FAO Meeting Report No. PL/1965/10/1
    WHO/Food Add./27.65


    The content of this document is the result of the deliberations of the
    Joint Meeting of the FAO Committee on Pesticides in Agriculture and
    the WHO Expert Committee on Pesticide Residues, which met in Rome,
    15-22 March 19651

    Food and Agriculture Organization of the United Nations
    World Health Organization

    1 Report of the second joint meeting of the FAO Committee on
    Pesticides in Agriculture and the WHO Expert Committee on Pesticide
    Residues, FAO Meeting Report No. PL/1965/10; WHO/Food Add./26.65


    Chemical name

         O,O-dimethyl S-[1,2-di-(ethoxycarbonyl)ethyl]
    phosphorodithioate; S-[1,2-di(ethoxycarbonyl)ethyl]
    dimethyl-phosphorothiolothionate; S-[1,2-di(ethoxycarbonyl)ethyl]
    dimethyl-phosphorothiolothionate; S[1, 2-bis
    (ethoxycarbonyl)ethyl]-O,O-dimethyl phosphorodithioate.


         Carbofos, malathon

    Empirical formula


    Structural formula



    Biochemical aspects

         Malathion is rapidly absorbed from the intestinal tract. Its
    metabolism has been studied in the hen, mouse, rat, cow and man.
    Malathion is oxidized to malaoxon, the active form of the compound,
    and is also hydrolysed to less toxic metabolites. Six to eight
    metabolites have been found, the main ones being in the urine,
    malathion mono- and di-acids. In eggs from treated hens and milk from
    cows treated with malathion, malathion or its metabolites were
    recovered (O'Brien, 1960; Heath, 1961).

         Malaoxon is a cholinesterase inhibitor in vivo and in vitro
    (I50 7 × 10-7) (Heath, 1961).

         The half time for the conversion in vivo of the reversibly
    inhibited form of the dimethylphosphorilated cholinesterase to the
    irreversibly inhibited form of this enzyme in the brain of chicken

    given malathion has been found to be 2 hours. The same half time was
    observed in vitro with the brain homogenate inhibited with paraoxon
    (Witter & Gaines, 1964).

         Simultaneous administration of malathion and ethyl p-nitrophenyl
    thionobenzenephosphate (EPN) results in a potentiation of the
    cholinesterase inhibitory effect of malathion in the mouse, rat and
    dog (Frawley et al., 1957; O'Brien, 1960).

         In a colony of rats showing an oral LD50 of 925 mg/kg for
    adults, the intragastric LD50 for a day-old rat was approximately 124
    mg/kg (Lu et al., 1965).

    Short-term studies

         Mouse. When malathion was added to the diet as 500 or 5000 ppm
    for 6 weeks or after the administration of 5 oral doses of 500 mg/kg
    the production of antibodies against B. Pertussis was not affected
    (Benes et al., 1963).

         Rat. Groups of 10 males were given Malathion at 100 or 500 ppm
    in the diet or dipterex at 60 or 300 ppm for 6 weeks and this was
    followed by the administration of both compounds at the same time.
    During the experiment erythrocyte cholinesterase fluctuated around
    100% of the initial values. At the end of the experiment, in
    comparison with the control group, the adrenals weighed more and
    showed hypertrophy of both cortex and medulla, the intensity of which
    was related to the concentration of the two substances in the diet
    (Benes & Cerna, 1965).

         In another experiment 95% technical malathion was fed to 3 groups
    of male rats, 10 animals per group, for 33 days at the levels of 100,
    1000 and 5000 ppm. No sign of toxicity was observed, nor any deaths.
    Food intake and weight gain in the groups fed 100 and 1000 ppm were
    higher than in the control group; groups fed 5000 ppm showed no
    difference from the controls. Cholinesterase activity was determined
    in 6 animals from each group. Activity was normal in the 100 ppm
    group. Erythrocyte cholinesterase activity was 68% of normal in the
    1000 ppm group, and in the 5000 ppm group plasm cholinesterase
    activity was 78% and erythrocyte activity 22% of normal. At all levels
    no depression of brain cholinesterase activity was found (American
    Cyanamid Co., 1955).

