FAO Meeting Report No. PL/1965/10/1 WHO/Food Add./27.65 EVALUATION OF THE TOXICITY OF PESTICIDE RESIDUES IN FOOD The content of this document is the result of the deliberations of the Joint Meeting of the FAO Committee on Pesticides in Agriculture and the WHO Expert Committee on Pesticide Residues, which met in Rome, 15-22 March 19651 Food and Agriculture Organization of the United Nations World Health Organization 1965 1 Report of the second joint meeting of the FAO Committee on Pesticides in Agriculture and the WHO Expert Committee on Pesticide Residues, FAO Meeting Report No. PL/1965/10; WHO/Food Add./26.65 MALATHION Chemical name O,O-dimethyl S-[1,2-di-(ethoxycarbonyl)ethyl] phosphorodithioate; S-[1,2-di(ethoxycarbonyl)ethyl] dimethyl-phosphorothiolothionate; S-[1,2-di(ethoxycarbonyl)ethyl] dimethyl-phosphorothiolothionate; S[1, 2-bis (ethoxycarbonyl)ethyl]-O,O-dimethyl phosphorodithioate. Synonyms Carbofos, malathon Empirical formula C10H19O6S2P Structural formulaBIOLOGICAL DATA Biochemical aspects Malathion is rapidly absorbed from the intestinal tract. Its metabolism has been studied in the hen, mouse, rat, cow and man. Malathion is oxidized to malaoxon, the active form of the compound, and is also hydrolysed to less toxic metabolites. Six to eight metabolites have been found, the main ones being in the urine, malathion mono- and di-acids. In eggs from treated hens and milk from cows treated with malathion, malathion or its metabolites were recovered (O'Brien, 1960; Heath, 1961). Malaoxon is a cholinesterase inhibitor in vivo and in vitro (I50 7 × 10-7) (Heath, 1961). The half time for the conversion in vivo of the reversibly inhibited form of the dimethylphosphorilated cholinesterase to the irreversibly inhibited form of this enzyme in the brain of chicken given malathion has been found to be 2 hours. The same half time was observed in vitro with the brain homogenate inhibited with paraoxon (Witter & Gaines, 1964). Simultaneous administration of malathion and ethyl p-nitrophenyl thionobenzenephosphate (EPN) results in a potentiation of the cholinesterase inhibitory effect of malathion in the mouse, rat and dog (Frawley et al., 1957; O'Brien, 1960). In a colony of rats showing an oral LD50 of 925 mg/kg for adults, the intragastric LD50 for a day-old rat was approximately 124 mg/kg (Lu et al., 1965). Short-term studies Mouse. When malathion was added to the diet as 500 or 5000 ppm for 6 weeks or after the administration of 5 oral doses of 500 mg/kg the production of antibodies against B. Pertussis was not affected (Benes et al., 1963). Rat. Groups of 10 males were given Malathion at 100 or 500 ppm in the diet or dipterex at 60 or 300 ppm for 6 weeks and this was followed by the administration of both compounds at the same time. During the experiment erythrocyte cholinesterase fluctuated around 100% of the initial values. At the end of the experiment, in comparison with the control group, the adrenals weighed more and showed hypertrophy of both cortex and medulla, the intensity of which was related to the concentration of the two substances in the diet (Benes & Cerna, 1965). In another experiment 95% technical malathion was fed to 3 groups of male rats, 10 animals per group, for 33 days at the levels of 100, 1000 and 5000 ppm. No sign of toxicity was observed, nor any deaths. Food intake and weight gain in the groups fed 100 and 1000 ppm were higher than in the control group; groups fed 5000 ppm showed no difference from the controls. Cholinesterase activity was determined in 6 animals from each group. Activity was normal in the 100 ppm group. Erythrocyte cholinesterase activity was 68% of normal in the 1000 ppm group, and in the 5000 ppm group plasm cholinesterase activity was 78% and erythrocyte activity 22% of normal. At all levels no depression of brain cholinesterase activity was found (American Cyanamid Co., 1955). Acute toxicity Animal Route LD50 mg/kg References body-weight 90% 99% technical technical Rat, male Oral 940-1156* 