The WHO programme for the evaluation and testing of new insecticides

INTRODUCTORY PAPER Bull. Org. mond. Sante) 1971, 44, 11-22Bull. Wld Hlth Org.f The WHO Programme for the Evaluation and Testing of New Insecticides J. W. WRIGHT' For many years the World Health Organization has been engaged in a programme for the control of vector-borne diseases, and in 1960 established a programme for evaluating and testing new insecticides, the special objectives being to find new compounds that would overcome the problem of insecticide resistance and not lead to further contamina- tion of the environment. This paper reviews the present status of control of some of the principal vector-borne diseases of man and describes the WHO programme for insecticide evaluation and testing and the results it has achieved. The WHO programme for evaluating and testing new insecticides has developed against a background of unprecedented activity in the control of vector- borne diseases. The discovery of the synthetic residual insecticides more than three decades ago revolutionized the whole approach to vector control, and for the first time in the history of public health it became possible to contemplate the control or even the eradication of many of the major diseases of man. Since that time the public health use of pesticides has steadily increased, to such an extent that the control of all vectors of health importance now depends on the application of these chemicals. However, the development of resistance in arthro- pods has tended to temper the original optimism on disease control, and no simple solution to this phenomenon has yet been found. The problem has been further aggravated by changes in man's behaviour, particularly by his crowding into urban conglomerations, and in some parts of the world this has given rise to a marked increase in the incidence of vector-borne diseases, such as filariasis and dengue haemorrhagic fever. An increasingly great effort is now being made to develop genetic and biological control methods, but it appears that no procedure currently under study is likely to be operational in less than 10 years. An added difficulty is the obvious and urgent need to replace the persistent chemicals now routinely used to control the blackfly and the tsetse fly by others that are more susceptible to biological degradation. 1 Chief, Vector Biology and Control, World Health Organization, Geneva, Switzerland. Taking all points of view into consideration, it would appear that the only practicable and effective solution to vector control today is the systematic development, evaluation, and testing of new com- pounds, relating these to the ecology of the vector to be controlled and using techniques and equipment that will bring about the minimum contamination of the environment, and the greatest degree of safety to man and animal, compatible with effective control. In the light of these facts it may be of value to examine the present status of control of some of the major disease vectors. PRESENT STATUS OF VECTOR CONTROL Malaria Malaria eradication has been one of the most successful global public health programmes ever undertaken. Of the 1 800 million persons who lived in malarious areas before this programme was started, 1 000 million have now been freed from the threat of the disease. The infection has been almost eliminated in most of the temperate areas, and national malaria programmes are in operation in almost all Asian and Latin American countries. In Africa, where special administrative and biological problems exist, a more gradual approach is being made, the present goal being malaria control rather than eradication. These programmes have in the past been based almost completely upon the applica- tion of DDT to the interior walls of houses. At the height of the programme approximately 60 000 tons 2612 -1 - 2 J. W. WRIGHT of this insecticide were used annually. The amount used each year has now dropped to about 35 000 tons and will continue to decrease as the malaria eradica- tion programme advances to its objective. However, the availability of DDT remains essential for the continuation of the malaria eradication programme. The Director-General of the World Health Organiza- tion has stated that the withdrawal of this insecticide would be a major public health tragedy in which vast populations in the malarious areas of the world would be exposed to a resurgence of epidemic and endemic malaria. Although anopheline mosquitos throughout the world have been exposed to DDT for almost two decades, in only 1% of the areas treated have they developed sufficient resistance to make it impossible to interrupt malaria transmission. These few areas are at the moment relatively well defined, but there is no evidence that the situation will not change. Where DDT resistance has occurred, dieldrin, HCH, malathion, or propoxur have been successfully substituted, but none of these has been entirely satisfactory, owing principally to high cost. No biological control procedure is yet available to replace or supplement insecticides in malaria eradica- tion or control, although some success has been reported with carnivorous fish in the USSR, Iran, and other countries. Research is being undertaken in many parts of the world on the genetic control of anophelines, but no technique is approaching an operational stage. Consequently, new insecticides are urgently required to meet the challenge of resistance