The predicted and observed decline in onchocerciasis infection

The predicted and observed decline inonchocerciasis infection during 14 years ofsuccessful control of Simulium spp. in West AfricaJ. Remme,' G. De Sole,2 & G.J. van Oortmarssen3

In 55 villages from the well-protected central area of the Onchocerciasis Control Programme in West Africa(OCP), skin snip surveys have been carried out at regular intervals since the programme started, and thelatest round ofsurveys was undertaken after 12-14 years ofsuccessful vector control. The observed trendsin the prevalence and intensity of onchocerciasis infection in cohorts of adults were compared with thetrends predicted using a host-parasite model. After 12-14 years of control the community microfilarial load(CMFL) was close to zero in all villages. During the last few years of control, the prevalence of infectiondeclined at an accelerated rate, and this was predicted by the model. There was generally good agreementbetween observed and predicted trends. The predictions were based on an estimated average duration ofinfection of 10.4 years, which corresponds to a mean reproductive lifespan for Onchocerca volvulus of9-9.5 years, and an upper limit of 15years for 95% of the infections. Differences between the observed andpredicted data included the trend in CMFL between the first and second surveys, which in 18 villages didnot show the predicted decline. Furthermore, the observed final decline in prevalence was faster thanpredicted in the north-eastern part of the central OCP area. After 14 years of vector control, the level ofonchocerciasis has fallen to such a low level that consideration is being given to ending larviciding.

IntroductionSince it began in 1975, the strategy of the Onchocer-ciasis Control Programme in West Africa (OCP) hasbeen to reduce transmission ofthe parasite Onchocercavolvulus to insignificant levels by means of vectorcontrol, and to maintain these levels for sufficientlylong to allow the initial reservoir of the parasite tofall so low that vector control can be safely inter-rupted (1).

The period of successful vector control requireddepends primarily on the duration of onchocerciasisinfection, which is a function of the reproductivelifespan of the adult 0. volvulus and the longevity oftheir microfilariae. In 1975, little was known aboutthe duration of onchocerciasis infection. The onlyrelevant information was from East Africa, wherecross-sectional skin snip surveys in three foci showedthat active onchocerciasis infection still persisted9-11 years after the elimination of the vector, whilethere was no active infection in another focus 18

' Chief, Biostatistics and Information Systems Unit, Onchocer-ciasis Control Programme in West Africa (OCP), B.P. 549,Ouagadougou, Burkina Faso. Requests for reprints should besent to this author.2 Chief, Epidemiological Evaluation Unit, OCP, Ouagadougou,Burkina Faso.

3Biomathematician, Institute of Public Health and SocialMedicine, Erasmus University, Rotterdam, Netherlands.

Reprint No. 5079

years after interruption of transmission (2). As aresult, vector control in the OCP area was originallyplanned to last for a period of 20 years (3).

Since 1975, epidemiological follow-up surveyshave been undertaken in selected indicator villagesfrom all the major river basins in the OCP area. Theobjective of these surveys was to evaluate the impactof vector control on transmission and on disease, andto document the decline in infection levels in thehuman population. For long-term planning and fund-ing of OCP, it was important to arrive at accurateestimates of the reproductive lifespan of 0. volvulusand to predict how many years of vector control werestill needed before the parasite reservoir would fall toan insignificant level. Epidemiological modelling wasused to evaluate the data. The first step was thedevelopment of a force-of-infection model foronchocerciasis (4) that facilitated a better under-standing of age-specific epidemiological trends andresulted in new statistical methods for the com-parative analysis of the epidemiological data. The useof this model led also to a preliminary estimate of 11years for the average duration of onchocerciasisinfection.

