Mutation scanning methods for the analysis of parasite genes

Rapid identification of cyst (Heterodera spp., Globodera spp.) and root-knot (Meloidogyne spp.) nematodes on the basis of ITS2 sequence variation detected by PCR-single-strand conformational polymorphism (PCR-SSCP) in cultures and field samples

Cyst and root-knot nematodes show high levels of gross morphological similarity. This presents difficulties for the study of their ecology in natural ecosystems. In this study, cyst and root-knot nematode species, as well as some ectoparasitic nematode species, were identified using the second internal transcribed spacer (ITS2) sequence variation detected by polymerase chain reaction-single-strand conformational polymorphism (PCR-SSCP). The ITS2 region was sufficiently variable within the taxa investigated to allow species to be separated on the basis of minor sequence variation. The PCR primers used in this study were effective for 12 species with three genera within the Heteroderinae (Globodera pallida, G. rostochiensis, Heterodera arenaria/avenae, H. ciceri, H. daverti, H. hordecalis, H. mani, H. schachtii, H. trifolii, Meloidogyne ardenensis, M. duytsi and M. maritima). However, pathotypes of Globodera pallida and G. rostochiensis could not be distinguished. The method was tested at two coastal dune locations in The Netherlands (one in the lime-poor dunes of the north and one in calcareous dunes of the south) to determine the population structure of cyst nematodes. At each site, cyst nematodes were associated with three plant species: two plant species on the foredune (Elymus farctus and Ammophila arenaria) and one plant species occurring further inland (Calamagrostis epigejos). Two species of cyst nematodes, H. arenaria and H. hordecalis, were found. H. arenaria associated with vigorous A. arenaria and H. hordecalis in association with degenerating A. arenaria and C. epigejos. The field survey demonstrated that in coastal dunes abiotic factors may be the important affecting the distribution of cyst nematodes.

Genetic heterogeneity of ribosomal internal transcribed spacer in clinical samples of Leishmania donovani spotted on filter paper as revealed by single-strand conformation polymorphisms and sequencing

A polymerase chain reaction and single-strand conformation polymorphism determination (PCR-SSCP) was used to detect deoxyribonucleic acid sequence polymorphisms in the transcribed non-coding regions between the small and large sub-unit ribosomal ribonucleic acid (rRNA) genes in Leishmania donovani from 63 clinical samples collected in eastern Sudan, between April 1997 and October 1998. Specific Leishmania primers were used to amplify the internal transcribed spacer (ITS) regions of L. donovani isolates directly from clinical samples spotted on filter papers. Amplification products were subsequently analysed by SSCP. Eleven polymorphic patterns were detected in the first part of the spacer, the ITS1 region, and were sequenced. Most of the changes were due to deletions of adenine bases and AT pairs within the first 192 nucleotides of the ITS region. This is the first application of PCR-linked SSCP analysis for the detection of population variation with direct display of sequence variation in parasitologically positive clinical samples spotted on filter paper. Culturing the parasite is thus not required, which is beneficial particularly in epidemiological studies based on field work where obtaining cultures can be extremely difficult.

Single-strand restriction fragment length polymorphism analysis of the second internal transcribed spacer (ribosomal DNA) for six species of Eimeria from chickens in Australia

Species of Eimeria from chickens from Australia were characterised using a polymerase chain reaction-linked restriction fragment length polymorphism (PCR-RFLP) approach. The ribosomal DNA region spanning the second internal transcribed spacer (ITS-2) was amplified from genomic DNA by PCR, digested separately with three restriction endonucleases (CfoI, Sau3AI and TaqI) and the fragments separated by denaturing gel electrophoresis. The PCR products amplified from the six species varied from approximately 70 to 620 bp on agarose gels, with differences in size and number of bands among species, but no apparent variation within a species. The PCR-RFLP analysis of ITS-2 amplicons on denaturing gels gave characteristic profiles for individual species (except for minor variation in profiles within some species). The results indicate that ITS-2 contains useful genetic markers for the identification of six Eimeria species occurring in Australia.

