The course of reinvasion by simulium damnosum S.L. into the Eastern areas of the onchocerciasis control programme during the rainy season 1983 and investigations on the vectorial efficiency and dispersal of different fly-populations in Togo and Benin

i,i THE COURSE OF REINVASION BY SIMULIUM DAMNOSUM S.L. INTO THE "E 4 t I E E U = 4 g E=4 ! = g E = I E E = g ! g ! g g E B q I4 ! I : = q g N I B g L = g Eq_G SAUUE =A U B M IHE_BAINY SEASON 1983 AND INVESTIGATIONS ON THE VECTORIAL EEEIEIENOY.AND DISPERSAL OF DIFFERENT FLY-POPULATIONS IN TOGO AND BENIN A. RENZ (WH0 consultant 15 July to 30 September 1983) INSTITUTE OF TROPICAL MEDICINE, UNIVERSITY OF T()BINGEN, FRG I f,1 I I Ii- t/ Contents: page: 1. INTROIIUCTION 2. MATERIAL AND METHODS.. 2,L, Study area and fly-catching sites................ ?.2. Identification and dissection of the flies..... 2.3, Selection of sites for the studjes on the dispersa'l ........ 2.4. Prospections of Simulium breeding sites....... 3. RESULTS 3.1. The course of the reinvasion in 1983 3. 1.1. Meteorological data. 3.1.2. Biting rates at fly-catching sites in Togo 3.1.3. Biting rates at fly-catching sites in Benin.... 3.1.4. Mont.hly averages of the daily biting rates at some fly-catching sites from May 1979 to September 1983.... 14 3.2, Prospect'ion of breed'ing rivers and identification of larvae 16 3.3. Seasonal changes in the species composition of the larval populations at T6t6tou and Djodji. ....... . ..... 16 3.4. Comparison of the biting rates during the rainy and dry- seasons 1979 to 1983: Trend of decreasing biting rates..,,. ZZ 3.5. Results from the identification and dissection of the flies 25 3.5.1. l'he occurrence of different members of the S. damnosum s.l. conrp'lex in Togo and Benin ......,.T zS 3.5.2. Comparison of the results from the dissection of fresh clr alcohol-fixed and stained flies....... ZB 3.5.3. Parous and infection rates of various f'ly-populations. 30 3.5.4. Varjations in the infection rate of parous flies from MO d M0 1982 and 1983. ...... 33 3.6. The dispersal of f'ly-populations from the river in diffe- rent bjocl imatic zones ......................... .. 36 3.7. The influence of mermithid-infections on the fly-popula- tions at B6t6rou and T-et6tou ....'. ,,....4I 4. DISCUSSIoN. ..... ..45 2 5 5 7 7 9 10 10 10 11 72 t ",1 '.1 i-g It il. : ,1 ;l tJ,l iipijii l- ,l .t; ,r ' j-', h ri, lrii ,'1,, ,:ili, :i:I 1 rii,tiri i; -._ l .i i t I n 1 I 5. ACKNOWLEDGEMENTS .. 53 7 . C0NCLUSi0NS. .. ....., 57 8. RECOMI"IENDATIONS ...58 #Ei, ffi t FI ilir,' t F' t F,' I Ir hi -3- These vectors can be distinguished on the basis of spec'ific inversions on their larval salivary gland chromosomes (VAJIME & DUNBAR 1975), but adult flies can only be classed, as belonging to one of the categor ies S. damnosum s.s./S. sirbanum , S. squamosuJn and S. soubrense/ S. sanctipaul i by using morphological methods (GARMS et al. L9B2). The occurrence of these different vectors in space and time, and their respective contributjon to reinvasion and transmission cf onchocercjasis in different bioclimatic zones of Togo and Benin were to be investigated in the light of the fo'llowing questions : - Which species are coming to bite on man and what is the amount of transmission by each vector ? - l^lhere are the potenbial source areas of reinvading flies ? - What are the differences in the vectorial efficiency of the different vectors in different b'iocl imatic zones ? Particular attention should be paid to the northern limits of the areas of distribution during dry- and rainy season and to the vectoria1 efficjency of the S. soubrense/S. sanctipauli flies. These vectors are assumed to transmit the rrajn-forest formr of onchocerciasis, which is found associated with a tow degree of pathogen'ity to the eye, even at places where the transmission is very high (ATP over 10,ooo,OMS 1982). In the Ivory Coast, S. soubrense/S. sanctipauli larvae became reslstant to the common'ly used insecticides (temephos and chlorphoxim, GUILLET et a1. 1980) and it is therefore crucial to know, whether these flies - or one of their subspecies- could repopu'late, if resistant, the former breedi ng s'ites of S. damnosum s.s./S. sirbanum in treated areas of the savanna. The djspersal of flies away from the river and, in particular, the flight range of old, parous and infective flies delimits the extension of the areas, where the human population is exposed to disease trans- mission. This is particularly true for the reinvasion flies which come to bite on nran only near to fast-f'lowing rivers, but'not when migra- ting over-land (RENZ, 1983). Studies on the dispersa'l of f1y-popula- tions were therefore designed to provide information on the differential flight-range of nuliiparous and parous flies and to compare the beha- viour of different species in various bioclimatic zones of Togo. -5- 2. MATERIAL AND METHODS 2.L. Study area and f'ly-catching sites (cf. Fig. 1) The course of the reinvasion was monitored at the same daily fly-catching sites as during the previous studies: At M0 ii M0 (r. I'ld), Landa Pozanda (r. Kara) in Togo and at Gbass6 (r. Bou'li) in Benin. Daily fly-catches started at these sites in May and con- tinued until the end of September. Additional information came from the routine OCP-fly-catching sites which were visited at weekly intervals. All breeding sites inside the OCP area were treated at weekly intervals by aircraft, starting in m'id-May in Togo (Landa Pozanda) and by the 29th of June in Northern Benin (Gbass6). AbateR insecti- cide was used for these treatments. 