Criteria for the definition of "onchocerciasis freed area" and a tolerable level of its transmission

---r-- I'ileetlng of Representatj.ves of ltratlonal Onchocerciasis Commi ttees Abitt.ian. 15-17 June 197? O],]CHOCXRC IAS IS COIITROI, PROGRAI ]i.,8 IiI T.m YOITA N]VM BT.SI}I AREA CRITIRIA FOR m{]r'DEFIIIITION 0F IIOIVCHOCF,RCIASIS IREE} AREASII A]'ID A TOIFA.AB],E I,EVEI, O} rTS IRA]ISI{ISSION cNo.77/8 INTRODUCTION It is the atm of the OncJ:ocerciasis Control ?rogranrme in the Yolta River Sasin Area, to enable the resettlement and exploitation of areas that have been deserted. beeause of the d.isease arid that are of econonical i-mportarce for the parbicipat ing countries " Even r.urd.er the uost favourable conditj-ons, the d.isease will not disappea.r before fifteen to twenty Jreors have passed' because of the longevity of theparasi-te. However, a resettleuent of populations should if possible take place before that, and rvittr.out any risk if an area may be considered rrfreedrr from trarlsrnissi-on of onchocerciasis. However if onJy partial eontrol of transmission is achieved under less favouzable conditions ar:d the disease lrersists, though less intenser a tolerable Ieve1 has to be d.efined at ','rihich the parasite load is reduced to a point where ocular arid general complications do not appear a4ymore and thr-rs no socio- economic conseqlrences ensue. fhe sitr.ration wil} renain unstable for several years because of swv"iving parasites that were transmitted before the beginning of control operations. It wiIl be important to i4entify the cnsuir:g risk for the resettled' populations if the veetor control operations shoulci come to a premature end. the problem of defining criteria for establishing rronchocerciasis freedrl areas or a tolerable leve1 oi the disease has become urgent beeause of tire successful results achieved. in vector control since 1974 in oc?, and' the progress in plaruring and implenaentirrg develpment projects in the zones concerned,. [he matter was raised du.ring the fourth session of JCC in Ouagadolrgou in December 1976. CRITERTA OF IIBDNATION FROU ONCHOCERCIASIS It is obvious that the onb, fo:ma1 criterion of an area freed froro oncho- cerciasis is the eradication of the disease in that area. fhe nedical evaluation should detect no more cases, and migrants infeeted from outside could be treated appropriately on a:r indirrid.ual basis. This id.ea1 objective will only be achieved following long-terIo vector control measures and will demand. about fifteen vears of continuous efforb. In Kerya, Iloberts et aI. (lSU ) still fourrd liutng arlult wonns arll dermal m:icrofi- lariae eleven years after f,hs slimination of the vector in the Kodera focus. Eighteen years after this elimination, he found on\y dead wor1ns and concluded tfrat tfre disappearance of the disease 1s abtained afber 11-17 years of completeJy internrptecl transmlssion. AccozdinglJ ft is now possible to d.efine the conditions that perurit the conclusion that the goal has been aehieved. An area can be con- sidered rrsaferr from onchocerciasis if there is nr risk of vlgw irmnlgrants and chj-Id.ren borrr since the beginning of control operations contractir:g the disease i-n the area. Eradication is coroplete when the disease is burnt-out j-n all pre- viously infected cases" fhe criteria of liberation of an area are fu.lfilled when : a) intomologlcaLly: Transmlssion is permanently intermpted; this is the major and absolute criterirn. -2- b).itpideq*logically: iftren inciclence of the disease is zero. l{o new cases of onchocerci-asis should. be detected, by parasitological nethods or by ophthal- mological examinations or, eventual11., by irmunologj-caI tests. A11 this is the logical consequence of interrupted