Current Diagnosis & Treatment in Infectious Diseases

Section VII - Parasitic Infections

84. Giardia

Stephanie Boade Silas MD

DeVon Hale MD

Essentials of Diagnosis

  • Key symptoms include initially profuse and watery diarrhea progressing to foul-smelling and often greasy stools that float.
  • It is the most common pathogen in waterborne diarrheal illness.
  • Patients at highest risk include infants, young children, travelers, and immunocompromised patients.
  • In North America, the Rocky Mountains and mountainous regions of the northwest, northeast, and British Columbia are notorious Giardiareservoirs.
  • Giardiasis is diagnosed either by identification of cysts or trophozoites on wet mounts of fresh stool or duodenal specimens or by antigen detection using enzyme-linked immunosorbent assay or immunofluorescence techniques.

General Considerations

Giardia, a genus of primitive eukaryotes, is a flagellated enteric protozoan of the class Zoomastigophorea. Giardia lamblia, also known as Giardia intestinalis or Giardia duodenalis, is the species known to infect humans. Its name comes from Vilem Lambl, who first reported the organism in 1859. However, the first description of G lamblia came from Anton von Leeuwenhoek in 1681, while examining his own stool during an episode of diarrhea.

  • Epidemiology.G lamblia is a global enteric pathogen. It is the most prevalent enteric parasite in the United States and Canada, and populations at highest risk include infants, young children, travelers, and immunocompromised patients. Between 1965 and 1984, the Centers for Disease Control and Prevention documented 90 outbreaks, making G lamblia the most common pathogen in waterborne diarrheal illness. Giardiasis plays a role in malnutrition and growth retardation in the developing world.

In some areas of the developing world, the overall prevalence of Giardia infection is as high as 20–30%, whereas prevalence in industrialized nations is 2–5%. Age-specific prevalence increases from infancy through childhood before falling in adolescence, and children < 10 years old in developing nations have a 15–20% prevalence of G lamblia infection. The highest giardiasis prevalence is seen in the subtropics and tropics. G lamblia accounts for < 5% of traveler's diarrhea, with increased risk after travel to southeast and south Asia, tropical Africa, Mexico, South America, and areas of the former Soviet Union, particularly St. Petersburg. In North America, the Rocky Mountains and the mountainous regions of the Northwest, Northeast, and British Columbia are notorious G lamblia reservoirs.

G lamblia is ingested orally, and transmission has been associated with contaminated water, person-to-person spread, and, less often, food-borne transmission. Most outbreaks are related either to untreated water or to inadequately purified water. The G lamblia cyst is particularly well suited to survive in cold water and is relatively resistant to chlorine.

Person-to-person transmission is related to poor fecal–oral hygiene. Children in day care facilities have an infection prevalence of ≤ 50%, and sexually active male homosexuals, regardless of HIV status, have a prevalence of 20%. Increased numbers of infections are also found in individuals in custodial situations. Food-borne cases related to infected food handlers have been increasingly reported.

Studies have confirmed the presence of a vast animal reservoir. The first associations involved the beaver as a source of water contamination. Subsequently, DNA similarities have been found in Giardia isolates from humans and both domestic and wild animals, including beavers, cattle, cats, coyotes, dogs, gerbils, and sheep.

  • Microbiology.The G lamblia life cycle has two stages, the trophozoite and the cyst. The pear-shaped trophozoite lives freely in the small bowel lumen and is 9–21 µm long × 5–15 µm wide (Figure 84-1). It has a convex dorsal surface, a flat ventral surface with a sucking disk, and four pairs of posterior flagella. Absorption of nutrients occurs through the dorsal surface. The sucking disk is composed of an array of microtubules containing tubulin, microribbons containing the protein giardin, and other contractile proteins. Within the trophozoite is a posteriorly placed median body and two anterior nuclei each with a prominent karyosome, giving G lamblia its characteristic facelike appearance. The trophozoite divides by binary fission, doubling in 9–12 h in culture. An aerotolerant anaerobe lacking mitochondria, the trophozoite scavenges phospholipids, fatty acids, cholesterol, and pyrimidine. It metabolizes glucose to ethanol, acetate, and carbon dioxide. In vitro growth is optimized by conditions comparable with those found in the small intestine, including the presence of biliary lipids, intestinal mucus, epithelial cells, and low oxygen tension.

