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    PESTICIDE RESIDUES IN FOOD - 1997


    Sponsored jointly by FAO and WHO
    with the support of the International Programme
    on Chemical Safety (IPCS)




    TOXICOLOGICAL AND ENVIRONMENTAL
    EVALUATIONS 1994




    Joint meeting of the
    FAO Panel of Experts on Pesticide Residues
    in Food and the Environment
    and the
    WHO Core Assessment Group 

    Lyon 22 September - 1 October 1997



    The summaries and evaluations contained in this book are, in most
    cases, based on unpublished proprietary data submitted for the purpose
    of the JMPR assessment. A registration authority should not grant a
    registration on the basis of an evaluation unless it has first
    received authorization for such use from the owner who submitted the
    data for JMPR review or has received the data on which the summaries
    are based, either from the owner of the data or from a second party
    that has obtained permission from the owner of the data for this
    purpose.



    METHIDATHION (addendum)

    First draft prepared by
    A. Moretto
    Istituto di medicine del Lavoro Universita degli Studi di Padova
    Padua, Italy

         Explanation 
         Evaluation for acute reference dose 
              Acute toxicity 
              Observations in humans 
         Comments
         Toxicological evaluation relevant for establishing an acute
         reference dose 
         References 

    Explanation

         Methidathion was last evaluated toxicologically by the 1992 JMPR
    (Annex 1, reference 65)when an ADI of 0-0.001 mg/kg bw was established
    on the basis of an NOAEL of 0.14 mg/kg bw per day in three-month,
    one-year, and two-year studies in dogs. The ADI was based on studies
    in which effects on the liver were observed. The CCPR requested the
    JMPR to establish an acute reference dose for methidathion in view of
    its high acute toxicity. New studies submitted for review were
    evaluated by the present Meeting.

    Evaluation for acute reference dose

    1.  Acute toxicity

     Rats

         Groups of three to six male and female Crl:CD(SD)BR rats were
    given technical-grade methidathion (purity, 93.2%) as single doses of
    0, 4, 8, 16, 20, 25, or 30 mg/kg bw by gavage in corn oil. The acute
    oral LD50 was about 25 mg/kg bw. An abbreviated functional
    observational battery of tests, including observations of mortality,
    signs in the home cage, during handling, and in the open field, and
    righting reflexes in air, were conducted. Signs consistent with
    cholinergic poisoning (e.g. lachrymation, salivation, diarrhoea,
    tremors, ataxia, and muscle fasciculations) were observed in males at
    doses > 8 mg/kg bw and in females at doses > 16 mg/kg bw. Two of
    three females at 30, one of five males and three of six females at 25,
    and one of six males at 20 mg/kg bw died. The NOAEL for signs of
    cholinergic overstimulation was 4 mg/kg bw, and the highest non-lethal
    dose was 20 mg/kg bw, when the estimated time to peak effect for toxic
    signs was 2 h. Males recovered faster than females, and many of the
    signs had abated 4-6 h after treatment (Leahy, 1993).

         Groups of five male and five female Crl:CD BR VAF/Plus rats were
    given technical-grade methidathion (purity, 93.2%) as single doses of
    0 or 0.5-35 mg/kg bw in corn oil by gavage. Body weights, clinical
    signs, and liver and brain weights were recorded. Serum, erythrocyte,
    and brain chlinesterase activities were measured 4 h after treatment.
    All males at 35 mg/kg were dead by day 3, and four females at 20 mg/kg
    died within one day. The signs of cholinergic overstimulation included
    decreased activity, tremors, absence of pain reflex, excessive
    salivation, dyspnoea, lachrymation, red-stained face, discoloured
    urogenital area, soft stools, few faeces, meiosis, and fasciculations
    and were observed in animals treated with doses > 5 mg/kg bw. The
    peak effect was seen 2 h after treatment. There was no meaningful
    effect on body-weight gain or on liver or brain weights. The NOAEL for
    cholinergic signs was 3 mg/kg bw.

