Public Health Importance of Giardiasis

Giardiasis exerts a significant public health impact because of the high prevalence and disease burden of the infection, its propensity in causing major outbreaks and emergency responses, and its effects on growth and cognitive functions of infected children. Giardiasis is also a common disease in livestock and companion animals; thus, it is of veterinary health importance.

Giardia Species

Species of the genusGiardia infect numerous hosts, ranging from mammals to amphibians and birds. The taxonomy ofGiardia was reviewed previously (42,180,260). Currently, sixGiardia species are accepted by most researchers. Among them,G. agilis,G. ardeae,G. muris,G. microti, andG. psittaci infect various animals, whereasG. duodenalis infects humans and many mammals (Table5). A unique species is probably also present in reptiles, as aG. duodenalis-like parasite was found in lizards. This parasite, however, mostly lacked median bodies and had binucleated cysts, and it was consideredG. varani (279). Although it is expected that fish have uniqueGiardia spp., a recent study of cultured and wild freshwater and marine water fish in Australia showed the occurrence of zoonotic (assemblages A and B) and artiodactyl-specific (assemblage E) genotypes ofG. duodenalis andG. microti (300). It was not clear whether the fish were infected with theseGiardia spp. or merely served as mechanical vectors for the dissemination of waterborneGiardia cysts. Thus,Giardia species differ significantly in host range, withG. duodenalis having the broadest host range and greatest public health significance.

The species namesGiardia duodenalis,Giardia intestinalis, andGiardia lamblia are used interchangeably in current literature referring to the same organism (296). BothG. duodenalis andG. intestinalis are used in equal frequency in referring to theGiardia species infecting most mammals, including humans, their companion animals, and livestock, and opinions differ regarding the legality of the nameG. intestinalis, largely because of different interpretations of the complex rules of the International Code of Zoological Nomenclature (180,259). For purposes of consistency,G. duodenalis is used in this review. In the medical field,G. lamblia is still commonly used to discussGiardia species infecting humans (180,259). Because numerous recent biological and genetic analyses have shown that the sameGiardia species present in humans are also found in a range of other mammalian species (296), there is no taxonomic basis for the use of the nameG. lamblia, which was preempted by bothG. duodenalis andG. intestinalis.

AlthoughG. duodenalis is the only species found in humans and many other mammals, including pets and livestock (261) (Table5), it is now considered a multispecies complex. Historically, allozyme analyses placed all isolates from humans into two genetic assemblages (assemblages A and B) encompassing at least four genetic clusters (groups I to IV) (reviewed in reference181). Phylogenetic analyses of a large set of nucleotide sequence data from the small-subunit (SSU) rRNA gene and several housekeeping genes coding for glutamate dehydrogenase (gdh), β-giardin (bg), elongation factor 1 alpha (ef1α), and triosephosphate isomerase (tpi) confirmed the genetic uniqueness of assemblages A and B. Additional lineages ofG. duodenalis from animals were identified: assemblages C and D from dogs, assemblage E from artiodactyls, assemblage F from cats, and assemblage G from rodents (42,261) (Fig.1 and see Fig. S1 and S2 in the supplemental material).

In view of the observed host specificity and genetic characteristics of theG. duodenalis assemblages, attempts have been made recently to resurrect some of the previously used species names based on host occurrence (180). Thus, it has been proposed to adopt the names ofG. duodenalis for assemblage A,G. enterica for assemblage B,G. canis for assemblages C and D,G. bovis for assemblage E,G. cati for assemblage F, andG. simondi for assemblage G (180,260,261). This suggestion seemingly makes logical sense and could lead to reduced confusion inGiardia taxonomy and a better understanding of giardiasis epidemiology. Because of the uncertainty related to the identity of the parasites at the initial description of these species, in compliance with the International Code of Zoological Nomenclature, redescriptions of theseGiardia species that incorporate modern biological and genetic data may be needed before the proposed newGiardia species nomenclature can be accepted by the research community. This probably should include morphological descriptions and measurements of the cysts and trophozoites, data on natural host spectrum and, if any, infectivity in animal models, other biological characteristics (prepatent period, patent period, and infection site in the gastrointestinal tract, etc.), and a summary of the nucleotide sequence uniqueness of the assemblage. Such data are already available for most of the proposedGiardia species, although they are scattered in the literature. The only difficulty is probably in the naming of the species that infects canines, as lumping assemblages C and D into oneGiardia species may be problematic; the sequence divergence between the two assemblages is of an order of magnitude similar to that separating the other assemblages, and at some genetic loci, such astpi, they do not form a monophyletic group (see Fig. S2 in the supplemental material).

