The spores of the bacteria that causes AFB (Bacillus larvae) are viable
   for decades. A couple of years ago, the susceptibility (to
   oxytetracycline HCL (the active ingredient of Terramycin) of cultures
   from old spores (1924) was compared to the susceptibility of cultures
   from current (1994) spores.
 
   Shuimanuki H and D. Knox. 1994. Susceptibility of Bacillus larvae to
        Terramycin. ABJ Vol 134 No 2 p 125-126.
 
   Current AFB was found to be just as susceptible as old-time AFB.
 
   That result seems better than should have been hoped for, since
   beekeepers' use of oxytetracline for 40 years was sometimes nearly a
   recipe for developing bacterial resistance (ie. half-treat active
   infections, use the same product all the time,  use uneven or haphazard
   doses, then throw the half-empty package in the truck til next year,
   etc.). For some reason, resistant strains didn't survive in the field
   (or maybe they were infrequent enough that beekeepers weeded them out
   after the hives died).
 
   There's been a recent change, though, that makes the appearance of
   oxytetracycline resistant AFB more likely: widespread, nearly continuous
   use of antibiotic extender patties. This is the one ingredient that was
   missing: a continuous selection pressure.
 
   While continuous vegetable oil patties may be recommended for tracheal
   mite control, and while the same patty can be used to apply antibiotic,
   antibiotic extender patties should not be used for months at a time.
   (The vegetable oil (not the antibiotic) is the active ingredient for
   tracheal mites).
   Long-term (months) continuous use of antibiotic extender patties can
   result in antibiotic residues appearing in an extracted honey crop, and
   will act to select antibiotic-resistant strains of AFB.
 
   Antibiotic extender patties can be a useful tool to apply
   oxytetracycline, but should be used (where they are permitted) only for
   short periods (just as other methods: medicated syrup, dry application,
   etc. would be used).
 
   Kerry Clark, Apiculture Specialist
   B.C. Ministry of Agriculture, Fisheries and Food
   1201 103 Ave
   Dawson Creek B.C.
        V1G 4J2  CANADA          Tel (250) 784-2231     fax (250) 784-2299
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