         In a subsequent phase, the doses administered were 0, 0.5, 1,
    2.5, 5, or 10 mg/kg bw, and cholinesterase activity was measured in
    plasma, erythrocytes, and brain about 4 h after treatment. The results
    are shown in Table 1. Thus, single oral doses of methidathion caused
    acute signs and cholinesterase inhibition. The NOAEL for
    cholinesterase inhibition was 1 mg/kg bw (Glaza, 1994).

         Crl:CD-BR VAF/Plus rats were given single doses of
    technical-grade methidathion (purity, 93.2%) at 0, 1, 4, 8, or 16
    mg/kg bw in corn oil by gavage. As a positive control, carbaryl was
    administered at 30 mg/kg intraperitoneally. Body weights, food
    consumption, and clinical signs were recorded. A complete functional
    observational battery of tests and figure-eight maze activity counts
    were performed before treatment, at the time of peak effect (1.5 h
    after treatment with methidathion and 0.5 h after carbaryl), and one
    and two weeks after treatment. Serum and erythrocyte cholinesterase
    activities were measured before treatment, at the time of peak effect,
    and 14 days after treatment. Acetylcholinesterase activity in regions
    of the brain (cerebellum, striatum, and cerebral cortex or
    hippocampus) were measured at the time of peak effect and one and two
    weeks after treatment. Necropsies were performed, and selected nervous
    tissues examined histologically. 

         All animals survived. In comparison with controls, the
    body-weight gain of males at 16 mg/kg bw was decreased. Food
    consumption was decreased in animals of each sex at this dose during
    the first week. Changes in clinical signs and in the results of
    functional observational tests and maze activity were seen only at the
    time of peak effect at doses > 8 mg/kg bw in males and > 4 mg/kg
    bw in females. The changes consisted of typical cholinergic signs and
    decreased body temperature. Similar changes were seen in
    carbaryl-treated animals. The cholinesterase activities at the time of
    peak effect and two weeks after treatment are reported in Table 2.
    Acetylcholinesterase activity in the cerebral cortex or hippocampus
    was inhibited in males at 1 mg/kg bw; however, this effect was not
    considered to be of toxicological significance because no inhibition
    was seen in acetylcholinesterase activity in erythrocytes or in other
    regions of the brain at this dose, the activity in controls was


        Table 1. Mean cholinesterase activity (mU/ml, standard deviation in parentheses) in rats 4 h after treatment 
    with methidathion

                                                                                                            

    Dose           Males                                        Females
    (mg/kg bw)                                                                                              
                   Plasma         Erythrocytes   Brain          Plasma         Erythrocytes   Brain
                                                                                                            

    0              316 (53)       1411 (145)     1593 (260)     821 (128)      944 (126)      1659 (63)
    0.5            332 (139)      1329 (268)     1715 (152)     857 (181)      1107 (170)     1549 (56)
    1.0            300 (41)       1282 (386)     1687 (178)     800 (186)      979 (126)      1626 457)
    2.5            282 (26)       1177 (328)     1676 (423)     857 (206)      769 (48)       1016 (102)*
    5.0            186 (19)*      1119 (238)     1138 (249)*    668 (109)      559 (97)*      605 (98)*
    10.0           169 (26)       1037 (190)     744 (91)*      477 (77)*      618 (t3)*      477 (135)*
                                                                                                            

    * Statistically significant in comparison with the control group, Dunnett's  t test,  p < 0.05

    Table 2. Mean cholinesterase activity (g/ml) after administration of methidathion to rats, at the time of peak 
    effect (1.5 h after treatment) and at week 2

                                                                                                                      

    Dose           Serum               Erythrocytes        Cerebellum          Cortex              Striatum
    (mg/kg bw)                                                                                                        
                   1.5 h     2 weeks   1.5 h     2 weeks   1.5 h     2 weeks   1.5 h     2 weeks   1.5 h     2 weeks
                                                                                                                      
