Sand fly identification and screening for Leishmania spp. in six provinces of Thailand

Sergentomyia khawi: a potential vector for Leishmania and Trypanosoma parasites affecting humans and animals and insecticide resistance status in endemic areas of Songkhla, southern Thailand

BACKGROUND

Sand flies serve as crucial vectors in various medical and veterinary diseases. Sand fly-borne diseases pose a significant public health burden globally, as the causative agents can infect a diverse range of hosts, leading to severe consequences such as leishmaniasis and sand fly fever. Additionally, the widespread use of insecticides for agricultural purposes and mosquito control is not specifically targeted at sand flies, potentially leading to resistance development. We investigated sand fly species, their potential role as vectors of various parasitic agents, and insecticide resistance in the endemic regions of Natawi and Sadao districts in Songkhla, Thailand.

METHODS

Sand flies were collected using CDC light traps. The collected sand flies were then identified to species level using molecular techniques. Subsequent analyses included the detection of pathogens and the identification of pyrethroid resistance mutations within the voltage-sensitive sodium channel (Vgsc) domain IIS6 gene, followed by sequence analysis.

RESULTS

The study identified nine sand fly species belonging to the genera Phlebotomus and Sergentomyia. The DNA of Sergentomyia khawi was the only species found to test positive for one sample of Leishmania orientalis in Sadao district. This finding represents the first detection of L. orientalis in Thailand. Moreover, three samples of Leishmania martiniquensis and four samples of Trypanosoma sp. were found in the Natawi district. No I1011M, L1014F/S, V1016G, or F1020S mutations were detected in Vgsc gene.

CONCLUSIONS

The results of this study provide valuable information on sand fly species and the continuous circulation of Leishmania spp. and Trypanosoma spp. in Songkhla, southern Thailand. Moreover, the development of geo-spatial information on vectors, parasites, and insecticide resistance in sand flies has the potential to provide well-informed risk assessments and evidence-based guidance for targeted vector control in Thailand. These results can serve as a foundation for integrating the One Health approach, which is crucial for disease control, considering the diverse ecological interactions among human and/or animal reservoir hosts, parasites, and sand fly vectors.

Trypanosoma infection and bloodmeal analysis in post-feeding sand flies across Thailand

Phlebotomine sand flies are recognized as a primary vector of Leishmania and are also suspected vectors of Trypanosoma. The transmission cycle of these parasites relies on the distribution of sand fly vectors, parasites, and reservoir animals. This study aimed to detect Leishmania and Trypanosoma DNA and identify the sources of bloodmeals in post-feeding sand flies captured across Thailand. A total of 42,911 field female sand flies were collected from 11 provinces across Thailand using CDC light traps. Among these, 253 post-feeding sand flies were selected for analysis. The predominant species in this study was Sergentomyia khawi (33.60 %). The DNA was extracted from individual female sand flies. Polymerase chain reaction (PCR), specific to the internal transcribed spacer 1 (ITS1) and the small subunit ribosomal RNA (SSU rRNA) gene regions were used to detect the presence of Leishmania and Trypanosoma DNA, respectively. Additionally, cytochrome c oxidase subunit I (COI) gene region was utilized to identify the sources of host bloodmeals. Leishmania DNA was not detected in any specimens. The analysis of SSU rRNA sequences revealed the presence of Trypanosoma DNA (11.46 %, 29/253) in sand fly samples. Among these samples, T. noyesi (1.58 %, 4/253) was identified in Idiophlebotomus longiforceps and Phlebotomus asperulus, Trypanosoma Anura01+02/Frog2 (1.18 %, 3/253) in Se. khawi, and Trypanosoma Anura04/Frog1 (8.70 %, 22/253) in Se. khawi, Se. hivernus and Grossomyia indica. Bloodmeal analysis utilizing the COI gene revealed a diverse range of vertebrate hosts' blood, including bird, bat, frog and sun skink. Our findings confirm the presence of Trypanosoma DNA and identify the sources of bloodmeals from vertebrate hosts in various sand fly species, suggesting their potential as possible vectors for Trypanosoma in Thailand. Furthermore, our study is the first to provide molecular evidence using the COI gene to identify frogs as a host blood source for sand flies in Thailand. Further studies focusing on the isolation of live parasites in sand flies to confirm vector potential and examining the role of animal reservoirs will enhance our understanding of the host-parasite relationship and enable more efficient control for disease transmission.