and the special needs of Africa. These should persist and be effective for not less than 3 months on mud surfaces, they should be safe to man and animal at operational dosages, and they should be reasonable in cost. Chagas' disease It is estimated that 30 million people suffer from Chagas' disease. This infection occurs mainly in the rural areas of Latin America and is associated with certain cultural patterns and poor socio-economic conditions. At the moment there is no way of effecting a radical cure of the disease or of providing immunological protection against it. The interrup- tion of transmission depends upon the improvement of housing and the application of insecticides to dwellings and adjacent structures. The compounds showing the greatest promise up to now have been HCH, dieldrin, and propoxur, but no large-scale national programme is under way. The main difficulties encountered have been reinfestation, high operational costs, the lack of ovicidal effect in any available insecticide, and inadequate ecological information. A good technique for evaluating infes- tation is also needed. Desirable properties in new insecticides for the control of reduviid bugs are rapid action, persistence of at least 3 months, ovicidal characteristics, and low toxicity to vertebrates. Biological or genetic control are not considered to offer any promise for the control of this vector. Typhus The control of the body louse still depends largely upon the application of DDT to infested persons. Where resistance has appeared, malathion has been an effective substitute. However, the first indications of malathion resistance in the body louse have appeared in Burundi. Effective alternatives with an acceptable level of safety to man are currently being sought among the carbamates that have low toxicity for mammals. The need for alternative insecticides is likely to continue indefinitely. Plaguie The nature of bubonic plague as it exists today, with foci scattered throughout the world, provides ample opportunity for a recrudescence of the disease. Recent outbreaks have demonstrated that plague can break out of these foci and affect large numbers of individuals. Provided they are instituted at an early stage, flea control measures are still considered to be the most effective means of controlling outbreaks of flea-borne plague. Until recently DDT was used almost universally for rat flea control. However, resistance has necessitated the substitution of organo- phosphorus and carbamate compounds in many parts of the world. Here also, insecticides will be required indefinitely. The biological control of fleas is not considered to be feasible. Growing urbanization (with municipal sanitation programmes often lagging behind urban growth) and the spread of single-crop culture have had unfortu- nate consequences, not the least of which is the increase of both urban and rural rodent populations. Virtually no new satisfactory acute rodenticide has appeared in the last two decades and, worse yet, resistance to the anticoagulant group is rapidly spreading among rat populations in the United Kingdom and Denmark, and has appeared else- where. The development of specific, effective new rodenticides is urgently required. 12 WHO PROGRAMME FOR EVALUATING AND TESTING NEW INSECTICIDES 13 Onlchocerciasis Onchocerciasis, transmitted by the blackfly, is a disease of very considerable importance in Africa and Latin America. It has been estimated that as many as 20 million persons suffer from it and it is considered to be the greatest single cause of blindness in Africa. A number of successful control pro- grammes have been carried out using DDT as a larvicide, particularly in East Africa, but no opera- tion using adulticides has been completely successful. Plans are being prepared for a large-scale blackfly control programme involving seven countries in West Africa, and in support of this WHO is evaluat- ing biologically degradable substitutes for DDT that could be successfully applied from aircraft as a floating formulation. Such a formulation, applied at low dosages, floats until it reaches larval resting sites-usually in rapids-where it mixes in the main body of water in droplet form. Of the compounds examined thus far, methoxychlor has shown the greatest promise for use in this technique. The treat- ment of river systems with any chemical has obvious limitations and much research is still required to develop appropriate insecticides for application in this type of ecosystem. No biological or genetic pro- cedure for the control of the blackfly is in sight. Filariasis Bancroftian filariasis is a disease that is spreading with great rapidity in many tropical countries. Its transmission is associated with poor sanitation, its vector, Culex pipiens fatigans, breeding in highly polluted waters. The refractoriness of this mosquito to control by the chlorinated hydrocarbons has, until recently, prevented the mounting of large-scale control programmes. Research performed by the WHO Filariasis Research Unit in Burma has demonstrated, however, that the larvae may be effectively controlled by certain organophosphorus compounds, notably fenthion and Dursban.t Nation- al programmes are now being based upon these chemicals in Asia, but a series of suitable degradable substitutes will be needed to counter the resistance that may develop. Such compounds should have no cross resistance one to the other and should have a persistence