Omethoate was evaluated by the Joint Meeting in 1971, and
reviewed in 1975, 1978, 1979, and 1981 (Annex 1, FAO/WHO, 1972a,
1976a, 1979a, 1980a, and 1982a). A toxicological monograph was
prepared by the Joint Meeting in 1971 (Annex 1, FAO/WHO, 1972b) and
monograph addenda were prepared in 1975, 1978, 1979, and 1981 (Annex
1, FAO/WHO, 1976b, 1979b, 1980b, and 1982b). In 1981, the Joint
Meeting requested carcinogenicity studies in a rodent species at
higher dose levels than were used in the study that was reviewed,
while further mutagenicity studies were deemed to be desirable. A
chronic toxicity study on mice, a teratogenicity study in rabbits, and
additional mutagenicity studies have become available in the meantime,
and are summarized in this monograph addendum.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
Special studies on the inhibition of cholinesterase activity in mice
Groups of BOR:NMRI mice (50 males and 50 females/group) were
given diets containing omethoate (97.1% purity) at concentrations of
0, 1, 3, or 10 ppm for 4 weeks. Animals were observed twice daily and
weighed once a week. Plasma, erythrocyte, and brain cholinesterase
activity were measured on days 1, 3, 7, 14, and 29 of the treatment on
10 males and 10 females per dose group.
Appearance, behaviour, and mean body weights were not
significantly different between the control and the treated groups. No
mortality occurred. Plasma cholinesterase activity in both sexes was
significantly lower than in controls in the 10-ppm group after the
first determination. Erythrocyte cholinesterase activity was depressed
in females from day 3 onward at 10 ppm, and on day 14 at 3 ppm; in
males it was depressed only on day 29 at 10 ppm. Brain cholinesterase
activity of both sexes was significantly lower than in controls at all
test times at 10 ppm and at nearly all test times at 3 ppm. In
addition, males in the 1-ppm group also had significantly lower brain
enzyme activity on days 1 and 3; however, an opposite trend was
observed on day 7.
The 1-ppm dose level was the no-effect level on cholinesterase
activity in this study (Kroetlinger, 1982).
Special studies on teratogenicity
Groups of 14 pregnant New Zealand White rabbits were dosed daily
by oral gavage with omethoate (96.8% purity), dissolved in distilled
water, at dose levels of 0, 0.1, 0.3, or 1.0 mg/kg b.w. from day 6 to
day 18, inclusive, of gestation. On day 29 of gestation the females
were sacrificed and the uterine contents examined. The reproductive
tract, complete with ovaries, was dissected out and the following
recorded; weight of gravid uterus, number of corpora lutea in each
ovary, number of implantation sites, number of resorption sites
(classified as early and late), number and distribution of live and
dead foetuses in each uterine horn, weight and sex of individual
foetuses, individual placental weights, and external, internal and
skeletal abnormalities of individual foetuses. Whole-blood
cholinesterase activity was determined prior to dosing on day 6 of
gestation and 2 hours after dosing on day 18 of gestation.
The general condition of control and treated females was
comparable throughout the study. Maternal mean body weights and
corrected day-29 body weights were unaffected by the treatment.
Mortality, the incidence of abortions and total-litter losses, and the
number of pregnant females with viable young on day 29 were not
altered by treatment. Whole-blood cholinesterase activity was
significantly depressed only among the 1.0 mg/kg b.w. females,
compared both to the pre-dosing level and to the control post-dosing
There were no treatment-related differences between the control
and the treated groups with respect to the corpora lutea count,
implantations, male and female viable young, early and late
resorptions, pre- and post-implantation losses, or foetal and
Examination of foetuses at necroscopy on day 29 of gestation or
following skeletal investigation revealed a number of non-dose-related
findings of the type and incidence previously recorded in this strain
of rabbit and in the laboratory that performed the study.
The 1.0 mg/kg b.w. dose level was the no-effect level for
embryotoxic/teratogenic effects (Tesh et al., 1982).
