American foulbrood (AFB) is an infectious brood disease caused by the spore-forming bacterium Bacillus larvae. It is the most widespread and destructive of the brood diseases, afflicting queen, drone, and worker larvae alike. Adult bees, however, are not affected by AFB. Bacillus larvae occurs in two forms: vegetative (rod-shaped bacterial cells) and spores. Only the spore stage is infectious to honey bees. Larvae less than fifty-three hours old become infected by swallowing spores present in their food. Older larvae are not susceptible. The spores germinate into the vegetative stage soon after they enter the larval gut and continue to multiply until larval death. New spores form after the larva dies. Death typically occurs after the cell is capped, during the last two days of the larval stage or first two days of the pupal stage.

Figure 32. A brood comb from a colony with American foulbrood, showing a typical scattered and irregular pattern of capped and uncapped cells.

Brood combs in an infected colony have a scattered and irregular pattern of capped and uncapped cells. Infected cells are discolored, sunken, and have punctured cappings. This "pepperbox" appearance contrasts with the yellowish-brown, convex, and entirely sealed cells of a healthy brood comb. Dead larvae change gradually from a healthy pearly white to light brown and then to a dark coffee-brown. With American foulbrood, this color change is uniform over the entire body. Within a month or so, these dead larvae dry

Figure 33. American foulbrood - larval remains "rope out" when a match stick is inserted and withdrawn. (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)

down into brittle scales that are almost black. Each scale contains as many as 100 million spores. The scales lie flat along the lower walls of the cells with the rear portion curving partway up the bottom of the cell. House bees cannot completely remove the scales from the cells. During the early stages of decay, up until about three weeks after death, the dead larvae have a gluelike consistency. The cell mass may string out for an inch or more when a toothpick is inserted and withdrawn; this is known as the "ropy stage." When death does not occur until the pupal stage, pupae undergo the same changes in color and consistency as larvae. In addition, a pupal tongue sticks up from the remains toward the top wall of the cell; this is one of the most characteristic symptoms of American foulbrood.

Only a few dead larvae or pupae will be seen when the colony is first infected by the disease. Once established though, AFB disseminates rapidly through the hive. If left unchecked, AFB spreads quickly to other healthy colonies at the same location and in nearby apiaries.

Nurse bees within the hive inadvertently feed honey contaminated with spores to young larvae, which perpetuates the disease. As the number of brood cells increases with the scales of dead larvae, which are spore reservoirs, housecleaning bees also aid in spore dispersal. Honey supplies within the brood chamber soon become contaminated as honey is stored in these spore-laden cells. Bees also transfer honey from the brood chamber to the supers above, thus spreading disease throughout the entire hive.

Figure 34. Scale of American foulbrood showing remains of pupal tongue. (Photo by M. V. Smith, University of Guelph, Guelph, Ontario.)

As the infection weakens a colony, the colony can no longer defend itself against robbers from strong colonies in the area. Spore-contaminated honey is spread quickly from hive to hive.

Another way American foulbrood is transmitted is through the interchange of combs between hives. When this disease is not recognized in an apiary, combs from a diseased hive inadvertently may be: (1) used in making splits or increases, (2) used in exchanging brood and food between hives, and (3) mixed with combs from other hives during honey extraction. In addition, the beekeeper's hive tool and gloves may spread AFB from hive to hive.

American foulbrood spores are highly resistant to desiccation, heat, and chemical disinfectants. These spores can remain virulent for more than forty years in combs and honey. Therefore, honey should not be purchased from other sources to feed bees. Only feed combs of honey if you are absolutely sure they are disease-free. An inexperienced beekeeper should not buy bees or equipment that have not been examined by an inspector or someone else familiar with the disease. Even a stray swarm from an infected colony may carry AFB.

American foulbrood cannot be transmitted to humans and has no effect on honey for human consumption. Because of the highly contagious and devastating action of the disease, every beekeeper should know the symptoms and be able to recognize AFB in its early stages. If you suspect disease and need help in diagnosis, contact your local apiary inspector or:

Chief of Apiary Inspection
Pennsylvania Dept. of Agriculture
Bureau of Plant Industry
2301 N. Cameron Street
Harrisburg, PA 17120
(717) 787-4843

Samples of diseased comb for laboratory tests can be sent to Harrisburg at the above address or to:

Beltsville Bee Laboratory
Building 476 BARC-Fast
Beltsville, MD 20705

Select a sample of brood comb about 4 inches square that contains a large number of affected cells. Mail it in a strong cardboard box without an airtight wrapping. Do not use aluminum foil or plastic bags. Samples that are crushed, wet from condensation, or moldy because of improper packaging make diagnosis impossible.

