Giardia intestinalis (Giardiasis)

Pathogen and Pathogenesis

Giardia species are distributed globally and have been detected in nearly all classes of vertebrates, including domestic animals and wildlife. Six Giardia species are currently recognized but Giardia intestinalis (also known as G. duodenalis and G. lamblia ) is the only species found in humans. Molecular typing of G. intestinalis strains reveal that human strains belong to 2 of 8 specialized genetic groups referred to as assemblages (A to H), which are species specific. Assemblages A and B infect primarily humans and primates, although other mammalian hosts can be infected as well. Assemblages C through H do not appear to infect humans. These findings suggest that the potential for zoonotic transmission of G. intestinalis may not be as high as thought previously.

Giardia has two major stages in its life cycle: the trophozoite, the replicative stage within the host, and the cyst, the relatively inert infectious stage capable of prolonged survival in a variety of environmental situations. ^, After ingestion of G. intestinalis cysts, an incubation period of 3–25 days (median, 7−10 days) ensues before onset of symptoms. Excystation of Giardia cysts occurs in the proximal small intestine, releasing trophozoites that undergo repeated mitotic division and form environmentally resistant cysts in response to bile salts and other conditions. Trophozoites are 10–20 μm in length and 5–15 μm in width, with a convex dorsal surface, two nuclei, four symmetrically placed flagella originating at the anterior pole of the nucleus, and a ventrally located adhesive disk ( Fig. 265.1 ). The disk is composed of microtubules, microribbons, and cross-bridges, consisting of proteins referred to as giardins, and is responsible for attachment to the intestinal epithelium and subsequent colonization. To establish an infection, it is necessary for Giardia to attach to enterocytes. However, the parasite does not invade enterocytes or other tissues. Rotavirus coinfection may act synergistically to enhance pathogenicity by allowing for more successful attachment. ^, Trophozoites divide by longitudinal binary fission, with a doubling time of 6–12 hours in vitro.

As detached trophozoites pass through the intestinal tract, they encyst to form smooth, oval-shaped, thin-walled cysts that measure 8–12 μm in length and 8–10 μm in width and are passed in feces. In patients with watery diarrhea and rapid transit time, trophozoites may be found in stool specimens but, once passed, they are not stable. Studies in adults show that ingestion of as few as 10–100 fecally derived cysts is sufficient to initiate infection, and studies in children document person-to-person transmission in childcare centers. Giardia cysts are infectious immediately on being excreted in feces and remain viable for 3 months in water at 4°C. ^, Freezing decreases infectivity, whereas heating, drying, and seawater appear to eliminate the ability of the organism to infect.

The exact mechanisms used by Giardia to cause diarrhea have not been fully elucidated but are thought to be related to several factors, including (1) the number of organisms ingested, (2) the specific infecting strain, (3) nonantibody protective factors in the gastrointestinal tract, and (4) the immune response of the host. Morphologic damage to intestinal epithelial cells and their brush borders can occur after colonization, with the degree of mucosal abnormality correlating with severity of diarrhea. ^, Varying degrees of villous atrophy have been observed in intestinal biopsy specimens from people with giardiasis, with most having normal or relatively mild villus shortening. ^, This microvilli disorganization may explain some of the physiologic changes with giardiasis. Giardia also increases the rate of enterocyte apoptosis, degrades enterocyte cytoskeleton, disrupts tight junctions, and alters enterocyte ion secretion, which contribute to secretory diarrhea. ^, Disaccharidase deficiencies, usually of lactase, in the microvillous membrane have been documented in humans and in the mouse model of G. muris infection. ^, Protein, fat-soluble vitamins, and D-xylose malabsorption also are associated with Giardia infection in humans. ^, Decreased intestinal absorption of oral antimicrobial agents can occur and lead to treatment failure of other infections. Simultaneous colonization of the small intestine with Giardia and Enterobacteriaceae may contribute to malabsorption by deconjugation of bile salts in some patients. Production of enterotoxin has not been demonstrated. In vitro and animal studies suggest that Giardia colonization alters both the composition of microbiota throughout the host gut and the host’s mucosal immune response to its commensal repertoire. These changes can persist well after pathogen clearance and could explain the development of postinfectious syndromes.

