Studies on the Intake of Microfilariae by their Insect Vectors, their Survival, and their Effect on the Survival of their Vectors

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

BACKGROUND

Loiasis 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.

RESULTS

We 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.

CONCLUSIONS

This 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.

The improvement of maintenance conditions for wild-caught Chrysops silacea and the production of infective larvae of Loa loa

Previous reports have shown that Chyrsops silacea survive poorly in captivity. Only 5% survive for more than 12 days, which is the time required for Loa loa microfilariae to develop into infective larvae. Improved maintenance conditions for C. silacea for the production of infective larvae were studied. Wild-caught female C. silacea were allowed to feed to repletion on mandrills, (Mandrillus sphinx), which were microfilaremic with human L. loa or on uninfected laboratory rats. Under optimal conditions 30-50% of flies engorged. The survival rates of fed and unfed flies were compared under a variety of conditions. Survival rates varied considerably under the various maintenance conditions. Ingestion of either rat or mandrill blood made no significant difference to fly survival. Optimal conditions were found to be as follows: one to two flies per sterilized jar with damp sand, constant access to sterilized 10% sucrose, a temperature of 23-29 degrees C and 84-98% relative humidity. Under these conditions 42% of the flies survived until day 12, when maximal numbers of infective larvae were present in the head.