Commercial Bt Strains as a Putative Cause of Infections in Humans

The few data that are available on Bt in humans suggest that under highly unusual circumstances this bacterium might be an opportunistic pathogen.6 Very few cases of statistically reliable adverse effects associated with human exposure to Bt insecticides are known, and even these consist only of temporary skin and mild throat irritation in persons who apply spray. There is only one case where a serious illness was associated with occupational exposure to Bt. In this case, a farmer splashed in the face with a commercial preparation of Bt developed an ocular ulcer.38 Exposure to Bt was characterized as the cause of the condition since this species was isolated from a swab of the farmer's eye 13 days following exposure. However, there was substantial evidence that Bt may not have been responsible because the ulcer was not examined directly for the presence of Bt and it was not realized that spores of this bacterium might persist in the eye without vegetative growth. Absence of vegetative growth would make it unlikely that Bt caused the ulcer. Clearance from eyes was subsequently investigated and it was determined that Bt administered to rabbit eyes was able to persist for at least a week. Persistence was dose-dependent and repeated flushing did not completely remove all of the initial inoculum.3940

In another case, Bt was isolated from burn wounds on a human and from water used to treat these wounds.41 Although the isolates produced parasporal bodies composed of proteins of 141, 83, and 81 kDa, these isolates were not toxic to Pieris bras-sicae (Lepidoptera) or Aedes aegypti (Diptera), and could not be serotyped because they did not have flagella. The latter is an important distinction because all Bt strains used in commercial formulations have flagella. This demonstrates that these isolates originated from an environmental source, apparently the water used to treat the wound, and not from commercial products. In addition, even if the source was water, it is highly questionable whether the Bt actually could cause an infection in intact skin, as commercial isolates were not infectious when applied to abraded skin of rabbits. This case as well as other putative mammalian infections were recently reviewed and critically assessed by Siegel.6 Based on these studies and analyses, there is no evidence that Bt strains from commercial products cause infections that lead to diseases of any significance in humans.

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