transrni-ssion, but epi- demiological conflmration of the entouological results ls absoluteJy neeessaly. Method.s of assessnent a) Entouological: In order to assess the entonological situation a large nursber of catehing points scattered throughout the Programme area 1s visited. on a weekly basi-s. [he standard procedure is for an eleven hours catch, O70O-18OO hours, that is to say during the vrhole period of biting activity, to be mad.e by a team of two vector collectors working alternate hor:.rs. The leveI of surv-eilltince durirlg the wet season 1975 was as No l{o % of Catching ?oints : of Catching Days : lf Caught Plies Dissected : JuIy 2?-7 1 288 72 Augrrst 256 1 425 70 September 295 1 600 75 follows : October 290 1 524 75 N.B: Catching effort increased between JuI1' arxl September as newJy createcl catching points in Phase III iTest were brought into operation for the first time. In rtaking an asses$rent fron this large anou:et of entomological data a nrmber of pararoeters are considered. These include : - number of flies caug'ht par rtan/aay - age colrposition of the flIy population (/6 parous) - percentage of infective flies - ni.mber of infective O. volvqlus larwae per infective f1y. i{onth}ye se&sor&l arrd aru:r.Lal suomaries of this d.ata are routine\y compi-Iecl. example of an arrnual sunrutarlf is sho'wn in Annex I which gives details of the 3agr6 catching point on the rflhite v'oIta rlver durirrg the first year of Phase control operatlons. In orrler to i-nterrrrpt the transmisslon of onchocerciasis it is unnecessary to totally eredicate the vector - an unattainable objective in ar\y case. lriore important is the red,uction of the nunber of infective larwae transnj-tted to a a sufflciently Low Ieve1.. It is considered that to assess the vectorial importance of any S. lqs4gsuun population the uost useful oarameter is the Arunua,l Transrniss:-on potentff@ This is 15s ssf,"imeted nrmber of infective O. volvulus larvae received. by a rnan exilosed for eleven hours a d.alr througout the year at a particular site. For this purpose only third stage larrrae found in tl:e head capsule of the fly are deemed to be infective. the transmission potential is calculated for each month sepazately and the monthly totals sunmecl to obtain the AlP. flrom the Table(Annex I ) a su,n of the monthly totals g:Lves an AT? of 1 15 whereas a stzaight a,nnual average is only 7j. An I] -3 - lhe AIP probab\y represents something close to the maxjmrm possible tzans- mi-ssion for a given site, as 1t is inconceivable that arly villager would be exposed' to vector bites to the ertent that a vector collector is. tr\.rrbher, catchit:g points have usually been chosen in order to marcimise the catch. ihere-fore, it seems likely.ttat the.incidence of infectionwill fall to zero beforethe A?? d.oes so (see (r) uefow). Several]inritin8 factors are involved. It 1s 1ike\y that only a fractionof the third' stage larrae for.mcl ln the head capsules rf flles are successflrllypassed. into the body of the vertebrate host. Dven then there may be a s,r""esifu1i-urnunological reaction by the ylost' *tould the overall nr.rmber of infective laryaesuccessfully transmitted be sufficiently reduced then there i-s a possibility thatthe different sexes of adult wolgtrs fail to make contact i-rr the veztebrate host. In orrrer that the Anrual Tr"an.Enission Potentiar (nfP) should. be reasor:ab]yreliable catches of flies a^:rd dlssectlons need to be carried out regularly thro'ugh-out the potential tranmission season. Three catches per month should prov-id.e an adequate basis for the caleulation of the AtP, providing that a statistically representative sample of flies is d.issected by a competent technieiap. At present the entomological