Detection of Genetic Variation in Taenia solium

Genetic Diversity of Taenia solium and its Relation to Clinical Presentation of Cysticercosis

In this perspectives paper, we discuss fertilization strategies for Taenia saginata and Taenia saginata asiatica as well as heterogeneity in Taenia solium, the causative agent of human cysticercosis. Two different genotypes of T. solium (Asian and Afro/American) were confirmed by mitochondrial DNA analysis approximately two decades ago. Since then, outcrossings of the two genotypes have been identified in Madagascar where the two genotypes are distributed sympatrically. Outcrossings were confirmed by the presence of discordance between mitochondrial and nuclear DNA. Since multiple tapeworm infections are common in endemic areas, outcrossing events likely occur quite frequently. Therefore, mitochondrial DNA from T. solium specimens collected from humans and pigs in endemic areas should be analyzed. If variations are found between specimens, nuclear DNA analysis should be performed to confirm the presence of discordance between mitochondrial and nuclear genes. Additional outcrossings likely add complexity to understanding the existing genetic diversity. Serological surveys are also recommended since serodiagnostic glycoprotein can also differentiate between the two genotypes. Viable eggs from different genotypes or from hybrids of two different genotypes should be used for experimental infection of pigs or dogs in order to observe any pathological heterogeneity in cysticercosis development. Although genetic diversity of T. solium is expected to result in clinical heterogeneity of cysticercosis in humans and pigs, there is currently no evidence showing that this occurs. There are also no comparative experimental studies on this topic. Therefore, studies evaluating the link between parasite heterogeneity and clinical outcome are warranted.

Genetic variability of Taenia solium cysticerci recovered from experimentally infected pigs and from naturally infected pigs using microsatellite markers

The adult Taenia solium, the pork tapeworm, usually lives as a single worm in the small intestine of humans, its only known definitive host. Mechanisms of genetic variation in T. solium are poorly understood. Using three microsatellite markers previously reported [1], this study explored the genetic variability of T. solium from cysts recovered from experimentally infected pigs. It then explored the genetic epidemiology and transmission in naturally infected pigs and adult tapeworms recovered from human carriers from an endemic rural community in Peru. In an initial study on experimental infection, two groups of three piglets were each infected with proglottids from one of two genetically different tapeworms for each of the microsatellites. After 7 weeks, pigs were slaughtered and necropsy performed. Thirty-six (92.3%) out of 39 cysts originated from one tapeworm, and 27 (100%) out of 27 cysts from the other had exactly the same genotype as the parental tapeworm. This suggests that the microsatellite markers may be a useful tool for studying the transmission of T. solium. In the second study, we analyzed the genetic variation of T. solium in cysts recovered from eight naturally infected pigs, and from adult tapeworms recovered from four human carriers; they showed genetic variability. Four pigs had cysts with only one genotype, and four pigs had cysts with two different genotypes, suggesting that multiple infections of genetically distinct parental tapeworms are possible. Six pigs harbored cysts with a genotype corresponding to one of the identified tapeworms from the human carriers. In the dendrogram, cysts appeared to cluster within the corresponding pigs as well as with the geographical origin, but this association was not statistically significant. We conclude that genotyping of microsatellite size polymorphisms is a potentially important tool to trace the spread of infection and pinpoint sources of infection as pigs spread cysts with a shared parental genotype.

Taenia solium, the pork tapeworm, is a major cause of epilepsy in developing countries. Although it has been deemed a potentially eradicable pathogen, it remains prevalent in rural communities. This two-part study aims to evaluate the utility of three microsatellite markers previously reported, to identify parasites and to establish relationships among them. In the first study, we evaluated the genetic variability of the progeny of two individual tapeworms by infecting two groups of three pigs each. We found variation of 8% and 0% in the two groups with respect to the parental tapeworm, indicating that the cysts source may be identifiable. Next, in the second study we described the genetic relationships among tapeworms obtained from four carriers and cysts obtained from eight naturally infected pigs in a rural community. We demonstrated that pigs can have two types of cysts, suggesting multiple infections. In addition, we found relatedness between 6 pigs and one tapeworm identified in the community. Our results indicate the potential for microsatellite markers to identify genetic relationships between parasites and thereby establish routes of transmission. It is likely that the limited number of microsatellites prevented us from establishing relatedness with more precision. Therefore, further evaluation of additional microsatellites is recommended.

