Genetic differentiation of cercariae infrapopulations of the avian schistosome Trichobilharzia szidati based on RAPD markers and mitochondrial cox1 gene

Molecular identification of Trichobilharzia species in recreational waters in North-Eastern Poland

Background

In Europe, avian schistosomes of the genus Trichobilharzia are the most common etiological agents involved in human cercarial dermatitis (swimmer's itch). Manifested by a skin rash, the condition is caused by an allergic reaction to cercariae of nonhuman schistosomes. Humans are an accidental host in this parasite's life cycle, while water snails are the intermediate, and waterfowl are the final hosts. The study aimed to conduct a molecular and phylogenetic analysis of Trichobilharzia species occurring in recreational waters in North-Eastern Poland.

Methodology

The study area covered three water bodies (Lake Skanda, Lake Ukiel, and Lake Tyrsko) over the summer of 2021. In total, 747 pulmonate freshwater snails (Radix spp., Lymnaea stagnalis) were collected. Each snail was subjected to 1-2 h of light stimulation to induce cercarial expulsion. The phylogenetic analyses of furcocercariae were based on the partial sequence of the ITS region (ITS1, 5.8S rDNA, ITS2 and 28SrDNA). For Radix spp. phylogenetic analyses were based on the ITS-2 region.

Results

The prevalence of the Trichobilharzia species infection in snails was 0.5%. Two out of 478 (0.4%) L. stagnaliswere found to be infected with Trichobilharzia szidati. Moreover, two out of 269 (0.7%) snails of the genus Radix were positive for schistosome cercariae. Both snails were identified as Radix auricularia. One of them was infected with Trichobilharzia franki and the other with Trichobilharzia sp.

Conclusions

Molecular identification of avian schistosome species, both at the intermediate and definitive hosts level, constitutes an important source of information on a potential threat and prognosis of local swimmer's itch occurrence, and helps to determine species diversity in a particular area.

Zoonotic Threats: The (Re)emergence of Cercarial Dermatitis, Its Dynamics, and Impact in Europe

Cercarial dermatitis (CD), or "Swimmer's itch" as it is also known, is a waterborne illness caused by a blood fluke from the family Schistosomatidae. It occurs when cercariae of trematode species that do not have humans as their definitive host accidentally penetrate human skin (in an aquatic environment) and trigger allergic symptoms at the site of contact. It is an emerging zoonosis that occurs through water and is often overlooked during differential diagnosis. Some of the factors contributing to the emergence of diseases like CD are related to global warming, which brings about climate change, water eutrophication, the colonization of ponds by snails susceptible to the parasite, and sunlight exposure in the summer, associated with migratory bird routes. Therefore, with the increase in tourism, especially at fluvial beaches, it is relevant to analyze the current epidemiological scenario of CD in European countries and the potential regions at risk.

Mammalian and Avian Larval Schistosomatids in Bangladesh: Molecular Characterization, Epidemiology, Molluscan Vectors, and Occurrence of Human Cercarial Dermatitis

Schistosomiasis is a neglected tropical disease (NTD) caused by blood flukes (Schistosoma spp.). Schistosomatids affect a wide array of vertebrate hosts, including humans. In the present study, multiple species of schistosomatids were identified by isolating schistosomatid cercariae (SC) from naturally infected snails. We also described different biotic and abiotic factors influencing SC infections in snails and reported human cercarial dermatitis (HCD) for the first time in Bangladesh. A total of 22,012 snails of seven species: Lymnaea auricularia, L. luteola, Indoplanorbis exustus, Physa acuta, Viviparus bengalensis, Brotia spp., and Thiara spp., were collected and examined. Among these snails, 581 (2.6%) belonging to five species: L. luteola, L. auricularia, P. acuta, I. exustus, and V. bengalensis, were infected with SC. The rate of infection was the highest for L. luteola (11.1%), followed by L. auricularia (5.3%), and was the lowest for V. bengalensis (0.4%). Prevalence in snails was the highest in September (16.8%), followed by October (9.5%) and November (8.8%), and was the lowest in colder months, such as January (1.8%) and February (2.1%). Infections with schistosomatids were more common in larger snails and snails collected from sunny areas. We confirmed the presence of Schistosoma indicum, S. incognitum, S. nasale, S. spindale, and Trichobilharzia szidati by PCR and sequencing. Through a questionnaire survey, we detected HCD in 214 (53.5%) individuals, and the infection rate was almost equally distributed across all professions. Collectively, the present results suggest that lymnaeid snails are the main vector for Schistosoma spp. prevalent in Bangladesh, and schistosomatids with zoonotic potential are also prevalent.