The force-of-infection model was based on cer-tain simplifications, notably the assumption that theduration of onchocerciasis infection was constant,which made it inappropriate for the prediction of thefinal decline in the parasite reservoir. A more sophis-ticated model, based on the simulation of individuallife-histories of human hosts and adult female worms,

Bulletin of the World Health Organization, U (3): 331-339 (1990) (9 World Health Organization 1990 331

J. Remme et al.

was therefore developed. The first predictions withthis host-parasite model were made in 1985, andthese indicate that onchocerciasis infection should bevirtually eliminated 15 years after interruption oftransmission.8 Since then, the model has been used toanalyse and evaluate various epidemiological trends(5-7).

From December 1988 to February 1989, OCPcarried out epidemiological evaluations in indicatorvillages from the original programme area, which hadbeen under vector control for 12-14 years. Because ofthe predictions made by the host-parasite model, itwas expected that the results would provide impor-tant information on the impact of vector control onthe parasite reservoir and on the reproductive life-span of 0. volvulus. Here, we report the trendsobserved for the intensity and prevalence of infectionduring 12-14 years of successful vector control andcompare them with the predictions that were made in1985 using the host-parasite model.

Materials and methods

Skin snip surveys

In all epidemiological surveys in the OCP area, twoskin snips are taken from the iliac crest of eachperson covered. The snips are incubated in distilledwater for 30 minutes, subsequently examined micro-scopically for the presence of 0. volvulus microfilariae(mf), and the numbers of microfilariae per snip (mf/s)are counted (8). The results are recorded on standardforms.

Study populationSelectlon of villages. The epidemiological follow-upsurveys that were carried out from December 1988 toFebruary 1989 in the original OCP area covered 90villages. In the present analysis, only villages in well-protected areas were included: excluded were thosefrom the eastern and western border area that havebeen reinvaded by infective Simulium spp. from out-side the OCP area (5). Also excluded were villagesfrom foci where there had been a relapse in transmis-sion during the control period and those from areaswhere vector control had been incomplete. Further-more, only villages that had had at least four follow-up surveys during the control period were included.This left a total of 55 villages, which represented allthe major river basins from the well-protected centralOCP area (see Fig. 1). Eighteen of these villages are

' Onchocerciasis Control Programme in the Volta River Basinarea: progress report of the World Health Organization for 1985.Unpublished WHO document OCP/JPC/85.1.

located in the Phase I area, where vector controlstarted in 1975 and has been continued since then.Eleven villages are in the Phase II area, wherecontrol started in 1976; and 26 villages from thePhase III area, which officially came under control in1977. The duration of control in the villages in thePhase III area of the Koulpeolgo river valley inBurkina Faso was taken as 13 years, because larvi-ciding along the White Volta in 1976 resulted in thecomplete protection of the neighbouring Koulpeolgobasin (9).

Cohort populations. Here, we describe the decline inonchocerciasis infection after interruption of trans-mission, with reference to the reproductive lifespan of0. volvulus. The approach used was to analyse thetrend in the prevalence and intensity of onchocer-ciasis infection in cohorts of adults aged 20 years ormore at the precontrol survey (4). Only adults whohad undergone a skin snip examination at eachsurvey were included in the analysis. Since mostindicator villages were small and some villages hadbeen surveyed as many as seven times, this selectioncriterion occasionally resulted in small cohorts. Atotal of 1536 adults were included in the analysis, andthe average cohort size was 28 (range, 9-89).

Host-parasite model

The host-parasite model was developed specificallyto analyse the impact of vector control on theparasite reservoir in human populations. Thisapproach uses the technique of microsimulation (10),which allows great flexibility in modelling thedynamics of onchocerciasis infection. The parametersconcerned with the initial endemicity level, exposureheterogeneity, longevity, and the level of microfilariaeproduced by adult female parasites are first specified,and the model is then used to obtain trends inmicrofilarial loads in human populations (expressedas the distribution and summary statistics ofmicrofilarial counts in skin snips). Furthermore, themodel also provides information about the distribu-tion of adult female parasites, distinguishing betweenalive and dead, and calcified and non-calcifiedworms. The model is described in more detail in theAnnex.

