Classification of AFB
A colony is said to have AFB when one or more of its larvae are fed enough spores to kill them.
In some colonies, the beekeeper may never be aware that a diseased larva or pupa has been produced. The house bees may remove diseased larvae as fast as they become infected, so all the beekeeper sees is a spotty brood pattern (Fig. 13).
Figure 13: Spotty brood pattern
The house bees may do this effectively enough to eliminate the infection from a colony.
Infections where no visual symptoms appear in the colony are called “inapparent” or “subclinical” infections. However, it is not possible to differentiate between a colony that is contaminated with spores and has no diseased larvae or pupae, and one that has a subclinical infection.
We may never know how many colonies develop subclinical infections of AFB and eliminate the disease by themselves.
Many beekeepers have noticed colonies that appear to recover from AFB. Having found a colony with a single diseased larva or pupa, they check the colony a few days later before burning it and are then unable to detect any sign of the disease.
It pays to be vigilant
The danger for beekeepers is that although some colonies may eliminate the disease completely, for other colonies the recovery will only be temporary. Because there may still be large numbers of AFB spores in such colonies, they may develop visual symptoms of the disease at a later date, and the symptoms will be severe enough to destroy the colony.
Obviously, colonies with subclinical infections are a factor in the spread of AFB. Even if beekeepers carefully check every frame of brood for diseased larvae and pupae before removing frames to place in other colonies, they cannot guarantee that they are not going to spread the disease.