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Viruses

Sacbrood

Sacbrood virus (SBV)

Severity : Moderate Spring and early summer, or during dearthReview in progress
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Key points

  • A viral brood disease caused by SBV, a very widespread iflavirus, often without symptoms.
  • Infected nurse bees pass the virus to young larvae through the brood food they produce.
  • The dead larva forms a fluid-filled sac, then a dry "gondola" scale that does not stick.
  • No rope and no smell: that is what sets it apart from foulbrood.
  • No treatment: nectar flow, strong colonies, varroa control and requeening if it recurs.

Sacbrood is the easiest viral brood disease to recognise: the larva dies just after capping and turns into a small fluid-filled bag that can be lifted out of its cell in one piece. In European honey bees, the disease usually stays limited to a few cells and often clears up on its own once the nectar flow arrives.

Yet it is often confused with European foulbrood, or even with American foulbrood. The same virus also affects adult bees, quietly, and a closely related form has caused devastating epidemics in the Asian honey bee, Apis cerana.

The virus

Sacbrood virus (SBV) is a small, non-enveloped RNA virus in the family Iflaviridae, genus Iflavirus1, 6. It is one of the most widespread bee viruses: it has been detected on every continent where Apis mellifera lives, usually in colonies with no symptoms13.

Spread by nurse bees

Leslie Bailey’s work showed that SBV multiplies in adult bees without making them visibly ill, and that it builds up mainly in the head, especially in the hypopharyngeal glands, which produce brood food4. An infected nurse bee therefore hands the virus to larvae along with their food.

A dead larva contains a huge amount of virus. The young workers that remove it become infected in turn, become nurses and close the cycle4. Other routes exist:

  • vertical: the virus has been found in the ovaries and eggs of infected queens6;
  • through stores: pollen, honey and larval remains can stay infectious for several weeks6;
  • between colonies: swapping combs, drifting and robbing13;
  • through varroa: its presence is linked to higher SBV prevalence, but it appears to act only as a carrier6.

In winter, without brood, the virus persists in adult bees. A colony can carry it for a long time without symptoms9, 13.

Why the larva becomes a sac

To become a pupa, the larva must shed its last skin. In an infected larva this moult fails: moulting fluid builds up between the body and the skin, which does not come away. The larva dies at the prepupal stage, stretched on its back inside a sac of virus-laden fluid1, 9.

Impact on the colony

On the brood

Most of the time only a few cells are affected, but severe forms do exist. A documented case in France describes a heavily affected colony with several aggravating factors, including varroa: it recovered within six weeks, but produced no more honey that season and remained at higher risk of collapsing over winter11.

On adult bees

Infected adults show no visible signs, but their behaviour changes. According to Bailey and Fernando, infected workers start flying earlier than healthy bees, and most infected foragers do not collect pollen; the few that do bring back pollen loads laden with virus. These bees also cope less well with cold5. Young infected bees eat little pollen and, in the laboratory, live about as short a time as bees deprived of pollen4.

In the Asian honey bee: Thai sacbrood virus

In Apis cerana, the picture is very different. A strain isolated in Thailand was described in 1982 as Thai sacbrood virus (TSBV)7.

These strains have caused major epidemics in Asia, including India, China, Vietnam, Thailand and South Korea; according to a recent review, losses of 95 to 100% of A. cerana colonies have been reported6. In India, surviving colonies remained weakened and less prolific8. In South Korea, an epidemic began in 200912. The Asian strain can also reach Apis mellifera: in 2016 it was linked to very high mortality in apiaries in Shandong province, China6.

Recognising the disease

Symptoms

  • Patchy brood, with sunken, perforated or removed cappings.
  • Larva stretched on its back, head raised towards the capping.
  • Sac sign: the tougher skin stays intact. Lifted out with tweezers, the larva comes out whole, like a small bag of fluid.
  • Colour: pearly white, then pale yellow, then dark brown, starting at the head and thorax9.
  • Gondola sign: as it dries, the larva forms a flattened dark brown to black scale with a raised head, shaped like a boat. This scale does not stick to the cell wall9.
  • No particular smell, no rope; adult bees look normal.
Photo needed — Larva lifted out of its cell with tweezers, laid on a light background, showing the intact fluid sac and darkened head.
Photo needed — Patchy brood comb with several uncapped cells showing "gondola" larvae, head raised and dark brown.

