Bt-treated crops may induce allergies.

Bacillus thuringiensis (Bt), a moth-killing bacterium that farmers use as an insecticide, has been considered nontoxic to all but a few types of insect larvae. It may pose some health risk for people, however. A new study of Ohio crop pickers and handlers finds that Bt can provoke immunological changes indicative of a developing allergy.

With long-term exposure, affected individuals might develop asthma or other serious allergic reactions, notes study leader I. Leonard Bernstein of the University of Cincinnati College of Medicine.

During more than 30 years of use, Bt has exhibited little human toxicity. However, "its potential allergenicity had never been carefully addressed," Bernstein says. So, he studied farm workers before and after fields were sprayed.

In the July ENVIRONMENTAL HEALTH PERSPECTIVES, his team demonstrates Bt's allergenicity. Before the pesticide's application, 4 of 48 crop pickers, about 8 percent, had a positive skin test to Bt, indicating a sensitivity that can lead to an allergy. One month after harvesting Bt-sprayed celery, parsley, cabbage, kale, spinach, and strawberries, half the pickers tested positive. That share climbed to 70 percent within another 3 months.

Workers with less direct exposure proved less likely to develop Bt sensitivity. Of 34 packers who washed and crated Bt-treated crops, just 5, or 15 percent, had positive skin tests after the spraying. Among 44 field hands working 3 miles away from Bt-sprayed fields, only 5, or 11 percent, tested positive.

Blood tests confirmed that many workers who tested positive also had immunoglobulin E antibodies to the strain of Bt sprayed. These antibodies can signal a developing allergy. Hay fever sufferers, for instance, often produce such antibodies 4 or 5 years before symptoms such as sneezing develop.

"We'll take a look at this study," notes Chris Klose, a spokesman for the American Crop Protection Association in Washington, D.C. If the new study's findings are confirmed, "the [pesticides] industry would be concerned," he says.

"In terms of consumer safety, there is probably also reason for concern," says Brian Baker of the Organic Materials Review Institute in Eugene, Ore. Gardeners and others "should remember Bt is a pesticide and show it the same respect they would other pesticides," he adds.

Though the data show that Bt "has the potential to elicit allergic responses," the pesticide was "not horribly allergenic," observes coauthor Mary Jane K. Selgrade of the Environmental Protection Agency in Research Triangle Park, N.C. However, the new data are prodding the agency to develop standardized assays so that microbial-pesticide developers can rank the relative allergenicity of their products. Indeed, Selgrade notes, if what makes Bt allergenic is not what makes it pesticidal, developers might one day genetically manipulate Bt to make it less worrisome.