surveillance network adequately fulfiIIs greseconditi-ons. It ls important, i:ov,rever, that this level of surweillance acti-r-ity isnaintained in regions where the control progrararne seeus to be a success, at lelstuntil the results of follow-up epidemiological si.lrreys are ayaj-Iab1e. b) Eg]-$gryi--olo8iea1: It i-s neeessary to confi:m the interruption of tranmissionby establi"irirrg that this entomologieal observatlon "o""""porrd s to iuedicalfindlngs (aain criterion of riberation frou oncrroc"r"i;;i;). -- Thi-s can be confirraed. by I The Parasitologist who should confirru tl:.at the ineldenee of the disease is zero,i.e. no new caaes of onchocerciasis are being detected.. Ihis can best be d.emon-strated by the absenee of infeetions anor6' children, bozre slnce the beginrr-ir:g ofthe control operations, as they Lra.ve had no possibiiity to get the disease piLo,to_the control prcgr€rme. Attention should be paid to the possibilit-v of mj-cro-filariae transmitted in utero, as demonstrateri by Brinlonar:n et al. (lbl5.).Positive cases rvi11 thus have to be discussed in relation to their exact agc,degree of maternar infection arrd. entcmological situation. Another virgin population is constituted. by imrnigrantse settled in an area ?:t":. the begir:n:ing of control operetionsr.and. corti.ng iro-u non-onchocerca.l regj-ons(i{ort}tern Upper-,'o1ta, i.1a1i, rali or lliger). A paras:-tological exaroination aial asociological j-nvest:gati-on shoulc'i elirnimte t}:.ose with a previons tenporary stayi:e end,emic areag. Concern.:ing the autochthonous populations, th,: negative cases shou14 be care-ffrI1y followed after the first exani-:oation. fhe fact {tiat tfrey rnay becomepositive after the first exami:aation does not necessarj-1y sigrrtqf that they havebeen infected after the beginnlng of vector control. tirly ,"y reveal a 1atentinfection that was not d.etected et the first control. The parasitological work should use the urost sensitive diagnostic technique on the population most 1iabIe to infection. In this respect, following fr-rrther studies, immunological methods may become the most iuportant diagnostic tool inli8ht infections. Taking into account the lncubation period of the di-sease andthe difficulty to reveal a tight infeetion with present techniques, a J-year inte:r-val seems to be a minimum to achieve a reliable evaluation of the cument situation. -4- lhe 0phthakaologist will complete the laboratory tlata by confi:ming thatparasitologically negative cases shorr no ernid.ence of nlcrofilariae in the eye ortypical onchocerclasis lesions. In cases where a punctate keratitls is the on\y sign of onchocerciasls the speci-ficity is relative. Onchocercal pr.rnctate kerati-tis is considered. to be a slgn of early and /or light infection and. will conse-quent\y become important diagnostic crj,terion when the level of tranEni sslon has been si-gnificantly reduced.. However, this rpacity in itself is not trnthognomon:Lc and uay glve rise to diagnostic errors lrrd.er ceztain clrcumstances. In prerrlously infected cases ocular onchocerciasis may prCIgress in splte of an intermptecl tm.nsmission, on accourrt of i-rreversible lesions, ard the risk of blindness vril1 accorrdingly not be zero in that population. SUTdT,IARY: Criteria of onchocerciasis freed. areas arrd. associated reuarks After 15 years of Campaign During Campaign Incidence of onchocerciasis = ZETO Il,egression of prevalence in relation to d.eath of adul-t wol!:lis mI[O]'tOlOGf Arrntral t ::ansm:L s sion potential = zeTo Possible persistance non-infected. flies Prevalence of active onchocerciasls = zero Serolory nay reveal traces of previous disease freatment of i::fected m:igzants Prevalence of a"ctive ocular onchocerciasis = ZeTO Risk of