This a viral infection of the brood caused by Iflavirus. It becomes visible when the diseased larva fails to pupate after being sealed in its cell. Fluid accumulates between the body of the larva and the unshed skin, forming a sac. This disease is actually relatively common, mostly during the first half of the brood-rearing season and can often go unnoticed as it often affects a small percentage of the brood. As such it does not usually cause severe colony loss.
Infected larva appear to develop normally until after being capped over.

The larva then turns from its normal pearly white to a pale yellow colour
After dying it begins to dry out, turning a dark brown to black colour, giving rise to the characteristic ‘Chinese slippers’ or ‘gondola-shaped’ scales
As they die the workers will uncap the cells to expose them, creating an uneven brood pattern with the discoloured, sunken or perforated cappings scattered through the brood cells
The skin of the dead larva turns into a tough plastic-like sac filled with fluid, hence the name (the sac can be carefully removed by using a pair of tweezers)

Image Courtesy The Animal and Plant Health Agency (APHA), Crown Copyright

This virus can be spread via Varroa, through the intervention of the beekeeper by transferring material from infected colonies to a healthy colony, and by the feeding behaviour of nurse bees. Bees robbing infected colonies can also spread the disease.

Re-queening the colony can help to alleviate the symptoms of sacbrood and controlling Varroa mite populations will help to control the spread of the virus.

Image Courtesy The Animal and Plant Health Agency (APHA), Crown Copyright