MURDOCH RESEARCH REPOSITORY · 2012. 12. 17. · Although given a new species 139 name, C....

44
MURDOCH RESEARCH REPOSITORY This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination. The definitive version is available at http://dx.doi.org/10.1016/j.vetpar.2011.07.016 Thompson, R.C.A. and Smith, A. (2011) Zoonotic enteric protozoa. Veterinary Parasitology, 182 (1). pp. 70-78. http://researchrepository.murdoch.edu.au/5728/ Copyright: © 2011 Elsevier B.V. It is posted here for your personal use. No further distribution is permitted.

Transcript of MURDOCH RESEARCH REPOSITORY · 2012. 12. 17. · Although given a new species 139 name, C....

  • MURDOCH RESEARCH REPOSITORY

    This is the author’s final version of the work, as accepted for publication following peer review but without the publisher’s layout or pagination.

    The definitive version is available at http://dx.doi.org/10.1016/j.vetpar.2011.07.016

    Thompson, R.C.A. and Smith, A. (2011) Zoonotic enteric protozoa. Veterinary Parasitology, 182 (1). pp. 70-78.

    http://researchrepository.murdoch.edu.au/5728/

    Copyright: © 2011 Elsevier B.V.

    It is posted here for your personal use. No further distribution is permitted.

    http://dx.doi.org/10.1016/j.vetpar.2011.07.016�http://researchrepository.murdoch.edu.au/5728/�

  • Accepted Manuscript

    Title: Zoonotic Enteric Protozoa

    Authors: R.C.A. Thompson, A. Smith

    PII: S0304-4017(11)00485-7DOI: doi:10.1016/j.vetpar.2011.07.016Reference: VETPAR 5944

    To appear in: Veterinary Parasitology

    Please cite this article as: Thompson, R.C.A., Smith, A., Zoonotic Enteric Protozoa,Veterinary Parasitology (2010), doi:10.1016/j.vetpar.2011.07.016

    This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

    dx.doi.org/doi:10.1016/j.vetpar.2011.07.016dx.doi.org/10.1016/j.vetpar.2011.07.016

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    Zoonotic Enteric Protozoa1

    2

    R.C.A. Thompson*, A. Smith3

    4

    WHO Collaborating Centre for the Molecular Epidemiology of Parasitic Infections, 5

    School of Veterinary and Biomedical Sciences, Murdoch University, South Street, 6

    Murdoch, Western Australia 6150, Australia.7

    8

    *Corresponding Author: WHO Collaborating Centre for the Molecular Epidemiology 9

    of Parasitic Infections and the State Agricultural Biotechnology Centre, School of 10

    Veterinary and Biomedical Sciences, Murdoch University, South Street, Western 11

    Australia 6150, Australia. Tel: 08 9360 2466. Email: [email protected]

    13

    Abstract14

    A growing number of enteric protozoan species are considered to have zoonotic 15

    potential. Their clinical impact varies and in many cases is poorly defined. Similarly, 16

    the epidemiology of infections, particularly the role of non-human hosts, requires 17

    further study. In this review, new information on the life cycles and transmission of 18

    Giardia, Cryptosporidium, Entamoeba, Blastocystis and Balantidium are examined in 19

    the context of zoonotic potential, as well as polyparasitism and clinical significance.20

    21

    Keywords: Enteric protozoa, Zoonoses, Transmission, Foodborne, 22

    Polyparasitism, Clinical significance, Giardia, Cryptosporidium, Entamoeba, 23

    Blastocystis, Balantidium.24

    25

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    1. Introduction27

    28

    Humans are susceptible to infection with numerous species of protozoa that colonise 29

    the intestinal tract. An increasing number of these protozoa are being shown to have 30

    zoonotic potential although their clinical significance varies. Apart from Entamoeba 31

    histolytica and Balantidium coli, they do not invade the mucosal tissues or other 32

    organs. Cryptosporidium has an epicellular association with the host cell, principally 33

    in the mucosal epithelium, but cannot be considered to be invasive. In most cases, the 34

    pathogenesis of most enteric protozoa is poorly understood and with some species, 35

    such as E. coli, opinions vary as to whether they should be considered as parasites.36

    37

    In this review, we update the latest information on individual species of enteric 38

    protozoa, and then examine transmission, polyparasitism and clinical significance on 39

    a broad scale.40

    41

    2. The parasites42

    43

    2.1. Giardia44

    45

    The molecular characterisation of Giardia isolates from different species of 46

    mammalian hosts throughout the world has confirmed the existence of host specific 47

    species and two species with broad host ranges and which are zoonotic (Table 1). A 48

    revised taxonomy has consequently been proposed which largely reflects the species 49

