Persistence of Tetracycline Activity in Medicated Syrup Stored in Honeybee Colonies in Late Spring

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Abstract
Summary Some beekeepers in northern Saskatchewan, Canada, have reported recurrent infection in their colonies by Streptococcus pluton, cause of European foul brood disease, in spite of preventive treatment with syrup containing oxytetracycline antibiotics. Ineffective or understrength formulations of the commercial tetracyclines used had been suspected as a contributing cause. However, microbiological assays with Bacillus subtilis spore suspensions conducted on a tetracycline-containing powder actually used showed antibiotic activity equivalent to the specifications on the labels. Oxytetracycline activity was destroyed in medicated syrup by exposure in an externally placed glass jar in 24 hours at a maximum temperature of 32°C, while tetracycline persisted for more than 24 hours at a maximum temperature of 39°C. Oxytetracycline activity persisted in treated syrup in externally placed feeder jars, painted black to exclude sunlight, for periods of at least 6 days, even though the syrup temperature reached 40°C during daylight hours. Oxytetracycline-treated syrup marked with a dye was fed to and stored in the combs by a test colony in Prince Albert, Saskatchewan, in late June 1972. No antibiotic activity was detected in the marked syrup stored in the combs later than 2 weeks after feeding. Similar tests with two colonies in Ottawa in July, 1973, disclosed very rapid dilution of the syrup when stored in the combs. Only two samples of marked syrup contained antibiotic activity detectable by the B. subtilis assay two days after feeding began.

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