How infection occurs
An AFB infection does not occur simply because a single bacterial spore enters a colony. Infections usually only result when millions of spores are introduced.
Many of the spores either end up stored in honey or find their way into the stomachs of adult bees, where they are removed from the colony when the bees defecate outside. Some spores may be fed to larvae, but not in high enough numbers to create an infection.
Colonies can still be infected without showing signs of diseased larva or pupa
Many colonies can be found that contain small numbers of AFB spores but never have a diseased larva or pupa. The hive may become contaminated because the colony has robbed out an infected colony, infected bees have drifted into the colony, or because a beekeeper has added contaminated equipment.
Disease criteria according to NZ law
The presence of AFB spores in a honey bee colony, or even in the gut of a larva, does not necessarily mean that the colony or the larva is diseased. A larva is not considered to be diseased until bacteria kill the larva, either before or after pupation. Likewise, a colony is not considered by New Zealand law to be diseased until it contains a diseased larva or pupa. Colonies that contain spores, but not diseased larvae, can be thought of as being “contaminated”, rather than diseased.
They are at risk of developing AFB, however, and should be managed appropriately (see eliminating AFB in beekpeeing outfits).
Colonies that are contaminated with AFB spores, but have no diseased larvae or pupae, have an increased risk of developing AFB.