Benefits and Hazards of Insecticides to Public Health W. J. HAYES, JR., and S. W. SIMMONS Communicable Disease Center, Public Health Service, Federal Security Agency, Savannah, Ga.
Striking improvements in the control of insect-borne diseases in recent years are largely due to DDT. Improvements in agricultural insect control are due to inorganic insecticides, insecticides of vegetable origin, DDT, and an ever-widening range of new organic insecticides. Use of promising new insecticides must wait on knowledge of their toxicity.
T h e effectiveness of some of the new insecticidal agents against an extremely wide v a r i ety of disease vectors has made possible the control of many important diseases which previously defied all practical efforts at control and has brought within the realm of possibility the ultimate eradication of certain diseases. Diseases which careful scientific study indicates are subject to control b y insecticides include malaria, plague, epidemic typhus, murine typhus, and enteritis due to Shigella. Diseases which w i l l probably be subject to control b y insecticides but have not yet been adequately tested include sandfly fever, dengue, urban yellow fever, bartonellosis, cutaneous leishmaniasis, Chagas' disease, filariasis, trench fever, and louse-born relapsing fever. Some of the virus encephalitides sleeping sickness, and visceral leishmaniasis may also be susceptible of control. ;
Control of Malaria and Plague More data showing epidemiological statistics for insecticide-treated areas and for control areas have been published on malaria than on any other disease. W i t h few exceptions, these data indicate a remarkable degree of control surpassing anything reported previously. F o r example, after a single year of operations, Viswanathan and Rao (31), working i n Bombay Province, India, were able to report control of 7 4 % or better based on spleen and parasite rates of children and adults i n sprayed as compared to control villages in the Kanara District and 3 0 % or better i n the less severely affected Dharwar District. The infant parasite rate, which more nearly reflects new cases of malaria, was reduced b y 9 4 % i n the K a n a r a District and apparently b y 100% i n the Dharwar District. (These examples are very incomplete. Since the preparation of this paper, a much more extensive résumé of disease control with insecticides has been presented at the meetings of the American Society of Tropical Medicine, Memphis, Tenn., November 6 to 9, 1949. The résumé will be published.) Essentially similar results have been reported i n more limited studies i n Panama, where a reduction i n the parasite rate from 5 2 % i n the control area to 14.8 i n the treated area was obtained (27), and i n Puerto Rico, where the rate was reduced from 5.8 to 0 . 9 % in one year (12, 25). I n Panama and Costa Rica, Macready (15) reported that the m a laria rate of hospitalized employees was reduced on the average b y 5 3 % for each 1000 a d missions. I n Peru, the parasite rate was less than 1% after treatment, where it had 56
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ranged from 11 to 2 8 % before the use of D D T (4). I n a small area of the Volturno River Valley i n Italy, the parasite rate was reduced from 21 to 1% i n one year (1). A n improve ment of about 5 0 % was noted i n Veneto Province i n Italy following treatment with D D T (23). Good results have been reported i n New Guinea (2) and i n the New Hebrides Is lands (33), especially among military personnel. I n Mauritius, i n the Indian Ocean, a carefully controlled study showed the parasite rate of the general population to be reduced b y two thirds and that of the children by one half (26). In the United States, the incidence of malaria is low and it has been falling gradually for many years. However, the current large program of house spraying conducted by the states i n cooperation with the Communicable Disease Center is believed to be increasing that rate of fall. Coupled with auxiliary techniques, the house spraying program, if con tinued, has an excellent chance of eradicating malaria from the continental United States i n the not too distant future. The first control of plague through the use of D D T was accomplished by the U . S. A r m y i n D a k a r i n November and December 1944 and i n Casablanca i n J u l y 1945 (8). The outbreak i n D a k a r had been active since A p r i l 20, 1944, and its complete control was apparently almost entirely due to the universal application of D D T i n the native quarters to persons, beds, floors, walls, and premises generally. Following this experience, D D T was used more promptly i n Casablanca. Macchiavello (13, 14) considers, on the basis of his experience i n Peru, that the use of D D T followed by the use of 1080 (sodium fluoroacetate) may be the method of choice i n the control of epidemics of bubonic plague. Pollock (20) using D D T alone successfully controlled an epidemic of plague i n Haifa i n J u l y 1947.