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    nomenclature reported by early workers in the field (Monis et al., 2009; Thompson 50

    and Monis, 2011). 51

    52

    The application of multilocus genotyping to Giardia from human and other 53

    mammalian hosts in different parts of the world has clearly demonstrated the 54

    occurrence of zoonotic species in human and non-human hosts (dogs, cats, livestock 55

    and wildlife) in the same geographical areas supporting the potential for zoonotic 56

    transmission (rev. in Thompson, 2011; Leonhard et al., 2007). Although such studies 57

    do not provide evidence of actual zoonotic transmission, a number of studies in 58

    defined endemic foci have provided convincing evidence involving dogs and humans 59

    (Traub et al., 2004; Inpankaew et al., 2007; Salb et al., 2008). Although the focus of 60

    these studies has been on the dog as a reservoir of human infection, several reports 61

    demonstrate that ‘reverse zoonotic transmission’ (zooanthroponotic) is an important 62

    factor that must be considered in understanding the epidemiology of Giardia 63

    infections. Humans are considered to be the source of infection in non-human 64

    primates and painted dogs in Africa, marsupials in Australia, beavers and coyotes in 65

    North America, muskoxen in the Canadian arctic, house mice on remote islands and 66

    marine mammals in various parts of the world (Graczyk et al., 2002; Sulaiman et al 67

    2003; Moro et al., 2003; Appelbee et al., 2005, 2010; Kutz et al., 2008; Dixon et al., 68

    2008; Teichroeb et al., 2009 Thompson et al., 2009, 2010b, 2010; Ash et al., 2010; 69

    Johnston et al., 2010). These reports raise important issues for conservation. This is 70

    because we do not understand the impact Giardia may have on what are possibly 71

    naïve hosts that may have been exposed to the parasite relatively recently as a 72

    consequence of habitat disturbance and human encroachment (Thompson et al 2010a; 73

    Johnston et al., 2010). In contrast, in Aboriginal communities in isolated regions of 74

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    northern Australia, reverse zoonotic transmission undoubtedly occurs between 75

    humans and dogs but the fact that the dogs are frequently infected with their own 76

    host-adapted species of Giardia, G. canis, demonstrates that competitive interactions 77

    likely limit opportunities for zoonotic species to establish in dogs (Hopkins et al.,78

    1997, 1999; Thompson and Monis, 2004). However, the fact that dogs have contact79

    with young children passing Giardia cysts, as well as discarded nappies/diapers, 80

    means that dogs are likely to act as mechanical transmitters of zoonotic Giardia since 81

    their coats are likely to be contaminated with cysts.82

    83

    Although competitive interactions between different species/assemblages of Giardia84

    has been proposed to explain the predominance of single assemblage infections in 85

    both dogs and cattle, this may reflect the consequences of mixed infections in 86

    endemic foci where the frequency of transmission is very high. In other situations 87

    where transmission is sporadic mixed infections may co-exist and have been 88

    increasingly reported from multilocus studies in several countries in humans, dogs 89

    and cattle (e.g., Hussein et al., 2009; Sprong et al., 2009; Dixon et al., 2011; Covacin 90

    et al., 2011). 91

    92

    The two zoonotic species/assemblages of Giardia are geographically widespread and 93

    as more isolates are genotyped some patterns are emerging on host occurrence. 94

    Recent studies in dogs have shown that it is not possible to extrapolate from one 95

    geographical region to another in terms of the prevalence or species/assemblage 96

    composition of Giardia infections in dogs (Ballweber et al., 2010; Covacin et al., 97

    2011). Until recently, comprehensive data on the assemblage distribution of Giardia 98

    in dogs in the USA was not available, and studies in Europe had suggested that 99

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    assemblage B has a predominantly human distribution (Sprong et al., 2009). 100

    However, a recent study of dogs infected with Giardia in the USA found a higher 101

    frequency of infections with assemblage B than assemblage A, which has not been 102

    reported elsewhere (Covacin et al., 2011). This suggests that in North America at 103

    least, we cannot assume that assemblage A is the most common of the zoonotic 104

    assemblages found in non-human hosts. Indeed in wildlife, assemblage B often 105

    predominates (e.g., Johnston et al., 2010) whereas in cattle, assemblage A is most 106

    often reported (Sprong et al., 2009). However, there is extensive genetic sub-107

    structuring within assemblage B, and it is possible that some subgroups are commonly 108

    associated with zoonotic infections than others. As regards Giardia infections in 109

    humans, there is some evidence of geographic sub-structuring (Wielinga et al., 2011) 110

    and assemblage B may be more common in isolated and/or community settings where 111

    the frequency of transmission is high (Thompson 2000). Under such circumstances, 112

    the parasite is likely to be exposed to greater selection pressure in terms of exposure 113

    to antigiardial drugs and competitive interactions which might explain why evidence 114

    of recombination in Giardia is mostly confined to assemblage B isolates (Lasek-115