bU.:edness due to onchocerciasis, Ii mi ted to self-perpetuatlng ].esi-ons Arunual transmi ssion potential red.uced to a very 1on, 1eveI ,Sign:LficantJy redueed fly po lation 0H{t5IAIi.f otocY Incidence of oeular rrf. = z€roin v-irgJ-n populati-on Regression of prevalence and i-ntensity of nf. in the eye Persistir:g irreversibJ-e lesions and risk of blindness -5^ A TOI,EITABIE INilTEI OF CNCHOCI;RCIASIS I'There control of the f1;r population renains incomplete and tzansrrrlssion pepersists, it should be established lvhether a tolerable leve1 of the d.isease exists which woulrl al1ow resettlement prograumes to proceed. without risk. A tolerable level of onchocerciasis may be defined by the -eUseqcg Sf.-qeqfqaf,complj-cations of socio-ecoryqic j-mportance. fhis definition will be based essen- tia1Jy 6r:a ophthalmological data but parasitological and entomological find.i::gs are also of import&rlc€. O[rhtha]no1ory: Ocular onchocerciasis m,xy be considered as tolerable if there j.s only a limited nr.mber of microfilariae in the eye and no serious arrd/or i.:=.ever- sible ocular lesions. [his rvouJ-d inp]J no risk of blinlness or rrisual handioap due to onchocerciasis. flhe signs and lesions of ocular onchocerciasis can be classified as follows: - sclerozing ke::atitis optic atzoptry (post-neuritic a:rd,/or conseeutive choroitloretinal lesi"ons iritisx N.B: 'x An irltls roay in itself be a reversible lesi-on, but neverbheLess its coexistance with onchocerciasis sigr:-ifles a severe and long-stand.ing ocular lnvolvement. Because of the possible serious sequelae of an anterior uveitis(synechlae, secondary cata::aci and/or glaucoua) it should be considered as apotentlalJy serious lesion. A tole:rable Ieve1 of microfilaria"l invasion of the eye has to be defined.A preli-nlnary working approxlmation has been suggested by Duke & .fimderson (lg7Z * 1976) conc}.rdi:tg that more than 20 rof. in the anterior chamber or more than 50 mf.in one cornea consti-tute a risk factor of blindness. Ihe imporbance of nf. inthe anteri-or ehanber as a risk factor has also been r:nderlined. by Ro1land (lSl+) and Br-dde" (lSlS). Ihe figures mentioned above have to be confi:med and it uight be uore relevent to calculate the total amor"m.t of mf. in the anterior segnent ofthe eye a;od relate them to parasitological paraneters. Irnmunological factors rrrEry also play an important ro1e, but in this field almost no investigations have been done so far. Different studles on a tolerable 1evel of ocular onchocerciasis may be env:lsaged: a) a longitudinal str.rd,y of a non-infected population moiring into an area of ]owtransrnission. fhe lncid.once and amouu:t of microfilarial j-nvasion of the eye should be carefl:lly studied., eonconltant with the absence of lesions. Hovlever, a study of this lcind will take time, probably 5-'loyears, before ar4y reli-ab1e results are obtaj-ned and it may be difficult to define and folIow-up a population in an area of lntense mlgrertion. fhe potential arrnual tranmission must be well-defined and. stable. Irreversible - microfilariae in the anterior ehauber - microfilariae i:e the cornea, dead or 1iv:Lng - pwrctate keratitis -6- b) a transversal study of stable populations llving 1n an area with low level of transnni ssion could gi-ve a more rapid answer to the question of a threshold leveI of transrn'i ssion, e.g. where ocular onchocerciasis exists, but only in a mi-ld fols. RellabJ-e and contj:ruous entomological d.ata are necessarXr for this study, a.niL the stability of tbe level- of trenmission established over several years. tttis study strould be caried. out outside the OCP progranme area. Results from the Cameroons(Unte et a1. 