Criteria for the definition of "onchocerciasis freed area" and a tolerable level of it's transmission

a'r/ ocvsrAc.5.1OVGIOCERCIASI S CO}ITROtr ?ROGRA}.INTE]N IIIE VOITA RIVER BASIN AREA SCIEI{IIFIC A}TD TESIINICAI /OVISORY COMMIITE]I Fifth meeting Ouagadougou, 1-4 March 1977 CRITERIA FOR TIIE DEFI]SITICIV 03 ''ONSIIOCERCIASIS FRED ABEASII AI'ID A TOIDBABLE IETEIT 0F ITS TBAI{$IISSICII INTRODUCTION It is the aim of the Onchocercj-asis Control Programme in the Volta Rlver Basj-n Area, to enable the resettlernent and exploj-tation of areas that have been d.eserted, because of the clisease and that are of econo&icaI importance for theparbicipat ing count ries. Even urrder the most favourable eonditions, the disease vsi11 not disappear before fifteen to twenty years have passed because of the longeuity of the pazasite. However, a resettleuent of populations should if possible take place before that, and ivlthout any risk if an area may be consid.ered. rrfreedrrfrom transrnission of onchocerciasis. However if only partial eontrol of transmission is achieved ulder less favourable corditions and the disease persists, though less intenser a tolerable leve1 has to be defi:red. at which the parasj-te load 1s reduced to a point rvhere ocular and general complications do not appear ar{ymore and thus no socio- economic consequences ensue. fhe situati-on will renain unstable for several years because of stu'v-iring parasites that were transrnitted before the begir:::ing of control operations. It vrill be important to identif) the cnsuing risk for the resettled populations 1f the vector control operations should come to a premature end.. The problem of definirig criteria for establishing rronehocercj-asis freed.tr areas or a tolerable 1evel of the disease has become urgent because of the successful results achieved. in vector control since 1974 ln OC?, and the progress in planning and iuplementing develtrxnent projects in the zones concemed.. [he matter was raised du-rirrg the fourth session of JCC in Ouagadougou in December 1976. CRTTERTA OF IIBEnATION FROI.I OI{C]IOCERCIASIS It is obvious that the onlv fo:maI criterion of an area freed from oncho- cerciasis is the eradieation of the disease in that area. The nedical evaluation should detect no more cases, arrd migrants infeeted from outside could. be treated appropriately on an individual basis. This ideal objective will only be achieved following long-terro vector control measures and will demar:d about fifteen years of continuous effort. In Kerya, Roberts et aI. (W ) still formrd liuing adult wonns ard. denna]- microfi-lariae eleven years after fils s}.'imination of the vector in the l(odera focus. Eighteen yeaxs after this elimi-nation, he founrl onl;r dead worrns ar:d concluded that the disappearanee of the disease is o}:tained. after 15-17 years of conplete\y interruptetl transmission. Accordingry ft is now possible to d.efine the corrdltionsthat pe:mit the conclusi-on that the goal has been achieved. An area can be eon- sid.ered. rrsaferr from onchocerciasis if there i-s no risk of nsv/ irrmigrants and child.ren born slnce the beginn-ir:g of control operations contracting the dj-seasej-n the area. Eradication is complete when the disease j-s burr:t-out in all pre- riously infected cases. The criteria of liberation of an area are fir].filled when : t a) .CIntomolJgically,: [re,nsmission is permanently intern-rpted; this is the najor and absolute criterion" -2- b) EpiAemiologically: iilhen inciclence of the disease is zero. l[o new cases of ffiu1d'bedetected.byparasitoIogica1nethod'sorbyophtha1- mological exantinations orr erentual\r, by iromirnological tests. All this is the logical consequence of interru.pted transrnj-ssion, but epi- demiological confj-rmation of the entouological results is absolutely necessalf. Methods of assessr,rent