Research to vector ecology

WORLD HEALTH ORGANIZATION AFRICAN REGION E)GERT ADVISORY COMMITTEE Third oeeting Geneva 13-17 S cember 1982 ORGANISATION MONDIALE DE LA SANTE REGION DE L'AFRIQUE oCP/EAC3.6 ORIGINAL: ENGLISH ONCHOCERCIASIS CONTROL PROGRAT{ME IN THE VOLTA RTVER BASIN AREA PROCRAMME DE LUITECONTRE L'ONCHOCERCOSE DANS LA REGION DU BASSIN DE LA VOLTA T6l.: 333.12 - 329.10. 338.57 - T6ldx ONCHO 5241 UV RESE/TRCH 0t{ VECT0R ECOLOGY 1. FliAht perfonnance of S. dannosum complex s'ecies: fne aim of this study was to obtain basic data relatin6l to the flight capacity of .L--gAi@, S. soubrerse/S. sanctiDauli ed L-gi@,, uith a view ro assessing their relative migration potential. It has been shown that aLl tnree cytospecies str.rdied are capable of rnakin3 extensive single flights t+hen tested on a flight ni11; that flights r.re::e possible over a nunber of ,1 ays of adult life; that blood fed, wild carrght fenales clruld fIy for long peri-ods arrcl that sugar feeding was both essential for long fllghts and co'rld facilitate further flight: on the same day if rnade available after the first fli'-tht. Field studies have shown that S. sirbanum and to certain extent S. squamosum and S. soubrense/S. sanctioauli are capable of making rnigratory fliShts. '.'/ind-assisted niga.tion is the most like1y dispersal mechanism of those flies assuming that flights in excess of about 50 Isn takes a relatively snort time (one or two days). If migration takes longer tir:re, and there is evj.dence to suggest thls in that inrnigrant flies are old, parous in,lividuals often carrxrin6 ma-trrre Onchocerca larva€r then although wind-assisted flight is sti1l probable, refuelling would be necessary after each 1e6 of the rnigration. In this way dishances of hundreds of kilometres ccul<I be covered if repeated. wind-assisted displacements are interposed between bouts of rgar feeding ard perhaps even blood feeding and reprodtr.ctive activity. The neas'rred perforrnance of the fenales was similar to that of the other two speci.es. This is interesting as only small numbers of botir forest cytospecles are frund r,rithin the OCP savanna areas. This may lndica.te that these insects migate in srnaller numbers or that the nea.';red flight perforrnance does not necessarily reflect behavioural willingness or physiological abilit, to m:ke long fli:hts ueder na.tura.l conditions. 2Newly emerged unfed S. sirbanr:m females performed mrch better than equivalent sar,rples of the other two species and this may be a real difference associated with adaptation to savanna conditions. Differences between cytospecies, with respect to t re longest flight durations recorded, indicate that there is scope for testing whether these d.ifferences relate to differences in behavior:r, fuel stora6e and utilization and.for to environmental effects. Studies are undetnray to investigate storage and utillzation of fuel for flight and the effect of environmental factors (tanperatrre, humidity) as well as the effect of parasite burden on flight performance. 2. S J &perimental treatrnents of potential source areas have so far been the only successful means of locating the ori3ine of reinvading biting fenales of the S. darnnosu.n complex. The pattern of reinva:ion in the To6o area of the OCP provided an excellent cpportrrnitl,' for euch a study of experimental treatrnents. [he Togo situa.tion is particularly interestin8 as the rnajoritSr of the reinvading flles are S. squamosrm, a species wirieh can be identlfied morphoIo.3ically with some reliability. ft ha.s been fc,.mcl that S. s-.u.arnosum breeding sites:re ccnfined to mor:ntainous regions and to rivers close to the hills to the south of the OcP boundarlr. As b.3.g@ was (froo experience in the west of the OCP area) considered^ to be only a short d.istance migrant, the aim of the experimental treatments was to eheck wilettter the reinvasion could be controlled by a limited exten:ion of the trea:ed'area. rn 1!go the experimental area was designed t4'ing to protect the i'16 valIey by treating S. squa.rnosurn breed ing sites r,rithin a range of a-bout 100 kn of iil6 