1975) i-nd.icate that there nagr be a high prevalence of onchocerciasis but stiIl no bllndness due to the disease when the arrnual transrnission potential is about 500 j-nfecti-ve 1a:nrae/uaa/year. However, in this str.:dy a very limited populatlon was e:canlned (lZ5 persons) ana sever€ lesions in fo:m of scle:nozir:g keratitis and iritis were formd.. fhe absence of blindness may ttren be due to a emall populatlon a^nd the life expectancy of blind people. Dete:mirri-ng a threshold would be of utmost impoz,bance for resettlement a^nct adrrising on populatj-ons at risP. i:r regions within the programne area where a Iow level of tran,<mission may persist. c) a 1eve1 of tren.qmission that is tolerabl-e for a popul"ation prerriously non-infec- ted may wel1- provolc an evolution of alread.y existing severe lesions in hearrilyjrrfected cases. [he progresslve r:ature of irreversible ocu]ar lesions ls hnowa (A.raaen 1 975) but the natural evolution of an early sclerozing keratitis or fr.rndal lesi-on nay possibly change with sigrrificantly reduced. or internrpted tranm:lssion. ftrls has to be confirmed. by a longitudinal study of severe lesions in relation to the anrrual t::ansrmlssion potential.* In the absence of chemotherapy there will be a rrlost generationrr with regarrl to bllndness, because of the self-perpetuating terrlency of severe ocular onchocerciasis lesions. fhe results from Kerlya (noUerts et aI. 1957) indicate a relatj-ve\y higher bllndness rate in previously tryper-ende- mic areas, even 11 and 18 years after the elim'ination of the vector. However, it will be most important to assess to what ertent this problen roay be recluced. by vector control, especially when plaruring an economical clevelopnent ln prerriousJy heavily jnfected areas. Parasltolorys Ihe essentiaL point wi-I1 be the d.efinltion of a level of prevalence amd nlcrofikrrial density consistent with ophthalmological- crj-teria mentioned above. flhe relationship between the de:mal m:lcrofi]-arial density amd ocular lesions has not yet been clear\r established. a) tfrere is a certain correlation between the existanee of ocular onchocercj-asis lesions and the ruicrofllarial density et the outer canthus. However, this geneml correlati-on has no absolute value as an individual risk factor. U) ttrere is no strlct epid.emiologi-caI relationstrip between blind,ness rate and the mean microfilarial density, calculated on the basi-s of 2 snips at the iliac crest. c) ttre combination of the three parameters, prevalence of the disease, mean micro- fi1.arial density and blindness rate may peru:it to identify an area of rflow risHt representing a tolerable level- of er:deralcity for the population as : Homogenous vlllages with a prevalence of less tban \Ui,t a mean m:lcrofil"arial density of less than 10 mf .r/sn:ip and proporbion of blindness not exceedj:rg the one caused. by tlisord.ers other than onchocerclasis. * TLre influence of 15 years of relative vector control on ocular has been studied. in a focus in MaIi (tfryfefors & Rolland 1976). ficarrt reduction of the prevalence of mf . in the eye was fowrd, conclusions can be drawn concerrring the severe lesions because ground d.ata. onchocerciasis A hj.ghly signi- but no defi:rite of limited back- -7 - NB.1 ltre micrcfilarial ctensity in this contert is based on the examj.nation of 2 s:;Lps in tlistilled water after 30 minutes. To arrive at the corresponding clensi-ty j-n saline solution after 24 hours, these figures have to be ntrlti- plied by a factor of approxi.uatdXr 2.5. NB.2 [hese fig:.res represent the average by viLLage a,nd ca,nnot be d.irectly applied. to irdirntdua,ls. EVen if a satisfactory comelation to the ophthalmological criteria can be establishd I a tolerable level of the dj-sease from a netlical polnt of vienr nust take into