a) Entomological: In ord.er to assess the entomological situation a large number of catcfiine points scattered. throughout the Prograurme area is visited on a weeltly basis. Ihe standard. procedure is for an eleven hours catch, OTOO-180O hours, that is to say during the whole period of 'oiting activity, to be mad.e by a team of two vector collectors working alternate hours. fhe Ievel of surweillance dr-rring the wet season 1976 was as No of Catching ?oj-nts : No of Catching Days : /, of Caught Flies Di-ssected . July August 227 255 1 288 1 425 72 70 September 295 1 600 follows : October 290 1 524 7'73 ld.B: Catching effort increased. between JuI1' sr6 September catching points in Phase III ilest were brought into first time. as newly created operation for the In making an assessoent from tiris large amourit of entomological data a nr.mber of parameters are considered. fhese inelude : - nrmber of fl.ies caught par naxy'aay - age compositi-on of the fly population - percentage of infective flies - m.mber of infective O. volvulus larvae (/l parous ) per infective f\y. I',Ionth1y, seasonal and. example of an anrtual Bagr6 catching point control operations. arurual siummaries of this d.ata are routine\y compiled-. An sllrrrrialy is shown in Arueex I which gives details of the on the -rilhite '/olta rlver cluri4g the first year of Phase II In ortLer to interrupt tire tranm:Lssi-on of onchocerciasis lt is unnecessar]r to totally eredicate the vector - an i.urattainable objective in anJr case. l,iore important is the red.uction of the nwober of infective larvae tranmltted. to a a sufficiently low le-rel. It is consid.ered that to assess the veetorial imporbance of any S. damno9um population the most useful parameter is the Anniral Transrnission Potential (ATP). fhis is 16" gsf,inated ngrnber of infeetive O, volvulus larvae received. by a maJe exposed for eleven hours a day througout the year at a particular site. For thls purpose only third stage lanvae for.u:d. in the head capsule of the f1y are deemed to be infective. Ihe transndssion potential is calculated for each month sepa::ateIy and the monthly totals si.momed to obtain the AT?. 3?om the Table (.O.rrnex I ) a sr.ur of the monthl;r totals glves an A'IP of 115 whereas a straight ar:m.laI average is only 7J. -3 - fhe At? probab\y represents something close to the naximum possible trans- m:lssi-on for a given si-te, as 1t is inconceivable that aqy vlIlager would be exposed. to vector bites to the erbent that a vector collector j-s. tr\.r:ther, catching points have usual\y been chosen in onler to ma:rim'i se the catch, flLrere-fore, it seems 1ike1y that the incidence of lnfection will fall to zero before the ATP d.oes so (see (f ) uefow). Several 1imif,fi1g factors are i-nvolved. It is likely that only a fraction of the third. stage laz'rae found ln the head capsules of flies are successful\y passed lnto the body of the vertebrate host. Even then there raay be a successfuli.munological reaction by the i1ost. Srou1d the overall nmber of lrrfective la:nrae successfully tmnsmitted be sufficiently reduced then there is a possibility that the d.i-fferent sexes of adult worrrrs fai]. to make contact in the vertebrate host. In order that the Annrral Tran.$nission ?otentiaf (Afp) shou].d. be reasonably reAiable catches of flies and d.issectlons need to be carried. out regularLy through- out the potential tranmission season. Tkrree catches per month shoulci provide an adequate basis for the calculatlon of the Af?, providing that a statistical\y representative sample of flies is d.issected by a competent teehn:icj-a{.. At present the entomological surweillance netv,rork a.dequately fu1fills these conditions. It ls important, hovlever, that this 1eveI of surrreillance activity is maintained until the in regions nhere the control prograrnne