Giardia undergoes frequent antigenic variations during infection of humans and animals during encystations and during culture in vitro. At any given time, only one of up to 270 unique and immunodominant variant-specific surface proteins (VSPs) is expressed, and VSP repertoire varies among Giardia strains and assemblages. Switching is regulated by RNA interference and is important in this parasite’s ability to cause chronic infections or reinfect the same host. ^, Besides VSPs, Giardia trophozoites produce several other virulence factors, including attachment and flagellar motility proteins; arginine deiminase and ornithine carbamoyl transferase, which may reduce nitric oxide production by host cells; secreted proteases that degrade host cytokines, mucins, and cytoskeletal proteins; and recently proposed tenascin-like proteins, which may play a role in damage of epithelial cells and tight junctions. ^, Giardia secretes several metabolic enzymes similar to those of the host that are crucial for salvaging lipids and nucleic acids that the parasite requires for replication. Additionally, Giardia- secreted proteins have recently been shown to alter the transcriptional profile of intestinal epithelial cells, modulating transcripts involved in cell metabolism, apoptosis, and immune function.

Protective immunity is successful in combating the infection in >85% of affected individuals and depends on both innate and adaptive responses. Innate mechanisms include mucus, peristalsis, nitric oxide production, mast cells, and dendritic cells. ^, Both B and T lymphocytes appear to play a role in protective immunity, as shown by the predisposition to giardiasis in patients with common variable immunodeficiency and in children with X-linked agammaglobulinemia. ^, More recently, it has been demonstrated that interleukin (IL)-6, which supports B-lymphocyte maturation, class switching to immunoglobulin A (IgA), and development of Th17-type CD4 ⁺ T cells, plays a major role in pathogen clearance. Although Giardia induces production of proinflammatory cytokines such as IL-6, this results in little to no intestinal inflammation. Antibody targets that have been shown to induce protective immunity or mitigate infect severity in animal models include variant surface proteins (VSPs), α-1-giardin, and cyst wall proteins. ^,

Anti- Giardia immunoglobulin G (IgG) can be found in >80% of patients during symptomatic infection. ^{, ,} IgG antibody tends to persist, thus limiting usefulness in distinguishing current from past infection in individual patients. An association between the presence of anti- Giardia IgG and protective immunity is not clear. Serum IgM antibodies increase early in infection and decrease rapidly after 2–3 weeks. ^, Serum IgA antibodies can be absent in symptomatic people; measurement has limited diagnostic value. Secretory IgA (sIgA) is presumed to be important in host protection. Human milk is protective against giardiasis in infants; this protection correlates with the amount of specific anti- Giardia sIgA . Human milk is lethal to Giardia trophozoites through action of free fatty acids, which are liberated from milk triglycerides by bile salt−stimulated lipase. Long-term CD4 T-lymphocyte−driven cellular immunity may occur variably after Giardia infection, with cross-reactivity between assemblages A and B.

Genetic manipulation of this pathogen has historically been difficult owing to its binucleate, tetraploid genome. Significant advances have been made in developing knock-out and knock-in methods, such as using cre/lox-, Morpholinos, and CRISPR/Cas-9 systems, which are allowing targeted studies of the genes required for key processes such as excystation and encystation.