Mapping occurrence of Taenia solium taeniosis/cysticercosis and areas at risk of porcine cysticercosis in Central America and the Caribbean basin

Background

This study aimed to map the occurrence of Taenia solium taeniosis/cysticercosis at national level within Central America and the Caribbean basin, and to map the distribution of porcine cysticercosis at first-level administrative subdivision level (department level) and the porcine population at risk. This zoonotic parasite is believed to be widely endemic across most of Latin America. However, there is little information readily available for Central America and the Caribbean basin. Taenia solium has been ranked the most important foodborne parasitic hazard globally and within endemic areas is a common cause of preventable epilepsy.

Methods

We conducted a structured literature search in PubMed, supplemented and crossed-referenced with relevant academic databases, grey literature, and active searches in identified literature, to identify all records of T. solium presence in Central America and the Caribbean basin between 1986 and April 2017. To retrieve grey literature, government entities, researchers and relevant institutions across the region were contacted in an attempt to cover all countries and territories. Identified records containing data on porcine cysticercosis were geo-referenced to identify department level distribution and compared to modelled distributions of pigs reared under extensive production systems.

Results

We identified 51 records of T. solium at the national level, covering 13 countries and an additional three countries were included based on World Organisation for Animal Health (OIE) reports, giving a total of 16 countries out of 41 with evidence of the parasite’s presence. Screening records for porcine cysticercosis data at the departmental level confirmed porcine cysticercosis presence in 11 departments across six countries (Colombia, Guatemala, Honduras, Mexico, Nicaragua and Venezuela).

Conclusions

When comparing these results to areas where pigs were kept in extensive production systems and areas where no information on porcine cysticercosis exists, it is apparent that porcine cysticercosis is likely to be underreported, and that a substantial part of the regional pig population could be at risk of contracting porcine cysticercosis. More detailed information on the distribution of T. solium and accurate burden estimations are urgently needed to grasp the true extent of this zoonotic parasite and the public health and agricultural problems it potentially poses.

Electronic supplementary material

The online version of this article (10.1186/s13071-017-2362-7) contains supplementary material, which is available to authorized users.

Immunopathology in Taenia solium neurocysticercosis

Neurocysticercosis is a clinically and radiologically heterogeneous disease, ranging from asymptomatic infection to a severe, potentially fatal clinical picture. The intensity and extension of the parasite-elicited inflammatory reaction is a key factor for such variability. The main features of the inflammatory process found in the brain and in the peripheral blood of neurocysticercosis patients will be discussed in this review, and the factors involved in its modulation will be herein presented.

Molecular analyses reveal two geographic and genetic lineages for tapeworms, Taenia solium and Taenia saginata, from Ecuador using mitochondrial DNA

Tapeworms Taenia solium and Taenia saginata are the causative agents of taeniasis/cysticercosis. These are diseases with high medical and veterinary importance due to their impact on public health and rural economy in tropical countries. The re-emergence of T. solium as a result of human migration, the economic burden affecting livestock industry, and the large variability of symptoms in several human cysticercosis, encourage studies on genetic diversity, and the identification of these parasites with molecular phylogenetic tools. Samples collected from the Ecuadorian provinces: Loja, Guayas, Manabí, Tungurahua (South), and Imbabura, Pichincha (North) from 2000 to 2012 were performed under Maximum Parsimony analyses and haplotype networks using partial sequences of mitochondrial DNA, cytochrome oxidase subunit I (COI) and NADH subunit I (NDI), from Genbank and own sequences of Taenia solium and Taenia saginata from Ecuador. Both species have shown reciprocal monophyly, which confirms its molecular taxonomic identity. The COI and NDI genes results suggest phylogenetic structure for both parasite species from south and north of Ecuador. In T. solium, both genes gene revealed greater geographic structure, whereas in T. saginata, the variability for both genes was low. In conclusion, COI haplotype networks of T. solium suggest two geographical events in the introduction of this species in Ecuador (African and Asian lineages) and occurring sympatric, probably through the most common routes of maritime trade between the XV-XIX centuries. Moreover, the evidence of two NDI geographical lineages in T. solium from the north (province of Imbabura) and the south (province of Loja) of Ecuador derivate from a common Indian ancestor open new approaches for studies on genetic populations and eco-epidemiology.

