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Am J Trop Med Hyg. 2013 April 3; 88(4): 795–802.
PMCID: PMC3617872

Development of Three PCR Assays for the Differentiation between Echinococcus shiquicus, E. granulosus (G1 genotype), and E. multilocularis DNA in the Co-Endemic Region of Qinghai-Tibet plateau, China

Abstract

To investigate echinococcosis in co-endemic regions, three polymerase chain reaction (PCR) assays based on the amplification of a fragment within the NADH dehydrogenase subunit 1 (ND1) mitochondrial gene were optimized for the detection of Echinococcus shiquicus, Echinococcus granulosus G1, and Echinococcus multilocularis DNA derived from parasite tissue or canid fecal samples. Specificity using parasite tissue-derived DNA was found to be 100% except for E. shiquicus primers that faintly detected E. equinus DNA. Sensitivity of the three assays for DNA detection was between 2 and 10 pg. Ethanol precipitation of negative PCR fecal samples was used to eliminate false negatives and served to increase sensitivity as exemplified by an increase in detection from 0% to 89% of E. shiquicus coproDNA using necropsy-positive fox samples.

Introduction

Human cystic and alveolar echinococcosis caused by infection with the metacestodes of the tapeworms Echinococcus granulosus (sensu stricto) and Echinococcus multilocularis, respectively, are among the most pathogenic helminth zoonoses.1 These species occur sympatrically on the Qinghai-Tibet plateau of western China and as a result human cystic (CE) and alveolar (AE) echinococcosis are co-endemic in Tibetan pastoral communities.2,3 In addition a new species, Echinococcus shiquicus was recently described in wildlife (Tibetan fox, Vulpes ferrilata and plateau pikas, Ochotona curzoniae). However, the zoonotic potential (if any) of this species is currently unknown.2,4 The occurrence of these three species in this particular part of the world highlights the importance of studies on transmission ecology and epidemiology and the need for the development of species-specific diagnostic assays for the detection of these Echinococcus species within both intermediate and definitive hosts.

Although protocols for the detection of E. granulosus and E. multilocularis have been published by many authors,510 only three polymerase chain reaction (PCR)-based assays have previously been developed and validated for the detection of E. granulosus11,12 and E. multilocularis DNA13,14 from dog or fox fecal samples, respectively. A recent assessment of the E. granulosus PCR assays failed to fully reproduce the species and/or subspecies specificity reported by the original authors.15 Moreover, the species-specific optimization of the E. granulosus PCR tests11,12 pre-dates the description of E. shiquicus4 and thus reduces their diagnostic value within the Echinococcus multi-species co-endemic regions of western China.

In the case of E. multilocularis, primers optimized for the detection of E. multilocularis DNA from feces13,14 were found to cross-react with DNA of E. shiquicus (B. Boufana, unpublished observations). The specificity of other primers optimized for the detection of E. multilocularis in definitive host feces12 was not assessed against DNA of E. shiquicus. Recently, a PCR-restriction fragment length polymorphism protocol was described for the differential diagnosis of tissue-derived DNA of E. shiquicus, E. granulosus (G1 and G6 genotypes), and E. multilocularis.16 However, in that study, the uniqueness of the restriction sites and specificity compared with those of other E. granulosus genotypes (sensu lato) or Taenia species was not assessed. The currently available PCR assays for detection of Echinococcus species have not been assessed in relation to potential cross-reactions with E. shiquicus in China and PCR tests using feces rather than DNA from purified eggs have not been described. We now report on the development of three species-specific PCR (uniplex) assays for the detection of E. shiquicus, E. granulosus (G1), and E. multilocularis.

Materials and Methods

Primer design.

Nucleotide sequences of the NADH dehydrogenase subunit 1 (ND1) mitochondrial gene of E. shiquicus (GenBank accession no. AB208064), E. granulosus genotype 1 (G1) (GenBank accession no. AF297617), and E. multilocularis (GenBank accession no. AB018440) as well as those of related species namely E. equinus (G4) (GenBank accession no. AF346403), E. ortleppi (G5) (GenBank accession no. AB235846), E. canadensis (G6, GenBank accession no. HM563036; G7, GenBank accession no. AB235847, G8, GenBank accession no. AJ237643, and G10, GenBank accession no. AF525297), E. vogeli (GenBank accession no. AB208546), E. oligarthrus (GenBank accession no. AB208545), Taenia multiceps (GenBank accession no. GQ228818), T. hydatigena (GenBank accession no. GQ228819), T. pisiformis (GenBank accession no. AJ239109), T. ovis (GenBank accession no. AJ239103), and T. crassiceps (GenBank accession no. AF216699) were retrieved from sequences deposited in the GenBank database (http://www.ncbi.nlm.nih.gov) and aligned using ClustalW (http://align.genome.jp). Forward and reverse primers were designed manually within the sequences of the three relevant target Echinococcus species to amplify a species-specific diagnostic fragment within the ND1 gene. Primers were validated through the use of BLAST biosoftware (www.ncbi.nlm.nih.gov/BLAST/). The high-performance liquid chromatography purified primers were synthesized by MWG-Biotech (Ebersberg, Germany).

