First molecular identification and report of genetic diversity of Strongyloides stercoralis, a current major soil-transmitted helminth in humans from Lao People's Democratic Republic

Feline strongyloidiasis: An insight into its global prevalence and transmission cycle

The potential cross-transmission of Strongyloides stercoralis between dogs and humans has become an increasing focus of strongyloidiasis research and control programs. However, the role of cats and wild felids in the maintenance and transmission cycles of human and canine strongyloidiasis has received sparse attention. Feline strongyloidiasis epidemiology remain enigmatic. We conducted a systematic review and meta-analysis to assess the global prevalence of Strongyloides spp. in felines and reviewed cross-species infection studies to elucidate the transmission cycle of some feline Strongyloides species. Literature searched from seven databases identified 42 eligible prevalence studies published between 1985 and 2024. Of these, 44 datasets from 40 studies were included in the meta-analysis. Using a random effect model combined with the Rogan-Gladen method, we estimated the pooled global prevalence of Strongyloides spp. in felines at 13.3% (95% CI: 8.3-18.3%), with rates of 12.2% (95% CI: 6.7-17.8%) in domestic cats (Felis catus) and 20.0% (95% CI: 14.9-25.2%) in wild felids. Feline strongyloidiasis was distributed across all six WHO regions, with Africa (49.7%; 95% CI: 40.0-59.3%) and the Western Pacific (46.9%; 95% CI: 42.6-51.1%) showing the highest pooled prevalence. Subgroup analysis revealed a significantly higher prevalence of Strongyloides infection in stray domestic cats (29.2%; 95% CI: 6.3-52.1%) compared to pet cats (9.3%; 95% CI: 3.7-14.9) and shelter cats (4.4; 95% CI: 0-9.0). Historical cross-species transmission studies demonstrated variable susceptibility of cats to human- or canine-derived S. stercoralis. It remains inconclusive whether cats act as a reservoir for S. stercoralis infection in humans or vice versa. Feline strongyloidiasis is a prevalent condition in wild, stray, pet and shelter cats. Much of the available prevalence data does not discriminate to species level, and the role of cross-species transmission in feline S. stercoralis infections remains obscure. Future studies would benefit from utilising molecular genotyping tools to enable species-level phylogenetic differentiation.

From past to present: opportunities and trends in the molecular detection and diagnosis of Strongyloides stercoralis

Strongyloides stercoralis is a soil-transmitted helminth that is mainly found in the tropical and subtropical regions and affects approximately 600 million people globally. The medical importance of strongyloidiasis lies in its capacity to remain asymptomatic and chronically unnoticed until the host is immunocompromised. Additionally, in severe strongyloidiasis, hyperinfection syndrome and larva dissemination to various organs can occur. Parasitological techniques such as Baermann-Moraes and agar plate culture to detect larvae in stool samples are the current gold standard. However, the sensitivity might be inadequate, especially with reduced worm burden. Complementing parasitological techniques, immunological techniques including immunoblot and immunosorbent assays are employed, with higher sensitivity. However, cross-reactivity to other parasites may occur, hampering the assay's specificity. Recently, advances in molecular techniques such as polymerase chain reaction and next-generation sequencing technology have provided the opportunity to detect parasite DNA in stool, blood, and environmental samples. Molecular techniques, known for their high sensitivity and specificity, have the potential to circumvent some of the challenges associated with chronicity and intermittent larval output for increased detection. Here, as S. stercoralis was recently included by the World Health Organization as another soil-transmitted helminth targeted for control from 2021 to 2030, we aimed to present a review of the current molecular techniques for detecting and diagnosing S. stercoralis in a bid to consolidate the molecular studies that have been performed. Upcoming molecular trends, especially next-generation sequencing technologies, are also discussed to increase the awareness of its potential for diagnosis and detection. Improved and novel detection methods can aid in making accurate and informed choices, especially in this era where infectious and non-infectious diseases are increasingly commonplace.

Prevalence of intestinal parasitic infections and genetic differentiation of Strongyloides stercoralis among migrant workers from Myanmar, Lao PDR and Cambodia in northeastern Thailand

Intestinal parasitic infections (IPIs) remain a public-health problem worldwide, including in countries of the Lower Mekong subregion. Increases in human migration from neighboring countries might cause reemerging parasitic infections, leading to spread of parasites in the landscape. Here, we conducted a cross-sectional study to identify the prevalence of IPIs in migrant workers from Myanmar, Lao PDR, and Cambodia who were dwelling in Nakhon Ratchasima Province, northeastern Thailand. The identification of Strongyloides species and genetic differentiation of worms from migrant workers with different countries of origin was also assessed. Fresh stool samples were collected from 338 migrant workers and examined for evidence of IPIs using agar plate culture (APC) and the formalin-ethyl acetate concentration technique (FECT). Among those nine samples positive for nematodes by APC, the Strongyloides or hookworm species present was confirmed using the polymerase chain reaction (PCR) followed by DNA sequencing. This revealed eight cases of Strongyloides stercoralis infection and one of Necator americanus. Fifty-one out of 338 individuals (15.09%) were positive for IPIs using FECT and APC. Eggs of Opisthorchis-like flukes were the most common parasite (11.83% of samples), followed by S. stercoralis (2.37%), Entamoeba coli (1.50%), hookworm (0.89%), Taenia sp. (0.60%) and Hymenolepis nana (0.30%). The genetic differentiation of S. stercoralis recovered from migrant workers with different countries of origin was analyzed. Specimens of S. stercoralis isolated from workers from Lao PDR, Cambodia and Myanmar were genetically similar to those sequenced from Thailand. However, there were population-genetic differences between S. stercoralis from these Southeast Asian countries and other regions of the world. This study demonstrated that IPIs were prevalent in migrant workers in the northeastern region of Thailand. Our findings provided molecular confirmation of the presence of S. stercoralis and explored the genetic differentiation of S. stercoralis from those infected migrant workers. An effective anti-parasitic drug should be provided for migrant workers and its administration enforced.

