Methylmercury Seed Dressings

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Methylmercury seed dressings

Seed dressings were devised to prevent diseases caused by a wide variety of seed-borne plant-pathogenic fungi , to protect the germinating seeds against secondary infections, and to increase crop yields. Various chemicals , including several heavy metals , have been used as fungicides to treat seeds since the end of the nineteenth century. The effectiveness of these fungicides was greatly increased when aryl organomercurials were introduced around 1914. They had a wider spectrum of fungicidal activity than nonmercurial formulations and were used extensively until the mid-1980s to control fungus diseases through their application as seed dressings on many grains, vegetables, and nuts, as well as to protect fruit trees, rice, turf grasses, and golf courses . However, with the introduction of the more effective alkyl mercury compounds in the 1930s, especially methyl mercury and ethyl mercury, severe poisoning incidents followed. In developing countries hundreds of people died or became incapacitated due to either the consumption of grains treated with alkylmercury compounds or meat from animals that had eaten such treated seeds.

Poisonings from eating alkylmercury-treated grains occurred on several occasions in various parts of Iraq. Destitute, illiterate rural families either did not understand the words or poison symbols on the bags of grain or did not believe government warnings that it was unsafe to eat. In some instances the families fed some of the grain to chickens and swine first. When they did not observe poisoning symptoms in the livestock and poultry after a few days, the farmers became convinced that the warnings were false and the grain was safe to eat. However, depending on the amount of methyl mercury consumed, there is a latency period of weeks or months between exposure and the development of poisoning symptoms.

When the seed grain was ground into flour, baked into bread, and consumed by the rural victims, both sexes and all ages were affected. The ingested quantities of methylmercury ranged from small amounts that produced no overt effects to lethal doses. Fetuses suffered the most damage. Among the rest of the population, the severity of the neurological and psychiatric symptoms was almost directly proportional to the amount of bread consumed. In cities where bread was produced from government-inspected flour mills, not a single case of poisoning was reported.

The first documented incident occurred in Iraq in 1956. Of the two hundred persons afflicted, seventy died. In 1960 an estimated 1000 persons were affected in a similar incident and over 200 died. During the 1960s other smaller but similar episodes of alkylmercury poisoning occurred on a more limited scale in West Pakistan, Guatemala, Ghana, and in other countries such as Mexico and the United States. The total death toll in these countries was 42 with approximately 197 individuals less seriously affected.

But the most serious outbreak of poisoning from eating methyl mercury-poisoned bread occurred in Iraq early in 1972. In October and November of 1971 a total of about 73,000 tons of high-yield Mexipac wheat seed grain and 22,000 tons of barley seed grain, all treated with alkyl mercury fungicide , had been distributed by cooperatives to farmers for planting throughout the country. Some of this grain, as before, was used to prepare homemade bread. The Iraqi government estimated that the treated grain was distributed to no more than five percent of the rural population, about 200,000 people. Most of the fatalities occurred within three months after the end of the exposure, although a few long-term illnesses resulted in fatalities as well.

By March 1973, up to 40,000 persons, residents of every province, were unofficially estimated to have been poisoned. The total number of casualties will never be known precisely because hospitals were quickly overloaded, and many victims did not have access to them. In addition, most of the poisonings occurred in rural areas, and many were not reported to authorities. The government officially acknowledged that 6,530 persons were hospitalized and 459 died. These figures were not confirmed because news reporters were denied entry to the country and the movements of foreigners were restricted. However, tourists reported that large numbers of Iraqis suffered brain damage, blindness, and paralysis. Since then, with the exception of a few follow-up scientific reports published between 1985 and 1989, hardly any new information relative to the long term health effects on the thousands of victims has been published or released by the Iraqi government although this was the largest such tragedy of this kind. Because of the highly toxic nature of the alkyl mercurials, as well as the severity of the accidents caused by misuse of the treated seeds, they were banned in 1970 in the United States and many other countries.

See also Agricultural chemicals; Birth defects; Heavy metals and heavy metal poisoning; Minamata disease; Plant pathology; Teratogen; Xenobiotic

[Frank M. D'Itri ]



D'Itri, P. A., and F. M. D'Itri. Mercury Contamination: A Human Tragedy. New York: Wiley-Interscience, 1977.


Bakir, F., et al. "Methylmercury Poisoning in Iraq." Science 181 (1973): 230241.

"Conference on Intoxication Due to Alkyl Mercury-Treated Seed." World Health Organization 53 (Suppl.) (1976): 138.

Greenwood, M. R. "Methylmercury Poisoning in Iraq: An Epidemiological Study of the 1971-72 Outbreak." Journal of Applied Toxicology 5 (1985): 148159.