The biomedical criteria for resettlement in the Volta River Basin Onchocerciasis Control Programme area (OCP): report of a scientific advisory panel working group, Geneva, 6-8 June 1977

WORLD HEALTH ORGANIZATTON ORGANISATION MONDIALE DE LA SANT6 THE BIOMEDICAL CRITERIA FOR RESETTLENIET.IT IN THE VOLTA BASIN ONCHOCERCIASIS COIITROL PROGRAMIIIE AREA (OCP) REPORT OF A SCIENTIFIC ADVISORY PAI.IEL WORKING GROUP Geneva 6-8 June L9?7 COTtrENTS LIST OF PARTICIPANTS INTRODUCTION 2. FACTORS TO BE CONSIDERED WHEN FORMUT,ATINC CRITERIA FOR RESETTLEMEI{T @ 2.L 2.2 2.3 2.4 ocefsl.Iftt.r ORIGIML: ENGLISH Page 2 Background to the problem Objectives of the SAP Working Group Sources of relevant information and data The underlying considerations for determining criteria 3 4 4 5 5 5 7 7 7 3. THE TOI.,ERABLE LEI/EL OF ONCHOCERCIASIS INFECTION IN RESETTLEMET.IT AREAS Definition of a tolerable level Monitoring the prevalence and incidence of Onchocerca volvulus infections 4 CRITERIA FOR DETERMINING ACCEPTABLE RISKS OF TRANSMISSION IN RESETTLEMEI.IT AREAS 4.I Calculation of the risk of transmission 4.1.1 Annua1 Biting Rate (ABR) 4.I.2 Annual Transmission Potential (ATP) 4.2 Operational criteria defined for use in OCP resettlement . 5. CONCLUS IOI{S 6. RECOMMI{DATIONS REFERENCES ANMX I - List of the working documents presented at the meeting ANNEX 2 - Methods used in OCP for obtaining data required in transmission studies 3.1 3.2 I 8 I 8 8 9 I to { ) The iseue of this document does not con6titute formal publication. lt should not be reviewed, abstracted or quotod without the agreement of the World Health Organization. Authore alone are responsible for views expressed in signed articles. Ce document ne constitue pas une publication. ll ne doit faire l'Objet d'aucun compte rendu ou rdsum6 ni d'aucuno citation 6ans l'autorisation de I'Organieation Mondiale de la Sant6. Les opinions exprimdes dans los articles slgn6s n'engagent que leurs auteurs. I ocPfsAPflr.t page 2 LIST OF PARTICIPANTS Dr R. W. CROSSKEY, Department of Entomology, British Museum (NaturaI History), L9.l@, United Kingdom ([4pg1t-eg1) Mr C. GARREIT-JONES, 16 Conduit Road, AbinEdo.l, Oxon, United Kingdom Or H. KN0TTGEN, Director, Institute of Tropical Medicine, Ttlbingen, Federal Republic of Germany Professor G. S. NELSON, Director, Department of Medical Helminthology, London School of Hygiene and Tropicat Medicine, Lo_t{o_q' United Kingdom Dr B. PHILIPPON, Directeur de 1'Institut de Recherches sur I'Onchocercose, Organisation de Coordination et Coop6ratio:r porr la Lutte contre les Grandes End6mies, Bouak6, Ivory Coast Mr A. RENZ, Institute of Tropicat Medicine, Ttlbingen, Federal Republic of Germany Dr J. L. STOCKARD, Office of Health, Technical Assistance Bureau, Department of State, Agency for International Develop.nent, Washington, D.C., United States of America (Observer) WHO Secretariat Dr T. LEPES, Director, Division of Malaria and other Parasitic Diseases, Headquarters, Ggqe_lq, Switzerland (Chairman) Dr J.B. DAVIES, Vector Control Unit, Onchocerciasis Control Programme, 9qqg149gggq' Upper Volta Dr B. O. L. DUKE, Filarial Infections, Division of Malaria and other Parasitic Diseases, Headquarters, Geneva, Switzerland Mr J. DUPPENTHALER, Unit of Health Statistical MethoCology, Headquarters, !.eneva, Swi tzer land Mr J. D. M. MARR, Onchocerciasis Control Prograrnme, Crenqv_1' Switzerland Dr J. A. ODETOYINBO, Disease Prevention and Control, WHO African Regional Office, Brazzaville, Congo Dr E. ONORI, Onchocerciasis Control Programme, Headquarters, Ge_nev_a, Switzerland ( Eg91e_tary ) Dr A. PROST, Epidemiological Evaluation Unit, Onchocerciasis Control Prograrure, O"qSA@Cg, Upper volta Dr B. THYLEFORS, Epidemiological Evaluatio:r Unit, Onchocerciasis Control Programme, Orlgado_ggou, Upper volta Mr J. F. WALSH, Vector Control Unit, Oncho,cerciasis Control Programme, Ouagadouglg, Upper Volta a t I ocPfsAPfTT pa ge 1. INTRODUCTION The Onchocerciasis Control Programme (OCP) in the savanna areas ot' seven West African countries was originally planned as a vector cortrol operation. At the time when the PIan of Operations was drawn up, it was known that larviciding could significantly reduce the density of the vector, Simulium damnosum s.1., to