The situation as regards epidemiological evaluation

ONCHOCERCIASIS CONTROL PROGRAMMI IN THE VOLTA RTVER BASIN AREA PROCRAMME DE LUTTE CONTRE L'ONCHOCNRCOSE DANS LA RECION DU BASSIN DE LA VOLTA Til.: 333.12 . 329 l0 - 338.57 - Tdldx ONCHO 5241 UV EXPERT ADVISORY COMMITTEE Third Meecing Geneva, 13-17 S eptember 1982 NTE SITUATION AS REGARDS E PIDEI'fi OLOGICAL EVALUATION Ihe thoughEs seE ouE below are in the naEure of comnenEaries on the follow-up of the reconmendations and remarks regarding the Epidemiological EvaluaEion Unit made at the second meeEings of the Expert Advisory CoruniEtee and the Joint Progranrne ConrniEtee. 1. 0n the correlation beEween epidemiological and entomological indices for an undersEanding of the co Ierable imits of infecEion A correlat,ion between Ehese two parameters has already been established in previous studies and has been used in defining onchocerciasis endemicit,y levels. Today, however, given the remarkable successes achieved in vecEor control, wtrich have been confirmed by epidemiological evaluaEion daEa, che idea of an ilonchocerciasis-freet'zone deserves to be clearly defined. To do so, we have first to define conEinuous Eransmission, residual Eransmission, and Ehe incerruption of cransmission. WORLD HEALTH ORCANIZATION AFRICAN RECION ORGANISATION MONDIALE DE LA SANTE REGION DE L'AFRIQUE ocP/EAC3 .2 (ocP82.6) ORIGINAI, : FRENCH |, Io L977, the Scientific Advisory Panel defined Ehe EolerabiliEy threshold as an annual t.ransmission potent.ial (ATP) not exceeding lOO infecEive larvae of O. voLvulus/..y'y"". and an anntral biting rate (ABR) noE exceeding IOOO tLiesfnady"...I From the epidemiological point of view, che fundamenE,al qualitative sign from among Ehe other crit,eria was the absence of any ne$, case of infection among children below the age of five years. In Ehis connexion, chere are five facE,ors which need Eo be considered. (a) The correlation becween catching poincs aE wtlich ATP(1OO and ABR(1OOO and the epidemio- Iogical indices is noc easy Eo establish. In the aEtempt, we examined the inhabitant,s of seven villages for wtrich ATP is (lOO and ABR<1OOO. Partial processing of che daEa reveals a progressive reducElon of the epidemiological data wtrich are largely age-related and above a1I related to the length of Eime for wtrich Eransmission is interrupEed, which is of prime imporEance in that: (i) ic does not necessari.ly correspond Eo the durarion of effective treaEment of a traEercourse; (ii) it may be marred by the effects of reinvasion and resistance;(iii) Ehe variabilicy brackec of the epidemiological indices is beEEer appreciated through its precise determinaEion. It may be considered that Ehe epidemiological effects of the same ATP and ABR races are dissimilar as between Ehe foresE, and che savanna. Leaving aside Ehe nuisance caused by the blackflies and Ehe bites, Ehe rpublic health" impacE of very high ATP and ABR does noc have the same effect.s in foresE zones and in the savanna. I 1 ,o"u.".,. ocef seef tl .t. l- ocPf Feca.2 (ocP82 .6) Page 2 (b) Ihe real epiderniological value of che det.ection of a new case of onchocerciasis in five- year-old children, in relaEion Eo flucEuaEions in sampling, when all chat is needed in order to fall tr"ithin an age-group in which such deEecEion is significant is a variation of one or two years. (c) Ihe anti-incidence parameter, wtrich we have been led Eo consider through being faced with Ehe increasing number of individuals wtro were previously positive and wtro have become negaEive wiEhout any chemoEherapy or migraEion. Even when