Sequence and secondary structure variation in the Gyrodactylus (Platyhelminthes: Monogenea) ribosomal RNA gene array

Nucleotide sequences were determined for the rRNA internal transcribed spacers 1 and 2 (ITS1 and 2) and the 5' terminus of the large subunit rRNA in selected Gyrodactylus species. Examination of primary sequence variation and secondary structure models in ITS2 and variable region V4 of the small subunit rRNA revealed that structure was largely conserved despite significant variation in sequence. ITS1 sequences were highly variable, and models of structure were unreliable but, despite this, show some resemblance to structures predicted in Digenea. ITS2 models demonstrated binding of the 3' end of 5.8S rRNA to the 5' end of the large subunit rRNA and enabled the termini of these genes to be defined with greater confidence than previously. The structure model shown here may prove useful in future phylogenetic analyses.

Genomic and genetic research on bursate nematodes: significance, implications and prospects

Molecular genetic research on parasitic nematodes (order Strongylida) is of major significance for many fundamental and applied areas of medical and veterinary parasitology. The advent of gene technology has led to some progress for this group of nematodes, particularly in studying parasite systematics, drug resistance and population genetics, and in the development of diagnostic assays and the characterisation of potential vaccine and drug targets. This paper gives an account of the molecular biology and genetics of strongylid nematodes, mainly of veterinary socio-economic importance, indicates the implications of such research and gives a perspective on genome research for this important parasite group, in light of recent technological advances and knowledge of the genomes of other metazoan organisms.

PCR assay for the specific amplification of Oesophagostomum bifurcum DNA from human faeces

Oesophagostomiasis in humans due to infection with Oesophagostomum bifurcum (nodule worm) is of major human health significance in northern Togo and Ghana where the human hookworm, Necator americanus, also exists at high prevalence. Accurate diagnosis of O. bifurcum infection in humans is central to studying the epidemiology and controlling the parasite. To overcome limitations of current copro-diagnostic methods, we have developed an alternative, molecular approach. Utilising genetic markers in the second internal transcribed spacer (ITS-2) of ribosomal DNA, we have established a two-step, semi-nested PCR method for the specific amplification of minute amounts (fg) of O. bifurcum DNA from human faecal samples. Using a panel of 155 well-defined faecal and DNA samples, the assay achieved a sensitivity of 94.6% and a specificity of 100%. This PCR assay will be useful for the diagnosis of O. bifurcum infection and as a molecular tool for elucidating the epidemiology of human oesophagostomiasis.

Intraspecific and interspecific variation in the second internal transcribed spacer (ITS-2) sequence for Metastrongylus (Nematoda: Metastrongyloidea) detected by high resolution PCR-RFLP

Metastrongylus species are important parasites of free-range pigs and wild boar, but little is known about the genetic make-up of natural populations. This study was undertaken to examine sequence variation in the internal transcribed spacer 2 of ribosomal DNA within and among three species of Metastrongylus using PCR-linked restriction fragment length polymorphism analysis. In contrast to many other species of bursate nematodes, significant intraspecific variation was detected in restriction fragment length polymorphism profiles among individual worms. In spite of this, it was possible to identify the three species by their distinctive restriction profiles. The findings suggest that the internal transcribed spacer 2 region should be useful for analysing population variation within Metastrongylus species.

Sequence-based analysis of enzymatically amplified DNA fragments by mutation detection techniques

The accurate analysis of molecular variation is important in a range of disciplines of parasitology. Although conventional DNA techniques have overcome some of the limitations of traditional approaches, some can be relatively expensive and/or cumbersome to use when large sample sizes require analysis, and some cannot accurately resolve or define nucleotide variation. Using selected examples of applications to parasites, Robin Gasser and Xingquan Zhu discuss some PCR-based mutation detection techniques and their advantages over conventional analytical methods.

Genotyping Taenia tapeworms by single-strand conformation polymorphism of mitochondrial DNA

To overcome limitations in identifying tapeworms of the genus Taenia by traditional approaches, we have established a single-strand conformation polymorphism (SSCP) method utilizing two different regions of mitochondrial (mt) DNA as targets. The NADH dehydrogenase 1 and the cytochrome c oxidase subunit I genes were amplified from genomic DNA by polymerase chain reaction (PCR), denatured and subjected to electrophoresis in mutation detection enhancement gels. SSCP analysis achieved delineation among eight different species of Taenia from different hosts based on characteristic profiles and enabled the detection of intraspecific variability in profiles for some taxa. This SSCP-based typing method has important implications for taxonomy, diagnosis and for studying the genetic structure of Taenia populations.

