Stephanie Boade Silas MD
DeVon Hale MD
Essentials of Diagnosis
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.
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.
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.
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.
BOX 84-1 Giardiasis
From 30% to 50% of individuals with acute giardiasis will progress to have chronic giardiasis.
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
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
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
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.
Abramowitz MD: Drugs for parasitic infections. Med Lett Drugs Ther 1995;37:971.
Backer HD: Effect of heat on the sterilization of artificially contaminated water. J Travel Med 1996;3:1.
Farthing MJG: Giardiasis. Gastroenterol Clin North Am 1996;25:493.
Hill DR: Giardia lamblia. In Mandell GL, Bennett JE, Dolin R: Principles and Practice of Infectious Diseases, 3rd ed. Wiley, 1995.
Marshall MM et al: Waterborne protozoan pathogens. Clin Microbiol Rev 1997;10:67.
Schwartz DA, Mixon JP, Owen RL: Giardiasis. In Connor DH, Chandler FW: Pathology of Infectious Diseases. Appleton & Lange, 1997.