of 7-10 days in polluted water. To ensure interruption of the transmission of filariasis, a minimum of 7 years of uninterrupted vector control are necessary. Yellow fever and dengue haemorrhagic fever The control of the urban vector of yellow fever and haemorrhagic dengue rests almost entirely upon the use of larvicides, of which Abate t is the most effective and safe. However, no satisfactory sub- stitute for this insecticide is available at present. In some areas of Latin America, the use of Abate for Aedes eradication is supplemented by the application of a residual organophosphorus or carbamate insecticide in the vicinity of larval habitats. The ultra-low-volume application of organophosphorus compounds, such as malathion, has proved effective for the control of the sylvatic vectors of yellow fever -e.g., Ae. simpsoni in Ethiopia-and for reducing the densities of adult Ae. aegypti in urban areas where the transmission of dengue haemorrhagic fever occurs-e.g., in Bangkok, Thailand. This technique has considerable promise for the more general control of adult mosquito vectors of disease, such as the vectors of Japanese B encephalitis. In all cases more effective alternatives will be required well into the future because of the possibility that resistance may develop. Of all mosquitos, Ae. aegypti and C. p. fatigans have shown themselves to be the most susceptible to genetic control. The practical feasibility of this technique is being studied by a WHO Research Unit in India, but this pro- gramme is still in a preliminary stage. Trypanosomiasis The control of the adult tsetse fly, which infests large tracts of valuable land in Africa and which is both a serious public health hazard and a major veterinary problem, has, for more than a decade, depended on the use ofDDT or dieldrin. It has been estimated that about 60 tons of the former are used annually. The need for biologically degradable substitutes for DDT is urgent and the World Health Organization has begun an examination of promising materials now in production. Extensive efforts are being made to develop a procedure for the genetic control of this insect. Pest insects Pest mosquitos, particularly salt-marsh species, developed resistance to the chlorinated hydrocarbons at an early date and these insecticides have been almost entirely replaced by those in the organo- phosphorus group. However, in many areas, notably California, resistance has emerged to all available materials and a return is being made to source reduction, involving engineering and water manage- ment techniques. Although fly control is still t Names against which this symbol appears are identified in the Glossary on pages 445-446. J. W. WRIGHT feasible in most parts of the world through the application ofcertain organophosphorus compounds, in some areas, such as Denmark, resistance is now almost general. A similar situation exists in Australia with the cattle tick. Household pests such as the cockroach, the bed- bug, and the cat and dog fleas may still be controlled by currently available materials, but reports of resistance are continually increasing. EVALUATION AND TESTING PROGRAMME In 1960, faced by the challenge of insecticide resistance and the problem of environmental con- tamination, the World Health Organization estab- lished a programme for evaluating and testing new insecticides. This was later enlarged to include other pesticides required in public health. The chemical industry, universities, and governmental institutes were invited to submit new compounds for examina- tion. The response has been most encouraging. Forty-three manufacturing companies in eight different countries associated themselves with the scheme by submitting large numbers of newly synthesized materials. These were supplemented by contributions, in the form of groups of chemicals of novel structure, from five universities and institutes in three countries (see Annex 1). Each candidate compound is subjected to a series of seven evaluation stages, each succeeding stage demanding more exacting criteria of effectiveness and safety. Three of these stages of evaluation are performed in the laboratory and four in the field. By meeting the criteria for each successive stage, a compound advances to the next higher level of testing, until finally it qualifies for large-scale field evaluation. Thus, it moves from early assessment under controlled laboratory conditions to ultimate evaluation under the normal stress of the field environment. Compounds accepted into the scheme are given a number with an OMS prefix-for example, OMS-1 is malathion. The chemical identity of each compound remains confidential to the World Health Organization and the directors of collaborat- ing laboratories until the contributor authorizes its disclosure. Data emanating from the programme are analysed and stored in the WHO computer and are routinely fed back to suppliers and the laboratories. Seven laboratories serve as WHO reference centres and perform the investigations required for Stages I-IV. Details of these are given in Annex 2. Six WHO field research units working in six different countries are responsible for the studies required at advanced levels of evaluation. The locations of these units are listed in Annex 3. Standard techniques and criteria for Stages I-IV have been agreed upon by the directors of collaborat- ing laboratories, who meet annually. Consequently, the results obtained by workers