Identification and Characterization of Microsatellite Markers Derived from the Whole Genome Analysis of Taenia solium

Background

Infections with Taenia solium are the most common cause of adult acquired seizures worldwide, and are the leading cause of epilepsy in developing countries. A better understanding of the genetic diversity of T. solium will improve parasite diagnostics and transmission pathways in endemic areas thereby facilitating the design of future control measures and interventions. Microsatellite markers are useful genome features, which enable strain typing and identification in complex pathogen genomes. Here we describe microsatellite identification and characterization in T. solium, providing information that will assist in global efforts to control this important pathogen.

Methods

For genome sequencing, T. solium cysts and proglottids were collected from Huancayo and Puno in Peru, respectively. Using next generation sequencing (NGS) and de novo assembly, we assembled two draft genomes and one hybrid genome. Microsatellite sequences were identified and 36 of them were selected for further analysis. Twenty T. solium isolates were collected from Tumbes in the northern region, and twenty from Puno in the southern region of Peru. The size-polymorphism of the selected microsatellites was determined with multi-capillary electrophoresis. We analyzed the association between microsatellite polymorphism and the geographic origin of the samples.

Results

The predicted size of the hybrid (proglottid genome combined with cyst genome) T. solium genome was 111 MB with a GC content of 42.54%. A total of 7,979 contigs (>1,000 nt) were obtained. We identified 9,129 microsatellites in the Puno-proglottid genome and 9,936 in the Huancayo-cyst genome, with 5 or more repeats, ranging from mono- to hexa-nucleotide. Seven microsatellites were polymorphic and 29 were monomorphic within the analyzed isolates. T. solium tapeworms were classified into two genetic groups that correlated with the North/South geographic origin of the parasites.

Conclusions/Significance

The availability of draft genomes for T. solium represents a significant step towards the understanding the biology of the parasite. We report here a set of T. solium polymorphic microsatellite markers that appear promising for genetic epidemiology studies.

Taenia solium, the pork tapeworm, is an important pathogen as it is a major cause of acquired epilepsy in developing countries. The parasite was eliminated from most developed countries decades ago due to improvement in sanitary conditions but it remains a common infection across Asia, Africa and Latin America. Identification of genetic variants within T. solium will enable to study the genetic epidemiology, distribution and movement of this parasite within endemic communities, which will ultimately facilitate the design of control strategies to reduce the health and economic burden of disease.

Microsatellites have been used in other parasites to identify genetic variants. In this study, we partially sequenced the genome of T. solium and identified microsatellites widely distributed in the genome using bioinformatics tools. We evaluated the distribution of these microsatellites collected from 20 tapeworms from the north and 20 tapeworms from the south of Peru. We identified seven polymorphic microsatellites, and evaluated their capacity to differentiate genetic variants of T. solium. Interestingly, tapeworms from the North and South of Peru showed different genotypes, suggesting its use as a potential marker to differentiate geographic origin.

Genetic variation in the Cytb gene of human cerebral Taenia solium cysticerci recovered from clinically and radiologically heterogeneous patients with neurocysticercosis

Neurocysticercosis (NC) is a clinically and radiologically heterogeneous parasitic disease caused by the establishment of larval Taenia solium in the human central nervous system. Host and/or parasite variations may be related to this observed heterogeneity. Genetic differences between pig and human-derived T. solium cysticerci have been reported previously. In this study, 28 cysticerci were surgically removed from 12 human NC patients, the mitochondrial gene that encodes cytochrome b was amplified from the cysticerci and genetic variations that may be related to NC heterogeneity were characterised. Nine different haplotypes (Ht), which were clustered in four haplogroups (Hg), were identified. Hg 3 and 4 exhibited a tendency to associate with age and gender, respectively. However, no significant associations were found between NC heterogeneity and the different T. solium cysticerci Ht or Hg. Parasite variants obtained from patients with similar NC clinical or radiological features were genetically closer than those found in groups of patients with a different NC profile when using the Mantel test. Overall, this study establishes the presence of genetic differences in the Cytb gene of T. solium isolated from human cysticerci and suggests that parasite variation could contribute to NC heterogeneity.