The model was quantified using literature dataand by fitting it to the observed epidemiologicalresults for the first 8-9 years of control in the OCParea from 1975 to 1984. The quantification outlinedbelow was obtained and has been used since 1985 topredict the epidemiological trends during the vectorcontrol period. The mean longevity of infection wasestimated to be 11 years for non-calcifying and 8years for calcifying worms. In accordance with the

332 WHO Bulletin OMS. Vol. 68 1990.

The predicted and observed decline In onchocerclasis Infection In West Africa

Fig. 1. Location of the study villages In the well-protected central OCP area (CMFL= community microfilarlal load).

results of nodulectomy surveys (9), it was assumedthat 20% of the adult worms would eventually cal-cify. The estimated variability of the longevity oninfection was such that 95% of longevities were lessthan 15 years. After a prepatent period of 1 year, theaverage contribution of a female worm to the skinsnip count remained constant at 5.2 microfilariaeuntil the worm was 8 years of age. Subsequently, themicrofilariae contribution was decreased annually by0.4 microfilariae per adult female worm in order tosimulate the observed reduction in productivity ofaged female worms (9). The standard deviation forthe human exposure index was 0.86 and the co-efficient of variation for the dispersal of micro-filariae in skin snip counts was 0.84.

Data analysisThe skin snip data were analysed on personal com-puters using routine programs developed by OCP.

WHO Bulletin OMS. Vol. 68 1990.

The precontrol endemicity level of a village is charac-terized by the community microfilarial load (CMFL),which is the geometric mean number of microfilariaeper snip in adults aged at least 20 years (4). TheCMFL was also used to describe the intensity ofonchocerciasis infection in cohort populations duringthe control period. The observed proportion of positiveskin snips in a cohort during the last survey wascompared with the proportion predicted for villageswith the same precontrol endemicity level and dura-tion of control. The goodness-of-fit between observedand predicted proportions was tested using a x2 testwith Yates' correction for continuity. In villages thatexhibited a major increase in CMFL between the firstand second survey, the CMFL for the second surveywas taken as the basis for determining the endemicitylevel and the prediction of the epidemiological trendsduring the remaining control period.

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Results

Predicted trends In onchocerclasis Infection Incohorts of adults

The predicted trends in the CMFL for different initialendemicity levels are shown in Fig. 2. These predic-tions are the result of simulations in which theprecontrol force-of-infection was taken to be 0.18,0.51, 1.45, 2.75, and 3.95 infections per person peryear, respectively. The results indicate that after adelay equal to the prepatent period, the CMFLshould fall almost linearly during the first 8 years ofcontrol and tail off to values close to zero after 12-14years. The absolute decline in the CMFL depends onthe initial endemicity level.

Fig. 3 shows the predicted trends in theprevalence of infection in cohorts of adults-thehigher the initial endemicity level, the longer it takes

Fig. 2. Predicted trend In community microfilarial load(CMFL) during the control period for different endemicitylevels (mf/s = number of microfilarlas per skin snip).

90 -

80 -

70-

60 _

Z..50

LL40

20 2 4 6 8 10 12 14 16 18 20

No. of years of vector control

Fig. 3. Predicted trends In prevalence of microfilarlaeduring the control period for different initial endemicitylevels.

100

X 90

* 80

o 70

* 60a o

E 40-

3030

o 20

et 10

0


Fig. 4. Observed trends in community microfilarlal load(CMFL) during 14 years of vector control (mf/s = numberof microfilarlae per skin snip).

70

60

so

I40

30

20Z

10

0 2 4 6 a 10 12 14 16 18 20

No. olys ofvedsronrol

before the prevalence begins to fall but the morerapid is the rate of final descent. The host-parasitemodel predicts that onchocerciasis will have virtuallydied out after 15 years of control, when only very fewpositive skin snips should be found, mainly in villageswith the highest precontrol endemicity levels.

