Larvae affected by the virus die after the cell has been capped. Nurse bees pierce the capping to expose the dead larvae and the raised mouth parts of the larvae are a sign of sacbrood. The affected larvae change colour starting at the mouth parts from white through to yellow and dark brown. Some affected larvae develop a sac of liquid at the anal end.
The brown larvae may dry to form wrinkled, brittle scales which are easily removed from the cell. Drone brood may also be affected with sacbrood disease.
Cells still capped after the surrounding brood has emerged may indicate that the disease is present. Sacbrood is most commonly seen in the early spring and the signs of the disease disappear with warmer weather and a good honey flow.
The first appearance of sacbrood should not be confused with American foulbrood disease. The distinguishing characteristics are that the brown larvae will not ‘rope’ as with American foulbrood disease and the beekeeper should test for ‘rope’ effect. Scales caused by sacbrood are easily removed while those caused by American foulbrood disease adhere to the cell wall and cannot be removed in one piece. Where American foulbrood and sacbrood diseases occur in a hive at the same time, the visual signs and ‘rope’ effect of American foulbrood disease may become less obvious.
It is essential any sign of disease is reported and a piece of comb, together with slide smears is sent for laboratory confirmation. Correct diagnosis will enable the beekeeper to manage the disease and prevent further spread within the apiary.