Genetic polymorphism in Taenia solium metacestodes from different Brazilian geographic areas

The aim of the present study is to investigate genetic polymorphisms in Taenia solium metacestodes from different Brazilian geographical areas and to relate them to antibody recognition in serum samples of neurocysticercosis (NC) patients. Metacestodes were obtained from the Distrito Federal (DF), Bahia, Minas Gerais (MG) and São Paulo (SP) regions of Brazil. Samples of human sera from 49 individuals with NC, 68 individuals with other helminthiasis and 40 healthy volunteers were analysed (157 individuals in total). Antigens were prepared and used in enzyme-linked immunosorbent assay and western blotting assays to detect specific immunoglobulin G antibodies. Genetic distances between metacestode populations were analysed using random amplified polymorphic DNA (RAPD) analysis. Our results show that there was a higher frequency of reactivity in the DF region in the sera from NC patients (p < 0.05), while discrimination between active and inactive NC was seen only in extracts from the MG and SP regions (p < 0.05). Using RAPD, the sample from the DF region presented a greater increase compared to the other regions. A relationship between genetic polymorphisms among T. solium metacestodes from different areas in Brazil and the differences in antibody detection in patients with NC were established.

Immunodiagnosis of neurocysticercosis: ways to focus on the challenge

Neurocysticercosis (NCC) is a disease of the central nervous system that is considered a public health problem in endemic areas. The definitive diagnosis of this disease is made using a combination of tools that include imaging of the brain and immunodiagnostic tests, but the facilities for performing them are usually not available in endemic areas. The immunodiagnosis of NCC is a useful tool that can provide important information on whether a patient is infected or not, but it presents many drawbacks as not all infected patients can be detected. These tests rely on purified or semipurified antigens that are sometimes difficult to prepare. Recent efforts have focused on the production of recombinant or synthetic antigens for the immunodiagnosis of NCC and interesting studies propose the use of new elements as nanobodies for diagnostic purposes. However, an immunodiagnostic test that can be considered as "gold standard" has not been developed so far. The complex nature of cysticercotic disease and the simplicity of common immunological assumptions involved explain the low scores and reproducibility of immunotests in the diagnosis of NCC. Here, the most important efforts for developing an immunodiagnostic test of NCC are listed and discussed. A more punctilious strategy based on the design of panels of confirmed positive and negative samples, the use of blind tests, and a worldwide effort is proposed in order to develop an immunodiagnostic test that can provide comparable results. The identification of a set of specific and representative antigens of T. solium and a thorough compilation of the many forms of antibody response of humans to the many forms of T. solium disease are also stressed as necessary.

Protein and antigen diversity in the vesicular fluid of Taenia solium cysticerci dissected from naturally infected pigs

Cysticercosis caused by Taenia solium is a health threat for humans and pigs living in developing countries, for which there is neither a flawless immunodiagnostic test nor a totally effective vaccine. Suspecting of individual diversity of hosts and parasites as possible sources of the variations of the parasite loads among cysticercotic animals and of the limited success of such immunological applications as well as, we explored and measured both in nine cases of naturally acquired porcine cysticercosis. For this purpose, 2-Dimensional IgG immunoblots were performed by reacting the sera of each cysticercotic pig with the antigens contained in the vesicular fluid (VF) of their own cysticerci. We found an unexpectedly large diversity among the proteins and antigens contained in each of the nine VFs. Also diverse were the serum IgG antibody responses of the nine pigs, as none of their 2D- immunoblot images exhibited the same number of spots and resembled each other in only 6.3% to 65.3% of their features. So large an individual immunological diversity of the cysticercal antigens and of the infected pigs´ IgG antibody response should be taken into account in the design of immunological tools for diagnosis and prevention of cysticercosis and should also be considered as a possibly significant source of diversity in Taenia solium´s infectiveness and pathogenicity.