a level at which very little or no transmission of Onchocerca volvulus would take place, but, in view of the fact that no Crugs are available for mass chemotherapy, it was realized that a reservoir of parasites would remaln r-n the human populatio.r for many years. As the Programme was the first attemp t to control S. damnosum- transmitted onchocerciasis on a large scale, no previo.rsly-establi.shed operational criteria were avai.lable to evaluate its progress. The results obtained during the first two years of operation have clearly shown that, in almost all rivers, local breeding of the vectors can be suppressed by larviciding, and that fly biting rates can thus be reduced to very low numbers. Experience has also shown that the long-range flying ability of the vectors may Iead to reinvasion of certain breeding sites, thus making it necessary to mai.ntain permanent surveillance and to be ready with emergency measures for the application of larvicides whenever and wherever required. However, the people Iiving near those riverine areas previously abandoneC because of onchocerciasis have realized the effectiveness of the vector control operations, and, in many places, they have begun a spontaneous resettlement. The OCP Scientific and Technical Advisory Committee (STAC), during its meeting I-4 March 1977, considered the question of criteria for a safe and organized resettle- ment of population in the river valleys where the numbers of Simulium flies have been successfully reduced. The Committee felt that thi.s sub3ect was of such importance that a special Scientific Advisory Panel (SAp) Working Croup should be appointed in the near future to define the criteria for resettlement and to Cevise methods for estimating the tolerable level of transmission which would not cause irreversible eye lesions. The duly appointed SAP Working Group met in Geneva from 6-8.June l9??, under the Chairmanship of Dr T, Lepes, Director of the Division of Malaria and other Parasitic Diseases, who also opened the meeting. He emphasized that the OCP i.s a successful operational prograrune, but one which now needs clearly defined criteria for ensuring that the resettlement and development of presently uninhabited river valleys - one of its primary obJectives - can proceed with the least risk of Onchocerca volvulus infection. Mr Marc Ls. Bazin, Director of the Onchocerciasis Control Programme, attended the opening session of the meeting and was introduced by Dr Lepes. In a short address, Mr Bazin reminded the Gro.rp of the enormous size of the multi-national OCP undertaking and pointed out that many socio-econo.ric projects in the area may not see the light of day unless it can be shown that, in the context of onchocerciasis, resettlement may take place in safety. He also emphasized that any good results obtained by OCP will be of great interest in other areas where control of oncho- cerciasis may be undertaken in the future. The Programme, whieh has the advantage of being able to call upon all available expertise, must therefore set a gocd examplein this respect. I 3 22 ocPfsAPfTT.r page 4 FACTORS TO BE CONSIDERED WHEN FORMULATING CRITERIA FOR RESETTLEMENT I q€Esl""nd to _Ihe_p.lqqlem The Onchocerciasis Control Prograrune was conceived as a pablic health programme closely concerned with the resettlement and economic potential of certain river valleys in the West African savanna which are at present uninhabited or underpopulated. Its main aim is to ensure that onchocerciasis, which is believed to be a major cause of depopulation in these valleys, does not continue to impede the socio-econo;nic development of fertile and potentrally productive areas, or to form a serious health hazard in the OCP area as a whole. Human resettlement of the river valleys is an essential part of the @P i.f its original intentions are to be fulfilled. The larviciding campalgn being carried out in the OCP area is very successful against aIl the locaI cytospecies of the Simulium damnosum comLp tex ( predo;ninant ly S. damnosum s. str. and S. sirbanum) which are known to be vectors of O. volvulus. However, as the programme has developeC, it has become clear that complete eradication of the vectors from the Programme area is unlikely