Giardia duodenalis Assemblages

Among assemblages ofG. duodenalis (Fig.1 and see Fig. S1 and S2 in the supplemental material), assemblages A and B have the broadest host specificity, having been found to infect humans and various other mammals (42,261,296). Assemblage A is frequently found in livestock (cattle, water buffalo, sheep, goats, alpacas, and pigs) and companion animals (dogs, cats, and horses) (Tables3 and4). In comparison, assemblage B is less frequently reported for livestock and companion animals, with only a few reports of infection of cattle, sheep, horses, dogs, cats, and rabbits (Tables3 and4). Assemblage A and, to a lesser extent, assemblage B are commonly found in wild animals, with the exception of beavers and muskrats, which seemingly have a high occurrence of assemblage B (Table6). Both assemblages A and B are commonly reported to infect humans (Tables1 and2). Because they are found in humans and numerous species of mammals, both assemblages A and B are considered to have broad host specificity and can be transmitted zoonotically (41,261,296). The host range of assemblages A and B is shown in Table5, and their distributions in humans and various animals are shown in Tables1 to4 and6.

There is substructuring within assemblage A, which consists of mostly two subgroups, subgroups or subassemblages AI and AII. The separation of subgroups AI and AII was initially made by allozyme analysis and is supported by phylogenetic analyses of assemblage A sequences at thegdh locus (Fig.2). The existence of numerous subtypes related to subgroups AI and AII, however, made the separation of subgroups AI and AII less obvious at some other loci, such astpi andbg (see Fig. S3 and S4 in the supplemental material). Recently, a third subgroup within assemblage A, subgroup AIII, was identified and appears to be associated mostly with wild hoofed animals (37,219,282). It has significant sequence differences from subgroups AI and AII at all loci examined thus far (Fig.2 and see Fig. S3 and S4 in the supplemental material). More recently, based on an analysis of thetpi,gdh, andbg loci, subgroup AIII was found in three cats, four cattle, and 45 wildlife but has not been found in dogs, goats, sheep, pigs, and humans thus far (247). In comparisons with subgroup AIII, subgroups AI and AII always form one cluster with high bootstrap values in phylogenetic analyses of nucleotide sequences of all loci (Fig.2 and see Fig. S3 and S4 in the supplemental material). In contrast, there is no clear subgrouping within assemblage B (293) (Fig. S5 to S7).

Assemblages C, D, E, F, and G have strong host specificities and narrow host ranges. Assemblages C and D have been found mostly in dogs and other canines (foxes and coyotes) and canine-related animals (seals) (Table4). Assemblage E has been found largely in cloven-hoofed domestic mammals (cattle, water buffaloes, sheep, goats, and pigs) (Table3). Assemblages F and G have been found mostly in cats and rodents, respectively (Tables4 and6). However, there are occasional exceptions to the host specificity. Thus, assemblages C and D were reported in a few cats (196,214) and humans (264), assemblage D was also reported in two pigs (149,247), assemblage E was reported in cats (154,214,247) and humans (83), and assemblage F was reported in one pig (20) and seven humans (87). It is commonly believed that humans are infected only with assemblages A and B (Tables1 and2), and the recent identification of unusualG. duodenalis genotypes, such as assemblages C, D, E, and F in humans (83,87,264), requires thorough genetic characterizations of the parasites.

