Entomological surveillance of onchocerciasis in the context of devolution of OCP

cOnchocerciasis Control Programme in West Africa ProgrammedeLuttecontrel'onchocercoseenAfriquedel'ouest rorNr pRocRAMME coMMrrrEE Jpc - CCp coMlrEcoNl?'xl3l]. PT'oGRAMMEOffice of the Chairman JOINT PROGRAMME COMMITTEE Fourteenth session Luxemboure. 7-10 December l9a3 JPCl4.9(ocP/EAcl4.9) bntctNel: FRENGH September 1993 Provisional aeenda item 7 ENTOMOLOGICAL SURVEILLANCE OF ONCHOCERCIASIS IN THE CONTEXT OF DEVOLUTION OF OCP I. STATE OF THE ISSUE Epidemiological surveillance, mainly centred on parasitological surveys (skin snip)' and ivermectin treatment of onchocerciasis-patiunt, "r. at ptesent (apart from awareness-raising campaigris) the only onchocerciasis-relatJfields of activity selected in the context of devolution of OCp. In the original Programme .t.., ii.iieater par.tof which has been freed through vector control alone, this tongitudinal epide*iof-og]..i suiveitlance for early detection of a possible recrudescence of the infection is by far tii-ir.in activity in the context of devolution. However' the parasitological surveys are coming up ,or. and mLre 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 consideiably the microfilarial loads in the treated subjecs; - reluctance oi the populations to accept this technique' reluctance which will undoubtedly increase more and more; -rbsence,atpresent,ofanatternativetechniqueforearlydetectionofapossible t'ecrudescence of infection' Faced with this situation, for which efforS are being made to- remedy. partly- through awareness-raising campaigns to get tt e plprUtions to better rinderstand the validity of the skin snips, it seems appropriate to see *trettrer incntomological surveillance adapted to the particular context of devolution would not usefuily complement the present parasitoloS,ical surveys conducted in the human populations and theieby serve as an addiiional tool for an early detection of recrudescence of onchocercal infection' By entomologicat surveillance is meant here the catching of blackfljesand their dissection with a view to foof,irie fo, ihe disease" e;tho;.;, O,'volvulUs; this entomological surveillance has to lead to the measurement of entomorogil.r indic.sjhe interpretation of the results and decision-making on a possibility or r..rui.!.Ln.. or infection will be the final stage of this whole process. JPCl4.9(ocP/EACl4.9) Page 2 to see whethen (a) The different staS,es of the above-mentioned activities will therefore be dealt with now 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. TECHNIQUES INVOLVED 2.1. Bleckfly cetching The countries can easily adopt the method of calching by villagers yttdii present by OCP in post-control entomological studies; O. oi.i..ii'e is io t"i"h-tt many blackflies as possible' 2.2. Blackfly dissection It will consist in separating the parous from nulliparous blackflies and then looking for the pathogen onlyin the head-s of tfrJ p.rourLi..kflies. This technique has also been well mastered by OCP. Just one experienced OCP techni.i.n ..n, by this technique, dissect' on average' 100 blackflies in 45 minutes. National-team tectrnicians, retrained beforehand, will undertake these dissections without difficultY. 2.3. Onchocerceldentificatiog. 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 ihe 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 resuls of simulations made with the cpidemiological model of .onchocerciasis transmission (oNCHosIM) developeo uv ocp have enabred the atove-mentioned entomological parameter to be adopted, with a criticat biacrfiy infectivity threshold that makes it possible' during the first two years which follow tti..rt"iion of larviciding in a given zone' to confirm' ,i ,ri 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 infefion, the following iu.ttiont would have to be answered beforehand: (a) what is the critical infectivity threshold beyond which, many years after the cessation of vector .on,roiiv bCP, the possi6itity of recrudescence of infection inthedevolutionzoneconcernedcouldbementioned? (b) How many blackflies need to be dissected for a given point before establishing valid rate of infectivitY' ? JPCl4.9 (ocP/EAcl4.e) Page 3 (c) (d) Are we authorized to institute large-scale iverdrectin treatment in a zone on the basis of this critical blackfly infectivity threshold alode? Should this critical infectivity threshold be perceived sotely as an alarm signal that has to lead, initially, to cpidemiological ,uiu.yt in the human populations of the 5 zone concerned? CONCLUSION There are, at presentr some relatively simple entomological techniques which could be easily used by the naiional teams of the participiting Countries within the framework of an entomologicat surueiiian.. ,f,.t would validly compteme-nt tt e present epidemiological surveillance with a view to tr,. ."iJv i.iection of s posiiule 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 votvulus. 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 recrudes;;; iron.t witi frigf Uiacffly-densities, formerly hyperendemic' subjected tolntense human migratory movements"')' However, before encouraging the OCP Participating Countries to .include these entomological surveillance activitieiinitreir devolution plans, it would be appropriate' on the one hand, to define tt r .iiti..r ity 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 attituOe to adopt in the face of this critical infectivity threshold' The reflections have to be cSntinued in this sense and should lead to the preparation o-f a practicat entomological surveill.n.. guiA. to be made available to the countries concerned' It woutd then be possible to recommena ao.*oro-*l*rioto"i.d surveillance of onchocerciasis to be conducted in the context of deuolut'i6iihis *ould streiflhen the ability of the countries to detect, early, possible recrudescence of the infection and control it more effectively' t.