to be achieved. It is true that most of the Programme area is now entirely or almost entirely glmg.llum-free, and can be maintained that way, but it appears impossible to prevent the seasonal, long- distance movement of parasitized vector flies into so,ne parts of the control area from the uncontrolled periphery. Although recolonization of the control area by the invading flies is prevented by continued larviciding, plans for resettlement and develop;nent in the OCP area cannot be based on the presumption of vector eradication; r.t must therefore be assumeC and accepted that, in spite of the very high degree of continuing vector control, soine transmission of the causal parasite, O. volvulus, wiIl continue in the programme area. The reasons for this are: (i) the continuing sporadic occurrence of small numbers of vector !!ggf,!g, resulting from reinvasion from outside and,/or temporary local breakdowrsof control within the OCP area; (ii) the persistence for up to 15 yea rs of the O. volvulus parasite reservoir in the untreated human population living in the @P area; and (iii) the immigration of persons infected with O. volvulus As vector eradication is not a practical option, the only realistic goal to aim for is the one which has already been attained, and is now being maintained by continued larviciding, namely a very high degree of vector control. This in turn has led to a very great reduction, and in many places to a complete cessation of transmission of the parasite O. volvulus. It is impossible yet to assess the full impact of vector control and reduced transmission on the clinical manifestations of onchocerciasis di.sease in man. The slow development of the disease, and theexcepticnal longevity of the parasite, prevent the effects of control from being immediately evident cIinically. Furthermore, because the disease is progressive, many severe cases of eye disease due to onchocerciasis may be expected to develop in the OCP area in persons who were already heavily infected with the parasite before the control carpaign began. This may tend to 3onvey the false impression that control is less effective than it really is. The long prepatent period of O._ no_lv_qlus infection makes it difficult to obtain early medical evidence as to whether or not transmission is continuing in a controlled area. This, combined with the fact that there is litt1e or no human population in the valleys intended for resettlement, means that direct evidence of the transmission risk in resettlement valleys is at present unobtainable. The risk of transmission at this stage in the resettlement areas can only be estimated by an indirect approacht using entomological and parasitological data derived from vector studies. Howevert such data must be continuously supplemented by observations on incidence of new infections and changes in parasite intensities among the poputation in the control area. ocef seef tt.t page 5 In practice it cannot be expected that resettlement and socio-economic develop- ment projects should wait upon the acquisition of adequate medical evidence of the reduction of transmission to an acceptably low leveI. It is necessary to decide now upon what levels of transmission are likely to occur, and what level can be considered acceptable, on the basis of indirect evidence from the entomological surveillance. This is made the more urgent by the fact that some resettlement is already takinB place within the OCP area without advice from the Programme. Hence, criteria must now be established from which it can be inferred that transmission of Onchocerca volvulus has been reduced to a level at which onchocerciasis is considered to be medically tolerable. For this purpose the 'tolerable level' is taken to be one at which onchocerciasis ceases to be a disease producing severe ocular lesions and becomes, ipstead, Iittle more than an incidental parasitosis. 