Isolation, propagation and characterisation of Cryptosporidium

This review consists of 11 papers presented at the Consensus Conference on Cryptosporidium in Water (Parasitology Stream), held in Melbourne, Australia, from 5 to 6th October 1998. The conference was sponsored by the Water Services Association of Australia, the Australian Water and Wastewater Association, and the Collaborative Research Centre for Water Quality and Treatment. The papers summarise the advantages and disadvantages of various contemporary technologies applicable to parasite propagation and biochemical/molecular characterisation. Studies have detected distinct genetic differences between clinical isolates from humans and animals, and it is hoped that comprehensive documentation studies will facilitate the identification of environmental isolates in the not too distant future.

PCR-based technology in veterinary parasitology

DNA technology is having a major impact in many areas of veterinary parasitology. In particular, the polymerase chain reaction (PCR) has found broad applicability because its sensitivity permits enzymatic amplification of gene fragments from minute quantities of nucleic acids derived from limited amounts of parasite material. This paper discusses some recent applications of PCR-based methods to parasites and highlights their usefulness or potential for those of veterinary importance. The focus is on PCR tools for the accurate identification of parasites and their genetic characterisation, the diagnosis of infections, the isolation and characterisation of expressed genes, the detection of anthelmintic resistance, and mutation scanning approaches for the high resolution analysis of PCR products.

Screening for nucleotide variations in ribosomal DNA arrays of Oesophagostomum bifurcum by polymerase chain reaction-coupled single-strand conformation polymorphism

We exploited the high resolution capacity of polymerase chain reaction (PCR)-coupled single-strand conformation polymorphism (SSCP) to screen for sequence variation in the second internal transcribed spacer (ITS-2) of ribosomal DNA (rDNA) among 77 individuals representing Oesophagostumum bifurcum from human or Mona monkey hosts from Africa. SSCP analysis revealed distinct profiles among some of the individuals, and sequence analysis of representative samples defined different ITS-2 sequence types attributable to polymorphism at particular nucleotide positions. However, there was no unequivocal sequence difference between O. bifurcum individuals from humans and that from monkeys. This provided some support for the hypothesis that the parasite from the two hosts represents a single species and that the sequence microheterogeneity detected in the ITS-2 rDNA region represents population variation, although the findings were insufficient to reject the proposal that the parasite represents different species. Overall, the results showed the usefulness of the SSCP-sequencing approach for studying the genetic variation in O. bifurcum populations and indicated its potential to study macromolecular evolution and elucidate population differentiation at the molecular level.

Screening for different genotypes of Echinococcus granulosus within China and Argentina by single-strand conformation polymorphism (SSCP) analysis

Single-strand conformation polymorphism (SSCP) analysis was employed for the direct visual display of genetic variability in mitochondrial DNA (mtDNA) fragments within and among populations of Echinococcus granulosus from the People's Republic of China and from Argentina. Fragments of the NADH dehydrogenase I gene (NDI) and the cytochrome c oxidase subunit I (COI) were individually amplified from parasite DNA by polymerase chain reaction, denatured and subjected to SSCP analysis. Using NDI and COI fragments, samples representing different genotypes could be readily identified based on characteristic SSCP profiles. The results demonstrate the utility of SSCP for the direct visual display of nucleotide variation in mtDNA of E. granulosus prior to DNA sequence analysis. The approach compares favourably with existing genotyping procedures and provides a reliable and technically reproducible method for the routine laboratory identification of Echinococcus isolates.

Characterisation of Ascaris from human and pig hosts by nuclear ribosomal DNA sequences

The sequences of the nuclear ribosomal DNA region spanning the first internal transcribed spacer, the 5.8S rRNA gene and the second internal transcribed spacer were determined for Ascaris samples from pigs and humans from different geographical regions. The sequences of the 5.8S gene and the second internal transcribed spacer were the same for all samples examined, whereas all Ascaris samples from humans had six (1.3%) nucleotide differences in the first internal transcribed spacer compared with those from pigs. These differences provided some support for the existence of separate species of Ascaris or population variation within this genus. Using a nucleotide difference within a site for the restriction enzyme HaeIII, a PCR-linked restriction fragment length polymorphism method was established which allowed the delineation of the Ascaris samples from pigs and humans used herein. Exploiting the sequence differences in the first internal transcribed spacer, a PCR-based single-strand conformation polymorphism method was established for future analysis of the genetic structure of pig and human Ascaris populations in sympatric and allopatric zones.

