Sponsored jointly by FAO and WHO


    Joint meeting of the
    FAO Panel of Experts on Pesticide Residues
    in Food and the Environment
    and the
    WHO Expert Group on Pesticide Residues
    Geneva, 3-12 December 1979



    Omethoate was evaluated in 1971 and reviewed in 1975 and 1978.  The
    ADI established in 1971 was a temporary basis because no long-term
    studies were available.  Data on subacute dermal toxicity, acute and
    subacute inhalation toxicity and on pharmacokinetics were received and
    are reviewed, and summarized in this monograph addendum.


    Absorption, Distribution and Elimination

    The distribution and elimination of 14C-omethoate was investigated in
    the rat.  After oral administration of 0, 3, 5 or 10 mg/kg omethoate,
    96-97% of the radioactivity was eliminated within 48 hours in urine
    and 1-2% in faeces.  Over this period, about 1% of the radioactive
    dose was exhaled as 14CO2.  Intravenous injection of 0.3 mg/kg
    resulted in a similar rapid elimination pattern.  Maximal tissue
    residues were reached one hour after administration.  After the first
    eight hours, about 18% of the residual radioactivity was observed in
    the body. After 2 days, less than 0.55% of the administered dose was
    observed. From quantitative data and whole body autoradiography, a
    relatively homogeneous distribution of 14C-activity was observed,
    with the exception of the thyroid, in which a 10-20 fold higher
    concentration was found (Weber, et al., 1978).


    Acute Inhalation Toxicity

    Groups of 10 Wistar rats of each sex were exposed by inhalation to
    aerosols of omethoate and observed for 14 days.  Estimates of the
    LC50 are summarized in Table 1.

    Table 1.  Exposure by inhalation of rats to aerosols of omethoate


    Exposure time       LC50 (mg/m3)  Lowest concentration causing
                                          clinical signs of poisoning

    1 hour              1000                        338
    4 hours             300 (male)                 <60
                        220 (female)                60
    5 × 4 hours         <49                         13

    Clinical signs of poisoning were observed for up to 10 days after
    exposure.  Acetylcholinesterase depression in plasma and erythrocytes,
    measures in the multiple-exposure experiment, was found to be
    dose-related.  Cholinesterase activity was depressed at all dose
    levels tested.  Three days after the last exposure, no enzyme
    inhibition was noted in plasma.  This recovery was not noted with
    erythrocyte cholinesterase.  No treatment-related pathological effects
    were observed in the lungs (Thyssen, 1978).

    Subacute Dermal Toxicity

    Groups of rabbits (3 male and 3 female rabbits per group) were
    dermally administered 0, 2.5 or 20 mg/kg body weight omethoate in
    aqueous solutions for three weeks (7 hours/day, 5 days/week) to the
    intact or abraded skin.  No dose-related effects were observed on body
    weight or a variety of clinical chemistry, hematological, or
    urological parameters.  The gross weight of tissues and organs (heart,
    lungs, liver, spleen, kidneys, adrenals, thyroid, testes and ovaries)
    did not deviate from control values.  Pathological examination of
    these organs, as well as of the uterus, epididymis, and skin showed no
    changes attributable to dermal omethoate treatment.

    Inhibition of acetylcholinesterase activity in plasma, erythrocytes,
    and brain was observed in those rabbits receiving 20 mg/kg.  After 1-3
    days of treatment, the enzyme depression was accompanied by clinical
    signs of poisoning in animals with abraded skin.  In the lower dose
    group, only females with abraded skins showed an inhibition of brain
    cholinesterase activity (Flucke and Luckhaus, 1979).


    Additional information was reviewed confirming that omethoate-induced
    inhibition of cholinesterase is the most predominant toxicological
    parameter as was previously noted in acute and subacute experiments.

    Omethoate is rapidly absorbed following oral administration, and
    elimination is complete within a few days.  The residual
    concentrations found in specific organs of the body decreased very
    rapidly after administration, with the exception of the thyroid gland,
    in which elimination was relatively slow.

    The meeting was informed that the previously required long-term study
    was complete, and the report was to be made available shortly.  The
    existing temporary ADI was therefore extended.


    Level Causing No Toxicological Effect

    Rat:  1.0 ppm in the diet, equivalent to 0.05 mg/kg body weight
    Dog:  1.6 ppm in the diet, equivalent to 0.12 mg/kg body weight


    0-0.0005 mg/kg body weight


    Required by June 30, 1980:

    Report of the long-term study.


    Information on the possible accumulation of omethoate in the thyroid.


    Flucke, W. and Luckhaus, G. Folimat-Wirkstoff (S 6876, Omethoat).
    Subkuter kutaner Toxizitätsversuch an Kanichen. (1979) Unpublished
    report submitted by Bayer AG.

    Thyssen, J.  S 6876 (Folimat-Wirkstoff).  Akute Inhalationstoxizität.
    (1978) Unpublished report submitted by Bayer AG.

    Weber H., Patzschke, K., and Wegner, L.A.  Omethoat-14C
    (Folimat(R)-Wirkstoff).  Biokinetische Untersuchungen an Ratten.
    (1978) Unpublished report submitted by Bayer AG.

    See Also:
       Toxicological Abbreviations
       Omethoate (WHO Pesticide Residues Series 1)
       Omethoate (WHO Pesticide Residues Series 5)
       Omethoate (Pesticide residues in food: 1978 evaluations)
       Omethoate (Pesticide residues in food: 1980 evaluations)
       Omethoate (Pesticide residues in food: 1981 evaluations)
       Omethoate (Pesticide residues in food: 1984 evaluations)
       Omethoate (Pesticide residues in food: 1985 evaluations Part II Toxicology)