Genetic variability of the 18 kDa/HP6 protective antigen in Taenia saginata and Taenia asiatica: implications for vaccine development

Genomic characterization of the genes encoding the Taenia 18 kDa/HP6 protective antigens was carried out for Taenia saginata and T. asiatica using 42 taeniid isolates comprising 23 samples of T. saginata, 13 samples of T. asiatica and 6 samples of T. solium. The corresponding sequences from all taeniid isolates were PCR-amplified with specific primers and then sequenced. All the genes, and other described taeniid gene homologues, had the same genomic structure. Surprisingly, the T. saginata TSA18 gene showed nucleotide variability within the 23 samples analyzed. This resulted in two distinct genotypes with 96% DNA sequence similarity and deduced amino acid sequences with 21 substitutions, mainly located in the second exon which contains the fibronectin type III domain. In regards to T. asiatica, the 18 kDa gene (TASI18) was very similar to the T. saginata antigen homologues, both at the DNA and deduced amino acid sequence levels, and the TSOL18 gene was conserved among T. solium isolates as previously described. The implications of these findings on the future development of taeniid vaccines are discussed.

Genetic diversity of Taenia solium cysticerci from naturally infected pigs of central Mexico

This study was designed to explore if each individual case of naturally acquired porcine cysticercosis, living in different geographic rural areas of central Mexico, is caused by one or more different specimens of Taenia solium tapeworm. The genetic variability among cysticerci from the same pig and that from different pigs was assessed by random amplified polymorphic DNA markers (RAPDs), through the percentage of polymorphic loci, the number of effective alleles, the expected heterozygosity and the Shannon index. The parasite population's reproductive structure was estimated through the association index (I(A)), and the degree of genetic differentiation and variation was determined using AMOVA. Using six different random primers, and a total of 181 cysticerci from 14 pigs, 88 different loci were amplified: 85% were polymorphic between pigs and 24% within pigs. The phenogram grouped the cysticerci into eight major clusters, with differences in the genetic distances among all cysticerci from 14 pigs ranging from 0.78 to 1. Most of the cysticerci grouped in accord with their different geographical origin and with their pig of origin. The similarity matrix produced from the phenogram (obtained by UPGMA) and the original similarity matrix yielded a good cophenetic correlation (r=0.82317, P=0.0004), which suggests that the phenogram accurately represents the original genetic similarities between isolates. The combination of I(A) (0.0-0.089) with the genetic diversity index (0.009-0.073) supports the idea that DNA diversity in T. solium cysticerci of naturally infected pigs is within the range expected from a recombination process occurring during sexual reproduction. The small genetic diversity found within the cysticerci of each pig (33.81%), when compared with that between pigs (66.19%), indicates that pigs are rarely infected by different tapeworms. It would then appear that porcine cysticercosis courses with effective concomitant immunity, as occurs in ovine cysticercosis.

Transcriptome analysis of Taenia solium cysticerci using Open Reading Frame ESTs (ORESTES)

BACKGROUND

Human infection by the pork tapeworm Taenia solium affects more than 50 million people worldwide, particularly in underdeveloped and developing countries. Cysticercosis which arises from larval encystation can be life threatening and difficult to treat. Here, we investigate for the first time the transcriptome of the clinically relevant cysticerci larval form.

RESULTS

Using Expressed Sequence Tags (ESTs) produced by the ORESTES method, a total of 1,520 high quality ESTs were generated from 20 ORESTES cDNA mini-libraries and its analysis revealed fragments of genes with promising applications including 51 ESTs matching antigens previously described in other species, as well as 113 sequences representing proteins with potential extracellular localization, with obvious applications for immune-diagnosis or vaccine development.

CONCLUSION

The set of sequences described here will contribute to deciphering the expression profile of this important parasite and will be informative for the genome assembly and annotation, as well as for studies of intra- and inter-specific sequence variability. Genes of interest for developing new diagnostic and therapeutic tools are described and discussed.