0utside the OCP area, the dynamics of the f1y-populations were examined, in Togo, at Landa Mono (r. Mono) and Fazao (r. Anie) by three days of catches per week and at Djodji (r. Gban Hou) and T6t6tou (r. Mono) by two days per week. In Benin, flies were caught at three days per vreek at B'elt6rou (r. Ou-em'e) and at two days at Kaboua (r. Okpara). The flies were caught according to OCP routine and were brought alive to the laboratory for subsequent identification and dissection. From each f'ly-catch'ing site, the catch of one day per ureek was dis- sected by the sector staff in Atakpam6, Kara and Parakou, up to a maximum number of 60 flies per catch, for the routine of the 0CP. Another dayrs catch was send to the laboratory'in Kara for species- identification and subsequent dissection. *fi l i; ihr ',\:i 72.?. Identification and dissection of the flies A11 flies caught during one day per week at Landa Pozanda, MO a MO, Landa Mono, Fazao, Diodii and T6t6tou in Togo and from Gbass'e, BEtErou and Kaboua in Benin were separated into nu]li- parous and parous flies, up to a maximum number of 110 f'lies per day and s'ite. Nulliparous and parous flies were preserved in 70 % alcohol until further examination and dissection. The species- identification of alcohol-preserved flies was done using the same morphologica'l criteria as in 1982 (RENZ 1983, and Table 1). The results of the examinations were recorded on modified OCP data sheets ('fiches 2A', Fig. 2). After the identification, the flies were placed into the wells of a mjcrotitre plate and were stained as fo1lows (NELS0N 1958, GARMS et al. 1982): - 30 min destilled water - 1-3 days l,'layer's H'dmalaun - 1-3 days tap water (1 0/oo fungicide Nipagine was added) - 1-3 hours 25 % acetic acid Then the flies were dissected on a slide in one drop of a mixture of 50 % glycerol and 5o % acetic aci d (25%). Infections of the flies were recorded on the modified data sheet. All dissected parous flies were reexamined for fjlarial infections under a hllLD compound m'icroscope at the end of the day. 2.3. Selection of the sites for the studies on the dispersal The studies on the dispersa) of the fly-populations away from the river were carried out at Djodji (6,/7. and 15./L6.8. 83)' MO e MO G3./14.8. 83) and at Alamassou (16./17. 9. 83). Five to seven f1y-catching sites were selected, in the vicinity of each vil'lage, according to the fo1lowing criteria: one site at the usual 0CP-fly-catching point, which had been situated at the place with the presumed highest b'iting rate. A second sjte was chosen at a distance of about 100 m fronr the river, and another at a distance of about 300 m, along the road or the path lead'ing from the v'il1age to the river. One catching site was located inside the village under a tree. Yet other sites were placed at rainy season tributaries or at a waterhole near to the vi'l1age. F'igures 8 to 10, which give the .r ,i + M ti 't $ t { I t t i- i+ Ii ; t, H7l\f: tr') ;'i ir. [: tr i',, hl li tr ,i' I tr - Il-, i .-. I f , r f._ l: i_r Lt, r i_x" l #l l+, tl i, hi ir: t:l It*l ,.li ; ,l l -9- location of these catching sites , were drawn after aeria'l photographs from the land registry office in Lom6. However, the region of Ala- massou was drawn from a map 1:200.000, according to prospection of the area by feet. 2,4, Prospection of Simu'lium breeding sites by road. S. damnosum s.l. larvae were collected at breedin g sites in Togo but there was not enough time to do so in Benin. Most of the larvae samples, the identifications of which are mentioned in this report, stem from prospections of the OCP sector teams or from the insecticide screening team in Lom6. The 'larvae were fixed in cold Carnoy and were send to Mr Fiasorgbor (0uagadougou) for cytotaxonomic identifi- cati on. . tl ll,I,! .! I ,i tt. l; I ,,1 i, l rll' , li r'i i i"ta { i rl .* '{ J I r'l ;, d '+ f,I fi ii ,f rt I rlt n- { f I h Il!t 'f t :l' - 11 - 3,1.2. Biting rates at f 'ly-catching sites in Togo (Fig. 3 ) At Landa Pozanda, the biting rates were much'lower in 1983 than during the previous years. For the total of the rainy season (May to September), the biting rate (1215 flies) was at its lowest level ever recorded and was even lower than during the two rainy seasons of experimental treatments of the suspected source areas of the reinvading flies in 1980 (32i0 flies) and 1981 (1629 flies) (Table 5). In 1982, the application cycles of insecticide to the river Kara were interrupted during the months May to August. This exp'lajns the very high biting rate in 1982 (9985 flies). Daily biting rates, ex- ceeding 20 flies per man per day were only observed in the second half of the month of August and the highest daily biting rate was observed on the 20th of August (82 flies/man/day). The dynamics of the biting rates were similar to those observed at Md ii Md and Landa Mono, but in May, more flies were caught at Landa Pozanda than one might have expected from the biting rate observed at these sites. At M0 a M6, rnore flies were caught in 1983 (18,487 flies) than in 1982 (14,503 flies), but this number was still lower ihan the biting rates in the years L976 to 1979. The maximum daily biting rate was recorded on 13.8.83 (647 flies/man/day) at M0 ri M6, on the 15.8.83 at Landa Mono (432 tlies/man/day) and on the L2.7,83 at Fazao (528 fl ies/man /day). 3.1.3. Biting rates at fly-catching sites in Benin (F'ig 4 ) Similarly to the previous years, Gbass6 continued to show biting rates exceeding the critical 1evel of 1000 flies per man per year by many times. For the tota'l of the rainy season (May to September), the estimate biting rate was 23,9?2 f'lies. The fly-population bui'lt- up later in this year than during the preceding years and reached high leve'ls (786 flies/man/day) by the end of June. These flies presumably cane from local breeding sites, since the Sota was only treated star- ting the 29th of June. Fi g. 13-4 d,UooF(Jo d,U 6 =UFdUVI r} @(,l 'g rOfo L JoL r0, r0 . T AJ @Oi ra, e r(u5o L =e r€, +, rluEI rl @(,l o o a/, L rO, 6a r., N @o o €o L r0 6o ..5 F =(5 J .a z. L^J co z. cr) @ql rl o z. C\J @O) "-< U =,) g tt Ii,It t ilfI rl{{l i H I f, I f,t' I I i t f, ll fl )it N fi .I d l) fi I t,I I I I ,', t lr T d !, c E o tn z.O tJ) trJtn H d. t -F (5 Hi oo 99 O(fO€) o o()g,oooo croc, o9q qoo9 o ooo E 666 6 oo d6cl oN io 6 q 6 (! Fro o --an ; .o N H N d.u m <>Fo o ciU6 UFGU vt o6 o ,ltL6oIo L 6Joo6v @Ol =oL rO L fI rOL rO :4 o @o Loa L Loo @o 5 o rl L f oof oN d. Jo a trJ HF a tdo F6 =(l, = J .a C5 z, F co J cl <J' cr) Lr- U .2 6E c E (u oooc)oFEEssEsoEEEoEEEoSsBB@o$oN=oNmNa@h i Fig. 5 -15- I c'.) OJ qr r-l 2 2oFlrlFtrlF noo o =o co v =od, Lr.lFlrJ @ o z. t\ c) o- o GJ @ Ol r-{ Fa cc qr Fa .2 - -) 6 -) .2 - o -O .) a a o a n orl{rl(rl c(l cr) c +J co =r!o (u -c +J l+- o ah OJ cr) rOL OJ rO x E +Jc o E (U -C +, (+- o VIc o +J ,O l. rO rttc o at7 ,o(u U7 lar I o @ Ol Fa or r\ ot Fa Ut tL hO O O A O O O r< O O O O O O F{ OC) Oa(rOC)C)c)FtOC|C)(])dOOA \.AAdFl C(, E 0J (r- ffi F.