brid.ge. However, although prospections slrowed that the treatments had been fully effective in the experimental area, they lid not reduce the number of flies to an a.cceptable Iow level. It had to be consi.iered. that flies came from fariher sources such as the Atakpamd region and the ccntinuous breel:ng sites of S. seuamos,.:m in the Astikawkaw system. ,The experimental treatrnents of 1981 were therefore planned to cover aII bree<li.ag sites of S. s'luanosrln eorrth and sorrth-west of the reinvaded areas of Togo in the proposed extension areas of Ghana west of the lalce and in southerrr Togo. The results of the experimental treatments trere satisfactory in the Souti:-wes tetn area where no treatment failtrfes ',.rere obse::rred. Problems arose uith smaller tributaries and source streams of these rivers in the nogntainous areas of To5io. Dre to the difflcult terrain, with deep steep-slded va11eys, often covered by high forest, only a few check points were accessible, even by helioopter. Resrrlts of experimental treatnents of the l"ono River were never satisfaetorl'. This is ass'.rmed to be ,4ue to wrong dosage because treat'nents of the tributa.ries worked well. Lesri.lts on the effect of experimental treatment on biting poErlations were satisfacto::y in tl:e areas vest of the mountains, particularly in Ghana. At some places biting densitles vrere suppressed to extresreLy low levels (o5oa3i: 4lI flies/man/aay in 1!80 to 1? ttLes/nanfday in 1981). As expected, the densrties along the lower Itlono basin could. not be redueed. The results were better on the middle course of the llono where only ferv flies were caught eompared with the two previous years. This part of the river started to flow very late and no treatnent failrlres l,rere obse:rred. i/ithin the OCP area at 1116 River (i,i8 triage) ttre biting density started to increase in IIe.y as drripg the paet two 1'qars. 4.t the time of experimental treatments the daily catcnes decreased to low levels. Hol.,ever, the results were not completely satisfzctcrT., particularly during the second half of July when daily catches uere in excess of 100 flies/day. At Kara River (Landa-Pozanaa) tne bi+,ing densities were s:-lcceesfu111' kept at a ver? 1o,l level throughout the experimental treatnents. llhe species composition of bj.ting population at l'16 River (tt6 Uriages) and Kara River (Lanaa-Pozanda) was similar to that of 19?9 anrt 1!80. The vast majorit.... of females eaught at t'16 bridge l{as !.r.-.S@. Some S. soubrense/S. sanctiparrli were obtained in tugtrst. irgain more than he.lf the flies caught at Landa-Poz.r-da were@. T t 4It can be concluded that there was no substiurtial reinvasion of the 116 and Kara valle]'s during the tirne of experimental treatment periode in 1981 and that the results may have been influenced by favourable natr.ral conditions, particularly at Landa-Pozanda. As no larvicidal treatment activities will take place in the southern extension zone in 1t82, studies are being carried out on the natr:ra1 dynanics of the migratlng fly population for comparison with the results obtained ln 1979. Three additional catching points have been established in the mountain area along the Togo-Ghana bord.er to st'rdy the biting d.ensities and the trans- mission potential of, fly poprlations in the centre of the S. sonamosum a.rea, particularly the source areaa of the Asuakawkaw and. Kon6. 1. Vectorial capacitv of S. soubrense/S. sanctlpauli t/ith the appeErrErnce of resistance to ternephos and chlorphoxim of the species pair in fvory Coast the qr.1g51ion aroae whether the females of those flies normally occuring in the forest and pre-forest zone would be potential vectors of 9t-gg}33i}g. in eavanna areas particularly in p:rotected aavanna aneas of the OCP which they lnvade during the rainy season. In 1981 preliminary studiee on the vectorial capacity of 9r.