DIMETHOATE EXPLANATION Dimethoate was evaluated for acceptable intake by the JMPR in 1963 and was also reviewed in 1965, 1966, 1967 and 1984 (Annex 1, FAO/WHO, 1964, 1965a, 1967a, 1968a, 1985a). In the absence of a complete data base the 1984 JMPR meeting replaced the ADI of 0-0.2 mg/kg bw. (established at the 1967 meeting, based on a no observed effect level in humans of 0.2 mg/kg/day for plasma cholinesterase) by a temporary ADI at a lower level (0-0.002 mg/kg bw.). The following investigations were required by 1987: 1. Submission of the on-going toxicological studies sponsored by the "Dimethoate Task Force". 2. A dog study of at least six month's duration. The on-going studies have become available and have been summarized in this monograph addendum. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOLOGICAL DATA Toxicological Studies Special studies on carcinogenicity See under long-term studies Special studies on teratogenicity Rats Groups of 25 pregnant female rats (CrL: COBS CD (SD) BR strain) received 0, 3, 6 and 18 mg dimethoate (purity 97.3%)/kg bw. by gavage from day 6-15 of gestation. The fetuses were delivered by caesarian section on day 20 of pregnancy. The dams were observed daily for clinical signs, mortality and body weight. The number of corpora lutea, implantations and live and dead fetuses were recorded. The fetuses were counted and weighed and examined for visceral and skeletal abnormalities. Salivation and hair loss was observed in females at all dose levels. Females of the highest dose group also had brown facial staining, small rounded feces (also at 6 mg/kg bw.), hypersensitivity, body tremors and unsteady gait. Body weight gain and food consumption were significantly reduced. No malformations were found. There was no obvious indication of an adverse effect of dimethoate treatment on mean incidences of both visceral and skeletal anomalies, although at all dosages an unspecific increase was observed relative to control values (which were, however, lower than concurrent control values in recent preceding studies) (Edwards et al., 1984) Rabbits Groups of 16 pregnant female rabbits received 0, 10, 20 and 40 mg dimethoate (purity 97.3%)/kg bw. in 1% methylcellulose by gavage on day 7-19 of gestation. The fetuses were delivered by caesarian section on day 29 of pregnancy. The dams were observed daily for clinical signs, mortality and body weight. The number of corpora lutea, implantations and live and dead fetuses were recorded. The fetuses were weighed and sexed and examined for visceral and skeletal abnormalities. Muscle tremors, an unsteady gait, and a increased incidence of animals showing cold ears after dosing and abnormal feces were observed at 40 mg/kg. During dosing body weight gain and food consumption were significantly reduced at 40 mg/kg. A slightly lower body weight gain was observed at 20 mg/kg, while at 10 mg/kg mean weight gain was higher. Fetal weight was significantly decreased at 40 mg/kg, with a tendency at 20 mg/kg bw. Teratogenic effects were not observed (Edwards et al., 1984). Special studies on mutagenicity Dimethoate was not mutagenic in the studies considered in this monograph addendum (see table 1). Special studies on skin sensitization Technical dimethoate (97.3% purity) was not a skin sensitizer in guinea pigs when rested by the patch technique (Buehler test) (Madison, 1984). Long-term studies Mice Groups of 50 male and 50 female B6 C3 F1 mice were fed diets containing 0, 25, 100 and 200 ppm dimethoate (purity > 96.71%) for 778 weeks. Satellite groups of 10 male and 10 female mice were fed diets containing dimethoate at the same dietary levels and sacrificed after 51 weeks of treatment. Observations included clinical signs, mortality, body weight, food consumption, hematology and cholinesterase activities (only after 51 weeks) in plasma and erythrocytes. Organs were weighed and comprehensive histopathology was performed (from the satellite animals only those dying intercurrently or sacrificed in a moribund state). Retarded growth was observed in males at all dose levels during the first months of the study and in females at 200 ppm in the beginning. Towards the end of the study the body weights of the females at all dose levels