Bulinus senegalensis and Bulinus umbilicatus Snail Infestations by the Schistosoma haematobium Group in Niakhar, Senegal

Freshwater snail-borne parasitic diseases in Africa

BACKGROUND

Freshwater snails are the first obligatory intermediate hosts in the trematode life cycle. Several parasitic diseases transmitted by these snails are endemic in Africa, and their distribution closely follows that of the intermediate hosts. These diseases represent a major public health problem and cause significant socio-economic losses in Africa, particularly schistosomiasis and fascioliasis. In this review, we will describe the main roles of freshwater snails in the life cycle of trematode parasites, and the geographical distribution of these diseases in Africa. We will also discuss the different techniques for detecting parasitic infections in snails, as well as the various methods of controlling snails and the larval stages of parasites.

METHODS

We carried out a literature search for articles dealing with parasitic diseases transmitted by freshwater snail hosts in Africa. The search was conducted in databases such as PubMed, Web of Science and Google Scholar using various search terms combined by Boolean operators. Our search was limited to peer-reviewed articles less than 10 years old. Articles published to date in the fields of control of parasitic diseases transmitted by freshwater snails were included. Results were presented in narrative and in table format.

RESULTS

The results of the database search identified 1007 records. We included 84 studies in this review. These studies generally focused on freshwater snails and the diseases they transmit. We described the geographical distribution of 43 freshwater species belonging to nine snail families, as well as the parasites that infect them. Several methods for diagnosing parasites in their snail hosts have been described, including microscopic and molecular methods, as well as antibody and protein barcode-based techniques. Molluscicides have been described as the main strategy for snail control.

CONCLUSION

This study highlights several elements of knowledge about diseases transmitted by freshwater snails and their distribution. A good understanding of snail infection detection techniques and existing control methods is an essential component in adapting control strategies for these diseases.

Urogenital schistosomiasis in schoolchildren in the lake zones of Kankossa and Oued Rawdha, southern Mauritania: The first parasitological and malacological survey

BACKGROUND

Urogenital schistosomiasis due to Schistosoma haematobium is a major public health problem in Mauritania, but little is known about its epidemiology in many areas of the country, particularly in the lake zones. The objectives of the present parasitological and malacological study were to assess the prevalence and intensity of urogenital schistosomiasis among school children in Kankossa and Oued Rawdha lakes, southern Mauritania, and determine the species of intermediate host snails and the prevalence of snails with schistosome.

METHODS

A school-based epidemiological survey was conducted in two villages in the lake areas of Kankossa and Oued Rawdha. Urine samples were collected from 450 state primary school children and Koranic school children and examined for the presence of S. haematobium eggs using filtration technique. Water bodies adjacent to human settlement were surveyed for Bulinus and Biomphalaria snails that may potentially be intermediate hosts of S. haematobium. Morphological, molecular, and proteomic (i.e. matrix-assisted laser desorption ionization time-of-flight mass spectrometry [MALDI-TOF MS]) identification of collected snails were conducted, and their infection status was assessed by real-time polymerase chain reaction (RT-PCR) using the highly repetitive DraI gene.

RESULTS

The prevalence of urogenital schistosomiasis was 35.6% and 15.8% in Kankossa and Oued Rawdha villages, respectively, corresponding to 'moderate' prevalence (i.e., 10-49% infected schoolchildren). Urogenital schistosomiasis prevalence was higher in boys (30.0%) than in girls (21.2%; P < 0.05), and in Koranic schools pupils (37.1%) than in state schools (20.5%; P < 0.05) pupils. Multiple regression analysis showed that sex (odds ratio [OR]: 1.64; 95% confidence interval [95% CI]: 1.06-2.57; P = 0.03) and Koranic school level (OR: 1.79; 95% CI: 1.06-3.04; P = 0.03) were independently and significantly associated with urogenital schistosomiasis. Based on molecular and proteomic identification, both B. senegalensis and B. umbilicatus colonized the water bodies of Oued Rawdha, whereas both B. forskalii and B. truncatus colonized those of Kankossa. The DraI RT-PCR detected S. haematobium complex DNA in 8 of 66 (12.1%) analysed snails: one B. truncatus and one B. forskalii in Kankossa and five B. senegalensis and one B. umbilicatus in Oued Rawdha.