DNA extraction from parasite tissue.

All tissue-derived DNA used in this study was extracted using the DNeasy Blood and Tissue Kit (Qiagen, Hilden, Germany) according to the manufacturer's instructions. The DNA extracted from adults of E. shiquicus (Tibetan fox, V. ferrilata, China), protoscoleces of E. granulosus (G1 sheep genotype, Tunisia), and adult tapeworms of E. multilocularis (dogs, China) was verified by sequencing and used as controls in this study.

DNA extraction from host feces.

CoproDNA was extracted from dog and fox fecal samples using the QIAamp DNA Mini Stool Kit (Qiagen) implementing the procedure recommended to process 1–2 g of feces with adjustment of lysis buffer volume. The suspension was heated in a water bath for ~25–30 minutes and then processed according to the manufacturer's instructions.

Echinococcus shiquicus PCR protocol (Es PCR).

For the amplification of E. shiquicus DNA, 0.3 μM of each primer (EsF50, 5′ TTA TTC TCA GTC TCG TAA GGG TCC G 3′ and EsR73, 5′ CAA TAA CCA ACT ACA TCA ATA ATT 3′) was used in a 50 μL reaction volume containing 5× manufacturers Flexi reaction buffer (Promega Ltd., Southampton, UK), 200 μM of each deoxynucleoside triphosphate (dNTPs; Bioline, London, UK), 2 mM MgCl2 and 2.5 U GoTaq polymerase (Promega Ltd.). The mastermix was overlaid with mineral oil and the cycling profile was as follows: 5 min at 94°C for 1 cycle, and then 36 cycles each consisting of 30 s at 94°C, 50 s at 60°C, and 30 s at 72°C. The diagnostic product was 442 bp in size.

Echinococcus granulosus PCR protocol (G1 PCR).

Amplification was performed in a 50 μL reaction volume with 5× manufacturers Flexi reaction buffer (Promega Ltd.), 200 μM of each deoxynucleoside triphosphate (dNTPs; Bioline), 0.3 μM of each primer (Eg1F81, 5′ GTT TTT GGC TGC CGC CAG AAC 3′ and Eg1R83, 5′ AAT TAA TGG AAA TAA TAA CAA ACT TAA TCA ACA AT 3′), 2 mM MgCl2 and 2.5 U GoTaq polymerase (Promega Ltd.). The mastermix was overlaid with mineral oil and the reaction was subjected to an initial incubation of 5 min at 94°C for 1 cycle, followed by 36 cycles each consisting of 30 s at 94°C, 50 s at 62°C, and 30 s at 72°C to amplify a species-specific 226 bp fragment.

Echinococcus multilocularis PCR protocol (Em PCR).

The 50 μL reaction volume contained 5× manufacturers Flexi reaction buffer (Promega Ltd.), 200 μM of each deoxynucleoside triphosphate (dNTPs; Bioline), 0.3 μM of each primer (EmF19/3, 5′ TAG TTG TTG ATG AAG CTT GTT G 3′and EmR6/1, 5′ATC AAC CAT GAA AAC ACA TAT ACA AC 3′), and 2 mM MgCl2. 2.5 U of Hotstart GoTaq polymerase (Promega Ltd.) was used to overcome primer multimerization. The mastermix was overlaid with mineral oil and the thermal cycling profile consisted of 5 min at 94°C for 1 cycle, followed by 35 cycles as follows: 30 s at 94°C, 50 s at 53°C, and 30 s at 72°C to amplify an E. multilocularis-specific 207 bp fragment.