Epidemiology

Giardia is the most commonly identified human intestinal parasite in the US and Canada. However, the burden of disease associated with Giardia in resource-poor countries is less clear. In a longitudinal, population-based evaluation of diarrhea in children aged <5 years living in Africa and Asia, Giardia was not associated with diarrhea at any site or in any age stratum. Giardia has been a nationally notifiable gastrointestinal tract illness in the US since 2002. Reporting of giardiasis occurs in 44 states (exclusions are Tennessee, Oklahoma, Kentucky, Mississippi, North Carolina, and Texas). ^, Age-specific prevalence rates are highest in children aged 1–9 years, followed by people aged 35–49 years. Giardiasis incidence rates have remained relatively stable in the US since 2002 at 7.2–8.7 cases per 100,000 population, with 19,927 cases reported in 2010 (7.6 cases per 100,000 population). ^, There are regional variations in incidence, with the Midwest and Northwest reporting the highest rates of giardiasis. Most cases are reported between May and September ( Fig. 265.2 ) and are associated with the summer recreational water season and with camping. From 2006–2010, males accounted for 56% of reported cases. Although reporting of giardiasis is required by 44 states, because diarrheal diseases often are underreported, these rates underestimate the disease burden attributable to Giardia.

Transmission of G. intestinalis is more common in certain groups, including (1) children and employees in childcare centers, (2) travelers to disease-endemic areas of the world, (3) close contacts of infected people, (4) people who swallow contaminated drinking water or recreational water, (5) people exposed to infected domestic or wild animals (e.g., dogs, cats, cattle, deer, and beaver), (6) people who take part in outdoor activities (e.g., camping or backpacking) who consume unfiltered or untreated water, and (7) men who have sex with men. The relative contribution of person-to-person, animal-to-person, and foodborne and waterborne transmission to sporadic human giardiasis is not well understood. Ingestion of surface water treated by faulty or inadequate purification systems has been a commonality of waterborne outbreaks. In addition, drinking untreated mountain stream water is a major risk for hikers. From 2009–2010, Giardia was identified as the causal agent of 1 (1%) of 81 reported recreational water-associated outbreaks and of 5 (11%) of 45 reported drinking water and other nonrecreational water source−associated outbreaks of gastroenteritis in the US. ^, Although Giardia is one of the most frequently identified parasites associated with waterborne disease outbreaks in the US, foodborne outbreaks of giardiasis are reported rarely. ^{, , , ,} During 1998–2010, only 18 (0.2%) of 9020 of foodborne outbreaks with a known etiology were associated with Giardia . ^, Because the infectious dose in adults is only 10–100 cysts, person-to-person spread also occurs, particularly in areas of low hygienic standards, crowding, and frequent fecal-oral contact. Person-to-person spread occurs frequently in childcare centers and in families of children with diarrhea. As with other enteropathogens, individual susceptibility, lack of toilet training, crowding, and fecal contamination of the environment all contribute to transmission of Giardia in childcare centers.

Clinical Manifestations

After excystation and colonization, a broad spectrum of clinical manifestations can occur, including (1) asymptomatic excretion of organisms, (2) acute infectious diarrhea, or (3) chronic diarrhea with failure to thrive or persistent gastrointestinal tract symptoms ( Table 265.1 ). Symptoms vary by age, with symptomatic infections more frequent in children than in adults. ^, Short-lasting, acute diarrheal disease with or without low-grade fever, nausea, and anorexia is usual; an intermittent or more protracted course occurs occasionally and is characterized by diarrhea, abdominal cramps, bloating, malaise, flatulence, nausea, anorexia, and weight loss. ^{, , ,} Stools can be profuse and watery initially and later become greasy, foul smelling, and buoyant. Blood, mucus, and fecal leukocytes are absent. Varying degrees of malabsorption can occur, and abnormal stool patterns can alternate with periods of constipation and normal bowel movements. Giardiasis should be considered in any patient with (1) a history of contact with an index case or a young child who attends a childcare center; (2) recent travel to an endemic area; or (3) persistent gastrointestinal tract symptoms, including diarrhea, malabsorption, crampy abdominal pain, abdominal bloating, unexplained gastrointestinal tract symptoms, failure to thrive, or weight loss. Extraintestinal involvement is unusual, but trophozoites occasionally migrate into bile or pancreatic ducts. Infrequently associated conditions with giardiasis include reactive arthritis, urticaria, and retinal changes.

