First draft prepared by
    M. Watson
    Pesticide Safety Directorate, Ministry of Agriculture, Fisheries and
    Harpenden, Hertfordshire, United Kingdom


         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.



    Special studies on delayed neurotoxicity


         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,

         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


         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).


         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.


    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.


    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)