The PCR procedures were carried out in fully equipped molecular laboratories using dedicated equipment to prevent amplification of extraneous DNA. Negative controls (PCR grade water) were included in all experiments to monitor for contamination. A Stratagene (La Jolla, CA) Robocycler was used for all cycling profiles. The PCR products were resolved on a 1.5% (w/v) agarose gel (Bioline) in 1× Tris-Borate-EDTA buffer (Severn Biotech, Kidderminster, UK) at 110 V, stained with gel red DNA dye (Cambridge BioSciences, Cambridge, UK), and visualized using Syngene G:Box gel documentation system (Cambridge Biosciences). Validation of the PCR tests was made against defined panels of parasite tissue-derived DNA and using DNA extracted from infected canid fecal samples as described below.

Cloning and sequencing.

In an attempt to ascertain the identity of the amplified products, representative DNA fragments amplified by each PCR assay were prepared for sequencing. These were Tris-acetate-EDTA (TAE)-gel purified using PurLink™ quick gel purification Kit (Invitrogen, Paisley, UK) and cloned into the TOPO TA cloning vector (Invitrogen) according to manufacturer's instructions and subsequently transformed into competent Escherichia coli cells. Plasmid minipreps were prepared using the Wizard Plus DNA purification system (Promega Ltd.) and DNA from the recombinant plasmids was commercially sequenced using M13 vector-specific primers (Beckman Coulter Genomics, Essex, UK).

DNA sequence analysis.

Nucleotide sequences were analyzed using the FinchTV software package (Geospiza, Seattle, WA) and compared with those in the GenBank database through the use of BLAST biosoftware (www.ncbi.nlm.nih.gov/BLAST/).

Parasite tissue samples.

The performance of the E. shiquicus PCR, E. granulosus G1 PCR, and E. multilocularis PCR was assessed using DNA extracted from parasite material as shown in Table 1.

Table 1
Parasite tissue DNA used for the assessment of the performance of the Echinococcus shiquicus, E. granulosus G1, and E. multilocularis PCR assays

Canid fecal samples.

The DNA extracted from feces of naturally infected canids was used to assess the performance of the E. shiquicus, E. granulosus G1, and E. multilocularis primers in detecting coproDNA (Table 2). The DNA extracted from feces collected from negative Tibetan foxes (V. ferrilata) (N = 4), from endemic necropsied negative dogs from Libya (N = 6) and Kenya (N = 1) and from necropsied negative red foxes (Vulpes vulpes) from France (N = 4) was included in the panel tested by the E. shiquicus, E. granulosus G1, and E. multilocularis primers, respectively. Three to 10 μL of DNA was used as template and 20 μL of the amplified PCR product were loaded onto agarose gel for electrophoresis.

Table 2
DNA extracted from feces of naturally infected dogs, red (Vulpes vulpes) and Tibetan (Vulpes ferrilata) foxes used in assessment of coprodetection of Echinococcus shiquicus, E. granulosus G1 and E. multilocularis PCR assays

Evaluation of PCR specificity.

Specificity of the three PCR assays (Es PCR, G1 PCR, Em PCR) was assessed using parasite tissue DNA extracted from cestodes (Table 3). The presence of cestode-specific DNA (Taenia species, Echinococcus species, Dipylidium caninum) used in evaluation of assay specificity was ascertained through the amplification of a 373 bp fragment within the 12S rRNA gene using cestode-specific primers.8,17 In addition, tissue-derived DNA of E. granulosus G1 and E. multilocularis was used to check the E. shiquicus primers, whereas that of E. shiquicus and E. multilocularis, and E. shiquicus and E. granulosus G1 were used to ascertain specificity of the E. granulosus G1 and E. multilocularis PCR assays, respectively. An identical set of samples were tested at least three times for each PCR assay with identical results observed each time. Representative results are shown here.

Table 3
Parasite tissue DNA extracted from Echinococcus and Taenia spp. used to assess the specificity of Echinococcus shiquicus, E. granulosus G1, and E. multilocularis PCR assays

Assessment of specificity using coproDNA.

