1.2. Disease symptoms

The 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.

Figure 1

 

Fig. 2. The infected larva loses its internal pressure and becomes flaccid, ultimately leaving a dark scale. Photo: Kaspar Ruoff.

Figure 2