Molecular evidence for cryptic species within Cylicostephanus minutus (Nematoda: Strongylidae)

The nucleotide sequences of the first and second internal transcribed spacers of nuclear ribosomal DNA were determined for adults of Cylicostephanus minutus from different geographical origins. The lengths of first and second internal transcribed spacer sequences ranged from 370 to 372bp and 215 to 216bp, respectively. Pairwise sequence comparisons revealed that some individuals of C. minutus had identical first and second internal transcribed spacer sequences, whereas others differed by 3.0% and 7.4% in their first and second transcribed spacers, respectively. Some individuals with sequence differences originated from the same host. The levels of difference within C. minutus were higher than that between the morphologically distinct species, Cylicostephanus goldi and Cylicostephanus longibursatus (0.8% for the first internal transcribed spacer and 3.8% for the second internal transcribed spacer). The data provide support for the proposal that C. minutus represents a complex of at least two species. In order to study the population genetic structure of C. minutus, a PCR-linked single-strand conformation polymorphism technique was also established.

Multilocus enzyme electrophoresis: a valuable technique for providing answers to problems in parasite systematics

The aim of this review is to highlight the effectiveness of the technique of multilocus enzyme electrophoresis in answering questions relating to the systematics of parasites and to highlight errors in the way the technique has been used and the results interpreted. We have approached this topic by answering specific questions that we have been asked by colleagues and students not necessarily familiar with the technique, the method of data analysis and its application. Although the technique has been applied to provide answers for taxonomic and population genetics studies, it remains under-utilised, perhaps because of recent advances in newer molecular technology. Rather than not acknowledge or dismiss the value of more traditional technology, we suggest that researchers examine problems in the systematics of parasites by the comparison of data derived from morphological, biochemical and molecular techniques.

Distinguishing Oesophagostomum dentatum from Oesophagostomum quadrispinulatum developmental stages by a single-strand conformation polymorphism method

At some life-cycle stages, it is impossible to distinguish between the two species of porcine nodular worm, Oesophagostomum dentatum and Oesophagostomum quadrispinulatum, using morphological features. A PCR-based single-strand conformation polymorphism technique was established to overcome this limitation. The rDNA region spanning the second internal transcribed spacer was amplified by PCR from genomic DNA from morphologically well-defined adult worms. The PCR products were then denatured and subjected to electrophoresis in a non-denaturing gel matrix. Single-strand conformation polymorphism analysis of the products generated characteristic and reproducible patterns for each of the two species and allowed their unequivocal delineation. The single-strand conformation polymorphism was also applied effectively to assess the purity of nine laboratory-maintained cultures of infective third-stage larvae believed to be monospecific for O. dentatum or O. quadrispinulatum. The analysis showed that all six O. dentatum cultures were indeed monospecific, whereas the three cultures believed to be monospecific for O. quadrispinulatum were either a mixture of O. dentatum and O. quadrispinulatum larvae or pure O. dentatum larvae. These findings demonstrated the usefulness of the single-strand conformation polymorphism approach for the routine monitoring of the purity of parasite "lines" and indicated its value for studies on the population biology of porcine nodular worms.

Characterisation of anisakid nematodes with zoonotic potential by nuclear ribosomal DNA sequences

Larvae of three species of anisakid nematode from fish, Anisakis simplex, Hysterothylacium aduncum and Contracaecum osculatum, were characterised genetically using a molecular approach. The nuclear ribosomal DNA region spanning the first internal transcribed spacer, the 5.8S gene and the second internal transcribed spacer was amplified and sequenced. The lengths of the first and second internal transcribed spacer sequences of the three species ranged from 392 to 449 bp and 262 to 347 bp, respectively, whereas the 5.8S sequence was 157 bp. For the three species, the G+C contents for the three regions of ribosomal DNA ranged from 42.4 to 52.2%. While no intraspecific variation was detected in the second internal transcribed spacer or 5.8S sequence of any species examined, one polymorphic nucleotide position was detected in the first internal transcribed spacer sequence for A. simplex and H. aduncum. The extent of sequence differences in the first (approximately 34-45%) and second (approximately 50-53%) internal transcribed spacers among the species was greater than in the 5.8S gene (approximately 3-5%). Based on the sequence differences, PCR-based restriction fragment length polymorphism and single-strand conformation polymorphism methods were established for the unequivocal delineation of the three species. These methods should provide valuable tools for studying the life-cycle, transmission pattern(s) and population structure of each of the three anisakid nematodes examined herein, and for the diagnosis of anisakiasis in humans and animals.