A subset of the coproDNA extracted from canid fecal samples was used to assess the copro-specificity of the three PCR assays. CoproDNA amplified using the E. granulosus G1 (N = 10) and E. multilocularis (N = 11) PCR was used to determine the copro-specificity of the E. shiquicus primers. The E. granulsous G1 primers were checked for specificity using coproDNA extracted from feces of E. multilocularis (N = 11) or E. shiquicus (N = 3) parasitologically infected hosts that had also amplified using the E. multilocularis and E. shiquicus PCR assays, respectively. In a similar manner E. shiquicus (N = 3) and E. granulosus G1 (N = 5) coproDNA positive samples amplified using the E. shiquicus and E. granulosus G1 PCR was used to test specificity of the E. multilocularis primers. In addition, DNA extracted from feces of dogs experimentally infected with T. multiceps and verified by sequencing was used. Furthermore, sequenced coproDNA extracted from dogs naturally infected with D. caninum and Mesocestoides corti collected at necropsy (Tunisia) was used to assess the copro-specificity of the three PCR assays.

Evaluation of PCR detection sensitivity.

Detection sensitivity for the PCR assays was determined by using twofold serial dilutions (2,500–0.6pg) of tissue DNA extracted from adult tapeworms of E. shiquicus, protoscoleces of E. granulosus, and adults of E. multilocularis. Three separate sets of dilutions were tested per PCR assay. In addition, the detection limit of the E. granulosus G1 PCR was evaluated using a negative fecal sample (1 gram) spiked with 1–5, 10, 100, or 1,000 E. granulosus eggs isolated from worms retrieved from a naturally infected Chinese dog. Echinococcus granulosus DNA extracted from feces collected 28–37 days post-infection from experimentally infected Australian dingoes (N = 10) was also used to determine copro-sensitivity and to investigate pre-patent DNA detection. These dingoes formed part of a study that was approved by the Animal Experimentation Ethics Committee (AEEC) of the Australian National University, Canberra, Australia. They were given a standard dose of 40,000 protoscoleces from cysts contained in livers and lungs of sheep slaughtered in an abattoir located in Goulburn NSW, Australia (DJ). Copro-sensitivity of the E. multilocularis PCR assay was assessed using DNA extracted from fecal samples of naturally infected Tibetan dogs (N = 6) and French red foxes (V. vulpes) (N = 16) from which worm burdens of E. multilocularis had been counted after purgation or at necropsy, respectively (Table 4). In the case of the Tibetan dogs, fecal samples were collected before purging.

Table 4
Sensitivity of Echinococcus multilocularis ND1 primers in detecting coproDNA extracted from fecal samples of Chinese purged dogs and French necropsied red foxes

Ethanol precipitation of coproDNA.

Ethanol precipitation was carried out whenever coproDNA amplification was negative. It was assumed that there was no target DNA, too little target DNA in a given fecal sample or that the presence of inhibitors and/or non-target DNA may have interfered with the amplification. To illustrate the effect of ethanol precipitation on PCR amplification, nine necropsy-confirmed coproDNA samples of E. shiquicus infected Tibetan foxes (V. ferrilata), which were initially negative by PCR were tested. CoproDNA was precipitated using Pellet Paint Co-precipitant (Merck, Merck Chemicals, Nottingham, UK) according to the manufacturer's instructions. Concentrated DNA pellets were re-eluted in Qiagen Stool Kit elution buffer (elution volume depended on the initial sample volume and concentration factor) and between 3 and 10 μL were used as template in a new PCR reaction. If samples were still negative following ethanol precipitation they were diluted and retested to remove PCR-inhibitory substances.

Validation of ND1 PCR assays.

The E. granulosus G1 PCR and the E. multilocularis PCR tests developed at the University of Salford and were assessed by our collaborator (G. Umhang) in their independent laboratory for specificity and detection sensitivity. This was carried out to overcome inter-laboratory discrepancies that may be generated as a result of the use of different PCR machines and/or reagents and allow for final optimization if necessary.

The DNA extracted from E. granulosus G1 protoscoleces and E. multilocularis adults was used as controls in the two PCR assays (G1 PCR, Em PCR). A Veriti thermal cycler (Applied Biosystems, Inc.) was used and PCR products were resolved on a 1% (w/v) agarose gel (Promega Ltd.) in 5× Tris-Borate-EDTA buffer (Promega Ltd.) at 110 V, stained using SYBR safe DNA stain, and visualized using Gel Doc XR (Invitrogen). Independent panels of parasite species-derived DNA were used by the French laboratory. Specificity of the E. granulosus G1 and E. multilocularis PCR assays was assessed using tissue DNA extracted from the following species (stage, concentration, host): E. equinus (protoscoleces, 59 ng/μL, horse), E. canadensis G6 (protoscoleces, 5.69 ng/μL, camel), T. hydatigena (cysts, 12.2 ng/μL, sheep), T. pisiformis (adult, 1.73 ng/μL, V. vulpes), T. taeniaeformis (adult, 36.9 ng/μL, V. vulpes), Mesocestoides sp. (adult, 7.27 ng/μL, V. vulpes) and Toxocara sp. (adult, 5.5 ng/μL, V. vulpes). The DNA of the previous parasites was obtained from laboratory panels that had been verified by sequencing. The level of detection of the E. granulosus G1 PCR and the E. multilocularis PCR was assessed using tissue-derived DNA ranging from 10 ng to 10 pg.