Control of Typhus Probably the most dramatic accomplishment i n public health i n the last quarter of a century was the control of major epidemics of louse-borne typhus i n densely populated, heavily infested populations under wartime conditions. This accomplishment was made possible by a great body of research before World W a r I I and b y the efforts of at least a dozen military and civilian agencies during that war. Methods of louse control with M Y L (pyrethrins, 0.2%; N-isobutylundecylenamide, 2.0%; 2,4-dinitroanisole, 2.0%; and phenol S, 0.25%) and D D T powders were first field tested i n the Near Eastern and African theaters and finally proved i n the epidemics i n Naples (3, 24), i n German concentration camps (δ, 7), and i n K o r e a and Japan (21). Although immunization was of value espe cially i n protecting expert personnel carrying out control work, it has been pointed out by Bayne-Jones and others that it was the judicious use of insecticidal powders that actually stopped the epidemics. It has been estimated that there were 5,000,000 cases of typhus i n Russia alone following World War I. B y contrast, the number of civilian cases i n the Near East, N o r t h Africa, Italy, Germany, Korea, and Japan during W o r l d W a r I I ap pears to have been less than 500,000. The murine typhus control programs of the individual states, i n cooperation with the United States Public Health Service, probably represent the largest unified attack on murine typhus ever undertaken. Since actual operations were begun i n September 1945, a total of 1413 tons of 1 0 % D D T dust has been applied i n 1,105,006 premises treatments in 156 counties of 10 states. Control of the tropical rat flea, Xenopsylla cheopsis, the chief vector of murine typhus, has been 8 0 % or better under field conditions. A s a result of the programs, murine typhus has decreased more rapidly i n the treated counties where it was originally more frequent. There has, during the same period, been a decrease i n the offi cially untreated counties which,originally suffered less from the disease. This may be ex plained, i n part, by D D T treatment, ratproofing, rat eradication, and general sanitation carried on i n the ' 'untreated" counties b y the local government without state support or by private enterprise and, i n part, by a natural decline i n the disease due to unidentified causes. The decrease i n murine typhus i n 61 counties of 9 states dusted each year, 1946 to 1948, was 8 4 % for the first 10 months of 1948 based on the first 10 months of 1945 before treatment was begun. The comparable figure for the officially untreated counties was 6 1 % . The rapid control of small outbreaks of typhus transmitted b y lice but presumably of the murine variety has been reported (6,17).
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Control of Other Diseases The control of diarrheal diseases through the control of flies b y insecticides has been noted i n association with several large scale programs—for example, the antimalaria program i n Greece (30) and the antisandfly program i n M a l t a (22). The only program of fly control b y insecticides, i n which the effects on diarrheal diseases were carefully followed and subjected to statistical analysis, was carried out by the Public Health Service i n towns i n the lower R i o Grande Valley. This study showed that a significant reduction i n the amount of infection, disease, and death resulted from the degree of fly control which was obtained. The effect on infections due to Shigella was greater than that on infections due to Salmonella (32).
Several other diseases have already apparently responded to vector control with i n secticides, although the conditions i n which the outbreaks occurred did not permit complete statistical evaluation of the control. Control of sandfly fever i n M a l t a was reported by Semple (22). Hertig and Fairchild (9) have noted the virtual cessation of new cases of both cutaneous leishmaniasis and bartonellosis following sandfly control with D D T i n Peru. There is every reason to suppose that dengue, urban yellow fever, and filariasis would respond to control of their mosquito vectors and that Chagas' disease and trench fever and louse-borne relapsing fever would also respond. W i t h somewhat more reservation, one may predict the control of some of the virus encephalitides, trypanosomal sleeping sickness, and visceral leishmaniasis. Public health officers are concerned with the control of arthropods as vectors of h u man disease. Important as disease control may be, food production is even more important. Biologically, organisms are more often and more completely limited b y their food supply than b y disease. The total health of our population is affected b y anything which reduces our food supply. The chemical control of food-destroying insects is, therefore, a necessary part of any survey of the benefits of insecticides to public health. Of course, the methodology and conduct of such control are not the responsibility of the public health profession, but the necessity of such control must be considered here, just as the necessity for maintaining the health of farmers and their families must be considered in a comprehensive outline of agriculture. Just how much human food is saved from insect destruction is difficult to estimate. I t is thought that 1 0 % or more of the total crop in this country with an annual value of about $13,000,000,000 is lost through insect damage (11, 16). The damage to stored food products alone is said to average 5 % even i n countries with well developed technical services (10). The earlier estimates of losses may be too low i n view of the increased production which may now actually be obtained by the judicious use of i n secticides (18). The toxicity of a substance is its capacity for