Ehe great.esE cauEion is exercised in the analysis of such daca, the proporEion aEEributab[e Eo error is minimal relaEive Eo Ehe large number of individuals for wtrom the t.est, has become negaEive. Can qre than speak of parasiEo- logical cure? (d) The absence of microfilariae in the anEerior segment of the eye of previously infected individuals, wttich is frequently recorded in Ehe course of ophthalmological examinations. Furthermore, the decline in the prevalence of ocular lesions is rapid in the region in wtrich Eransmission is inEerrupEed. It declined from 58.2% before che launching of vecEor control oPeraEions co 34.2% in 1982, wtrich is a 41.37. reducEion. Irreversible ocuLar lesions have declined from 19.3% co 15.1%, r"rhich is a 21.87. reduction. Itre blindness rare has declined from 5.8% co 4.7%, which is a 19% reducrion (Table 1). (e) Ttre remarkable morLality of O. volvulus females. Ie has emerged from an analysis of 7I1 female $rorms Ehat the mortaliEy raEe, wtrich was L2.8% in a zone in which Eransmission had not been inEerruPEed, was L9.L% aft,er Ewo years' interruption in Eransmission, climbed Eo 2L.9% after three years and reached 69.4% after six to seven years' interruption in Erans- mission (Table 2 and Fig. 1). In the light of Ehese new data, we have Eo acknowledge that the old criEeria used in defining an "onchocerciasis-free zone'r are in need of re-examinaEion. It would therefore be desirable for a contmission to pronounce on the concepEs of conEinuous transmission, resid.ual transmission, and interrupcion in transmission before review'ing the basic criEeria for thedefinition of an lonchocerciasis-free zone',. 4 I { 2 On Ehe inununoLoqical diaqnosis of hocercias is In the search for an inrnunological diagnostic meEhod, several biological samples have been sent to the LaboraEory of ParasiEology and ExoEic Pathology in Grenoble for Ehe develop- menE of a metabolic anEiSen; Eo the University of Lille for the producEion of monoclonal antibodies; to Ehe Swiss Tropical InstiEute in Basle for the development, of an irmnunological technique; and Eo the London School of Hygiene and Tropical l,ledicine, from which we have received an unpurified anEigen which has been tested in Ehree villages in an uncont,rolled zone in che Republic of Mali; Ehe dat.a from Ehese villages are being analysed; Eo Ehe Laboratory of ImmunoparasiEology, aE Ehe Walter and Eliza Hall InstituEe of Medical Research, Melbourne, AusEralia; Ehe preliminary results should be enlarged upon by result.s from a second baEch sera recently dispaCched. For the momenE Ehese sEeps caken by us have noE yielded the desired resul6s and ehe Programme is considering the comnissioning of research on an immunologlcal diagnosEic meEhod Ehrough conEraccual agreements chaE would speed up rhe work by giving Ehe researchers moreincentive. 3. On sEudy of the [o vj.ty of adult worms Progress has been made in keeping of O. volvulus females in virro (Dr Schultz-Key). What now has to be done is co improve the keeping of the worms in vit.ro without che addition of serum for immunological reasons, and Eo study spermaceleosis. I / t h ocef rl.ct.z (ocP82 .6) page 3 4. On the differentiation of savanny'forest sE.rains Success was initially achieved in che isolaEion of five O. volwlus strains from biological. maEerial from Ehe Programne area (Dr H. Flockhart). Of the various enzymes t.ested, only lacEate dehydrogenase (LDH) revealed a difference between the forest strains and the savanna sErain. Also currenEly under investigation is the different.iaEion of st,rains by histochemical sEaining of infective larvae in blackflies (Dr Omar). 