Observed trends In CMFL in cohorts of adultsFig. 4 shows the observed trends in CMFL in cohortsof adults in all 55 indicator villages. Comparison withthe predictions shown in Fig. 2 indicates that theoverall trend is quite similar, and that after 12-14years of control the observed CMFL was almost zeroin all the villages. Since it is difficult to use Fig. 4 toassess the relative trends in CMFL, the villages weresubdivided into the following groups according totheir initial endemicity level: 30-90 mf/s, 10-30 mf/s,and 3.3-10 mf/s. In addition, there was one hypo-endemic village with a pretreatment CMFL of 0.2 mf/s and a prevalence in adults of only 20%.

The observed trends in CMFL are shown in Fig.5 for each of these three endemicity groups. Overall,the trend was very much as predicted for each of thethree groups and the relative trend was the same forall endemicity levels. However, there were differencesbetween the predicted and observed trends for severalvillages. The most striking differences concerned thechange in CMFL between the first and secondsurvey. In each group there were several villages forwhich the CMFL hardly changed, or even increased,between the first and the second survey. This wasobserved for 18 villages. In all but three of thesevillages the CMFL followed the predicted trend fromthe second survey onwards; for these three villages,however, the CMFL level did not change until the

WHO Bulletin OMS. Vol. 68 1990.33

The predicted and observed decline In onchocerciasis Infection In West Africa

Fig. 5. Observed trends In community microfiiariai load(CMFL) for different Initial endemicity levels (mt/a = number of microfilarlao per skin snip). (a) Precontrol:CMFL > 30 mf/s (b) Precontrol: 10 mt/s < CMFL < 30 mt/s.(c) Precontrol: CMFL <10 mt(s).

30

24

6

Fig. 6. Observed trends in the prevalence of micro-filarlse for different Initial endemicity levels. CMFL=community microfilarlal load; mf/s=number of micro-bilarial per skin snip. (a) Precontrol: CMFL >30 mt/s. (b)Precontrol: 10 mt/s <CMFL<30 mt/s. (c) Precontrol:CMFL<10 mt/s.

0

0

0

v0

In

-_

0

0.400

au

v9

0 2 4 6 8 10 12 14 16 18 20


third survey (after 7-8 years of control), but this wasfollowed by a subsequent rapid decline. These 18villages are marked in Fig. 1, and were found in mostriver basins in the central OCP area; no clear geogra-

phical pattern emerges, although the proportion ofsuch villages was high in the extreme north-easternfocus in Niger.

Obsrved trends In the prevalence of mkrofilariaeIn cohors of adults

The observed trends in the prevalence ofmicrofilariae in cohorts of adults is shown in Fig. 6,

1Uoi

80-

60-

40-

20-

,, )VXW. -,1-Tt7P,X100

80 - ----b

110 0 < Observed:

40 aPredicted:--CIFLlIO mf/s

, -.;;--CMFL=3 mf/s

20

0 2 4 6 8 10 1 14 16 18 20


for the same precontrol CMFL endemicity levels asin Fig. 5. The predicted accelerated fall in prevalencewas observed in all villages, and the rate of declinedepended markedly on the initial endemicity level.Most remarkable was the sudden drop in prevalencefor the group of villages with a precontrol CMFL>30 mf/s (Fig. 6(a)). The decrease in prevalence wasnever significantly slower than predicted and thedecline was faster in several villages. The differencebetween the observed and predicted number ofpositive skin snips at the last survey was statistically


\. ~~~Observe d

\a

Predicted

ed:

Led:UFL-30 mf/sFL=IO mf/s

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J. Remme et al.

significant for three of the most northern villages inthe Phase II area (P< 0.001 for one village and P< 0.05for two villages) and for nine villages in the Phase IIIeast area in Burkina Faso and northern Togo (P< 0.01for six villages and P<0.05 for three villages). Therewere no significant differences for the 18 villages inthe Phase I area, where the observed trends followedthe predictions most closely. However, the predictedprevalences for the last survey in the Phase I area,which had been under vector control for 14 years,might have been too low to detect a significantdifference. The predicted and observed prevalenceswere therefore also compared for the penultimatesurvey, which was undertaken after 10-12 years ofcontrol in 15 of the 18 villages, and after 8 years ofcontrol in the remaining three villages. No statis-tically significant differences were found.

