A
Cochrane Review in 2000 found metronidazole to be the most efficacious longer duration choice of
antimicrobials but with significant heterogeneity between trials. Of the single dose therapies, which had
obvious excellent adherence, tinidazole was found to have superior efficacy over the other choices.55
Pregnant women do not have great options for treating Giardiasis. Many of the above agents
have theoretical side effect on the fetus. For most women where the disease is mild, hydration and
nutrition can be sustained throughout the pregnancy until delivery at which time treatment can be
initiated. Luminal agents are a reasonable option as systemic absorption is minimal. Metronidazole has
been used in pregnancy to treat trichomoniasis and the teratogenic effects appear to be minimal and
concentrated in the first trimester. Therapy, if absolutely necessary, should be administered during the
second or third trimesters. Metronidazole is also excreted in breast milk. The American Academy of
Pediatrics recommends giving a single 2 g dose in nursing mothers and to discontinue breast-feeding for
12-24 hours.
Treatment Failure
It is important to determine the cause, if possible, of a treatment failure. The first important
distinction is to determine if an actual infection exists or if the patient is experiencing post-Giardia
11
lactose intolerance. In order to demonstrate that lactose intolerance is the cause of the symptoms, stool
examinations should be performed documenting the absence of cysts. Rarely does one need to proceed
to gastric biopsy or aspiration especially if this was not required for the initial diagnosis. If cysts are still
seen, then a search for the cause of treatment failure should be undertaken. These include a true cure
with a subsequent reinfection (assess risk factors), inadequate drug levels (usually from poor adherence),
actual antimicrobial resistance, parasite sequestration in the gallbladder or pancreas, and an underlying
immunodeficiency. Usually when a relapse occurs due to reinfection or poor adherence, the initial
regimen can be attempted again if the patient did not experience side effects limiting the adherence.
However if resistance is suspected, a course of the same drug (using a higher dose and/or longer
duration) or a switch to another drug from a different class generally is effective. However, sometimes
patients require combination therapy. The combination of albendazole and metronidazole has been
reported to have 100 percent efficacy in patients who failed a standard approach.56 Rarely several
different combinations or approaches will need to be tried before the patient recovers. In a study by
Nash et al, the combination of quinacrine and metronidazole was successful after several failed attempts
with other agents in five out of six patients in a case series (see below). This has been supported by
other case series. Sensitivity assays showed susceptibility to both agents alone and an additive effect
when used together.57
Prevention
As with most infectious diseases, prevention remains the most effective means by which the
incidence of disease can be reduced. The foundation of this approach requires basic tenets of hygiene
and water treatment.
Treatment of public water supplies in the developed world is a complex process that requires, in
addition to chlorination, flocculation, sedimentation and filtration. Chlorination alone can be sufficient
12
to kill Giardia cysts but the efficacy depends upon water temperature, clarity, pH, and contact time.
Higher chlorination levels of 4 to 6 mg/liter may be required depending upon these conditions. In other
parts of the world, travelers should consider the water to be contaminated. Boiling water to 70 degrees
Celsius for 10 minutes alone is sufficient to kill all cysts and at higher altitudes longer periods of boiling
may be necessary. In situations where boiling is impossible, halogenation is an effective alternative.
This includes chlorine-based methods and iodine-based methods. When using halogenation,
disinfection must be carried out for at least eight hours. Again, the recommended contact times should
be extended if certain variables are present such as very low water temperature or significant turbidity.
Filtering is also a valid alternative. Pore size should be no greater than one micrometer. It is also
important to remember that water also used in the washing and preparing of food or for brushing teeth
should be treated in a likewise manner.
It has not been effectively proven that the treatment of asymptomatic carriers reduces the overall
transmission in endemic settings and high-risk populations. Because the treatment of children is not
without side effects, many public health experts have recommended only treating those who have
symptoms. However, if there is recurrent diarrhea in a day care center which cannot be controlled by
improved hygiene and exclusion of symptomatic children, one can consider screening all children and
treating those infected. Also in resource-poor settings where malnutrition exists and growth
development may be compromised by infection, there may be some utility in treating in this
circumstance.58 However, treatment must be weighed against the significant likelihood that reinfection
within a short period of time is inevitable. Additionally, those persons who may be carriers and
responsible for food-handling or the cause of an outbreak should be treated. Breast-feeding does appear
to confer a benefit to children by transferring passive mucosal immunity and by antiparasitic effect of
the fatty acids found in the milk.59 Sexual transmission can be decreased by avoidance of oral-anal and
oral-genital sex if one of the partners is suspected to be infected. Currently, an effective immuno- or
chemophrophylactic strategy for giardiasis does not exist.
Conclusions
Giardiasis although not a major cause of mortality throughout the world does impact upon
various populations in significant ways. Certainly, immunosuppressed populations suffer rather difficult
courses of the disease and often require months of various antibiotic courses to clear the infection.
However, children in developing countries endure the greatest impact as the result of recurrent infection
and the subsequent malnutrition and growth retardation that ensues. The key features of diagnosis
include recognizing the populations at risk for acquiring the infection and a laboratory confirmed
presence of the parasite in a patient’s stools. Treatment sometimes requires more than one regimen and
rarely combinations of two antiparasitics. Improving personal hygiene and reducing exposure can make
the greatest impact on an individual level. From a development perspective, public health and water
treatment are critical components necessary for endemic areas to maintain a sustained effort against the
disease.
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