    Nesselquist et al., 2009; Thompson and Monis 2011).116

    117

    It is still not yet clear whether the different species/assemblages of Giardia are 118

    associated with differences in pathogenesis. Although differences have been reported 119

    in human populations infected with assemblage A vs B, no clear picture has emerged 120

    (Thompson and Monis 2011). This may be particularly relevant to the two main 121

    disease syndromes of diarrheoa and failure to thrive. Similarly, it is not known 122

    whether there is any difference in the clinical outcome in dogs infected with the 123

    zoonotic assemblages or G. canis.124

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    125

    2.2. Cryptosporidium126

    127

    Current evidence indicates that the main reservoirs of zoonotic Cryptosporidium 128

    remain livestock, with the potential transmission of C. parvum (= C. pestis [see 129

    Slapeta 2011), including the so-called cervine genotype, to humans via contaminated 130

    water or through direct contact with livestock (Robertson et al 2010; Gormley et al.,131

    2011). Sub-genotyping continues to reveal genetic sub-structuring within C. parvum 132

    but whether this is reflected in variation in host specificity and zoonotic potential 133

    remains unclear. The recent demonstration of zoonotic transmission to humans in the 134

    U.K of a newly described genotype of Cryptosporidium from rabbits (Chalmers et al.,135

    2009) has raised concerns that humans may be at risk of Cryptosporidium infection 136

    from rabbits in other geographical areas, as recently proposed in Australia where the 137

    rabbit genotype has been identified (Nolan et al., 2010). Although given a new species 138

    name, C. cuniculi, by Robinson et al (2010), it is genetically very close to C. parvum 139

    and C. hominis and thus it may be prudent to reconsider the taxonomic status of all 140

    three ‘species’ in the future, particularly given the intra-specific genetic variability 141

    that has been described. In contrast, available evidence suggests that the evolving 142

    nomenclature for what appear to be host specific forms in different species of 143

    domestic and wild animals, is likely to be stable and largely reflective of the early 144

    taxonomy (see O’Donoghue, 1995) proposed before the advent of molecular 145

    characterisation.146

    147

    Phylogenetically, Cryptosporidium has been shown to have closer affinities to 148

    gregarine protozoa than the coccidians (Barta and Thompson 1996; Borowski et al., 149

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    2010). This is reflected in the life cycle, developmental biology, metabolism and host 150

    parasite relationship of Cryptosporidium and will require a reappraisal of its biotic 151

    potential, particularly in terms of survival, particularly in aquatic environments.152

    153

    2.3. Entamoeba154

    155

    Of the zoonotic amoebae, Entamoeba histolytica is undoubtedly of most clinical 156

    significance and is considered by some authorities as the only amoeba parasitic in the 157

    human intestine (Schuster and Visvesvara, 2004). However, the public health risk 158

    from zoonotic transmission of E. histolytica would appear to be minimal. Dogs can be 159

    a potential source of human infections following coprophagy of human faeces, 160

    although this reverse zoonosis is unlikely to result in significant environmental 161

    contamination since E. histolytica rarely encysts in dogs (Eyles et al., 1954; Barr, 162

    1998). Similarly, E. histolytica is found in non-human primates (Schuster and 163

    Visvesvara, 2004) but the risk to public health would appear to be minimal as such 164

    infections appear to again reflect reverse zoonosis.165

    166

    A number of other potentially zoonotic species of Entamoeba have been reported in 167

    humans including E. coli, E. polecki and E. hartmanni ( Meloni et al., 1993; Stensvold 168

    et al., 2011). They have generally been considered to be ‘non-pathogenic’ and 169

    ‘commensals’ but increasing evidence of their widespread and common occurrence, 170

    particularly as co-infections with other enteric protozoa and helminths, warrants a re-171

    evaluation of their potential clinical significance. Of the three species, E. coli is one of 172

    the most commonly reported enteric protozoa in human surveys (Chunge et al.,1991; 173

    Ouattara et al., 2008; Youn, 2009; Boeke et al., 2010), and apart from non-human 174

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    primates (e.g. Howells et al., 2011) has been recovered from dogs and marsupials 175

