TRIAZOPHOS
First draft prepared by
M. Watson
Pesticide Safety Directorate, Ministry of Agriculture, Fisheries and
Food
Harpenden, Hertfordshire, United Kingdom
XPLANATION
Triazophos was previously evaluated by the Joint Meeting in
1982, 1986 and 1991 (Annex I, references 38, 47, 62). In 1991 the
temporary ADI of 0-0.0002 mg/kg bw was extended in view of the
uncertainty regarding the potential for triazophos to cause delayed
neurotoxicity. The 1991 Meeting also noted that previous
investigations of the effects of antidotes to acute triazophos
intoxication were inadequate (Annex I, reference 64).
The results of a new neurotoxicity study, a re-evaluation of
histopathology from a previous study, and the results of a study to
investigate the action of antidotes to triazophos poisoning are
summarized in this monograph addendum.
EVALUATION FOR ACCEPTABLE DAILY INTAKE
BIOLOGICAL DATA
Special studies on delayed neurotoxicity
Hens
A 90-day neurotoxicity study in hens at dietary concentrations
of 0, 50, 110 or 250 ppm was evaluated at the 1991 JMPR (Annex I,
reference 64). Morphological lesions characteristic of
neurotoxicity were reported in the spinal cord and peripheral
nerves. The study report stated that lesions were more severe and
seen more frequently in hens treated with triazophos than in
controls. The 1991 Meeting concluded that it was difficult to
interpret these histopathology findings since, most unusually,
lesions were frequently seen in the spinal cord, but very
infrequently seen in the peripheral nerves of control animals.
Sections of spinal cord and peripheral nerves from control animals
and animals treated with triazophos and TOCP were re-examined by
three pathologists, in different laboratories (one of which was the
pathologist originally responsible for evaluation of the slides) in
order to investigate the possibility that lesions previously
considered to be due to treatment with triazophos, may in fact have
been due to variation in background pathology.
Re-examination of sections of spinal cord revealed that there
was a marginal increase in the group mean severity of histopathology
score in animals treated with 250 ppm triazophos in comparison with
controls. However, when compared with the marked increase in
severity seen in the positive control group administered TOCP (these
slides were originally not available to the study pathologist), the
marginal increase among hens treated with 250 ppm triazophos was
considered to represent normal variation in the level of background
lesions, rather than a response to treatment with triazophos. In
the peripheral nerves, findings for animals treated with triazophos
were essentially similar to those seen in controls. There was an
increase in severity of mean histopathology scores in the tibial
nerves in hens treated with TOCP (Leist et al., 1993).
In a new acute delayed neurotoxicity study, triazophos was
administered by single oral gavage to groups of 15 white Leghorn
hens at dose levels of 0, 2.5, 5.0 or 10 mg/kg bw. A positive
control group (TOCP, 750 mg/kg bw) was also included. Plasma
cholinesterase activity was measured in all surviving hens pretest,
24 and 48 hours after dosing and 8, 10 and 21 days after dosing. At
48 hours, 10 days and 21 days after dosing up to 3 hens from each
group were sacrificed and brain and spinal cord tissue collected for
estimation of acetyl cholinesterase and NTE. At 21 days after
dosing, all surviving hens were killed and histopathology of brain,
spinal cord and peripheral nerve was performed on up to 6 hens from
each group. Clinical signs of reaction to treatment such as
sedation, excitement, frightened behaviour, abnormal posture,
dyspnea and ataxia were seen in a dose-dependent manner within 24
hours after dosing with triazophos. These animals recovered during
the following 24-48 hours. In contrast, animals treated with TOCP
were largely normal during the first week after dosing, but showed
impairment of motor function and ataxia, with increasing severity,
during weeks 2 and 3. Two hens treated with 10 mg/kg bw triazophos
died shortly after dosing and two hens treated with TOCP were killed
on humane grounds one day prior to scheduled sacrifice. Plasma
cholinesterase activity was inhibited by triazophos, in a dose-
dependent manner. The positive control agent, TOCP, also inhibited
plasma cholinesterase, but the inhibition was longer-lasting than
that seen with triazophos and there was some indication of an
adaptive response by day 21. Acetyl cholinesterase activity in brain
was not inhibited by either triazophos or by TOCP. NTE activity (in
brain and spinal cord) was inhibited by TOCP from 48 hours after
dosing, with signs of recovery by day 21. There was no inhibition
of NTE in hens treated with triazophos. Histopathological
examination revealed signs of axonal degeneration and myelin
breakdown in hens treated with TOCP, while the findings in hens
treated with triazophos were minor and did not differ from those
observed in the untreated control group. The report concluded that
the administration of a single dose of triazophos to hens was not
associated with any indication of delayed neurotoxicity (Mahl,
1992).
The design of this study was criticized by the present Meeting,
in that if atropine and other protective measures had been used,
higher doses could have been used in order to maximize the potential
for exposure to the nervous system.