In addition to the above-described accepted assemblages, several novel genotypes have been reported.Giardia cysts isolated from U.S. seals were reported to belong to a new genotype based on a sequence analysis of thegdh gene (86), and the same genotype was found in gray seals, harbor seals, and a gull in a later study and was recently named assemblage H (151) (Fig.1). However, the existence of this new assemblage was supported by the sequence analysis of onlygdh but nottpi (151), and thegdh sequence of the new genotype is placed outside theG. duodenalis cluster in phylogenetic analyses (Fig.1). OneGiardia isolate from a southern brown bandicoot (Isoodon obesulus, commonly known as quenda) in Western Australia was reported to be a novel genotype based on a phylogenetic analysis of the SSU rRNA (GenBank accession numberAY309064) andef1α (accession numberAY309065) gene sequences. Those authors believed that this isolate constituted a distinct species, because phylogenetic analysis shows it to be distinct from other recognized species such asG. microti,G. psittaci,G. ardeae, andG. muris (7). This new genotype was identified in another six quenda specimens in a recent study (262). Two novel genotypes ofG. duodenalis were found in house mice on Boullanger Island, Australia, based on a sequence analysis of a 130-bp region of the SSU rRNA gene (183), although the sequence data were not presented. All these new genotypes need to be characterized extensively, at least at the common loci used for genotypingG. duodenalis.

TABLE 7.

Genotyping Tools

The SSU rRNA gene is a commonly used marker for the species and assemblage differentiation ofGiardia, and different regions have been targeted for different applications. The variable 5′ and 3′ ends of the SSU rRNA gene locus can be used for identifying the relatively closely related assemblages, whereas the more conserved regions would provide sufficient information only for the differentiation ofGiardia species. Thus, when the SSU rRNA gene locus is used for assemblage differentiation, primer selection should be careful because the products by some primer sets are too small to differentiate allG. duodenalis assemblages. For example, some primers (118,283,291) amplify only the first 1/5 or the last 1/10 of the gene (∼1,400 bp in total). Using one such set of primers (118), cat isolates were genotyped as assemblage A in one study (30) and assemblage F in another study (79) due to the fact that assemblage F is mostly identical to assemblage A in the fragment under analysis (293). The identification of unusual assemblages (assemblages C, D, E, and F) in humans involved mostly the use of SSU rRNA gene-based tools. One reason for targeting small fragments of the SSU rRNA gene is the difficulty in the PCR amplification of the locus; the use of dimethyl sulfoxide or special PCR buffers designed for GC-rich targets is frequently needed for efficient PCR amplification of the target.

Thetpi,gdh, andbg loci are also common genotyping markers. Because most primers amplify 40% to 60% of thegdh gene and 60% of thetpi andbg genes, these loci have been used for both genotyping and subtyping. However, inconsistent genotyping results are sometimes generated among different loci. In a study of human and dog specimens in a tea-growing community in India, SSU rRNA gene sequencing suggested that assemblages C and D were the dominant genotypes in humans but were absent in dogs. The finding of assemblages C and D in humans was not supported by a sequence analysis of the specimens at theef1α andtpi loci (265).

The occurrence of infections with mixed assemblages in humans and animals is common. For 908 isolates from human and various animals characterized at two or more loci, mixed infection ofGiardia assemblages was found for 121 (13.3%) isolates. Among them, mixed infection was found for 46 of 392 (12%) human specimens and 45 of 134 (34%) dog specimens analyzed. Mixed infection was also found in cat, cattle, goat, sheep, pig, and wildlife specimens. Mixed infection involving assemblages A to E is especially common. In some dogs, the concurrent occurrence of three assemblages (assemblages A, B, and C or B, C, and D) was seen (247). Therefore, assemblage-specific PCR assays are now used to assess more accurately the occurrence of mixed infections. The usage of such a system targeting thetpi gene in analyses of specimens from dairy and beef calves led to the identification of mixed assemblage A and E infections in 31% of positive calves and a more frequent occurrence of assemblage A in dairy calves (59% of all positive samples) (91). Primers specific for assemblage A or B based on thetpi gene were reported and should be useful for an accurate assessment of the occurrence of mixed infections in clinical specimens (14,31,223). Such data will improve our understanding of the clinical spectrum of giardiasis, intensity of giardiasis endemicity in study areas, tracking of infection sources, and identification and differentiations of outbreaks of giardiasis.