The immune response in Taenia solium cysticercosis: protection and injury

This article reviews current knowledge on the innate and acquired immune responses in human Taenia solium neurocysticercosis, highlighting the conditions that appear to be favourable for the survival or destruction of the parasite and for the benefit or injury to its host.

Genetic polymorphism in Taenia solium cysticerci recovered from experimental infections in pigs

Taenia solium cysticerci recovered from naturally infected pigs from Mexico, Honduras and Tanzania show a clonal structure and local lineages with probable events of genetic recombination without genetic flow within them, as revealed by RAPD. To evaluate genetic polymorphism from cysticerci recovered from experimental infections, 4 pigs were infected with T. solium eggs obtained from tapeworms released by 3 human carriers, a 10-year-old female, a 25-year-old female, and a 44-year-old male, the 4th pig was infected with a mixture of eggs from the 3 tapeworms. Each pig was orally inoculated with 50,000 eggs. After 16 weeks pigs were humanely euthanized and cysticerci were excised. Parasites recovered from each pig were analyzed by RAPD. The proportion of polymorphic loci and the mean heterozygosity as well as a dendogram and an analysis of principal coordinate and minimum spanning tree were obtained. All four pigs developed viable cysticerci; the percent infection was obtained from the ratio of the number of eggs used for infection and the number of cysticerci counted in each pig after necropsy. Infection varied from 0.2 to 4.2%. The values obtained for the proportion of polymorphic loci (0.14-0.55) and the average of expected heterozygosity (0.06-0.22) in the present experimental infection had a broader range than those reported in the literature from natural infections. The dendogram obtained clustered cysticerci into two main groups; the minimum spanning tree allowed to corroborate the data obtained in the dendogram and gave a better discrimination because in a three-dimensional plot it was easier to see that all cysticerci from each tapeworm were clustered amongst themselves. The results obtained could be hypothetically explained because environmental factors and genetic selection agents present in nature influence natural infections but do not participate in experimental ones.

Taenia solium and Taenia saginata: identification of sequence characterized amplified region (SCAR) markers

Cysticercosis is one of the most important zoonosis, not only because of the effects on animal health and its economic consequences, but also due to the serious danger it poses to humans. The two main parasites involved in the taeniasis-cysticercosis complex in Brazil are Taenia saginata and Taenia solium. Differentiating between these two parasites is important both for disease control and for epidemiological studies. The purpose of this work was to identify genetic markers that could be used to differentiate these parasites. Out of 120 oligonucleotide decamers tested in random amplified polymorphic DNA (RAPD) assays, 107 were shown to discriminate between the two species of Taenia. Twenty-one DNA fragments that were specific for each species of Taenia were chosen for DNA cloning and sequencing. Seven RAPD markers were converted into sequence characterized amplified region (SCAR) markers with two specific for T. saginata and five specific for T. solium as shown by agarose gel electrophoresis. These markers were developed as potential tools to differentiate T. solium from T. saginata in epidemiological studies.

Improvement of the synthetic tri-peptide vaccine (S3Pvac) against porcine Taenia solium cysticercosis in search of a more effective, inexpensive and manageable vaccine

Vaccination of pigs may curtail Taenia solium transmission by reducing the number of cysticerci, the precursors of adult intestinal tapeworms in humans. Several antigen preparations induce protection against porcine cysticercosis in experimental settings but only one subunit vaccine (S3Pvac) has been tested and proved effective in the field against naturally acquired disease. Besides improving of the vaccine's effectiveness, significant reductions in production costs and in the logistics of its administration are necessary for the feasibility of nationwide control programs. This review highlights the development of several versions of S3Pvac aimed to increase effectiveness, reduce costs and increase feasibility by novel delivery systems and alternative routes of administration.

Neurocysticercosis: clinical, radiologic, and inflammatory differences between children and adults

BACKGROUND

Human neurocysticercosis (NC) is caused by Taenia solium larvae lodged in the central nervous system. NC is clinically heterogeneous, ranging from asymptomatic infection to severely incapacitating and even fatal presentations. Although NC affects adults and children, age-related factors have not been thoroughly studied.