Special studies on antidotes
Rats
A single dose of 100 mg/kg bw triazophos was administered
orally to groups of 10 female Wistar rats. When no therapeutic
measures were taken, 7 animals died within one day of dosing and a
total of 9/10 animals died within four days. Clinical signs prior
to death were typical of acetyl cholinesterase inhibition and
included irregular breathing, exophthalmus, twitching, trembling and
clonic convulsions. Further groups of animals received additional
treatment with antidotes (atropine sulphate alone, or atropine
sulphate combined with obidoxime chloride or 2-PAM) by
intraperitoneal injection as listed below:
Antidote Dose (mg/kg bw)
Atropine sulphate 50
Atropine sulphate + obidoxime 50 + 70
Atropine sulphate + 2-PAM 50 + 70
An initial treatment with antidote was administered 10 minutes
after dosing with triazophos, this was followed by further doses of
75%, 50% or 25% of the initial dose as considered necessary
according to signs of reaction to treatment observed. In animals
given antidote treatment with atropine sulphate alone, clinical
signs and mortality were initially delayed during the first 8 hours
following dosing, but a total of 9/10 rats died within 2 days of
treatment. No mortality occurred in the groups treated with the
combinations of atropine plus obidoxime or atropine plus 2-PAM.
Clinical signs of intoxication were observed soon after dosing in
these animals, but treatment with the antidotes quickly achieved
improvement in condition and recovery was generally complete within
3 days. It was concluded that antidote treatment with atropine
sulphate alone proved unsuitable as a therapeutic measure after
intoxication with triazophos, whereas the two combinations atropine
sulphate plus obidoxime and atropine sulphate plus 2-PAM proved to
be efficient antidotes, producing rapid and comparable therapeutic
effects (Ehling, 1993).
COMMENTS
Re-examination of sections of spinal cord and peripheral nerve
from a 90-day study in hens (which the 1991 JMPR meeting had found
difficult to interpret) revealed that lesions previously considered
to be due to treatment with triazophos were in fact most probably
variations in background pathology and were not consistent with
delayed neurotoxicity. In a new acute delayed neurotoxicity study
in hens, there was no indication that treatment with triazophos was
associated with any induction of delayed neurotoxicity. However,
the Meeting criticized the study design, in that higher doses could
have been used in order to maximize the potential for exposure to
the nervous system.
In an investigation of antidote treatment to triazophos
intoxication in rats, the expected results were obtained, a
combination of atropine and oxime proving to be efficient antidotes.
The Meeting concluded that, despite shortcomings in the design
of the neurotoxicity studies, the total available database indicated
that triazophos does not have potential to cause delayed
neurotoxicity following dietary exposure. An ADI was allocated
using the NOAEL from the human volunteer study reviewed in 1982 and
1991, using a 10-fold safety factor. This ADI was supported by the
NOAEL from a 52-week study in dogs, using a 100-fold safety factor.
TOXICOLOGICAL EVALUATION
Level causing no toxicological effect
Mouse: 30 ppm in the diet, equal to 4.5 mg/kg bw/day
(2-year study) (1991 JMPR)
Rat: 3 ppm in the diet, equal to 0.17 mg/kg bw/day
(2-year study) (1991 JMPR)
27 ppm in the diet, equal to 2-3 mg/kg bw/day
(multigeneration reproduction study) (1991 JMPR)
Dog: 4 ppm in the diet, equal to 0.12 mg/kg bw/day
(1-year study) (1991 JMPR)
Human: 0.0125 mg/kg bw/day (3-week study) (1982 & 1991 JMPR)
Estimate of acceptable daily intake for humans
0-0.001 mg/kg bw
Studies which will provide information valuable in the continued
evaluation of the compound
Further observations in humans.
REFERENCES
Ehling, G. (1993) Triazophos - Effects of antidotes tested on female
wistar rats. Unpublished report submitted to WHO by Hoechst AG.,
Frankfurt-am-Main, Germany.
Leist, K.-H., Alison, R.H., Prentice, D.E., Deutschlander, N.,
Weber, K. (1993) 3-month subchronic delayed neurotoxicity (feeding)
study with triazophos substance technical (Code: HOE 002960 00 ZD93
0002) in the hen. Re-evaluation of the neurotoxic potential.
Unpublished report submitted to WHO by Hoechst AG., Frankfurt-am-
Main, Germany.
Mahl, A. (1992) Delayed neurotoxicity study following single oral
application with triazophos substance technical (HOE 002960 00 ZD93
0002) in laying hens. Unpublished report from RCC, submitted to WHO
by Hoechst AG., Frankfurt-am-Main, Germany.
See Also:
Toxicological Abbreviations
Triazophos (JMPR Evaluations 2002 Part II Toxicological)
Triazophos (Pesticide residues in food: 1982 evaluations)
Triazophos (Pesticide residues in food: 1983 evaluations)
Triazophos (Pesticide residues in food: 1986 evaluations Part II Toxicology)
Triazophos (Pesticide residues in food: 1991 evaluations Part II Toxicology)