BackgroundLymphatic filariasis (LF) is a leading cause of disability in South Pacific regions, where >96% of the 1.7 million population are at risk of LF infection. As part of current global campaign, mass drug administration (MDA) has effectively reduced lymphatic filiariasis prevalence, but mosquito vector biology can complicate the MDA strategy. In some regions, there is evidence that the goal of LF elimination cannot be attained via MDA alone. Obligate vector mosquitoes provide additional targets for breaking the LF transmission cycle, but existing methods are ineffective for controlling the primary vector throughout much of the South Pacific, Aedes polynesiensis.Methodology/Principal FindingsHere we demonstrate that interspecific hybridization and introgression results in an A. polynesiensis strain (‘CP’ strain) that is stably infected with the endosymbiotic Wolbachia bacteria from Aedes riversi. The CP strain is bi-directionally incompatible with naturally infected mosquitoes, resulting in female sterility. Laboratory assays demonstrate that CP males are equally competitive, resulting in population elimination when CP males are introduced into wild type A. polynesiensis populations.Conclusions/SignificanceThe findings demonstrate strategy feasibility and encourage field tests of the vector elimination strategy as a supplement to ongoing MDA efforts.

SummarySemi-annual mass DEC chemotherapy combined with vector control at the beginning of the programme, has been administered on the remote island of Maupiti (French Polynesia) since 1955. The results of two surveys in 1985 and 1989, reporting 0% micro®laraemia, led to the hope that the eradication of lymphatic ®lariasis had been achieved. We combined parasitological criteria (micro®laraemia by membrane ®ltration), immunological (antigenaemia and serum levels of speci®c IgG antibodies) and molecular (PCR-based evaluation of infection in mosquitoes) techniques and found only good control of the parasite: We found residual micro®laraemia in 0.4% of the sample (mean level in carriers: 101.2 mf/ml), antigenaemia in 4.6% (mean level in positive persons: 714.4 units/ml) and speci®c IgG in 21.6% (including in one very young child). In addition, an infection rate of 1.4% was calculated in the Aedes polynesiensis vector population. These data, obtained in 1997 just before a hurricane, were partially con®rmed in 1999 (0.1% of infection rate in the vector). Together with the possibility of some resistance to DEC, various epidemiological factors critical for the eradication of lymphatic ®lariasis are discussed.keywords lymphatic ®lariasis, Wuchereria bancrofti, diethylcarbamazine, control correspondence Dr Ngoc

In French Polynesia, the cyclopoid copepod Mesocyclops aspericornis was inoculated into burrows of the land crab Cardisoma carnifex and into tree holes, drums, wells, and tires. It successfully reduced larval populations of Aedes polynesiensis and/or Aedes aegypti by 91-99%. Mesocyclops aspericornis has persisted to date for 29 mo in crab holes and for 60 mo in some wells, tires, and tree holes. As M. aspericornis does not effectively reduce Culex populations, copepod suspensions can be mixed with Bacillus thuringiensis var. israelensis, without detriment to the copepod, to effectively kill larvae of both genera. Its oligophagic feeding pattern, ease of mass production or collection, and high fecundity within probable temperature and salinity ranges of 23-30°C and O-4% make M. aspericornis a good candidate as a biological control agent.

In most of the 130 islands of French Polynesia, the stenotopic mosquitoes Aedes aegypti (the main local vector for the viruses causing dengue) and Aedes polynesiensis (the main local vector of Wuchereria bancrofti) share many breeding sites in water containers such as discarded cans, coconut shells, buckets and water-storage pots and drums. In addition to selective application of insecticides, non-polluting methods of controlling these mosquitoes have been evaluated during the last decade in two main ecological situations: (1) villages, where Aedes breeding sites are typically peridomestic; and (2) flooded burrows of land crabs, the major source of Ae. polynesiensis throughout the South Pacific region. Large-scale trials of biological control agents, such as mosquito fish (Gambusia affinis and Poecilia reticulata) and copepods (Mesocyclops aspericornis), and of integrated-control strategies have demonstrated the efficacy of certain techniques and control agents against the target Aedes populations in some village situations. Generally, mechanical methods (the use of layers of polystyrene beads against mosquito larvae and pupae, and screening against adult mosquitoes) were more efficient than use of the biological control agents. By integrating several methods of control, mosquito densities (as measured by human-bait collections and larval surveys) were reduced significantly compared with the results of concurrent sampling from untreated villages, and control remained effective for months after the interventions ceased. In land-crab burrows, the first attempts to control Aedes larvae used bacterial agents (Bacillus thuringiensis) and predatory copepods gave disappointing results. Mesocyclops aspericornis could be an effective control agent if the burrows were constantly flooded, but most burrows dry out and refill periodically, so copepod populations do not survive. As it proved difficult to reach all corners of the long sinuous burrows with any control agent, larvicidal (chlorpyrifos-methyl) baits were developed for foraging crabs to carry into their burrows. This novel technique proved to be effective and could become the method of choice for treating crab burrows. Further research is underway to find the optimum (biological or chemical) larvicidal ingredient for the crab bait. Despite the ecological and logistical challenges of controlling the Aedes vectors of the dengue viruses and W. bancrofti in so many scattered islands, the French Polynesian experience indicates that relatively simple methods can be integrated and applied effectively and economically. Operationally, however, success also depends on a strong political commitment and on at-risk communities that are sufficiently motivated to maintain a good level of Aedes control.

On Tereia Island, Rangiroa Atoll, 14,321 land crab burrows were treated with the copepod Mesocyclops aspericornis from January to June 1986, to control larvae of Aedes polynesiensis marks. In October 1987, the entire island of Tereia was retreated (17,300 burrows), and the neighboring island, Voisin, was left untreated as a control. From 5 to 15 mo after treatment, burrows with M. aspericornis contained an average of 2 Ae. polynesiensis immatures compared with 97 immatures from untreated burrows. Long-term larval control was successful in low-lying areas where burrows remained wet or were reflooded. Although there may have been other contributing factors, the major reason for lack of persistence of M. aspericornis in burrows over the entire island appeared to be poor resistance to desiccation. From all treated burrows in October 1987, M. aspericornis subsequently was found in 89.5, 39.1, and 24.1% of burrows sampled 5, 8, and 15 mo after treatment, respectively. The broad-scale results for Tereia indicated that there was no reduction of adult biting indices when compared with Voisin. Mark-release experiments on four occasions indicated that Ae. polynesiensis had a limited flight range and that the probability for interisland movement was low.