in different parts of the world are comparable. At Stages V-VII, where the number of compounds is limited, standardized evaluation procedures are also used but these some- times require modification for different species and varying ecological conditions. The overall structure of the Evaluation Pro- gramme and the role of each participating laboratory or research unit are illustrated in Fig. 1. Stage I As will be seen, Stage I screening of insecticides is performed at the University of Illinois, and consists essentially of the establishment of dosage-mortality curves against susceptible and resistant mosquitos and houseflies. Adults and larvae of two species of mosquito are used: dieldrin-resistant Anopheles albimanus and susceptible C. p. fatigans. Observa- tions on houseflies are made with a susceptible strain (NAIDM), a strain resistant to chlorinated hydro- carbons (SP), and a strain resistant to both organo- phosphorus compounds and chlorinated hydro- carbons (SC). The criteria for acceptance at this stage are shown in Table 1. The laboratory at the University of Illinois investigates the potential of promising insecticides to produce resistance in C. p. fatigans and the housefly by subjecting these species to 25-50 generations of selection pressure. It also undertakes preliminary assessment of biological degradability using a model ecosystem. The primary screening of chemosterilants is the responsibility of the Entomology Research Division of the Agricultural Research Service, US Department of Agriculture, Gainesville, Florida. Stage II Stage II evaluation consists of the testing, by four different laboratories, of samples of compounds that have shown promise at Stage I. Based wherever possible on data provided by the contributing manu- facturer or institute, the Toxicological Research Unit of the Medical Research Council Laboratories 14 WHO PROGRAMME FOR EVALUATING AND TESTING NEW INSECTICIDES 15 Fig. 1 Structure of the evaluation programme and roles of collaborating laboratories Pesticide MlanuLfacturers |& University Laboratories University of Illinois Urbana ENTOMOLOGY L ,sDA, Ei I) TIPR USDHIW, CDC, TDi) Toxicology Research Unit Gainesville 'orton Savannah NletI.Res.Coun.Lab.Carshalton INO1010r0 _ NTONIOL0GX ENTOMOLOGY TOX I COLOGY DI;E)IW, CDC, 10D S avannah ENTlOMIOLOGY IPSI'P'R I Aruslia ENTOMIIOOGY | V11() JtE RI horea Taiwan FNTOMIOILOGY Centre Muraz Bobo-ountilasso FENTOMOLOGY \1110 XCIII -I SIlIO 1A ARL kOaduna.l WilO JELURU )ar-es-sla;am K\aTdu10a Korea/ Taiwan E1NTON1 I,( )(jI C tXIICAL FOltUI ,ATIONS NITO:110LOGY _ . _ __ X ro>;~~~~CO IC C lGY r- WHO ACXRU-2 7 WTiO ARU Bangkok ENTOMOLOGY kenv a ENTOMIOLOGY TOXICOLOGY OPERATIONAL EXPERIENCE 11'11) Mtalaria Fradication Fiel(l Trials EPIDE IOIL(GY at Carshalton, England, studies the oral and dermal toxicity and the neurotoxicity of the compounds. Repeated doses are sometimes given to ascertain whether a given compound has a cumulative effect. Definite criteria for acceptance are not laid down at this stage. A decision as to whether a chemical will proceed to further stages depends upon its projected use (i.e., as a larvicide or an adulticide), the extent to which human populations may be exposed, the insect involved, the dosage and the method by which application will be made, the formulation, and the stability of the material. The laboratory at the Tropical Pesticides Research Unit, Porton, England, measures the physical properties of the insecticides and establishes its residual effectiveness on different types of building material, using An. stephensi as the test insect. The US Department of Agriculture laboratory in Gainesville, using a variety of techniques, makes a preliminary evaluation of the effectiveness of the insecticide against mosquito larvae and adults, houseflies, body lice, fleas, ticks, and bedbugs. Evaluations are also performed by the Technical Development Laboratory of the Center for Disease Control in Savannah, Georgia, which concentrates on mosquito adults and larvae, adult houseflies, and triatoma. This laboratory carries out a preliminary assessment of the effectiveness of new rodenticides. Table 1 Criteria for acceptance at stage I Exposure Insect Type of treatment period Criteria for acceptance (hours) adult mosquito residue 1 24-hour mortality of 50 % at 1 6 Pg/cm2 larval mosquito larvicide 24 50 % kill at 0.1 ppm adult housefly topical application 24 24-hour mortality of 50 % at 1 4g/fly SuIIrce Stage I (Screening l'ests) Stages II & III (Iaboxatorv & -Simulatel F'ield Tests) Stage IV (F ield Tests) ;takieUe (E illage ri)al) (ta,e Ii F ieI( 1'riaIl) Slt ge) V l I (aftIIge-sIal I' ial ) USDA), lIIt) Gainesv ilo E1NTOMOILOGY __I 11lO AtIC blarngkok INTIttOt OGY m'N1011.G\ I J. W. WRIGHT Table 2 Criteria for acceptance at stage 11 Insect Type of treatment Laboratory Exposure Criteria for acceptanceperiod Adult and larval mosquitos and adult houseflies residue space sprays space sprays LC95 determination residual larvicide residue residues on plywood space sprays baits baits impregnated cords residues on plywood powders on cloth residues on paper residues on paper residues on paper US Department of Agriculture, Gainesville, USA Tropical Pesticides Research Unit, Porton Down, Salisbury, England Technical Development Laboratories, Center for Disease Control, Savannah, USA Technical Development Laboratories, Center for Disease Control, Savannah, USA US Department of Agriculture, Gainesville, USA