Species diversity of phlebotomine sand flies and sympatric occurrence of Leishmania (Mundinia) martiniquensis, Leishmania (Leishmania) donovani complex, and Trypanosoma spp. in the visceral leishmaniasis focus of southern Thailand

Autochthonous leishmaniasis in Thailand has recently been a public health concern due to an increasing number of new clinical cases. Most indigenous cases were diagnosed with Leishmania (Mundinia) martiniquensis, and Leishmania (Mundinia) orientalis. However, some doubts regarding vector misidentification have arisen and need to be elucidated. Accordingly, we aimed to assess the species composition of sand flies and determine the molecular prevalence of trypanosomatids in the transmission area of leishmaniasis in southern Thailand. In the present study, a total of 569 sand flies were caught from the vicinity of a visceral leishmaniasis patient's house in Na Thawi District, Songkhla Province. Of these, 229 parous and gravid females consisted of Sergentomyia khawi, Se. barraudi, Phlebotomus stantoni, Grassomyia indica, and Se. hivernus, accounting for 31.4%, 30.6%, 29.7%, 7.9%, and 0.4%, respectively. However, Se. gemmea, which has previously been proposed as the most abundant species and putative vector of visceral leishmaniasis, was not found in the present study. Based on ITS1-PCR and sequence analysis, two specimens of Gr. indica and Ph. stantoni showed positive amplification of L. martiniquensis and L. donovani complex, respectively, the first one being presumed indigenous and the second one being not. Anuran Trypanosoma was also molecularly detected using SSU rRNA-PCR and ubiquitously found in sixteen specimens of four dominant sand fly species except for Se. hivernus. The obtained sequences could be phylogenetically categorized into the two major amphibian clades (An04/Frog1 and An01+An02/Frog2). The existence of the monophyletic subgroup and distinct lineage suggests them as novel Trypanosoma species. The TCS network analysis of these anuran Trypanosoma sequences also revealed high haplotype diversity (Hd = 0.925 ± 0.050), but low nucleotide diversity (π = 0.019 ± 0.009). Furthermore, the living anuran trypanosomes were microscopically demonstrated in a single specimen of Gr. indica, supporting the vectorial capacity. Importantly, our data confirmed the scarcity of Se. gemmea and also uncovered, for the first time, the co-circulation of L. martiniquensis, L. donovani complex, and suspected novel anuran Trypanosoma spp. in phlebotomine sand files, implicating their potential role as vectors of trypanosomatid parasites. Therefore, the novel data from this study would greatly facilitate the comprehension of the complexity of trypanosomatid transmission and the establishment of prevention and control measures for this neglected disease more effectively.

Molecular taxonomy of phlebotomine sand flies (Diptera, Psychodidae) with emphasis on DNA barcoding: A review

The taxonomy and systematics of sand flies (Diptera, Psychodidae, Phlebotominae) are one of the pillars of research aimed to identifying vector populations and the agents transmitted by these insects. Traditionally, the use of morphological traits has been the main line of evidence for the definition of species, but the use of DNA sequences is useful as an integrative approach for their delimitation. Here, we discuss the current status of the molecular taxonomy of sand flies, including their most sequenced molecular markers and the main results. Only about 37% of all sand fly species have been processed for any molecular marker and are publicly available in the NCBI GenBank or BOLD Systems databases. The genera Phlebotomus, Nyssomyia, Psathyromyia and Psychodopygus are well-sampled, accounting for more than 56% of their sequenced species. However, less than 34% of the species of Sergentomyia, Lutzomyia, Trichopygomyia and Trichophoromyia have been sampled, representing a major gap in the knowledge of these groups. The most sequenced molecular markers are those within mtDNA, especially the DNA barcoding fragment of the cytochrome c oxidase subunit I (coi) gene, which has shown promising results in detecting cryptic diversity within species. Few sequences of conserved genes have been generated, which hampers higher-level phylogenetic inferences. We argue that sand fly species should be sequenced for at least the coi DNA barcoding marker, but multiple markers with different mutation rates should be assessed, whenever possible, to generate multilocus analysis.