Figure 84-1. The G lamblia trophozoite is distinctive with its pear-shaped body (10–20 µm long and 7–10 µm wide). The location of the two nuclei with central karyosomes and the small curved parabasal body give it a facelike appearance.

Encystation occurs in the small bowel, possibly because of high concentrations of bile salts and elevated pH. The highly resistant cyst is passed out of the host into the environment where trophozoite division occurs within the cyst. The mature G lamblia cyst is an oval structure (8–12 µm long × 7–10 µm wide) with four nuclei and an acid phosphatase-positive periphery encased in a thin wall that is composed primarily of N-acetylgalactosamine (Figure 84-2). Hundreds to thousands of cysts may be excreted per gram of stool. After ingestion and exposure to gastric acid and pancreatic enzymes, excystation releases two trophozoites to resume the cycle.


Figure 84-2. The Giardia cyst is an oval structure (8–12 µm long × 7–10 µm wide) with four nuclei and an acid phosphatase-positive periphery encased in a thin wall of N-acetylgalactosamine.

  • Pathogenesis.G lamblia infection requires the oral ingestion of as few as 10 cysts. Excystation, promoted by gastric acid, releases trophozoites, which then multiply and colonize the upper small bowel. Trophozoites attach to the brush border enterocytes by two proposed mechanisms. First, the ventral disk may be involved in attachment by either contractile proteins or flagellum-mediated hydrodynamic forces. Second, a receptor-ligand interaction mediated by lectin has been suggested. The attachment process enables the trophozoite to avoid peristalsis.

The exact mechanism of injury causing disease is uncertain, but several observations have been made. First, the brush border is disrupted by microvilli injury and villous atrophy, which cause a disaccharidase deficiency. It has been postulated that this injury may be caused by a proteinase or mannose-binding lectin. Second, increased epithelial turnover in the crypts has led to altered absorption, which may be caused by immature enterocytes. T lymphocytes may contribute to this crypt hyperplasia, which is also observed in graft vs host disease. Third, decreased bile salt concentrations with consequent diminished pancreatic lipase activity and impaired solubilization of fat has been reported in giardiasis patients. The trophozoite, although unable to deconjugate bile salts, does have an uptake mechanism for bile salts that, in low concentrations, stimulate growth. Low-bile-salt concentrations in giardiasis patients may also result from deconjugation by simultaneous colonization with Enterobacteriaceae or yeasts. This increased colonization of anaerobic and aerobic bacteria in giardiasis has not been uniformly reported, however, with all confirmatory studies coming only from India. Fourth, G lamblia infection inhibits trypsin. Thus, disaccharidase deficiency, immature enterocytes, and both lipase and trypsin inhibition suggest that the diarrhea in giardiasis is primarily malabsorptive. Evidence supports neither mucosal invasion nor the presence of an enterotoxin in the pathogenesis of giardiasis.

The immune response to G lamblia infection is initiated by antigen uptake into macrophages in Peyer's patches. This action generates both an antibody and a cellular response. Although serum immunoglobulins M and G are lethal to G lamblia by the classical complement pathway, secretory immunoglobulin A (IgA) appears to be more important in clearing and preventing infection. Intraluminal IgA can prevent adherence, and chronic giardiasis is associated with the failure to make IgA. G lamblia has been found to make an IgA protease that is protective to trophozoites.

A cellular immune response is also generated and shown in mice to be necessary for both cytotoxicity and coordination of IgA secretion. As already mentioned, the T-cell response may also contribute to the pathogenesis of G lamblia because the mononuclear cell submucosal infiltrate is associated with flattened villi and crypt hypertrophy.