were significantly increased. At all dose levels cholinesterase activity in plasma as well as in erythrocytes was significantly decreased in both sexes (at 25 ppm only 10 to 30% respectively in both sexes). The inhibition was greater for cholinesterase in erythrocytes in all cases. Absolute as well as relative ovary weight were significantly decreased in females of all dose groups at termination of the study. Other changes in organ Table 1. Special studies on the mutagenicity of dimethoate Type of test Test object Concentration Purity Results References of dimethoate In vitro CHO/HGPRT Chinese 1000 µg up to 97.3% Negative Johnson et al., mutation assay hamster 3500 µg/ml 1984 (with and without ovary metabolic activation) In vivo Dominant lethal NMRI mice 5, 10 and 20 mg/kg 96.89% Negative Becker, 1985 test (administered (1) orally, 5 days) Micronucleus CD-1 mice 55 mg/kg 97.3% Negative Sorg, 1985 test (administered i.p. (1) twice 24 hr apart) dissolved in 0.9% saline Metaphase Rat bone i.p. 15, 75 97.3% Negative San Sebastian, analysis assay marrow cells & 150 mg/kg (1) 1985 (1) Positive control yielded positive results weights were not consistent or dose related and are probably induced by the higher body weight at all dose levels (especially in females). At histopathology hepatocellular vacuolization was observed in males and females at 200 and 100 ppm. Extramedullary hematopoiesis in the spleen was observed in males at 200 ppm and slightly in males at 100 and 25 ppm and in females at all dose levels. No enhanced tumour incidence was observed (Hellwig, 1986a). Rats Groups of 50 male and 50 female Wistar rats were fed diets containing 0, 5, 25 and 100 ppm dimethoate (purity > 96.71%) for 24 months. Satellite groups of 15 male and 15 female rats received diets containing dimethoate at the same dietary levels and were used, together with a satellite group of 20 rats/sex receiving a diet containing 1 ppm dimethoate, for hematological, clinico-chemical and urinalysis examinations. All satellite groups received the diet for 24 months also. Observations included clinical signs, mortality, food consumption, body weight, ophthalmoscopy, hematology, clinical chemistry, cholinesterase activities in plasma, erythrocytes and brain and urinalysis. Surviving animals were sacrificed after 24 months. Organs were weighed and comprehensive histopathological examinations were made. Towards the end of the study mortality was increased in females at 100 ppm. Body weight gain was decreased in males and females at the highest dose group during the first 12 months. Hemoglobin and hematocrit values were significantly decreased in males and females at 100 ppm (tendency in males at 25 ppm) and mean corpuscular hemoglobin concentration was increased. The number of erythrocytes was significantly decreased in males at 100 ppm (tendency at 25 ppm) and the number of leucocytes was significantly increased in females at 100 and 25 ppm and in males at 100 ppm only. Plasma cholinesterase activity was significantly decreased (about 50%) in male and female rats at 100 and cholinesterase activity in erythrocytes and the brain were significantly decreased at 5 (males only), 25 and 100 ppm in both sexes (erythrocytes: max. 35%, 55% and 90%, respectively; brain: max. 23%, 40% and 60% respectively). In females, potassium values in blood were significantly decreased at 100 and 25 ppm and alanine amino-transferase activity was significantly increased at 100 ppm. In males of the same dose group, total protein was significantly increased and relative ovary weight in females was significantly decreased. No enhanced tumor incidence was observed. The NOAEL in this study is 1 ppm, equivalent to 0.05 mg/kg bw (Hellwig, 1986b). COMMENTS Toxicology The 1984 JMPR replaced the ADI of 0.02 mg/kg bw by a temporary ADI of 0.002 mg/kg bw because the data on which the former ADI was based were considered inadequate. The meeting was aware that new toxicological studies were under way. These studies were reviewed by the present meeting. Maternal toxicity and fetotoxicity were observed in teratogenicity studies in rats and rabbits, but no teratogenic effects. In 1984 the Joint Meeting concluded that dimethoate was mutagenic in a limited