causing injury, whereas the hazard of a substance is the probability that such injury will actually occur. Factors that influence the capacity of a given compound to cause injury include its residual action; its acute toxicity; its subacute and chronic toxicity (including not only the accumulation of the toxin or its metabolites in the tissues but also the accumulation of tissue damage with or without the accumulation of the toxin) ; its solvent and particle sizes, as used; its relation to diet; and the timing of its application. The toxicity of a compound may vary for different species, for different ages, and for the two sexes. Factors that influence hazard include not only all the aspects of toxicity but also the method of use, the degree of warning, such as odor given by the compound itself ; the degree of education of the public ; and many other subtle factors. There is nothing basically new i n our present problem, but there are a great variety of new insecticidal compounds and formulations available, and there is a greatly increased demand for their use. Just as the benefits of insecticides to public welfare fall into two classes, so also do the hazards fall into two groups : first, those which are traditionally and legally the responsibilities of local, state, and federal health officers, and, second, those which are traditionally and legally the responsibilities of other authorities. The two groups are often impossible to separate. Of direct interest to the public health professions are the following:
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Acute poisoning i n the household b y actual ingestion or food contamination Acute poisoning of handlers and operators Acute poisoning i n industry Subacute or chronic poisoning of consumers through residues i n fruit, vegetables, meat, or dairy products or through improper household use Subacute or chronic poisoning of handlers and operators Subacute or chronic poisoning i n industry (Health officers may also be notified when household pets are poisoned.) Of equal importance to the public health are hazards of acute or chronic poisoning of farm animals and poultry; insecticide residues i n fruit, vegetables, meat, or dairy products; phytotoxicity; reduction of soil fertility; and disturbance of the balance of nature through selective destruction of wildlife. These latter hazards are largely the responsibilities of agriculturalists and conservationists but are mentioned here for completeness. The great interdependence of public health and agriculture can be no better illustrated than by the problem of insecticide residues i n food. Compounds, vehicles, and spraying schedules adapted for insect control must be selected by the agriculturalist i n such a way as to ensure an acceptable product at harvest. However, once the product is harvested and offered for human food, it becomes the concern of various public health agencies. The health of the agricultural operator who applied the insecticide is also a matter for medical and public health attention. The benefits which have been pointed to i n disease control are largely due to D D T . The benefits which have been pointed to i n agricultural insect control are due to the inorganic insecticides, to insecticides of vegetable origin, to D D T , and to an ever-widening range of new organic insecticides. I n both fields, and particularly i n disease control, the use of several promising new insecticides must be severely limited or entirely omitted because so little is known of their toxicity that they cannot be used with confidence. A t least the following groups are interested, i n one capacity or another, i n the toxicity and hazard presented by insecticides : the public health and medical profession; the chemical industry; the agricultural industry, including farmers and state and federal departments of agriculture (registration and labeling) ; the Food and D r u g Administration and similar state authorities (prohibition of contaminated food i n commerce) ; the food industry, including canners, frozen food packers, and green grocers; the armed services; v a r i ous research groups; and the public generally. Broadly speaking, the most important factors i n the safe use of economic poisons are adequate information and appropriate care b y all persons having any contact whatsoever with the poisons. Safety may be promoted but not ensured b y voluntary control, such as the self-discipline of industry; by legal control, such as regulation of sale, labeling, and distribution ; and b y economic control, such as the refusal of food processors to buy from farmers or dealers food containing excessive residues for which no adequate method of decontamination is known. The principal features of the Federal Insecticide, Fungicide, and Rodenticide Act have been briefly and clearly presented b y Perry (19) ; the full text and interpretations of the act are also available (28, 29).
Conclusions Because the benefits of insecticides to the public health both i n a restricted and i n a broad sense are very great, i t should be our purpose to use the new as well as the older compounds i n such a way as to avoid injury and at the same time obtain the maximum benefits. Some think that efforts should be made to simplify the nomenclature and to encourage industry to do proper toxicological research and supply methods for analysis before a new compound is submitted for registration. The study of antidotes should be encouraged. I t is clear that research i n the health hazards as well as the benefits of insecticides must be intensified and that the efforts of different laboratories should be better coordinated. Education of industry, of handlers and operators, of the medical and public health profession, and of the public must be advanced.
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