5 On the UniE ,s icipation in the health acEivirie of Member StaEes Parcicipation by the Programne in health acEiviEies, and even in the setting up of primary health care in Member SEates is a very delicate maEEer and exEremely limited having regard Eo Ehe organizat,ional probrems raised by such participacion. Nevertheless, Ehe Programne already has co its credit,: - the producEion of a reporc on Ehe result,s of a questionnaire that is readily usable by village health workers tn zones freed of onchocerciasis. Ttris reporE has been widelybrought Eo Ehe aEEention of l'lember Statesl - the detection and Ereatment of condit,ions currenEly encounEered in Ehe villages; - health education in the villages, which is beginning Eo have an effect; - che drafting of an experimental schistosomiasis treatment prococol in collaboraEion with che Unit on Schistosomiasis and other Helminthic Infect,ions of che WIIO parasitic Diseases Prograrune, with che object of finding an effective, cheap t.reatment. 6 che raised level o 1i in che bl The Progranune has underEaken a sEudy of the raised level of morEality in the blind inthree villages in Benin and Ewo in Upper Volta. Medlcal, ophthalmologlcal and biologicalinvestigations were carried ouE on blind people and their immediate circle in these.rill"g"".Their environment was very closely scrutinized and a posE-mortem hras conducEed on those blindpersons wtto died in the inEerval between che last Ewo visits of che epidemiologic.al Eeam.The processing of the dat,a is now in a quiEe advanced stage. Ttre results are being analyse<1. should they prove significanE, a wider-scale study could be undertaken. A{ A A ,,|l (E tl oH (Yl N H @ LO (n o\d@rn d t'\ tn -tJ N -t(n tr NO ln rn d O\ \O Od)d E (n \o dl \o N N@ o!nd El IHH H AJ .h (E or o\ H N6t l/t (f| d F{ N!n N (n r\ o d (n F{(Yi ft{ N No\ \t N F{ @ E dtN d\t 6t (no (n$ d =IH l-{ H c, o(! o. ..f, H ON @ r'\ d rn l'-tn coo !n \O d o l'.-(r| H O\N |rlt\ d @\O Ot rl $\O O\ON r\ -f, d\oN H H C, th(! Or \t H coo(n \O c-l (n @\o Ord o\tnd o ln(fl tq o\ Or o.\td :rn rn (7t d t- F{ N t'\ N \ON +\O N H o att (0 Ar \o H @F\ O\ \O d 6t tn I.n @\O(n -tH (ft (fl('') tr @o(n.n F{ .hcl ON z l'\ C.l €t coN O c.t r\ \O d o o oo(o d .,{ E>(,U|+{(l o00 "{lr {J oUl!oE>AQ z.A .A @ q,(!)kotr z(6H.OodotrHa>..{Hood d ch tr,tc rfl .i{ r+{ c r{Ao(,)oooO6..{O.'{O(,lOt&OOOOoo tr k trd aoc, o oHdodl{F{&(u0(0(!(u HOOOT0.lt/]HtrkokA{OAOAH o o ot{OA(!l{.lJdotra > .'.{o(rd oUl.oU,O r+{ .H r+{ d 'Ho(,oooHt!'r{Hqr..{O(,lO u) (J O OO OH tr t{ C-{ C F{(rHr{ilE{(!o(u(,(E 4(u(J(u5c, -l r{ tr |ral kAOAOAzOrrr ocPf EAc3.2 (ocP82 .6) Page 4 U)4 H H *, IrlAo Ffo&2Zt55*&UItl :l9E 9H ov L&;3Eq3 tsa -'6t'l rc EEH rrt(-) zH u) zoHH =Jo rlr I ,l o (E C,) E q, 60(o +J c 0) o H c, o- o t{(g th olr 60 ..{ r+{ o H iir 3lzl o F{do([([ .{EtJ6(u0ErqH lt [[ EhH I ocPf EAc3.2 (ocP82.6) Page 5 / TABLE 2. EVOLUTION OF fiIE MORTALITY OF FEMALE O. VOLW LUS WORMS FIG. I. EVOLUTION OF PERCENTAGE MORTALITY OF O. VOLWLUS FEMALES AS A FIJNCTION OF TIME FOR WI{ICH TRANSMISSION INTERRUPTED IN EIGHT VILI^A,GES IN TITE PROGRAMME REGION ?' r / ? t 7" mortality P f 70 60 50 40 30 20 10 I Phases DuraEion of effective vec Eor conErol Number of vi I lages Number of nodules Total number TdOIINS l,Ia Ie worms FemaLe Irorms Sex ratio Percentage of dead female IilOrms I III.W IV Wes t. Extension 6-7 years 3 years 2 years o 3 t 1 3 29 62 38 I70 53 L82 L52 75L L7 68 68 274 36 LL4 84 477 2.2 L.7 L.2 I 7 69.4 2L.9 19.1 L2.8 8 299 r 138 427 7Lr 2 3 4 5 6 years ?