A comparison of reinvasion by simulium damnosum s.l. females in 1982 and in the three preceding years: control prospects

) 7 I ONCHOCERCIASIS CONTROL PROCRAMME IN THE VOLTA RTVER BASIN AREA PROCRAMME DE LUTTE CONTRE L'ONCHOCERCOSE DANS LA RECION DU BASSIN DE LA VOLTA Til.: 333 l2 - 329.10 - 338.57 - Tdl6x ONCHO 5241 UV EXPERT ADVISORY COMMITTEE Third Meeting Geneva 13-17 S tember 1982 A COMPARISON OF REINVASION BY SIMULIIJ}T DA}INOSUM s.I. FEMALES IN 1982 AI.ID IN THE TTTREE PRECEDING YEARS CONTROL PROSPECTS 1. Introduction Reinvasion is a well-known phenomenon in the Progranune, having reappeared regularly Eowards every rainy season since 1975. It is essenEially certain regions adjacenE t.o unEreated zones thaE are affecced. Reinvasion affeccs parEicular reaches of cerEain wat.ercourses, and the poinEs of impacc on Ehe affected rivers are now well known. Scudies of the phenomenon carried ouE since 1975 have shown EhaE Ehree species of che S damnosum complex are more parEicularly involved in reinvasion: two savanna species, S. damnosum s.s. and S. sirbanum rP lus S. squamosum , whose t'migratory capaciEy" is, however, less Ehan that of the first Ewo species in terms of the distances covered. In some insEances the same research has revealed the starting points of the invading blackfly populaeions, Lhus enabling che problem Eo be tackled by prompt larviciding. It has also revealed the localized behaviour of the reinvading females, which were found Eo be confined to the gallery forest along che affected waEercourses, noE radiating out much. Only che approaches Eo the waEer- courses in a sErip no more chan 3.5 km wide would be affected wtren che level of Eransmission on the river bank was hyperendemic. However, now EhaE resisEance Eo organophosphorus insecticides is becoming general in rhe forest, and sub-forest foci in the south-west. of the Programne area, wtrich may be breeding grcunds for reinvading blackflies, reinvasion control st,rat,egy is becoming complicated. 2. AspecEs of reinvasion beEween 1979 and 1982 Before^1979, some watercourses within the limics of the initial Prograrrne area (654 OO0 kmz) were heavily reinvaded, and in the lighc of our present knowledge these could be grouped geographically into five main rrreceptionrr foci of reinvading blackflies: Ehe western focus : the $ratercourses affect,ed are the Wassouloubal6, the Bago6, che Faya and the upper Sassandra basin; Ehe western cent,ral focus: with the L6raba , Ehe WhiEe Bandama and ics Eribur.ary, Ehe Bou and the upper Marahou6; che southern focus: consisEi WORLD HEALTH ORGANIZATION AFRICAN RECION ORCANISATION MONDIALE DE LA SANTE RECION DE L'AFRIQUE ocP/EAC3 .4 (ocP82. 10) ORIGINAL: FRENCH between Tagadi and Bui; ng of a stretch of some IOO km of che lower Black Volca ocP/EAc3.4 (ocP82. ro) Page 2 Ehe south-eastern focus: comprising Ehe M6 and the upper course of some of the Eribucaries of che Oti; the eastern focus: incorporaEing the tributaries of the river Niger in Benin, the middle reaches of which are parEicularly affected. In 1979 and the years that followed, wtren insecticides were used Eo treat the phase IV rivers (onchocerciasis foci in the south-rrest and south of Ivory Coast), such watercourses as the upper Bago6, che Wtrite Bandama, the L€raba and the lower Black Volta were practically unaffect.ed by reinvasion or only very slightly affected. VecEor denslEies were very Low and cransmission was int.errupted. Consequently, Ehe reinvasion of the wesEern cenEre and Ehe south of the inicial Prograrme area appeared Eo have been brought under control in three con- secutive rainy seasons (L979, l98o and 1981). On the other hand, reinvasion continued in che west, the south-east and the easE. Ihese latter reinvasions rrere used for pin-point studies, as indicaEed in the intro-duction, to arrive ac a betEer undersEanding of the phenomenon. However, r"rhat. is remarkableis the suppression of the $resEern central and southern reinvasion foci following the treatment of potential sources of deparEure. 3. Features of reinvasion in 1982 3.1 Reinvasion appeared in 1982, as every year, true Eo the st.arE of the rainy season. However, wtten compared with the three previous years, ttris reinvasion had suffered the con- sequences of double resistance to Eemephos and chlorphoxim, the only two larvicides for use in simuliid conErol in aIl seasons available to OCp. Because of this, Ewo sit.uations arose, at. least in the lead-up to the rainy season inApril-May and in Ehe hrestern zone. For the momenE there has been no change in reinvasion inthe eastern zone by comparison wiEh reinvasions in the previous years, unless it be inintensity, r.rtrich appears Eo be lower this year than in past years. Ihe first of the situations in che western zone is like thae wtrich prevailed before che emergence of double resistance to the organophosphorus insecticides used by the prograrmre.Ihe valleys of the waEercourses habicually preferred by blackfly populations developing inplaces outside the Prograntrne area have noc been spared. The rivers concerned are tributariesof che Niger in Mali ptus the upper basin of che Sassandra, Eogether consEiEuc,ing Ehe wesEern reinvasion focus. Ihis is a sustained reinvasion. The second situation is a consequence of the double resisEance and of the experimental use of Teknar (R) as a substicute simuliid larvicide in place of cemephos and chlorphoxim.As this Larvicide cannot be used during the high-waEer period, iE has been difficulc Eo dealwith the breeding sites of onchocerciasis vectors in those phase IV regions recognized as sources of the reinvasion of the upper Bago6, Ehe white Bandama, the LEraba and the lowerBlack Volta, or in other words che wesEern central and southern foci. As a result, Ehese watercourses srere affected by reinvasion in the run-up Eo Ehe 1982 rainy season, Ehe earlinessof which was resPonsible for equally early reinvasion cornnrencing in the first. and second weeksof April- However, the reEurn co the use of Abate for che Ereatment of pot.ential sources ofdeparture in Phase lV enabled this reinvasion, rrtrich $ras on the wane in June on the L6raba andthe wtrite Bandama, to be brought under cont,roI. The fact Ehat iE persisted on the lowerB1ack Volta in June may possibly be a consequence of faulty larviciding of che lower Como6, which is regarded as a possible source of reinvasion. 