    (Campos Filho et al., 2008; Youn, 2009). It also exhibits extensive genetic diversity 176

    with at least two major subtypes. Although previous reports have considered E. 177

    polecki to be zoonotic (Barnish and Ashford, 1989), recent molecular studies suggest 178

    this species is restricted to humans whereas E. hartmanni is potentially zoonotic with 179

    infections reported in non-human primates (Stensvold et al., 2011).180

    181

    2.4. Blastocystis182

    183

    Blastocystis is an emerging pathogen in terms of its association with disease and 184

    zoonotic potential (Vassalos et al., 2008; Boorom et al., 2008; Parkar et al., 2010). 185

    Now that robust molecular tools are available to detect and characterise Blastocystis, 186

    it has been shown to be both more common and genetically diverse than previously 187

    envisaged (Parkar et al., 2010). Blastocystis is a ubiquitous parasite of humans usually 188

    associated with chronic infections, the prevalence of which increases with age. In 189

    addition to humans, Blastocystis is common in numerous species of wild and 190

    domestic animals (Parkar et al., 2007, 2010; Yoshikawa et al., 2008).191

    192

    As has been shown with Giardia and Cryptosporidium, Blastocystis comprises a 193

    genetically variable complex of interspecific and intraspecific variants and a stable 194

    taxonomy has yet to be established. Humans are susceptible to infection with a variety 195

    of genetically distinct subtypes some of which have also been reported in domestic 196

    and wild animals (Parkar et al., 2007, 2010; Yoshikawa et al., 2008). Studies in 197

    defined endemic foci, such as zoos, have provided convincing evidence of zoonotic 198

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    transmission (Parkar et al., 2010). However, the frequency of zoonotic transmission 199

    and risk factors have still to be clearly defined.200

    201

    2.5. Balantidium202

    203

    This is the only ciliate known to cause infections in humans and is considered to be 204

    zoonotic with pigs as the asymptomatic natural reservoir, although other domestic 205

    livestock such as cattle may be infected in some areas (Bilal et al., 2009). Non-human 206

    primates are also susceptible to infection (see Howells et al., 2011) but their role in 207

    zoonotic transmission is likely to be minimal.208

    209

    Balantidium is most common in tropical and subtropical regions (Zaman, 1998; 210

    Farthing et al., 2003; Owen, 2005) and the risk of infection in humans is in 211

    communities that have a close association with pigs (Schuster and Ramirez-Avila 212

    2008). However, infection in humans is rarely reported (Farthing et al. 2003; Schuster 213

    and Visvesvara, 2004; Conlan et al. 2011). The parasite is invasive in humans and can 214

    cause serious disease but cases are reported sporadically (Schuster and Visvesvara, 215

    2004; Conlan et al., 2011). The low reported prevalences, even in pig rearing 216

    communities (Owen 2005; Kirkoyun Uysal et al., 2009) would suggest that even if 217

    cysts are ingested from the environment, the parasite rarely establishes ‘patent’ 218

    infections. However, the only reported waterborne outbreak originating from 219

    contaminated pig faeces, resulted in 110 people infected (Walzer et al., 1973 [in 220

    Farthing]). This would suggest that sporadic and/or isolated infections usually go 221

    unnoticed or that sojourn in water may enhance infectivity to humans in some way.222

    223

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    3. Transmission224

    225

    3.1. Pathways of transmission226

    227

    Specific transmission pathways for enteric protozoan parasites can be difficult to 228

    determine though some routes are more common than others. Direct transmission via 229

    the faecal/oral route is likely to be the most common form of transmission, whether 230

    zonotic (see above) or direct person to person. However, indirect transmission, where 231

    infection results through the mechanical transmission of oo/cysts on, for example, 232

    flies (Szostakowska et al., 2004) or other animals such as dogs or livestock, or by the 233

    contamination of food or water sources (Smith et al., 2007; Karanis et al., 2007; 234

    Nyarango et al., 2008), poses a significant threat particularly in the developing world. 235

    Quantifying the level of protozoan parasite infection using commonly applied metrics 236

    such as the DALY (eg Pruss et al., 2002) is impractical unless gross non-specific 237

    estimates are required as distinguishing between all possible sources of infection and 238

    the confounding impact of polyparasitism is rarely if ever possible (Payne et al., 2009; 239

    King 2010). Epidemiological analysis of focal point contaminations such as water or 240

    specific food contamination events can provide good estimates of the spread and 241

    impact of parasite infection, but on a global scale, due to the uncertainty caused by the 242

    scarcity of data (Payne et al., 2009) the impact of these occurrences remains poorly 243

    understood.244

    245

    3.2. Enhancing transmission246

    247

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    Children are a particularly high risk group for infection with enteric protozoan 248

    parasites although the relative risk for infection with different parasites varies 249

    between different geographic regions of the world (Figure 1). Interestingly, based on a 250

    major review of the number of reports in the scientific literature (Smith and 251

