Afrotropical sand fly-host plant relationships in a leishmaniasis endemic area, Kenya

Discovery of the vector of visceral leishmaniasis, Phlebotomus (Artemievus) alexandri Sinton, 1928, in Kenya suggests complex transmission dynamics

Visceral and cutaneous leishmaniasis are endemic to specific regions due to the ecological preferences of phlebotomine sand flies and Leishmania spp. transmission. Sand fly entomological data in northern Kenya are scarce due to limited studies and neglect of leishmaniasis. The aim of this study was to investigate: (i) sand fly diversity and distribution; (ii) occurrence of Leishmania DNA within sand flies; and (iii) blood-meal sources of sand flies in Laisamis, northern Kenya. We conducted an entomological survey during February and March of 2021 in five areas of Laisamis sub-county using standard CDC light traps. A total of 1009 sand flies (394 male and 615 female) were morphologically identified, and representative samples verified by PCR amplification and sequencing of the cytochrome c oxidase subunit 1 (cox1) gene. Similarly, we identified blood-meal sources and Leishmania DNA in female sand flies by PCR amplicon sequencing of the vertebrate cytochrome b (cyt b) gene and internal transcribed spacer 1 (ITS1) of the 28S rRNA gene, respectively. Sergentomyia clydei (59.8%) was the most abundant sand fly species. Though collected mainly from one locality (Tirgamo), 14.8% of samples belonged to Phlebotomus (Artemievus) alexandri Sinton, 1928. We detected DNA of Leishmania major in 5.19% of Ph. alexandri, whereas Leishmania adleri DNA was detected in S. clydei (7.51%), Sergentomyia squamipleuris (8.00%), and Sergentomyia africanus (8.33%). Nine of 13 blood-fed sand flies had obtained blood from humans, of which 33.3% had L. major DNA. Both Ph. alexandri and S. clydei primarily fed on humans and could potentially be involved in the transmission of cutaneous leishmaniasis. The findings of this study contribute to the understanding of sand fly vector populations and their potential to transmit leishmaniasis in the area.

Sandfly Blood-Feeding Habits and Competence in Transmitting Ntepes Virus, a Recently Discovered Member of the Genus Phlebovirus

Phleboviruses transmitted by sandflies are among emerging public health threats. A novel Phlebovirus named Ntepes virus (NTPV) was recently described and found to infect humans from a wide geographic area in Kenya. However, the entomologic risk factors of this virus such as the potential vectors and the transmission cycles remain poorly defined. This study assessed the ability of the colonized sandfly Phlebotomus duboscqi to transmit NTPV and determined the bloodmeal host sources of field-collected sandflies from the area where NTPV was found in Baringo County, Kenya. Five-day old laboratory-reared P. duboscqi were orally challenged with an infectious dose of NTPV (≈106.0 pfu/ml) and incubated for up to 15 days postinfection. Individual sandflies were dissected into abdomens, legs, and salivary glands and screened for the virus infection by cell culture. Of the 205 virus-exposed sandflies, 19.5% developed non-disseminated infections in the midgut, with no evidence of virus dissemination or transmission in legs and salivary glands, respectively. The midgut infection rates decreased with increasing extrinsic incubation period (Spearman's correlation, ρ = -0.71). Blood-fed specimens analyzed by polymerase chain reaction (PCR) and sequencing of a region of the mitochondrial 12S rRNA, revealed almost exclusive feeding on humans (98%) represented by the sandflies Sergentomyia schwetzi, S. clydei, S. antennata, S. squamipleuris, S. africana, and Phlebotomus martini. One specimen of S. clydei had fed on cattle (2%). These findings suggest P. duboscqi is an incompetent laboratory vector of NTPV. The high human-feeding rate by diverse sandfly species increases the likelihood of human exposure to pathogens associated with these sandflies. Assessment of the susceptibility of Sergentomyia species to NTPV is recommended given their high human-feeding tendency.

Contemporary exploitation of natural products for arthropod-borne pathogen transmission-blocking interventions

An integrated approach to innovatively counter the transmission of various arthropod-borne diseases to humans would benefit from strategies that sustainably limit onward passage of infective life cycle stages of pathogens and parasites to the insect vectors and vice versa. Aiming to accelerate the impetus towards a disease-free world amid the challenges posed by climate change, discovery, mindful exploitation and integration of active natural products in design of pathogen transmission-blocking interventions is of high priority. Herein, we provide a review of natural compounds endowed with blockade potential against transmissible forms of human pathogens reported in the last 2 decades from 2000 to 2021. Finally, we propose various translational strategies that can exploit these pathogen transmission-blocking natural products into design of novel and sustainable disease control interventions. In summary, tapping these compounds will potentially aid in integrated combat mission to reduce disease transmission trends.