    Males
    0              390       359       1530      1488      3.13      3.31      8.22      6.24      25.32     27.95
    1              392       415       1482      1516      2.74      3.20      4.81**    8.07      27.00     30.80
    4              316       398       646**     968*      1.48**    3.26      2.60**    8.09      7.08**    37.60
    8              272**     462       394**     1224      0.99**    3.09      1.01**    6.50      4.12**    19.12
    16             231**     394       248**     1084      0.69**    2.79      0.51**    5.94      2.32**    17.88
    Females
    0              1128      1225      1852      1548      3.10      3.10      7.78      7.26      28.96     15.36
    1              874       1128      1690      1456      2.69      2.95      6.74      7.11      26.52     35.84
    4              858       1237      610**     1440      1.22**    2.95      2.26**    7.41      7.76**    17.92
    8              863       1370      336**     1208      0.77**    2.84      0.82**    6.63      2.32**    28.68
    16             800       1320      260**     1252      0.53**    2.81      0.50**    6.16      1.52**    21.52
                                                                                                                      

    10 animals at 1.5 h; 5 animals at 2 weeks
    Statistically significant in comparison with the control group, Dunnett's  t test: *  p < 0.05, **  p < 0.01
    

    relatively high; the measurements were variable, and there were no
    detectable clinical signs of cholinergic dysfunction. There were no
    treatment-related findings at necropsy or histopathologically in the
    central or peripheral nervous system. It is concluded that single oral
    doses of methidathion caused acute neurobehavioural effects and
    cholinesterase inhibition but no histopathological lesions. The
    overall NOAEL was 1 mg/kg bw (Chang & Richter, 1994).

    2.  Observations in humans

         No inhibition of plasma or erythrocyte cholinesterase activity or
    any other effect was observed in groups of eight men who took daily
    oral doses of 0.04 or 0.11 mg/kg bw methidathion in capsules for six
    weeks. No low-observed-adverse-effect level (LOAEL) was determined
    because cholinesterase was not inhibited (Coulston, 1970; reviewed in
    Annex 1, reference 67).

         In addition to two cases of massive poisoning (Teitelmann et al.,
    1975; Zoppellari et al., 1990) reviewed by the 1992 JMPR (Annex 1,
    reference 67), a third clinical case of a massive overdose was
    described in the literature. A 52-year-old man ingested an estimated
    31 g of a commercial product containing methidathion. He was found
    comatose and was treated with gastric lavage, pralidoxime, atropine,
    and prolonged mechanical ventilation. Six hours after ingestion, his
    plasma cholinesterase activity was 545 IU/ml (normal range, 5500-13
    500). The signs had disappeared by day 8, and treatment was
    discontinued; however, the patient showed relapse of cholinergic
    symptoms, and the treatment was resumed. The patient recovered after
    22 days. An electrophysiological study performed on day 15 showed some
    evidence of axonal polyneuropathy (no details reported). On day 30,
    the plasma cholinesterase activity was still low, at 3390 IU/ml. Three
    months after the incident, slight indication of peripheral neuropathy
    persisted, as evidenced by hyporeflexia and distal hypoaesthesia. The
    plasma cholinesterase activity had returned to 6150 IU/ml by day 90
    (Cantais et al., 1993).

         In the case reported by Zoppellari et al. (1990, reviewed in
    Annex 1, reference 67), there was slight liver involvement, with
    transient jaundice (4.3 mg% of total bilirubinas compared with normal
    values up to 1.2 mg%) but no other biochemical indication of liver
    damage.

    Comments

         The NOAEL for behavioural changes in rats after a single oral
    dose was 3 mg/kg bw. The NOAEL for the inhibition of brain
    acetylcholinesterase activity measured 4 h after treatment was 1 mg/kg
    bw in males and 2.5 mg/kg bw in females.