Technical Development Laboratories, Center for Disease Control, Savannah, USA US Department of Agriculture, Gainesville, USA Technical Development Laboratories, Center for Disease Control, Savannah, USA US Department of Agriculture, Gainesville, USA Technical Development Laboratories, Center for Disease Control, Savannah, USA US Department of Agriculture, Gainesville, USA US Department of Agriculture, Gainesville, USA Tech nical Development Laboratories, Center for Disease Control, Savannah, USA Technical Development Laboratories, Center for Disease Control, Savannah, USA Triatoma, lice, fleas, bedbugs, and ticks Technical Development Laboratories, Center for Disease Control, Savannah, USA 1 hour 30 seconds momentary 24 hours 24 hours 1 hour 30 minutes momentary 24 hours 1 hour 1 hour 2 hours 24 hours 24 hours 24 hours 24 hours 70 % 24-hour mortality for 8 weeks 95 % mortality at concentra- tion of 0.005-0.01 % LCso = 0.05 % or less variable, dependent on toxicity of compound for mammals; should normally be 1.0 ppm or less kill of 95 % for 6 weeks 90 % mortality for 4 weeks 70 % mortality for 1 month LCso < 1 % 90 % mortality at concentra- tion of <0.005 % 70 % mortality at 24 hours 100% mortality for 2 months 70 % mortality for 2 weeks 90 % mortality for 2 weeks 90 % mortality for 4 weeks 90 % mortality for 4 weeks 90 % mortality at 20 mg/mi2 The criteria for acceptance at Stage II are shown in Table 2. Stage III Following laboratory tests at Stages I and II, compounds progress to simulated field trials in Stage III at Gainesville and Savannah. The criteria for acceptance at Stage III are given in Table 3. Stage IV In Stage IV the effect of compounds on naturally occurring insect populations is assessed for the first time (see Table 4). The tests carried out at this stage lead to the exposure of spray-men and other person- nel, and the Carshalton laboratory thus obtains additional information upon which to base recom- mendations for the protection of operators and others handling the compounds. Formulations are adult mosquitos larval mosquitos adult houseflies adult and immature Triatoma adult lice adult fleas adult bedbugs ticks 1 6 WHO PROGRAMME FOR EVALUATING AND TESTING NEW INSECTICIDES Table 3 Criteria for acceptance at stage IlIl adu mo, Insect Type of treatment It residue (interior squitos application) residue (exterior application) residual fumigant larval mosquitos adult lice fleas Laboratory Adult and larval mosquitos, lice, and fleas Technical Development Laboratories, Center for Disease Control, Savannah, USA Exposure Criteria for acceptanceperiod (hours) 12 90 % mortality for 8 weeks 70 % mortality for 8 weeks 70 % mortality for 6 weeks in ventilated huts residual larvicide Technical Development Laboratories, Center for 24 70 % or greater mortality drums Disease Control, Savannah, USA for 8 weeks small containers 24 70 % mortality 12 weeks pre-flood (field) 24 70 % or greater mortality for 8 weeks sleeve (powder) dust US Department of Agriculture, Gainesville, USA examined at Porton and Savannah, and these laboratories also begin work on the establishment of analytical procedures. Hut tests are carried out against anophelines at Arusha, Tanzania; at Bobo- Dioulasso, Upper Volta; and at Savannah, Ga., and Gainesville, Fla., USA. The Gainesville and Savan- nah laboratories also perform studies at Stage IV against adult and larval mosquitos, other than anophelines, with particular emphasis on Ae. aegypti. 24 24 90 % mortality for 1 week LC95= 5% WHO research units begin to play an important role at this point, trials being performed by the Aedes Research Unit in Bangkok, Thailand, against Ae. aegypti and C. p. fatigans; by the Japanese Encephalitis Virus Research Unit in Korea and Taiwan against the vectors of Japanese B encephali- tis; by the East Africa Aedes Research Unit in Dar es Salaam, Tanzania, against the vectors of yellow fever; and by the Anopheles Control Research Table 4 Scope of individual tests against various insects in stages IV, V, and VI Stage IVa experimental huts in single ponds, streams or containers in single barns or chicken houses in single houses in single stream or part of a stream on small group of people in single houses in single burrows or animal nests on naturally infested plots Stage V b village on population in an area of several acres or several city blocks on population in group of barns or in village on population in village on population in river basin or group of streams on infested village on infested areas on population in confined area on population of several acres Stage VI c several thousand houses in square miles of country or in an entire city in large area in several thousand houses over a large area of many square miles over a large population large-scale trial over infested community in a large area over a large area (l Small-scale field trial; first stage with field insects. b "Village" scale; first stage against an insect population. c Large-scale field trial. nsect adult mosquitos larval mosquitos houseflies reduviid bugs Simulium lice bedbugs fleas ticks _ T 17 J. W. WRIGHT Unit in Kaduna, Nigeria, against anophelines. Studies on formulations and tropical storage are also performed by this unit. Stage IV trials against houseflies have also been carried out in Denmark by the Government Pest Infestation Laboratory and in Italy by the Istituto Superiore di Sanita, and the Gainesville laboratory has completed