    Acute toxicity


    Animal               Route              LD50 mg/kg                  References
                                          90%          99%
                                       technical    technical

    Rat, male             Oral          940-1156*    4700-5843*    American Cyanamid Co., 1955
                                                                   Hazelton & Holland, 1953

    Rat, male             Oral          390-480*     1400-1845*    American Cyanamid Co., 1955
                                                                   Frawley et al., 1957
                                                                   Hazleton & Holland, 1953

    Mouse, male           Oral          720-886      3300-4060     American Cyanamid Co., 1955
                                                                   Hazleton & Holland, 1953

    Mouse, male           Oral                       2700-3320     American Cyanamid Co., 1955
                                                                   Hazleton & Holland, 1953

    Mouse, male      Intraperitoneal    420.474                    Hazleton & Holland, 1953

    Chicken               Oral         >850 (95%)                  American Cyanamid Co., 1955

    Calf                  Oral           80 (95%)                  American Cyanamid Co., 1955

    Cow                   Oral          560 (95%)                  American Cyanamid Co., 1955

    * Differences due to use of different vehicles.

         Ninety-eight per cent. technical malathion was fed to groups of 5
    rats for 8 weeks at levels of 100 and 500 ppm without any inhibition
    of whole-blood cholinesterase activity (Frawley et al., 1957).

         Ninety-five per cent. technical malathion was fed to 40 male and
    40 female rats for 5 months in a daily dose of 240 mg/kg body-weight
    (4000 ppm in the diet). Growth was normal and no signs of intoxication
    occurred. Ten wake after the beginning of the experiment, 18 females
    and 12 males were used for breeding. The average litter size from the
    treated females was smaller than in the controls and the number of
    newborn alive after 7 and 21 days was about half the number in the
    litters of the controls (Kalow & Marton, 1961).

         Chick. 95% technical malathion was fed to day-old chicks for 
    2 weeks at a level of 10 ppm. For the following 10 weeks they were
    divided into groups of 10 and fed 100, 1000 and 5000 ppm in their
    diets. The groups on 100 and 1000 ppm behaved normally and showed a
    similar growth rate and food consumption to the controls. Four animals
    died in the 5000 ppm group, and signs of intoxication and growth
    retardation were observed. At autopsy, no pathological lesions were
    found. Plasm and brain cholinesterase activity were significantly
    lowered in the 5000 ppm group (American Cyanamid Co., 1955).

         In a two-year study, 21 females were fed 250 ppm and 21 females
    and 6 males 2500 ppm. The 250 ppm group did not differ significantly
    from the controls. At the 2500 ppm level a decrease in plasma
    cholinesterase activity was found between the 195th and 465th day of
    experiment. The hens came later into production and laid slightly
    fewer eggs, but the hatchability was not influenced. The offspring
    showed no deformities. At autopsy no macro- or microscopical lesions
    were found (American Cyanamid Co., 1960).

         Man. Five male volunteers, 23-36 years old, took 8 mg of
    malathion in gelatin capsules daily for 32 days. No effect on plasma
    or erythrocyte cholinesterase activity could be detected. Five males
    took 16 mg daily for 47 days, also without any significant effect on
    cholinesterase activity. A daily dose of 24 mg taken by 5 males for 56
    days was followed by depression of the plasma cholinesterase activity
    2 weeks after the first administration. Maximum depression amounting
    to about 25% of the plasma cholinesterase activity occurred
    approximately 3 weeks after the cessation of administration. No
    clinically manifest side-effects were reported. Simultaneous intake of
    16 mg of malathion and 5 mg of EPN per day caused a light inhibition
    of cholinesterase activity (Moeller & Rider, 1962).

         No plasma or RBC cholinesterase depression was noted, in 10
    humans ingesting 3 mg EPN or 8 mg malathion daily for 32 days, nor in
    5 humans receiving 6 mg EPN for 88 days and 8 mg malathion for the
    last 44 days, nor in 5 humans ingesting 16 mg malathion for 88 days
    and 3 mg EPN for the last 41 days. However 10 humans ingesting 6 mg
    EPN and 16 mg malathion daily for 42 days showed a slight depression
    of both the plasma and the RBC cholinesterase (Rider et al., 1959).

    Long-term studies

         Rat. 65% technical malathion as a 10% or 25% wettable powder
    was mixed in the diets of groups of 20 male rats at the levels 100,
    1000 and 5000 ppm, and fed for 2 years. The mortality rate was not
    influenced, and at the 2 lower levels weight gain and food intake were
    comparable to those of the controls. Five thousand ppm reduced food
    intake and decreased weight gain. Cholinesterase determinations showed
    no inhibition at the 100 ppm level; with a diet containing 1000 ppm,
    36% inhibition of cholinesterase activity was found in the plasma, 73%
    in the erythrocytes and 37% in the brain, while at the 5000 ppm level,
    the plasma samples showed 80%, the erythrocytes 100% and the brain 77%
    inhibition. At autopsy neither gross nor microscopic examination
    revealed any pathological changes attributable to malathion (American
    Cyanamid Co., 1955; Hazleton & Holland, 1953).