         In another study of acute neurotoxicity in rats, changes in
    clinical signs, the results of a battery of functional observational
    tests, and maze activity were observed at the time of peak effect
    (about 2 h after treatment) at doses of 8 mg/kg bw and above in males
    and 4 mg/kg bw and above in females. Inhibition of
    acetylcholinesterase activity in various regions of the brain was
    found at doses of 4 mg/kg bw and above. Reduced acetylcholinesterase
    activity in the cortex and hyppocampus of a male treated with 1 mg/kg
    bw was not considered to be relevant. The overall NOAEL in this study
    was 1 mg/kg bw.

         In a human case of methidathion poisoning,, the estimated dose
    was more than 10 times the LD50 in rats. The patient recovered from
    the cholinergic toxicity and developed mild neuropathy; however, no
    details were given.

         The hepatic changes observed in short-term studies in dogs were
    not considered relevant for establishing an acute reference dose for
    humans, as no such changes were observed in the volunteer studies
    after repeated dosing for up to six weeks. Furthermore, no indication
    of hepatic toxicity (except slight, transient jaundice in one case)
    was seen in three cases of massive oral overdose which required
    atropine and oxime administration and assisted ventilation.

         The Meeting established an acute reference dose of 0.01 mg/kg bw,
    on the basis of an NOAEL in humans for inhibition of erythrocyte
    acetylcholinesterase activity of 0.11 mg/kg bw (highest dose tested)
    in a study reviewed by the 1992 JMPR (Annex I, reference 65), and a
    safety factor of 10. This value is supported by the NOAEL of 1 mg/kg
    bw in rats for the inhibition of brain acetylcholinesterase activity.

    Toxicological evaluation relevant for establishing an acute reference 
    dose

     Levels that cause no toxic effect

         Rat:      1 mg/kg bw (single oral administration, inhibition of
                   brain acetylcholinesterase activity)

         Human:    0.11 mg /kg bw per day (six-week study in volunteers,
                   highest dose tested)

     Estimate of acute reference dose for humans

         0.01 mg/kg bw

    References

    Cantais, I., Escarment, J., Ledantec, P., Deloffre, I., Stephannazi,
    J. & Quinot, J.-F. (1993) Poisoning by an organophosphate:
    Methidathion.  JEUR, 6, 100-102 (in French).

    Chang, J.C.F. & Richter, A.G. (1994) Acute neurotoxicity study with
    methidathion technical in rats. Unpublished report No. F-00178, dated
    15 February 1994, amended 22 February 1994, from Ciba-Geigy Corp.,
    Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop
    Protection AG, Basel, Switzerland.

    Glaza, S.M. (1994) Acute oral toxicity study of methidathion technical
    in rats. Unpublished report No. HWI 6117-235, dated 1 April 1994,
    amended 4 May 1994, from Hazleton Wisconsin, Madison, Wisconsin, USA.
    Submitted to WHO by Novartis Crop Protection AG, Basel, Switzerland.

    Leahy, C.L. (1993) Acute range finding neurotoxicity study with
    methidathion technical in rats. Unpublished report No. F-00177, dated
    7 December 1993, amended 22 February 1994, from Ciba-Geigy Corp.,
    Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop
    Protection AG, Basel, Switzerland.

    Teitelman, U., Adler, M., Levy, I. & Dikstein, S. (1975) Treatment of
    massive poisoning by the organophosphate methidathion.  Clin. 
     Toxicol., 8, 277-282.

    Zoppellari, R., Targa, L., Tonini, P. & Zatelli, R. (1990) Acute
    poisoning with methidathion: A case.  Hum. Exp. Toxicol., 9, 415-419.
    


    See Also:
       Toxicological Abbreviations
       Methidathion (ICSC)
       Methidathion (WHO Pesticide Residues Series 2)
       Methidathion (WHO Pesticide Residues Series 5)
       Methidathion (Pesticide residues in food: 1979 evaluations)
       Methidathion (Pesticide residues in food: 1992 evaluations Part II Toxicology)