Drastic reduction of populations of Simulium sirbanum (Diptera: Simuliidae) in central Sierra Leone after 5 years of larviciding operations by the Onchocerciasis Control Programme.

The major vectors of the blinding form of human onchocerciasis in West Africa are two blackfly species, Simulium sirbanum and Simulium damnosum s.s. (Diptera: Simuliidae), identified at the adult stage as the 'savanna group' of the Simulium damnosum complex. In 1988, in the central part of Sierra Leone, the average daily biting rate (females/man/day) by savanna blackflies (mostly S. sirbanum) during the peak of the dry season (April-May) was 59.9, making up 69.1% of total captures on average. There was evidence of a strong long-range immigration of adult females of S. sirbanum through eastern Guinea in the dry season, with a reverse movement towards Guinea in the rainy season. Therefore, in 1989, the World Health Organization's Onchocerciasis Control Programme (OCP) extended its vector control operations from central West Africa to rivers of central and northern Sierra Leone, and to rivers of eastern Guinea. Four years of efficient larviciding drastically reduced adult populations of S. sirbanum in Sierra Leone. In the peak of the dry seasons of 1993 and 1994, the average biting rate by savanna blackflies in central Sierra Leone had dropped to 1.0, making up only 4.3% of total captures on average. Yearly biting rates by S. sirbanum in central Sierra Leone were therefore reduced to 2% of their pre-intervention levels. Based on larval samples, the S. sirbanum has been replaced by two forest species, S. leonense in the south and S. squamosum in the north. Since 1992, it has been possible to calculate accurate transmission rates for blinding onchocerciasis, based on DNA-probe identifications. From 1993, the risk of transmission has not only been reduced by vector control but also by mass distribution of ivermectin to rural communities. In terms of control strategy, the authors conclude that larviciding operations could be alleviated in central Sierra Leone without increasing the risk of blinding onchocerciasis transmission, as long as the migration of S. sirbanum through eastern Guinea and northern Sierra Leone is prevented.

[The necessity of conducting to the end the activities of vector control in the onchocerciasis control program in west Africa: recall of the stakes and proposal of a minimum budget for the period 1998-2002]. / De la nécessité de mener à terme les activités de lutte antivectorielle du programme de lutte contre l'onchocercose en Afrique de l'Ouest: rappel des enjeux et proposition d'un budget a minima pour la période 1998-2002.

After a brief presentation of the Onchocerciasis Control Programme in West Africa (OCP), the authors realize the health and socioeconomic consequences that could follow a cessation of larvicide treatment before 2002 in the south-eastern and western extensions of the Programme. Taking into account that OCP activities are theoretically supported until 1997, but aware of financial constraints that will probably increase from now to 2002, this paper proposes an a minima estimation of the residual vector control activities for a "phasing out" spread out for five years (1998-2002). These estimations essentially concern the larvicide coverage, the insecticides used, the entomological surveillance, the logistical support and their financial aspects. As far as 48 U.S. $ million amount for 5 years are concerned, the budget allocated for vector control activities should not exceed the third of the global amount allocated to OCP for the actual fourth financial phase of the Programme (1992-1997).

[Control of onchocerciasis vectors in West Africa: description of the logistics adapted for a large-scale public health program]. / La lutte contre les vecteurs de l'onchocercose en Afrique de l'Ouest: description d'une logistique adaptée a un programme de santé publique de grande envergure.

The Onchocerciasis Control Programme (OCP) in West Africa, launched in 1974, includes 11 participating countries and covers more than one million square kilometres. The aim of the OCP is to control blinding onchocerciasis (river blindness) which is caused by the savannah strain of Onchocerca volvulus transmitted by the Simulium damnosum complex. There is no effective macrofilaricide, so vector control to prevent the transmission of the parasite remains the method of choice, despite the availability of ivermectin, a drug which controls ocular morbidity. The potential value of vector control has been demonstrated by the original programme: 14 years activity has eliminated the disease as a public health problem in the areas included. This strategy requires adapted logistical support involving (i) widespread insecticide coverage (27,000 km of river are treated by the OCP during the rainy season), (ii) frequent (weekly) application of larvicide and (iii) prolonged intervention due to the life-span of the worm in the human reservoir, estimated to be approximately 14 years. We describe the vector control operations and their organisation 20 years after the initiation of the OCP. The OCP can be divided into 5 areas of logistic activity. The first covers activities involving insecticide and fuel management for the OCP as a whole: assessment of the requirements for the following year, ordering from insecticide and petrol suppliers, stocking fuel and insecticide at the depots covering the area. The second activity is the treatment of rivers with insecticide. This includes treating the ground with larvicide, the aerial operations run by an independent company supplying 12 helicopters on contract to the OCP, and use of satellite beacons for retransmitting of hydrological data. The third activity is monitoring the impact of larvicides on both the target (adult and larval S. damnosum) and on other fauna (fish, crustaceans and other insects). The fourth activity is field data collection and its processing. This involves a data transmission network to facilitate stock management insecticide application and entomological and hydrological surveillance using computer systems. The fifth activity is the coordination of vector control operations, technical and administrative staff and estimations of the funds available to the Vector Control Unit. The logistic aspects of other large-scale-insect-control programmes world-wide are considered, and the possibility of using the OCP as a model for such programmes (both public health and agricultural) is assessed.