A next-generation (DNA) sequencing (NGS)-based method for identifying the sources of sugar meals in mosquito vectors of West Nile virus in Israel

Mosquitoes of the genus Culex comprise important vectors of pathogenic arboviruses in our region, including West Nile and Rift Valley Fever viruses. To improve our understanding of the epidemiology and transmission dynamics of arboviruses, we need to study the behavior and ecology of their vectors. The feeding patterns of the vector mosquitoes can be very useful in determining how and where to focus control efforts. For example, determining the preferred blood hosts of the females can assist in the implementation of potentially efficacious strategies for focused control of mosquito females. Determining the plants from which both sexes derive their sugar meals can comprise the initial step towards the formulation of efficient lures for trapping mosquitoes. In the past, plant meal identification was based mainly on chemical detection of fructose and microscopical observations of cellulose particles in mosquito guts. More recent studies have utilized DNA barcoding capable of identifying plant food sources. In the current study, we identify multiple plant species from which large numbers of mosquitoes obtained their sugar meals in one experimental procedure. We employed next generation DNA sequencing to sequence the chloroplast specific plant genes atpB and rbcL.

Can floral nectars reduce transmission of Leishmania?

Background

Insect-vectored Leishmania are responsible for loss of more disability-adjusted life years than any parasite besides malaria. Elucidation of the environmental factors that affect parasite transmission by vectors is essential to develop sustainable methods of parasite control that do not have off-target effects on beneficial insects or environmental health. Many phytochemicals that inhibit growth of sand fly-vectored Leishmania—which have been exhaustively studied in the search for phytochemical-based drugs—are abundant in nectars, which provide sugar-based meals to infected sand flies.

Principle findings

In a quantitative meta-analysis, we compare inhibitory phytochemical concentrations for Leishmania to concentrations present in floral nectar and pollen. We show that nectar concentrations of several flowering plant species exceed those that inhibit growth of Leishmania cell cultures, suggesting an unexplored, landscape ecology-based approach to reduce Leishmania transmission.

Significance

If nectar compounds are as effective against parasites in the sand fly gut as predicted from experiments in vitro, strategic planting of antiparasitic phytochemical-rich floral resources or phytochemically enriched baits could reduce Leishmania loads in vectors. Such interventions could provide an environmentally friendly complement to existing means of disease control.

Leishmania parasites infect over a million people each year—including over 200,000 infections with deadly visceral leishmaniasis—resulting in a greater health burden than any human parasite besides malaria. Leishmania infections of humans are transmitted by blood-feeding sand flies, which also consume floral nectar. Nectar contains many chemicals that inhibit Leishmania growth and are candidate treatments for infection of humans. However, these same compounds could also reduce infection in nectar-consuming sand flies. By combining existing data on the chemistry of nectar and sensitivity of Leishmania to plant compounds, we show that some floral nectars contain sufficient chemical concentrations to inhibit growth of insect-stage Leishmania. Our results suggest that consumption of these nectars could reduce parasite loads in sand flies and transmission of parasites to new human hosts. In contrast to insecticide-based methods of sand fly control, incorporation of antiparasitic nectar sources into landscapes and domestic settings could benefit public health without threatening beneficial insects. These findings suggest an unexplored, landscape-based approach to reduce transmission of a major neglected tropical disease worldwide.

Identification of Repellents from Four Non-Host Asteraceae Plants for the Root Knot Nematode, Meloidogyne incognita

Olfactory cues guide plant parasitic nematodes (PPNs) to their host plants. We tested the hypothesis that non-host plant root volatiles repel PPNs. To achieve this, we compared the olfactory responses of infective juveniles (J2s) of the PPN Meloidogyne incognita to four non-host Asteraceae plants, namely, black-jack (Bidens pilosa), pyrethrum (Chrysanthemum cinerariifolium), marigold (Tagetes minuta), and sweet wormwood (Artemisia annua), traditionally used in sub-Saharan Africa for the management of PPNs. Chemical analysis by coupled gas chromatography-mass spectrometry (GC/MS) combined with random forest analysis, followed by behavioral assays, identified the repellents in the root volatiles of B. pilosa, T. minuta, and A. annua as (E)-β-farnesene and 1,8-cineole, whereas camphor was attractive. In contrast, random forest analysis predicted repellents for C. cinerariifolium and A. annua as β-patchoulene and isopropyl hexadecanoate. Our results suggested that terpenoids generally account for the repellency of non-host Asteraceae plants used in PPN management.