Ideally, genotyping should be performed at the single-cyst level, as this will allow the differentiation between mixed infections and the occurrence of recombinants, because there is a possibility of genetic exchanges between isolates of assemblage A (57) or even between isolates of assemblages A and B (254). The use of real-time PCR appears to be promising in reaching this technically demanding level of sensitivity and specificity. A real-time PCR assay targeting thebg gene was developed. It could differentiate assemblages A and B ofG. duodenalis with a sensitivity of detecting an equivalent of one cyst ofG. duodenalis (105). More recently, three real-time PCR assays targeting thetpi,gdh, and open reading frame C4 sequences were developed to differentiate assemblages A and B. The assays had high specificity and detected DNA from a single trophozoite or cyst (9). When these assays and a TaqMan assay targeting thebg gene were used to analyze 30 human stool samples, a simultaneous detection of both assemblages was seen for 37 to 83% of specimens, depending on the genes targeted. PCR analyses of DNA extracted from single cysts purified by immunomagnetic separation from the same specimens showed that the simultaneous amplification of DNA of both assemblages was attributable to mixed infections. However, potential genetic exchanges between assemblages A and B were suggested by the detection of both assemblages in individual cysts (9). There is also a single-tube multiplex real-time PCR assay based on the SSU rRNA gene for the differentiation of assemblages A and B in fecal specimens but with a low sensitivity of an equivalent of 20 trophozoites per PCR (187).

Subtyping Tools

Assemblages A and B are zoonotic; therefore, genetic differences within them may provide information on the relationship of subtypes and hosts, the zoonotic potential of each subtype, contamination source tracking, outbreak investigation, as well as characterizations of transmission dynamics. Based on sequence analysis of thetpi,gdh, andbg genes, multiple subtypes of assemblage A were noticed. At thegdh locus, these subtypes can be grouped easily into three subassemblages, subgroups AI, AII, and AIII, and there are multiple subgroup- or subassemblage-specific polymorphisms (Fig.2). At thetpi andbg loci, subassemblage AIII also has many distinct nucleotide substitutions, but there is no formation of robust subgroup AI and AII clusters in phylogenetic analyses of the sequences (see Fig. S3 and S4 in the supplemental material). The initially identified two subassemblage-specific substitutions in thetpi gene apply only to the common A1 and A2 subtypes, which belong to subassemblages I and II, respectively. Thus, the A5 subtype (belonging to the AI subassemblage) (37) has atpi sequence more similar (two nucleotide substitutions versus four) to A2 (an AII subtype) than to A1 (an AI subtype).

To systematically characterize intra-assemblage genetic diversity and provide more-robust typing ofG. duodenalis, Caccio et al. proposed a subtype nomenclature system (Table8) for assemblage A based on multilocus genotype (MLG) analysis of thebg,gdh, andtpi genes (37). This would reduce confusions in subtype terminology and improve understandings of the host segregation of subtypes. Nevertheless, the subgroup AI, AII, and AIII classification system proposed was determined largely by the extent of nucleotide substitutions at thegdh locus, although concatenated sequences of thegdh,tpi, andbg genes were used in the phylogenetic assignment of the subassemblages. The designation of rare subtypes within the AI and AII subassemblages may need support from genetic characterizations of other loci.

The BIII and BIV subassemblages originally described by allozyme electrophoretic studies are not supported by DNA sequence analyses. The sequences from known subgroup BIII or BIV isolates are insufficient to assess the existence of subgroup BIII/BIV-specific sequence polymorphisms, and subtyping analyses of field isolates produced inconsistent subassemblages among different loci, most of which were not supported by bootstrap analyses (293) (see Fig. S5 to S7 in the supplemental material). Assemblage B has many more subtypes than does assemblage A.

Many subtypes are also seen in assemblage E. Although assemblage E also infects a range of hoofed livestock, there are no host-specific subgroups in nucleotide sequences of various genetic loci. There are insufficient data to assess the substructure of other assemblages (assemblages C, D, F, and G). Host-specific sequence polymorphism was not seen at thegdh locus when assemblage G isolates from rats and mice were examined (293).