Dideoxy fingerprinting: application to the genotyping of Echinococcus

Dideoxy fingerprinting is an efficient method for the detection of sequence variation in PCR-amplified DNA segments. It is a hybrid between single-strand conformation polymorphism and dideoxy sequencing, employing only one dideoxynucleotide in the sequencing reaction. Herein, we report the application of dideoxy fingerprinting to genetically type cestodes of the genus Echinococcus, utilising the mitochondrial cytochrome c oxidase subunit I as the gene sequence for analysis. All of the seven genotypes (G1, G4, G6, G8, O, V and M2) examined could be readily differentiated from one another by their characteristic and reproducible dideoxy fingerprinting profiles. Only subtle variation in profiles was detected among some of the eight isolates representing genotype G1, and no variation was detected between two samples of genotype G4 and of genotype M2. The capacity of dideoxy fingerprinting to detect all nucleotide variations over 150-250bp fragments indicates that it should be possible to distinguish among all of the genotypes of Echinococcus thus far described. Although employed herein to display sequence variation in the cytochrome c oxidase subunit I of Echinococcus, dideoxy fingerprinting could be used for the high-resolution analysis of nucleotide variations in other parasite genes, without the need for DNA sequencing. This has important implications for studying the genetic structure of parasite populations.

Analysis of sequence homogenisation in rDNA arrays of Haemonchus contortus by denaturing gradient gel electrophoresis

Testing different theories of concerted evolution experimentally has been hampered mainly due to the lack of appropriate model systems and technical limitations. In this study, we employed a denaturing gradient gel electrophoresis (DGGE) approach for the display and definition of nucleotide variations in the second internal transcribed spacer (ITS-2) of ribosomal DNA (rDNA) of the parasitic nematode, Haemonchus contortus. The ITS-2 was amplified from individual adult nematodes by PCR and subjected to DGGE. Of the 94 individuals (representing nine different populations) analysed, 13 different DGGE profiles were displayed. Eighteen bands representing those profiles were excised and sequenced. Sequencing defined 13 different types of ITS-2 with 12 nucleotide variations (4 transitions, 5 transversions, 1 insertion and 2 deletions) which could be related to particular positions of the predicted secondary structure for the ITS-2 pre-rRNA. The results showed that individuals of interbreeding populations of H. contortus can have rDNA arrays that are partially or fully homogenised for different sequence variants (despite interindividual variation), suggesting that the homogenisation process is driven mainly by intrachromosomal exchange. The findings also demonstrated the capacity of the DGGE-sequencing strategy to quantify the frequency of ITS-2 sequence types within individual nematodes from different populations without the need for cloning or Southern blot procedures. This has important implications for studying the mechanisms of sequence homogenisation in rDNA and pre-rRNA processing as well as for elucidating speciation events and population differentiation at the molecular level.

Display of sequence variation in PCR-amplified mitochondrial DNA regions of Echinococcus by single-strand conformation polymorphism

Echinococcosis, a disease caused by infection with the larval stage of a tapeworm parasite of the genus Echinococcus, is of major socio-economic importance, and studying genetic variability within and between Echinococcus populations has important implications for disease control and epidemiology. Various DNA approaches have been used to study Echinococcus genetics, but most methods do not allow the accurate display or definition of mutational/allelic variation. To overcome this limitation, we established a mutation scanning approach. Single-strand conformation polymorphism (SSCP) of two different enzymatically amplified mitochondrial (mt) DNA regions was evaluated using seven different genotypes of Echinococcus (defined as G1, G4, G6, G8, O, V and M2). The NADH dehydrogenase 1 gene (ND1) or the cytochrome c oxidase subunit 1 (CO1) were amplified by polymerase chain reaction from parasite DNA, denatured and directly subjected to electrophoresis in a non-denaturing gel matrix. Each of the seven genotypes examined could be delineated from one another based on characteristic and reproducible banding patterns. The results demonstrate the usefulness of SSCP for the direct visual display of sequence variation in mtDNA of Echinococcus without the need for DNA sequencing or restriction analyses, and indicate its potential for studying allelic variability in a range of other genes.

