A question often asked about long-term use of Terramycin® in American foulbrood control is whether there is risk that the causative organism, Paenibacillus larvae larvae, will become resistant? A 1994 study comparing the susceptibility to oxytetracycline HCL (the active ingredient in Terramycin®) of cultures from old spores (1924) to current spores showed no difference. (H. Shimanuki and D. Knox, American Bee Journal, Vol. 134, No. 2, pp. 125-126, 1994).
Mr. Kerry Clark, British Columbia’s provincial apiarist, wrote to the Bee-L list in November 1996 that the above result seems better than might have been hoped for. It meant that after 40 years of use, resistant strains of Paenibacillus larvae larvae were not surviving in the field. This was in spite of many observations on his part that beekeepers used uneven or haphazard doses and employed one product exclusively.
Unfortunately, Mr. Clark said, there’s been a recent change, making the appearance of resistance more likely: widespread, nearly continuous use of antibiotic extender patties. A reason for this is the adoption by many beekeepers of long-term vegetable oil patties for tracheal mite control. It seemed reasonable that beekeepers would also add Terramycin® foulbrood control in the same treatment to save labor. Thus, the one ingredient that was missing over the last four decades, continuous antibiotic selection pressure on the causative bacterium, is now present, Mr. Clark said. This use also provides much more opportunity for the antibiotic to wind up in the beekeeper’s honey crop. While vegetable oil patties may be recommended for tracheal mite control, and while the same patty can be used to apply antibiotic, the latter should be used only for short periods, he concluded.
Some of the practices reported by Mr. Clark may also have contributed to resistance development in Argentina. This adds to speculation that long-term exposure to Terramycin® is not without risk.
While at the Fifth Ibero Latin American Congress on Apiculture held May 30 through June 2, 1996 in Mercedes, Uruguay, I learned what can happen when AFB gets out of hand. Since it was first discovered in Argentina in 1989, the beekeepers in that country have gone through three stages in controlling AFB, according to Marcelo del Hoyo of the veterinary faculty, University of Buenos Aires. The first was denial, when the name American foulbrood was not uttered. When the disease was finally recognized in affected areas, beekeepers tried to combat the problem locally, but it was still ignored elsewhere in the country. The present stage is a mature condition where most good apiculturists have been exposed to the disease and have at least temporarily treated the problem.
The fact that Argentinean beekeepers went through the above phases, however, appears to have led to something more serious than in other parts of the world. It is the only country to my knowledge where AFB has become resistant to oxytetracycline. This was confirmed in a study reported at the Mercedes congress by A. Alippi and M. Aguilar (National University of Mar de la Plata). They used biochemical techniques (PCR) to analyze bacterial DNA. Resistance was found in only 28 percent of samples, but this translated to 58 percent of tested locations in the country. These results support the theory, according to the authors, that beekeepers used antibiotics to excess as an alternative to burning colonies. This behavior, along with inadequate doses of a wide range of products of dubious origin and quality employed as a preventative, appear to have favored the appearance of resistant bacteria.
Argentine researchers reported on another product, tylosin lactate (evidence of its effectiveness was first published by John Hitchcock and colleagues at the Laramie Bee Laboratory in the Journal of Economic Entomology. 63:1, pp. 204-207, February, 1970), which they say will control the disease as an alternative to Erythromycin and oxytetracycline. M. del Hoyo also listed a suite of controls now used by beekeepers, including burning colonies, using packages to move bees off contaminated hives, making splits into clean equipment, applying chemotherapy and various sanitation procedures (dipping in hot wax and lye solution and scorching supers). These must be used in conjunction with inspection at least every 90 days, he concluded, as one abandoned diseased colony can undermine a control program that has taken years to implement.