number of in vitro and in vivo studies reported in the literature. Since then, additional negative mutagenicity data have become available. It was concluded that dimethoate is mutagenic in bacterial tests, but not in mammalian cells or in in vivo tests. In long-term toxicity/carcinogenicity studies with mice and rats no indication of carcinogenicity was found. In the mouse study the lowest dose of 25 ppm produced decreased body weight and decreased cholinesterase activities in erythrocytes as well as slight extramedullary hematopoiesis in the spleen. Two years' administration of dimethoate to rats also revealed a decrease in body weight, decreased cholinesterase activities (in erythrocytes and brain) and slight anemia. No effects were observed at 1 ppm (equivalent to 0.05 mg/kg bw/day). The required dog study was not available. The 1984 evaluation of human data was based on the understanding that plasma CHE activity was measured. Re-examination of the data showed that they were in fact based on erythrocyte CHE and that a NOAEL was present. The present meeting therefore decided that an ADI could be established on the basis of these data. TOXICOLOGICAL EVALUATION LEVEL CAUSING NO TOXICOLOGICAL EFFECT Rat: 1 ppm in the diet, equivalent to 0.05 mg/kg bw/day Man: 0.2 mg/kg bw/day ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN 0-0.01 mg/kg bw. STUDIES WHICH WILL PROVIDE INFORMATION VALUABLE IN THE CONTINUED EVALUATION OF THE COMPOUND 1. Observations in man. 2. A one-year dog study. 3. An up-to-date reproduction study in rats. REFERENCES Becker, H., 1985. Dominant lethal study with dimethoate technical in the mouse. Unpublished report no.: 039003 d.d. September 12, 1985 from RCC, Research & consulting Company AG, CH 4452 Itingen/Switzerland. Submitted to WHO by Dimethoate Task Force, Milano, Italy. Edwards, J.A., Leeming, N.M. & Clark, R., 1984. Effect of dimethoate on pregnancy of the rat. Unpublished report no.: DTF 3/84245 d.d. April 13, 1984 from Huntington Research Centre. Submitted to WHO by Dimethoate Task Force, Milano, Italy. Edwards, J.A., et al., 1984. Effect of dimethoate on pregnancy of the New Zealand white rabbit. Unpublished report no.: DTF 4/84247 d.d. April 19, 1984 from Huntington Research Centre. Submitted to WHO by Dimethoate Task Force, Milano, Italy. Johnson, E., Caterson, C.R. & Allen, J.S., 1985. Mutagenicity testing of dimethoate in the in vitro CHO/HGPRT mutation assay. Unpublished report No. 0423 d.d. January 30, 1985 from American Cyanamid Co., Princeton, New Jersey. Submitted to WHO by Dimethoate Task Force, Milano, Italy. Hellwig, J., 1986a. Report on the study of the toxicity of dimethoate in mice after 78-week administration in the diet. Unpublished report no.: 75CO326/8242 from BASF, Department of Toxicology, Ludwigshafen/ Rhein, FRG, Submitted to WHO by Dimethoate Task Force, Milano, Italy. Hellwig, J., 1986b. Report on the study of the toxicity of dimethoate in rats after 24-month administration in the diet. Unpublished report no.: 70CO326/8241 d.d. October 9, 1986 from BASF, Department of Toxicology, Ludwigshafen/Rhein, FRG, Submitted to WHO by Dimethoate Task Force, Milano, Italy. San Sebastian, J.R., 1985. In vivo bone marrow cytogenetics rat metaphase analysis. PH 315-AC-00-84, Dimethoate CL 12880. Unpublished report d.d. August 29, 1985 from Pharakon Research International, Inc., Waverly, PA 18471. Submitted to WHO by Dimethoate Task Force, Milano, Italy. Sorg, R.M., 1985. Micronucleus Test (MNT). PM 309A-AC-004-84, Dimethoate CL 12880. Unpublished report d.d. March 1, 1985 from Pharakon Research International, Inc., Waverly, PA 18471. Submitted to WHO by Dimethoate Task Force, Milano, Italy.
See Also: Toxicological Abbreviations Dimethoate (EHC 90, 1989) Dimethoate (HSG 20, 1988) Dimethoate (ICSC) Dimethoate (FAO Meeting Report PL/1965/10/1) Dimethoate (FAO/PL:CP/15) Dimethoate (FAO/PL:1967/M/11/1) Dimethoate (JMPR Evaluations 2003 Part II Toxicological) Dimethoate (AGP:1970/M/12/1) Dimethoate (Pesticide residues in food: 1983 evaluations) Dimethoate (Pesticide residues in food: 1984 evaluations) Dimethoate (Pesticide residues in food: 1984 evaluations) Dimethoate (Pesticide residues in food: 1996 evaluations Part II Toxicological)