Results

Sequencing of ND1 gene product.

Sequencing of cloned fragments for each of the three uniplex PCR assays was undertaken to verify the identity of the generated amplicons. A 99% homology to E. shiquicus (GenBank accession no. AB159137) ND1 mitochondrial fragment was obtained when the E. shiquicus amplified product was blasted against the database. The products amplified by the E. granulosus G1 and the E. multilocularis PCR assays showed 100% homology for a fragment within the E. granulosus G1 genotype (GenBank accession no. HM636643) and E. multilocularis (GenBank accession no. EU704124) ND1 mitochondrial gene, respectively. Nucleotide sequences of the fragments amplified by the E. shiquicus, E. granulosus, and E. multilocularis PCR assays were deposited into GenBank under the following accession nos.: JN371772, JN371770, and JN371771, respectively.

Parasite tissue and coproDNA amplification.

The performance of the three uniplex PCR assays (Es PCR, G1 PCR, Em PCR) was 100% in terms of the detection of parasite tissue-derived DNA from the confirmed respective Echinococcus species. The respective diagnostic products for E. shiquicus (442 bp), E. granulosus (226 bp), and E. multilocularis (207 bp) were generated when DNA extracted from metacestode cysts of intermediate hosts, adult worms from definitive hosts, or from human infection was used as template (Figure 1). A similar picture was observed with coproDNA extracted from fecal samples derived from purged or necropsied confirmed naturally infected canids. Representative examples of copro-amplification are shown in Figure 1.

Figure 1.
Performance of three uniplex polymerase chain reaction (PCR) assays, (A) E. shiquicus (Es PCR), Lane M, DNA marker, lanes 1–2, tissue-derived DNA from adults, lanes 3–6, coproDNA from necropsied Tibetan foxes, lane 7, positive control, ...

Specificity of three uniplex PCR assays (Es PCR, G1 PCR, Em PCR) using parasite DNA and coproDNA panels.

Using both parasite tissue-derived DNA and coproDNA as template, 100% specificity was observed when DNA from E. shiquicus, E. granulosus, or E. multilocularis was checked against each other (Figures 2 and and3).3). Furthermore, no signals were observed when DNA of E. granulosus G1 and E. multilocularis was used against the E. shiquicus primers (Figures 2A and and3A).3A). The E. granulosus (G1) ND1 primers did not amplify DNA of E. multilocularis or E. shiquicus (Figures 2B and and3B).3B). Similarly, the E. multilocularis uniplex PCR did not amplify DNA of E. granulosus G1 or E. shiquicus origin (Figures 2C and and3C).3C). In addition, negative signals were obtained when tissue or coproDNA of T. multiceps and D. caninum or coproDNA of M. corti was used to check cestode genus specificity of the assays (Figures 2 and and3).3). The three sets of primers (Es PCR, G1 PCR, Em PCR) were also specific against tissue DNA extracted from T. hydatigena, T. pisiformis, T. ovis or T. crassiceps (Figure 2A2C). Echinococcus shiquicus primer specificity was compromised only against E. equinus (G4) DNA, when a faint positive signal was observed (Figure 2A). Echinococcus granulosus G1 and E. multilocularis primers gave negative signals when tested against DNA extracted from E. granulosus G3 (Buffalo strain), E. equinus (G4), E. ortleppi (G5), or E. canadensis (G6, G7, G8, and G10) parasite tissue isolates (Figure 2B and and2C).2C). The E. granulosus G1 and E. multilocularis primers were 100% specific in the hands of our collaborator (GU) including against tissue derived DNA of T. taeniaeformis and Toxocara sp.