Multilocus Genotyping Tools

The low resolution of current genotyping tools has limited their potential for the characterization of the transmission of human giardiasis (42). Thus far, the vast majority of studies have relied on the sequence characterization of human- and animal-derived cysts at one or two genetic loci (41). In early studies, there was a bias toward the use of the SSU rRNA gene target because of its multicopy nature and high degree of sequence conservation. This has led to some problematic interpretations of the data acquired. In addition, recent data have shown that inconsistent genotyping results may be obtained when different genetic loci are targeted. Thus, MLG analysis is increasingly used for the characterizations ofG. duodenalis from humans and animals (37).

Several recent studies using the MLG approach showed that some isolates of both human and animal origins could not be unequivocally assigned at the assemblage level (37,265). In one study where a sequence analysis of four genes (the SSU rRNA,bg,gdh, andtpi genes) of human and animal isolates was performed, the data generated showed that although congruent genotyping results were obtained at the four genetic loci for isolates of assemblage A, it was difficult to assign some assemblage B isolates from humans and nonhuman primates to an assemblage, because the result obtained at one locus was not consistent with data obtained at the other three loci. In addition, there were variable levels of intraisolate sequence heterogeneity in assemblage B isolates, which prevented the unambiguous identification of MLGs (37). Similar results were reported by other studies of human and animal isolates using some of the same targets (93,265). This inconsistency in genotyping results was more frequently reported for isolates from dogs, where, depending on genetic loci, isolates were typed as either host-adapted assemblages C and D or zoonotic assemblage B (265). This has important repercussions, as different conclusions may be reached. These findings also raise concerns about the interpretation of genotyping data based on single markers. It was reported that 15% of isolates genotyped (including 2,400Giardia sequences from the GenBank database) in the ZOOnotic Protozoa NETwork (ZOOPNET) had inconsistent typing between two markers among the SSU rRNA,bg,gdh, andtpi genes, and this inconsistency was observed predominantly for specimens from humans and dogs (247).

One possible factor contributing to the inconsistent genotyping among markers is the high level of occurrence of mixed infections as a result of the high prevalence of giardiasis in humans and animals. This was supported by the frequent detection of concurrent infections of both assemblages A and B in humans and the high level of occurrence of assemblage A infection in animals when assemblage-specific PCR was used for genotyping. The grouping of the parasites into individual MLGs was further complicated by the fact that many isolates exhibited double peaks at specific positions in electropherograms of otherwise clean sequences. In one recent study, double peaks in electropherograms were seen in assemblage B, C, D, and E isolates but never in assemblage A, F, and G isolates, suggesting that MLG analysis ofG. duodenalis might be more useful for the typing of assemblage A (154).

Epidemiologic Evidence for Zoonotic Transmissions

Few epidemiological studies have assessed the importance of zoonotic transmission in the occurrence of human giardiasis. In New Zealand, case-control studies of giardiasis did not identify contact with pets as a risk factor for children or adults, although contact with farm animals was associated with an increased risk of infection for adults (119,120). In agreement with this finding, the infection rate of human giardiasis in New Zealand was 23% higher in rural areas than in urban areas (241). In the United Kingdom, farm visitation was frequent among case patients, but specified exposures to dogs, cats, horses, cattle, and sheep was not a significant risk factor (248). One case-control study in eastern England found an association of giardiasis with exposure to farm animals and pets, particularly pigs, dogs, and cats (296). Other studies in the United States, Canada, and the United Kingdom did not show such an association (296).