Molecular typing of Strongyloides stercoralis in Latin America, the clinical connection

This study analysed Strongyloides stercoralis genetic variability based on a 404 bp region of the cox1 gene from Latin-American samples in a clinical context including epidemiological, diagnosis and follow-up variables. A prospective, descriptive, observational study was conducted to evaluate clinical and parasitological evolution after ivermectin treatment of 41 patients infected with S. stercoralis. Reactivation of the disease was defined both by clinical symptoms appearance and/or direct larvae detection 30 days after treatment or later. We described 10 haplotypes organized in two clusters. Most frequent variants were also described in the Asian continent in human (HP24 and HP93) and canine (HP24) samples. Clinical presentation (intestinal, severe, cutaneous and asymptomatic), immunological status and eosinophil count were not associated with specific haplotypes or clusters. Nevertheless, presence of cluster 1 haplotypes during diagnosis increased the risk of reactivation with an odds ratio (OR) of 7.51 [confidence interval (CI) 95% 1.38–44.29, P = 0.026]. In contrast, reactivation probability was 83 times lower if cluster 2 (I152V mutation) was detected (OR = 0.17, CI 95% 0.02–0.80, P = 0.02). This is the first analysis of S. stercoralis cox1 diversity in the clinical context. Determination of clusters during the diagnosis could facilitate and improve the design of follow-up strategies to prevent severe reactivations of this chronic disease.

Strongyloides genotyping: a review of methods and application in public health and population genetics

Strongyloidiasis represents a major medical and veterinary helminthic disease. Human infection is caused by Strongyloides stercoralis, Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi, with S.stercoralis accounting for the majority of cases. Strongyloides f. fuelleborni likely represents a zoonosis acquired from non-human primates (NHPs), while no animal reservoir for S. f. kellyi infection has been found. Whether S. stercoralis represents a zoonosis acquired from dogs and cats remains unanswered. Over the past two decades various tools have been applied to genotype Strongyloides spp. The most commonly sequenced markers have been the hyper-variable regions I and IV of the 18S rRNA gene and selected portions of the cytochrome c oxidase subunit I gene. These markers have been sequenced and compared in Strongyloides from multiple hosts and geographical regions. More recently, a machine learning algorithm multi-locus sequence typing approach has been applied using these markers, while others have applied whole genome sequencing. Genotyping of Strongyloides from dogs, cats, NHPs and humans has identified that S. stercoralis likely originated in dogs and adapted to human hosts. It has also been demonstrated that S. stercoralis is distinct from S. f. fuelleborni and S. f. kellyi. Two distinct genetic clades of S. stercoralis exist, one restricted to dogs and another infecting humans, NHPs, dogs and cats. Genotyping of S. f. fuelleborni has identified two separate clades, one associated with African isolates and another Indochinese peninsular clade. This review summarises the history and development of genotyping tools for Strongyloides spp. It describes the findings of major studies to date in the context of the epidemiology and evolutionary biology of these helminths, with a specific focus on human-infecting species.

High prevalence of opisthorchiasis in rural populations from Khammouane Province, central Lao PDR: serological screening using total IgG- and IgG4-based ELISA

BACKGROUND

Human opisthorchiasis, caused by Opisthorchis viverrini, is a public health problem in Southeast Asia and a major risk factor for cholangiocarcinoma. In Lao PDR, seroprevalence and the relationship between the number of O. viverrini eggs in infected people and specific antibody responses are still unknown. We evaluated and compared parasitological and serological screening methods in the community in an endemic area of opisthorchiasis in Lao PDR.

METHODS

Seroprevalence of O. viverrini-specific total IgG and IgG4 antibodies and their relationships with O. viverrini egg intensities were evaluated in Khammouane Province, central Lao PDR, using ELISA and a modified formalin ethyl-acetate concentration technique (FECT).

RESULTS

FECT stool examination revealed O. viverrini eggs in 70.3% (90/128) of individuals (95% CI 61.6 to 78.1%) while ELISA (based on total IgG and on IgG4 antibodies to O. viverrini) found 98.4% (95% CI 94.5 to 99.8%) and 89.8% (95% CI 83.3 to 94.5%) of sera, respectively. There was a positive and significant correlation between numbers of O. viverrini eggs per gram and levels of both IgG (R2=0.168, p<0.001) and IgG4 (R2=0.219, p<0.001) antibodies.

CONCLUSIONS

A high prevalence of human opisthorchiasis in Lao PDR was found using a new platform, serological screening in the community. This points to a need for sustainable control of this liver fluke infection.

