Overall Assessment of Bt Insecticide safety to Humans

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Numerous reports over the years, many cited above, have suggested that Bt strains used in commercial insecticides were the cause of either a few rare cases of human infection, food poisoning, or allergic reactions. Analysis of the data in these studies, however, reveals no substantive evidence that Bt strains originating from commercial bacterial insecticides ever caused disease in humans, and certainly there is no evidence that these strains caused any kind of significant infection or outbreaks of food poisoning. Thus, Bt insecticides must be considered among the safest, if not the safest, ever developed for humans and most nontarget organisms.

To keep the few reported cases of putative health effects of Bt in perspective, it should be remembered that this bacterium is ubiquitous in the environment and occurs commonly in soil, grain, on leaf surfaces, and in water. Probably most of the Bt and Bt-like strains found in food have their origin either in grain (hence their presence in pasta, bread, and processed foods that include flour) or milk. Moreover, given the widespread occurrence of Bt in soil, one could argue that exposure to Bt is nearly as common as exposure to soil. If Bt were a human pathogen that would generate concern, given its widespread occurrence in nature and handling by and exposure to many workers in agriculture, food processing, forestry, and pest control, we would expect serious illness caused by Bt in humans to be relatively common. However, even when humans in residential areas have been subjected to repeated aerial sprays of commercial formulations, there is not a single confirmed report of a significant human illness due to Bt. In addition, it must be realized that Bt formulations, due to their demonstrated safety, are (unlike chemical insecticides) allowed to be sprayed on crops for insect control just prior to harvest. In many regions of the world where fresh vegetable crops are marketed within a few days of harvest, these have been recently sprayed with Bt.41,48 This is especially true of vegetables grown using organic methods. It is quite common for vegetables treated with Bt, such as broccoli, tomatoes, cucumbers, cauliflower, and lettuce, to be eaten raw and with only minimal washing. In these cases, humans are directly consuming thousands of Bt spores and insecti-cidal crystals. Again, if Bt were the cause of upset stomachs or diarrhea or more serious diseases due to vegetative growth and enterotoxin production after consumption, this should be apparent from epidemiological studies of human populations or reports of visits to hospitals and physicians. From the various studies published over the past several decades, including the most recent and detailed studies from Denmark,47,48 we conclude the evidence is overwhelming that Bt strains used in commercial bacterial insecticides are safe for humans. In the context of any type of risk/benefit analysis, the benefits derived from the very narrow spectrum of activity of Bt insecticides far outweigh any putative risks due to their use. Moreover, additional environmental and health benefits accrue from the concomitant reductions in chemical insecticide usage associated with the use of Bt insecticides.

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