Several morphologically distinct Giardia species are recognized (Table 1). However, Giardia isolated from humans and other mammals are morphologically similar and therefore the taxonomic status and zoonotic potential are not clear and often controversial. Some investigators prefer G. duodenalis as the official name for the species infecting humans and other mammals. Other commonly used names are G. intestinalis and G. lamblia. G. lamblia--named after Lambl, a physician from Prague who fully described the trophozoite from stools of pediatric patients in 1859--is the more widely used in the medical literature.

Molecular analysis reveals a substantial level of genetic diversity between G. duodenalis isolates. For example, comparison of the 18S rRNA sequences reveals distinct groups which segregate more-or-less according to host species (Figure 1). The major exception are human isolates which fall into one of two major groups (designated assemblages A and B). Giardia grouping with assemblages A and B have also been isolated from other mammals and both exhibit a relatively broad host range. However, distinct subgroups within both assemblages A and B are predominantly derived from human isolates. These phylogenetic analyses have been carried out using more sequences and the basic conclusions from the earlier studies are supported (Monis and Thompson, 2003).

Seven major assemblages have been identified (Thompson and Monis, 2004). In contrast to assemblages A and B, G. duodenalis recovered from some animal groups exhibit a rather limited host range (Table 2). The existence of Giardia isolates with broad host ranges and narrow host ranges might explain some of the confusion concerning the zoonotic transmission of Giardia.The adaptation of many Giardia genotypes to specific hosts reduces the likelihood that human giardiasis is obtained from animal sources. Indeed, most molecular epidemiology studies indicate that zoonotic transmission of Giardia is not a major source of human infection (Monis and Thompson, 2003). Although zoonotic transmission of Giardia is possible, it appears to be rather infrequent.

Studies addressing correlations between virulence and genotype have not yielded consistent results. Some studies have found no differences in virulence between genotypes (Cedillo-Rivera et al, 2003). Other studies have found higher levels of virulence associated with assemblage A (Read et al, 2002), whereas other studies indicate assemblage B is more virulent (Homan and Mank, 2001).

In summary, the phylogenetic data suggest that G. duodenalis is a species complex composed of several species which are generally host specific. The development of reliable biological or epidemiological markers that can distinguish these species/genotypes will assist in the understanding of giardiasis and its management.

REFERENCES

R. Cedillo-Rivera, J.M. Darby, J.A. Enciso-Moreno, G. Ortega-Pierres, and P.L. Ey (2003) Genetic homogeneity of axenic isolates of Giardia intestinalis derived from acute and chronically infected individuals in Mexico. Parasitology Research 90: 119-123.

W.L. Homan and T.G. Mank (2001) Human giardiasis: genotype linked differences in clinical symptomatology. International Journal for Parasitology 31, 822-826.

P.T. Monis and R.C.A. Thompson (2003) Cryptosporidium and Giardia-zoonoses: fact or fiction? Infection, Genetics and Evolution 3: 233-244.

C. Read, J. Walters, I.D. Robertson (2002) Correlation between genotype of Giardia duodenalis and diarrhoea. International Journal for Parasitology 32, 229-231.

R.C.A. Thompson, R.M. Hopkins, and W.L. Homan (2000) Nomenclature and genetic groupings of Giardia infecting mammals. Parasitology Today 16: 210-213.

R.C.A. Thompson and P.T. Monis (2000) Variation in Giardia: implications for taxonomy and epidemiology. Advances in Parasitology 58, 69-137.

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