Protective immunity does not develop after a single infection, possibly because of genomic plasticity and significant antigenic diversity described in G lamblia isolates. However, increased prevalence in the young and decreased symptoms in long-term residents of endemic areas suggest at least partial immune protection. Infection in infants < 6 months old is rare, and human milk is protective because of the presence of antibodies and cytotoxicity from free fatty acids generated from milk triglycerides.

Although occurring in immunocompetent hosts, a predisposition to chronic giardiasis is reported in patients with X chromosome-linked agammaglobulinemia, lymphoid nodular hyperplasia, and common variable immunodeficiency with variable levels of hypogammaglobulinemia. Patients with earlier gastric surgery and decreased gastric acidity also have an increased susceptibility to infection. Of interest, patients with AIDS have no more severe illness than patients without AIDS, in contrast to the disparity seen in intracellular protozoal infections such as Cryptosporidium parvum.


After ingestion of G lamblia cysts, 5–15% of patients will have asymptomatic cyst passage, and 25–50% of patients will have diarrhea. From 35% to 70% of these patients will have no evidence of infection. The three manifestations of infection include asymptomatic cyst passage, self-limited diarrhea, and chronic diarrhea with associated malabsorption and weight loss. Factors related to each of these manifestations are unknown but are believed to be related to specific host factors, parasite load, and virulence variation among G lamblia isolates.


Clinical Findings

  • Signs and Symptoms.After ingestion of cysts, an incubation period of 3–20 days exists before symptom onset. At the time of presentation, patients have generally had symptoms for 7–10 days. The predominant symptom in acute giardiasis is diarrhea, occurring in 90% of patients, accompanied by generalized malaise (Box 84-1). Patients describe initially profuse and watery stools progressing to foul-smelling and often greasy stools that float. Flatulence, bloating, and abdominal cramps are frequent concerns as are belching, nausea, and anorexia. Weight loss and vomiting are less frequent, and fever is rare. Symptoms are usually self-limited to 2–4 weeks.
  • Laboratory Findings.Laboratory studies are notable for a normal peripheral leukocyte count without eosinophilia. Stool studies are negative for the presence of mucus, leukocytes, and blood.
  • Differential Diagnosis.C parvum, rotavirus, and toxigenic Escherichia coli may all have similar presentations.
  • Complications.Rare associations with urticaria and reactive arthritis have been reported in giardiasis. Biliary tract disease and pancreatitis have been reported, as have retinal arteritis and iridocyclitis. Gastric infections have been seen in G lamblia patients with achlorhydria.

BOX 84-1 Giardiasis




More Common

·  Diarrhea

·  Malaise

·  Flatulence

·  Abdominal cramps

·  Diarrhea

·  Alternating diarrhea with constipation

·  Abdominal pain worsening by eating

·  Malaise

Less Common

·  Nausea

·  Weight loss

·  Vomiting

·  Urticaria

·  Malabsorption

·  Macrocytic anemia

·  Lactose intolerance

·  Weight loss

·  Headache


From 30% to 50% of individuals with acute giardiasis will progress to have chronic giardiasis.

Clinical Findings

  • Signs and Symptoms.Profound malaise and lassitude are frequently reported. Patients experience diarrhea alternating with constipation, frequent stools, and abdominal pain associated with eating. Weight loss of 10–20% of body weight is common, with a 10-lb weight loss reported in 50% of patients. Headache has also been associated with the syndrome.
  • Laboratory Findings.Laboratory studies are significant for biochemical evidence of malabsorption. D-Xylose, fat, protein, vitamin A, and vitamin B12malabsorption have all been reported. Disaccharidase deficiency, particularly lactase, occurs in 20–40% of patients and may persist for several weeks beyond infection. Macrocytic anemia from folate deficiency and hypoalbuminemia has been reported.
  • Differential Diagnosis.Similar presentations can be seen in infections caused by coccidians such as C parvum, I belli, and Cyclospora spp., as well as infections caused by Strongyloides spp. and Entamoeba histolytica. Also, diagnoses of inflammatory bowel disease and irritable bowel syndrome should be considered.
  • Complications.Chronic giardiasis may play an important role in growth impairment and nutritional deficiency in the developing world.