Beavers are probably the animals most commonly implicated in the zoonotic transmission of giardiasis. This originated largely from investigations of waterborne outbreaks and reports of frequent giardiasis in hikers and campers, who recalled drinking water from streams or lakes. The first observation was made in 1976 with an outbreak of giardiasis in Camas, WA, when 128 townspeople had laboratory-confirmed giardiasis. A questionnaire survey revealed that 3.8% of residents had clinical giardiasis, whereas none of 318 residents in a control town were ill.Giardia cysts were recovered from water entering the city water treatment system and two storage reservoirs containing chlorinated and filtered stream water. Trapping in the watershed yielded three beavers infected withGiardia that was infective for beagle pups. Similar observations were made by subsequent investigations of waterborne giardiasis outbreaks in New Hampshire and Nevada, based largely on the finding ofGiardia cysts in beavers caught in source watersheds (reviewed in reference296). These reports did not provide concrete evidence of an involvement of beavers in human infections and reported merely circumstantial evidence based on beavers found at contaminated sites. The possible role of humans and other animals as a source of infection for beavers was not addressed in these reports. In a review of data from case-control studies, the association between the drinking of wilderness water and acquisition of giardiasis was deemed minimal (292).

The consumption of raw surface water clearly represents a significant risk for giardiasis (119). However, the contamination of such water supplies may result from humans, farm animals, and wildlife (112). A 2-year study assessed the significance of each source in the environmental contamination of surface water withGiardia cysts (112). Sewage effluent was shown to have the highest prevalence ofGiardia, although the concentration of cysts was minimal compared with that detected in cattle feces. Although the overall prevalence ofGiardia was lower in wildlife, giardiasis was prevalent in aquatic mammals such as beavers and muskrats. Nevertheless, an interpretation of these results in the context of the source of humanGiardia infections can be made only in conjunction with data on the distribution ofGiardia species and genotypes in these potential contamination sources (257).

Genetic Evidence for Zoonotic Transmissions

Numerous isolates ofGiardia collected from different host species in various geographical locations have been genotyped, and the occurrence of the same species/genotype in humans and other animals has been well demonstrated (181). Such data are indicative of zoonotic potential, and most experts would agree thatG. duodenalis strains are potentially zoonotic, especially assemblages A and B ofG. duodenalis. The use of genotyping and subtyping tools, especially the more recent MLG tools, in well-designed epidemiological studies, however, is needed before we can fully assess the human disease burden caused by zoonotic giardiasis (37).

The occurrence ofGiardia in aquatic wildlife in investigations of waterborne outbreaks of giardiasis, particularly of isolates that are morphologically identical toG. duodenalis, has been the single most important evidence implicatingGiardia as a zoonotic agent. Indeed, recent studies have shown the common occurrence ofG. duodenalis assemblage B in beavers and muskrats (78,249). However, there is little evidence to implicate these animals as the original contaminating source in waterborne outbreaks. It has been suggested that these animals are more likely to become infected from water contaminated with fecal material of human or even domestic animal origin, thus serving to amplify the numbers of the original contaminating isolate (257).

In view of the frequency of human-pathogenicGiardia genotypes, the public health risk of giardiasis from domestic animals appears to be small. This is the case at least for cattle in North America, Australia, and Europe, whereG. duodenalis assemblage E predominates (149,231,249,271,272). The human-pathogenic assemblages (assemblages A and, occasionally, B) in cattle may have to compete with the more common assemblage E. Similar conclusions can probably be drawn for other livestock such as sheep, goats, and pigs, where assemblage E is also the predominant genotype (Table3), or domestic pets such as dogs and cats, which have other host-adaptedG. duodenalis genotypes (assemblages C and D in dogs and assemblage F in cats) as the dominant parasites (Table4).

Most interests in the zoonotic transmission ofG. duodenalis focus on assemblage A, as it is the most common non-host-specific assemblage in animals (Tables3,4, and6). Subtyping data accumulated so far do not support a widespread occurrence of zoonotic transmission. The two most common subassemblages of assemblage A, subgroups AI and AII, appear to differ in host preference (247,296). Humans are more commonly infected with subgroup AII, although subgroup AI has also been seen in some areas or studies (Table9 ). In contrast, animals are commonly infected with subgroup AI, although subgroup AII is sometimes seen (Table10). More systematic characterizations of the less common subtypes within subgroups AI and AII, however, are needed to improve our understanding of the host specificity of subgroups AI and AII. The other subgroup, subgroup AIII, has thus far been found only in animals, mostly in wildlife (Table10). There are no clear geographic differences in the distribution of the two common assemblage A subgroups in humans; although subgroup AI was detected in humans at a high frequency in two studies in South America, two other studies conducted in the same countries found only subgroup AII in humans (Table9). Increasing the typing resolution and detailed epidemiological data are needed to determine what portion of human subgroup AI infections is the result of zoonotic transmission.

