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 are glistening white, but AFB-infected brood turn chocolate-brown and melt into a gooey mass on the floor of the cell.
They may exhibit a syndrome called 'pupal tongue' where the tongue protrudes to the top of the cell.
As the disease progresses, colonies may also display a pepper box symptom 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' 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 above 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.
Pictured above ia 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.