METHODS

We describe and compare the clinical, radiologic, and inflammatory features of pediatric and adult Mexican NC cases. Two hundred six NC cases (92 pediatric and 114 adult) diagnosed by computed tomography or magnetic resonance imaging were included.

RESULTS

Seizures were more frequent in children (80.4% versus 56.1%), and intracranial hypertension and headaches were more frequent in adults (27.2% versus 15.2% and 35.1% versus 21.7%, respectively). Different causes underlie the different distribution of seizures and intracranial hypertension in the 2 patient groups. In pediatric NC patients, single colloidal parenchymal cysts were the most common radiologic findings compared with adults in whom multiple viable parasites in the basal subarachnoidal cisterns or in the ventricles were seen. Cerebrospinal fluid inflammation was greater in adults than in children (P = 0.02).

CONCLUSIONS

This study documents significant age-related radiologic, clinical, and inflammatory differences in Mexican NC patients. Possible causes and relevance of these age-associated findings are discussed.

An epidemiological study of familial neurocysticercosis in an endemic Mexican community

Neurocysticercosis (NC) caused by Taenia solium is a frequent parasitic disease of the central nervous system. It is highly endemic in many developing countries, where many people are exposed but few become infected. Here, the relevance of age, gender, and genetic and exposure factors on NC susceptibility was studied in 649 inhabitants of a rural community of Mexico. Endemicity was confirmed by the high prevalence of pig cysticercosis (32.8%) and human seroprevalence (43.8%). Human NC cases were diagnosed by computerised tomography scans. A questionnaire to evaluate risk factors was applied and familial relationships between participants were registered. An overall NC frequency of 9.1% (59/649) was found. NC frequency increased with age but did not associate with gender. Most NC cases were asymptomatic. None of the evaluated risk factors were associated with NC. No familial aggregation was detected when studying all cases, although a significant relationship between mother and child in cases with multiple parasites was found. These findings point to the fact that human NC in high exposure conditions is not simply related to exposure factors and they do not support the participation of a major gene in single-cyst NC. Rather, our results point to a complex interaction of genetic and environmental factors involved in NC.

Molecular tools--advances, opportunities and prospects

Modern molecular technologies are having a substantial impact in many fundamental and applied areas of parasitology. In particular, polymerase chain reaction (PCR)-coupled approaches have found broad applicability because their sensitivity permits the enzymatic amplification of gene fragments from minute quantities of nucleic acids from tiny amounts of parasite material. Also, high-resolution electrophoretic and genomic methods are finding increased utility. This paper briefly discusses some developments and applications of DNA methods to parasites and highlights their usefulness or potential for those of veterinary importance. Selected examples of applications with implications in fundamental (systematics, population genetics, epidemiology and ecology) and applied (diagnosis, prevention and control) areas are presented. The focus is mainly on tools for the accurate identification of parasitic nematodes and protozoa of socio-economic importance, the diagnosis of infections and the detection of genetic variability using PCR-coupled mutation scanning technology.

Genetic variation in Taenia solium

Neurocysticercosis is a major zoonotic larval cestode infection that has a worldwide distribution and is of significant public health importance. Knowledge of the genetic structure of Taenia solium can be applied to the epidemiology and transmission of this disease, since genetic variants may differ in infectivity and pathogenicity. Molecular epidemiological approaches can also enable detailed studies of transmission. On a global scale, mitochondrial markers have differentiated between T. solium isolates from Asia and isolates from Africa/Latin America. Intraspecific variation in T. solium has been detected to some extent, using RAPD markers to differentiate between T. solium populations from different regions within Mexico. Markers currently available for T. solium have not been used to analyse genetic variation at the community or local level. The development of highly polymorphic markers such as microsatellites in T. solium can provide the means to examine genetic heterogeneity of tapeworm infection at the household, community and regional level. Preliminary studies suggest it is possible to analyse population genetic variation in communities using a range of polymorphic markers.