2 .2 qlrgtrl"_._qt_ ltlq qP__ryglk1_qc_q9w. Since the attainment and maintenance of a satisfactorily low level of O. volvulus transmission were seen to be essential before an area could be judged and declared safe for resettlement, the two main objectives of the Group were as follows: (i) to decide upon the level of human onchocerciasis that should be considered acceptable (the 'tolerable leveI') for resettlement; (ii) to decide uPon the critical level of O. volvulus transmission at or below which the tolerable level of human onchocerciasis will not be exceeded 2.3 Sources of relevant information and data In order to clarify its ideas on how the two tasks defined above could best be carried out, the Group assessed and discussed the currently available evidence pertaining to the problems of defining safety criteria for resettlement areas. Much of the data reviewed by the Group were contained in the working documents prepared by group members and tisted in Annex I. They derived from studies made inside and outside the OCP area. Other pertinent sources taken into account were the published papers relating to work in Cameroon, lvory Coast, Liberia, Mali, and Upper Volta by Duke (19681, 1968b), Duke eL_11_.(1972t 1975), Rolland <1972), Garms (1973), Thylefors & Rolland (19?6), Philippon <1977), and Prost (1977). 2.4 The underlying _c ar]glders !i"n"_tqlge le"qin ing c r i ter i a Onchocerciasis is a disease in which it is very difficult to determine the exact level of transmission to which any individual or community is exposed. However, in a major control operation against the disease, it becomes essential to develop methods of quantifying the risks of transmission that persist in the control area. without this the effectiveness of the control measures in suppressing or preventing the dtsease cannot be reliabty judged. In the OCP this is especially important because of the need to determine how safe the presently unsettled river valleys really are for resettlement. A large part of the Group's work was therefore to consider the multiplicity of complex and inter-related entomological, parasitological' ophthalmological and statistical factors which are involved in deciding how the amount of transmission can be measured and how it relates to the severity of the disease - in particular to the levels of infection that can be considered tolerable for OCP resettlement. ocefsaefzt.r page 6 At present the only way of quantifying the amount of transmission that could take place is by using a concept such as that introduced by Duke (1968a) and called by him the 'transmission potential'. This ls the total number of infective larvae(of O. v_olvulgs) which could be inoculated into one man in unit time if all the infective flies (S. damnosum s.1.) biti ng him were to transmit their total load of infective larvae. In practice, since Duke's publication, transmission potentials have been calculated on an annual basis referred to as the 'Annual Transmission Potential' or 'ATP'* for short. It is not precisely known how a calculated ATP relates to the actual amount of transmission to which any individual is exposed, but the use of ATPs is essential at present as providing the only means of comparing estimated transmission rates. Calculation of ATPs (see sub-section 4.1.2) has to be made from studies on vector biting rates and infectivity rates, and the acquisition of data for estimating levels of transmission (and therefore of the safety or otherwise of areas for resettlement) remains largely an entomological problem. There is some evidence that it might prove possible to gauge the transmission risk simply from fly bi.ting rates without having to dissect flies in order to record the infective O. volvulus larvae that they are carrying. However, the complications arising from widely varying parity rates in different sanples of vector flies make it unjustifiable to do this in the present state of knowledge, and the use of ATPs(which incorporate results from direct counts of infective O. volvulus larvae in the vector) must continue in the OCP. Nevertheless, the Group noted that, provided a reliable method could be found, there would be maSor practical advantages in using fly biting rates alone to assess the amount of transmission. This would avoid the need to make large numbers of fIy dissections, which at present places a significant strain on OCP resources. For practical reasons the OCP method of calculating ATPs is based on the numbers of infective O. volvulus larvae in the flies' heads. This procedure is open to criticism as it differs from the