The key to diagnosis of G lamblia infection is the identification of trophozoites or cysts, both of which can be seen in stools by standard ova and parasite exam (Table 84-1). Trophozoites have a short survival time outside the small bowel when not contained within cysts and are more likely to be seen in fresh wet mounts of liquid stool. Semiformed stool may be preserved in formalin or polyvinyl alcohol. Staining with trichrome or iron hematoxylin reveals cysts. Formalin or zinc flotation concentration techniques may increase the yield of diagnosis. Generally, one stool exam has a 50–70% diagnostic yield, which improves to 85–90% after 3 stools collected over 2–3 days because of cyclic shedding. Purged samples have no effect on diagnostic yield.

Enzyme immunoassay and direct fluorescent-antibody assay kits are commercially available for testing for G lamblia infection. Both methods have reported sensitivities of 87–100% and specificities of 99–100% when compared with microscopic stool examination. Advantages of these techniques include a decrease in both examination time and required technician training. Both direct fluorescent-antibody assay and enzyme immunoassay are particularly valuable when the sole diagnosis or exclusion of G lamblia is needed, as may occur during an epidemic or for screening purposes. Although cost of antigen detection techniques is similar to ova and parasite microscopic exams, microscopy allows for diagnosis of other possible pathogens. One commercially available direct fluorescent antibody assay kit does detect both G lamblia and C parvum, however.

Table 84-1. Laboratory diagnosis of giardiasis.1

·  The trophozoite is 10 µm × 15 µm with a convex dorsal surface, flat ventral surface, and 2 facelike nuclei

·  The cyst is 10 µm × 10 µm, thin-walled, and with an eccentric nucleus

·  Motile trophozoites are seen on microscopy of fresh stool specimens or on specimens obtained by duodenal aspirate, biopsy, or the string test

·  Cysts and trophozoites are observed on wet mounts of fresh stools with or without prior formalin concentration. Cysts can also be found in specimens preserved in formalin or PVA, using trichrome or iron hematoxylin staining

·  Antigen detection by ELISA is highly sensitive and specific and comparable in cost to standard stool O & P. It is best used when the sole diagnosis or exclusion of giardiasis is needed

·  Antigen detection by IFA has the same sensitivity and specificity as standard O & P, but is advantageous in small labs with less-trained technicians

·  DNA probes and PCR techniques are available but not yet useful clinically

1O & P, Ova and parasite examination; PVA, polyvinyl alcohol; IFA, immunofluorescent antibody assay

Other, more invasive techniques are rarely used but may contribute to diagnosis. The string test involves swallowing a capsule attached to a nylon string. The capsule sits in the jejunum for 4–6 h while the patient is fasting. The string is subsequently removed and examined for trophozoites by microscopy. A duodenal aspirate can be similarly examined. Also, a duodenal biopsy or endoscopic brushing can be examined for trophozoites, by using Giemsa stain.

Upper gastrointestinal aspirates can be cultured, but this test is generally not available clinically. Serology, too, has little clinical utility but may be helpful epidemiologically. Serum immunoglobulin M or IgA titers are indicative of recent infection as compared with IgG titers. Polymerase chain reaction and gene probe studies are still in experimental stages, with their most practical limitation being extraction of DNA from the stool sample.


Historically, quinacrine hydrochloride has been the drug of choice for the treatment of giardiasis, with 90% efficacy. However, this drug is no longer produced in the United States. Despite having never received a Food and Drug Administration indication for giardiasis, metronidazole is the first-line treatment, with 80–95% efficacy after a 7-day course (Box 84-2). Its efficacy is considered to be related to inhibition of attachment. Because of the disulfiram effect of metronidazole, patients should be warned that concurrent use of ethanol could cause flushing, tachycardia, and nausma. Tinidazole is also considered a first-line agent with 90% efficacy, but it is also not available in the United States.