    Thompson, unpublished report, Food Safety, Zoonoses and Foodborne Diseases, 252

    World Health Organization, Geneva) the risk of infection appears greater within rural 253

    environments than within urban areas; presumably because of the increased 254

    opportunity for both direct and indirect transmission to occur in areas with poor 255

    sanitation and higher contact rates with wildlife and domestic animal reservoirs of 256

    infection.257

    258

    Poor hygiene has been shown to be a crucial factor in enhancing the transmission of 259

    enteric protozoa (Ang, 2000; Alvarado and Vasquez, 2006; Diaz et al., 2006; 260

    Balciogul et al., 2007; Azian et al., 2007). However, the term ‘poor hygiene’ is broad 261

    and does not fully describe the range of risk factors involved. 262

    263

    Blastocystis, Cryptosporidium, Giardia, Entamoeba spp, and Balantidium can all be 264

    transmitted in water, including drinking water sources, recreational water, as well as 265

    lakes and streams. The impact of infection obtained from lakes and streams varies 266

    greatly depending on geographic location as there exists significant variation in the 267

    use of such water bodies throughout the world as well as within regions depending on 268

    socioeconomic status. For example the predominant use of lakes and streams in 269

    Europe is recreational whereas in Africa or Asia there is a much greater reliance on 270

    these water sources for drinking and in the preparation of food and personal hygiene. 271

    Waterborne transmission of parasites in the developed world is therefore more likely 272

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    to be the result of contamination, or a process failure within water utilities, industry or273

    in public places such as swimming pools (Welch, 2000; Stuart et al., 2003; Dawson, 274

    2005; Shields et al., 2008). In the developing world in particular, areas which are 275

    prone to flooding face an increased risk of waterborne infection particularly where 276

    basic sewerage systems such as long-drop latrines are common. The risk is further 277

    increased in those communities and societies where the keeping of animals (pets and 278

    livestock) within or near the home is common and the risk of infection with specific 279

    parasites including concurrent infections will vary culturally depending on the species 280

    of zoonotic reservoir involved (Hunter and Thompson, 2005). 281

    282

    3.3. Foodborne transmission283

    284

    Cases of foodborne transmission have until recently been difficult to determine, and 285

    linking infection to a contaminated food source remains so for the majority of 286

    scenarios. Small scale outbreaks, where the point of initial contamination may be the 287

    result of poor hygiene by an individual resulting in localised foodborne transmission 288

    to family members or the immediate community, are extremely common particularly 289

    in the developing world. Transmission resulting from contamination of food or drink 290

    that leads on to larger scale infections, such as an outbreak of cryptosporidiosis in the 291

    USA in 1993 arising from the consumption of infected fresh-pressed apple cider is a 292

    relatively much rarer event (Millard et al.,1994). Again, according to reports 293

    published in the primary scientific literature, the risk of becoming infected with 294

    Blastocystis from contaminated food appears low, although infection arising through 295

    poor hygiene practices of food handlers has a potentially much higher infection rate. 296

    For example, a survey of 150 apparently healthy food handlers in Bolivar State, 297

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    Venezuela, showed that 25.8% (n = 107) were in fact positive for Blastocystis298

    (Requena et al., 2003), whereas a review of 103 published reports concerning 299

    Blastocystis infection between 1990 and 2008 showed that none were able to 300

    conclusively ascribe infection to conventional foodborne transmission (Smith and 301

    Thompson, unpublished report, Food Safety, Zoonoses and Foodborne Diseases, 302

    World Health Organization, Geneva). However, other enteric parasites are readily 303

    transmitted on food and in some regions of the world farming and agricultural 304

    practices such as the use of human waste for fertilisation and inefficient or absent 305

    pasteurisation techniques positively enhance transmission. None the less, the majority 306

    of indirect transmission from contaminated food arises from more proximate sources 307

    of infection such as infected food handlers and poor hygiene, often resulting from 308

    overcrowded living conditions where clean running water is scarce, and also from 309

    children and adults with little or no health education.310

    311

    Foodborne transmission, whether arising as a result of agricultural practices or poor 312

    hygiene within households, or by food handlers, is undoubtedly responsible for a 313

    significant number of infections every year. Preliminary estimates derived from a 314

    literature review of published reports and case notes for Cryptosporidium and Giardia315

    occurrence, suggest that the number of cases of foodborne transmission resulting in 316

    infection by Giardia range from 13 million in the WHO derived Eastern 317

    Mediterranean region (EMR) to 76 million in the Western Pacific Rim (WPR) region 318