Figure 2.
Specificity of three uniplex ND1 PCR assays; (A) Echinococcus shiquicus (Es PCR), (B) E. granulosus G1 (G1 PCR), and (C) E. multilocularis (Em PCR). Lane M, DNA marker, lanes 1–7, tissue-derived DNA of Taenia multiceps, T. hydatigena, T. pisiformis ...
Figure 3.
Copro-specificity of three ND1 PCR assays; (A) Echinococcus shiquicus (Es PCR), Lane M, DNA marker, lanes 1–4, E. granulosus coproDNA from positive Welsh, Australian, Kenyan, and Kazak dogs, respectively, lanes 5-6, E. multilocularis-positive ...

PCR detection sensitivity.

Sensitivity for DNA detection limit was checked using dilutions of E. shiquicus, E. granulosus, and E. multilocularis tissue-derived DNA ranging from 2500–0.6pg. The E. shiquicus and E. multilocularis assays detected 2–10 pg of DNA (Figure 4A and and4C).4C). Echinococcus granulosus primers were capable of detecting between 5 and 10 pg of DNA (Figure 4B) This is in agreement with the results obtained by our collaborator (GU), which showed in his laboratory the detection level of E. granulosus and E. multilocularis to be 10 pg.

Figure 4.
Sensitivity of three uniplex ND1 PCR assays. The DNA detection limit using a twofold serial dilution of DNA extracted from adult tapeworms of (A) E. shiquicus (Es PCR), (B) protoscoleces of E. granulosus G1 (G1 PCR), and (C) adults of E. multilocularis ...

CoproPCR detection limit.

Echinococcus granulosus PCR primers (G1 PCR) were able to amplify DNA from negative dog fecal samples spiked with at least 1–5 eggs of E. granulosus (Figure 5). They were also able to detect pre-patent E. granulosus infections in dogs from 30 to 37 days post experimental infection (Figure 6). The sensitivity of the E. multilocularis primers (Em PCR) in detecting coproDNA extracted from red fox fecal samples with known worm burdens is shown in Table 4. The E. multilocularis primers detected DNA from fox feces with at least 45 worms. They were also able to amplify DNA from feces of a French fox that harbored immature adult infections (> 30,000 non-gravid E. multilocularis worms).

Figure 5.
Detection of Echinococcus granulosus DNA in egg-spiked fecal samples using G1 PCR. Lane M, DNA marker, lanes 1–4, represent coproDNA extracted from 1 gram of feces spiked with 1–5, 10, 100, and 1,000 eggs, respectively, lane 5, negative ...
Figure 6.
Sensitivity of Echinococcus granulosus PCR assay (G1 PCR) in detecting prepatent infections in experimentally infected Australian dingos. Lane M, DNA marker, lanes 1–10, coproDNA extracted 28–37days post infection, lane 11, positive control, ...

Ethanol precipitation of coproDNA.

To increase the concentration of target DNA in an extracted fecal sample, ethanol precipitation was used to maximize the chances of PCR amplification. Eight of the nine E. shiquicus fox necropsy positive coproDNA samples that were initially negative by PCR, gave positive signals with the amplification of the 442 bp diagnostic product following ethanol precipitation (Figure 7, ,1a1a and and2a).2a). In addition, when one of these ethanol precipitated fecal samples, which had only faintly amplified, was diluted and re-tested by PCR, a very bright band was obtained suggesting reduction of inhibition had occurred after dilution (Figure 7, ,1b1b and and2b2b).

Figure 7.
Effect of DNA ethanol precipitation and dilution on detection of Echinococcus shiquicus coproDNA (Es PCR). Lane M, DNA marker. CoproDNA extracted from Tibetan foxes, pre-ethanol (1a, 3a, and 3b) and post-ethanol precipitation (2a, 4a, and 4b); post-ethanol ...

PCR reproducibility.

The independent laboratory validation of the E. granulosus G1 and E. multilocularis ND1 PCR assays further confirmed the specificity of these two assays. Furthermore, the sensitivity level of both sets of primers was 10 pg of tissue-derived DNA.

Discussion

Human cystic (CE) and alveolar (AE) echinococcosis are co-endemic in Tibetan communities and in extensive high pasture areas of Sichuan and Qinghai Provinces of western China where local prevalence levels range from < 1 to > 9% (mean 3%) for both CE and AE.3 The Tibetan pastoral lifestyles are such that close associations with domestic or wild canid definitive hosts of E. shiquicus, E. granulosus, and E. multilocularis are maintained.16 Although the infectivity (if any) of E. shiquicus to humans is unknown, the availability of a specific copro-detection test for this fox parasite and its differentiation from the other two Echinococcus species (E. granulosus and E. multilocularis) occurring on the Tibetan plateau is useful for epidemiological studies.