method used by a number of workers elsewhere, who base the ATP measure on the total numbers of larvae found in the head, thorax and abdomen of the vector. However, provided that the basis of the calculation is recorded, it is possible to correlate ATPS that have been calculated by the different methods . The Group paid considerable attention to the complications caused by the occasional presence in the vector flies of filarial larvae other than those of O. volvulus. It is thought that those belonging to genera other than Onchocerca can be fairly easily distinguished, but there is a possibility that some infective larvae found in the vector flies and recorded as "indistinguishable from O. volvulus" may in fact be those of Onchocerca species other than volvulus. Studies on the various filarial parasites found in the different cytospecies of the S. damnosum vector complex are much needed because accurate identification of O. volvulus larvae is necessary for the calculation of ATPs. However, the erroneous incluston in the basic data of infective Orrchocerca larvae other than those of O. volvulus would actually bias the ATP favourably, i.e. so that the risk of transmission is over-estimated rather than underestimated. --r-SAi"-r".r-."i "t ttr. C"oup felt that this term is not completely appropriate,and certain possible alternative terms were sugBested and considered. It was decided to retain the term Annual Transmission Potential and its abbreviation ATP as these have already acquired currency through publication. The differential epidemiological si.gnificance of the several cytospecies of the S.-{tql"..u*. complex found in the OCP area is another complrcating factor involved in determininB the risk of transmission. The Group recoBnised that much remains to be done in investigating the real r61e of the individual cyto- species in relation to the epidemiology of onchocerciasis. Such factors as their differing degrees of zoophily and anthropophily are clearly important. The Group also co;rsidered some of the sociological aspects involved in resettlement, such as the siting of villages so as to minimize man-fIy contact,but it felt that this subJect was largely outside its brief. Nonetheless, theGroup thoJght itimportant to bear such factors in mind because of their influence on possible future transmission of o. volvulus from fry to man, if for any reason Ei"*.llj-r,. control were to n"ea-r. ao-*-"-after resettlement has occurred. 3. THE TOLERABLE LEVEL OF ONCHOCERCA VOLVULUS INFECTION IN RESETTLEMENT AREAS 3.1 Definition of a tolerable leve I ocPfsAPfTT.L page 7 poin t s def ined persons he outset In the context of OCP, and from the public health and socio-economic of view, a tolerable level of onchocerciasis infection in the community is as "The attainment and maintenance, throughout the oCp area and among allIiving therein who were not already infectec with Onchocerca volvulus at t of the campaign, of a zero incidence of the serious and irreversible ocular lesi.ons resulting from onchocerciasis: and consequently of a zero i.ncidence of serious visual handicap or blindness due to this disease,'. 3.2 MonitorinE the prevalence and incidence of Onchocerca volvulus infect ions From observations made in vitlages in the OCP area, using the standard OCp methods' absence of serious onchocercal eye lesions is associated, under stabilised conditions of transmission' with the following levels of o. volvulus infection: (a) a prevalence rate of onchocerciasis not reaching 50% until after 30 years of age; (b) a geometric mean density of microfilariae of less than lO (examination ofsnips in distilled water after 30 minutes); (c) a geometric mean number of microfilariae in the anterior chamber of the eye(after positioning the patient's head) of less than 2; (d) a geometric mean number of microfilariae in the cornea of less than 3; (e) no microfilariae in the urine (lO ml sample). These levels of infection with q:-v-o-Lv-q]us may therefore be considered as torerabrein the oCP area once a post-control balance oftransmission has been estabrished.Adequate monitoring should be carried out to determine the incldence cf new infectionsin the control area and to ensure that the ophthalmological and parasitologicallevels of infection defined above are not exceeded. To determine the incidence of new infections, the present standard methodsshould be supplemented wherever possible by more sensitive techniques for detectingmicrofilariae in the skin' such as countinB them after the skin snips have been leftfor 24 hours in saline. In addition, the use of the Mazzottj. test in incidence studies should be considered and further evaluated. It was accepted that present serological methods are neither sufficiently specific nor sufficiently sensitive to be of practical use in determining the incidence of new infections of O. volvulus. 