Entomological surveillance of onchocerciasis in the context of devolution of OCP

WORLD HEALTH ORGANIZATION ORGANISATION MONDIALE DE LA SANTE ONCHOCERCIASTS CONTROL PROGRAMME IN WEST AFRICA PRoGRAMME DE LUTrE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST EXPERT ADVISORY COMMITTEE Fourteenth session Ouaeadoueou. 7-ll June 1993 ocP/EACl4.9 (93/ tt9/devo/2.t7) ORIGINAL:FRENCH ENTOMOLOGICAL SURVEILLANCE OF ONCHOCERCIASIS IN THE CONTEXT OF DEVOLUTION OF OCP STATE OF THE ISSUE Epidemiological surveillance, mainly centred on parasitological surveys (skin snip), and ivermectin treatment of onchocerciasis-patients are at present (apart from awareness-raising campaigns) the only onchocerciasis-related fields of activity selected in the context of devolution of OCP. In the original Programme area, the greater part of which has been freed through vector control alone, this longitudinal epidemiological surveillance for early detection of a possible recrudescence of the infection is by far the main activity in the context of devolution. However, the parasitological surveys are coming up more and more against a number of difficulties the principal ones being: a decrease in the sensitivity of the skin snip technique because of the ivermectin treatment of the populations, which decreases considerably the microfilarial loads in the treated subjects; reluctance of the populations to accept this technique, reluctance which will undoubtedly increase more and more; absence, at present, of an alternative technique for early detection of a possible recrudescence of infection. Faced with this situation, for which efforts are being made to remedy partly through awareness-raising comapaigns to get the populations to better understand the validity of the skin snips, it seems appropriate to see whether an entomological surveillance adapted to the particular context of devolution would not usefully complement the present parasitological surveys conducted in the human populations and thereby serve as an additional tool for an early detection of recrudescence of onchocercal infection. By entomological surveillance is meant here the catching of blackflies and their dissection with a view to looking for the disease's pathogen, O. volvulus; this entomological surveillance has to lead to the measurement of entomological indices. The interpretation of the results and decision-making on a possibility of recrudescence of infection will be the final stage of this whole process. l. ocP/EACt4.9 Page 2 The different stages of the above-mentioned activities will therefore be dealt with now to see whether: (a) the techniques involved are easily utilizable by the Participating Countries in the context of devolution; (b) the results obtained can be used to prevent recrudescence of onchocerciasis or control it. 2. TECHNTQUES TNVOLVED 2.1. Blackfly catching The countries can easily adopt the method of catching by villagers used at present by OCpin post-control entomological studies; the objective is to carch-as many blackflies as possible. 2.2. Blackfly dissection It will consist in separating the parous from nulliparous blackflies and then looking for thepathogen only in the heads of the parous blackflies. This technique has also been well mastered by OCP. Just one experienced OCP technician can, by this technique, dissect, on average, 100 blackflies in 45 minutes. National-team technicians, retrained beforehand, wiil undertake these dissections without difficulty. 2.3. Onchocercaidentification The national teams could send their onchocerca samples to the DNA-probe identification laboratory in Bouake (Cote d'Ivoire) which, having been given, among other activities, the responsibility of separating O. volvulus from the other onchocerca, will thus become a reference centre for the Participating Countries. 3. SELECTION AND CALCULATION OF THE ENTOMOLOGICAL INDICES The number of infective blackfly females per thousand parous females can be adopted as the sole entomological index to be used. 4. INTERPRETATION OF RESULTS AND DECISIONS The results of simulations made with the epidemiological model of onchocerciasis transmission (ONCHOSIM) developed by OCP have enabled the above-mentioned entomological parameter to be adopted, with a critical blackfly infectivity threshold that makes it possible, during the first two years which follow the cessation of larviciding in a given zone, to confirm, or not, the decision to stop vector control definitively in the river basin concerned. If it is wished to use this entomological parameter in the context of devolution to report recrudescence of infection, the following questions would have to be answered beforehand: (a) What is the critical infectivity threshold beyond which, many years after the cessation of vector control by OCP, the possibility of recrudescence of infection in the devolution zone concerned could be mentioned? How many blackflies need to be dissected for a given point before establishing valid rate of infectivity ? (b) I(c) ocP/EAcr4.9 Page 3 Are we authorized to institute large-scale ivermectin treatment in a zone on the basis of this critical blackfly infectivity threshold alone? Should this critical infectivity threshold be perceived solely as an alarm signal that has to lead, initially, to epidemiological surveys in the human populations of the zone concerned? '(d) 5. CONCLUSION There are, at present, some relatively simple entomological techniques which could be easily used by the national teams of the Participating Countries within the framework of an entomological surveillance that would validly complement the present epidemiological surveillance with a view to the early detection of a possible recrudescence of onchocerciasis infection in the OCP devolution zones. Besides, by means of DNA probe, it would soon be possible, from a group of flies, to detect and quantify the presence of Onchocerca volvulus. This identification task could be entrusted to the above-mentioned reference centre. For better cost-effectiveness, this entomological surveillance could be limited to zones at high risk of recrudescence (zones with high blackfly densities, formerly hyperendemic, subjected to intense human migratory movements...). However, before encouraging the OCP Participating Countries to include these entomological surveillance activities in their devolution plans, it would be appropriate, on the one hand, to define the critical fly infectivity threshold which would be the signal of recrudescence of onchocercal infection in the human populations and, on the other hand, to decide on the attitude to adopt in the face of this critical infectivity threshold. The reflections have to be continued in this sense and should lead to the preparation of a practical entomological surveillance guide to be made available to the countries concerned. It would then be possible to recommend an entomo-eoidemioloeical surveillance of onchocerciasis to be conducted in the context of devolution. This would strengthen the ability of the countries to detect, early, possible recrudescence of the infection and control it more effectively.