seeus to be a success, at least results of follow..up epidemiological surweys are arailable. b) Epidemiological,: It is necessary to confi:m the intern:ption of transrissj_onby establlshirg that this entomological observatj-on corresponds to roed.icalfindings (main eriterion of liberation from onchocerciasis). this can be confirued b;.: The Parasitologist who should confiri:: tirat the j-neidence of the d.isease is zero,i.e. no new cases of onchocereiasis are beirg deteeted. lhis can best be deuon- stratecl by the absence of infections araong children, born since the beginn-lng ofthe control operations, as they ha"ve had no possibillty to get the dj-sease priorto the control progranme. Attention should be paid to the possibilit], of mj-cro-filariae traasmitted. in utero, as ilemonstraterl. by Brinlmann et aI. (lgl1).Positive cases rvi-ll thus have to be d.iscussed. in reJ.ation to their exact age, degree of mater:nal infection and entomological situation. Another v-irgin population is constituted by i:a'nalgrants, settLed ln an area after the begin:aing of controL oper"ationsr.and. corring fro-o non-onchocercal regi-ons(ITortlterrr Upper-:o1tar liali, Lta11 or iliger). A paras:-tological examination ai:d a sociological investlgation shou.lcl- elirnirrate those with a previou.s temporary stayilt endemic areag. Concerning the autochthonous populatiorrse thr: negative cases should. be care-ffr1ly followed after the first examjr:ation. fhe fact that they may becomepositive after the first examination does not necessarj-\y sign:tqf that they have been irrfected after the beginning of vector control. fhey may reveal a latent lrrfection that was not detected et the first eontrol. The parasitological work slioulcl use the most sensitive diagnostic technique on the population most liable to infection. In this respect, following flrrther studies, imroturological methocl.s may beeome the nost important diagnostic tool inIight infecti-ons. faking into accor.rnt the i-ncubation period of the d.isease and the difficulty to reveal a light infectlon with present technlques, a 5-year lnte:r- val seems to be a minian:m to achieve a reliable evaluation of the cr.urent sitlmtion. -4- lhe Ophthalmologist will comr:lete the laboratory data by confiming that parasitologically negative cases show no evidence of nicrofilariae in the eye or typi-eal onchocerciasis lesions. In cases irihere a punctate keratitis i.s tire only sign of onchocerciasis the specificity is rela.tive. Onchocercal punctate kerati- ti-s i-s consid.ered to be a sign of early ana /ov light infection and will conse-quently become important diagnostic criterion when the level of transmission has been significantly reduced. However, this opacity in itself is not pathognononlc arrd. uay give ri-se to diagnostic errors r.:nder ceztain circumsta,nces. In previously infected cases ocular onchocerciasls may prugress i.n splte of an interrupted trans"m:ission, on accor:at of irreversible lesions, ard the risk of blindness w"il1 accordi-ng1y not be zero in that population. STJMI,IARY: Criteria of onchocerciasis freed areas and associated remarks After '1! years of During Canpaign Canipaign Arrnual transnission potential = zeTo]f,TIOLIOIOGY Possible persistance of non--infected. flies Prevalence of onchocerciasis active = ZETO Serologr may reveal traces of prerrlous d isease Treatment of infected migrants Prevalence of active ocular onchocerciasls = ZeTO Risk of bli.:aclness due to onchocerci-asis, lfuntted to self*perpetuating ]-esions Incidence of onchocerciasis = ZoTO i'legression of prevalence in relation to ti.eath of adult woIlns Incidence of ocular In.f. = zerroin virgin population Regression of prevalence ar:d intensity of nf. in the eye PAMSIIOIOGY Persisting and. risk of imeversible lesions blirrdness Arunral tran.srni ssion potential