44 ocefsa,ef zz.r page 8 Transplacental passage of microfilariae has been demonstrated in the OCP area in a few infants and the significance of these observations needs to be evaluated in relation to incidence studies. CRITERIA FOR DETERMINING ACCEPTABLE RISKS OF TRANSMISSION IN RESETTLEMENT AREAS I Calculation of the risk of transmission Having defined what should be accepted as a tolerable level of O. volvulus infection in communities living in the OCP area, it is necessary to decide upon measurable critical leveIs of transmission which must not be exceeded if the tolerable level of onchocerciasis is to be reached and maintained. An estimate of the risk of transmission of Onchocerca volvulus from the Simutium vector to man may be made from (a) tne Annual Biting Rate (ABR) of the vector, i.€. the estimated number of Simulium damnosum s.I. com ing to bite one man who is exposed to biting during all or almost all daylight hours every day for one year; and(b) tfre Annual Transmission Potential (ATP), i.€. the estimated total number of infective larvae indistingu ishable from O. volvulus carried by the flies from which the ABR f:.gure is calculated. The methods currently used to calculate these two indr.ces are as follows, and the procedures adopted in the OCP to acquire data for these calculations are given rn Annex 2. 4. I. t n"tfql_ sr_Liqg_ Rql"__({qBl_ This is the annual total of the esttmated individual monthly biting rates that have been obtained from the formula: No. of flies caught x No. of days rn month No. of days worked 4.1.2 AnnuaI Transmission Potential (ATP) This is the annual total of the estimated individual monthly transmissio:'l potentials obtained from the formula: No. of days in month x No. of infective larvae _____.-____ x No. of days worked No. of flies caught No. of flies dissected Calculations for the ATP in the OCP area are based so1ely on the numbers of infective larvae indistingu ishable from O. volvulus in the flies' heads , and the recommended maximum level of ATP given in the following sub-section 4.2 is made on this bas is . * 4.2 Qge_ltl_qlsl_elr_!e: ria defined for use in OCP resettlement Although it is desirable to maintain the level of transmission as near to ze,"o as is possible in practice, the maximum tolerable leveI of transmission that can be considered as being compatible with resettlement in the OCP area is that associated with the following two criteria applied simultaneously over a period of at least two consecutive years before resettlement begins, and for an indefinite period thereafter Where Simulium biting densit].es are low, as in the OCP area, ATPs calculated in this way are Iiable to wide fluctuations resulting from the numbers of infective larvae in single fIies. Mathematical studies should be undertaken, using the available data on total f lies/man,,/year, total parous f.Iies,fnanfyear, total infective f lies/man,/year, and the mean numbers of infective larvae per infective fly, in order to determine whether reasonably reliabLe estimates of the ATP can be made from any of these para- meters. tocefsa,efn.r page 9 (i) (ii) AnnuaI Biting Rate not exceeding looO fliesf^anfV"ar Annual Transmission Potential not exceeding IOO infective larvae of 9.:"_1rqtd.^nf y"^r. In practice the two criteria will function as a single operational criterion for resettlement, since neither of them alone is sufficient to guard against an unacceptable risk of transmission. These critical levels, which are put forward on the basis of all the evidence currently availabte, should be subject to review as and when fresh data re obtained. It must be clearly understood that they relate only to African savanna areas where Simulim damnosum s. str. and S. sirbanum are the predominant vectors. 