CONCLUSION

Urogenital schistosomiasis is moderately prevalent in the lake zones of Kankossa and, to a lesser extent, Oued Rawdha, located in southern Mauritania. Mass drug administration campaigns with praziquantel should be conducted to reduce the prevalence of urogenital schistosomiasis among school-aged children in the lake zone of Kankossa and Oued Rawdha village. Further parasitological and malacological studies should be conducted in other villages located in the Mauritanian lakes in the southern Sahelian zones and the northern oasis areas to strengthen our knowledge of the current epidemiological situation and implement appropriate urogenital schistosomiasis control strategies.

Exploring the Adaptation of Bulinus senegalensis and Bulinus umbilicatus to the Dry and Rainy Season in Ephemeral Pond in Niakhar (Senegal), an Area of Seasonal Transmission of Urogenital Schistosomiasis

Bulinus snails surviving drought play a key role in the seasonal transmission of urogenital schistosomiasis, although our knowledge of their adaptation to dry season is still limited. We investigated the survival dynamic and infestation by the Schistosoma haematobium of Bulinus snails during the dry and rainy seasons in a single pond in an area of seasonal schistosomiasis transmission in Senegal. During the rainy season, 98 (94.23%) B. senegalensis and six (5.76%) B. umbilicatus were collected, respectively. In the dry season, B. umbilicatus outnumbered B. senegalensis, but all five (100%) B. senegalensis collected were viable and alive after the interruption of aestivation by immersion in water, while only 7 of 24 (29.16%) B. umbilicatus collected emerged from their dormant state. The rate of infestation with S. haeamatobium during the rainy season was 18.2% (19/104), while all the viable snails collected during the dry season were negative. B. senegalensis and B. umbilicatus have different seasonal dynamics with no evidence of maintaining S. haematobium infestation during the drought. Further studies including more survey sites and taking account both snails biology and ecological conditions are needed to better understand snail adaptation to seasonal changes and their ability to maintain S. haeamatobium infestation during drought.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry traces the geographical source of Biomphalaria pfeifferi and Bulinus forskalii, involved in schistosomiasis transmission

BACKGROUND

Freshwater snails of the genera Bulinus spp., Biomphalaria spp., and Oncomelania spp. are the main intermediate hosts of human and animal schistosomiasis. Identification of these snails has long been based on morphological and/or genomic criteria, which have their limitations. These limitations include a lack of precision for the morphological tool and cost and time for the DNA-based approach. Recently, Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight (MALDI-TOF) mass spectrometry, a new tool used which is routinely in clinical microbiology, has emerged in the field of malacology for the identification of freshwater snails. This study aimed to evaluate the ability of MALDI-TOF MS to identify Biomphalaria pfeifferi and Bulinus forskalii snail populations according to their geographical origin.

METHODS

This study was conducted on 101 Bi. pfeifferi and 81 Bu. forskalii snails collected in three distinct geographical areas of Senegal (the North-East, South-East and central part of the country), and supplemented with wild and laboratory strains. Specimens which had previously been morphologically described were identified by MALDI-TOF MS [identification log score values (LSV) ≥ 1.7], after an initial blind test using the pre-existing database. After DNA-based identification, new reference spectra of Bi. pfeifferi (n = 10) and Bu. forskalii (n = 5) from the geographical areas were added to the MALDI-TOF spectral database. The final blind test against this updated database was performed to assess identification at the geographic source level.

RESULTS

MALDI-TOF MS correctly identified 92.1% of 101 Bi. pfeifferi snails and 98.8% of 81 Bu. forskalii snails. At the final blind test, 88% of 166 specimens were correctly identified according to both their species and sampling site, with LSVs ranging from 1.74 to 2.70. The geographical source was adequately identified in 90.1% of 91 Bi. pfeifferi and 85.3% of 75 Bu. forskalii samples.