Somatic Dimorphism in Cercariae of a Bird Schistosome

Phenotypic polymorphism is a commonly observed phenomenon in nature, but extremely rare in free-living stages of parasites. We describe a unique case of somatic polymorphism in conspecific cercariae of the bird schistosome Trichobilharzia sp. “peregra”, in which two morphs, conspicuously different in their size, were released from a single Radix balthica snail. A detailed morphometric analysis that included multiple morphological parameters taken from 105 live and formalin-fixed cercariae isolated from several naturally infected snails provided reliable evidence for a division of all cercariae into two size groups that contained either large or small individuals. Large morph (total body length of 1368 and 1339 μm for live and formalin-fixed samples, respectively) differed significantly nearly in all morphological characteristics compared to small cercariae (total body length of 976 and 898 μm for live and formalin samples, respectively), regardless of the fixation method. Furthermore, we observed that small individuals represent the normal/commonly occurring phenotype in snail populations. The probable causes and consequences of generating an alternative, much larger phenotype in the parasite infrapopulation are discussed in the context of transmission ecology as possible benefits and disadvantages facilitating or preventing the successful completion of the life cycle.

Non-coding Regions of Mitochondrial DNA and the cox1 Gene Reveal Genetic Variability Among Local Belarusian Populations of the Causative Agent of Cercarial Dermatitis, Bird Schistosome Trichobilharzia szidati (Digenea: Schistosomatidae)

INTRODUCTION

The cercariae of avian blood flukes Trichobilharzia szidati (Digenea, Schistosomatidae) are known to cause cercarial allergic dermatitis ("swimmer's itch") in humans. Global epidemics can have significant impacts on local tourism-related economies in recreational areas. Little is known about the genetic polymorphism of the parasite population, or about the variability of the non-coding regions of mitochondrial DNA (mtDNA) and the possibility of using this as a genetic marker.

MATERIALS AND METHODS

The T. szidati cercariae were collected over 7 years from 33 naturally infected Lymnaea stagnalis snails from five sites at two neighboring lakes in Belarus. We investigated the variability of the short (SNR) and long (LNR) non-coding regions of mt DNA and the genetic diversity within the 1125-bp sequences of the gene for subunit 1 of cytochrome c oxidase (cox1).

RESULTS

In the SNR sequences, we found only length variability caused by changes in the number of bases in the mononucleotide tracts T6-T8. LNR demonstrates high variability in nucleotide sequence length (182-260 bp) depending on the presence of two long deletions of 59 and 78 nucleotides. Both mitochondrial loci (LNR and cox1) are characterized by high haplotype diversity (H = 0.922 and H = 1.0, respectively); the nucleotide diversity is significantly higher for LNR (π = 1.926 ± 0.443) compared to cox1 (π = 0.704 ± 0.059). Phylogenetic reconstructions based on the variability of each of the loci (LNR and cox1) and their concatenated sequences revealed their shallow structure and the absence of a correlation between the distribution of single-nucleotide polymorphisms and the geographic origin of parasites from two Belarusian lakes. We identified at last four weakly sublineages in the phylogenetic pattern of T. szidati. The carriers of each deletion have specific patterns for each of the two loci and form their own phylogeographic sublineages. An association between two fixed LNR substitutions and a fixed non-synonymous substitution in cox1 was found in four representatives of one lineage that had a short deletion in the LNR.

