Resistance and insecticides

WORLD HEALTH ORGANIZATION ORGANISATION MONDIALE DE I.A SANTE ONCHOCERCIASIS CONTROL PROGRAMIqE IN I,.JEST AFRICA PROGRAMIUE DE LUTTE CONTRE L'ONCHOCERCOSE EN AFRIQUE DE L'OUEST EXPERT ADVISORY COMMITTEE Ninth session Ouagadougou, 13-17 June 1988 ocP/EAC9.5 ENGLISH ONLY RESISTANCE AND ]NSECTICIDES RESISTANCE SITUATTON mid-1!87 to April, 1988. 1. Introduction During this period the resistance situation was very dynamic, with verysignificant spread into new areas and involvement of other species. At the seme time,the populations in some rivers reverted to apparently normal susceptibility. 7.2. Eastern Area rn September and October, 7987, a series of temephos treatments hrere undertakenin the Southern Extension Area of Togo and Benin in order to stop reinvasion ofPhase III West of the existing area. Especially in the 0uem6 and Okpara Rivers ofBeriin these treatments tvere not sufficiently effective and had to be abandoned beforethe fly numbers were reduced to low Levels. Susceptibility tests just before thetreatments showed a very high susceptibility to temephos. During and after thetreatments, susceptibility was much reduced, although it was stiff not at theresistance IeveI, when compared with resul-ts from other areas. ft was later foundthat temephos solutions had been overheated, leading to fal-sel-y high susceptibilityfigures' Despite this confusion, it is necessary to consider the Southern Extensionas a zone where S. damnosum s. str., the najor vector species now present, isresistant to temeprros. trrere is no evidence of resistance to other compounds,including chlorphoxim. Nor is there any evidence that the "beffa,, form ofS' soubrense or the "Djodji" form of S. sanctipauli is involved. The 1!gB rainyseason strategy relies on chlorphoxim. tt seems urrlikely that this resistanceoccurred by direct sel-ection during only J weekly cycles. Some nearby rivers inNigeria may have been treated with t"r"pho". El-sewhere in the Eastern Operational area, resistance reverted on the Oti Riveron the Togo-Ghana frontier and did not spread farther toward the North and East. TheSota and Upper Pendjari Rivers remained areas of wet-season tenephos use. However,the originar resistant S. damnosum s.str. population on the pru River in Ghanaremained resistant to temEptos in spite of two years of treatment with B.t. H-14. Aspecial effort is made to prevent this populatiln from colonizin; ;;h;.=ffiers uymigration. There seems to be no genetic interchange with nearby S. squamosumpopulations. 1. l. Western area 1 1 In the lrlestern Operational Area, in the Extension and Existing Areas, theseasonal use of temephos was more successfur, although it was necessary to increasethe dose somewhat. Temephos h,as fol-1owed by permethrin (Extension Area) or B.t.(Existing Area) . By the end of L987, """".p.iuiii.v ro remepho" ;";-;;lrli ffi.."normal in the Baou16 and Bago6 Rivers, parts of the upper Niger basin and in theNiger itserf at Tienfala (Banako). This was confirmed by a trial temephos treatmentat Tienfala. rn early 1988, the reversion continued, with the upper Niger Basinreturning to the pre-treatnent (1985) revels of susceptibility. ocP/EACg.5 page 2 Unfortunately, tests in the Bafing and Senegal River basins to prepare for firsttreatments showed resistance to temephos practically on the entire length of theBafing and downstrean in the Senegal. S. sirbanum and S. dieguerense (i sarranna sp.Ioca11y important in Western Guinea ana ptafi) are involved. This seems to havedeveloped IocaIIy due to treatments in the area of Manantali Dam. A "clean-out,roperation with B.t. H-14 and chlorphoxim has begun. Permethrin witl be used at higherdischarges' or a brief return to temephos may be attempted. It is remarkable thattests in late 1985 did not show this resistance, although certain aspects of the data were slightly aberrant. Finally the "bastions" of resistance in the south held firm. The populations ofthe Lower Como6 and Bandama Rivers remain resistant to temephos and (for forestspecies) chlorphoxim. 1.4. Summary The situation is summarized by the map (Fig.l). It shows a smarl- number of areas where resistance to temephos and/or chlorphoxin is constant in spite of lengthy suspensions of organophosphate use. Then there is the najority of the p"og"arnm! a"ea(including the Extensions) where resistance to temephos has occurred and wi1l occuragain if that product is used, but where temephos can be used for brief periods whenit is economicalty advantageous to do so. This strateg'y requires close monitoring ofsusceptibility for success. There remains a limited area where no resistance has beenseen' and some areas where data wilr be gathered during the next year. A11 the species of the complex excepting S. yahense have now been involved inresistance, although only the S. soubrense/san-EEiptli complex has shown resistanceto chlorphoxim. No evidence of resistance to pyrethroids, carbamates, or B.t. H-14 has beenfound Resistance did not influence epidemiologi-cal resurts in the Existing area. rn theextensions, it reduced the efficacy_of some 1987 anti-reinvasion treatments, andforced changing the strategy for 1988. The financial impact of resistance has been lessened by price reductions in thealternative compounds. rncreases in the price of temephos itseLf have also reducedthe cost differential between temephos and other treatments. The cost of meeting theresistance challenge is not prohibitive. However, the technicat complexity of theproblem remains. The capacity for rapid, precise determination of susceptibility mustbe maintained. 