red.uced to a very low 1evel Sigrrificant\y reduced flIy popu Iation OPI{IIIAL:IOIOGY tr A TOIERASItr I,EVEI OF OI{CHOCDRCIASIS \There control of the fl+r population renains inconplete ar:d transrclssion pe persists, it should be established whetlrer a tolerable leve1 of the disease exists which would. allow resettlement prograrnmes to proceed. without risk, Ato1erab1e1eve1ofonchocerciasismaybedefined'bythe-@ complications of sooio-econorr-l-c jm-portance. [his definition will be based essen- tially 6n ophthalmological data but parasltological afid entomological fir:d.lngs are also of importance. Ophthal-mol-o$r: Ocular onchocerciasis may be considered as tolerable if there is @;Ertea number of nicrofilariae j:r the eye and no serious ar:,lrlor irreve:l- sible ocular lesions. Thls would fmply no ri-sk of blind.ness or visual har:dioap due to onchocerciasis, The signs and lesions of ocular onchocerciasis can be classi-fied as follows: - sclerozing keratitis optic atroptry (post-neuritic a"nd.r/or consecutive chorold oretinal lesions iritis* N.B: * An j-ritls may in j-tself be a reversible lesion, but nevertireless its coeristance with onchocerciasis signifies a severe and long-standing ocular involvement. Because of the possible serious sequelae of arr anterior uveitis(syneehiae, secondary cataract and/or glaucoma) it siroul"d be consid.ered. as apotential\r serious losion. A tole:rabIe leve1 of m:icrofilarial invasion of the eye has to be defined. A preliminaqy worl<Lr€ approxj-mation has been suggested by Duke & l\:nd.erson (lg7Z *, 1976) concluding that more than 2O mf. jJl the anterior ehamber or more than 50 nrf. in one cornea constitute a risk factor of blindness. The importance of mf. in the anterior chamber as a risk factor has al.so been w:derlined. by Rolland (lll+) ar:d Budder, (tg?>). fhe figures mentloned. above harre to be confirned. and it night be more relevant to calculate the total ainount of nf. i:r the anterior segment of the eye amd relate them to parasitological parerueters. Inrormological factors IIlEty also play an important role, but in this field almost no i:rvestigations have been d.one so far. Different stud.ies on a-{6terab1e 1eve1 of ocular onchocerciasis may be envisaged: a) a longitudinal study of a non-infected population moving lnto an area of low transmission. The inci-dence an1 aroor.mt of micrrofilarial invasion of the eye should be carefully studied, concoraltant with the absence of lesj-ons. However, a study of this kind will take tjme, probably l-1Oyears, before any reliable results are obtained. and it may be dtffieult to d.efine and follow-up a population in an area of intense migration. The potential annual transri ssion must be well-definecl and stable. - microfilariae in the anterior chamber - uicrofilariae in the cornea, dead or liuing - punctate keratitis -6.- b) a transversal stud.y of stable poprlations living in an area with 1ow leve1 of tranmj-ssion could give a more zapid answer to the question of a threshold Ievel of tzensrni ssion, e.g. where ocular onchocerciasis exists, but onJy in a rn.ild. forrn. Beliable and continuous entomological data are necessary for this study, and the stabllity of the leve1 of tranmlssion established. over several years. tttis stud.y slrould be carried out outsid.e the OCP programme area" Results from the Cameroons (Out<e et a1. 