    (Fig 1). Similarly, preliminary estimates for Cryptosporidium suggest there may be as 319

    many as 6 million cases of foodborne transmission annually in the EMR region and 320

    up to 27 million in the African region. However, these estimates must be interpreted 321

    with extreme caution as the majority of publications upon which most estimates are 322

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    based simply do not, or cannot, ascribe infection to a particular source or event such 323

    as contact with contaminated food. Transmission between humans, either involving 324

    intermediate vessels such as contaminated food or water, or via direct contact, is 325

    ultimately responsible for most reported cases, with zoonotic parasites and parasite 326

    strains as well as zoonotic sources of human strains also a major source of infection in 327

    many parts of the world (Thompson 2000; Feltus et al., 2006; Ng et al., 2008; 328

    Thompson et al., 2008). 329

    330

    4. Polyparasitism331

    332

    The clinical impact of zoonotic enteric protozoan infections is greatest in the 333

    developing world where inadequate sanitation, poor hygiene and proximity to 334

    zoonotic reservoirs, particularly companion animals and livestock are greatest. In such 335

    circumstances, it is not surprising that infections with more than one species of enteric 336

    protozoan is common, and in fact single infections are rare (see above). 337

    Unfortunately, the impact on health of such concurrent/co-infections has not been 338

    adequately taken into account. Awareness of the significance of polyparasitism 339

    (concurrent/concomitant infections) in terms of malaria, schistosome infections and 340

    more recently gastrointestinal (GI) helminths, is now well established (Cox, 2001; 341

    Pullan and Brooker, 2008; Midzi et al., 2008; Koukounari et al., 2010; Supali et al 342

    2010). However, the impact of polyparasitism in the context of enteric protozoan 343

    infections has not been considered (Ouattara et al., 2008).344

    345

    Enteric protozoan infections are components of a complex ecological system, shared 346

    with helminths and bacteria. As such, we should not study the components of such an347

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    ecological system in isolation (e.g., see Rohani et al., 2003) especially when 348

    considering their impact on disease dynamics (Jolles et al., 2008). Polyparsitism will 349

    lead to the stimulation of different components of the immune system that may act 350

    synergistically, lead to competitive interactions and/or alter pathogenic behaviour of 351

    one or more species (Thompson and Lymbery, 1996; Cox, 2001; Mideo, 2009; 352

    Koukounari et al., 2010; Supali et al., 2010). These may have an additive and/or 353

    multiplicative impact on nutrition and pathogenicity (Pullar and Brooker, 2008) 354

    leading to an increase or decrease in clinical severity, and/or interfere with normal gut 355

    microflora. 356

    357

    Current measures of determining the impact on health of parasitic disease (DALY) do 358

    not take into consideration the “co-morbidities of polyparasitism” (Payne et al., 2009). 359

    Payne and colleagues suggest a new approach where co-infections with more than one 360

    infectious agent are defined as a specific disease, for example malaria, hookworm, 361

    malaria + hookworm etc. This would appear a logical approach and should be readily 362

    achievable with diseases such as malaria and hookworm where the pathogenic 363

    mechanisms of the individual aetiological agents are reasonably well understood. This 364

    would be much more of a challenge with most of the enteric protozoa since their 365

    pathogenesis is still poorly understood, particularly in chronic infections. However, it 366

    is an area of future epidemiological research that should be given priority.367

    368

    Available survey data, mostly from rural areas of developing countries, that provide 369

    data on the prevalence of mixed infections with enteric protozoa show that they are 370

    the rule rather than the exception and that children are at most risk with the majority 371

    of the populations studied infected with at least two species of protozoan (Chunge et 372

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    al., 1991; Ouattara et al., 2008; Nematian et al.,2008; Nguhiu et al., 2009; Aly and 373

    Mostata, 2010). The most commonly represented protozoa in concurrent infections 374

    are Giardia, Blastocystis, and Entamoeba coli, which are usually also found with the 375

    cestode Hymenolepis nana (Chunge et al., 1991; Meloni et al., 1993; Ouattara et al., 376

    2008; Nematian et al., 2008). Chronic helminth infections could have a significant 377

    influence over the immune response and hence susceptibility to other pathogens 378