DiscussionThe epidemiological follow-up surveys that wereundertaken after 12-14 years of successful vectorcontrol in the well-protected central OCP areaprovided valuable information on the impact of thecontrol on the human reservoir of 0. volvulus. Theintensity of infection in the community, as indicatedby the CMFL, was close to zero in all 55 villagessurveyed. In noncontrolled endemic areas, theseverity of ocular manifestations of onchocerciasis inthe community is directly related to the CMFL (11).The extremely low CMFLs after 12-14 years ofcontrol therefore leave no doubt that onchocerciasishas been eliminated as a problem of public healthimportance from the central OCP area (5).

The most significant finding in the present studywas the accelerated decline in the prevalence ofonchocerciasis infection, which fell in accordancewith the predictions made in 1985 using the host-parasite model. Particularly satisfying was themarked reduction in the prevalence in those villagesthat had the highest level of endemicity during theprecontrol period.b The observed trends suggeststrongly that vector control has practically interrup-ted transmission throughout the central OCP areaand that the parasite reservoir has been virtuallyeliminated. These trends are not inconsistent with theobservations made by Roberts et al. in East Africa(2), but they indicate that the required duration ofsuccessful vector control is significantly less than the20 years originally planned for OCP (3). In thisconnection it should be recalled that these resultsrefer exclusively to cohorts of adults, who were themost exposed and infected section of the populationbefore the start of the programme. The prevalence of

bSee footnote a, p. 332.

onchocerciasis in the total population was alwayslower, especially during the last surveys, becausechildren born since the start of control were free ofinfection and hardly any positive skin snips wereobtained from individuals below 25 years of age.

The host-parasite model is an importantimprovement over the force-of-infection model (4),because it includes the variability in the reproductivelifespan of the adult female worms and consequentlysimulates much more realistically the later years ofcontrol. The good agreement between the predictedand observed trends supports the basic assumptionsthat underlie the model, including the hypothesis thatthe reproductive lifespan of the adult female worm isthe main determinant of the epidemiological trendsfor onchocerciasis during a period of vector control.Furthermore, it indicates that the quantification ofthe model was based on realistic estimates of themost important parasitological parameters. Theestimated duration of infection between inoculationand the disappearance of the last microfilariae fromthe skin was 10.4 years on average, while thevariability factor indicated that 95% of all infectionshad a duration of less than 15 years. This corres-ponds to an average reproductive lifespan for 0.volvulus of 9-9.5 years, based on the assumption thatthe average microfilarial longevity is 1-1.5 years (12).It should be noted that these estimates represent onlyone possible quantification of the model; detailedsensitivity analysis is at present being undertaken todetermine the complete range of parameter valuesthat are consistent with the observed epidemiologicaltrends.

There were certain differences between theobserved and predicted trends which merit furtherdiscussion. For example, in 18 of the 55 villagessurveyed there was no decrease in CMFL betweenthe first and second survey, although the modelpredicts an almost linear decline after an initial delayof about 1 year, corresponding to the prepatentperiod. These differences may be due to randomvariations in the CMFL, in particular among thesmaller cohorts, or to an underestimation of skinmicrofilarial loads during the baseline surveys, whenmany technicians were still inexperienced. However,some discrepancies may reflect precontrol variationsin transmission that were not taken into account inthe model predictions, which were based on theassumption of an equilibrium situation with a con-stant force-of-infection during the precontrol period.In reality however, there may be considerable annualdifferences in the intensity of transmission as a resultof variations in hydrological conditions, which deter-mine the availability and productivity of Simuliumbreeding sites (13). A relatively high intensity oftransmission for a period of 1-2 years before the start


The predicted and observed decline In onchocerclasis Infection In West Africa

of control could explain the increase in CMFL be-tween the first and second survey, because most ofthe infections acquired during the last precontrolyears might still have been prepatent when the firstsurvey was carried out.