Special study on carcinogenicity
Groups of SPF mice of the BOR:CWF1 strain (50 males and 50
females per dose level) were fed diets containing omethoate (94%
purity) at levels of 0, 1, 3, or 10 ppm for 24 months. The animals
were inspected daily for clinical symptoms. Food consumption was
determined weekly. The body weights were determined weekly during the
first 14 weeks and at 3-week intervals thereafter. Necropsy was
performed on the mice that died or were sacrificed when moribund, and
on all the animals sacrificed at the end of the experiment. Heart,
lungs, liver, spleen, kidneys, and testes were weighed. A range of
organs and all changes detected during gross examination were
subjected to histopathological examination.
Appearance, behaviour, and activity were not significantly
different between the control and treated groups. Total and mean daily
food consumption were essentially the same among all the animals.
The body weights of the male mice of all the treated groups were,
for the most part, higher than those of the controls during the entire
experiment, whereas there were no differences from controls in the
body weights of the females.
Mortality and the frequency-distribution of mortality by death-
date-intervals were comparable among all the groups. Mortality at 18
months was between 12% and 27% for the males and between 14% and 31%
for the females. Absolute and relative organ weights showed no dose-
related and/or significant differences between control and treated
groups. Gross anatomical and histopathological examinations revealed a
range of non-neoplastic changes commonly observed in old mice.
Comparison of these changes by type, site, and frequency distribution
by sex and dose level gave no indications of any treatment-related
Neoplastic changes were found primarily in the lungs, liver,
adrenal cortex, and haemopoietic system. Neither the data concerning
the type, site, or frequency-distribution of tumours by sex and dose
level, nor the data regarding the number of tumour-bearing mice, mice
with benign tumours, mice with malignant tumours, or mice with both
benign and malignant tumours provided any indication of treatment
The somatic no-effect level in this study was 10 ppm, equal to
2.1 mg/kg b.w./day and 3.1 mg/kg b.w./day, for male and female mice,
respectively (Kroetlinger & Loeser, 1982).
Special studies on mutagenicity
Several mutagenicity assays have been carried out on omethoate
(Table 1); omethoate was found to be mutagenic only in gene-conversion
and mitotic-recombination tests in S. cerevisiae D7. All other tests
Special studies on delayed neuropathy
Hens (2-4/dose group) were dosed orally with omethoate dissolved
in corn oil at levels of 20 to 300 mg/kg b.w. (ca. 4-8 times the
unprotected LD50) under eserine and atropine protection. Omethoate
used was a sample that caused a fatal human poisoning accident.
AChE and neurotoxic esterase (NTE) acitivites of brain
homogenates were assayed 24 hours after dosing. Pair-dosed birds which
survived were observed for signs of ataxia for 3-4 weeks after dosing.
Hens dosed at 4 times the LD50 did not show any inhibition of
NTE at 24 hours, nor signs of ataxia for 3-4 weeks after dosing. Hens
dosed at 8 times the LD50 did not survive, despite treatment with
high doses of atropine; however, the NTE activity in the brain of
those animals who died within 36 hours was measured immediately after
death and was found to be normal. In all the birds acute cholinergic
symptoms were correlated with high inhibition of AChE in the brain. An
inhibition of 70% of brain AChE in a bird dosed with 20 mg/kg b.w. of
omethoate was not enough to produce detectable signs of acute
The in vitro inhibition activity of pure omethoate and of the
incriminated sample of omethoate were measured for NTE and AChE
activities using both hen and human brain enzymes. For both hen and
human tissue the I50 for AChE was in the range of 0.08-0.15 mM, so
that it would be virtually totally inhibited at 5 mM, the
concentration which caused no detectable inhibiton of NTE.
NTE and AChE activities were measured on samples of brain tissue
(cortex) taken during the post-mortem examination performed 24 hours
after death on a 30-year-old male farmer who was acutely poisoned by a
commercial formulation of omethoate. The NTE acitivity was within the
normal range, while AChE was strongly inhibited.