CONCLUSIONS

Our findings demonstrate that MALDI-TOF MS can identify and differentiate snail populations according to geographical origin. It outperforms the current DNA-based approaches in discriminating laboratory from wild strains. This inexpensive high-throughput approach is likely to further revolutionise epidemiological studies in areas which are endemic for schistosomiasis.

Identification of Bulinus forskalii as a potential intermediate host of Schistosoma hæmatobium in Senegal

Understanding the transmission of Schistosoma hæmatobium in the Senegal River Delta requires knowledge of the snails serving as intermediate hosts. Accurate identification of both the snails and the infecting Schistosoma species is therefore essential. Cercarial emission tests and multi-locus (COX1 and ITS) genetic analysis were performed on Bulinus forskalii snails to confirm their susceptibility to S. hæmatobium infection. A total of 55 Bulinus forskalii, adequately identified by MALDI-TOF mass spectrometry, were assessed. Cercarial shedding and RT-PCR assays detected 13 (23.6%) and 17 (31.0%), respectively, Bulinus forskalii snails parasitized by S. hæmatobium complex fluke. Nucleotide sequence analysis identified S. hæmatobium in 6 (11.0%) using COX1 and 3 (5.5%) using ITS2, and S. bovis in 3 (5.5%) using COX1 and 3 (5.5%) using ITS2. This result is the first report of infection of Bulinus forskalii by S. hæmatobium complex parasites in Senegal using innovative and more accurate identification methods to discriminate this snail and characterize its infection by S. hæmatobium.

Molecular characterization of schistosome cercariae and their Bulinus snail hosts from Niakhar, a seasonal transmission focus in central Senegal

Bulinus senegalensis and Bulinus umbilicatus, two sympatric freshwater snails found in temporal ponds in Senegal, were thought to be involved in the transmission of Schistosoma haematobium and/or Schistosoma curassoni. To better understand the role of these Bulinus species in the transmission of human and animal Schistosoma species, B. senegalensis and B. umbilicatus were collected in 2015, during a malacological survey, from a temporal pond in Niakhar, central Senegal. Snails were induced to shed cercariae on two consecutive days. Individual cercariae from each snail were collected and preserved for molecular identification. Infected snails were identified by analysis of a partial region of the cytochrome c oxidase subunit 1 (cox1) gene. Six individual cercariae shed from each infected snail were identified by analyses of the cox1, nuclear ITS and partial 18S rDNA regions. Of the 98 snails collected, one B. senegalensis had a mixed infection shedding S. haematobium, S. bovis and S. haematobium-S. bovis hybrid cercariae and one B. umbilicatus was found to be shedding only S. haematobium. These data provide molecular confirmation for B. senegalensis transmitting S. bovis and S. haematobium-S. bovis hybrids in Senegal. The multiple Bulinus species involved in the human urogenital schistosomiasis in Senegal provides a high force of transmission warranting detailed mapping, surveillance and regular treatment of at-risk populations.

Plasma-derived exosomal miRNA as potential biomarker for diagnosis and prognosis of vector-borne diseases: A review

Early disease diagnosis is critical for better management and treatment outcome of patients. Therefore, diagnostic methods should ideally be accurate, consistent, easy to perform at low cost and preferably non-invasive. In recent years, various biomarkers have been studied for the detection of cardiovascular diseases, cerebrovascular diseases, infectious diseases, diabetes mellitus and malignancies. Exosomal microRNA (miRNA) are small non-coding RNA molecules that influence gene expression after transcription. Previous studies have shown that these types of miRNAs can potentially be used as biomarkers for cancers of the breast and colon, as well as diffuse large B-cell lymphoma. It may also be used to indicate viral and bacterial infections, such as the human immunodeficiency virus (HIV), tuberculosis and hepatitis. However, its use in the diagnosis of vector-borne diseases is rather limited. Therefore, this review aims to introduce several miRNAs derived from exosomal plasma that may potentially serve as a disease biomarker due to the body's immune response, with special focus on the early detection of vector-borne diseases.