1975) ind.icate that thcre maJr be a high prevalence of onchocerciasis but still no blir:dness due to the dj.sease when the anrni.al t:ansission potential is about 5OO infective larirae/nan/year, However, in this study a Yery limited population was exarj-ned (tZ5 persons) and severe lesions in form of sclerozing keratitis and iritis were founrl. The absence of blj-nd.ness may then be due to a ma1l population and the life expectancy of blind people. Dete:m:inir:g a threshol-d would. be of utmost importance for resettlement and advising on populations at risk" in regions within the programme area where a 1ow level of tratrmi ssion may persi-st. c) a 1eve1 of transnlssion that is tolerable for a population prev-iously non-infec- ted nay well provoke an evolution of already existi-ng severe lesions in heari\y infected. cascs. The progressive nature of ir:reversible ocular lesions is ]qloBrr(fuAaen 1975) but the natural evolution of an early sclerozing keratitis or f\:::llal lesion uray possibly change with sigrrlficantly reduced or intern:pted. tzansission. This has to be confirmed. by a longitudj:ral study of severe lesions j:e relation to the annual transrnission potential.* In the absence of che;mootherapy there will be a ttlost generationrt with regard. to blindness, becauec of the self-perpetuating tendency of severe ocular onchocerciasis lesions. The results from Keqya (noUerbs et al. 1967) ind.icate a relatively higher blindness rate i-n previously hyper-end.e- mic areas, even 11 a"rrd 18 years after the efinination of the vector. However, it w111 be m6st important to assess to vrhat eztcnt this problem may be reduced. by vector control, especiatly when planni-ng eu:. economicarl- d.evelopment in prerriously heavlly infected areas. Pa:easitologr: The essential point will be the de:finitron of a leve1 of prevalence EffiITariat aensity consistsgt with oprrthalmotqglq"l "rlg@. The relationship between the d.e:mai microfilarial d.ensity artd ocular leslons has not yet been clear\r establish:d. a) there is a certain correlation between the existance of ocular onchocerciasis lesions and the microfilarial denslty et the outer ca.nthus. However, thi-s general comelation has no absolute value as an individual risk factor. b) ttiere is no strict epid.en-iological relationship between blindness rate and the mean nicrofilarial density, calculated. on the basis of 2 snips at the il-iac crest. c) tfre combination of the three parameters, prevalence of the disease, mean micro- filarial density and blind.ness rate may per"rn:it to id.entify an area of r'Iow risktr representing a tolerable leve1 of endenicity for the population as : Homogenous rrillages wj-th a prevalence of less than 3U/'t a mean microfilarial d.ensity of less than 10 mf./snl-p and. proporbi-on of blindness not exceeding the one caused by clisorders other than onchocerciasis. * flre influence of 11 years of rel-ative vector control on ocule.r has been studied in a focus in Mali (thytefors & RollarLd. 1976). ficarrt reduction of the prevalence of mf in the eye was fou:d, conclusions caJ] be drawn concerning the severe lesions because ground data. onchocerciasis A hiehly sign'i- but no definite of limited. back- -7 - NB.1 flre ulcrofilarial density in this context is based on the exam:lnation of 2 s'rlips in distilled. water after ]0 ninutes. [o a:rive at the corresponding d.ensity i-n saline solution after 24 hours, these figures have to be uulti- plied by a factor of approrj-nnte$ 2.5. NB.2 Tlrese figr.rres represent the average by village and caruaot be directly applied to ind.ivid.uals. Even if a sati-sfactory correlatlon to the ophthalmological criteria can be establ1skrdI a tolerable leve1 of the disease from a medical point of vj-ew must take into account also that : - there strou]-d be no m:j-crofilarLrria. TLris qrmpton is a sign of severity of the disease sigpj-fyi-ng a generalized disease which cannot be considered as rrtolerab1eil. Entomologr: The entomologist sliould work out the aru:uaI transmission potenti-a1 Consj-stent with the ophtha}nological criteria, i.e. the malcimln nr-mber of infective Iaryae that an.inlivid.ual can receive per year without developi.:ng severe ocular lesions. IEBESIOI'D OF TOLERAJ{CE AND STABIIITY The concept of stability or instability in hr.m.:n ecologr