Possible transmission of Strongyloides fuelleborni between working Southern pig-tailed macaques (Macaca nemestrina) and their owners in Southern Thailand: Molecular identification and diversity

Human strongyloidiasis is caused by Strongyloides stercoralis, S. fuelleborni fuelleborni and Strongyloides f. kellyi. Strongyloides fuelleborni is a soil-transmitted nematode parasite typically infecting non-human primates. The southern pig-tailed macaque (Macaca nemestrina) is distributed throughout the southern part of Thailand and could be a source of zoonotic transmission of this nematode. Here, we extracted DNA from Strongyloides speciescultured from the feces of southern pig-tailed macaques and their owners. Using PCR and sequencing of the extracted DNA, we compared the nucleotide sequences of these worms using portions of the 18S rDNA hypervariable region IV (HVR-IV) and the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene. Sequences from the 18S rRNA gene were obtained from worms from 23 southern pig-tailed macaques and from one owner. These sequences were identical with each other and with all East and Southeast Asian S. fuelleborni sequences (from Japan, Thailand, and Lao PDR) in the GenBank database. A median-joining network of published cox1 sequences (n = 123), in combination with the present 24 new sequences, represented 107 haplotypes distributed among six clusters, which corresponded to geographical localities but did not relate to host species. The S. fuelleborni cox1 sequences from some southern pig-tailed macaques and the one infected owner shared the same cox1 haplotype. This is the first evidence of likely zoonotic transmission of S. fuelleborni from a reservoir host, M. nemestrina.

Machine learning-based analyses support the existence of species complexes for Strongyloides fuelleborni and Strongyloides stercoralis

Human strongyloidiasis is a serious disease mostly attributable to Strongyloides stercoralis and to a lesser extent Strongyloides fuelleborni, a parasite mainly of non-human primates. The role of animals as reservoirs of human-infecting Strongyloides is ill-defined, and whether dogs are a source of human infection is debated. Published multi-locus sequence typing (MLST) studies attempt to elucidate relationships between Strongyloides genotypes, hosts, and distributions, but typically examine relatively few worms, making it difficult to identify population-level trends. Combining MLST data from multiple studies is often impractical because they examine different combinations of loci, eliminating phylogeny as a means of examining these data collectively unless hundreds of specimens are excluded. A recently-described machine learning approach that facilitates clustering of MLST data may offer a solution, even for datasets that include specimens sequenced at different combinations of loci. By clustering various MLST datasets as one using this procedure, we sought to uncover associations among genotype, geography, and hosts that remained elusive when examining datasets individually. Multiple datasets comprising hundreds of S. stercoralis and S. fuelleborni individuals were combined and clustered. Our results suggest that the commonly proposed 'two lineage' population structure of S. stercoralis (where lineage A infects humans and dogs, lineage B only dogs) is an over-simplification. Instead, S. stercoralis seemingly represents a species complex, including two distinct populations over-represented in dogs, and other populations vastly more common in humans. A distinction between African and Asian S. fuelleborni is also supported here, emphasizing the need for further resolving these taxonomic relationships through modern investigations.

Genomic studies on Strongyloides stercoralis in northern and western Thailand

Background

Strongyloidiasis is a soil borne helminthiasis, which in most cases is caused by Strongyloides stercoralis. Human infections with S. fuelleborni fuelleborni and S. fuelleborni kellyi also occur. Although up to 370 million people are currently estimated to be infected with S. stercoralis, this parasite is frequently overlooked. Strongyloides stercoralis is prevalent among humans in Thailand; however, S. fuelleborni fuelleborni has also been reported. Three recent genomic studies of individual S. stercoralis worms found genetically diverse populations of S. stercoralis, with comparably low heterozygosity in Cambodia and Myanmar, and less diverse populations with high heterozygosity in Japan and southern China that presumably reproduce asexually.

Methods

We isolated individual Strongyloides spp. from different localities in northern and western Thailand and determined their nuclear small ribosomal subunit rDNA (18S rDNA, SSU), in particular the hypervariable regions I and IV (HVR-I and HVR-IV), mitochondrial cytochrome c oxidase subunit 1 (cox1) and for a subset whole genome sequences. These sequences were then compared with each other and with published sequences from different geographical locations.

Results

All 237 worms isolated from 16 different human hosts were S. stercoralis, no S. fuelleborni was found. All worms had the common S. stercoralis SSU HVR IV haplotype A. Two different SSU HVR I haplotypes (I and II), both previously described in S. stercoralis, were found. No animal heterozygous for the two haplotypes was identified. Among the twelve cox1 haplotypes found, five had not been previously described. Based upon the mitochondrial cox1 and the nuclear whole genome sequences, S. stercoralis in Thailand was phylogenetically intermixed with the samples from other Southeast Asian countries and did not form its own branch. The genomic heterozygosity was even slightly lower than in the samples from the neighboring countries.

Conclusions

In our sample from humans, all Strongyloides spp. were S. stercoralis. The S. stercoralis from northern and western Thailand appear to be part of a diverse, intermixing continental Southeast Asian population. No obvious indication for genetic sub-structuring of S. stercoralis within Thailand or within the Southeast Asian peninsula was detected.