Diagnosis of Echinococcus species in definitive hosts is complicated by the fact that dog and fox hosts are frequently concurrently infected with Taenia species and/or other tapeworm species.18,19 Eggs of Taenia species in particular are morphologically identical under the light microscope to those of Echinococcus species. In addition, Taeniidae family members of both Echinococcus and Taenia genera are genetically similar and hence specific molecular diagnosis can prove to be challenging. The detection of Echinococcus species DNA whether in tissue or feces has historically relied on the use of the 12S rRNA gene.8,1214 However, other genes such as the U1 snRNA gene,5 cytochrome oxidase subunit 1,7 and a tandem repeat region11 have also been targeted. Our choice of the NADH mitochondrial gene (ND1) was taken to capitalize on the variability at the species level found within the Echinococcus genus.20,21 The usefulness of the ND1 gene in differentiating between closely related species has been recently applied for the molecular detection of Echinococcus species and genotypes DNA from formalin fixed/paraffin embedded clinical samples and for differentiation between cestode eggs.22,23

Using defined DNA panels extracted from parasite tissue or infected canid fecal samples, three PCR assays based on an ND1 gene sequence were developed for the detection of E. shiquicus (Es PCR), E. granulosus G1 (G1 PCR), and E. multilocularis (Em PCR). The number of samples used per panel in this work is comparable to those used in similar studies.1113 The E. shiquicus primers were shown to be highly species-specific with no PCR products detected when parasite DNA from the other related Echinococcus species or Taenia species were tested except for the amplification of E. equinus DNA. Although horses are frequent on the Tibetan plateau, to date E. equinus has not been described from either intermediate or definitive hosts from the plateau or indeed from any other part of China. Currently, the only E. granulosus species/genotypes recorded in China are E. granulosus G1 and G6 (E. canadensis).24,25 The possibility of cross-reactions with E. equinus if it occurs, cannot be ruled out but is not considered a likely occurrence.

Initially, a multiplex assay for the concurrent detection of E. shiquicus, E. granulosus, and E. multilocularis was developed (data not shown). However, because of the limited sensitivity of this multiplex, the three uniplex ND1 assays described here were used. These PCR assays (Es PCR, G1 PCR, Em PCR) were capable of detecting at least 2–10 pg of Echinococcus DNA. Similar results were obtained by our collaborator's (GU) laboratory where the E. granulosus G1 and E. multilocularis assays were able to detect 10 pg of tissue-derived DNA. The E. granulosus G1 primers were able to detect at least 1–5 eggs, which would equate to 8–45 pg of DNA.26 No such spiked fecal samples were available for E. multilocularis PCR primers, and instead we based our detection on fecal samples from necropsied red foxes with known worm burdens. This indicated that E. multilocularis primers appeared to require an infection of at least 45 adult E. multilocularis worms for a positive coproPCR signal to be observed. However, the sensitivity of the E. shiquicus, E. granulosus, and E. multilocularis PCR primers may differ when coproDNA rather than DNA derived from parasite tissue is assessed and would largely depend upon the quality and quantity of the template, the absence of inhibitors as well as the quantity of non-target DNA co-extracted with the Echinococcus species target DNA.15

Although no similar spiked fecal samples were available to test the copro-sensitivity of the E. shiquicus primers, the ethanol precipitation of PCR negative fecal samples from necropsy positive E. shiquicus foxes, increased the detection of E. shiquicus in fecal samples from 0% to 89%. The fecundity of E. shiquicus is known to be lower (< 100 eggs in a gravid segment)4 than that of E. granulosus or E. multilocularis and thus the amount of DNA in fecal samples would likely to be significantly reduced. Many methods have been used to improve the recovery of DNA from feces through the extraction of DNA directly from Echinococcus species eggs.2729 It should be emphasized that the PCR assays developed here detected coproDNA, which includes egg and parasite tissue-derived DNA. The use of 1–2 grams of feces as starting material and the use of ethanol precipitation of initially negative DNA fecal samples helped increase assay sensitivity. Using this ethanol precipitation method we have documented the first record of E. shiquicus DNA in dogs from Shiqu County.30

In the current study, we have also shown that the E. granulosus ND1 primers were capable of detecting prepatent infections, i.e., without the need for the presence of eggs. Furthermore, although based on fecal samples from one fox with > 30,000 worms, a similar pre-patent detection was observed for the E. multilocularis ND1 assay. This has previously been documented for Taenia and Echinococcus DNA15,3133 and may be useful in the surveillance of control programs when definitive host exposure is reduced.15 The development of these three uniplex ND1 PCR tests (i.e., Es PCR, G1 PCR, Em PCR) will serve to improve the detection and diagnosis of E. shiquicus, E. granulosus, and E. multilocularis, especially in definitive hosts. This will enable further investigation of the transmission biology of E. shiquicus and the epidemiology of cystic and alveolar echinococcosis in the unique highly endemic region of the eastern Tibetan Plateau, China.