In several villages in the Phase III east area andin the northern half of the Phase II area the observedepidemiological trends showed a significantly fasterdecline than predicted. Entomological data for 1975(3) suggest that larviciding in the Phase I area mayhave had an important effect on transmission in thePhase II area by eliminating populations of S. dam-nosum s.l. that would otherwise have been importantsources of reinvasion. Similarly, control in the PhaseII area may have reduced biting rates and transmis-sion in the Phase III east area. Also, the drought inthe early 1970s may have had an additional effect byseverely limiting vector breeding in many of the mostnorthern foci in the OCP area (9). The combinationof these factors could have been responsible for a

more rapid decline in the parasite reservoir in largeparts of the Phase II and Phase III areas. Alter-natively, vector control in the Phase I area and in thesouthern half of the Phase II area may not haveinterrupted transmission sufficiently during the firstyear of larviciding operations, with the result thatsuccessful control was only achieved during sub-sequent years. This would imply that the modeloverestimates the duration of infection.

Taken together, the observed and predictedtrends indicate that the parasite reservoir should bevirtually eliminated after 15 years of successful vectorcontrol, and in many areas even earlier. The questioncurrently facing OCP is when can the expensivelarviciding operations be stopped without running a

serious risk of recrudescence of onchocerciasis. Toanswer this, priority has been given to investigationsof blackfly feeding and transmission habits in orderto determine the significance of low microfilarialloads, and to the further development of epidemio-logical models to predict the risk of recrudescence.Predictions of this type cannot be made using thehost-parasite model, which does not address thetransmission cycle of 0. volvulus. Nevertheless, use ofthis model and the technique of microsimulation hasled to the recent development of a comprehensivetransmission model that allows the simultaneoussimulation of populations of humans and parasites,the dynamics of the vector population, and ofinterventions based on larviciding or chemo-therapy (14).

development of the model were made by Dr K.Y. Dadzie,Professor K. Dietz, Dr J. Grunewald, Professor J.D.F.Habbema, Dr M. Karam, Dr B. Philippon, and Dr G. Zerbo.The computer program for the model was developed atthe Institute of Public Health and Social Medicine, Eras-mus University, Rotterdam, under the supervision ofProfessor J.D.F. Habbema. The dedication and efficiencyof the staff of the Epidemiological Evaluation Unit and ofthe Biostatistics and Information Systems Unit, OCP, areacknowledged.

R6sum6Le d6cin de l'onchocercose pendant 14 ansde lutte rbussie contre Simullum spp. enAfrique de l'Ouest: previsions et observationsLa strategie du Programme de lutte contre l'on-chocercose en Afrique de l'Ouest (OCP) a 6ted'amener la transmission d'Onchocerca volvulus ades niveaux insignifiants grace a la lutte contre levecteur et de prolonger cette lutte jusqu'a ce quele reservoir de parasites ait diminu6 au point queles mesures prises puissent etre interrompuessans risque. Depuis le debut du programme, desenquetes de suivi epidemiologique ont ete entre-prises dans des villages indicateurs choisis pourevaluer l'impact de la lutte sur la transmission etsur la maladie, et pour attester le declin des tauxd'infestations dans la population humaine. Pour uneplanification et un financement a long terme, il etaitdevenu de plus en plus important d'obtenir desestimations precises sur la duree de fecondite d'O.volvulus et de predire combien d'annees la lutteantivectorielle devait encore etre poursuivie. Celaa necessite 1'emploi d'un modele epidemiologiquepour analyser les donn6es de I'evaluation.