Dogs are reservoir hosts for possible transmission of human strongyloidiasis in Thailand: molecular identification and genetic diversity of causative parasite species

Human strongyloidiasis is a deleterious gastrointestinal disease mainly caused by Strongyloides stercoralis infection. We aimed to study the possible transmission of S. stercoralis between humans and pet animals. We isolated Strongyloides from humans and domestic dogs in the same rural community in north-east Thailand and compared the nucleotide sequences of derived worms using portions of the mitochondrial cytochrome c oxidase subunit 1 (cox1) and 18S ribosomal RNA (18S rRNA) genes. Twenty-eight sequences from the 18S rRNA gene were obtained from worms derived from humans (n = 23) and dogs (n = 5), and were identical with S. stercoralis sequences (from Thailand, Cambodia, Lao PDR and Myanmar) published in the GenBank database. The 28 cox1 sequences from humans and dogs showed high similarity to each other. The available published cox1 sequences (n = 150), in combination with our 28 sequences, represented 68 haplotypes distributed among four clusters. The 28 samples from the present study represented eight haplotypes including four new haplotypes. Dogs and humans shared the same haplotypes, suggesting the possibility of zoonotic transmission from pet dogs to humans. This is of concern since dogs and humans live in close association with each other.

Strongyloidiasis in northern Vietnam: epidemiology, clinical characteristics and molecular diagnosis of the causal agent

Background

Strongyloidiasis is a health problem in Vietnam, but appropriate information is still limited. The aim of this study was to determine the prevalence, geographical distribution, epidemiological aspects, symptoms and other health indicators of Strongyloides stercoralis infections in patients from 27 provinces of northern Vietnam attending the Hanoi Medical University Hospital during 2016 and 2017.

Methods

Blood samples of 2000 patients were analyzed for S. stercoralis infection with an IgG ELISA test. Seroprevalence was analyzed by gender, age group, locality of origin (rural or urban areas) and symptoms. Stools from the seropositive patients were examined for the detection of worms which were subsequently used for species identification by morphology and rDNA ITS1 sequencing.

Results

A seroprevalence of 20% was detected, showing an increasing prevalence from young to older age groups but without significant gender difference. Seroprevalence was higher in rural areas than in urban areas, both in general and individually in all provinces without exception, and lower in the mountainous areas than in the large valley lowlands. The follow-up of the 400 patients showed eosinophilia in 100% of cases, diarrhoea in 64.5%, digestion difficulties in 58.0%, stomachache in 45.5%, stomach and duodenal ulcers in 44.5%, itching in 28.0% and fever in 9.5%. The prevalence of symptoms and signs were also higher in older age groups than in younger age groups. Worms were detected in stools of 10.5% of the patients. Sequencing of a 501-bp nuclear ribosomal DNA ITS1 fragment allowed for the verification of infection by Strongyloides stercoralis.

Conclusions

To our knowledge, this study is the largest survey of human strongyloidiasis in Vietnam so far and the first molecular identification of this nematode species in this country. Long-term chronicity may probably be usual in infected subjects, mainly in the older age groups.

From the feces to the genome: a guideline for the isolation and preservation of Strongyloides stercoralis in the field for genetic and genomic analysis of individual worms

Strongyloidiasis is a soil-borne helminthiasis, which, in spite of the up to 370 million people currently estimated to be infected with its causing agent, the nematode Strongyloides stercoralis, is frequently overlooked. Recent molecular taxonomic studies conducted in Southeast Asia and Australia, showed that dogs can carry the same genotypes of S. stercoralis that also infect humans, in addition to a presumably dog-specific Strongyloides species. This suggests a potential for zoonotic transmission of S. stercoralis from dogs to humans. Although natural S. stercoralis infections have not been reported in any host other than humans, non-human primates and dogs, other as yet unidentified animal reservoirs cannot be excluded. Molecular studies also showed that humans carry rather different genotypes of S. stercoralis. As a result, their taxonomic status and the question of whether they differ in their pathogenic potential remains open. It would therefore be very important to obtain molecular genetic/genomic information about S. stercoralis populations from around the world. One way of achieving this (with little additional sampling effort) would be that people encountering S. stercoralis in the process of their diagnostic work preserve some specimens for molecular analysis. Here we provide a guideline for the isolation, preservation, genotyping at the nuclear 18S rDNA and the mitochondrial cox1 loci, and for whole genome sequencing of single S. stercoralis worms. Since in many cases the full analysis is not possible or desired at the place and time where S. stercoralis are found, we emphasize when and how samples can be preserved, stored and shipped for later analysis. We hope this will benefit and encourage researchers conducting field studies or diagnostics to collect and preserve S. stercoralis for molecular genetic/genomic analyses and either analyze them themselves or make them available to others for further analysis.

Assessment of the global paradigms of genetic variability in Strongyloides stercoralis infrapopulations determined by mitochondrial DNA sequences

Microevolutionary data of Strongyloides stercoralis infrapopulations are regarded as a requirement for determining the global genetic structure and transmission paradigms of this neglected tropical nematode among the neighboring countries of the world. English databases were searched from 2010 to 2019, analyzing a total of 10 publications. The cytochrome c oxidase subunit 1 sequences of S. stercoralis isolated from Asian and African continents were subjected to calculate the diversity indices and genetic differentiation. A parsimonious haplotype network indicated a star-like trait a total of 106 (Homo sapiens) and 48 haplotypes (Canid) being grouped into four distinct geographical haplogroups. A significant genetic diversity was identified in human-derived S. stercoralis (Haplotype diversity: 0.78) and those with dog (Hd: 0.86) origins. Cladistic phylogenetic tree indicated the Japanese, Thailandish, and Myanmarese clades have a sister relationship with the Laotian clade. The statistically significant Fst values indicated that human S. stercoralis populations of Japanese-Thailandish, Japanese-Myanmarese, and Japanese-Laotian origins were genetically differentiated (Fst: 0.48430 to 0.54903). We conclude that a high gene migration of human strongyloidiasis is being unequivocally shared between the Laotion-Myanmarese and Laotion-Thailandish population pairs. The current findings enhance our knowledge to assess the transmission dynamics and the evolutionary patterns of S. stercoralis in various geographical regions of the globe; also it will serve as a basis for public health policy to control human strongyloidiasis particularly in immunocompromised individuals. Besides, the infected canids and other environmental reservoirs for zoonotic transmission of S. stercoralis to humans should be de-wormed along with their owners.