Single-strand conformation polymorphism (SSCP)-based mutation scanning approaches to fingerprint sequence variation in ribosomal DNA of ascaridoid nematodes

In this study, we assessed single-strand conformation polymorphism (SSCP)-based approaches for their capacity to fingerprint sequence variation in ribosomal DNA (rDNA) of ascaridoid nematodes of veterinary and/or human health significance. The second internal transcribed spacer region (ITS-2) of rDNA was utilised as the target region because it is known to provide species-specific markers for this group of parasites. ITS-2 was amplified by PCR from genomic DNA derived from individual parasites and subjected to analysis. Direct SSCP analysis of amplicons from seven taxa (Toxocara vitulorum, Toxocara cati, Toxocara canis, Toxascaris leonina, Baylisascaris procyonis, Ascaris suum and Parascaris equorum) showed that the single-strand (ss) ITS-2 patterns produced allowed their unequivocal identification to species. While no variation in SSCP patterns was detected in the ITS-2 within four species for which multiple samples were available, the method allowed the direct display of four distinct sequence types of ITS-2 among individual worms of T. cati. Comparison of SSCP/sequencing with the methods of dideoxy fingerprinting (ddF) and restriction endonuclease fingerprinting (REF) revealed that also ddF allowed the definition of the four sequence types, whereas REF displayed three of four. The findings indicate the usefulness of the SSCP-based approaches for the identification of ascaridoid nematodes to species, the direct display of sequence variation in rDNA and the detection of population variation. The ability to fingerprint microheterogeneity in ITS-2 rDNA using such approaches also has implications for studying fundamental aspects relating to mutational change in rDNA.

What's in that band?

The analysis of molecular variation in parasites has important implications for studying gene function and organisation, taxonomy, phylogeny and population genetics. Polymerase chain reaction-based mutation scanning methods can have significant advantages over some currently used DNA approaches for the analysis of allelic and mutational sequence variation in parasites. The present report describes briefly the principles of some of these methods, examines some of their advantages and disadvantages, and indicates their potential for applications in parasitology.

Detection of sequence variation in parasite ribosomal DNA by electrophoresis in agarose gels supplemented with a DNA-intercalating agent

This study evaluated the use of a commercially available DNA intercalating agent (Resolver Gold) in agarose gels for the direct detection of sequence variation in ribosomal DNA (rDNA). This agent binds preferentially to AT sequence motifs in DNA. Regions of nuclear rDNA, known to provide genetic markers for the identification of species of parasitic ascarid nematodes (order Ascaridida), were amplified by polymerase chain reaction (PCR) and subjected to electrophoresis in standard agarose gels versus gels supplemented with Resolver Gold. Individual taxa examined could not be distinguished reliably based on the size of their amplicons in standard agarose gels, whereas they could be readily delineated based on mobility using Resolver Gold-supplemented gels. The latter was achieved because of differences (approximately 0.1-8.2%) in the AT content of the fragments among different taxa, which were associated with significant interspecific differences (approximately 11-39%) in the rDNA sequences employed. There was a tendency for fragments with higher AT content to migrate slower in supplemented agarose gels compared with those of lower AT content. The results indicate the usefulness of this electrophoretic approach to rapidly screen for sequence variability within or among PCR-amplified rDNA fragments of similar sizes but differing AT contents. Although evaluated on rDNA of parasites, the approach has potential to be applied to a range of genes of different groups of infectious organisms.

A mutation scanning approach for the identification of hookworm species and analysis of population variation

To overcome limitations in the morphological identification of different developmental stages of hookworms to species, we have established a polymerase chain reaction-linked single strand conformation polymorphism technique (PCR-SSCP) utilizing the internal transcribed spacers (ITS) of ribosomal (r)DNA. These spacers were specifically chosen because they provide reliable species markers for strongylid nematodes. ITS spacers were amplified by PCR from DNA derived from individual parasites of seven species of hookworm, then denatured and subjected to electrophoresis in a mutation detection enhancement (MDE) (non-denaturing) gel matrix. PCR SSCP analysis showed that the single-strand ITS patterns produced allowed the unequivocal identification of all species. The method also allowed the direct display of sequence variation within some species where multiple individual worms were examined. These findings demonstrate the usefulness of the SSCP approach for hookworm identification, the detection of population variation and the direct display of sequence variation in rDNA.