Loa loa vectors Chrysops spp.: perspectives on research, distribution, bionomics, and implications for elimination of lymphatic filariasis and onchocerciasis

Abstract

BackgroundLoiasis is a filarial disease caused Loa loa. The main vectors are Chrysops silacea and C. dimidiata which are confined to the tropical rainforests of Central and West Africa. Loiasis is a mild disease, but individuals with high microfilaria loads may suffer from severe adverse events if treated with ivermectin during mass drug administration campaigns for the elimination of lymphatic filariasis and onchocerciasis. This poses significant challenges for elimination programmes and alternative interventions are required in L. loa co-endemic areas. The control of Chrysops has not been considered as a viable cost-effective intervention; we reviewed the current knowledge of Chrysops vectors to assess the potential for control as well as identified areas for future research.ResultsWe identified 89 primary published documents on the two main L. loa vectors C. silacea and C dimidiata. These were collated into a database summarising the publication, field and laboratory procedures, species distributions, ecology, habitats and methods of vector control. The majority of articles were from the 1950–1960s. Field studies conducted in Cameroon, Democratic Republic of Congo, Equatorial Guinea, Nigeria and Sudan highlighted that C. silacea is the most important and widespread vector. This species breeds in muddy streams or swampy areas of forests or plantations, descends from forest canopies to feed on humans during the day, is more readily adapted to human dwellings and attracted to wood fires. Main vector targeted measures proposed to impact on L. loa transmission included personal repellents, household screening, indoor residual spraying, community-based environmental management, adulticiding and larviciding.ConclusionsThis is the first comprehensive review of the major L. loa vectors for several decades. It highlights key vector transmission characteristics that may be targeted for vector control providing insights into the potential for integrated vector management, with multiple diseases being targeted simultaneously, with shared human and financial resources and multiple impact. Integrated vector management programmes for filarial infections, especially in low transmission areas of onchocerciasis, require innovative approaches and alternative strategies if the elimination targets established by the World Health Organization are to be achieved.Electronic supplementary materialThe online version of this article (doi:10.1186/s13071-017-2103-y) contains supplementary material, which is available to authorized users.

The Population Biology and Transmission Dynamics of Loa loa

“…They are found across the tropical rainforests of Central and Western Africa, alongside other Chrysops species, including Chyrsops langi, Chrysops centurionis, Chrysops zahrai and Chrysops longicornis. These species are not considered to be primary vectors of the human strain of L. loa, and instead are thought to contribute to the maintenance of the simian form of the infection (discussed below) through their crepuscular/nocturnal biting habits (simian L. loa exhibits nocturnal periodicity of microfilarial circulation in the peripheral blood), and preference for primate bloodmeals [10]. Whilst C. silacea has been more abundant than C. dimidiata at many of the sites studied [35], others have observed C. dimidiata to be more abundant [36], highlighting the importance of local ecological factors in underpinning species' abundance.…”

“…These include better resolution of the spatial distribution of loiasis (particularly in intermediate-and low-transmission areas), something that may be facilitated by the recently developed antibody[ 3 0 0 _ T D $ D I F F ] -detecting lateral-flow assay [88], as well as investigation of interventions other than ivermectin. Some work has already been carried out on this, including important exploration of vector control options [10], as well as investigations into albendazole as a treatment in instances where high blood microfilarial concentrations preclude treatment with ivermectin [89], but more research is needed.…”

“…Although these parasites appear to belong to the same species that infects humans (as evidenced by their capacity to produce fecund hybrids with the human-infecting parasites [60]) the overall contribution of simian hosts to human loiasis is considered minimal due to the differing biting habits of their respective vectors (C. silacea and C. dimidiata for human Loa; C. langi and C. centurionis for the simian strain) and the differing periodicities of microfilarial circulation in the peripheral blood (adapted to the biting habits of their vectors -diurnal for human Loa, nocturnal for the simian form) [10,61]. analysed to assess whether the ingested concentration of microfilariae from human blood affects tabanid Chrysops species mortality and lifespan.…”

“…Yet for L. loa, no such models exist, and little attention has been paid to the fundamental population biological processes that underpin model development. Previous reviews have focused on entomological, clinical, and immunological aspects of loiasis [9,10], but a review summarising the pertinent epidemiological, population biology, and transmission dynamics features of the parasite remains outstanding. In the context of loiasis' newly uncovered public health importance [7], this review addresses this absence, making the case for increased interest and effort in researching the disease, and advocating [ 2 9 4 _ T D $ D I F F ] for mobilisation of resources for its control using strategies guided by mathematical modelling.…”

Species Composition and Diurnal Activity Rhythm of Tabanids (Diptera: Tabanidae) at the Ivindo National Park and Its Environs

“…Tabanids are known mechanical vectors of animal Trypanosoma spp., like T. vivax, T. theileri etc. [3] as well as biological vectors (Chrysops silacea and C. dimidiata) of Loa loa filariasis [4,5,6].…”