Sacbrood

Sacbrood is a brood disease caused by a virus that usually runs a mild course. The dead larva turns into a fluid-filled sac and can be pulled out of the cell in one piece. Its main importance is that it can be confused with the notifiable foulbroods; making the distinction correctly prevents both needless panic and a real foulbrood going unnoticed.

What is it and how does it spread?

The agent is sacbrood virus (SBV). The virus reaches young larvae a few days old through the food fed by nurse bees; however, the larva dies after the cell has been capped, at the prepupal stage. The larva cannot complete its final moult; fluid collects between the old skin and the body, and the larva turns into a fluid-filled sac.

Young worker bees that clean out dead larvae pick up the virus and carry it in their food glands. These bees show no symptoms, but their lifespan may be shortened and they spread the virus while feeding larvae. The varroa mite can also carry the virus. Between colonies it is passed on by robbing, drifting bees and moving frames.

The virus is present without symptoms in many colonies. The disease usually appears in spring and early summer, when brood rearing is increasing rapidly; once the nectar flow begins it often subsides by itself.

How to recognise it

• Brood pattern: Scattered (patchy) brood; in between, cells with perforated or completely uncapped cappings.
• Position: The dead larva lies stretched out on its back along the cell; the head end is raised towards the roof of the cell. This appearance is likened to a boat, a gondola or a slipper.
• Colour: The larva turns from pearly white to yellow, then brown. The darkening starts at the head end.
• Sac: The larva's skin is intact; when pulled out carefully with tweezers it comes out whole, without tearing, as a fluid-filled sac.
• Scale: When it dries, a dark brown, boat-shaped scale with a raised tip remains. The scale does not stick to the cell wall and is easily removed.
• Smell: There is no distinct foul smell.

How it differs from similar diseases

How to confirm it in the hive

1. Open the capping of a suspect cell and look at the position and colour of the larva.
2. Pull the larva out gently with tweezers. If it comes out whole as a fluid-filled sac, sacbrood is very likely.
3. Do the matchstick test on another cell: push the matchstick into the remains and pull it out slowly. No thread should form.
4. Check the smell and whether the scales stick to the cell.
5. If it ropes, if there is a foul smell or if you are unsure, report it to the official veterinary service / local agriculture authority without moving the frame to another hive, and request a laboratory examination. A definitive virus diagnosis is also made in the laboratory.

What to do

In most cases the disease subsides by itself within a few weeks; colonies with strong hygienic behaviour clear out the dead larvae quickly.

1. Strengthen the colony. Check the honey and pollen stores; if there is a shortage, give syrup and a protein supplement.
2. Adjust the hive volume. Remove surplus frames that the bees cannot cover; prevent the brood from chilling.
3. Take heavily affected frames out of circulation. Melt down old combs containing many dead larvae and give foundation in their place.
4. If it is persistent and widespread, requeen. Requeening both creates a short break in brood rearing and changes the colony's genetics to a line with strong hygienic behaviour.
5. Control varroa. If the mite load is high, the virus spreads more easily.
6. Do not move frames. Do not give brood frames from a colony showing symptoms to other hives.

Treatment and medicines

There is no medicine for sacbrood. Antibiotics have no effect on the virus, leave residues in honey and must not be used. What overcomes the disease is a strong colony, good nutrition and hygienic behaviour.

Prevention

• In spring keep colonies well fed and at a strength matching the hive volume.
• Replace brood combs every few years.
• Use queens from lines with strong hygienic behaviour.
• Prevent robbing; reduce the entrances of weak colonies.
• Monitor the varroa level throughout the year.