Honey Bee Disorders: Bacterial Diseases
AFB is the most serious bacterial disease of honey bee brood and is caused by the bacterium Paenibacillus larvae. The disease is transferred and initiated only by the spore stage of the bacterium. The reason this disease is so serious is that the spores can remain viable and last indefinitely on beekeeping equipment. It is extremely contagious and spreads easily on contaminated equipment, hive tools, and beekeeper’s hands. A beekeeper’s best way to manage AFB is to avoid it.
Normal healthy larvae (Fig. 1) are glistening white, but AFB-infected brood turn chocolate-brown and melt into a gooey mass on the floor of the cell (Fig. 2). They may exhibit a syndrome called 'pupal tongue' where the tongue protrudes to the top of the cell, as shown in Fig. 2. As the disease progresses, colonies may also display a pepper box symptom (Fig. 3) where the cappings are perforated and sunken into the cell. When the larvae are brown and have not formed a hardened scale, the symptom of ropiness can be demonstrated. To do this, poke a stick into this mass, macerate it and withdraw it from the cell. If AFB is present the contents will 'rope out' (Fig. 4) up to one inch. This is the most definitive field test for AFB. As the dead larva dries, it becomes a black scale that adheres tightly to the cell floor. The comb in Figure 5 is being held so that sunlight is falling onto the longitudinal floors of the open cells; in many of the uncapped cells the scale can be seen as a black mass on the floor. See Figure 6 for a detailed view of black masses in comb. These scales are difficult to remove and remain a site for constant re-infection. A single scale can contain one billion spores, and it takes as few as 35 spores to trigger the disease. These scales are difficult to see and can easily be missed when purchasing used equipment. Colonies with high levels of AFB will have a foul odor similar to a chicken house. As more and more brood becomes infected and dies, the colony dwindles and eventually collapses.
One has an advantage if the beekeeper can purchase brand new hive equipment, install package bees, and maintain them perpetually in isolation from other apiaries. This, however, is not always practical or realistic. It always makes good sense to practice sanitation practices such as washing hands and hive tools between apiaries, avoiding used hive equipment of unknown or suspicious history, and avoiding feeding bees honey from unknown sources.
It is possible to breed for bees that are genetically resistant to AFB and other diseases. One of the most important characteristics in bees is the so-called 'hygienic behavior' – the ability of bees to detect and remove from the colony abnormal cells of brood. Hygienic queens are available from nationally-advertised queen breeders. See advertisements listed in American Bee Journal or Bee Culture.
Another tactic for preventing AFB and a similar disease, European foulbrood (EFB - see below), is biennial treatments of the veterinary antibiotic Terramycin™. However, this medication is no longer available over-the-counter; it is now by veterinary prescription only. It is fed as a mixture in either powdered sugar, sugar syrup, or in vegetable oil extender patties and limited to the months of September and February in Georgia. It is important to never feed Terramycin within four weeks of a nectar flow to avoid contaminating honey for human consumption.
The current restricted use of Terramycin is evidence of bacterial resistance that has developed in this country. In any case, it has always been preferable to rely on sanitation and resistant queen stock for the management of this disease.
AFB is regulated by the Georgia Department of Agriculture, and infected colonies are normally burned by state inspectors. Contact the Georgia Department of Agriculture state inspector at firstname.lastname@example.org for more information or to report an infected colony. The spores of the AFB bacterium are extremely persistent in contaminated comb and hive parts. Although resistant bee colonies may clean up visible signs of infestation, it is more typical for AFB to be incurable and essentially doom the colony. Beekeepers should never maintain 'hospital yards' in which they group AFB colonies together in isolation. Such yards simply serve as reservoirs of disease that will serve to contaminate apiaries for miles around. It is equally inadvisable to treat infected colonies with Terramycin. The antibiotic will simply obscure visible signs of the disease, but the symptoms will rapidly recur once the antibiotic is removed.
EFB is a bacterial disease of honey bee brood. It is considered less virulent than American foulbrood, and colonies sometimes recover from infection. Its field symptoms are easily confused with those of AFB, but there are important differences. Instead of being a healthy pearly white (Fig. 7), larvae with EFB appear off-white, progressing to brown, and are twisted in various positions in the cell (Figs. 8, 9,10). Larvae with EFB usually die before they are capped whereas larvae with AFB die after they are capped.
The sanitation precautions recommended in the section on AFB apply also to EFB. Likewise, bee stocks selected for hygienic behavior can be expected to minimize outbreaks of EFB. The disease sometimes goes away on its own at the onset of a strong nectar flow. The beekeeper may be able to control the disease by simulating a nectar flow (by feeding sugar syrup) and by requeening the colony.
Preventive biennial treatments with Terramycin antibiotic, as recommended in the section on AFB, will also prevent EFB. As with AFB, it is important to consider antibiotic treatments as a preventive measure, not a cure. Terramycin treatments in EFB-infected colonies may actually be counterproductive because the medication permits those infected larvae to survive which would otherwise perish. These survivors then persist in the colony as a source of contamination. If the infected larvae are instead permitted to die, the house bees eject them from the hive and with them goes the source of infection. The bacterium does not form long-lived spores that persist on hive surfaces.