Sacbrood is a brood disease caused by sacbrood virus. It takes its name from its most conspicuous feature: the dead larva becomes a fluid-filled sac.

It is confused with the foulbroods, yet it is a very different disease with far milder consequences.

What happens

The virus affects the larva at the stage of shedding its skin. The larva cannot shed the final skin; fluid accumulates between the skin and the body. As a result the larva cannot pupate and becomes a sac full of fluid.

Distinguishing appearance

• The sac: When drawn out carefully with tweezers the dead larva comes out whole, filled with watery fluid
• Position: The larva lies on its back with the head end raised. The shape is often compared to a slipper or a gondola
• Colour: The head end darkens first; the larva turns from yellow to brown over time
• Scale: On drying it leaves a curved scale that comes out of the cell easily

Differences from foulbrood

This distinction is critical, because the consequences are entirely different.

• Matchstick test: In American foulbrood the remains stretch like a thread. In sacbrood they do not
• Smell: The foulbroods have a sharp, foul odour. Sacbrood has no distinct smell
• Scale: In American foulbrood the scale adheres firmly to the cell. Here it lifts out easily
• Persistence: American foulbrood spores survive for decades. Sacbrood virus is not that durable

When it appears

It usually appears in spring, when the colony is expanding rapidly and under stress. Cool wet weather, a shortage of forage and congestion increase its frequency.

Its prevalence is low and in most colonies it remains at the level of a few cells.

What to do

In most cases the disease regresses on its own. There is no specific treatment and none is usually needed.

• Strengthen the colony. Feeding and adjusting the space are often sufficient
• Rely on hygienic behaviour. A healthy colony removes the affected larvae
• If it is widespread and persistent, requeen. A new queen creates both a brood break and a genetic change
• Renew comb. Take heavily affected frames out of circulation

Summary

Sacbrood is not as dangerous as its name suggests. Its real importance lies in the possibility of confusing it with the foulbroods. If no thread forms in the matchstick test and there is no smell, this is most probably what you are looking at.