+ I H:E.'Hl ts: E:l k,; B.i'F{i ffi [i; [:' F" I F, E V k, ,t l', t h, Ii. I Fr [.L: Lrir i t F t Ir h. F: Bi. F/IiI it*, i,' #, ri !:' Itk.ta k m, Ifi ffi ffi ,1i', $, , Iri.,it' i.i [, i L7 Tab. 3 CYTOTAXONOI'{IC IDBNTIFICATION OF SI}IULIU}I DAI'INOSUII S.I. LARVAE FRO}I I]RDDDING SITES IN TOGO AND BENIN DURING T}IE RAI}IY SDASON 1983 (Identifications by ltr.by thc Sector staffl theDr. Cheke and llr Renz) G. Fiasorgbor, larvae collectlonsinsecticide laboratory in Lom6, Date Site (niver) da si sq so sa so/sa Djodji (Gban Hou) 9.3. 83 24.5. 83 5. 7. 83 t2.7.83 19.7. 83 26.? .83 7.8.83 23.8.93 30. B.83 6.9. 83 2 7 16 o 1B 77 7? 13 4 3 1 4 5 4 3t ! 3B 72 5 28 16 74 13 77 5 a! \, 26. 11.82 77. 2.833. 3. 83 74" 6.837. 7.83 1O. B.B3 17. B.83 26. B.BJ 28. B.B3 20. 10.83 25, 8. e' 21,10.83 22. 10. 83 4. 7.Bs 25. B.B3 27. B.B3 il B6gba R t6't6tou tt It !t lt lt apidsrr T6t6tou (Iower Mono) Adjarala (Mono) 771 71 12 26 47 2 4 94 101 21 19 24 6B 27 1+ 5B 24 D 2il ll ll t, lt ll 7 === =================-====-====================================== Landa llono (upper ltono ) 2. 9.93 33 Ngamb6to Adjarala Kpcssi It 1 39 22 42 o Segboh6 16. 9.83 6B 3 3 i--=- = ==== === = ================================= ========= r l Date Site 19 (river) da Bl 6q Bo Ea so/sa Tchalou (tla) 27. B.B3 5B Goubi (Ogou) 23.9.83 2t Kati (sio ) B. 28. 8. 83 8. 83 3 15 tt 60 t f, i t. ,1, Ezim6 (our6) 8. B.B3 Brouffou (Gban IIou) 6.9"83 77 Ayagba (Gban IIou) G.9.83 20. g.B3 11.10.U3 1 l1 : a F .l 23 67 77 1i. k bi fr uh' h, H, +?,: V, tE! I I I t & h ffi* Hl- $l FJ ti, )- :i , k!r- r- t. [.r-, ir I l*:, 25. 8.83 B6t6rou (ou6nr6) I.ltbatakoukou (tr) 41 4 9. 27. 22'. 24. 1. g. 83 8.83 B. 83 B.83 g.g3 t7 75 49 119 27 7 t7 363 52 Aguigadji (ouam6) 23. B.B3 40 ,.t Bassa (ot<para) 25. B"B5 s86 -2L- vae were already identified at T'et6tou, when no such larvae were found in samples from Djodji. S. damnosum s.s. which was present in all samples of the dry- seasons in previous years, was not found in one (small) sample collected in March 1983. S. squamosum was al ways found at Djodji during dry- and rainy seasons, and it became the predominant species after the experimen- tal treatments in 1981. 0nly in thjs year, the former popu'lation of S. sanctipauli came back to its orj ginal size. At TEtEtou, S. damnosunr s.s. had arways been present during the rainy season at a proportion of 7-62 % and, during the dry season, at 5-6 %. The rest of the population was main'ry S. soubrense, S. sou brense/sanc ti paul i and (few) l. sanctipaul i (interbreeding 'Beffa form'). S. sirbanum and S. s uam0sum were only found very occasionally. However, during the rainy season 1983, no 'larvae of S. soubrense, S. soubrense/sancti paul i and S . sanctipauli were obse rved. Coinciden- taly' the biting rates were at the lowest'level ever recorded at this site, and mermithid-infections were found frequently in both larvae and adults of S. damnosum s.s.. a i ilt. ' L't [,i t ,l I I I l I -23- Tab. 5: Total S. damnosum s.l. biting rates during the rain.y seasons 1976 - 1983 ( I'lay to September onlY ) 1976 7977 1978 7979 1980 r981 t982 1983 td* Landa Pozanda L7.529 + 3.379 3. 119 4. 194 3.270 7.629 9.985 ++ t.275 *I* MO ii M0 22.638 +rf 28. 080 27.823 24.465 L7 .077 9.775 14. 503 18.487 *xx Gba ssb 40.669 49. 968 30.062 34.997 17. 381 10. s95 23.9225 Fazao 30.115 +++ 14 .4 16 +++ 4,992 10.747 33.725 Landa Mono 33.752 31.316 +++ 25.977 3. 616 16.t79 t5.L74 B6tBrou 19. 333 25.662 38. 704 16. 678 12.697 4. 616 Goubi r** ( 12. e3B ) 2.972 8.494 Djodj i . 108. 550 81. B8B +++ 27 .4s5 13. 703 75.677 88.141 32.484 16.781 21.275 T6t6tou Ka boua 42.246 59.433 69.426 73. 998 1 10. 949 27.870 13. 106 +: ++: +++: r! r*: xt{: no control no control in August S' no control during May and June 1983 experimental treatments without May 1976 (approx. 1.500 flies August and September 1978 only ! i n contro'l I ed area I H t I 1 1 1 I 1 J I i I i I i i I I l I 1 I l I l I I i I I .1 I J JI I l l I I 1 I t I i y i. 4l,- ffi h. i{I, :r H.ih. !r h,i ffi E1 EI Ii [', Ft F6i [, F F ft ffi BE.' tl ] t, F i I t I I 1 J I I I I I JIq It Il I .J I I I I I i ) -25- 3.5. Results from the identification and dissection of the flies 3.5.1. The occurrence of different members of the S. damnosum s.'1. complex in Togo and Benin During the studies in 1983, a total number of 1341 flies from vari- ous catching sites were examined and were classed into one of the three catego ries S. damnosum s.s./S. sirbanum, S.lgqlmqs um and S. soubrense S. sanctipauli. Together with the results from the identification of flies in 19BZ (RENZ 1983), these data were used to calcu1ate an esti- mate proportion of each vector group in the total biting rate, per rainy season and per year, for different catching sites in various regions of Togo and Benin (Table 7 and Fig. 6 ). The values for the biting rates per year and per rainy season were taken from the OCP microfiches data for those years, when no insecticide applications were done outside the programme area (1978/7979, L98L/ 1982, 19821 1983). An average biting rate per year is calculated, together with the pro- portion of the total b'iting rate during the rainy season. The relative proportion of the three vector groups was taken from the results of the identification of flies in 1982 and 1983 and