a series of trials against ticks in North Carolina. The National Institute of Communicable Diseases in Delhi, India, has collaborated in tests against fleas, and the responsibility for work on Simulium larvae has been accepted by the ORSTOM laboratory at Bobo-Dioulasso, Upper Volta, and by the New York State Museum and Science Service. The evaluation of compounds against Simulium has presented a number of problems. Since effective colonization of this insect species has not yet been achieved, primary evaluation has been made with larvae from field sources and has been carried out in channels or troughs with flowing water. This has introduced difficulties in respect to standardization of the test material. Stage V Stage V testing against anophelines is carried out principally by the WHO research units, although sizable trials have also been performed by the Gainesville and Savannah laboratories. The latter has carried out singularly successful studies on the control of houseflies in Georgia, on the control of Ae. aegypti in the Caribbean area, and on com- pounds for aircraft disinsection. The Gainesville laboratory has concentrated on materials for the control of adult mosquitos, the body louse, and ticks. The Anopheles Control Research Unit at Kaduna is responsible for village-scale trials on compounds against anophelines. The savannah area of Northern Nigeria, where this unit is located, represents a severe challenge to any insecticide, not only from the climatic viewpoint, but also because houses are built essentially of mud. The Aedes Research Unit in Bangkok evaluates compounds that might be applied for the control of Ae. aegypti in urban areas where breeding occurs mainly in man-made con- tainers. It has also undertaken studies on the potential value of the ultra-low-volume application of insecticides for the control of adult mosquitos, particularly in epidemic situations. Ultra-low- volume trials were performed in Ethiopia in a biotype where Ae. simpsoni is an important sylvatic vector of yellow fever. The Japanese Encephalitis Vector Research Unit, with branches in Taiwan and Korea, is concerned essentially with investigating compounds that might be applied in rice fields for the control of Culex tritaeniorhynchus larvae. This research is greatly complicated by the large-scale use of pesti- cides for rice culture. From 1962 to 1969 the Filariasis Research Unit in Rangoon, Burma, studied measures for the chemical control of the vector of Bancroftian filariasis, C. p. fatigans, with consider- able success. In addition to the normal entomological observa- tions, Stage V includes toxicological observations on persons who apply or handle insecticides and who live in treated houses. These studies, which are an integral part of the work of the Anopheles Control Research Unit I in Kaduna, have been instrumental in clarifying the potential hazard of compounds that have operational promise. Stage VI Insecticides that meet the criteria established for Stage V and that present no problems involving toxicity or operational use are submitted to advanced testing against anophelines at Stage VI. These trials involve a human population of up to 25 000 living in several thousand houses and are carried out by the WHO Anopheles Control Research Unit II now situated in Kisumu, Kenya. As far as possible, these applications are made to approximate field operating conditions. Four rounds of spraying are carried out, covering a full seasonal cycle, and toxicological observations are performed in each spraying cycle. It is possible therefore at this stage to analyse the stability and performance of the commercially pro- duced formulation, its suitability for application by locally employed spray-men using conventional equip- ments, its insecticidal effectiveness, and its safety. Stage VII Compounds passing Stage VI may then move to a full epidemiological evaluation, as has been done with malathion in Uganda and propoxur in Central America, involving populations of up to 100000 persons. Insecticides passing final stages of evaluation are considered by WHO Expert Committees when re- commendations are made for their use for disease or vector control. Additional studies As compounds progress through the programme and reach advanced stages, additional research is undertaken to determine the potential of the various 18 WHO PROGRAMME FOR EVALUATING AND TESTING NEW INSECTICIDES species against which they will be used to develop resistance. A typical study of this type performed by the University of Western Ontario, Canada, with C. p. fatigans and fenthion indicated that the devel- opment of resistance would be slow and that con- tinuous control for about 5 years could be expected without the emergence of resistance. Experience has shown that this prediction was correct. Research is also carried out, in different strains, on cross resistance between new compounds and those currently in use to which resistance has appeared or is developing. Attention is given to the relationships between physical properties, contact toxicity, and persistence of residues. Special studies were undertaken to develop a method of formulating dichlorvos that would enable the insecticide to be released at a slow and steady rate for residual fumigant action. Extensive research has been carried out on the formulation of larvicides for use under different conditions. The extent to which organo- phosphorus compounds and carbamates