4700-5843* American Cyanamid Co., 1955 Hazelton & Holland, 1953 Rat, male Oral 390-480* 1400-1845* American Cyanamid Co., 1955 Frawley et al., 1957 Hazleton & Holland, 1953 Mouse, male Oral 720-886 3300-4060 American Cyanamid Co., 1955 Hazleton & Holland, 1953 Mouse, male Oral 2700-3320 American Cyanamid Co., 1955 Hazleton & Holland, 1953 Mouse, male Intraperitoneal 420.474 Hazleton & Holland, 1953 Chicken Oral >850 (95%) American Cyanamid Co., 1955 Calf Oral 80 (95%) American Cyanamid Co., 1955 Cow Oral 560 (95%) American Cyanamid Co., 1955 * Differences due to use of different vehicles. Ninety-eight per cent. technical malathion was fed to groups of 5 rats for 8 weeks at levels of 100 and 500 ppm without any inhibition of whole-blood cholinesterase activity (Frawley et al., 1957). Ninety-five per cent. technical malathion was fed to 40 male and 40 female rats for 5 months in a daily dose of 240 mg/kg body-weight (4000 ppm in the diet). Growth was normal and no signs of intoxication occurred. Ten wake after the beginning of the experiment, 18 females and 12 males were used for breeding. The average litter size from the treated females was smaller than in the controls and the number of newborn alive after 7 and 21 days was about half the number in the litters of the controls (Kalow & Marton, 1961). Chick. 95% technical malathion was fed to day-old chicks for 2 weeks at a level of 10 ppm. For the following 10 weeks they were divided into groups of 10 and fed 100, 1000 and 5000 ppm in their diets. The groups on 100 and 1000 ppm behaved normally and showed a similar growth rate and food consumption to the controls. Four animals died in the 5000 ppm group, and signs of intoxication and growth retardation were observed. At autopsy, no pathological lesions were found. Plasm and brain cholinesterase activity were significantly lowered in the 5000 ppm group (American Cyanamid Co., 1955). In a two-year study, 21 females were fed 250 ppm and 21 females and 6 males 2500 ppm. The 250 ppm group did not differ significantly from the controls. At the 2500 ppm level a decrease in plasma cholinesterase activity was found between the 195th and 465th day of experiment. The hens came later into production and laid slightly fewer eggs, but the hatchability was not influenced. The offspring showed no deformities. At autopsy no macro- or microscopical lesions were found (American Cyanamid Co., 1960). Man. Five male volunteers, 23-36 years old, took 8 mg of malathion in gelatin capsules daily for 32 days. No effect on plasma or erythrocyte cholinesterase activity could be detected. Five males took 16 mg daily for 47 days, also without any significant effect on cholinesterase activity. A daily dose of 24 mg taken by 5 males for 56 days was followed by depression of the plasma cholinesterase activity 2 weeks after the first administration. Maximum depression amounting to about 25% of the plasma cholinesterase activity occurred approximately 3 weeks after the cessation of administration. No clinically manifest side-effects were reported. Simultaneous intake of 16 mg of malathion and 5 mg of EPN per day caused a light inhibition of cholinesterase activity (Moeller & Rider, 1962). No plasma or RBC cholinesterase depression was noted, in 10 humans ingesting 3 mg EPN or 8 mg malathion daily for 32 days, nor in 5 humans receiving 6 mg EPN for 88 days and 8 mg malathion for the last 44 days, nor in 5 humans ingesting 16 mg malathion for 88 days and 3 mg EPN for the last 41 days. However 10 humans ingesting 6 mg EPN and 16 mg malathion daily for 42 days showed a slight depression of both the plasma and the RBC cholinesterase (Rider et al., 1959). Long-term studies Rat. 65% technical malathion as a 10% or 25% wettable powder was mixed in the diets of groups of 20 male rats at the levels 100, 1000 and 5000 ppm, and fed for 2 years. The mortality rate was not influenced, and at the 2 lower levels weight gain and food intake were comparable to those of the controls. Five thousand