was weighted by the corresponding biting rate during these years to obta'in an average proportion of each vector at every site (Fig. 6). l4ost of all fljes came to bite during the rainy season (l4ay to September), and only 4 to 40 % of all flies came to bite during the dry season (November to April) . S. soubrense/S. sanctipaufi fIies were dominant at Djodjj, T-et6tou and Kaboua, S. squamosum was the predominant species at t'10 ri M0 and Fazao, and S. damnosum s.s./S. sirbanum at Gbass'e. Ilot enough flies were identified for the popula- tion at Landa Mono. These estimates represent only very crude values, since they were based on a very limited number of flies identified during a few months in two years (July to October 1982 and August to September 1983). More data should be used for these calculations, especially for the re- maining months of the year. When more data wou'ld be included, a simjlar calculation could be made for the amount of transmission by the diffe- rent vectors at the various sites. I HAT lrlN60u a -27 - ..'ctt"'"t' 5 lu (andio o'q s rd, :, o I I . tAltA-KARA .' a I , v; |4 0 Fig. 6 THE COMPOSITION OF FLY- POPULATIONS DURING THE RAI NY-SEASON s. d si S. s S. so/sa tl I ! rl'iJ-- ,1. , , I ABR 10" to4 +c p rD 0 t I Proportlon of total ABR durlng dry-season (lX.-lV. ) I I I I I I I I ,I t , a I a a I , , PLACE TBE RAINY.SEASON a _a A:H0iM0 B: Landa Poz. D: Fazao E: Landa Hono G: Kaboua H: GbassE M: DJodjl ll : T6tE tou 24. ooo 7,7oo 28.6oo 3l.3oo 73.600 37.3oo 140. ooo 6l.8oo data yet unsufficient COIONO o 50 km 9lt s7/ 967 eot 76/ 77/ 50r 65r ] t t , I t , I Io)6)r{ N(o aa@rc o$ aars to to rf aa t, @ D- FtO'$rcN t,) 3B H(o@(orf Fr 3g Nrccooo,od?{ H Ff A t{ F{ C) z .A a a frl tt) z EI ca o th !) H |f E H o t?lora $1. aa ?fr{ aalr)|o tC l. aaNOI Ob? aa(oto f{ i'r fits g$ Nrc@@ooHH o)N+ -29- Tab. 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"?aSn asRa;i orsCx= ofg5*9i xAC({i30.seg 8R 6 ra)(>ooo q)H (,t lnF)E trco $oHOr hr, 6 sIr o N NSg9oro ocrocr(>o dd(rc)OO I , a I a ''l''lE-9SS:";a <to .o H(' HIe$oo Eg*ddo SSxGI- HE;Br- 3-osoo c, lcoS.j O!-HOig*3R- OtN <)o<)o N6 oo NOia, o mooo @d octNO ooc)o NN oql di6 @h crt@oH(\ro I <t @ oN rt @ o @ 55-odo SE=oicro ES-..6Io Hagcosrc, rj 53VI-NNd Uhe-.Hoc, ffiBaFI* oo aoa ..- cON.6lcoH a:N3 a: ?-:?d@rN6 u'!o6r-A g.iscie ?"1 3^frg=tjg "l 9i-9s3tjg, ^:\ 9reS9:.9i. !Qco - Ag.-: gog \"1 S^$RI Adi @ doCNEess Nr @n o o{J -o- .6ssiiRi oOa.ar =d=os..noSoeNHvdv €oa=$ g 3'g\ e se ?HgEri3 oro6o5 ..?.thcoi.:r.jl<ti3dv oa.a. a N rt-o bo@ B9 -r O OJ 6o No 6 g*eI"oo oooo NddO co(o(ro ocrooo 6 h3 3@u : ' I I I I M :f- o 6 N (ro r@ 1l ooo6 GJ 'No?'(, ooo9 Oh (\'N ER-* RI^o ocoOrn C\IH orco?-a$Ngd3 Eoooc) t, oC)oae I s4 f\/l .: I:aa **T' ;T 6-o o oc 6 oC)o L oo 6 C o oo c o E c Ll- . .+. .+. ..4 . . o .. a . ^ ! .-O . NF .JEaE m+ .aFJF .J .ttso} .!rC>C .Oo+o69(r9U5(,uooLlot.Lra-rnSn6COCCh oFv?v N. HJ Jv}a -5OaF o-tt>l! OF +)9ar rJlJ0 o0,9r.-C cco PL o c\ oLG L o,llEf c co E u I oEEI L . .. 6.. . . @ to ..oo. .^60 . O! o . .J6€6 A.. 4 C .* NO O .Jc'u'- O.'F>{HO.FU\.--J o I 11, .vFul! .g(uE .E>gF .(l,(Utr a9!.-O>ooo4crtLE+r9J €C.F'FO e* I ! !, I tU ra- rt o L o .o ef = I L o,CI o6 E6 o !,o uo !, -31 - Tab.9 (udo>o dF QtaOE o!p o !a- o!LC EEp() DE! oru oOL o o >x odOL C >r J- 5LlFo 6A OEO.- GT-o op >o po (JF o(a- L C.e o3 o60 Lo(u op>u oaJo oola- LEL o o uo E E Eo EoI Uo ,oL6 CL la- o co +, L oo oL CL 0,EP q- o p E o oc ol, €o(., De6 o + ILooL960 aE H J = o- t-L) z a LJa t4Jd. co)oa a =aho z)OUI V' =J cod. tJ', tJ'l tn t1 =V'O z. =o a.r'l (r,ll cou C,.l IIF{ llI06 llI(v ll@ll ch llF{ llI =ltxll I v) ll z. lt c) llHll Fil(J tl LrJ ll o3ii E xll olt bllE =[ Foll;dlr F tJ- ll #I(,tt 2FilJII AlI) ll : v, ll a uJll *oail ElloClll ruIJII Q -1 | 2 sll eii El8ii 8l vll olll ol ..ll €l att ^,1LrJ ll irlHll JlJrr g1LL ll .(l ,,.il P() ll 't-[= U) ll 1(, uJll 6Fll q <(il.9d. lt i"Iu z, lt gOll 6Eii 5(JI u.l ll LL ll z, tl xll I oll z,n <il llan =llc) lld.I <ilo- ll sol ls ttB t^l n0ssPu,gtv oPz eJ OH9OH 9noq E)l n0Jql?8 nol?1?1 t lpolo PPuezod PPuel sol!s ttc hl nos s PUrP [v oezPJ cr.l P ct{ rnoqc)l noJ ?1 ?B n01?1El l lp0l0 PPUPZod epuPl solts ttE t n0sseuElv 0ezeJ AHSAH c noq Px noJgl ?B n01?1 ll t rpof0 Epuer0d EpuP'l ni ; .,{ :{' B: i l C\ -o|l,F ar I J l I I l I i I i j I i I 1 ] I ,1 I ) i .t I r ffi iiF^ ffi h*tr r{. ffi{ ffr F !'.{r.hi! E Ul H [r1 . tlt, [r, F F, E; E; [r]q, H H, L' H t i t l t, [, b t" I E t !r t' rf- I I + I I I I i l I -33- 3.5.4. Variations in the infection rate of parous flies from M0 ii ll0 l9B2 and 1983 The infectjon rate of parous flies at 140 d M0 was extreme'ly low during our studies in end of July, August and September. 0n an average,'less than 4 % of all parous flies were infected and less than 1 % were infective (Table 9). Almost all flies were identif ied as _S. sgr.r!lno_!_u11, whereas 2 out of 10 flies, identified as S. damnosum s.s./S. sirbanum were found infected. This phenomenon could be interpreted either by a 1ow suscepta- bility of S. s uamosum to infections by 0. vo]vulus, or by the fact, that these flies did not have the chance to feed on an.in- fected human population in these sparsely populated areas. A comparison of the jnfectjon rates at M6 d M6 during different months of the years 19E2 and i9g3 (Table 10) shows, that the .in- fection rate was rather high (14 - L6 %) at the beginning and at the end of the rainy season rgBZ, whereas the rates .in July and August were similar to ours (z - 4 %).lnfection rates urere h.igh, when the parous rate was lour (Fig. 7 ). This is in contrast to the usual observation, that jnfection rates do increase wjth in_ creasing parous rate of a population. From the data of GARMS et al. (1981), it can be concluded, that the proportion of S. damnosum t.../S. sirbanum in the population at M0 ii M6 was about 70 % at the beginning of the rainy season in May, decreased steadily until mid-June and was low in Jury (CHEKE & GARMS 1gB3). It appears there- fore, that s. squaqlrqrm has a lower abi'lity to transmit the disease in this area, if compared to S. damnosum s.s. /S. sirbanum. i, { , t_ YLj i./i l I 1 -35 Fi g. z' ggEBEllIIg!