3.2 A new situation as regards Ehe reinvasion of Ehe river l,lassouloubal6 arose in 19g2.simuriid density remained asconishingly low at the Yanfolila sit,e throughouE the reinvasionperiod, in contrast to previous years (since L977). Itris site appears to have escaped reinvasion in 1982, wttereas ordinarily iE is affected at the same Eime as Madina-Diassa on the Baou16 and Kankela on a tribuEary of the Bago6. There has been an ecological change at Ehelevel of the Yanfolila breeding site, rrhich was submerged beneath the wat,ers of the new I T -l ( ocPf1 c3.4 (0cP82.10) Page 3 Selingu6 hydroelecEric dam in the 1982 rainy season, rotrereas in previous years iE had noE been submerged. ftris observation would appear t,o be a verification ad absurdum of Ehe hypochesis Ehat reinvading blackflies would be atEracted by the breeding sites of the affected water- courses. Itre use of Bellec Eraps aE chis previously reinvaded site has yielded results lit,tle beEEer than catches on man. A detailed scudy of the macter has been undertaken on Ehe Baould (see section 4). 4. Study of linear disoersion in 1982 A detailed study is in progress on the Baou[6 and will be continued throughout the reinvasion period, i.e. for four monEhs, from May Eo August. Ten sites have been selecE,ed on the Baould and Ewo on iEs tribuEary, Ehe D6gou. In so far as possible, catches will be made on Ehe same days ac siEes more or less equidist,ant from Madina-Diassa, Eaken as a reference site. IE is also envisaged that there will be periodic rnapping of potenEial breeding siEes at periods when Ehere are significanE modifications in t}re discharge of watercourses. Itre first results of the study indicate Ehat Ehe upper course of the Baoul6 was Ehe firsE to be affected by invading blackflies in 1982. ltrereafter there rcas a gradual shift of che blackflies Eowards the middle reach in the Madina-Diassa regi.on. Ttre stretch affected extends from l,l.ar6na upsEream Eo Madina-Diassa and Niako dorensEream. It is on this sEretch beEween Mar6na and Niako that blackfly densities were highest EhroughouE Ehe reinvasion period. Higher upsEream and lower doqmstream the densities decline rapidly t.o the point of disappearance in Ehe Logognasso region, towards Odienn6 and in the Niagal6 region, norEh of Bougouni. Ttre first mapping of the breeding sites was carried out concurrently with Ehe arrival of the first wave of Ehe reinvasionl it indicates that che disEribution of breeding sites is more or less uniform on the Baoul6, but that they are closer together in che middle course affected by reinvasion. 5 Possible straEeeies for the control of reinvasion in the ProAramme area SErategic planning is dependenE upon whether or noE the potenEial sources of deparEure of reinvading blackflies are accessible. IE should be not,ed chat since the onchocerciasis vecEors involved in the reinvasion are species sensitive co organophosphorus insecEicides they can be aEEacked in Ehe larval stage in their breeding sites. A solut,ion had already been found for the western cenEral and southern foci at the Eime of che inclusion of Phase IV regions in the onchocerciasis control operaEions. Although these regions are in zones in wtrich forest. species have become resiscanE t.o insecEicides, the use of Abate has been effective againsE the reinvasion of Ehese foci. Studies carried ouE in 1981 on pot.enEial sources of departure for Ehe reinvasion of the south-easEern focus would seem to indicace Ehat these sources are in Ehe basins of the Asukawkaw and of the cributaries of the upper Mono in Ehe souEhern extension zone. Ic is considered Ehat appropriate larvicidal control could neutralize Ehe south-east.ern reinvasion focus in these regions. New feasibility studies are planned for 1983. As regards the wesEern and easEern foci, Ehe sources of departure remain unknown, although they are certainly out.side Ehe Progranrne limits. On Ehe other handr given Ehat, Ehe behaviour of reinvading females in the affected reaches is increasingly well known, and EhatiE is impossible to reach the sources of departure, the preferred tact,ic in Ehese foci could be some form of adulticidal control sEill to be defined in the light of the conclusions from the linear dispersion scudy now in progress. I tl- tk C) U o oan ar.r{ O o .F{ o rJ a o # d ct) U) tHoOE E +J (U .dkEooo ..{co 'iklr . ao trk c., !J ocPf FAc3.4 (ocP82 . ro) page 4 H o ah(, Pr trd ro oo0{JA oc..{ooof+{ r{ ,r,H(/),o 6(u 0>k(/)c,o.X"{dro odkodd(u>'O >E N t ,aJ(i"Cil\:-irl _ -^,)!: )_'t .r ,il ) , o 6 = @ =a z 9o G 2 o o o -ozIJ e =o FF = o = =eooGG z EA @ G t) o -F = E =eoo o F2o a 6e oL zo I\ L \. /,\ ,, .)t N g,' \ i o\ \ | .I i.(t I ) : I l.r. f \_.,\ 1/\ I ';, t! .: --I) n6 ii,!il ld. it .I t.'):, II!. /; /<) j\ r- t. i\ ( I ( !' 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