can be applied. either to the ecologi-ca1 rysteu consid.ered, or to the different states or situa- tions, which this rystem may assume. The stability of a system may corrdition the i:estability of these states or sitlations. Io give a physical illustration of what is meant: A footbaLl- filled with air can be "o*p"r"A io a stable system because of its capacity to return to its original slrape ,tter havir:g been d.efo:med by the foot of the sporbsman. fhe tenporary a3d hence instable d.efo:mation. carr be compared to the situation of the system trnd.er the jnfluence of an exterrral supply of energr' As thi-s suppS;r ceasesl the d.efo:mation will cease also, and the system will return to its poiqt of d'epar- ture. This corresponds to the ilefini-tion of stabillty in cybernetics (13-aus 1969). The instability of a situation j-s thus only an aspect of the stability of the system with regard. to a d.efined disturbance. An rxrstable system would be one which keeps the deforrnation inflictedl like an rm-lnflated. football. flhe system of trasmission of onchocerciasis comprises all ecological factors centred on ma^n, the parasite and the vector, that d.ete::naine their respectiv€ Ilultr- bers and contacts. The different states or sitr.ntions of the rystem would be the results of the vector control camPaign" As long as nan and. vector keep or re-estabtistr contact before the elimination of the para=ite in the hr.rman population, the situation wiJ.J- always be unstable. lkris means that a prevalence zate or mean mlcrcfj-lariaI cienslty resulting fmm a sma1l residual trasmission arid consid.ered. as tol-erabler maX not be taken as a reason for dlscontinuatlon of the vector control campaign. -B- It is however posslble, at least in theory, that as a result of vector control transm'issi-on decreases - without being completely internrpted. - to a 1eveI insuffi- cj-ent for the reprod.uction of the parasi-te, thus leadi-ng eventual\y to a point of no-return. Only a realJy operational mathematical mod.el could. assist us in deter- mining to what leve1 the tranmission arrd prevalence would have to be red.uced ard. for what period to reach this cri-tical point. It could indj-cate also the speed. at which the prevalenee vrould return to its original level j-n case of interruption of vector control operations or of major operational breaklown. Ihrls speed must be futportant, as can be jr:dged. from the following stud.ies : lst Stndy Ihis concerns three villages on the Banfora escanpment on the Como'd river inUpper-Volta. A larrricidal campaign had been financed in this area by the iuropea,:a Development tr\.md (fonAs Euz'op6en de D6veloppement = FD), between Jarruazy 1969 andOctober 1973, with interruptions from April to Jrme 1972 and, Jarruary to July 197i.Iarvicid.ing had starbed. again with the beginning of OCP vector control opezetj-onsin Febmary 1975. lhe temporary interrupti-ons of 1972 and 1973 };rad been follovred. by jncreased nrmber of flies caught; their theoretical arrnual mean wast 16.OOO/mxt in 1967, 1O.OO0 in 19691 600 in 197Ot then rose again to 5OO0 in 1972 and 4000 Ln 1973,(Nr.mbers of the two ca.tching points of Itarfiguela and. Boda.diougou taken together. There are no data for ig74). 0 The iriact of the eanopai-gn financed by FD and .the impact of its internrption on prevalendb can be seen from the follor,rlng table (lrost and Rolland. 1975) : Villages Crr.ule Rates Adjusted Rates1968/1969 Dec.1973 Oct. 1975x Dec. 1977 Oct.1975 Bod.adiougou tr'ab6tlougou KolokoIo 69.8 61.8 61,3 45.3 41.1 44,1 59.6 51 .6 61.2 40. - 78.- 76.4 56.5 51 .8 56.5 x No adjustment has been possible for data for 1968-1969. It canr be seen that the beneficial effect of the campaign 1968-1972, which was evident in 1971, was aruriJ:ilated by the inte:sral between the FID ard OCP campaigns and by the temporary interruptions of control o'perations in 1972 and 1977. 