Research review paperGiardia/giardiasis — A perspective on diagnostic and analytical tools
Introduction
Species of Giardia (Metamonada) are important parasitic protists that are transmitted by the faecal–oral route and cause gastroenteritis in vertebrates, including mammals, birds, reptiles and fishes (Adam, 2001, Filice, 1952, Thompson et al., 1998, Thompson and Monis, 2011). In mammals, including humans, giardiasis is mainly caused by Giardia duodenalis (syn. Giardia intestinalis and Giardia lamblia) (Thompson, 2004). This disease is usually transmitted from human-to-human (anthroponotic) or animal-to-human (zoonotic) (Xiao and Fayer, 2008, Feng and Xiao, 2011). In agricultural animals, for instance, giardiasis can lead to morbidity and economic losses (Olson et al., 2004), although asymptomatic infections are common (Geurden et al., 2010b). In humans, giardiasis is usually a self-limiting illness, characterised by diarrhoea, colic, headache, dehydration, malabsorption, weight loss and/or wasting (Buret and Cotton, 2011). This disease particularly affects children and immuno-deficient or -compromised people (Muhsen and Levine, 2012, Stark et al., 2011, Thompson, 1994). Nonetheless, asymptomatic infections are common in apparently immuno-competent individuals, particularly in developing countries (e.g., Farthing et al., 1986, Feng and Xiao, 2011, Haque et al., 2005, Mason and Patterson, 1987). Chemotherapeutics, such as metronidazole, fenbendazole and febantel, are commonly used to treat clinical cases, while other drugs employed include albendazole, nitazoxanide, furazolidone, tinidazole, quinacrine and ornidazole (Escobedo and Cimerman, 2007, Gardner and Hill, 2001, Huang and White, 2006).
Outbreaks of human giardiasis are commonly associated with child day-care centres, contaminated drinking water or swimming pools (Baldursson and Karanis, 2011, Karanis et al., 2007). Giardia cysts are quite resistant to disinfectants, such as chlorine, routinely used for water treatment (Betancourt and Rose, 2004). Waterborne outbreaks of giardiasis have been reported in developed countries, including the USA, Canada and, more recently, Norway (e.g., Baldursson and Karanis, 2011, Karanis et al., 2007, Nygård et al., 2006), the most notable being the outbreak in Portland, Oregon, USA, in 1954, which resulted in ~ 50,000 human cases (Karanis et al., 2007). Additionally, Giardia is a common etiological agent of traveller's diarrhoea (Ross et al., 2013). Given the relative resilience of cysts in water and the environment (Olson et al., 1999), the cost of chemotherapeutic compounds or regimens for treatment or vaccination in animals (O'Handley and Olson, 2006) and the socioeconomic impact of giardiasis, Giardia is recognised as a key waterborne pathogen impairing health and development, and hindering socioeconomic improvement in developing countries (Savioli et al., 2006, WHO. Microbial fact sheets. Guidelines for Drinking-Water Quality and Second edition. Geneva: World Health Organization, 2011b).
The identification and characterisation of Giardia is central to investigating and understanding the epidemiology of giardiasis. However, there are significant limitations in detection or diagnosis using conventional microscopic, biochemical, immunological and serological techniques (Dixon et al., 1997, Thompson, 2004), such that there has been a need for reliable and practical molecular methods. Using genetic methods, seven recognised species and eight genotypes, called assemblages A–H, have been reported from various vertebrate host groups (Thompson and Monis, 2012), although assemblage H requires further verification. These species and genotypes cannot be distinguished based on host origin or parasite morphology (Feng and Xiao, 2011, Thompson and Monis, 2011). In the present article, we provide an account of some key microscopic and immunological methods used for the detection or identification of Giardia, and review nucleic acid-based approaches for the diagnosis of giardiasis and analysis of genetic variation within and among species of Giardia. We also describe the advantages and disadvantages of some techniques, and emphasise the benefits of using molecular tools to achieve a better understanding of the systematics, epidemiology and population genetics of members of the genus Giardia, underpinning the prevention and control of giardiasis in animal and human host populations. Topics that are beyond the scope of this paper can be found in key review articles of Giardia/giardiasis (Ankarklev et al., 2010, Appelbee et al., 2005, Feng and Xiao, 2011, Fletcher et al., 2012, Gardner and Hill, 2001, Huang and White, 2006, Monis et al., 1996, Ryan and Cacciò, 2013, Smith and Smith, 1989, Thompson, 2004, Thompson and Monis, 2011, Xiao and Fayer, 2008).
Section snippets
Microscopy
Conventionally, the detection of Giardia cysts in duodenal, faecal, tissue, environmental and/or water (= biological) samples is achieved mainly by microscopic examination (e.g., Behr et al., 1997, de Souza et al., 2004, Dixon et al., 1997, Garcia, 2009, Goka et al., 1990, Huang and White, 2006, Mank et al., 1997, Sauch, 1985, Schuurman et al., 2007) (Fig. 1). A number of morphological features of the trophozoite, including median body shape and location, shape, ventrolateral disc length,
Nucleic acid techniques for diagnosis or genetic analysis
Since phenotypic methods (Section 2) cannot be used to distinguish different species or assemblages of Giardia, various nucleic acid-coupled methods have been developed to identify and detect genetic variation within and among members of this genus within clinical and environmental samples (Fig. 2). Although some methods rely on the specific in situ hybridisation of probes to particular genetic loci within the genome of Giardia cysts (Sub-section 3.1.), most techniques rely on the specific
Conclusions
Advances have been made in the development of specific and sensitive molecular tools for the identification and genetic characterisation of Giardia, and the diagnosis of giardiasis. In particular, the identification and classification of Giardia species and assemblages using genes, such as bg, C4, ef1α, gdh, tpi and/or vsp, in combination with more or less variable loci in ribosomal DNA (e.g., ITS and SSU), have assisted in elucidating the systematics, population genetics and epidemiology of
Acknowledgements
Funds from the Australian Research Council (ARC), the National Health and Medical Research Council (NHMRC) and Melbourne Water Corporation are gratefully acknowledged, as is support from the Alexander von Humboldt Foundation and the Freie University Berlin (RBG). Thanks are given to Georg von Samson-Himmelstjerna for comments on the manuscript. The authors also thank two anonymous reviewers for their constructive comments and suggestions. The authors have no conflict of interest.
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