are absorb- ed in man has been measured, and the mode of action of these materials in the human body has been investigated. As a result of such studies, detail- ed recommendations have been made on the protection required under operating conditions. The effect of mixtures of insecticides has been evaluated with the object of delaying or preventing the development of insecticide resistance, but without notable practical success. The possible use of synergists to enhance the effectiveness of residual compounds has also been investigated, but apart from their use in aerosols and with compounds for the control of body lice little success has been achieved. During the past 4 years the programme has been expanded to include the evaluation of rodenticides, because of the great need for effective acute poisons and to the development in many areas of resistance to certain anticoagulant compounds. These are evaluated by laboratories situated in Denmark, the United Kingdom, and the USA. A most encouraging development has been the increased flow of biologically effective material that has been developed specifically to avoid environ- mental contamination. By the end of 1970 almost 1 400 compounds of all types had passed through Stage I. Of this total, 1 265 were insecticides, the remainder being rodenti- cides, chemosterilants, synergists, and growth- inhibiting compounds. As the rodenticide evaluation programme has only recently been started, this group of compounds is in the early stages of testing, and it is still too early to assess the operational potential of any of them. No chemosterilant has yet been recommended for field control, although thiotepa,t metepa,t and apholate t have shown promise against houseflies and mosqui- tos in laboratory experiments in different parts of the world. At present it appears that priority in research on chemosterilants should be given to the demonstration of the practical feasibility of the technique rather than to the development of new materials. Growth-inhibiting chemicals offer promise and it is important that they be tested against a wide variety of insects as soon as possible. However, it is clear that a number of ecological, toxicological, formulation, and application problems will have to be solved before their use on an operational scale can be contemplated. The rate at which insecticides were received for evaluation during the past ten years will be seen in Fig. 2. The ratio of those provided by industry to those provided by collaborating universities and institutes was 14: 1. Between 1961 and 1965 a relatively large number of compounds was examined; this established a base-line for comparison and provided experience on the behaviour of the various groups of compounds under differing environmental conditions. The decline in numbers from 1965 onwards coincided with the first appearance of the financial and other difficulties that currently face the industry in the development of new pesticides. However, during this same period contributions from non-industrial sources increased. The chemical groupings of the different insecticides received will be seen in Fig. 3. The organophospho- rus compounds have been subdivided into chemical classes for easy reference. Almost all the carbamates were methyl derivatives. Fig. 4 shows the progress of the 1 265 compounds through the scheme, with special reference to their potential for malaria control or eradication. Slightly more than 300 compounds met the criteria that justified their being moved into Stages II and III. Of those left in abeyance, 50% had insufficient insecticidal action, 40% were considered to be too hazardous for indoor spraying, and 10% were with- drawn by the supplier. Of these 300 compounds, 82 were recommended for Stage IV evaluation, half against anophelines and half against other insects of public health importance. The main reason for rejection was again insufficient 19 20 J. W. WRIGHT Fig. 2 Compounds evaluated at stage 1, 1961-70 220r 200 180 - 120 100 \0 20 0) ___1_ ___ _ __ _ __ _ _ I _ I 19(;1 1962 19.'3 1964 196. 196(; 1967 1968 19469 19(70 Fig. 3 Chemical classifaction of compounds evaluated during 1961-70 271 z h19 1:~ 32 (CE1C11(' l( SS WHO PROGRAMME FOR EVALUATING AND TESTING NEW INSECTICIDES Fig. 4 Compounds evaluated for malaria eradication insecticidal activity, with 50O% falling into this category; however, the proportion of withdrawals by suppliers rose to 30%, whilst that related to possible hazard fell to 20%. The results obtained in the evaluation of these 82 compounds at Stages IV, V, and VI are analysed below. AduItlt anophelines Of the 41 compounds evaluated in huts against anophelines, 19 failed to meet the criteria for effectiveness and 2 were considered to be unsuitable because of a lachrymatory effect. The 20 assessed in villages at Stage V fell into the following chemical groups: 9 phosphorothioates, 10 methylcarbamates, and 1 dimethyl phosphate. On the basis of observations on spray-men and persons living in treated houses, two carbamates and one phosphorothioate were considered to be unsafe for routine indoor application. Four carbamates and three phosphorothioates were shown to have insufficient persistence on mud walls. Studies are now in progress on three phosphorothioates and one carbamate. Six compounds-malathion, propoxur, fenitrothion, dichlorvos, Mobam,t and Landrin t- were proposed for large-scale field trials. The trials of malathion and