Un modele hote-parasite a ete mis au point quiest base sur une simulation des antecedents desh6tes humains et des vers femelles adultes. Lemodele a et6 quantifie en 1985 en utilisant desdonnees publiees et les resultats epidemiolo-giques obtenus pendant les 8 a 9 premieresannees du programme de lutte. La long6vitemoyenne de l'infestation a ete estim6e a 11 anspour les vers non calcifiants et 8 ans pour les 20%de vers qui se calcifiaient. La variabilit6 6tait telleque la longevite de l'infestation etait de moins de15 ans pour 95% des vers. Apr&s soustraction dela long6vit6 moyenne des microfilaires, cela corres-pond i une dur6e moyenne de f6condit6 de 9 a 9,5ans pour 0. volvulus. Ce chiffre a et6 utilise depuis1985 pour predire les tendances epidemiologiques,la conclusion la plus importante 6tant que l'on-chocercose devrait etre pratiquement 'elimine'eapres 15 annees d'interruption de la transmission.


AcknowledgementsWe thank Dr E.M. Samba, Director, OCP, for his activesupport of our work. Important contributions to the

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De decembre 1988 a janvier 1989, I'OCP aentrepris une autre serie d'evaluations epidemio-logiques dans 55 villages indicateurs de la regioncentrale bien protegee, soumise depuis 12 a 14ans a une lutte antivectorielle reussie. Les tendan-ces observees en ce qui concerne la prevalence etl'intensite de l'onchocercose dans des cohortesd'adultes de ces villages concordaient en generalbien avec les previsions.

L'intensite de l'infestation, mesuree par lacharge microfilarienne communautaire, etait pro-che de zero dans les 55 villages. Cela confirmeque l'onchocercose a ete eliminee en tant queprobleme de sante publique important dans laregion centrale de l'OCP. Le resultat le plus signi-ficatif etait la reduction acceleree de la prevalencede l'infestation pendant les dernieres annees.Cette reduction etait egalement prevue par lemodele et les tendances laissent a penser que lalutte antivectorielle a pratiquement interrompu latransmission dans 1'ensemble de la region cen-trale de l'OCP et que le reservoir de parasites aete pratiquement elimine.

Deux types de differences sont apparus entreles tendances observees et les tendances prevues.Dans un tiers des villages, il n'y a pas eu dediminution de la charge microfilarienne commu-nautaire entre la premiere et la seconde enquete,bien que le modele ait prevu un declin a peu preslineaire apres une periode d'un an. Cela peut etredu' des variations aleatoires de la charge micro-filarienne, a une sous-estimation des chargesmicrofilariennes pendant les enquetes prelimi-naires et a des variations de la transmissionavant la periode de lutte. De plus, dans diversvillages du nord-est oiu la lutte a commence un oudeux ans plus tard, les tendances observeesetaient nettement plus rapides que celles quiavaient ete prevues. II est possible que lapremiere annee de mesures larvicides dansl'ouest ait eu un impact important sur la transmis-sion dans le nord-est. Cependant, il est egalementpossible que la premiere annee de mesures lar-vicides n'ait pas suffisamment interrompu la trans-mission, auquel cas la duree de l'infestation a etesurestimee par le modele.

L'association des tendances observees et pre-vues montre que le reservoir de parasites devraitetre pratiquement elimine apr6s 15 ans de lutteantivectorielle reussie, et meme plus rapidementdans de nombreuses regions. Le probleme qui sepose actuellement a l'OCP est de savoir quand lesoperations larvicides couteuses peuvent etrearretees sans qu'il y ait un risque serieux derecrudescence de l'onchocercose. Pour permettrede repondre a cette question, les etudes de

simulation dans la region de l'OCP ont recemmentete etendues, avec la mise au point d'un modelecomplet de transmission.

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4. Remme, J., et al. A force-of-infection model foronchocerciasis and its applications in theepidemiological evaluation of the OnchocerciasisControl Programme in the Volta River basin area.Bulletin of the World Health Organization, 64: 667-681 (1986).