Phlebotomine sand flies (Diptera: Psychodidae) and sand fly-borne pathogens in the Greater Mekong Subregion: a systematic review

Phlebotomine sand flies are proven or suspected vectors of several pathogens of importance, including leishmaniasis, bartonellosis and sand fly fevers. Although sand flies have a worldwide distribution, there has been limited research published on sand flies and sand fly-borne pathogens throughout the Greater Mekong Sub-region (GMS). This review followed the PRISMA guidelines to determine the biodiversity and presence of phlebotomine sand flies and their associated pathogens in the GMS, specifically Cambodia, Thailand, the Lao People’s Democratic Republic (Laos), Malaysia and Vietnam. A total of 1472 records were identified by searching electronic databases, scanning reference lists of articles and consulting experts in the field. After screening of title and abstracts, 178 records remained and were further screened for original data (n = 34), not having regional data (n = 14), duplication of data (n = 4), records not available (n = 4) and no language translation available (n = 2). A total of 120 studies were then included for full review, with 41 studies on sand fly-related disease in humans, 33 studies on sand fly-related disease in animals and 54 entomological studies focused on sand flies (5 papers contained data on > 1 category), with a majority of the overall data from Thailand. There were relatively few studies on each country, with the exception of Thailand, and the studies applied different methods to investigate sand flies and sand fly-borne diseases, impacting the ability to conduct meaningful meta-analysis. The findings suggest that leishmaniasis in humans and the presence of sand fly vectors have been reported across several GMS countries over the past 100 years, with local transmission in humans confirmed in Thailand and Vietnam. Additionally, local Mundinia species are likely transmitted by biting midges. Findings from this study provide a framework for future investigations to determine the geographic distribution and risk profiles of leishmaniasis and other associated sand fly-borne disease throughout the GMS. It is recommended that researchers expand surveillance efforts across the GMS, with an emphasis placed on entomological surveys, syndromic and asymptomatic monitoring in both humans and animals and molecular characterization of sand flies and sand fly-borne pathogens, particularly in the understudied countries of Cambodia, Vietnam and Laos.

Significance of eggshell morphology as an additional tool to distinguish species of sand flies (Diptera: Psychodidae: Phlebotominae)

Morphological characteristics of eggshells are important in sand fly ootaxonomy. In this study, eggshells from Phlebotomus stantoni Newstead, Sergentomyia khawi (Raynal), and Grassomyia indica (Theodor) sand flies collected in Chiang Mai province, Thailand were examined and characterized using light microscopy (LM) and scanning electron microscopy (SEM). Then, eggshell morphology of these three species was described for the first time. Each gravid female was forced to lay eggs by decapitation and the eggs were collected for SEM analysis. Egg laying females were identified by morphological examination and molecular typing using cytochrome b (Cytb) as a molecular marker. The chorionic sculpturing of Ph. stantoni eggs combines two patterns on the same egg: unconnected parallel ridges and reticular patterns. Sergentomyia khawi and Gr. indica have similar chorionic polygonal patterns, but their exochorionic morphology and aeropylar area are different. Results indicate that eggshell morphological characteristics such as chorionic pattern, exochorionic morphology, inter-ridge/boundary area, aeropylar area (including the number of aeropyles) and basal layer, can be useful to develop morphological identification keys of eggs. These can serve as an additional tool to distinguish species of sand flies. In addition, the chorionic sculpturing of the eggs of the three species of sand flies observed by LM is useful for species identification in gravid females with spermathecae obscured by eggs.