         Ninety per cent. technical malathion was fed as 25% wettable
    powder in the diet to 20 males at a concentration of 100 ppm, to 20
    males and 10 females at 1000 ppm, and to 20 males at 5000 ppm for 2
    years. Mortality rate, growth response and food intake were not
    influenced by any of these diets, except that there was some growth
    retardation when the concentration was 5000 ppm. Terminal
    cholinesterase determinations revealed 10.30% inhibition of
    cholinesterase activity in the plasma, erythrocytes and brain at 100
    ppm. At 1000 ppm, 60-95% inhibition of erythrocyte cholinesterase
    activity was observed. The 5000 ppm group showed total inhibition of
    erythrocyte cholinesterase activity and 60-95% inhibition of
    cholinesterase activity in plasma and brain (American Cyanamid Co.,
    1955; Hazleton & Holland, 1953).

         Ninety-nine per cent. technical malathion was fed for 2 years to
    groups of 3-4 rats and produced, at 1000 and 5000 ppm levels,
    inhibition of erythrocyte cholinesterase activity of the same order as
    did the 90% compound. The decrease in plasma and brain cholinesterase
    activity, however, was much less than that produced by 90% technical
    malathion (American Cyanamid Co., 1955, Hazleton & Holland, 1953).

    Comments on experimental studies reported

         The studies are extensive and have been carried out in several
    species including man.

         In view of the very high doses used in the short-term breeding
    experiments in the rat, the results of these experiments were not
    taken into account in arriving at the maximum acceptable daily intake
    for man.


    Level causing no significant toxicological effect in the rat and man

         Rat. 100 ppm in the diet, equivalent to 5 mg/kg body-weight per

         Man. 16 mg a day, equivalent to 0.2 mg/kg body-weight per day.

    Estimate of acceptable daily intake for man

         0-0.02 mg/kg body-weight.

    Further work considered desirable

         Reproduction studies in rats.


    American Cyanamid Company, New York (1955) Report on Malathion

    American Cyanamid Company, New York (1960) Malathion pharmacology
    and toxicology (Unpublished data)

    Benes, V. & Cerna, V. (1965) Czech. Hyg., 10 (In press)

    Benes, V., Pekarek, J. & Cerna, V. (1963) Czech. Hyg., 8, 3

    Frawley, J. P., Fuyat, H. N., Hagan, E. C., Blake, J. R. & Fitzhugh,
    O. G. (1957) J. Pharmacol, exp. Ther., 121, 96

    Hazleton, L. W. & Holland, E. G. (1953) Arch. industr. Hyg., 8,

    Heath, D. F. (1961) Organophosphorus poisons, Pergamon Press

    Kalow, W. & Marton, A. (1961) Nature, 192, 464

    Lu, F. C., Jessup, D. C. & Lavallée, A. (1965) Food & Cosmetics
    Toxicol., 3 (In press)

    Moeller, H. C. & Rider, J. A. (1962) Toxicol. Appl. Pharmacol., 4,

    O'Brien, R. D. (1960) Toxic phosphorus esters, Academic Press

    Rider, J. A., Moeller, H. C., Swader, T. & Devereaux, R. J. (1959)
    Chemical Res., 7, 81

    Witter, R. F. & Gaines, T. B. (1964) Biochem. Pharmacol., 12, 1421

    See Also:
       Toxicological Abbreviations
       Malathion (ICSC)
       Malathion (FAO/PL:CP/15)
       Malathion (FAO/PL:1967/M/11/1)
       Malathion (JMPR Evaluations 2003 Part II Toxicological)
       Malathion (FAO/PL:1968/M/9/1)
       Malathion (FAO/PL:1969/M/17/1)
       Malathion (AGP:1970/M/12/1)
       Malathion (WHO Pesticide Residues Series 3)
       Malathion (WHO Pesticide Residues Series 5)
       Malathion (Pesticide residues in food: 1977 evaluations)
       Malathion (Pesticide residues in food: 1984 evaluations)
       Malathion (Pesticide residues in food: 1997 evaluations Part II Toxicological & Environmental)
       Malathion (IARC Summary & Evaluation, Volume 30, 1983)