The conclusion was that omethoate is extremely unlikely to cause
delayed neuropathy in man (Lotti et al., 1981).
Table 1. Results of mutagenicity assays of omethoate
Test Test Range of doses or Result Reference
organism substance concentration tested
S. cerevisiae Omethoate 0.03-66.67 µl/ml* No significant Hoorn, 1982
S138, & (96.9% differences in
S211 alfa purity) revertant
S. cerevisiae Omethoate 0.03-66.67 µl/ml* Dose-related Hoorn, 1983
D7 (96.0% increases of
purity) frequencies of
Mouse lymphoma Omethoate 500-2000 µg/ml No significant Bootman &
L5178Y (96.9% without S-9 mix, increase in Rees, 1982
(TK +/-) purity) 500-5000 µg/ml mutation
cells - with S-9 mix frequency at the
in vitro TK gene locus*
Micronucleus Omethoate 2×6 mg/kg b.w., No significant Herbold,
test on (97.1 2×12 mg/kg b.w. differences 1981
mouse purity) between control &
with respect to
Table 1. (Con't)
Test Test Range of doses or Result Reference
organism substance concentration tested
E. coli (K12) Omethoate 625-10,000 µl/ No indication of Herbold,
p 3478 with (96.0% plate* potential for 1983
DNA repair purity) DNA damage
E. coli W3110
with intact repair
* With and without S-9 mix
After exposure to omethoate (96.9% purity) for 4 hours, a primary
irritant effect on the sheared uninjured skin of albino rabbits was
not observed. The test compound caused a slight conjunctival reaction
that was completely reversible within 7 days (Pauluhn, 1982).
Groups of Beagle dogs (6 males and 6 females/group) received
omethoate (97.1% purity; dissolved in acidulated water) daily for 12
months by stomach tube at dose levels of 0, 0.025, 0.125, or
0.625 mg/kg b.w. All the animals were observed several times daily for
appearance and behaviour. Food and water consumption were recorded
daily. Body weights were noted weekly. Reflex tests, ophthalmoscopic
examinations, body temperatures, pulse-rate measurements, haematology,
clinical chemistry, and urinalyses were carried out before the start
of the study and during weeks 6, 13, 26, 40, and 52 of treatment.
Cholinesterase activity in the plasma and erythrocytes was determined
1 week before the start and during weeks 1, 3, 6, 9, 13, 26, 40, and
52 of treatment. On autopsy, after 52 weeks of treatment,
cholinesterase activies in the brains of all the animals were
All the animals were normal in appearance and behaviour. WP signs
attributable to treatment were not observed. All the animals survived
the treatment. There were no significant differences between the
control and the treated groups with respect to reflex tests,
ophthalmoscopic examinations, body temperatures, pulse rates, food and
water consumption, mean body weights, haematology, clinical chemistry
(except for cholinesterase activity) or urinalysis.
A clear depression of plasma cholinesterase activity was observed
only in the 0.625 mg/kg b.w. dose group, ranging between 25% and 32%
in the males, and 16% and 29% in the females, in comparison to the
controls. The depression remained essentially constant over the entire
period of study.
A marked depression of erythrocyte cholestinerase activity was
measured in the males (17% to 40%) and females (22% to 40%) at the
0.625 mg/kg b.w. dose level, which varied only slightly during the
study. At the 0.125 mg/kg b.w. dose level, only the males in the first
third of the study showed slight (less than 28%) depression of
erythrocyte cholinesterase activity.
Brain cholinesterase activity was depressed in the males of the
0.125 mg/kg b.w. group (20%) and of the 0.625 mg/kg b.w. group (39%),
and in the females of the 0.625 mg/kg b.w. group (30%).
On autopsy, absolute and relative organ weights were not
significantly different between the control and treated groups. The
gross pathological and histopathological examinations did not reveal
The dose level of 0.625 mg/kg b.w. was the no-effect level for
somatic effects, while 0.025 mg/kg b.w. was the no-effect level for
erythrocyte cholinesterase activity (Hoffmann & Schilde, 1984).