2nd. Stlidy In'1968, Rolland (lglZ) visited the ma1I l.4rperendemic village of St. Pierre on the Kerali6 ri-ver, a tributary of the Black Volta in Upper-Volta. this village had been fourrdecl in 1967. The inhabitants had been recruited i-n several suxncun- d.ing villages, pazticu-larly in Iansila. lYe can get an idea on the prevalence of onchocerciasis in their rrillages of origin when we consider the percentages of nodure carriers - 1 /" at ra.:rsi1a, 1Q'f at tr'eIwe, Muaa, Dogo and 20';L at iarrgo,.,ra. Here are the data obtained by Ro1lar:d and his s6mrns6f,s 3 Age group Examined Positive skin Ocular lesions 5-910-14 15-29 30- I 5 7 20 7 4 6 20 5 7 6 16 Infection by age groups, ar:d ocular lesions, St. Pierre ilSki:l exam:ir:ation was nade by two sn:ips, one each at the shoulder and iliac cresb. ixamination of the anterior chamber of the eye was made by slit 1amp, and of the posterior chamber by direct ophthalmoscopy a"fter dilatation of the pupil. Ihe results i-rrdicated. : 1 ) The prevalence of onchocerciasis (g€") *", Upper-Volta vi1lage, ar:d was virtual\y the upwanls. the highest I have foml in arqy same in all age groups from 5 years 2) The number of ocular conplications was extremely high, considerir:.g the brief period of exposure. zt', of subjects sLrowed. microfilariae in the anterior cham- ber; 7€i showed. punctate keratitis, figures of the same oz'd.er as those found at Niarba and Tigr6, the two most heavily infected villages I have found in all Upper-Volta. Ear\y ocular lesj-ons pred.on-lnated, as night be expected, ra.me\r prrnctate keratitis and microfilariae in the anterj-or chamber. One woman presen- ted. an onchocereal iridocyclitis, and two subjects had. choroido-retinj-tis of presumed. onchocercal aetiolostr. In the strict sense, there is no intezmediate te:m between stabi-1ity and lnstability. It ray however be permitted to talk about degrees of stability aradinstability accord.ing to the speed of retuzn to the point of departure and to the qr.rantity of input needed to maintain the trensnission and prevalence at a small 1eve1. [he degree of instability of a particular sitr.ation can be j-nfluenced. by the followi.:ag factors : 1 ) TLre time interrral since the beginning of vector control-. An j-ncreasing inter- va1 will result in a decreasing parasJ-te reservoj-r by : - a dirdnution of the nr:rnber of positives hunan mortality and the repla.cement of non-infected age groups. - an aging of the parasite population and of adult vyorrns vrith ti:rre. r.rnder the effect of the population of younger an increasi-ng mortality 2) The importance of the parasite reseryoiro fn case of interruption of control operations, the speed of retuzr: to the original situation would be a function of the residual number of parasites and of the age composition of the latter. 7) The eventual changes in the degree of immunological tolerance assoclated with a reductj-on of the parasite l-oad.. 4) Tl:e arurual transission potential and its fluctuations. 5) The eventual re-invasion problens by nr-igration of flies from outside the control area. 6) The importance of population moyements betwecn controlled zones ard. zones wLrere transmission is continuing. 7) Ihe size of the population exposed. around a breding site. A big population size may dilute the transmission potential. B) fhe application of measures compleuentary to the larricidal carnpaign : - '10 - elimination of natu.ral or arbificial breeding sites by en'\rironment aI mod ificat ion - constzuction of village werls to di-ninish contact between nan ar:d rlver - the possibil-ity of anti--parasite treatment of onchocerciasispatients and. comm:nities. In conclusion, it can be raid that every epidem:iological situation, not cortespondfu:g to the deflnj.tive libezation as it