The asymptomatic carrier rate of G. intestinalis is estimated to be 3%–7% in the US and as high as 20% in southern regions. In two prevalence studies of children aged <3 years enrolled in childcare centers, Giardia cysts have been found in 21% and 26% of asymptomatic children, respectively. ^, Longitudinal studies have shown that asymptomatic infections are well tolerated. ^, Thus neither testing of case contacts nor treatment of asymptomatically infected people is indicated routinely.

Humoral immunodeficiencies, including common variable hypogammaglobulinemia and X-linked agammaglobulinemia, predispose humans to chronic symptomatic giardiasis. ^, People with sIgA deficiency appear to have an increased risk for Giardia infection. ^, Children with severe T-lymphocyte deficiencies from thymic dysplasia (DiGeorge syndrome) and patients with AIDS with a low CD4 ⁺ T lymphocyte count do not have persistent or severe diarrheal episodes from Giardia. People with cystic fibrosis have a higher incidence of Giardia infection, probably as a result of local factors such as an increased amount of mucus, which may protect the organism in the duodenum.

The effects of early-life exposure to Giardia have more recently been investigated in numerous global settings. Giardia in children, especially in the first 6 months of life, has been associated with malnutrition and stunting in multiple resource-poor countries. ^, Giardia also has been associated with postinfectious syndromes, including irritable bowel syndrome and chronic fatigue.

Diagnosis

A definitive diagnosis is established by detection of Giardia trophozoites or cysts in stool specimens, duodenal fluid, or small bowel tissue by microscopic examination using staining methods such as trichrome or direct fluorescent antibody (DFA) assays; by detecting soluble stool antigens using enzyme immunoassays (EIA); or by using molecular techniques including polymerase chain reaction (PCR).

The traditional method used was identification of trophozoites and cysts on direct smears or concentrated specimens of stool. The advantages of microscopy are that minimal equipment and reagents are required. However, the sensitivity of direct microscopy techniques depends on the method used (direct or concentrated stools), the number of samples examined, and the experience of the laboratorian. As excretion of cysts can be intermittent, sensitivity of detection can be increased by collecting stool samples on 3 separate days. Additionally, antimicrobial agents, antacids, antidiarrheal compounds, and certain enema and laxative preparations can decrease passage of Giardia in the stool. Thus such compounds should be withheld for 48–72 hours before stool collection for detection of the parasite.

As direct microscopy is operator dependent, diagnostics have been improved to include EIA and, more recently, PCR. Antigens shared by trophozoites and cysts excreted in stool specimens include the 65-kd antigen used in commercial diagnostic assays. ^{, , ,} DFA testing is a sensitive and specific method and is considered the gold standard by many laboratories. Commercially available rapid immunochromatogenic cartridge assays and EIAs are alternative antigen-based tests that have variable sensitivity and that can be performed rapidly in laboratories without access to fluorescent microscopy. The limitations of immunoassays has led many laboratories in the US to adopt molecular diagnostics methods. The advantages of molecular diagnostics include increased sensitivity and specificity, ability to test for multiple targets in one assay, rapid turnaround time, and the possibility for quantitation of the parasite load. ^,

When giardiasis is suspected and stool specimens are negative, the string test, duodenal aspiration, or biopsy may be considered. In a fresh specimen, trophozoites usually can be visualized on direct wet mount. Duodenal biopsy is the optimal method for diagnosis. The biopsy sample can be used to make touch preparations for identifying Giardia in tissue sections and for histologic examination and to identify abnormalities not associated with Giardia . Small intestinal biopsy should be considered in patients with characteristic clinical symptoms, negative stool and duodenal fluid specimens, and one or more of the following: abnormal radiographic findings (e.g., edema and segmentation in the small intestine), abnormal lactose tolerance test, absent sIgA, hypogammaglobulinemia, or achlorhydria.

Specific antibodies to Giardia have been detected and quantified by immunodiffusion, hemagglutination, immunofluorescence, and EIA. Although not typically used in routine diagnosis, serology may serve as a useful indicator of levels of infection within a community. Radiographic studies of the small intestine can show nonspecific findings such as irregular thickening of mucosal folds. The peripheral blood leukocyte count usually is normal; giardiasis is not associated with eosinophilia.