A global genotyping survey of Strongyloides stercoralis and Strongyloides fuelleborni using deep amplicon sequencing

Strongyloidiasis is a neglected tropical disease caused by the human infective nematodes Strongyloides stercoralis, Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi. Previous large-scale studies exploring the genetic diversity of this important genus have focused on Southeast Asia, with a small number of isolates from the USA, Switzerland, Australia and several African countries having been genotyped. Consequently, little is known about the global distribution of geographic sub-variants of these nematodes and the genetic diversity that exists within the genus Strongyloides generally. We extracted DNA from human, dog and primate feces containing Strongyloides, collected from several countries representing all inhabited continents. Using a genotyping assay adapted for deep amplicon sequencing on the Illumina MiSeq platform, we sequenced the hyper-variable I and hyper-variable IV regions of the Strongyloides 18S rRNA gene and a fragment of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene from these specimens. We report several novel findings including unique S. stercoralis and S. fuelleborni genotypes, and the first identifications of a previously unknown S. fuelleborni infecting humans within Australia. We expand on an existing Strongyloides genotyping scheme to accommodate S. fuelleborni and these novel genotypes. In doing so, we compare our data to all 18S and cox1 sequences of S. fuelleborni and S. stercoralis available in GenBank (to our knowledge), that overlap with the sequences generated using our approach. As this analysis represents more than 1,000 sequences collected from diverse hosts and locations, representing all inhabited continents, it allows a truly global understanding of the population genetic structure of the Strongyloides species infecting humans, non-human primates, and domestic dogs.

Strongyloidiasis is a neglected tropical disease caused by the human infective worms (nematodes) Strongyloides stercoralis, Strongyloides fuelleborni fuelleborni and Strongyloides fuelleborni kellyi. Little is known about the genetic diversity of these nematodes and the possibility of geographically isolated genetic types is a particularly interesting research question, given the unique life cycle of these worms which includes both sexual and asexual stages. We extracted DNA from human, dog and primate feces containing Strongyloides, collected from several countries representing all inhabited continents. Using next-generation sequencing, we analyzed three Strongyloides DNA fragments from these specimens; two fragments of the 18S gene and one from the cytochrome c oxidase subunit 1 (cox1) gene. Using this approach, we discovered some unique S. stercoralis and S. fuelleborni genotypes, and identified a previously unknown S. fuelleborni infecting humans within Australia. We compared our data to all 18S and cox1 sequences of S. fuelleborni and S. stercoralis available in public databases and identified several patterns relating to the global distribution of certain genotypes. This knowledge could allow us to infer the origin of human Strongyloides infections in the future, and assess the role certain animals (non-human primates and dogs) might play in the transmission of Strongyloides to humans.

Advances in the Molecular and Cellular Biology of Strongyloides spp

Purpose of Review

This paper constitutes an update of recent studies on the general biology, molecular genetics, and cellular biology of Strongyloides spp. and related parasitic nematodes.

Recent Findings

Increasingly, human strongyloidiasis is considered the most neglected of neglected tropical diseases. Despite this, the last 5 years has seen remarkable advances in the molecular biology of Strongyloides spp. Genome sequences for S. stercoralis, S. ratti, S. venezuelensis, S. papillosus, and the related parasite Parastrongyloides trichosuri were created, annotated, and analyzed. These genomic resources, along with a practical transgenesis platform for Strongyloides spp., aided a major achievement, the advent of targeted mutagenesis via CRISPR/Cas9 in S. stercoralis and S. ratti. The genome sequences have also enabled significant molecular epidemiologic and phylogenetic findings on human strongyloidiasis, including the first genetic evidence of zoonotic transmission of S. stercoralis between dogs and humans. Studies of molecular signaling pathways identified the nuclear receptor Ss-DAF-12 as one that can be manipulated in the parasite by exogenous application of its steroid ligands. The chemotherapeutic implications of this were unscored by a study in which a Ss-DAF-12 ligand suppressed autoinfection by S. stercoralis in a new murine model of human strongyloidiasis.

Summary

Seminal advances in genomics of Strongyloides spp. have transformed research into strongyloidiasis, facilitating fundamental phylogenetic and epidemiologic studies and aiding the deployment of CRISPR/Cas9 gene disruption and editing as functional genomic tools in Strongyloides spp. Studies of Ss-DAF-12 signaling in S. stercoralis demonstrated the potential of this pathway as a novel chemotherapeutic target in parasitic nematodes.