=!EISE9!=E4E9g!=!I9=IIIESII9!=E!IEg=4I=I9=!=!9 DURII,IG TITE RAINY SEASONS 1982 - 1988 ---====-- Percent of parous flies lnfectecl 1982 : x 19Bg : o x Oct 75 x May 10 I Septenrber r June r lugust ,( I I I 5 o O June JuIy r s $Hf,,s t mber tt' fi rl't,t. 40 50 60 70 80 90 94 of flies Parous i tlr ,&r ,FIil l: tA t ,. I .t I I$ :t ri.f I ;, !r :! 2 j ,l -l _i _l 'l r-i ;.1 i i ,l :l j iri i I 1 1 I -37- Fig. 8: D'ispersal of Fly-Populations at Djodji I f1y-catching site 72 road y, S. damnosum breeding slte .r/' dry tributary ..r" path ,f, treated breeding slte A farm 4^A ritt.s. Di stance from rlver Catchlng sl te Fl{es per man per day (I parous) 6.8.83 7.8.B3 15.8.83 16.8.83 Percentage of bitlng rate at the river: total f'lles parous only Gbqn Hou 5o t$ i A A JT Al 60 4 6 o I 4A4 DjedjL A4 A ( lfl l-i iiit IIr IY, ?t I,, ] A, Ihjj it [] i 'i+ i;r t: li_ I$t- IEi) H.i rf- j *l l; .tl tl -l ;t 5m l0m 100 m 375 m 1500 m 1700 m 2300 m (l) (21 (3) (4) (s) (6) (7) Gban Hou ford 150 m downstream at the road at the road yaterhole 0JodJl vlllage tri butary (3ot) 614 (3ot) slo (sst)44s (30r) 486 (34r) 40s (23r) 307 (l4r) ( l8:) ( ler) l2s7) (28r) 100 t 109 t 76/ 54 I t7t s5t t5r 100 t l3l t 54 I ?81 t4 I 481 t?t 396 234 t47 67 136327 (?5t) r23 (?6/) 43 (23r) -39- Fig. 1o : Dispersa'l of F1y-Popu'lations at Alamassou I fly-catching stte i/ roaa f S. a.rnotrr breeding slte -r' dry tributary .....' path 7( Vert"a breeding slteA r'arm A A vl llagc Dl sta nce from rlver Catchlng slte Flles per man per day (lparous)16.9.83 17.9.83 Percentage of bltlng rate at the rlver: total flles parous only 6 o "^rt tI \ \ t \\ \ 4 \ \ \ \ A 2 I t t \ 4 Tl[c/massou APnie --> t.r I 5m 10m 100 m 200 m 300 n 700 n 1000 m (l) (2) (3) (4) (5) (6) (7) 100 t 24 I 257 72 I 35t t2 I It6 t 100 t 87 ?4 I 57 1 25 I t4z 937 Anla rt Alamassou raplds 2 km upstream path to vlllage dry trlbutary Alamassou vlllage vlllage flc'lds dry tributary 1s8 (s6r) 64 (5sr) ll3 (4st) 78 (44t1 t23 (621',) 2e l?rtl s (Bot) 23 (30r) ls (73r) lB3 (4sZ) $ 'i # tr H h9tr ffi l&+ ffiili !'.! lr Hilk- Hi ?"tt:[: ' t.t. Ii r, t: r+ I{; LtL. Et tr l{, h It rri [, !+.lb. [i., tu'iS.. 7v. tl ffi[' HI ffi- fi; i'' u' h,}}' ldr. iH# i:. , trf !i tt H E H F..it1l ti fi' s., v,: -EI t1 t,' -41 - 3.7. The influence of mermithid-infections on the fly-populations at B'et6rou and T-et6tou A total number of 308 mermithid-infected flies were found at the dissection of 4,007 flies in 1982 and 1983. The vast majority of these flies were nulliparous and had atrophied ovaries, but a few parous flies were found infected too, and in one occasion, a deve'lo- ping stage of 0. volvulus was found besides a (sma1 I ) mermithid. These mermithid larvae resembled to Isornernris lairdi (MONDET et al. 1977). They were about 7 to 14 mm long and their average diameter was 0.1 nan. The total volume of these parasites (0.05 - 0.1 mm3; corresponds to almost 10 % of the unfed fly's size (-1 mm31. Most of the infected flies came from B6tErou (219 infections) and from T6t6tou (67 infections). Apart from this 'localized distribution of mermithid infections, different infection rates were observed for the three groups of vectors: 185 infected flies were c'lassed, accor- ding to their morphology, S. damnosum s.s./S. sirbanum, 119 S. squamo- sum and only 3 infections were recorded in S. soubrense/S. sancti pau'li . This phenomenon was studied in detaii at B-et6rou and T6t-etou: At T6t6tou, only 2 infected flies were found at the dissection of 551 flies in 19BZ and both fl'ies were classed S. damnosum s.s./S. sir- bqqum, although this species was very rare at this site in i9B2 (2/g = 2? % infected for S. damnosum s.s./S. sirbanum, 0llq= 0%forS. squamo- sum and 0/549 " 0 % for S. soubrenseE. saqctipauli ). In 1983, at the same site, only S. damnosum s.s./S. sirbanum flies were found and the infection rate was very high: 65 of 149 flies 1= 43.6 %) carried mermj- thid infections (Tab1e 11). At B6tarou, the corresponding infection rates were, in 1982,: 92/27L (= 34.0%) for S. damnosum s"s./S. sirbanum, tI6/252 (= q0.0%) for S. squamosum and only 3199 (= 3.0%) for S. soublrqnle/ S. sanctipauli. In 1983, sim'ilar infection rates occurred: 2L/L02 (=20.6%) for S. damnosum s_.s./S. sirlanum, 2/17 (= tt .B %) for S. squamosum and 0/6 (: 0%) for S. lqubrense/S. sanctipauli. It seems from these data, that S. soubrgnse/S. sqrctipauli flies are much less parasitized than the two other vector groups - either due to a reduced susceptability of their Iarvae or to an increased mortality of '!nfested larvae or adult flies. ; rl I ,i f 1,t -43- In order to examine the influence of the mermithid infection on the morphology of the infested flies, the ratios of the thorax/antenna length measurements were compared for uninfested and infested flies (Fig.1l). Mermithid infections were mainly found in f'lies with a ratio between 2.05 and ?,25 and only few infested flies had a ratio of more than 2.40. Hardly any flies with a ratio below 2.00 carried these parasites corresponding to the fact, that S. soubrense/S. sancti- pau'li were rarely infested. The decrease in the infection rate of flies with a ratio of over 2.15 might be explained by a reduction of the size of the thorax of infested flies by atrophy, whereas the length of the antenna is less influenced by parasitism: 46,5'l of all uninfested flies from B6t6rou and T6t6tou had a thorax-length of more than 1020 ,um, whereas only 24.2 % of the infested ones were found in this sameI group. F I 'I I I I I -4s- 4. DISCUSSION The reinvasion in 1983 The course of the reinvasion in Togo and Benin differed from the pattern of bit'ing activities at the reinvasion sites observed during the preceding years (GARMS et al. 1981, 1983; RENZ 1983), presumab'ly due to the long drought during