5. CONCLUSIONS Fotlowing the wide-ranging discussions that took place, the Group came to the following conclusions : (i) The data that have been, and are being, routinely obtained by the OCP are adequate to jr.rstify decisions on whether, in the context of onchocerciasist resettlement should take place in any particular area. (ii.) At this stage of knowledge it is not justifiable to rely on vector biting rates or parity rates alone in determining the risk of transmission, and therefore it is essential to have an evaluation, of the kind here ca1led the Annual Transmission Potential, which incorpor ates observations on the O. volvulus parasite itself. (iii) The principal criterion available at present for assessing the degree of safety of an area for resettlement shoutd be the existence of a very low or zero level of transmission by S._3rys.1. as determined from evidence of vector biting rates (ABR) (ATP). and of infective O. volvulus worm loads ln the vector (iv) There is need for further studies on the fitarial parasites found in S. damnosum s.1. and on the epidemiological relationships between O. volvglus and the various cytospecies in the S. damnosum comp lex. 6. RECOMMENDATIONS The recommendations of the Group are as follows (i) The principal criterion for safe resettlement should be derived from the Annual Biting Rates of the vector and the Annual Transmission Potentials. The values of these indices must be kept below the threshold likely to lead to serious ocular manifestations even if a small amount of transmission does cont inue. (ii) Resettlement should only be advised or approved if it can be shown that an Annual Biting Rate of not more than IOOO and an Annual Transmission Potential of not more than loo have existed at potentially dangerous places in the area(s) concerned for at least two consecutive years prior to intended resettlement, and if there are good prospects that both indices can be kept below these critical levels after resettlement. ocvfsa,vf zt.r page 10 (iii) Certification by oCP staff that an area fulfils the cri.teria of acceptable risk levet described above should be a pre-requisite for any government encouragement of organized resettlement proiects. In some situations it is to be expected that OCP may certify areas as fulfilling the criteria provided that certain additional precautions are taken in the development of the resettlement project. (i.v1 There should be no reduction in the entomological and epidemiological surveillance networks until the full OCP area has been under control for at least two years. (v) Once resettlement has taken p1ace, the parasitological situation in the human populations should be monitored by means of parasite surveys conducted at regular intervals among (a) previously uninfected persons living in the area, (b) those born after control began, and (c) immigrants. REFERENCES DUKE, B.O.L. (1968a) Studies on factors influencing the transmission of onchocerciasts IV. The biting cycles, infective biting density and transmission potential of "forestt' Simulium damnosum, &l.lryp-.__[aq. :gtq:r-g., Ql, ss-ro6 DUKE, B.O.L. (f968b) Studies on factors influencing the transmission of onchocerciasrs VI. The infective biting po tential of Simulium damnosum in different bioclimatic zones and its influence on the transmission potential, 92_, r64-L7O Ann. trop. Med. Parasit ! DUKE, B.O.L., MOORE, P.J., & ANDERSON, J. (1972) Studies on transmission of onchocerciasis. VII. A comparison of factors influencing the the Onchocerca volvulus transmission pote ntials of Simulium damnosum populat ions in four Cameroon rain- forest villages and the pattern of onchocerciasis associated therewith, AlL-_Lry.. Med. Parasit , 99, 219-234 DUKE, B.O.L., ANDERSON, J., & FUGLSANG, H. (1975) The Onchocerca volvulus transmission potentials and associated patterns of onchocerciasis at four Cameroon Sudan- savanna villages, Trope_nmed. Bqrgs_i3., 26, 143-154 GARMS , R. (1973) Quantitative studies on the transmission of q@e_g_yglvuIu1 by S: clegosgq in the Bong Range, Liberia, Z.fryg"{-_Parlal!., 24, 358-372 PHILIPPON, B . (19??) Etude de la transmission d'Onchocerca volvulus (teuckart, 1893) Ner1.1|ogl, 9@-!aeg) par qlryLium_qaqnosg Theobatd, l9O3 (qlg!.""'qfqqiida") en Afrique tropicale, Travaux et Documents de 1'ORSTOM, No. 63, ORSTOM, Paris PROST, A. (1977) te foyer d'onchocercose du Farako (arondissement central de Sikasso, R6publique du Mali) treize ans apr6s te d6but des op6rations de cont16le antisimulidiennes, WHO/ONCHof77.L34, t? pp. ROLLAND, A. (I9?2) Onchocerciasis in the village of St. Pierre: an unhappJ, experience of repopulation in an uncontrolbd endemic area, Trans._L- _E_.