Characterization of a non-sexual population of Strongyloides stercoralis with hybrid 18S rDNA haplotypes in Guangxi, Southern China

Strongyloidiasis is a much-neglected but sometimes fatal soil born helminthiasis. The causing agent, the small intestinal parasitic nematode Strongyloides stercoralis can reproduce sexually through the indirect/heterogonic life cycle, or asexually through the auto-infective or the direct/homogonic life cycles. Usually, among the progeny of the parasitic females both, parthenogenetic parasitic (females only) and sexual free-living (females and males) individuals, are present simultaneously. We isolated S. stercoralis from people living in a village with a high incidence of parasitic helminths, in particular liver flukes (Clonorchis sinensis) and hookworms, in the southern Chinese province Guangxi. We determined nuclear and mitochondrial DNA sequences of individual S. stercoralis isolated from this village and from close by hospitals and we compared these S. stercoralis among themselves and with selected published S. stercoralis from other geographic locations. For comparison, we also analyzed the hookworms present in the same location. We found that, compared to earlier studies of S. stercoralis populations in South East Asia, all S. stercoralis sampled in our study area were very closely related, suggesting a recent common source of infection for all patients. In contrast, the hookworms from the same location, while all belonging to the species Necator americanus, showed rather extensive genetic diversity even within host individuals. Different from earlier studies conducted in other geographic locations, almost all S. stercoralis in this study appeared heterozygous for different sequence variants of the 18S rDNA hypervariable regions (HVR) I and IV. In contrast to earlier investigations, except for three males, all S. stercoralis we isolated in this study were infective larvae, suggesting that the sampled population reproduces predominantly, if not exclusively through the clonal life cycles. Consistently, whole genome sequencing of individual worms revealed higher heterozygosity than reported earlier for likely sexual populations of S. stercoralis. Elevated heterozygosity is frequently associated with asexual clonal reproduction.

Prevalence and associated risk factors of Strongyloides stercoralis infection in Lower Myanmar

Background

Strongyloidiasis is prevalent in Southeast Asian regions along with other soil-transmitted helminthiases, but only limited present-day data was available for Myanmar.

Methods

A prevalence survey for Strongyloides stercoralis infection was conducted among villagers in rural areas of three townships located in the Lower Myanmar during 2014–2016 by agar plate culture method in combination with specific identification by molecular assays. Risk factors associated with S. stercoralis infection were assessed by analyzing questionnaires obtained from study participants.

Results

Strongyloides stercoralis was identified in 40 out of 703 participants (5.7% overall prevalence). The highest prevalence (14.4%) was observed in Htantabin, while other two communities (Thabaung and Thanlyin) had much lower prevalence (2.2 and 2.5%, respectively). Infection was relatively rare (1.2%) in younger generations under 20 years compared to older generations (9.5%). Even in Htantabin, none of the female residents under age 40 (n = 33) had infection. In adult Htantabin residents, those who answered that they do not wear shoes regularly had an elevated risk of infection (odds ratio = 2.50, 95% confidence interval = 1.03–6.08).

Conclusions

This study showed that there is still an on-going transmission of strongyloidiasis in Lower Myanmar. It is highly desirable that the soil should be free of fecal contamination by improving the management of fecal waste. Meanwhile, health education to promote shoe-wearing would be beneficial to reduce the risk of transmission, especially for those who have frequent and intense contact with soil.

Strongyloides stercoralis infection in imported and local dogs in Switzerland: from clinics to molecular genetics

Strongyloides stercoralis is a worldwide-distributed intestinal nematode affecting mainly humans and dogs. Canine strongyloidosis is generally characterised by diarrhoea, malabsorption and bronchopneumonia, and may be fatal in cases of impaired immunity. In recent years, molecular and epidemiological studies suggested that host-adapted populations of S. stercoralis with different zoonotic potential may exist. Clinical and subclinical cases of S. stercoralis infection have been increasingly diagnosed in imported (France, Belgium, Bulgaria) and locally born dogs in Switzerland, showing that this parasite is currently circulating in Europe. Three of these clinical cases will be described here. All three dogs presented severe disease, characterised by harsh diarrhoea, dehydration, vomiting, respiratory and/or neurologic signs, and needed intensive care and hospitalisation. One of these dogs was related to a Swiss breeding kennel, in which the infection was subsequently diagnosed in several other dogs. Faeces were analysed by three coproscopical methods including (i) the Baermann technique, which consistently identified the typical S. stercoralis first-stage larvae in both clinical and subclinical infections, (ii) the sedimentation-zinc chloride flotation and (iii) sodium acetate-acetic acid-formalin concentration (SAFC) methods, which allowed the additional identification of parasitic females and/or eggs in two of the clinical cases. Interestingly, S. stercoralis isolated from all three independent clinical cases exhibited an identical genetic background on the nuclear 18S rDNA (fragment involving hypervariable regions I and IV) and the mitochondrial cytochrome oxidase subunit I (cox1) loci, similar to that of zoonotic isolates from other geographical regions, and not to that of dog-adapted variants. Due to the clinical relevance and zoonotic potential of this parasite, the awareness of both diagnosticians and clinicians is strongly required.

Current high prevalences of Strongyloides stercoralis and Opisthorchis viverrini infections in rural communities in northeast Thailand and associated risk factors

Background

Two important helminths, Strongyloides stercoralis (an intestinal roundworm) and Opisthorchis viverrini (a liver fluke), are endemic in northeast Thailand. There have been national campaigns in place aimed at the control and eradication of soil-transmitted helminthiasis and opisthorchiasis in Thailand for several decades. However, these helminths still exist and raise concerns regarding public health. This study aimed to evaluate the current prevalence of S. stercoralis and O. viverrini infections in rural communities in northeast Thailand. The data from this study will be useful to improve strategies for future helminth prevention and control.