MLG tools have recently been used in assessments of the zoonotic transmission of giardiasis because of their highly discriminatory power. When 978 human and 1,440 animal isolates were characterized using sequence analysis of four loci (the SSU rRNA,bg,tpi, andgdh genes), the zoonotic potential of both assemblages A and B was apparent at the levels of assemblages and subassemblages at each locus. However, when isolates were defined using an MLG scheme, only two MLG types of assemblage A and none of assemblage B appeared to have occurred in both humans and animals (247). In contrast, three assemblage A MLG types that were previously detected in humans were present among 114Giardia isolates from various animals (pets, livestock, wildlife, and captive nonhuman primates) in Sweden when they were analyzed by an MLG technique targeting three genes (bg,gdh, andtpi) (154).

The finding of the same genotypes or MLG types in humans and animals is not by itself conclusive evidence that zoonotic transmission has taken place. A better assessment of zoonotic transmission can only come from studies that examine the dynamics ofGiardia transmission between humans and animals living in the same household or localized focus of endemicity. In Aboriginal communities in Australia, SSU rRNA gene sequences from 13 humans and nine dogs identified assemblages A and B in humans and assemblages C and D in dogs. Only one dog had an assemblage B isolate, suggesting that zoonotic transmission between humans and dogs, if present, was infrequent, and the dog could have acquired the assemblage B infection from a human source (265). Similarly, in a community in which giardiasis is highly endemic in Peru, multiple examinations of dogs and humans in 22 households showed the presence of assemblages A and B in 167 human specimens genotyped and assemblages C and D in 67 canine specimens genotyped (57a). In contrast, in a study conducted in 20 temples and surrounding communities in Bangkok, Thailand, of 13Giardia isolates from dogs and 3 from humans, 1 dog and 2 monks in the same temple had assemblage A isolates (129). In another similar study in the same area, 42 and 35Giardia-positive specimens from dogs and humans, respectively, were chosen for genotyping. Assemblage A was shown to be the most common genotype in dogs (79%), followed by assemblages D (31%), B (21%), and C (12%). Likewise, 73% of humans were infected with assemblage A, followed by assemblage B and assemblage C. Based on the high level of occurrence of assemblage A in both dogs and humans, dogs were considered reservoirs for human giardiasis in temple communities in Bangkok (264).

More recently, subtyping studies of isolates from humans and animals living in the same community or area were conducted to further assess the likelihood of the zoonotic transmission of giardiasis. In a socially deprived small Rom community in Italy where dogs roamed freely,G. duodenalis specimens from children and dogs both had only subassemblage AI (168). Likewise, one child and a dog living in the same household were found to be infected with a subassemblage AI strain in a Brazilian study (287). In contrast, in Assam, India, subassemblage AII was found to be the dominantG. duodenalis assemblage in both humans and a dog living on a tea estate (265,266), and in one household, two isolates from humans and one isolate from a dog all belonged to subassemblage AII. Evidence for zoonotic transmission was further supported by epidemiological data showing a highly significant association betweenGiardia infection of humans and the presence of aGiardia-positive dog in the same household. Unfortunately, it was not clear how many subassemblage AII cases in dogs in the community represented active infection. It was noticed that over 30% of dogs in the community hadAscaris lumbricoides eggs in high intensities in their feces (265,266). AsA. lumbricoides is a human-specific pathogen and dogs were sheddingA. lumbricoides eggs after coprophage of human feces, some dogs might have acted as mechanical disseminators ofGiardia subtype AII rather than being actively infected with this more human-adapted parasite.

Molecular Epidemiology of Giardiasis in Animals

Molecular Epidemiology of Giardiasis in Humans

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