ACKNOWLEDGMENTS

We thank the following individuals for material used in this study. Christine Budke (dog fecal samples, Shiqu County, Sichuan, China), Paul Torgerson (dog fecal samples experimentally infected with T. multiceps and necropsied dogs from Kazakhstan), Francis Raoul (E. multilocularis negative fox fecal samples, France), Imad Buishi (E. granulosus necropsy positive and negative samples from Kenya and Libya; E. granulosus natural infections from Wales, UK), Maiza Campos-Ponce (E. granulosus spiked fecal samples), Adrian Casulli (DNA of E. granulosus G3 Buffalo strain, Italy), Kevin Shaddick (E. equinus protoscoleces from horse hydatid, Bristol abattoir, Bristol, UK), Abullah Rafiei (E. canadensis G6 protoscoleces from camel hosts, Iran), P. Dubinsky (E. canadensis G7 protoscoleces and germinal layer, Slovak Republic), D. McManus (E. canadensis G8 protoscoleces, Minnesota, USA), A. Lavikainen and A. Oksanen (E. canadensis G10 protoscoleces, Finland), W. Rebaï and Z. Ben Safta (E. granulosus metacestodes from confirmed CE patients, Hopital La Rabta, Tunis), Li Tiaoying (human metacestodes of E. granulosus and E. multilocularis from China), Akira Ito (molecular identification of E. shiquicus adult worms from necropsied Tibetan foxes), Bruno Gottstein for providing E. equinus and Taenia taeniaeformis to our collaborator, Ariel Nadich in Buenos Aires (for help with validation of earlier primer prototypes), and Benoit Combes (for support in Malzeville). We also acknowledge the inclusion of samples commercially analyzed through Cestode Diagnostics at the University of Salford (ku.ca.droflas@scitsongaidedotsec). These were E. granulosus G1 sheep germinal layer from the Falkland Islands (Steve Pointing, Ministry of Agriculture, Port Stanley), E. granulosus G1 metacestode from a guenon monkey (Nic Masters, UK), E. multilocularis metacestode from a macaque monkey (Mark Stidworthy, UK), and E. ortleppi G5 metacestode from a Philippine spotted deer (Aiden Foster and Suzi Bell, AHVLA; Angela Potter and Bob Lawrence, West Midlands Safari Park, UK). Finally, we thank Qiu Dongquan of Sichuan CDC for help in facilitating the collection of samples in China.

Footnotes

Financial support: The research described here was supported by a grant (RO1 TW001565) from the National Institutes of Health and Natural Science Foundation (USA) Ecology of Infectious Diseases Program and by the Sichuan Centre for Disease Control, Chengdu, China.

Authors' addresses: Belgees Boufana and Philip Craig, Cestode Zoonoses Research Group, School of Environment and Life Sciences, University of Salford, United Kingdom, E-mails: ku.ca.droflas@anafuoB.B and ku.ca.droflas@giarC.S.P. Gérald Umhang and Franck Boué, Anses Rabies and Wildlife Laboratory, Technopole Agricole et Veterinaire, Malzeville, France, E-mails: rf.sesna@GNAHMU.dlareG and rf.sesna@EUOB.knarF. Jiamin Qiu and Xingwang Chen, Institute of Parasitic Diseases, Sichuan Centre for Disease Control and Prevention, Chengdu, Sichuan, China, E-mails: moc.361@54nimaijuiq and moc.uhos@820gnawgnixnehc. Samia Lahmar, Service de Parasitologie, Ecole Nationale de Medecine Veterinaire, Thabet, Tunisia, E-mail: rf.oohay@aimaslrd. David Jenkins, School of Animal and Veterinary Sciences, Charles Sturt University, New South Wales, Australia, E-mail: ua.ude.usc@sniknejjd.

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