ppm reduced food intake and decreased weight gain. Cholinesterase determinations showed no inhibition at the 100 ppm level; with a diet containing 1000 ppm, 36% inhibition of cholinesterase activity was found in the plasma, 73% in the erythrocytes and 37% in the brain, while at the 5000 ppm level, the plasma samples showed 80%, the erythrocytes 100% and the brain 77% inhibition. At autopsy neither gross nor microscopic examination revealed any pathological changes attributable to malathion (American Cyanamid Co., 1955; Hazleton & Holland, 1953). Ninety per cent. technical malathion was fed as 25% wettable powder in the diet to 20 males at a concentration of 100 ppm, to 20 males and 10 females at 1000 ppm, and to 20 males at 5000 ppm for 2 years. Mortality rate, growth response and food intake were not influenced by any of these diets, except that there was some growth retardation when the concentration was 5000 ppm. Terminal cholinesterase determinations revealed 10.30% inhibition of cholinesterase activity in the plasma, erythrocytes and brain at 100 ppm. At 1000 ppm, 60-95% inhibition of erythrocyte cholinesterase activity was observed. The 5000 ppm group showed total inhibition of erythrocyte cholinesterase activity and 60-95% inhibition of cholinesterase activity in plasma and brain (American Cyanamid Co., 1955; Hazleton & Holland, 1953). Ninety-nine per cent. technical malathion was fed for 2 years to groups of 3-4 rats and produced, at 1000 and 5000 ppm levels, inhibition of erythrocyte cholinesterase activity of the same order as did the 90% compound. The decrease in plasma and brain cholinesterase activity, however, was much less than that produced by 90% technical malathion (American Cyanamid Co., 1955, Hazleton & Holland, 1953). Comments on experimental studies reported The studies are extensive and have been carried out in several species including man. In view of the very high doses used in the short-term breeding experiments in the rat, the results of these experiments were not taken into account in arriving at the maximum acceptable daily intake for man. EVALUATION Level causing no significant toxicological effect in the rat and man Rat. 100 ppm in the diet, equivalent to 5 mg/kg body-weight per day. Man. 16 mg a day, equivalent to 0.2 mg/kg body-weight per day. Estimate of acceptable daily intake for man 0-0.02 mg/kg body-weight. Further work considered desirable Reproduction studies in rats. REFERENCES American Cyanamid Company, New York (1955) Report on Malathion American Cyanamid Company, New York (1960) Malathion pharmacology and toxicology (Unpublished data) Benes, V. & Cerna, V. (1965) Czech. Hyg., 10 (In press) Benes, V., Pekarek, J. & Cerna, V. (1963) Czech. Hyg., 8, 3 Frawley, J. P., Fuyat, H. N., Hagan, E. C., Blake, J. R. & Fitzhugh, O. G. (1957) J. Pharmacol, exp. Ther., 121, 96 Hazleton, L. W. & Holland, E. G. (1953) Arch. industr. Hyg., 8, 399 Heath, D. F. (1961) Organophosphorus poisons, Pergamon Press Kalow, W. & Marton, A. (1961) Nature, 192, 464 Lu, F. C., Jessup, D. C. & Lavallée, A. (1965) Food & Cosmetics Toxicol., 3 (In press) Moeller, H. C. & Rider, J. A. (1962) Toxicol. Appl. Pharmacol., 4, 123 O'Brien, R. D. (1960) Toxic phosphorus esters, Academic Press Rider, J. A., Moeller, H. C., Swader, T. & Devereaux, R. J. (1959) Chemical Res., 7, 81 Witter, R. F. & Gaines, T. B. (1964) Biochem. Pharmacol., 12, 1421
See Also: Toxicological Abbreviations Malathion (ICSC) Malathion (FAO/PL:CP/15) Malathion (FAO/PL:1967/M/11/1) Malathion (JMPR Evaluations 2003 Part II Toxicological) Malathion (FAO/PL:1968/M/9/1) Malathion (FAO/PL:1969/M/17/1) Malathion (AGP:1970/M/12/1) Malathion (WHO Pesticide Residues Series 3) Malathion (WHO Pesticide Residues Series 5) Malathion (Pesticide residues in food: 1977 evaluations) Malathion (Pesticide residues in food: 1984 evaluations) Malathion (Pesticide residues in food: 1997 evaluations Part II Toxicological & Environmental) Malathion (IARC Summary & Evaluation, Volume 30, 1983)