-_Llqp.:_@._EyG..,6q, 913-915 THYLEFORS, B. & ROLLAND, A. (1976) Aspects oculaires d'un village onchocerquien de savane africaine aprEs treize ans de lutte antisimulidienne, wHo/oNCOf76.L2g (mimeographed unpubtished document, t2 pp.) a ocefseef n.r page ll ANNEX I I. LEPES, LIST OF THE WORKINC DOCMMNTS PRESENIED AT THE MEETING T. Considerations for the operational criterla required for the evaluation of the Onchocerciasis Control Programme Some considerations on ct'iteria for resettlement in the2 3. HAMON, DUKE, Dr B. O. L OCP area J. considerations on the determination of tolerable levels of OnChocerca vo1vuluS residual transmission in the resettLement zones 4 5 6 WAISH, J. F. Notes relating to the working paper of Dr J' Hamon THYLEFORS, B. & PROST, A. Aspects on a tolerable level of onchocerciasis WALSH, J. F., DAVIES, J.& LE BERRE, R. Methods of entomological evaluation currently in use by VCU with suggestions for establishing criteria for advising on resettlement and development proiects DAVIES, J. & GARMS, R. The effects of reinvasion and of local failures in control on the A.T.P. REM, A. Levels of ATP in a savanna area of northern Cameroon 7 8 9 10. 11. 12. 13. 14. 15. PHILIPPON, B. & QUIr.r.F:rr':ERE, D. Inf luence of Simulium cytotypes in the transmission of O. volvulus in the OCP area GARMS, R. Influence of vector infections with Onchocerca spp, other than O. volvulus on the evaluation of the transmi.ssion of the disease(Presented by Dr Da vies ) NEISON, G. S. Methods of detecting new and light human infection(parasitological, immunological, Mazzotti tests) DIETZ, K. Mathematical model of O. volvulus transmission: (a) data(Paper notavailable and required; (b) predictive possrbiliti.es spoken to) PROST, A. Epidemiological evaluation in space and time in the Onchocerciasis Control Programme. Present situation and future prospects. ONORI, E. Requirements for parasitological surveillance network in space and time in the OCP area. PHILIPPON, 8., WALSH, J. F., DAVIES, levels of ATP in the OCP area. maps. ) J . B. & LE BERRE, R. Present and Past(Paper presented in the form of wall 3 I * Note - Papers are listed in agenda of the meeting. t I the thematic order in which they were taken on the page 12 ANl!E-L2_ ocefstef zt.r METHODS USED IN OCP FOR OBTAINING DATA REQUIRED IN TRANSMISSION STUDIES 1. FIy catches Catches are on the basis of one man-day of 1I hours worked from O7.OO-18.OO hours local time (by two men working alternately). Fties are caught individually in tubes when settling to bite the vector collector, and are segregated for recording purposes into one-hourly catches. 2 . IrrLef _o_!_qqq_qay_: _te&{ At each site throughout the year at least one whole day's catchper week is used to compute the ABR and ATP indices. However, where pre-control experience shows that flies are seasonally absent, catches may be suspended throughout that perrod. 3' llv d issec t ions As many flies as possibLe from each hour's catch are dissected, as soon as possible after capture. Dissected flies are classed as parous or nulliparous after examination of their ovaries and fat-bodies. Parous flies are further dissected and the presence of all stages of filarial parasites in them is recorded. The presence, in the head, thorax and abdomen, of infective larvae identified as indistinguishable from O. volvulus is recorded separately. Only the infective larvae in the head are considered when computing the ATP 4. Sites selected These are the ones at which fly densities and the risk of transmission are believed to be highest, giving due regard to adequate Beographical coverage. Most selected sites are on the banks of rivers. t a a t