1.2. Disease symptoms
field diagnosis of EFB is based on the visual inspection of brood combs and
detection of diseased larvae (see section 4). The general symptoms observed in
a colony suffering from EFB are irregular capping of the brood; capped and
uncapped cells being found scattered irregularly over the brood frame (known as
pepper pot brood). The youngest larvae that die from the infection cover the
bottom of the cell and are almost transparent, with visible trachea. Older
larvae die malpositioned and flaccid in their cells; twisted around the walls
or stretched out lengthways (Fig. 1). The colour of affected larvae changes
from pearly white to pale yellow, often accompanied by a loss in segmentation.
More advanced symptoms can manifest as further colour changes to brown and greyish
black (Fig. 1), sometimes ultimately leaving a dark scale (Fig. 2) that is more
malleable than those typically found with American foulbrood (AFB).
Fig. 1. Malpositioned and discoloured larvae in brood with symptoms of European foulbrood. Photo: Eva Forsgren.
Fig. 2. The infected larva loses its internal pressure and becomes flaccid, ultimately leaving a dark scale. Photo: Kaspar Ruoff.