-gg[gg9/ S. sanctioauli were earied out in the pre-forest zone and the Guinea and Sud.al savanna in lvory Coast, Togo and Benin. However data obtained drrring these stud,ies are too few to draw final conclusions. Ilevertheless, there areindicationsthatmightbeon1ypoorvector8 of onchocerciasis in t.re sav€ulna areas in Ivory Coast and there are also indications that in the aava,nna zone they are less anti:ropophilic than in the pre-forest area. A a 5In Togo and Benin preliminaeT results showed that the infection rate of 9- soubrenselS. sarctirarrll ("Beffat' forur) seens to decrease fron thesouthtothenorth.I{owever,inmixed'poplr1ationof@ and the latter pair carrl' srgnifieantly higher numbers of infective la::\rae. I.Iored.etai1edstud.iesonthevectoria1capaci't3rof and their role in the transnission of onchocerciasis are being carried out in various bioclimatic zones in Ivory Corst, Togo and. Benin drrring the rainy and reinvasion seaaon of the current year. Beside investigations on the natu.ra.l lnfection rates and the infective lerval load of S. soubrense/S. sanctipauli their vectorial capacity is also being stud.ied experimentally. 1. RearinE and Co zation Studies Simmons and Edman (fggZ) established the first rmrltiple generation colony of S. da.rnnosum s.1. at the Unlversi ty of l4assachusetts Later investigations in the OCP Rearing Laboratory at .r-kosombo showed that the form colonized was ttBeffatr, the Eastern fo:m of S. soubrense/S. sanctioauli. this cytotSrpe therefore represents the first onchocerciasls vector to be identified as a rabo:ratorx' *.1trrg form as werr as the first member of the .). ccmplex to be colonized. Populations of the Beffa fozm from certain sites in Togo and Benin were found to have a high laboratory rmting rate. This suggested. that long-term colonization, at least of certain populations of this form, was possibre. However, simmons and Frlman (rgaz) re-oorted a shift in the male: female ratio of their colony from 1:1 in the FI to O.0J:1 in the F{ generation. Inbreeding iegression was suggested as a possible cause of these anomalies. Recoloniza.tion of the Beffa form wa.s therefore carried out to study this ootentially serious obstacle to colonization. a 6colonies of trBeffat' were therefore started fnon onry one or two females and then subjected to the marlmr-n possible lnbreed.ing. One colony started with a single fenale fron Benin sr:rrived. for five generations, one eg6 batch only being reared each generation. Another colony fron Togo started wlth tro egg batches produeed thirt generation aduLts. Although these two tiny coLonies could not be pemanentl'y naintained^, they did not d.evelop such d.efects as the upeet ser ratio obeenred. by Simons a.nd. E<lnan. The maintenance of a colony for five generations (eome F6 enbryos developed but failed to hatch) despite the paucity of adults obtained with such snall scale colonlsationr provea that adult hantlling techniques are nov a.d.equate. lltrisr together rith the absence of a shlft in the sex ratio in either colony in spite of extreme inbreedtng, suggests that long-term colonization of the Beffa forn is feasibl_e. An attempt to produce a nore permanent colony ln Africa by worklng on a much larger scale Is therefore in prog?ess in collabora- tion with tlasaachuEette University. A naJor etunbling block to succeEsful colonlzation is the fact that rnany laboratory raid eggs fail to d.evelop. Howeyer, two different ovlposition technlques have been compared and a recent nethod which stimulates natural conditions gave alnost twlce the fertillty rate (l}16) of an ee^rlier method (fiit). Because nares of the Beffa form mate ver11 actively in captlvity, suceessful cross-nating has been lndueed. betveen nBeffatr and two other memberE of the S. dasrrosum complex. Ore of these, S. seuamosum, ie not close1.y related to (Beffat'. !r-eg@ offsprings have been reared through to the acult stage. Investigations are in progress to cross Beffa nales fron Togo (which show no insecticide resistance) with resistant S. soubrense females from the fvory Coast, to learn nore about the lnheritance of resistance. Recent prog?e88 in rrnd.erstanding factors affectlng the rate of egg 6evelop- nent ehould facilitate the transportation of S" damrosum s.I_. eggs to collaborating: laboratories without the la::rrae hatching en noute. {