Colonies infected with American foulbrood should be destroyed by burning. The bottom board, hive bodies, supers, inner covers, and outer covers may be saved, disinfected, and reused. Before burning, diseased colonies should be killed in the evening after all foraging activities have ceased. The synthetic pyrethroid insecticide, resmethrin SBP 1382®, is approved for killing diseased colonies whose frames and combs will be destroyed. This insecticide, formulated as a 1 percent aerosol, is available from bee supply dealers. A ten- to fifteen-second application across the top of the frames under the inner cover is recommended. After treatment, hive exits should be sealed; all bees should be dead after thirty minutes.

Dig a pit 18 inches deep and wide enough to hold all combs and equipment to be burned. Build a fire in the pit. Set your unopen hive close to the pit and drop all combs and dead bees into the fire. After everything has been completely burned and the area cleaned of small pieces of comb or bees, cover the ashes with dirt.

Equipment that was saved (bottom boards, hive bodies, and covers) should be scraped to remove all propolis and wax, then scrubbed with a stiff brush and hot soapy water. Dispose of the wash water and burn the scrapings so they are not accessible to the bees. After scraping and scrubbing, all equipment should be either fire scorched or completely immersed in a boiling lye solution. Prepare your lye solution (sodium hydroxide) by mixing 1 pound of lye with 10 gallons of water. Boil the equipment for twenty minutes; wooden parts can be damaged by longer exposure. Weaker Solutions may not remove all of the wax and propolis from the equipment. Remember that lye solutions are caustic and can cause severe burns. Before using, read the label carefully and observe all precautions.

A blowtorch is suitable for scorching small quantities of equipment. Burn the surface until it is light brown and be sure to hit the corners. For large quantities of hive bodies, brush the inside surfaces with kerosene. Stack the hive bodies with the metal rabbets facing downward on top of each other, five to eight supers high. Ignite the stacks and allow them to burn long enough to lightly char the wood. Another approach is to fill the stack with wadded sheets of newspaper sprinkled with kerosene. Place an outer cover on top of the stack to smother the fire when you are finished.

Terramycin® (oxytetracycline HCL) is the only drug approved for use as a preventive treatment against American foulbrood. This antibiotic does not kill Bacillus larvae spores, but prevents or delays their growth when present in low concentrations in the food fed by workers to susceptible larvae. While this treatment allows individual larvae to survive, it does absolutely nothing about the virulent spores in the contaminated equipment. Thus the disease usually reappears once drug feeding stops.

Do not feed medication to your colonies when there is any danger of contaminating the honey crop. All drug feeding must stop at least four weeks before any surplus honey flow. Every precaution should be taken to ensure that no antibiotic will ever be present in honey taken from the hive.

Preventive Terramycin treatments are normally made early in the spring, at least one month before the first major nectar flow, and again in the fall after the honey crop has been removed. Because Tamarind is relatively unstable in honey or syrup solutions, it should be fed as a dust mixed with powdered sugar. Approved methods of treatment include mixing one part TM-5 with one part powdered sugar; one part TM-10 with two parts powdered sugar; one part TM-25 with five parts powdered sugar; or one part TM-2O with twenty parts powdered sugar. The TM-25 formulation is sometimes referred to as the soluble powder animal formulation. About 1 tablespoon of the drug-sugar mixture should be applied on the edges or ends of the top bars of the brood chamber. Do not sprinkle the mixture so that it drops into cells containing young larvae, as this can kill them. Two treatments, seven to ten days apart, are commonly made in the early spring and fall. A commercial ready-to-feed drug mixture also is available.

The secret of success in controlling American foulbrood is to find the disease in its early stages, burn all honey and combs, and disinfect the equipment before the disease can spread to other colonies. No treatment is considered totally effective for permanent control of American foulbrood. Therefore, the beekeeper should always be alert for possible recurrence.