the dry season and to the 'late on-set of rains this year. In view of the'limited period, covered by th'is consultantship (end of July to September), a detailed description of the reinvasion and of the participation of different vectors in this phenomenon could only be given by combining these results with those of Dr Cheke's consultantship, which covers the period May to end of July, but which was not yet available to me unti'l this data. By the beg'inning of these studies in July, the biting rate was stjll unusually 1ow at Landa Pozanda, but high biting rates, exceeding 300 flies/man/day were observed along the river MO at Bagan and Md a'M6. The majority of flies were S. squamosum and some of them probably came from untreated breeding sites in the nearby tributarjes Loukoulou and Sako of the river M0, or from breeding sites in the affluents of the upper Mono.Only very ferv f]ies were caught at Landa Pozanda before August, and the increase in the biting rate at this site in late August coinc'idates rather with the pattern of biting rates observed in northern Benin than with the dynamics of the f1y-populations at Fazao, Landa Mono or l'ld ii M6. This indicates two separate sources of rejnvading ff ies at Landa Pozanda, similar to the results of CHEKE & GARMS (1983) for the years 1980 and 1981. Local failures of the treatments of the river Kara or of its tributaries are most probably responsible for the flies caught during May and 0ctober. The total bit'ing rate during the rainy season at Landa pozanda (1,215 flies during May to September) was not much over the'tolerable' level of 1000 flies per man per year, but this very positive result should be compared with the biting rates at other sites in this region (A16h6rid6, Kapayando, K6tao...) in order to see whether this is re- presentative for the whole area. In Benin, the biting rate at Gbass'e continued to be exeed'ingly high, thcugh the situation at Zougou and, in part.icular, at Alibori and K6r6- mou was much better than the preceding years. The high b'it'ing rate at Gbass6 in June and July is probably due to'local breed'ing, since the r*"' ie' i'r r Ir if, [F,;ta " lfi..r f,r rt, tr ffi., lr-': H; l't' 't*. i i I3 ni$,i T i,, :it - {r lrp, ffi r4!t1 ,:!" t ,+"i.: . ,l , Ir tl , ri lv F,tl Bi V fi ii, ls. F $ tr t i, t i I ffi EL' ffii r.3 t$ ii i -47- this group is not found in Nigeria or cameroon. If this observation could be confirmed, then it could be tried to attack the remaining breeding sites of this vector in Togo and Benin during the dry season in order to see, how long it would take until these sites would be repopulated. The Volta Lake could possibly form a natural barrier for the repopulation, if the breeding sites in the east would have been treated. The rBeffa' form of S. soubrense/S. sancti paul i The different proportions of larvae, assigned according to cyto- taxonomic identification to either 's. soubrense, or s. sancti paul i ' or to S. soubrense/sanctipauli is inconsistent with the supposition of MEREDITH et al. (1983), that the popu.lations in Togo and Benin are a single interbreeding group and that the diagnostig inversions occure in Hardy-we'inberg equi I ibrium. If this wou'ld be the case, one should expect an hjgher proportion of'S. sqlctipauli'at T6t6tou and of 'S._soubrensg' at Mb'et-ekoukou (Tables 3 and 4), and at Djodji, S._ soubrense/sancti paul i shou'ld ha ve been seen in much higher propor- tions (Tab1e 4 ) . As rvel'l , the adu'lt S. soubrense/s. sancti paul i fl ies from Djodji had,'in 1982 (RENZ, i9B3) and in 1983, dark (D,E) wing- tufts in a great nrajority, whereas pale (A,B) tufts should be found in about 30 % of all typical Beffa females (RENZ 1993, MEREDITH et al. 1983). Such pale tufts were frequently found at T6t-etou during the preced'ing years, whilst also the Beffa populations at Kaboua always showed darker wing-tufts (RENZ 1983, GARMS pers. comm.). A possible explana- tion for this phenomenon would be, that, even if this group forms an interbreeding popu'lation in Togo and Benin, there exist adaptations of subpopulations to different environmenta't conditions in the various areas. The inversion 4,6.7,, typ ical for 'S. sancti paulil is more fre- quent in'rain-forest'types of breeding sites and the inversion 4.6.('S. soubrense') seems to be better adapted to'savanna,types of sites. This is similar to the distribution of these cytotypes in the Ivory Coast (QUILLEVERE 1979), although the differences in the vectorial effi- ciency of the two types in the Ivory coast vrere not observed for the Beffa populatjon. More should be known about the occurrence of djfferent types within this popu'lation in Ghana, Togo and Benin, in particular as concerns the eastern areas of the occurrence of S. soubrense and S. sancti- ffi: j H-i $'-i *.-, ' ili I' *{ t I , 1 (i ltl i l pauli sensu Vajime and Dunbar (1975) and the western areas of the Beffa form. I) 1 I I) -49- Di f ferent S. damnosum s . 'l . - 0. vol vul u complexes in Togo and Benin ?s \ tl f. 1ii 1"t The different manifestation of onchocerciasis in rain-forest and savanna might be explained by the existence of different parasite- vector complexes (DUKE et al . lg7T, 1975, OMS 1gg3), the one in the savanna being highly pathogenic to the eye and transmitted by S. damno- sum s.s./S. sirbanum , the other in the rain-forest being 'less dangerous to the eye and transmitted by S. soubrense/S. sancti pauli or S. yahense in the Ivory coast (0MS 1983) and by s. squamosum in cameroon (DUKE tglz). Unfortunately, there are not enough ophthalmological r pdFdsito'logica'l and c]inical data available from Togo and Benin, that rea11y could sustain the co-existence of two different manifestations of the disease there. In the northern savanna areas, where the transmission is (or by now, was) maintained by S. damnosum/S. sirbanug, onchocerciasis is as danger- ous to the eye as it is in other savanna areas of west Africa (BA, pers. comm.), but