Reduction in the susceptibility to Temephos of larval populations of the simulium damnosum complex in the Ivory coast

-/---. WORLD HEALTH ORGANIZATION AFRICAN REGION ORGANISATION MONDIALE DE LA SANTE REGION DE L'AFRIQUE \ \ !! ONCHOCERCIASIS CONTROL PROGRAMME IN THE VOLTA RTVER BASIN AREA PROGRAMME DE LUTTE CONTRE L'ONCHOCERCOSE DANS LA REGION DU BASSIN DE LA VOLTA EXPERT AD\[SORY COMMITTEE ocP/EAc1.1 First meetlng ORIGINAL: FRENCH Geneva. 9-12 September 1980 REDUCTION IN THE SUSCEPTIBILITY TO TEMEPHOS OF I.ARVAL POPULATIONS OF TIIE SIMULIT,I"I DA}INoSI,M coMPLEx IN l]TE IVORY COAST Ttris phenomenon has appeared on the 6O-km section of the Bandama River sltuat,ed beEween the Taabo dam and the town of Tiassald (lower prefunaginal vector site on the rlver). Here the Bandama ls several hundred metres wide;its rate of flow, partly regularized slnce therc,ompletlon of the Taabo dam in July 1979, now varies fron 12o to 3oo m'/second. The river forms several extensive complexes of rapids where thc waters divide into a labyrlnth of channels and secondary arms whlch are difficult to reach and havefluctuating currents and flows; the largesE is the Gauthier Fa1ls complex dor^rnstream from the confluence of the Bandama with its tribut,ary the Nzl . The stretch between /houati and Tiassal6 was successfully treated wlth DDT in the 196os (Le Berre et a1.) to protect the workers in rlversldeplantations where the densities of bltirng blackfly exceeded SOoofura,6.f d,ay.This was essentially a nuisance problem because, although the prevalence of onchocerca may be fairly high, the disease is of the forest type wigh an almost complete absence of severe eye lesions and blindness. The species concerned were S.sanctlpauli (65%), S.soubrense (+ 3O7.) and S.damnosum s.1.Ihe proportion of S.sanctipauli felI gradually towards the north in favourof S.soubrense and farther on S.damnosum s.1. Treatment wlth E,emephos inslx consecutive weekly cycles was trled out by the Prograume in 1976 durlng a reinvasion study, withrspectacular results: Ehe initial biting populagions of over 15Oo femaLes/aadday were reduced to zero; in the absence of anyfurther treatmenE Ehere was then very slow and delayed repopulation over several months, indicaEing quiEe considerable isolatlon of the blackflypopulations. F t 2 I Ttre wtrole of the lower Bandama (downstream from Lake Kossou) was lncluded in the Prograurne's regular temephos treatment.s as from March 1979. Ttre results were verry satisfactory, because in 1979 the monthly averages fell to 1 bite/maty'a^V at, Taabo and O.7 aE the Gauthier Falls. , ., However, from early 19BO (March in particular), the densities of biting females gradually rose, and by mid-April reached almost ZooOfrlratf day at the Gauthler Falls, over 4OO at Taabo and almost 15O at, Tlassa16. At the same time, every week slnce January positive Iarval sltes and hlgh levels of nulliparous females lrere found at one point or anoEher on thls stretch, indlcating that blting blackflies are produced Ioca1ly. The regutar weekly treatments with temephos were suspended from the beginning of May and several posslble causes of failure of the treatments were tested in turn and ellninated: lack of precislon in the larvicide applicatlong (conducted by helicopter instead of fixed-wing aircraft since rrld-March)r- under-dosage resulting from lncorrect adjustment of the \rater leve1 gauges following the constructlon of the Taabo dam, modlficatlon of the physico-chemical envlronment below this darn because of the creatlon of a reservoir, deterioration ln the temephos batch in use at the time, modlfication of 'the formulation provided by Procida (tn June this was provisionally replaced, without any greater suceess, b5l a forrmrlaElorr f,rom American Cyanamid). Finally, in late May and early June, it, was shown (Guitlet et a1.) that susceptibllity to temephos of S.damnosum s.1. on the lower Bandarna had dropped by amounts ranging from 2.5 fold Eo 38.7 fold since L977 (ratio of LDoo o values after and before treatment), so Ehat less than half theLaiii6 in the sites are eliminated with dosages 4 to 5 time's hlgher than the operational temephos concentration (O.O5 ppdlO theoretlcal minutes). The surviving larvae are S.sanctipauli (* 8O7.) and s soubrense G n7") in proportions different from those which applied before the treaEments. It was then declded- to replace temephos wlth chlorphoxirn, the only tested substitution product (operational trials on the River Nzi in 1978). As from 10 June, a series of six successive weekly treaEments hras applied aE the Gauthier Falls with a dosage of 0.o25 ppny'10 minutes. Logicarly, the ent,ire stretch from Taabo to Tiassal6 should have been treaE,ed with chlorphoxim, but some of Ehe Programme's stocks had become damaged with time and since Ehe manufacEurer (Bayer) provides this product only on request lt was possille to treat only the Gauthier Falls, v*rere catches had exceeded 2ooo fematesfrr,adaay during Ehe interruption in May and June. At Taabo lAerial treatment of large labyrinths of rapids is a highry delicate operaEion because of the large number of changeable currents: since March -despite sysEemaEic entomological training of Ehe pilots, weekly adjustment of Ehe insecticide dosages and the creaEion of many secondary EreaEment poinEs - it. is not uncommon to find that some arms or channels are only partially affected by the larvicide, even when using chlorphoxlm. Larval surveillance ln such labyrinths is hazardous and difficult, moreover, and even by reverting to t,reaEmenE and conErols by river it seems difficult to guaranE,ee full cont.ror of complexes of breeding places such as the Gauthier Falls. J { -.i 3 and Tiassal6, catches had not exceeded 2oo and 150 femares/mrry'a^yrespectlvery durlng the suspenslon of the treatments, and the treatmentshad remained consistently effectlve upstream from Taabo Lake and on theRlver Nzi' once again the results were spectacular, because the figuresfell frorn 23fi btte"/.^ty'ary on 14 June to 99 on 7 July and to 1 on21 July, wtrile the females disappeared from Tiassal6, from mid-June onwards. Ilowever, the situation in Taabo, r.*rere temephos was sti1l being used,gradually worsened and lL was decided, despite the deflnite risks Lnvolved,to revert to temephos at the Gauthier Falrs after six weekly cycles ofchlorphoxim and at the same tfune to change over to chlorphoxim on the Taabosection. This operatlon was carried out at the end of .lury. since thattime, despite a slight recrudescence, the populations of biting femaleshave remained at a fairly low level at the Gauthier Falls, whereas atTaabo they are proving difficult to contain because of logisEic difficultiesln spraying. To sum up, resistance to temephos has appeared unforeseeably afterabout one year of regular treatments. Presumably tts appearance is relatedto the comparative isolation of the blackfly populations and to theexistence locaIIy of a forest speci.es @ipauri) rrittr utrlch the prograunnewas not confronted before its exten"ion i, 1979. rtre phenomenon is apparently limited to the lower Bandama and doesnot seem to have spread upstream from Lake Taabo and the Bandama_Nziconfluence. Both on the Bandama and on the Marahou6 a plan for close surveillanceof the biting populations, for control of the breeding places by river,for identification of species and for control of susceptibility to thecustomary insectlcides has now been put into operation As soon as the Progranme's stock of chlorphoxim has been reprenished,the whole of the lower Bandama will be rreated ,i.i' at i" i"."t"ia". Thisoperation will be accompanied by surveillance of the aquatic fauna. )r.