CONCLUSIONS

This study clarified the phylogeographic structure of the Belarusian population of T. szidati. Our data provide the basis for the use two mt markers in large-scale population studies of the parasite, as well as for studying the molecular evolution of coding and non-coding mtDNA in trematodes.

First Record of Trichobilharzia physellae (Talbot, 1936) in Europe, a Possible Causative Agent of Cercarial Dermatitis

Several species of avian schistosomes are known to cause dermatitis in humans worldwide. In Europe, this applies above all to species of the genus Trichobilharzia. For Austria, a lot of data are available on cercarial dermatitis and on the occurrence of Trichobilharzia, yet species identification of trematodes in most cases is doubtful due to the challenging morphological determination of cercariae. During a survey of trematodes in freshwater snails, we were able to detect a species in the snail Physella acuta (Draparnaud, 1805) hitherto unknown for Austria, Trichobilharzia physellae; this is also the first time this species has been reported in Europe. Species identification was performed by integrative taxonomy combining morphological investigations with molecular genetic analyses. The results show a very close relationship between the parasite found in Austria and North American specimens (similarity found in CO1 ≥99.57%). Therefore, a recent introduction of T. physellae into Europe can be assumed.

First confirmed record of Trichobilharzia franki Müller & Kimmig, 1994, from Radix auricularia (Linnaeus, 1758) for Austria

Avian schistosomes are of medical and veterinary importance as they are responsible for the annually occurring cercarial dermatitis outbreaks. For Austria, so far, only Trichobilharzia szidati Neuhaus 1952 was confirmed on species level as causative agent of cercarial dermatitis. Here we present the first record of Trichobilharzia franki Müller & Kimmig 1994 in Austria. The species was detected during a survey of digenean trematodes in Upper Austrian water bodies. Furthermore, we provide DNA barcodes of T. franki as well as measurements of several parasite individuals to indicate the intraspecific diversity. We also recommend the usage of an alternative primer pair, since the “standard COI primer pair” previously used for Schistosomatidae amplified an aberrant fragment in the sequence of T. franki. Overall, our study shows how limited our knowledge about occurrence and distribution of avian schistosomes in Austria is and how important it is to acquire such a knowledge to estimate ecological and epidemiological risks in the future.

Microphallus ochotensis sp. nov. (Digenea, Microphallidae) and relative merits of two-host microphallid life cycles

A new digenean species, Microphallus ochotensis sp. nov., was described from the intestine of Pacific eiders (Somateria mollissima v-nigrum) from the north of the Sea of Okhotsk. It differs from other microphallids in the structure of the metraterm, which consists of two distinct parts: a sac with spicule-like structures and a short muscular duct opening into the genital atrium. Mi. ochotensis forms a monophyletic clade together with other congeneric species in phylograms derived from the 28S and ITS2 rRNA gene. Its dixenous life cycle was elucidated with the use of the same molecular markers. Encysted metacercariae infective for birds develop inside sporocysts in the first intermediate host, an intertidal mollusc Falsicingula kurilensis. The morphology of metacercariae and adults was described with an emphasis on the structure of terminal genitalia. Considering that Falsicingula occurs at the Pacific coast of North America and that the Pacific eider is capable of trans-continental flights, the distribution of Mi. ochotensis might span the Pacific coast of Alaska and Canada. The range of its final hosts may presumably include other benthos-feeding marine ducks as well as shorebirds. We suggest that a broad occurrence of two-host life cycles in microphallids is associated with parasitism in birds migrating along sea coasts. The chances that migrating birds would stop at a site where both first and second intermediate hosts occur are relatively low. The presence of a single molluscan host in the life cycle increases the probability of transmission.

