Key points
- A viral brood disease caused by sacbrood virus (SBV).
- The dead larva forms a fluid-filled sac with its head raised like a "gondola".
- No rope in the matchstick test: that tells it apart from American foulbrood.
- Often short-lived in spring; requeening helps if it recurs.
Sacbrood is the easiest viral brood disease to recognise. In European honey bees it usually stays limited and often disappears once a nectar flow starts.
The agent
Sacbrood virus (SBV) is very widespread, usually without symptoms. Larvae become infected through the food given by nurse bees. An infected larva cannot moult into a pupa: it dies just after capping, and fluid builds up under its skin1.
In the Asian honey bee, Apis cerana, one form of the virus causes heavy losses.
Symptoms
- Perforated cappings, patchy brood.
- Larva stretched on its back with its head raised towards the capping, like a “gondola”.
- Tough skin: the larva comes out of the cell whole, like a small fluid-filled bag.
- Pearly white colour turning yellow and dark brown, starting at the head.
- No particular smell and no rope.
Diagnosis
The sac sign, obtained by gently lifting out the larva, is usually enough. PCR confirmation in a laboratory is possible2.
What to do
There is no treatment. Measures target the colony:
- Strengthen the colony and make sure pollen and nectar stores are good.
- Remove heavily affected combs.
- Requeen if the disease returns every year.
- Control varroa, which helps viruses spread3.
References
- Chen Y.P., Siede R. (2007). Honey bee viruses. Advances in Virus Research 70: 33-80.
- de Miranda J.R. et al. (2013). Standard methods for virus research in Apis mellifera. The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(4).
- FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.