on the other side, there exists hardly any site in the two countries, where S. damnosum s.s./S. sirbanum is not, at.least partially, involved in the transmission. Our data, stemming from the rainy-season do not permit to estimate the relative proportion of transmission, attri- butable to the various vectors, but this could be attempted by combining the results from different sources, i.e. OCP-data, the reports of GARMS et al. (1979-83), CHEKE (1983), BARBIERo (1983) and the results from the cytotaxonomic identification of larvae samp'les from nearby breeding sites. Probably, the most typical 'rain-forest' situation is found at Djodji. Most of the transmissionr p€F€nrial at this site, is maintained by S. soubrense/S. sancti pau'li and on'ly few infective S. squamosum lvere found, but nevertheless, S. damnosum s.s. probably represented l/4 of the population and of transmission (?), as it is indicated by the composition of the larvai popu'lation jn the Gban Hou (Table 4 ). The ATP at this site was higher than 10,ooo before the break-down of the f1y-population in 1981 and from the results of the study on the dis- persa'l of the flies, it can be estimated, that the human population at the village Diodii is exposed to almost 50 % of the biting rate measured at the river. According to all experience, such a situation would result in an intolerable manifestation of severe eye-lesions and blindness, if encountered in the savanna. However, the medical examination of 411 people from thjs villages showed 'on'ly' a prevalence of 7l,z %, an average 1 I 1 .,1 1 t i 1 l 1 'I t 1 1 I i , i I t dlH'Frt ffi Er. H,, E. ffip. l HI Eri is h. t.['i.: fir H. F,i; i !, H Ft t,, r, tdr h, It+ [' ir tl i3 I! F* [. E; Fiiit ; rli leit FSt H'r tI'!llt' li, lr :i; , Ij ftil :ur :+i rd' il ).f HJ ,d: -&, i: ffi F tft f, il.'+-, #,t, T'{ q -51 - The vectorial efficiency of different vectors A def in'ition of the term 'vectorial ca ci of S. damnosum s.1., will be presented in a seperate paper (RENZ, in prep.), together with a detailed discussion of the parameters, based on the data from the present investigations. In the following, I shall therefore on'ly compare the proportion of infected and infective flies of different groups, with special regard to the group S. soubrense/S. sancti paul i . In the Ivory coast, s. sanctipaul!- has a reduced ability to transmit onchocenciasis because of its 'low parous rate (GARMS 19g3, QUILLEVERE 1979) and its high degree of zoophily (TRA0RE et at. 19Bz). 0n1y ?.8 % of all parous flies identified, according to their morphology, as S. soubrense/S. sancti paul i , carried infections of 0. volvulus in the savanna areas of northern Ivory Cost and its vectorial role in this region was thought to be practically zero (TRA0RE et al. 1992). The data from Togo are different in so far, as the anthropophily of the Beffa-form is high, as seen by the high infection rates (23.5 % of al1 parous) and by the propensity of these vectors to feed on man (BARBIERO 1983). The parous rate was more than 30 % at all sites. However, no infective fly of this group was identified amongst the re- invasion-flies and this could indicate a reduced life-expectancy in the northern savanna areas. Another interesting fact is the low infection rate of s. soubrense/S. sanctipau'li at B6t6rou (z.g %), but this should be confirmed by more data. 0n an average, 1oo0 biting flies of this group carried 123 infective larvae, which is a value a bit lower than the one given by QUILLEVERE (1919) for S. soubrense in the ,zone de contact forEt-savane' in the Ivory Coast (204 'larvae). This djfference is mainly due to the different number of infective'larvae per infective fly (see be'low). No obvious difference could be detected for the susceptibility to infections by 0. volvulus for S. soubrense /S. sancti pauli from different sites in Togo and Benin. The experimenta'l-transmission data c'learly show, that this vector allows the development of'savanna'and 'forest,micro- filarjae without difference (RENZ 1983, BARBIERO 1983), and the variations in the proportion of wild-caught infected parous flies, observed at Diodii (41.7 /" of parous infected), T6t6tou (23,9%) and Kaboua (17.2%), a-53- 5. ACKNOHLEDGEMENTS I wish to express my thanks to Dr E.M. Samba, Director of the Onchocerciasis control programme, for the invitation to carry out these studies in Togo and Benin. Drs J. Grunewald, B. philippon, D. Kurtak, S.E.0 Meredith piovided invaluable assistance through- out the study. I am specially indepted to the sector and Subsector chiefs, Messrs A. som6 c. Adjounou, A. Gon6, A. Koumouvi, p. Gbaguidi, D.Otledraogo and Dr. A. Akpoboua and their staff for their continuous cooperation and support. The skilled technica'l assistance of Messrs Gomina Elimdabalo and G. Mbila was highly appreciated. I am grateful to Mr. G. Fiasorgbar for the cytotaxonomical identification of the Simul ium larvae. :i l* E:: *r.lr, , ir' i,t sti:" #.il, 1l IE e!. t^ ffi H E, .l ffi ffi ti' fi. I F It'II : )i I I I , I $.r I['' h. "a!*. ,,.1- .., 1 -55- GARMS, R., R.A. CHEKE, M. KERNER: Investigations in 1980 on the reinvasion by Simulium damnosum s.l. into the Eastern areas of the 0nchocerciasis Control Prograrme, including experimen- tal treatments of potential sources.(1980) 77 pp (unpublished document) GARMS, R., R.A. CHEKE, M. KERNER: Studies in 1981 of the rein- vasion by Simu'lium damnosum s.l. into the Eastern areas of the Onchocerciasis Control Progranrne and the resu'lts of experimental treatments of potential sources. (1981) 22 pp (unpublished docu- ment) + supplement GARMS, R., R.A. CHEKE, C.G. VAJIME, S. SOl^lAH: The Occurrence and Movements of Different Members of the Simulium damnosum Compl ex In Togo and Benin. Z. Angew. Zool. 69 (2), (1982) 219-236 GARRETT-J0NES, C.: The human blood 'index of malaria vectors 'in relation to epidemiological assessment. Bulletin WHO 30 (1964) 241-?61 GUILLET, P., H. ESCAFFRE, M. 0UEDRAOG0, D. QUILLEVERE: Note preliminaire sur une resistance au temephos dans le comp'lexe Simulium damnosum (S. sanctipau'li et S. soubrense ) en C0te d'Ivoire (Zone du Progranme de lutte contre lr0nchocercose dans I a Region du Basin de la Vo1ta ) . l^lH0/VBC/&l.784 (1980) 9 pp (unpublished mimeographed document) MEREDITH, S.E.0., R.A. CHEKE, R. GARMS: Variation and distri- bution of forms of Simulium soubrense and S. sanc tipaul i in l.lest Africa. Ann. Trop. l'led. Parasi to1. 