The elimination of the onchocerciasis vector from the island of Bioko as a result of larviciding by the WHO African Programme for Onchocerciasis Control.

The island of Bioko is part of the Republic of Equatorial Guinea and is the only island in the World to have endemic onchocerciasis. The disease is hyperendemic and shows a forest-type epidemiology with low levels of blindness and high levels of skin disease, and the whole population of 68,000 is estimated to be at risk. Control of onchocerciasis began in 1990 using ivermectin and this yielded significant clinical benefits but transmission was not interrupted. Feasibility and preparatory studies carried out between 1995 and 2002 confirmed the probable isolation of the vector on the island, the high vectorial efficiency of the Bioko form of Simulium yahense, the seasonality of river flow, blackfly breeding and biting densities, and the distribution of the vector breeding sites. It was proposed that larviciding should be carried out from January to April, when most of the island's rivers were dry or too low to support Simulium damnosum s.l., and that most rivers would not need to be treated above 500 m altitude because they were too small to support the breeding of S. damnosum s.l. Larviciding (with temephos) would need to be carried out by helicopter (because of problems of access by land), supplemented by ground-based delivery. Insecticide susceptibility trials showed that the Bioko form was highly susceptible to temephos, and insecticide carry was tested in the rivers by assessing the length of river in which S. damnosum s.l. larvae were killed below a temephos dosing point. Regular fly catching points were established in 1999 to provide pre-control biting densities, and to act as monitoring points for control efforts. An environmental impact assessment concluded that the proposed control programme could be expected to do little damage, and a large-scale larviciding trial using ground-based applications of temephos (Abate 20EC) throughout the northern (accessible) part of the island was carried out for five weeks from 12 February 2001. Following this, a first attempt to eliminate the vectors was conducted using helicopter and ground-based applications of temephos from February to May 2003, but this was not successful because some vector populations persisted and subsequently spread throughout the island. A second attempt from January to May 2005 aimed to treat all flowing watercourses and greatly increased the number of treatment points. This led to the successful elimination of the vector. The last biting S. damnosum s.l. was caught in March 2005 and none have been found since then for more than 3 years.

Susceptibility of Simulium damnosum complex larvae to temephos in the Tukuyu onchocerciasis focus, southwest Tanzania.

Tukuyu onchocerciasis focus was earmarked for vector control using insecticide against larval stages. Susceptibility tests of mature larvae of Simulium damnosum s.l. vectors to temephos insecticide were carried out before and after two years of insecticide treatment of rivers within Tukuyu onchocerciasis focus, south-western Tanzania. The tests were done in 1999/2000 and 2004 using WHO standard methods. Mature larvae were exposed to 9 concentrations of temephos active ingredient, from the weakest 0.00975mg/litre to the strongest of 2.5mg/l. Each test concentration and control was run in duplicates of 25 larvae each, set for three hours in a cool temperature. After incubation, test solution was discarded and larval condition checked. Numbers of larvae in each category were recorded and used to determine mortality rate for each concentration as well as for the LC50 and LC95. A total of 1,666 larvae were tested, 942 during the pre- and 724 post-treatment. Results showed that both pre and post-treatment samples were susceptible, attaining 100% mortality at the diagnostic dose of 1.25mg/l, and LC50 between 0.129-0.34mg/l pre - and 0.144-0.211 mg/l (95% CI, P < 0.05) post- treatment. These values fall within the standard diagnostic dose of < or = 0.4mg/l for susceptible S. damnosum s.l populations. It was concluded that the endemic S. damnosum population was susceptible to temephos before and after two years of intermittent field application. Temephos was thus recommended for continued use in onchocerciasis vector control in the Tukuyu focus, to complement Community Directed Treatment with Ivermectin, but close monitoring of vector susceptibility should be done.