The results of an adequate chronic toxicity/oncogenicity study in
mice, in which neither oncogenic effects nor other somatic damage were
observed, were provided to the meeting.
A no-effect level of 1.0 mg/kg b.w. was determined for
embryotoxic/teratogenic effects in the rabbit.
Delayed neuropathy was not observed in hens. No potential for
delayed neuropathy was observed in man.
Omethoate was found mutagenic only in gene-conversion and
mitotic-recombination tests in S. cerevisiae D7, whereas other tests
The Meeting estimated an ADI for omethoate.
LEVEL CAUSING NO TOXICOLOGICAL EFFECT
Rat: 1 ppm in the diet, equivalent to 0.05 mg/kg b.w.
Dog: 0.025 mg/kg b.w./day.
ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN
0 - 0.0003 mg/kg b.w.
FURTHER WORK OR INFORMATION DESIRED
Observations in man.
Bootman, J. & Rees, R. S 6876: Investigation of mutagenic activity in
(1982) the TK +/- mouse lymphoma cell mutation system. Unpublished
report No. 82/BAG 027/488 from Life Science Research,
England. Submitted to WHO by Bayer F.R.G.
Herbold, B. S 6876 (Omethoate, Folimat(R) active ingredient):
(1981) Micronucleus test on mouse to evaluate S 6876 for mutagenic
potential. Unpublished report No. 10,021 from Institute of
Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G.
Herbold, B. S 6876 (Omethoate, Folimat(R) active ingredient): Pol test
(1983) on E. coli to evaluate for DNA damage. Unpublished report
No. 12,126 from Institute of Toxicology, Bayer AG. Submitted
to WHO by Bayer F.R.G.
Hoffman, K. & Schilde, B. S 6876 (Omethoate): Chronic toxicity to dogs
(1984) on oral administration (twelve-month stomach tube study).
Unpublished report No. 12,561 from Institute of Toxicology,
Bayer AG. Submitted to WHO by Bayer F.R.G.
Hoorn, A.J.W. Mutagenic evaluation of S 6876 (omethoate) in the
(1982) reverse mutation induction assay with Saccharomyces
cerevisiae strains S138/S211-alfa. Unpublished final
report No. E-9030 from Litton Bionetics, The Netherlands.
Submitted to WHO by Bayer F.R.G.
Hoorn, A.J.W. Evaluation of S 6876 (c.n. omethoate; a.i. of Folimat(R))
(1983) in the induced mitotic crossing over and gene conversion
assay in Saccharomyces cerevisiae strain D7. Unpublished
final report No. E-9127 from Litton Bionetics, The
Netherlands. Submitted to WHO by Bayer F.R.G.
Kroetlinger, F. S 6876 (Omethoate, the active ingredient of Folimat(R))
(1982) Study of the inhibition of cholinesterase activity in mice.
(4-week feeding experiment). Unpublished report No. 11,235
from Institute of Toxicology, Bayer AG. Submitted to WHO by
Kroetlinger, F. & Loeser, E. S 6876 (Omethoate, the active ingredient
(1982) of Folimat(R)): Chronic toxicity study on mice. (2-year
feeding experiment). Unpublished report No. 11,161 from
Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer
Lotti, M., Ferrara, S.D., Caroldi, S., & Sinigaglia, F. Enzyme studies
(1981) with human and hen autopsy tissue suggest omethoate does not
cause delayed neuropathy in man. Arch. Toxicol., 48,
Pauluhn, J. S 6876 (Omethoate, the active ingredient of Folimat(R)):
(1982) Study of the irritant/corrosive effect. Unpublished report
No. 11,977 from Institute of Toxicology, Bayer AG. Submitted
to WHO by Bayer F.R.G.
Tesh, J.M., Ross, F.W., Wightman, T.J., & Wilby, O.K. S 6876: Effect
(1982) of oral administration upon pregnancy in the rabbit. 2. Main
study. Unpublished report No. 82/BAG 023/111 from Life
Science Research, England. Submitted to WHO by Bayer F.R.G.