Methods

A cross-sectional study was conducted from December 2016 to June 2017 in Mueang Khon Kaen district in Khon Kaen, Thailand. The participants were selected using a simple random sampling method. Demographic data were collected using a questionnaire. Stool samples were collected and processed using agar plate culture to determine the presence of S. stercoralis infection and an in-house formalin-ethyl acetate concentration technique to determine the presence of O. viverrini and other intestinal parasite infections (IPIs).

Results

In total, 602 persons were enrolled. However, only 526 were analyzed for S. stercoralis and 387 for O. viverrini risk factors. The overall prevalence of S. stercoralis infection was 23.0% (95% confidence interval [95%CI]: 19.4 to 26.6). The prevalence of O. viverrini infection and IPIs other than S. stercoralis was 20.4% (95%CI: 16.5 to 24.8). The prevalence of O. viverrini infection was 19.4% (95%CI: 15.6 to 23.7). Male sex was significantly associated with S. stercoralis infection [Adjusted Odds Ratio (aOR) 4.0; 95%CI: 2.5 to 6.2; P-value < 0.001]. Males were significantly more likely to be infected with O. viverrini and other IPIs (aOR 4.1; 95%CI: 2.3 to 7.2, P-value < 0.001).

Conclusions

This study demonstrated that the updated prevalence of intestinal parasite infections is still high in rural communities in northeast Thailand, especially that of strongyloidiasis and opisthorchiasis.

First molecular identification of Strongyloides fuelleborni in long-tailed macaques in Thailand and Lao People's Democratic Republic reveals considerable genetic diversity

Strongyloides fuelleborni is a soil-transmitted nematode parasite of non-human primates. The worm is prevalent also in human populations in Africa and South-East Asia. In this study, we amplified and sequenced a portion of the 18S ribosomal RNA gene (rRNA) and of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of Strongyloides adult males recovered from faecal samples from long-tailed macaques (Macaca fascicularis) in Thailand and Lao PDR. The prevalence in Thailand was 31.1% (55/177) and in Lao PDR it was 62.1% (41/66), with an overall prevalence of 39.5% (96/243). All 18S rRNA sequences that we obtained (n = 96) showed 100% identity with published S. fuelleborni sequences. The 96 cox1 sequences that we obtained represented 32 new haplotypes. When included with the 17 previously known haplotypes from S. fuelleborni, the cox1 sequences fell into four clusters, which had clear geographical structure. This is the first molecular confirmation of S. fuelleborni in long-tailed macaques in Thailand and Lao PDR. Clearly, awareness needs to be raised of the zoonotic potential of S. fuelleborni. A monitoring programme should be organized, taking into account the role of reservoir hosts (i.e. monkeys) in the natural background of human strongyloidiasis caused by S. fuelleborni.

An eleven-year retrospective hospital-based study of epidemiological data regarding human strongyloidiasis in northeast Thailand

Background

Human strongyloidiasis is a chronic and persistent gastrointestinal disease caused by infection with soil-transmitted helminths of the genus Strongyloides. The aim of this research was to obtain diagnostic prevalence regarding strongyloidiasis in northeast Thailand through a hospital-based study.

Methods

Patients’ demographic data and the results of stool examinations conducted using the formalin ethyl acetate concentration technique were collected from the parasitology laboratory records at Srinagarind Hospital in Khon Kaen, Thailand. The relevant information from years 2004 to 2014 was collected and descriptively analyzed.

Results

Of a total of 22,338 patients, 3889 (17.4%) had stool samples that tested positive for Strongyloides larvae. The highest prevalence was 22.8% (95% CI = 19.6–26.2%) in the year 2004. This percentage progressively decreased, reaching 11.2% (95% CI = 10.2–12.4%) in 2013 and remaining stable at 12.9% (95% CI = 11.8–14.1%) in 2014. Males (2741 cases) had double the positivity rate of females (1148 cases). The prevalence of infection was highest (25.9%; 95% CI = 24.5–27.3%) among patients that were 51–60 years of age.

Conclusions

Areas endemic for strongyloidiasis should be emphasized under the national helminth control program and health education campaigns. Nationwide assessments should also be performed regarding Strongyloides infection, including risk factors, treatment, and prevention. The diagnostic laboratory data presented here identify the geographical focus of disease to be the northeastern region of the country. Further targeted surveillance using more sensitive methods will almost certainly reveal a higher individual disease burden than found in this report.