77 (1983) 62l-640 M0NDET, 8., G.0.Jr. POINAR, J. BERNAD0U: Etude du parasitjsme des simulies (Nematoda) en Afrique de l'Ouest. IV. Decription de Jso- mermi s lai rdi n. s p. 'Pd rasite de Simu'lium damnosum. Can. J. Zool .55, (L977) 20tt-2017 NELS0N, G.: Staining of filarial larvae in insects before dessection. Bu1l. l^lld Hlth 0rg. f9, (1958) , 204 0MAR, M.S.: Report on studjes on the histochemica'l differentiation of filarial larvae in Simulium damnosum s.l. and on the vectorial capacity of the main species of the complex in Togo and Ben'in. Report, Consultant OCP (1982)(unpublished document) J l i I l I 1 I I 1 I I l i f 1 ir". #4 B Ett -57- 7. CONCLUSIONS Reinvasion into the south-eastern flanc of the Ocp occurred, like during the preceding rainy seasons, at the same sites and by the same vector groups, but at a much reduced biting rate, along the river. Kara(at Landa Pozanda) in Togo and along the rivers l'16krou (at K6r6mou), Alibori (at Alibori) and Bouri (at Zougou) in Benin. In contrast to these sites, high numbers of biting f]ies, exceeding those measu_ red during the past two years, were observed at Gbass6 (r. Sota, Benin) and at MO ii M0 (r. M0, Togo), partially being of local origin. The pattern of the dynamics of the biting rates at the reinvasion r sites and the species-composjtjon of the reinvading popu'latjons indicate at least three different sourccs of reinvading flies: Those caught at M0 A MO were mainly S. squamosum, during the period of this consultant- ship, and came presumably from untreated rivers in the south. At. Landa Pozanda, the different pattern of the biting rate and the higher pro- port'ion of S. damnosum s . s. /5, sirbanum indicates source areas in the upper Mono and its tributaries or in the Benin, although some local breeding might be responsible for the biting rates in May and October. AII flies identified from reinvasion areas in Benin were S. damnosum s.s. S. sirbanum and probably came in August from the upper 0u6m-e basin and from yet unidentified breeding sites (river 0lim of from Nigeria ?), whereas those observec at Gbass6 during June and Ju'ly presumably came from untreated breeding sites jn the river Sota. S. soubrense S. sancti ul i dis appeared completely from its former breeding sites in the lower l,lono, after these sites fall dry during April i983, and it was replaced at these sites by an almost pure popu- lation of !.-dqmno-sum s,,s., 0t a much rower density of population, how- ever' Coincidentally, the population was heavily parasitized by mermi- thids, both in the larva'l and adult stage. At Djodji, the popu.lation of S. soubrense S. sancti ul i recovered and came back, almost two years after the experimental treatment of this area, to its original level (> s00 gg/man/day;. The vectorial efficiency was highest for S. soubrense/S. sancti paul i(6 tZS infecti ve larvae per looo biting flies), followed by s. damnosum s.s./S. sirbanum (l SA larvae), but uras lowest for S. s uamosum (6 zg 'larvae), proba b1y as a result of the 1ow receptivity and the high pro_ t, -59- (f1ies) could st'i'll be examined. Actually, those flies on'ly classed nul'liparous or parous during the past rainy season, are identified and di ssected at Ti.ibingen. The occurrence and the species-composition of fly-populations should be studied durjng the dry seasorr €Sp€cially during the months of March and Apri'1, and the few remaining breeding sites should be mapped in view of a future larviciding campaign during this period of the year. There is fonded hope, that the populations of S. soubr:ense/ !. sa4ctipau'li coul d be substantia'll y reduced with a long-lasting effect by only few treatments and there is enough evjdence, that a possible re-popuiation of the treated sites by s. squamqsum or s. dam- nosum s. s./S. s'irbanum would probably result in a reduced rate of disease transmission. However, the needs of the insecticide-screening team at Lom6 should be duly consjdered. The different receptivity of S. squamosum to infections by 0. vol- vulus could be investigated in a series of cross-transmiss'ion experi- ments, by feeding flies from M0 ii M0 and southern and northern areas of Ivory Coast (and possibly from the Kumba area in Cameroon) on human infections from the same and different areas. Such experiments would indicate, whether there exist different strains of S. squamosum ( 'in this case: S. squqmosum s.l.) or of 0. volvulus (: s.l. ?) - or even different strains of both parasite and vector. The trend of decreasing biting rates, observed at most of the catch'ing sites in Togo and Ben'in during the last years, should be compared.to more data from other f1y-populations in similar zones of Ghana and Ivory Coast. The variation of the ABR at a given site could be compared to other parameters'like the river's water level and dis- charge, the species composition of the fly-population and the density of human and anjmal populations around this site. The differences in the morphology and in the caryotype of various S. soubrense S. sancti uli ('Beffa') populations merit further atten- tion, in particular in the western zones of the distribution of this form, where it approaches the areas of distribution of s. soubrense and S. sq4ctipauli sensu Vajime & Dunbar. The influence of nrermithids on the size of the f'ly-populat'ions and the reasons for the low proportion of parasitized S. soubrense/S. sancti- I $ s It I i I gj':r'l I I l i I 1 pauli flies should be investjgated further.