account also that 3 - there strould be no microfilaruria. flris qrnpton is a sigu of severity of the disease signisring a generalized clisease which car:not be considered as rrtolerablerr. Entomologr: TLre entomologist should. work out the anrnral traxrgission potential Consistent with the oBhtha}nologieal- criteria, i.e. the CIad-uun nrmber of infective larrae that ag.ind.ividtral can receive per year without eteveloping severe ocuLar leslons. TTIHESIOI'D OF IOUENA}ICE AI{D SIABIIIIY [Lre concept of stability or insbabllity in htman ecolory can be applied either to the ecological rysteu considered, or to the different states or situa- tlons, which this system uaY assume. []re stabj-]ity of a system nay cond.ition the instability of these states or sitr.ntions. Io give a physical illustration of what is meant: A football- fillecl with air can be -onpared. 1o a stable system because of its capaci-ty to return to its original strape after havir:g been d,efolsed by the foot of the sporbsnaJx. The temporary and hence instable ttefomation can be coupared. to the situation of the system tpd.er the influence of an erternal supptry of enerry. As thls supp\r ceases, the d.efo:mation wiLl cease also, and. the system will return to its poigt of tlepar- ture. Ihis eorresponds to the ctefi-rrition of stability in cybernetics (tOaus 1969). ftre instability of a situation is thus only an aspect of the stability of the systen with regarri to a clefj-ned disturbance. An rrnstable rystem would be one which keeps the defortatlon inflictedl f-ike an rxrlnflated. footbalI. lhe system of transmlssion of onchocerciasis comprises a1l- ecological factors centred. on matrr, the parasite and. the vector, that deterai:re thelr respectiv€ 1Iuo- bers a.nd contacts. flhe different states or situations of the qrstem would be the results of the vector control ca,mpaign. As long as rnarr and vector keep or re-establish contaet before the elirnina.tion of the parasite in the hr:roan population, the situatlon will always be unstable. Ihis meaps that a prevalence rate or meaJll microfllarial density resulting from a malL residr:al trangission and consid.ered as tolerable, Inay not be taken as a reaaon for discontinuation of the vector control campaign. -8 - It ls however possible, at least in theory, that as a result of yector control tranmlssion decreases - wlthout being conpletely interrupted - to a level insuffi- eient for the reprod.uctlon of the pazasite, thus leading eventuaS.ly to a point of no-retuzn. Only a real\y operational mathematieal model couId. assist us i-n cteter- nining to vyhat level the tran$ni ssion a.:ed prevalenee would. have to be reduced and. for what period. to reach this critical poi-nt. It coulil jnclicate also the speed at which the prevalence would return to its origlnal leveI in case of intermption of vector control operations or of najor operer.tional breakd.ovrn. Thj-s speed must be lnportant1 os ca.rl be Judged from the foJ-J-owi:rg str;dies : 1st Study Thj-s concerns three v:ll1-ages on the Sanfora escamplent on the Cono'd river in Upper-Yo1ta. A larrricidal canpalgn had been fi:ranced in thj.s area by the .iJuropean Develolment tr\md (f'onas Europ6en de D6veloppeuent = IED), between Jarl:ary 1969 andOctober 1973, w:ith internrptions fron April to June 1972 afi, Jarrrrary to Ju\y 197i. Ia,rricitling had started again with the beginning of OCP vector control operati-ons i.n Febnnty 1975. Itre temporary lnternrptions of 1972 and 1971 }:an been followed by i.:rcreased u:mber of fLies ear:ght; their theoretical anrnral mean was z 16.OOO/wn Ln 1967 , 1O.OOO in 1969, 5OO in 19701 then rrcse agaln to 5OOO tn 1972 ard 4OOO in 1971.