What is Sacbrood Disease?
Sacbrood is a viral infection causes by the Sacbrood Virus. The virus itself is common but only causes disease in bees that are genetically susceptible. It is usually seen in the early spring and affects larvae about two days old.
The virus multiplies in the body tissues of the larvae as they grow. A grey, grainy fluid builds up under the skin of the larva. The skin can become a loose, fluid filled ‘sac’ around the body of the larvae, hence the name ‘Sacbrood’.
Like some other brood diseases, larvae usually die in the pre-pupal stage, after the cell is sealed. Sacbrood infected larvae also dry out into scales. In advanced cases, combs show irregular brood pattern similar to foulbrood diseases. However, it can easily be distinguished from American Foulbrood through the ropiness test – Sacbrood will not rope.
Identifying Sacbrood Disease
- Cells still capped after the surrounding brood has emerged may indicate Sacbrood
- In Sacbrood, you will find dead larvae lying fully stretched out against the lower walls of their cells
- The heads will be raised towards the upper cell walls, their mouthparts at the cell’s opening end
- The larvae will turn from white, to yellow, to dark brown and black, starting at the mouthparts
- Head growth is usually retarded in Sacbrood infected larvae
- The skin of the larvae will toughen and form a fluid fill ‘sac’ around the larva’s body tissues
- The dead larvae will eventually wrinkle and dry out to form a brittle scale that can easily be removed from the cell
- Both the infected larvae and scales have a characteristic ‘gondola’ or ‘banana’ shape
How is it spread?
- Sick and recently dead larvae contain a large amount of virus and are highly infectious
- However, the dead larvae become less infectious as they dry out
- Like most brood diseases, it is spread within the hive by cell cleaning bees
- These bees become infected when they remove the remains of diseased larvae
- The virus is believed to accumulate in the cleaning bees’ hypopharyngeal glands
- When these bees are older, they become nurse bees and infect new larvae through feeding
- The virus may be carried over from season to season by adult bees
- Drifting and robbing bees can spread the disease between hives
- However, it’s unhygienic practices by beekeepers that is most likely to cause spread
Management of Sacbrood Disease
It’s essential that you report brood diseases to your relevant state authority. Lab testing for the disease may also be available in order to confirm infection.
There is no method of treating virally infected bees. Re-queening with new queens, which come from hives that show resistance to the disease, may reduce the frequency or severity of outbreaks. These queens will likely boost the hygienic behaviour of the workers too.
Hygienic hive management by the beekeeper is essential to limit or prevent the spread of Sacbrood. Do not introduce bees or equipment from unknown sources. You should also avoid exposing honey combs or equipment to robber bees. This means storing excess equipment where bees can’t access it. Label hives and any extra combs and supers removed for winter, so you can return them to the same hive each year.
Signs of the disease usually disappear with warm weather and a good honey flow.
Sources: Sacbrood disease of bees at agric.wa.gov.au, amazingbees.com.au and The Australian Beekeeping Guide
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