Behavioural responses of Phlebotomus duboscqi to plant-derived volatile organic compounds

Phlebotomine sand flies are vectors of Leishmania parasites that cause leishmaniases. Both sexes of sand flies feed on plants primarily for sugars, although the chemical cues that mediate attraction to host plants remain largely unknown. Previously, using coupled gas chromatography-mass spectrometry, the authors identified several volatile organic compounds (VOCs) common to preferred host plants for selected Afrotropical sand flies from the Fabaceae family. Of the identified volatiles, the significance of the monoterpenes linalool oxide, ocimene and p-cymene and the benzenoid m-cresol, p-cresol in sand fly behaviour is unknown. In olfactometer assays, the authors tested these compounds singly and in blends for their attractiveness to Phlebotomus duboscqi, cutaneous leishmaniasis vector in Kenya. In dose-response assays, single compounds increased the responses of males and females over controls, but their optimum attractive doses varied between the sexes. Two five-component blends, referred to as Blend-f and Blend-m for females and males respectively, were formulated and tested in dose-response assays against 1-octen-3-ol (positive control). The results of the present study showed that males and females were significantly attracted to varying levels of the two blends. In pairwise assays, the authors evaluated the most attractive of these blends to each sex (i.e., Blend Am for male against Blend Bf for female), revealing that males were attracted to both blends at varying levels, whereas females were indifferent. The study's results demonstrate that plant-derived VOCs can be exploited for sand fly management.

Plant sugar feeding patterns of wild-caught Aedes aegypti from dengue endemic and non-endemic areas of Kenya

A fundamental understanding of plant sugar feeding behaviour in vector populations can lead to the development of ecologically effective vector monitoring and control strategies. Despite previous studies on mosquito-plant interactions, relatively few have been conducted on the dengue vector Aedes aegypti (Diptera: Culicidae). The authors studied Ae. aegypti-plant interactions at two sites of varying dengue endemicity in Kenya: Kilifi (endemic) and Isiolo (non-endemic). Using chemical and molecular assays [DNA barcoding targeting the chloroplast ribulose-1,5 bisphosphate carboxylase/oxygenase large chain (rbcL) gene], the authors show that at the two sites plant feeding in this mosquito species: (a) varies by sex and season; (b) results in the acquisition of diverse sugars, and (c) is associated with diverse host plants in the families Fabaceae, Malvaceae, Poaceae and Rosaceae. These results reveal insights into the plant sugar feeding patterns of wild-caught Ae. aegypti and provide a baseline for future studies on the olfactory basis for host plant attraction for the development of vector monitoring and control tools.

Chemical control and insecticide resistance status of sand fly vectors worldwide

BACKGROUND

Phlebotomine sand flies are prominent vectors of Leishmania parasites that cause leishmaniasis, which comes second to malaria in terms of parasitic causative fatalities globally. In the absence of human vaccines, sand fly chemical-based vector control is a key component of leishmaniasis control efforts.

METHODS AND FINDINGS

We performed a literature review on the current interventions, primarily, insecticide-based used for sand fly control, as well as the global insecticide resistance (IR) status of the main sand fly vector species. Indoor insecticidal interventions, such as residual spraying and treated bed nets are the most widely deployed, while several alternative control strategies are also used in certain settings and/or are under evaluation. IR has been sporadically detected in sand flies in India and other regions, using non-standardized diagnostic bioassays. Molecular studies are limited to monitoring of known pyrethroid resistance mutations (kdr), which are present at high frequencies in certain regions.

CONCLUSIONS

As the leishmaniasis burden remains a major problem at a global scale, evidence-based rational use of insecticidal interventions is required to meet public health demands. Standardized bioassays and molecular markers are a prerequisite for this task, albeit are lagging behind. Experiences from other disease vectors underscore the need for the implementation of appropriate IR management (IRM) programs, in the framework of integrated vector management (IVM). The implementation of alternative strategies seems context- and case-specific, with key eco-epidemiological parameters yet to be investigated. New biotechnology-based control approaches might also come into play in the near future to further reinforce sand fly/leishmaniasis control efforts.