Susceptibility of Simulium damnosum complex larvae to temephos in the Tukuyu onchocerciasis focus; southwest Tanzania

Tukuyu onchocerciasis focus was earmarked for vector control using insecticide against larval stages.Susceptibility tests of mature larvae of Simulium damnosum s.l. vectors to temephos insecticide were carried out before and after two years of insecticide treatment of rivers within Tukuyu onchocerciasis focus, south-western Tanzania. The tests were done in 1999/2000 and 2004 using WHO standard methods. Mature larvae were exposed to 9 concentrations of temephos active ingredient, from the weakest 0.00975mg/litre to the strongest of 2.5mg/l. Each test concentration and control was run in duplicates of 25 larvae each, set for three hours in a cool temperature. After incubation, test solution was discarded and larval condition checked. Numbers of larvae in each category were recorded and used to determine mortality rate for each concentration as well as for the LC50 and LC95. A total of 1,666 larvae were tested, 942 during the pre- and 724 post-treatment. Results showed that both pre and post-treatment samples were susceptible, attaining 100% mortality at the diagnostic dose of 1.25mg/l, and LC50 between 0.129-0.34mg/l pre - and 0.144-0.211 mg/l (95% CI, P<0.05) post- treatment. These values fall within the standard diagnostic dose of ≤0.4mg/l for susceptible S. damnosum s.l populations. It was concluded that the endemic S. damnosum population was susceptible to temephos before and after two years of intermittent field application. Temephos was thus recommended for continued use in onchocerciasis vector control in the Tukuyu focus, to complement Community Directed Treatment with Ivermectin, but close monitoring of vector susceptibility should be done

A new simple technique for rearing F1 progeny from females of the Simulium damnosum Theobald complex.

Laboratory rearing of Simulium damnosum complex species enables female adults to be indirectly identified from their larval progeny. A simple, readily transportable rearing apparatus was developed for investigations by the Onchocerciasis Control programme by using unbreakable, easily packed and reassembled parts and by feeding the larvae at least partly with living green algae (Chlorophyceae) cultured in the laboratory. Although the larvae are sensitive to excess nitrogen compounds, the use of algae and a large volume of water obviated the need for a separate purification system in which water was circulated by an A/C electrical centrifugal pump between a container and an inclined trough in which most of the larvae developed. The rate of development was rapid giving rise to full size larvae (suitable for cytotaxonomic determination), pupae and adults. The adults were attracted into a collecting device by light. Developments to protect the larvae during electrical power failures included a slow-drying wooden rearing trough and an automatic switch to an alternative D/C power supply and pump. This technique has been in general use in O.C.P. laboratories in Benin, Ghana, Ivory Coast and Upper Volta and many thousands of larvae have been reared.

Studies on biting Simulium damnosum s.l. at a breeding site in the Onchocerciasis Control Programme area during and after an interruption of insecticidal treatments.

The increase of population of biting S. damnosum s.l. at a breeding site situated within the Onchocerciasis Control Programme area of West Africa following a temporary cessation of insecticidal treatments was studied by making every day collections on human bait at five catching sites and for 2--3 days per week at seven others, between 17 January and 9 April 1977. Observations on pupal and larval densities, water and air temperatures, relative humidity and water level were also made. The results showed that there was an interval of 23 days between the last application of insecticide and the appearance of the first nulliparous biting flies. This coincided with the finding of the first S. damnosum larvae. Subsequently, the biting densities increased following a geometric progression from about 10 flies per day to a maximum of 188 per day, giving a daily rate of increase of 1.07. Ten days after the re-application of insecticide, biting densities had fallen to about 5% of the pre-control level giving a daily rate of decrease of 0.74. Very little dispersion was observed with biting rates falling to 5% at 3000 m. perpendicular to the river. At these sites nullipars predominated. Conversely along the axis of the river densities at 1000 m away had to only 68% and were composed mainly of parous flies. Studies on diel activity showed the greatest biting activity between 15.00 h and 18.00 h with a lesser period of activity between 07.00 and 10.00 h. The significance of these observations in future planning of control strategy is discussed.

The laboratory maintenance and rearing of Simulium damnosum complex species as a research tool for the Onchocerciasis Control Programme in the Vola River Basin.

The laboratory maintenance and rearing of Stimulium damnosum complex species and the cytotaxonomic determination of reared larvae, has been useful to OCP in the following fields of investigation: 1. Species identification of reinvading females. 2. Distribution of biting populations of S. damnosum complex species. 3. Identification of eggs and small larvae found within the OCP area. 4. Host preferences of species within the complex. 5. Vector potential of S. damnosum complex species for Onchocerca volvulus and O. ochengi. 6. Provision of identified material for morphological investigation. The technique of indirect adult identification using reared larvae, may eventually by largely superseded by progress in morphological taxonomy.