5. Twelve years of onchocerciasis control in WesternAfrica. World health statistics annual, 1987. Geneva,World Health Organization, 1987, pp. 19-22.

6. De Sole, G. et al. Epidemiological results of 12 yearsof onchocerciasis vector control in West Africa. LaMedicina Tropicale nella Cooperazione alloSviluppo, 4: 1-4 (1988).

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13. Le Berre, R. Contribution a l'6tude biologique et6cologique de Simulium damnosum Theobald, 1903(Diptera, Simuliidae). Paris, M6moires ORSTOM,1966.


The predicted and observed decline In onchocerciasis Infection in West Africa

14. Plalsler, A.P. et al. ONCHOSIM: a computer programfor modelling the transmission and control of on-chocerciasis Computer methods and programs inbiomedicine, 31: 43-56 (1990).

Annex

Simulation of onchocerclasis infectlon In cohorts ofadults using the host-parasite model

Strcture of the host-parasite model. The model de-scribes the following aspects of onchocerciasis in ahuman population: new infections of individuals,with the possibility of multiple infections; the lon-gevity of adult female parasites in human hosts; andthe age-specific contribution of each adult femaleparasite to the microfilarial density in the skin. Here,only a fixed human cohort is considered, and apartfrom aging, all other human population dynamics(e.g., birth, death, and migration) are neglected.

infectons. The human population is heterogeneouswith respect to the risk of becoming infected withonchocerciasis. Part of this heterogeneity is modelledexplicitly by defining age- and subpopulation-specificexposure. Thus, Exa (a,, s,) denotes the relative ex-posure for the i-th person in age-class ai and sub-population s;. Further individual variation is takeninto account by using the exposure index, Exi, whichfollows a continuous Weibull-type probability distri-bution with mean = 1.0 and standard deviation sEX,i.

The force-of-infection (foi) denotes the rate atwhich a person with an average exposure acquires anew infection with onchocerciasis. Only successfulinfections are considered, i.e., the inoculation of afemale parasite that succeeds in becoming a matureworm and produces microfilariae. For an individual,i, the infection rate foij is given by:

foij = foi * Exi1 - Exa(a,, s;)The foi needs to be specified and can vary for

different time intervals. Typically, foi is assigned a

constant value in a precontrol situation, and equalszero in the case of complete interruption of transmis-sion as a result of vector control.

Worm longevity and microtilarial ofspring. For eachparasite in a human host the microfilarial production(mo) is a function of worm age (w), determined fromthe moment of inoculation. The prepatent period, thelongevity of microfilariae, and the mating probabilityare not modelled explicitly, and should be taken intoaccount in specifying the function mo(w).

A distinction is made between non-calcifyingand calcifying parasites, and each is assigned aspecific distribution of the longevity, Ti. Dead wormscan be identified for some time, Td, which alsodepends on their state of calcification. Both Tl andTd are governed by a Weibull-type distribution, forwhich the mean and standard deviation should bespecified.

For a human host with a given microfilarialload, skin snip counts show some variation becauseof the dispersal of microfilariae in the body and thevariability that is inherent in the skin snipping andcounting procedure. This variability is also modelledaccording to a Weibull-type distribution.

Simulition. The model is implemented using a com-puter program that is based on the technique ofmicrosimulation (10), whereby individual life-histories of human hosts and adult parasites in eachhost are simulated. Events in the life-histories, e.g.,human birth, human death, infection, and death ofadult parasites, are generated by random selectionfrom the corresponding probability distributions.Skin snip counts are simulated for each member ofthe human population at specified times, to give age-specific microfilarial load distributions and CMFLvalues.

In evaluations of the impact of vector controlfollowing a stable endemic situation, the initial force-of-infection is simulated for a period of at leastTl + Td years before the start of control, to obtain astable parasite distribution in the human population.

WHO Bulletin OMS. Vol. 68 1990. 339

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The predicted and observed decline in onchocerciasis infection