    (Rodriguez et al., 1999). This was concluded in the context of GI nematode infections 379

    but consideration should also be given to H. nana which is rarely found on its own in 380

    developing areas of the world and usually co-infects with one or more enteric 381

    protozoan species. Depending on the endemic area, E. histolytica and E. dispar may 382

    also occur in mixed infections. In addition, other potentially zoonotic protozoa, such 383

    as Chilomastix mesnili and Endolimax nana that may have been dismissed as having 384

    no clinical consequence and not reported, are also probably common (e.g. Chunge et 385

    al., 1991) and in the context of polyparasitism should be considered.386

    387

    5. Clinical significance388

    389

    The clinical impact of Entamoeba histolytica and Cryptosporidium in humans are 390

    well understood. They have clearly defined individual effects on the host and the 391

    resulting morbidity will be worsened in an additive way when other enteric protozoa 392

    are present. The severity of Cryptosporidium infections is greatest in individuals with 393

    an impaired or deficient immune system. Much less is known about the clinical 394

    impact of other enteric prototozoa particularly in chronic and mixed infections.395

    396

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    The symptoms associated with Giardia infections vary greatly but have mostly been 397

    documented on the basis of individual infections with the expression of short-lasting 398

    diarrhoea the most common manifestation (Eckmann, 2003). However, there is 399

    increasing realisation that the clinical impact of Giardia is greatest in children, 400

    particularly in developing countries or disadvantaged communities where poor 401

    hygiene and the proximity of animal reservoirs favour a high frequency of 402

    transmission and the establishment of chronic infections (Thompson, 2009). In such 403

    situations, nutrition may be suboptimal and Giardia infections contribute to poor 404

    growth and development (‘failure to thrive’), zinc and iron deficiency, as well as 405

    predispose to the development of allergic diseases (Muniz-Junqueira and Queiroz, 406

    2002; Muniz et al., 2002; Nematian et al., 2008; Hesham et al., 2005; Savioli et al.,407

    2006; Thompson, 2008; Boeke et al., 2010; Duran et al., 2010; Quihui et al., 2010). 408

    Importantly, diarrheoa is not necessarily a symptom in such chronic infections (Nash 409

    et al., 1987; Chunge et al., 1991; Flanagan 1992; Rodriguez-Hernandez et al., 1996; 410

    Troeger et al., 2007), which are often shared with enteric parasites that contribute to 411

    the syndome of failure to thrive, such as Entamoeba coli, Blastocystis and 412

    Hymenolepis nana (see above). Reports from around the world suggest there are 413

    differences in the clinical outcome in humans of infections with assemblage A (G. 414

    duodenalis) and assemblage B (G. enterica) (Thompson and Monis, 2011). However, 415

    it is difficult to directly compare between reported studies because of differences in 416

    design and populations sampled. It has been suggested that infections with 417

    assemblage A are usually short lasting, acute infections often associated with 418

    diarrhoea whereas infections with assemblage B are more common in community 419

    settings or localised endemic foci where the frequency of transmission is high 420

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    resulting in chronic infections that may contribute to poor growth (Thompson and 421

    Monis, 2011).422

    423

    There is now increasing recognition that Blastocystis is pathogenic, at least in 424

    humans, although the circumstances and mechanisms associated with disease are not 425

    understood (Boorom et al., 2008). Similarly, the spectrum of disease manifestations 426

    associated with Blastocystis infections have yet to be clearly defined. Many of the 427

    documented symptoms are non-specific and may be overlooked but the realisation 428

    that Blastocystis is most often associated with chronic infections and that prevalence 429

    increases with age, demonstrates that such symptoms may be indicative of a range of 430

    disorders, often sporadic, not necessarily associated with an enteric pathogen, for 431

    example inflammatory bowel disorders (Boorom et al., 2008). As with Giardia, it is 432

    possible that some ‘strains’ of Blastocystis are more often associated with overt 433

    disease than others (Boorom et al., 2008). The developmental cycle of Blastocystis is 434

    not completely understood. It comprises a range of morphologically pleiomorphic 435

    stages and it is not clear whether all are always represented and whether some are 436

    more pathogenic than others.437

    438

    Most attention on the clinical effects of zoonotic enteric protozoa has focussed on 439

    infections in humans. Comparatively little is known about the clinical consequences 440

    of infections in domestic animals and livestock.441

    442

    Although infections with Giardia, and to a lesser extent Cryptosporidium, are 443

    common in dogs and cats, infections are usually asymptomatic (Thompson et al., 444

    2008). Giardia infection in ruminants is also often asymptomatic, but may also be 445

  • Page 19 of 42

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    associated with the occurrence of diarrhoea and ill-thrift in calves (O’Handley et al.,446

    1999; Geurden et al., 2006), as well as production losses (Olson et al., 1995). In this 447

    respect, the effects of polyparasitism, with Giardia as one member of an often diverse 448

    parasite community in livestock (O’Handley and Olson, 2006), requires further study. 449