Second-line agents include furazolidone and paromomycin sulfate. Furazolidone, a nitrosourea with 80% efficacy, may be particularly useful in children because it comes in a liquid form. Paramomycin sulfate is an aminoglycoside with 60–70% efficacy. Because of its poor oral absorption, it may be beneficial for use in pregnant patients. Ideally, treatment for giardiasis in pregnancy should be delayed until after delivery. Metronidazole may be safe after the first trimester, however. Variable sensitivities of isolates to drug regimens occur, and resistance to metronidazole and furazolidone has been reported.

Other drugs with reported benefit against G lamblia infection include some antidepressants, fusidate sodium, D- and DL-propranolol, mefloquine, doxycycline, and rifampin. Albendazole, an anthelmintic benzimidazole derivative, may prove to be effective in treating giardiasis and may be especially useful in developing countries for dual coverage.

BOX 84-2 Treatment of Giardiasis


First Choice

·  Metronidazole, 250 mg three times daily × 5–7 d

·  Tinidazole1 2 g × 1

·  Quinacrine (Mepacrine)1 100 mg three times daily × 5 d


Second Choice

·  Furazolidone, 100 mg four times daily × 7–10 d

·  Paromomycin, 25–30 mg/kg/d divided three times daily × 5–10 d


·  Metronidazole, 5 mg/kg three times daily × 7 d

·  Furazolidone liquid, 6 mg/kg divided 4×/day for 10 d


·  Delay until after delivery if possible

·  Metronidazole may be used after first trimester—see above

·  Paromomycin, see above for dosing

1Not available in the United States.


The natural history of untreated giardiasis is unknown. Chronic persistent diarrhea may develop in a small number of patients, some of whom, particularly children, may develop malnutrition and growth impairment. The prognosis of treated giardiasis, however, is excellent.

Prevention & Control

Because of the worldwide presence of Giardia spp., vast human and animal reservoirs, and environmental resilience, total elimination of giardiasis is not expected. Instead, prevention depends on focusing on the primary sources of infection, including water contamination and person-to-person contacts (Box 84-3). Chlorine kills cysts in warm water, and public water supplies should undergo chlorination, flocculation, sedimentation, and filtration. DNA techniques may be valuable in screening filtered water for cysts.

BOX 84-3 Prevention & Control of Giardiasis

Prophylactic Measures

·  Proper treatment of public water supplies including chlorination, flocculation, sedimentation, and filtration

·  In the wilderness, bring water to a rolling boil

·  Halogenation with chlorine or iodine tablets may be effective with warm water

·  Filters with < 1–µm pores can be used as well

Isolation Precautions

·  No specific need for isolation

·  Good hygiene practices

In the wilderness or in the developing world, water should be boiled before consumption. Inactivation is immediate at 100°C, and water need only be brought to a rolling boil. At altitudes of ≥ 10,000 ft, where the boiling point is 90°C, water can still be safely disinfected by simply bringing it to a boil. The margin of safety is further ensured by the time taken for the water to heat to this level and subsequently cool because keeping water at 70°C for 10 min also results in 100% inactivation. Halogenation with iodine or chlorine tablets has proven useful but may not be effective in cold water. The questionable efficacy of halogenation is best exemplified by reported cases of giardiasis associated with chlorinated swimming pools. Water filters with pores < 1–2 µm may also be effective in preventing giardiasis.

Person-to-person spread of giardiasis could be lessened by improved hygiene, especially in those at high risk. However, evidence is not yet available to support the treatment of asymptomatic patients. Further, no vaccines for giardiasis are available. Vaccine development is limited because the initial infection itself does not confer protective immunity to the patient.



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