Genomic diversity of cercarial clones of Himasthla elongata (Trematoda, Echinostomatidae) determined with AFLP technique

The aim of this study was to reveal genomic diversity formed during parthenogenetic reproduction of rediae of the trematode Himasthla elongata in its molluskan host Littorina littorea. We applied amplification fragment length polymorphism (AFLP) to determine the genomic diversity of individual cercariae within the clone, that is, the infrapopulation of parthenogenetic progeny in a single molluskan host. The level of genomic diversity of particular cercariae isolates from a single clone, detected with EcoR1/Mse1 AFLP reaction, was significantly lower than the variability of cercariae from different clones. The presence of intraclonal genomic diversity indicates a nonsexual shuffle of alleles during parthenogenesis in the rediae of H. elongata. The obtained polymorphic AFLP fragments were long enough to detect the sequences that may be responsible for clonal genomic variability. Based on this, AFLP can be recommended as a tool for the study of genetic mechanisms of this variability.

Phylogenetic analysis of nasal avian schistosomes (Trichobilharzia) from aquatic birds in Mazandaran Province, northern Iran

Nasal schistosomes are trematodes in the family Schistosomatidae, many members of which are causative agents of human cercarial dermatitis (HCD). Little is known about the species diversity and distribution of nasal dwelling schistosomes of water birds, particularly in countries outside of Europe; even less is known in countries like Iran. Nasal schistosomes are of particular interest since these species migrate via the central nervous system to the nasal cavity once they penetrate their host. Thus, there must be efforts to determine the incidence of HCD due to nasal schistosomes. HCD outbreaks are reported seasonally in Mazandaran Province, northern Iran, an area well known for rice cultivation leading to increased person contact with water and infected snails. Such places include favorable habitat for both domestic ducks year round, and wild migratory ducks in the winter through spring. Recent reports have detected the presence of both nasal and visceral schistosomes in ducks in this area but with little species characterization. In this study, we examine a diversity of aquatic birds to determine the distribution, prevalence and bird host use of nasal schistosomes. We apply for the first time a molecular identification and phylogenetic analysis of these schistosomes. From 2012 to 2014, the nasal cavity of 508 aquatic birds from Mazandaran Province were examined that included species in Anseriformes, Gruiformes, Charadriiformes and Phoenicopteriformes. Nasal schistosomes were found in 45 (8.9%) birds belonging to Anseriformes (Anas platyrhynchos and Anas clypeata). Phylogenetic analysis of the nuclear internal transcribed spacer 1 rDNA and the mitochondrial cytochrome oxidase1 gene of isolated eggs revealed that all samples grouped in a sister clade to the European Trichobilharzia regenti. However, Trichobilharzia from this study were more similar to a unique haplotype of Trichobilharzia, isolated from the nasals of an A. clypeata in France. The genetic and phenotypic differences between the species found herein and T. regenti from Europe, may prove with additional data to be a distinct species of Trichobilharzia.

The genomic proliferation of transposable elements in colonizing populations: Schistosoma mansoni in the new world

Transposable elements (TEs) are mobile genes with an inherent ability to move within and among genomes. Theory predicts that TEs proliferate extensively during physiological stress due to the breakdown of TE repression systems. We tested this hypothesis in Schistosoma mansoni, a widespread trematode parasite that causes the human disease schistosomiasis. According to phylogenetic analysis, S. mansoni invaded the new world during the last 500 years. We hypothesized that new world strains of S. mansoni would have more copies of TEs than old world strains due to the physiological stress associated with invasion of the new world. We quantified the copy number of six TEs (Saci-1, Saci-2 and Saci-3, Perere-1, Merlin-sm1, and SmTRC1) in the genome and the transcriptome of old world and new world strains of S. mansoni, using qPCR relative quantification. As predicted, the genomes of new world parasites contain significantly more copies of class I and class II TEs in both laboratory and field strains. However, such differences are not observed in the transcriptome suggesting that either TE silencing mechanisms have reactivated to control the expression of these elements or the presence of inactive truncated copies of TEs.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Active chi-like sequences are present in the ITS1 region of polyembryonic adult Collyriclum faba trematodes encysted in pairs

