Vector elimination project: Progress report (1999 - 2000)

ABSTRACT

A combination of factors unique to Bioko (Republic of Equatorial Guinea) renders it possible to stop onchocerciasis transmission pernanently through the elimination of the local vector, a member of the Simulium damnosum complex. These factors include its geographical isolation, its small size, the very few rivers that will require treatment and even for those that do, the short stretches that have to be treated, the close proximity of rivers that need treatment and the very low river discharges which means that relatively small quantities of insecticide will be needed. These facts were established by a study funded by WHO/TDR in 1996 and the subsequent project was adopted for implementation by APOC. Three main activities were initiated in the first one and half years (the period under review) and these were; development of the necessary skills and capacity in Bioko, pre-control entomological data collection and an impact assessment studies of the effect of Temephos on non-target aquatic fauna of Bioko. Six Guineans received training in all aspects of entomological data collection foronchocerciasis. control and are now competent enough to conduct these exercises with very minimum supervision. The data collection was initiated in April 1999 at four sites located on the most important rivers in the northem inhabited part of the island. Regular fly catches and dissections have been carried out since then with each site being visited once each week. A total of 21,713 flies were caught during 199 visits and 16,988 of them were dissected to find Onchocerca volvulus infections. When the data was analysed according to sites, it revealed that biting was intense throughout the period at all sites. The highest annual biting rate (ABR) was 60,698 bites/person/year recorded at Barleycorn on R. Apu and the lowest ABR recorded was 21,849 at Sampaca (R. Sampaca). Periodicity in fly densities were observed with most biting occurring during the rainy season when the rivers are as expected most productive. With regards to transmission the highest recorded index of annual transmission (i.e., ATP) of 1,447 infective larvae/person/year was at Sampaca and the lowest was 385 at Balacha de Riaba. These figures are low if considered that similar values are recorded monthly at sites on mainland West Africa which have had no history ofonchocerciasis control and but have similar biting rates. Bioko has a 10 year history of ivermectin distribution and this was thought to be the reason for the observed low transmission. Seasonal variation in transmission was also observed with most transmission occurring in the dry season, which is also not surprising because of increased proportions of old flies in S. damnosuz populations during this season. The environmental impact assessment studies also provided baseline information of densities and diversity of fish, prawns and macro- invertebrate fauna of Bioko. The main findings were that these were poor if compared to the richness of the fauna of rivers on the mainland. The major conclusions drawn from the studies on the impact of Temephos were that neither mortalities nor harm to fish and prawns should be anticipated at the treatment dosages envisaged and that the overall impact on the macro- invertebrate fauna was not that high as to prevent their recovery after treatment. Ground treatment of rivers is planned for the coming 2000 - 2001 dry season. The main aim is to establish the extent to which it could complement any aerial larviciding operations. The detailed planning has been completed (the plan of action is attached to this report). All the rivers on the island have been coded, the treatment points have been identified, treatment maps have been prepared and other activities, including procurement of spraying equipment, insecticide, etc have been initiated. A strategic meeting at APOC/FIQ is planned after the ground larviciding operation for evaluation and to decide on the future direction of the project.