Interactions between Bacillus thuringiensis subsp. israelensis and Fathead Minnows, Pimephales promelas Rafinesque, under Laboratory Conditions

Interactions between Bacillus thuringiensis subsp. israelensis and fathead minnows, Pimephales promelas, were studied in laboratory exposures to two commercial formulations, Vectobac-G and Mosquito Attack. Mortality among fatheads exposed to 2.0 × 10⁶ to 6.5 × 10⁶ CFU/ml with both formulations was attributed to severe dissolved oxygen depletion due to formulation ingredients rather than to direct toxicity from the parasporal crystal. No adverse effects were observed at 6.4 × 10⁵ CFU/ml and below. Fathead minnows rapidly accumulated high numbers of spores with 1 h of exposure to 2.2 × 10⁵ CFU of Mosquito Attack per ml, producing whole-body counts of 4.0 × 10⁶ CFU per fish. Comparison of counts on gastrointestinal tract samples and whole-body samples and high numbers of spores in feces indicated that ingestion was the major route of exposure. B. thuringiensis subsp. israelensis spore counts decreased rapidly after transfer of fish to clean water, with a drop of over 3 orders of magnitude in 1 day. Spores were rarely detected in fish after 8 days but were detectable in feces for over 2 weeks. These findings suggest that fish could influence the dissemination of B. thuringiensis subsp. israelensis, and possibly other microbial agents, in the aquatic environment.