(Nr.unbers of the two ca"tctring polnts of l(azfiguela a$d Boalad.lor.rgou taken together. Itrere are no tlata for 1974). The iuact of the canpaign fj-nanced. by F@ and the 'impact of tts J.nternrption on prevalence can be seen from the follwing table (frost and Rol"Lard 197il 3 Villageo Czude Ra.tes AdJusted Rates1968/1969 Dec. 197, Oct.1975x Dec. 1977 0ct.1975 Bod.adlougou Fab6dougou Kolokolo 69.8 61.8 51.3 45.7 43.1 44.1 59.6 51.6 61.2 40.- 74.- ,6.4 56.5 5'i .8 56,5 x No adjustment has been possible for d.ata for 1968-1959. It carr be seen that the beneficial effect of the campaign 1968-1972, ntrioh was errldent in 1973, was amnihilated by the intenral between the FED and OC? campaigns and by the temporary internrptj-ons of control operations in 1972 or:il 1973. 2nd Study In 1968, Rolland (lSlZ) visitea the malL lryperendemlc willage of St. Pie:re on the Kera1i6 river, a tributary of the 31ack Volta ln Upper-Vo1ta. Tttis village harl been foundecl in 1967. Ihe lnhabitants had been recnritetl in geveral sumoun- ding v:L1rages, partj-cul"arly in [ansila. TIe can get an idea on tbe prevalence of onclrocerciasis in their v:tlIages of origln r*ren we eonsider the pereentages of nodule earriers - 1 /. at Tansila, 1l'fr at Fe1we, Mtrna, Dogo and 20 'ii at [amgouma. Here are the data obtained by Rolland ar:d his conments : Age group E'xam:ined ?ositive skin Ocular lesions ,-9 10-14 15 -29 ,o- 7 5 7 20 7 4 6 20 5 3 6 15 Infection by age groupsr and oeular lesionsr St. Pierr€ -9- rrskin elcamination was made by two snips, one each at the shoulder and iliac cres'b. Xxamination of the anterior chamber of the eye was nacle by slit 1amp, and of the posterior chamber by direct ophthalmoscopy a"fter dllatation of the pupil. lhe results ind.icated. : 1 ) The prevalence of onchocerclasis (g€.) was Upper-Volta viIlage, and was rrirtually the upward.s. the highest I have found j-:: aqy sam.e in all age groups from 5 years 2) The mrmber of ocular coriplicatj-ons was erbremely high, consideri:rg the brief period of exposure. 2{, of subjects showed mi-cr"ofilariae i-n the anterlor c}ram- aer; 34' showed. punctate keratitis, fi-gures of the same o:rd.er as those found. at Niarba and Tigr6, the two most heavily infected villages I have for.urd in all Upper-Vo1ta. Early ocular lesions pred.ominated, as might be expected, namely pr:nctate keratitis a"rad. microfilariae in the anterior charnber. One wonan presen- ted an onchocercal iridocyclitis, and two subjects had choroido-retinitis of presrmed onchocercal aetiologrrr. In the strict sense, there is no intersed.iate term between stability and instability. It nay.however be pezmj-tteri to talk about d.egrees of stability and instabiU-ty aecording to the speed of return to the point of departure and to the qr.nntity of input needed to ma:irrtain the transraission a.:cd preyalence at a smalI 1eve1. lhe degree of lnstability of a parbicular situatj-on can be influenced by the following factors : 1) [Lre time interval since the beglruring of vector control. An increasi:rg i-nter- va1 w111 result in a decreasi-ng parasite reservoir by : - a diminution of the number of positives r.md.er the effect of hurnen mortality and the replacement of the population of younger non-infected. age groups, - an aging of the pamsite population and an increasing mortality of adult woflns with tlme. 2) The importance of the parasite reseryoir, In case of J-nternrption of control operationsl the speed of return to the original situation would be a fi.mction of the resid.ual nr.ruber of parasites and of the age composition of the latter. 7) fhe eventual changes j.n the degree of inmunological tolerance associated with a red.uction of the parasite Ioad.. 4) fLre annual tren"mTi ssi-on potenti-a} and its fluctuations. 