Different but overlapping populations of Strongyloides stercoralis in dogs and humans-Dogs as a possible source for zoonotic strongyloidiasis

Strongyloidiasis is a much-neglected soil born helminthiasis caused by the nematode Strongyloides stercoralis. Human derived S. stercoralis can be maintained in dogs in the laboratory and this parasite has been reported to also occur in dogs in the wild. Some authors have considered strongyloidiasis a zoonotic disease while others have argued that the two hosts carry host specialized populations of S. stercoralis and that dogs play a minor role, if any, as a reservoir for zoonotic S. stercoralis infections of humans. We isolated S. stercoralis from humans and their dogs in rural villages in northern Cambodia, a region with a high incidence of strongyloidiasis, and compared the worms derived from these two host species using nuclear and mitochondrial DNA sequence polymorphisms. We found that in dogs there exist two populations of S. stercoralis, which are clearly separated from each other genetically based on the nuclear 18S rDNA, the mitochondrial cox1 locus and whole genome sequence. One population, to which the majority of the worms belong, appears to be restricted to dogs. The other population is indistinguishable from the population of S. stercoralis isolated from humans. Consistent with earlier studies, we found multiple sequence variants of the hypervariable region I of the 18 S rDNA in S. stercoralis from humans. However, comparison of mitochondrial sequences and whole genome analysis suggest that these different 18S variants do not represent multiple genetically isolated subpopulations among the worms isolated from humans. We also investigated the mode of reproduction of the free-living generations of laboratory and wild isolates of S. stercoralis. Contrary to earlier literature on S. stercoralis but similar to other species of Strongyloides, we found clear evidence of sexual reproduction. Overall, our results show that dogs carry two populations, possibly different species of Strongyloides. One population appears to be dog specific but the other one is shared with humans. This argues for the strong potential of dogs as reservoirs for zoonotic transmission of S. stercoralis to humans and suggests that in order to reduce the exposure of humans to infective S. stercoralis larvae, dogs should be treated for the infection along with their owners.

A possible origin population of pathogenic intestinal nematodes, Strongyloides stercoralis, unveiled by molecular phylogeny

Humans and dogs are the two major hosts of Strongyloides stercoralis, an intestinal parasitic nematode. To better understand the phylogenetic relationships among S. stercoralis isolates infecting humans and dogs and to assess the zoonotic potential of this parasite, we analyzed mitochondrial Cox1, nuclear 18S rDNA, 28S rDNA, and a major sperm protein domain-containing protein genes. Overall, our analyses indicated the presence of two distinct lineages of S. stercoralis (referred to as type A and type B). While type A parasites were isolated both from humans and dogs in different countries, type B parasites were found exclusively in dogs, indicating that the type B has not adapted to infect humans. These epidemiological data, together with the close phylogenetic relationship of S. stercoralis with S. procyonis, a Strongyloides parasite of raccoons, possibly indicates that S. stercoralis originally evolved as a canid parasite, and later spread into humans. The inability to infect humans might be an ancestral character of this species and the type B might be surmised to be an origin population from which human-infecting strains are derived.

First molecular identification and genetic diversity of Strongyloides stercoralis and Strongyloides fuelleborni in human communities having contact with long-tailed macaques in Thailand

The parasitic nematodes, Strongyloides stercoralis and Strongyloides fuelleborni, can infect humans and non-human primates. We amplified and sequenced a portion of the 18S ribosomal RNA gene (rRNA) and of the mitochondrial cytochrome c oxidase subunit 1 (cox1) gene of Strongyloides from humans in the study area in Thailand, where people have frequent contact with long-tailed macaques. Fresh stool samples were obtained from 213 people and were examined using the agar plate culture method. The overall prevalence of Strongyloides infection was 8.92% (19/213). From a total of 19 worms (one per infected person), 18 adult males had 18S rRNA sequences identical with that of S. stercoralis and one adult female had a sequence almost identical with that of S. fuelleborni. A median-joining network of cox1 sequences revealed nine new haplotypes from S. stercoralis, and an overall haplotype diversity (Hd) of 0.9309. The single haplotype of S. fuelleborni was also new and contributed to an overall haplotype diversity for that species of 0.9842. This is the first molecular identification of S. stercoralis and S. fuelleborni in a human community having contact with long-tailed macaques in Thailand. It is also the first report of S. fuelleborni infecting a human in Thailand.

Genetic diversity of Taenia saginata (Cestoda: Cyclophyllidea) from Lao People's Democratic Republic and northeastern Thailand based on mitochondrial DNA

BACKGROUND

Taenia saginata is a tapeworm found in cattle worldwide. Analysis of genetic diversity in different geographical populations of T. saginata not only helps to understand the origin, transmission and spread of this organism, but also to evaluate the selection pressures acting on T. saginata and how it is responding to them. However, there are few reports of the genetic variability of T. saginata populations in different regions of the world, including Lao PDR and Thailand. We report the genetic diversity of T. saginata populations in Lao PDR and northeastern Thailand together with sequences of T. saginata from other countries deposited in GenBank.

RESULTS

Mitochondrial cox1 sequence analysis revealed that 15 and 8 haplotypes were identified in 30 and 21 T. saginata isolates from Lao PDR and northeastern Thailand, respectively. Fifty-three haplotypes were identified from 98 sequences. Phylogenetic tree and haplotype network analyses revealed that global isolates of T. saginata were genetically divided into five groups (A, B, C1, C2 and D). Taenia saginata isolates from Lao PDR and northeastern Thailand belonged to either Group A or B. Taenia saginata from western Thailand clustered in groups C1, C2 and D, and populations from the northeast and western Thailand were found to be genetically distinct. Taenia saginata isolates in Lao PDR and Thailand were also found to be genetically diverse but the degree of genetic differentiation was low.

CONCLUSIONS

Taenia saginata populations from Lao PDR and northeastern Thailand are genetically distinct from the population in western Thailand and it is proposed that T. saginata has been dispersed by different transmission routes in Southeast Asia.