Invited reviewGiardiasis as a re-emerging infectious disease and its zoonotic potential
Introduction
Giardia is one of the most common and best known, yet poorly understood, parasitic organisms. The protozoa that collectively comprise the genus Giardia have intrigued biologists for over 300 years since Antony van Leeuwenhoek first discovered the organism [1]. Members of the genus are ubiquitous, inhabiting the intestinal tracts of a multitude of vertebrate species [2]. They are flagellated protozoans belonging to the class Zoomastigophorea and order Diplomonadida. However, the phylogenetic affinities of Giardia have been a matter of controversy for a number of years [2]. Until recently, support had grown for considering Giardia as representing a pivotal ‘missing link’ between the prokaryotes and eukaryotes, but recent biochemical evidence has led to a novel emerging notion that amitochondriate organisms, such as Giardia, may not represent an ancestral lineage after all, and that the amitochondriate condition is secondarily derived [3].
Although species of Giardia inhabit the intestinal tracts of virtually all classes of vertebrates, Giardia duodenalis (syn Giardia intestinalis; Giardia lamblia) is the only recognised species found in humans and most other mammals, including dogs, cats, cattle, pigs, sheep and horses [2], [4], [5], [6], [7], [8]. Giardia is not invasive and lives and multiplies by asexual multiplication on the lumenal surface of the small intestine of its vertebrate host [2]. The organism produces environmentally resistant cysts which are voided in the faeces and transmitted directly, or via water or food, to another host, with infection resulting from ingestion. Water is increasingly recognised as an important vehicle for the transmission of Giardia, and giardiasis along with cryptosporidiosis represent the major public health concerns of water utilities in developed nations [9].
The pathogenesis of Giardia is not clearly understood, but appears to involve villous atrophy and damage to the microvilli. These changes are in turn correlated with brush border enzyme deficiencies and impaired absorption, which return to normal levels of activity once the infection has resolved [10], [11]. However, the symptoms, which include persistent diarrhoea, abdominal pain and rapid weight loss, are highly variable [2] and may not be evident in a significant proportion of infected individuals [12]. The risk factors for clinical giardiasis are poorly understood and undoubtedly comprise both host factors and the ‘strain’ of the parasite [13], [14]. The immune status of the host appears to influence susceptibility to infection, as well as the severity of clinical signs [15]. Conversely, different strains of the parasite have also been shown to vary in virulence [13], [14].
G. duodenalis has a global distribution and is the most common intestinal parasite of humans in developed countries. In Asia, Africa and Latin America, about 200 million people have symptomatic giardiasis with some 500 000 new cases reported each year [16]. It is also a frequently encountered parasite of domestic animals, especially livestock, dogs and cats, and numerous species of wild mammals have been documented as hosts of Giardia [2], [4], [17], [18], [19].
Section snippets
Child care and community settings
Despite the organism's long history and the attention it has received in the laboratory, there are a number of public health issues which have resulted in Giardia being recognised as a re-emergent infection. Initially, this was because of the association of Giardia with outbreaks of diarrhoea in child day care centres, the use of which is increasing in developed countries and is a major factor in disease emergence [20], [21]. Recent social and workforce changes in developed countries have
Taxonomy and genotypic characterisation
Although the genetic heterogeneity of parasites within the G. duodenalis morphological group has been widely recognised for over 10 years, it is only relatively recently that a consensus has emerged as to which genetic variants constitute major genotypic groupings [60]. Studies comparing isolates of Giardia from different hosts and geographical locations have identified a number of major groups which are genetically identical and distributed world-wide [61].
This major advance in our
Zoonotic transmission
The direct genotypic characterisation of Giardia isolates has made a major contribution to our understanding of the host specificity of different genotypes. Some of the genotypes appear to have a limited host range, whereas others appear to be infective to a wide range of host species (Fig. 1). This has also allowed a predictive assessment to be made about determining sources of infection in outbreak situations [72], as well as zoonotic transmission and the likely interaction between
Molecular epidemiology
Different levels of genetic discrimination can be revealed and utilised in infectious agents by using appropriate techniques and regions of DNA [33], [83]. At the finest level, ‘fingerprinting’ approaches utilising micro/minisatellites or RFLP/IGS-PCR, may allow genotypes or clonal lineages to be followed over extended periods in order to better understand inter- and intra-familial transmission at a localised level in endemic communities, as well as in longitudinal studies. Such approaches can
Concluding comments
Giardia continues to thrive in terms of its ubiquity and the interest it continues to generate amongst researchers. Giardia is also seen as an emerging problem in a variety of epidemiological settings; due to its prevalence as a parasitic infection of children, particularly those attending day care or living in community settings, and a recognition of the high levels of infection seen in domestic animals throughout the world. These issues have been reflected in novel approaches to anti-giardial
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