Phylogeny of Taenia: Species definitions and origins of human parasites

Phylogeny is fundamental as it constrains explanations about history and forms our foundation for recognizing and diagnosing species. In the absence of such a framework taxonomists historically relied on intuitive processes, personal judgment and authority, often embracing a typological view of species that disregarded otherwise unequivocal historical and biological criteria. Species of Taenia are among the most characteristic tapeworms infecting carnivores and humans as definitive hosts and indeed Taeniidae is unique among the Eucestoda in requiring 2 obligate mammalian hosts for transmission; a high percentage (>80%) of life cycles have been completely elucidated among the approximately 45 species and nominal subspecies of Taenia. Until recently there had been no comprehensive attempts at reconstruction of a phylogeny among these important parasites. Such analyses have allowed us to explore the origins and evolution of those independent species of Taenia that occur in humans (T. saginata, T. asiatica, and T. solium) and to understand the ecological and historical processes serving as determinants of biogeography and host-association. These studies supported the status of T. asiatica as a valid species and diagnosed a relationship as the sister-species of T. saginata. These conclusions contrasted with a diversity of opinions that would subsume T. asiatica as a subspecies. Recognition of a species constitutes a specific and testable hypothesis, is not an arbitrary decision and is most appropriately assessed in the context of phylogenetic or historical data. Considering macrospecies, a process has been outlined by Brooks and McLennan [Brooks DR, McLennan DA. The nature of diversity: an evolutionary voyage of discovery. University of Chicago Press: Chicago; 2002] as follows: (1) Discovery: a systematist describes the species; (2) Phylogenetic reconstruction; (3) Evaluation I: do sister-species show geographical overlap-are they sympatric or allopatric (use phylogeny+geographical distributions)? (4) Evaluation II: are sister-species reproductively isolated based on information from natural history, ecology and reproductive biology? Species may be viewed in the context of microevolutionary and macroevolutionary processes. For instance, microspecies are defined in ecological time and involve populations and contemporary process that are potentially reversible (reticulate). In contrast, macrospecies as exemplified by T. saginata and T. asiatica are divergent lineages resulting from processes in evolutionary time where an ancestor has undergone a permanent split that is non-reversible (non-reticulate). Applying these criteria in evaluation of T. saginata and T. asiatica, it becomes clear that in evolutionary time these represent historical lineages with independent spatial and temporal trajectories, having separated from a common ancestor near 0.78 to 1.71 MYBP in Africa, or Eurasia. In ecological time, sympatry, reproductive isolation, and differences in life history evident for T. saginata and T. asiatica as observed in China, and perhaps other regions of Southeast Asia, further serve to validate these taeniids.

Mitochondrial DNA diagnosis for taeniasis and cysticercosis

Molecular diagnosis for taeniasis and cysticercosis in humans on the basis of mitochondrial DNA analysis was reviewed. Development and application of three different methods, including restriction fragment length polymorphism analysis, base excision sequence scanning thymine-base analysis and multiplex PCR, were described. Moreover, molecular diagnosis of cysticerci found in specimens submitted for histopathology and the molecular detection of taeniasis using copro-DNA were discussed.

Portrait of Human Tapeworms

The tapeworms of the genus Taenia that infect human beings are T. solium, T. saginata and T. saginata asiatica. Taenia solium and T. saginata exhibit unequivocal features that characterize them; in contrast, only recent DNA studies, morphological characteristics, and epidemiological and sanitary aspects indicate that T. saginata asiatica is a subspecies of T. saginata. These 3 tapeworms occur in humans in their adult stage, and the intermediate hosts are pigs for T. solium and T. saginata asiatica and cows for T. saginata. Their identification is crucial considering the migratory increase from Asia to the Western Hemisphere and the fact that these tapeworms coexist in the same environment in Asia; furthermore, it is estimated that movement in both directions across the United States-Mexico border exceeds 200 million persons per yr, and thus, opportunities for acquiring and transporting T. solium infections are multiplied. It is not easy to distinguish among these tapeworms; therefore, a comparative diagram of the 3 parasites is shown in this article, which will facilitate their identification. All morphological features, some of which allow for identification, are clear and can be easily distinguished among the 3 tapeworms.