Most of the following information is taken from the New Zealand Beekeepers web site and from a booklet that they have online called Elimination of American Foulbrood Without the Use of Drugs. The book is fairly detailed and quite thorough so I have picked several topics that are covered in the book and have based the following article from my understanding of the subject
American Foulbrood is caused from the bacteria Bacillus Larvae. The life cycle goes through several stages and each stage is important for the beekeeper to understand. We generally see only the active stage when infection is present and recognizable in the hive. Dormant AFB is difficult to recognize but can have a serious impact on the beekeepers operation. Just as important as the recognition of AFB is the knowledge of how the disease is spread.
AFB infects primarily the developing Larvae of the hive. Young developing larvae either are fed brood food (honey/pollen mixture) that is contaminated with the bacteria spores or the spores are present in the comb where the egg is laid. In either case the larvae become infected with the bacteria and die before they mature into young emerging bees. Most of the time death of the larvae occurs after the bees have capped the brood and the larva dies under the capping. It is at this point that the infected and dead larvae are removed from the comb by the nurse bees and the contamination is spread through the hive. In the process of removing the dead larva the nurse bee contaminates the mouth that is used to feed the healthy larvae. Since bees routinely share food and move throughout the hive, the honey and stored pollen becomes contaminated with bacteria and the bacterial spores.
Bacterial spores are a dormant phase of the bacteria's life cycle. The shell of the spore is a tough and resistant membrane that will allow the bacteria to survive under harsh conditions. It is logical to compare the spore to the seed of a plant, capable of remaining alive during conditions that are unacceptable to the plant itself (winter, lack of rain...) in the case for the AFB spores, the viability extends for up to 50 years. The spores are not affected by cold temperatures and can survive in abandoned equipment or encapsulated in wax or propolis until they are exposed to young larvae by ingestion.
Symptoms of AFB should be recognizable by the beekeeper if the beekeeper has periodic close examinations of the brood pattern within the hive. Things to look for include the cappings of the brood that appear to be perforated or have a hole chewed in the center. Nurse bees will do this after the larva dies in an attempt to remove the body. The larva is somewhat liquefied and is difficult to remove. If a small stick is inserted into the infected cell and the larva is stirred slightly the stick is covered with sticky goo that will cling to both the stick and the remaining larva in the cell. The contents of the cell will rope out up to an inch from the cell when the stick is removed. There may be as few as 2 or 3 of these cells on a pattern of brood so it is necessary to have a very close look at the capped brood to find this. Since the dead larva is difficult to remove it often dries up to form a dead scale or hardened layer of dead larval remains in the cell. This is difficult for the worker bees to remove and the bees will be unable to clean the cell well enough to make it acceptable for the queen to lay an egg in. The brood pattern will have holes in it of unlaid cells. This may be easier to spot and should cause the beekeeper to examine the hive more closely. Decaying brood smells bad and a heavily infected hive may have a smell to it that has been described as a glue pot odor.
Keep in mind that the antibiotic Terramycin is used to treat AFB but only works on the active stage of the life cycle. The spores are unaffected by this drug and persist in the hive in a waiting game with the beekeeper. AFB is undetectable to most beekeepers in hives treated with antibiotics but can still be present. When the season comes along that the beekeeper does not treat the hive with antibiotics the spores will become active. Many beekeepers that have a few hives would rather know that they have a problem than have an underlying infection ready to contaminate other hives in the area. In either case the cure for AFB in the active stage is destruction of the hive by burning all frames and comb, and scorching all of the woodenware with a torch.
Most beekeepers concentrate on the prevention of the spread of the disease. It is most useful to know what are the factors in getting a case of AFB into your clean apiary. I have listed the most common factors in order of most risk to the ones of least risk. Since Alaska has a very low incidence of AFB some of the factors listed won't seem as great as if they would be in more populated areas.
- Transfer of frames of brood between hives.
- Mixing supers between hives during the next season
- Allowing wet supers to be cleaned out that are from AFB hives
- Robbing bees
- Feeding commercial pollen or honey to a clean colony
Low on the list is: drifting bees, tools, hands and clothing