propoxur have been completed, and these compounds are now in operational use for malaria control and eradication. Dichlorvos, when applied from dispensers, was shown to be unsuitable as a residual fumigant in African huts because their open construction gave rise to excess ventilation. Work is not yet complete on fenitrothion and supplies of Mobam t and Landrin t are awaited from the manufacturers. Mosquito larvae Culex pipiens fatigans. Eighteen compounds, of which one was a pyrethroid, were evaluated at Stage IV for the control of C. p. fatigans in polluted water, involving 32 different formulations. The greater part of this work was done by the WHO Filariasis Unit in Rangoon, Burma. It was observed at an early date in this phase of the evaluation programme that the members of one group of carbamates were generally ineffective as larvicides and that most of the organophosphorus compounds were rapidly decomposed by organisms occurring naturally in larval habitats, the most important of these being Bacillus subtilis. Those compounds possessing the ability to withstand such decomposi- tion are diazinon, fenthion, and Dursban.t Following a successful Stage V trial with fenthion in 1967 a large-scale programme (Stage VI) for the control of C. p. fatigans was successfully completed in Rangoon in 1969. The techniques developed by the Filariasis Research Unit based upon the use of fenthion are now being used in a national filariasis programme in Burma and by other countries with a Bancroftian filariasis problem. Three additional larvicides are now being examined for use in the control of C. p. fatigans by the WHO Research Unit in Bangkok. Aedes aegypti. Since the control of the larval form of Ae. aegypti requires the application of an insecti- cide to man-made containers, including those used for the storage of drinking water, the inherent toxicity of the insecticides for man, in addition to high effectiveness at low dosages, has been an over- riding factor in this portion of the programme. Only three compounds were considered to meet the criteria justifying their Stage IV evaluation. Of these, Abate t has been by far the best because of its unique characteristics, and it is now undergoing a Stage VI trial in Bangkok. A search for possible substitutes for it is also being carried out. 21 J. W. WRIGHT Other mosquito species. Since the larval control of anophelines frequently involves the treatment of water that might be used by man and animals, emphasis has been placed on toxicity in evaluating compounds for this purpose. Of the materials that have been examined, Abate t has shown itself to be the most suitable. However, the examination of eight other compounds is at present under way at the Anopheles Control Research Unit I in Kaduna, Nigeria, with special attention being given to materials that could be used during the dry season when mosquito populations are at their lowest. The Japanese Encephalitis Vector Research Unit is testing a group of organophosphorus compounds for the control of Culex tritaeniorhynchus. Applica- tion will be made from the air using a variety of formulations. Body lice. The requirements that materials for the control of body lice be of extremely low toxicity for man and of high insecticidal effectiveness have excluded all but a small number of compounds. Both malathion and carbaryl have met the criteria; studies on Abate t and Mobam t are still in progress. Fleas. It has been demonstrated that most organo- phosphorus and carbamate compounds with high insecticidal activity are suitable for flea control. The choice of a compound therefore rests on safety to man, the status of resistance to the chlorinated hydrocarbons, and the availability of the material locally. Compounds that have passed through the scheme or have been recommended by the WHO Expert Committee on Insecticides are carbaryl, diazinon, and malathion. Annex 1 COLLABORATING UNIVERSITIES AND INSTITUTES Fordham University, New York, N.Y., USA (Professor University of Illinois at Urbana-Champaign, Urbana, D. J. Hennessy) Ill., USA (Professor R. L. Metcalf, Head, Department Hebrew University of Jerusalem, Jerusalem, Israel of Zoology) (Prof.D.Bergmann) Commonwealth Scientific, Industrial, and Research(Professor E. Organization, Chemical Research Laboratories, Mel- Israel Institute for Biological Research, Ness-Ziona, boume, Australia (Mr G. Holan, Principal Research Israel Scientist) Annex 2 COLLABORATING LABORATORIES Department of Entomology, University of Illinois at Urbana-Champaign, Urbana, Ill,. USA Entomology Research Division, Agricultural Research Service, US Department of Agriculture, Gainesville, Fla., USA Technical Development Laboratories, Center for Disease Control, US Public Health Service, Savannah, Ga., USA Tropical Pesticides Research Unit, Porton Down, Wilts., England ORSTOM/OCCGE Team, Centre Muraz, Bobo-Diou- lasso, Upper Volta Tropical Pesticides Research Institute, Arusha, Tanzania Toxicology Research Unit, Medical Research Council Laboratories, Carshalton, Surrey, England Annex 3 WHO RESEARCH UNITS PARTICIPATING IN THE INSECTICIDE EVALUATION AND TESTING PROGRAMME Aedes Research Unit, Bangkok, Thailand East Africa Aedes Research Unit, Dar es Salaam, Tanzania Anopheles Control Research Unit, Kaduna, Nigeria Anopheles Control Research Unit No. II, Kisumu, Kenya Japanese Encephalitis Vector Research Unit, Taipei, Taiwan Japanese Encephalitis Vector Research Unit, Seoul, Korea 22