Collyriclum faba (Plagiochiida: Collyriclidae) adults occur in pairs within subcutaneous cysts. Here, we tested the extensive C. faba infrapopulation for five DNA loci known to display variability among Central European C. faba individuals. The infrapopulation tested shared 100% similarity in four of the five mitochondrial and nuclear DNA loci tested. Contrariwise, the internal transcribed spacer 1 (ITS1) loci in all but one individual differed from each other. We found only 0.0-1.5 base substitutions per 1,000 sites within the cysts, while we found 0.7-9.0 substitutions between the cysts of the single host and 3.0-9.0 substitutions when comparing C. faba individuals isolated from different host individuals. We observed the most of the ITS1 variability within 48 bp repetitive sequences featured by the chi-like sequence 5'-GCTTGTCTGCC-3' at their beginning. Similarly to the extensive C. faba infrapopulation examined, we determined the presence of highly variable number of repetitive sequences within the ITS1 locus of C. faba isolated from multiple host species and from various geographic locations. While similar variability was observed earlier in mutually unrelated specimens of several Schistosomatidae and Microphallidae species, here, we for the first time document it among multiple individuals of a single infracommunity possessing single mitochondrial haplotype. Lower ITS1 evolutionary divergence rates observed between individuals within the cysts when compared to those between the cysts suggest that the recombination occurs at multiple stages of the life cycle. We propose DNA recombination involving chi-like sequences to serve as a general feature shared by multiple families of digenetic trematodes to increase genetic diversity of their polyembryonic populations infecting their definitive hosts.

Genetic differentiation of invasive Aedes albopictus by RAPD-PCR: implications for effective vector control

Aedes albopictus is an invasive mosquito species of great concern to public health as it is responsible for the biological transmission of several pathogens causing dengue, chikungunya, yellow fever, etc. In 2009, this mosquito was detected for the first time in Agra City. This study represents the first genetic analysis of A. albopictus from India. Random amplified polymorphic DNA (RAPD) was used to study the genetic structure of A. albopictus in four populations from different larval habitats. Seven RAPD primers produced 141 loci. The results displayed rich genetic variation among larval populations which is evident from high value of genetic differentiation (G ST), i.e. 0.280, indicating a very great genetic differentiation. Effective migration rates were observed to be 1.28, depicting a limited gene flow. According to analysis of molecular variance (AMOVA), the genetic distance between populations was significant (P < 0.05), showing a very high intrapopulation variation (96%) with only 4% variation among populations. Average genetic distances between populations were also calculated using PopGene software. Nei's average genetic distance between these populations was 0.112 (0.05-0.18). The cluster analysis technique of unweighted pair-group mean analysis (UPGMA) method of arithmetic averages was used to develop the phylogenetic tree which clearly shows two clusters of different larval habitats. The findings highlight high genetic differentiation indicating a slight migration rate confirming the recent introduction of this species in Agra region.

Pathogenicity of Trichobilharzia spp. for Vertebrates

Bird schistosomes, besides being responsible for bird schistosomiasis, are known as causative agents of cercarial dermatitis. Cercarial dermatitis develops after repeated contact with cercariae, mainly of the genus Trichobilharzia, and was described as a type I, immediate hypersensitivity response, followed by a late phase reaction. The immune response is Th2 polarized. Primary infection leads to an inflammatory reaction that is insufficient to eliminate the schistosomes and schistosomula may continue its migration through the body of avian as well as mammalian hosts. However, reinfections of experimental mice revealed an immune reaction leading to destruction of the majority of schistosomula in the skin. Infection with the nasal schistosome Trichobilharzia regenti probably represents a higher health risk than infections with visceral schistosomes. After the skin penetration by the cercariae, parasites migrate via the peripheral nerves, spinal cord to the brain, and terminate their life cycle in the nasal mucosa of waterfowl where they lay eggs. T. regenti can also get over skin barrier and migrate to CNS of experimental mice. During heavy infections, neuroinfections of both birds and mammals lead to the development of a cellular immune response and axonal damage in the vicinity of the schistosomulum. Such infections are manifest by neuromotor disorders.