    Infections with Cryptosporidium in calves can be clinically significant with profuse 450

    diarrhoea, depression and anorexia, and is often life threatening in housed animals 451

    where stress associated with overcrowding may predispose to disease (Thompson et 452

    al., 2008).453

    454

    The impact of enteric protozoa on wildlife is not known, but there is evidence of an 455

    association with clinical disease of infections with Giardia and Balantidium in 456

    primates (Graczyk et al., 2002). The stress associated with captivity may further 457

    predispose to disease, particularly in the case of infections of Cryptosporidium in 458

    wildlife. This incredible diversity of species and ‘strains’ of Cryptosporidium, 459

    Giardia and Balantidium in wildlife clearly warrants further study in terms of their 460

    potential impact on wildlife health. Habitat loss as a result of human encroachment 461

    can lead to a variety of stressors (e.g. increased competition for food, mates and 462

    refuge, as well as greater exposure to predators) that may exacerbate the clinical 463

    consequences of infection with enteric protozoa in wildlife (Thompson et al., 2010a; 464

    Johnston et al., 2010; Howells et al., 2011).465

    466

    6. Conclusions467

    468

    Although tremendous advances have been made in the development of 469

    molecular epidemiological tools, their practical potential in terms of controlling 470

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    enteric protozoan infections has yet to be realised. In both developed and 471

    developing countries there has been little progress in identifying risk factors for 472

    infection and understanding the outcomes of infection. This is particularly so for 473

    the developing world where these infections are most common and the 474

    frequency of infection very high. In such environments, the epidemiology of 475

    diseases caused by enteric protozoa is poorly understood in terms of 476

    transmission dynamics and sources of infection. Importantly, we must not 477

    consider enteric protozoan parasites in isolation in terms of impact on their 478

    hosts. A child or dog, on good planes of nutrition, harbouring a single infection 479

    with Giardia in a developed country represents a very different host parasite 480

    relationship to that in a developing country where Giardia rarely occurs as a 481

    single infection but is accompanied by several other protozoa, nematodes and 482

    cestodes in an environment usually deficient in some way, in terms of nutrients 483

    available to the host. Unravelling the interactions and impact on their hosts in 484

    such circumstances is an important challenge for the future.485

    486

    Acknowledgements and conflict of interest487

    488

    The Australian Research Council and Western Australian Department of Environment 489

    and Conservation have supported some of the work reported here. The authors declare 490

    that there is no conflict of interest.491

    492

    References493

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    934

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    Arnold.949

    950

    951

    952

    953

    954

    955

    956

    957

    958

    959

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    960

    961

    962

    963

    964

    Table 1. 965

    Proposed and existing species in the genus Giardia966

    Species Hosts

    G. duodenalis

    (=Assemblage A)

    Wide range of domestic and wild mammals

    including humans

    G. enterica

    (=Assemblage B)

    Humans and other primates, dogs, some

    species of wild mammals

    G. agilis Amphibians

    G. muris Rodents

    G. ardeae Birds

    G. psittaci Birds

    G. microti Rodents

    G. canis

    (=Assemblage C/D)

    Dogs, other canids

    G. cati

    (=Assemblage F)

    Cats

    G. bovis

    (=Assemblage E)

    Cattle and other hoofed livestock

    G. simondi

    (=Assemblage G)

    Rats

  • Page 41 of 42

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    967

    968

    969

    970

    971

    972

    973

    Fig. 1. Estimated prevalence of Blastocystis, Cryptosporidium, Giardia and 974

    Entamoeba within children from rural and urban regions for each or the World Health 975

    Organisation (WHO) defined regions of the world: European Region (EUR), South-976

    East Asian Region (SEAR), Western Pacific Region (WPR), African Region (AFR), 977

    Region of the Americas (AMR) and Eastern Mediterranean Region (EMR). 978

    979

    980

    981

    982

    983

    984

    985

    986

    987

    988

    989

    990

    991

  • Page 42 of 42

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    992

    993

    994

    995

    996

    997

    998

    999

    1000

    1001

    1002

    0

    50

    100

    Pre

    val

    en

    ce

    WPRRural

    0

    50

    100

    Pre

    val

    ence

    SEARRural

    Urban

    0

    50

    100

    Pre

    val

    en

    ce

    EURRural

    Urban

    0

    50

    100

    Pre

    vale

    nc

    e

    EMRRural

    0

    50

    100

    Pre

    val

    ence

    AFRRural

    0

    50

    100

    Pre

    val

    ence

    AMRRural

    1003

    Cover page author's versionzoonotic enteric protozoa