5) The eventual re-j-nvasion problems by migration of flies from outsid.e the contrcl area. 6 ) The l-mportance of popuLa,tion movements between controlled. zones a.:cd. zones where tm.nmissi-on is continuing. 7) The size of the population ertrrosed around a breedlng site. A bi-g population size may dilute the tiansrni ssion potential. B) the application of measr.rres compleurentary to the larrrici-da1 ca:npaign : -10- - elim'ination of riatural or artificial breed.ing sites by environnental mod ifieation - const:mction of village wells to diministr contact betrreen mar:. and river - the possibility of anti-parasite treatment of onqtrocerciasis ptient s and commxrities. In conclusion, i-t can be caid that every epideniological situation, not corresponding to the definitive llberation as i-t has been defined, is an wrstabLe one. Every mocliflcation in the paraneters su€Bested. will have a profound. reper- cussion - either good. or bad - on the evolution of the system. An inereased importance coulcl be glven to ueasures liable to increase the stabillty of an epidemiologieal situation resulti:rg from a reduced transmission. But all intermption of control on the attairment of a threshold of tole::ance would have drau'etic consequences for the populations re-instalIed in conficlence. lfe can only speeulate on the qr:arrtity of peramnent input necessary to naintain the tzanrs- uission below a dangerous threstrold,. 2. 1. 2). 4. 6. 7. B. 9, BIBIIOGRAPIry A}TDERSON J. & FUGISANG H.(1;gl>) Ocular onchocerciasis, risk factors for bLlncln'ess. oI,TCHO^il? /75 ,ta BRTIfiCIANN U.K., KRE}/ER ?" ?RESSIIUS G.[. & SA\7AD0G0 B' Transplacental transmission of uicrofilariae of onchoeerca Yolvulus'ffiesse) BRII{KLIAN}I U.K. Urlne exa,mtnation for microfilariae vlJ-lage of }trortherrr Ghana. Doc. OCP/fiPr/76.16 7p, (0. volvulus) in a BUCK A.A.1 AIIDERSON R.I.1 KAIIAIA K. & Iil[CHCoCK Jr. J.C.(fgOg) oncfrocerciasis 3 some new epidemiologie and cl-inlcal findings. Results of an epidemlologie study in the republ-ic of Ichad. Am. J. Trop. l.{ed. Hyg.l 1Br 2, 2L7-27O Bf,TDDEN F.H.(lglt) watural h.istory of ocular onchocerctasis. o][cfio,/,?P /75 .7O DIIKE B.O.I.(fgZe) Ocular onchocerciasis-risk factors, treatment and prevention. YrR/"8r,/75.7- DIIIG B.O.l., ANDEP.SOII J. & -TUGISANG II. l;-;Ol>) The 0nchocerca Volvulus transmission potentials and assocfa@hocerciasis at four Ca,meroon Sud.an-savanna vlllages .[ropenmed-?arasit,, 26 t I-43-154 KIAUS G.(fSOS) Yrorteruuch der Kybernetik. Frankfurt and. Ha"uburgl Sischer Bllcheret SROST A. ii;ia) ie royer dtonchocercose du Farako (arrondisseuent central de Sikasso, rdp. du Lfa1-i) treize ans aprbs 1e d.6but des opdrati-ons d,e controle antisiuulidielllr€sr ;;;:- ociTniit?e.i---irp. ("o"" presse s6rie yrHo/oNCHo) PROST A" & ROIIAND A.(J:gll) Ovaluatlon du rdservoir de parasite humain d.ronchocerca volvuLus dans 1e foyei de Banfora (Haute-Vo1ta) aprbs slx ans de campagne anti-simu1-idienrLe' *"rp. ron6ot. ministbr" ae 1a sant6l Ouagadougou 1975t I9p' tr l-0. BibJ.iograPhyr Page 2 LI. RoBEB-TS J.M.D.1 IIEIIIIAI{N E., CoCKnT, C.trT. & IIGIToN R.3.(fSe7) onchocerciasj.s in Kenya 91 11r and 18 years after elimination of the vector, 'jHO 3u11. 77 2 f95-2f,2 ]''2. ROIIAND A.(tglz) Onchocerciasis in the viJ.J"age an u-nhappy experienee of repopulation endemic area.[rans. Roy. Soc. Trop. I{ed. I{yg . , 66 , of St. Pierre :in an uncontrol,led 6, 9f5-9f5 17. ROIIATtrD A.(1r9l+) Resultats de deux exanens b six ans cllintervalle dans deux cle Haute-Vo1ta. 3u11. OL{S' 5:-.t 7t 257-261 oculaires pratiquds villages onchocerguiens I+, THTIDIIORS B. & ROIIAND A.(WlA) aspeets oculaires 6lun viLlage onchocerquien d.e savane africaine aprbs treize ans de lutte anti- sj-mu].idienne. 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