By Daniel Wallis

NAIROBI, July 26 (Reuters) - Agricultural scientists unveiled a cheap kit on Thursday to let African farmers test crops for a deadly poison that makes them unfit to eat and costs the continent millions of dollars in lost exports.

Aflatoxin, a toxic chemical produced by a fungus, develops on maize, groundnuts, sorghum and cassava during hot weather and droughts. In large quantities it can cause cancer in humans, and it can also be fatal for animals.

Authorities in Europe and the United States reject food imports that exceed strict aflatoxin levels, and rich nations routinely test crops for the poison.

But African producers have found that far too expensive, so the new method developed by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) cuts the cost of each test to just $1 from $25 previously.

"It's available as a small, simple kit that can be used even in the most remote rural farms to monitor grains and nuts and improve storage techniques to avoid serious contaminations," ICRISAT, part of the U.S.-based Consultative Group on International Agricultural Research, said in a statement.

"The end result is safer products for consumers and higher returns for African farmers."

More than 5 billion people in poor nations are constantly exposed to aflatoxins by unknowingly eating infected foods, the scientists say, so cutting contamination of African crops could offer considerable health benefits.

In Kenya three years ago, about 125 people died after eating aflatoxin-infected maize, the third major outbreak to hit the country's staple food since 1981, U.S. researchers say.

The new kit has been used successfully in Malawi, which saw its status as a major groundnut exporter hammered in the 1970s by aflatoxin outbreaks. Now its nuts are back on the shelves of big supermarkets in Britain and the Fair Trade market.

"We have put another strong weapon in the hands of poor farmers to fight a problem that was making it particularly hard for African agricultural products to get fair treatment in international markets," ICRISAT head Dr. William Dar said.

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