Transferring frames of brood the second most common cause of the spread of AFB
Beekeepers often transfer frames of brood from one colony to another, either to boost the populations of small colonies, or to reduce the strength of colonies in an effort to prevent swarming.
Unfortunately, transferring a frame of brood from an AFB-infected colony to a clean colony is a very effective way of spreading AFB. To put it in context, a colony may need to be fed five million AFB spores to become infected. However, one diseased larva or pupa can contain 2,500 million spores, or 500 times the number required to initiate an infection.
If a frame of brood has 50 or more diseased larvae/pupae, and the house bees come into contact with large numbers of spores in removing the dead brood, the chances of sufficient spores being fed to a healthy larva to set off a new infection are very high.
The transferring of brood frames is probably the second most common way AFB is spread between colonies in New Zealand. Swapping brood frames between hives is only less important than extracted honey supers because brood frames are exchanged less frequently.
Swapping frames of brood between colonies is the most guaranteed method of transmitting AFB between colonies.