has been d.efjned., is an wstableone. Every modification in the paraneters suggested wj-II have a profor:nd, reper- cussioa - either good or bad - on the evolution of the system. An j-ncreased importance could. be given to measures liable to i::crease the stability of an epideniological situation resulting from a reduced transnission.But all j-nterruption of control on the attaj:uoent of a threshoLd of tole::ance wouLd.have dr^a$gtic consequences for the populations re-installed in conficterlce. We can only speculate on the quantity of pe:marrent input necessary to maintain the trens- mission below a dengerrcus threshold. BIBIIOGRA?}IY 1. AI{DERSON J. & FUGISANG H.(39lf) OcuLar onchocerclasisl risk factors for bl-lndness. oNCHO/1i{P /75 ,tA 2. BRTNKILANN U.K., KR[.I[ER ?,, ?R]lS[Hirs G.T. & SAY'IADOGO B. franspLacental transmission of mlcrofilariae of onchocerca voIvulus.ffiesse) 5. 7. 4. 6, 'l . B. BUCK A.A.e AMERSON R.I.1 KA\/ATA K. & IIIICECoCK Jr. J.C'(fg6g) onchocerciasis : some new epidemiologie and, cJ-inical flnd.ings. Resul-ts of an epidemiologi.e study in the repubJ-lc of [chad , Am. J. Trop. L[ed. HYg,, AB, 2, 21-7-270 BUDDEN F.H.(lrSlS) ratural history of ocuLar onchocerciasis. O1[CHO,/i7P /75.7O DIIKE 3.0.I.(nle) Ocular onchocerciasis-risk factors, treatment and preventi-on. yrR/PBL/76.1 DIrKE 8.0,1., ANDIRSoII J. & IUGISANG H.(1:SlS) tfre Onehoeerca Vo]-vulus transmission potenttals and asgoclaEeil-patterns of onchocercj-asis at four Cameroon Sudan-sayanna vl1lages. Tropenmed-Parasit,, 26 t L41-L54 KIAUS G.(39AS) worterbuch der Kybernetik. tr'ran"k-furt and. Hamburg, Flscher B0cherei BRIIIKI.{AN}tr U.K. Urine exa"roination for microfllariae rnlllage of ltrortherr: Ghana. Doc. OCP/EPL/76 .16 7p. (0. volvulus) in a humaln 9, ?ROS[ A.(fgZO) !e foyer dronchocercose du 3arako (aryondlssement central de Sikasso, r6p. du Inal.i) treize ans aprbs 1e d.6but d.es op6rations de contrOle antisi-mulid.iennes' Doe . ocP/EPi/76 .7 I1p. ( sous presse s6rie l?Ho,/oNCHo) 10. PROST A. & ROIIAND A.(fgZl) Evaluatlon du 16servoir de paraslte d.tonchooerca volvulus dans 1e foyer de Banfora aprbs slx ans de campagne anti-simulldienne. (naute-vo1ta) Rapp. rondot. ministbre d.e l-a sant6, Ouagatlougou 4975, 19p. LI. RoBEnTS J.M.D.1 NEUMANN E.' CUCI(EI C.yr. & 1fS0f) onchocerciasis in Kenya 9, 11, and after elimj-nation of the vector, ',iHO BuI1. ROIIAI\TD A.(llll+) Resultats de dleux exartrens d slx ans d.lintervalle dans deux de Haute-Vo1ta. 8u11. OMS, 51, 5t 257-261 Bibliographlr page 2 IrIG]TON R.B. L8 years 77, ].95-2]-2 ]-.2. RottAlID A .(1.:glZ) Onchocerciasis in the viJ.lage of St, Pj-eme : an unhappy experience of repopulation in an uncontrolled endemie area. Trans. Ro;r. SOc. Trop. I/Ied. IIyg., 66, 61 9L3-9L5 17. L4. oculaires pratiquds vlI1ages onchocerquiens TI{TIEFORS B. & ROIIAN' A.(WlA) Aspects oculaires ctlun vi1-1age onchocerquien tle sanane africaine aprbs treize ans de lutte anti- simu]-idierrne. VHO/O]iCHO/7A .tZ9 rn I p. E{ o X+,>r r(od odll gr\EioErld .rr odAA (\@tr\o6lll-oo':& ooo<)(f\0 oN o Frto+>o .rl oo0+ q{\od ,14 o rl +r\lrtro rld .liX o d.{2du \oo lrleN <l!t I .r'{ 4t oo odqr> MHt{ r{d r( ooEts Itlrl$la\osl F N +, _oE:olo 0,{ Xl O.H dEil.{ H drtAI ali Hoo o .+Or{Za rrrr::r88rR: .aooooo r+ o oor{ox>drl\rR{,uoo o0aH rtE'il t\\tha ttl8tr.oF-F-o\o o \o.". o r{j5 o(5o otr2a OlllFC)6lrOO \o oOnr-l tXodL\\A d@Pr ! E .aAAo\ooNr--loOO@O<)o\aoootl No\ a :Jot4.t{odt.a. l-f\c\\t Nr N E o + oAq .@ Or{ c{(nN r.r\ rnl-(r\r\tr-* N \o\o N d F b0I hrl r{ ..1 O()i{+r+, dttdAEAE l--(If@r+r)m\O C\ O tr. tr\ \{) <frl oo(r<>ot_ r\ .t o rn N N + rJo-C cqdo60 +r-{ :,ori dF.Eqo OOOrJ\(\r-rNnN NlJ\Lr\ @\o GI @qro <oo odZo 6O<)@(r\O\cOrjto\\tnr N c'\ E g oE 3{H3ggBEEB32HH H ,i oac \o ..- o\ Ld o}{ Frit o E] B.H t & o) n{d o\oo A o ,Ea EI o rJo L) >{ldt slEI BI .rl EI2t <l l.lE(, rq o r{ ra