2. INSECTICIDES 2.1. Strategy of insec ticide use 19BB The generil strategy proposed to EAc.8 was sucessfutly used in L9BT. For therainy se.so.r1 of 1988,-irrl "ur" g".r.rar outline wir-r be used: a) where no resistance to temephos has occurred, temephos wiII be used, with areserve of a replacement compound b) where tenephos resistance has occurred, but has reverted, temephos wi1l beused at high river fIow, followed by an alternative to stop the build-up oftemephos resistance B. r.t-or the dry season, H-14 is used universally. ocP /EAcg .5 page 3 c) where temephos resistance remains at a high level, chlorphoxim/permethrinalternation will be used by preference. If chlorphoxim rlsistance exists, orif agricul-tural use of pyrethroids is heavy in a given area, carbosulfan willbe used. 2.2. Larvicide Screenins Activities nid-1987 to . 1988 - Classica1 chemical larvicides A' Selection of a possible pyrethroid replacenent of permethrin in case ofresistance. Trough and river testing (with hydrobiological monitoring) were completed for thethree candidates (biphenthrin, cyphenothrin, urra la-mbda-cyhalothrin). The EcologicalGroup approved operational trials of all three. Enquiries are out to the threemanufacturers concerning cost and availability of sufficient quantities for triars. B. Selection of a replacement for phoxim The final delivery of chlorphoxim will be made by Bayer in late 1988. Bayerproposed phoxin as a replacement. Unfortunately, the formulations tested to date areconsiderabry less active against the target splcies than chlorphoxim. Their use isnot economicalry feasibre. Arso, they could probably be more ttxic to the non-targetfauna than chlorphoxim at the necessary dose. C. New compounds Pyraclofos (OMS 3040) is a new organophosphate under deveropment by the Takedacompany in Japan. rt has similar activity to iemephos, but does not show cross-resistance with temephos-resistant rarvae. The effect on the non-target fauna issimilar to that of chrorphoxim. oMS 3o4o is thus a prospective replacement forchlorphoxim in L99o. An operational trial has been approved, but cannot be carriedout in 1988 because the manuflacturer is still completing production facilities. D. New formulations of operational compounds A considerable number of formulations were tested. A 25lz EC of permethrin fromFMC corporation was found to be adequate, allowing FMC to iia on the 1!gB permethrinorder' 0n the other hand, a novel, water-based, permethrin formuLation from Wel1comewas found to be much less effective than the standard sorvent-based EC formul-ation.concentrated permethrin formulations were also tested and shown to be effective, butwere not j.ntroduced for operational reasons. - B.r. H-14 The manufacturers were quite active during the year, attempting to produce moreactive products. Both sandoz and Abbott produred industrial lois of ,,4x,, productsdesigned to be effecti-ve at one half rhe dose (0.6;;li--10-ri;i-of rhe currenroperational products (Sandoz Teknar HPD and Abbott Vectobac 12 AS used at 1.2 ng/L-70min) ' Lab-sca1e samples of these products had even shown good resurts at one-thirdthe normar dose -(o'4 mg/-10 min). However, in the large-scale operational trial-, theminimum dose of both products was found to be o.B mgTi-1o-;i;.-it-"o, a severe problemof settling in storage was discovered. As a resurt, neither product was introducedinto operationar use. since the beginning of r9BB, the sane two firms have beenworking onrr3xrr products designed to be errective at 0.8 mg/1-10 min and not settLein storage. River testing of ihese sanples will begin shortry. oCP/EAC9 .5 page 4 Samples were also provided by Solvay, FMC, Roussel-Uclaf and Becker Microbial Products. None of them was competitive with the operational products. Opening of the bids for the first 1tB8 order of B.t. H-14 confirmed that theprice of the "2X" product wil_I renai_n about $ 3 per litre. - IGR's The industry continues to develop new compounds in this group. Unfortunately, no compound effective at a feasible dose has yet been found. This is particularly true of the chitin-inhibiter compounds. Anong the juvenoid-type compounds, OMS 30L9 andfenoxycarb are still undergoing testing. OCP sti1I hopes to obtain an operational IGR. 2.3 Reoreani sation of screeninE tivities Due to a reduction in the number of new compounds available and the necessity to regroup resources for the extensions, there will be a reduction in staff during 1!88 A11 activities will be maintained, but some (screening of classical chemical compounds, some B.t. H-14 fornulations, and IGR's) will be done on contract.OCp staff responsible for others (screening of B.t. 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