Do not confuse

The most common confusion is with European foulbrood9.

Criterion Sacbrood American foulbrood European foulbrood Chilled brood
Stage affected Prepupa, just after capping Older larva or pupa, under capping Mainly young uncapped larvae All stages, pupae included
Appearance Fluid sac, tough skin, head raised Sticky brown mass, slumped Yellowish, twisted, “melted” larva Greyish to black larvae and pupae
Matchstick test No rope Elastic rope No true rope No rope
Smell None Strong Often sour Possible putrid smell
Scale Gondola, easy to remove Flat, firmly stuck Soft, loosely attached No typical scale
Distribution Scattered cells Scattered cells Scattered cells Patches at the edge of the brood nest
Context Spring, dearth Any season Spring, summer Cold snap, population drop

Chalkbrood, for its part, produces chalk-hard mummies. If you suspect foulbrood, have it tested: American foulbrood is a regulated disease.

Diagnosis

In the apiary

  1. Look at the comb: spot uncapped or sunken cells.
  2. Lift out a larva with tweezers: if it comes out whole, sac-shaped, sacbrood is very likely.
  3. Do the matchstick test on another suspect cell: no elastic rope argues against American foulbrood.

The sac sign is often enough. But a colony can carry SBV and foulbrood at the same time: if the picture is atypical or the brood heavily affected, a laboratory test is needed6.

In the laboratory

  • RT-PCR and quantitative PCR: the reference methods. They detect viral RNA and, in quantitative form, measure viral load2, 10. A high load in diseased larvae confirms the disease; simply detecting the virus in adults is commonplace.
  • Serology: immunodiffusion was long used for routine diagnosis, while ELISA can handle many samples6, 9.

A piece of comb containing affected larvae is usually sent, possibly with bees from the colony13. In some countries, SBV PCR is only done on request and at the beekeeper’s expense13.

How it changes through the season

Outbreaks mainly appear in spring and early summer, or during a dearth: the colony is then rearing a lot of brood with relatively few young bees9. Infection usually recedes in summer, when the nectar flow sets in and workers clean out the affected cells9. The virus then overwinters in adult bees and can reappear the following spring.

Regulation

In France, sacbrood is not a notifiable disease. It is not on the World Organisation for Animal Health (WOAH) list of bee diseases14. Rules vary between countries. This status only holds if the diagnosis is certain: a suspicion of American foulbrood must be reported.

Prevention and what to do

There is no medicine against SBV. Measures target the colony3, 13.

  • Secure food supply. A colony well supplied with nectar and pollen clears affected cells faster. During a gap in the flow, supportive feeding is recommended13.
  • Strengthen weak colonies with healthy capped brood or by uniting: the more young bees, the faster the clean-up.
  • Remove heavily affected combs and render them: the virus is heat-sensitive13.
  • Requeen if it recurs. Repeated susceptibility often reflects poor hygienic behaviour, which is partly genetic13. Requeening also creates a brood break that interrupts the virus cycle6.
  • Control varroa, which is linked to higher SBV prevalence6, 11. See the Varroa factsheet.
  • Limit spread: no brood combs from an affected colony to a healthy one, no robbing. See Apiary biosecurity.

In practice: what beekeepers actually do

Sacbrood often causes more worry than harm. Here is how beekeepers actually handle it, without any treatment.

Hands-on steps

  • Rule out foulbrood first: larva lifted with tweezers, matchstick test on another cell, smell of the comb.
  • Record the colony and how widespread the symptoms are, to spot a recurrence.
  • Check stores of pollen and honey around the brood.
  • Come back one to two weeks later to check the cells have been cleaned out.

Common strategies

1. Wait for the flow. For a few cells in a strong colony, most beekeepers do nothing more: symptoms often disappear as nectar and pollen come in.

2. Strengthen. For a weak or clearly affected colony, it is common to add a frame of healthy capped brood, unite two weak colonies, or feed if forage is lacking.

3. Requeen if it comes back. A colony that shows sacbrood two springs running is often given a new queen from a hygienic line.

4. Watch varroa. A sacbrood episode is a good moment to count mites: high infestation makes every viral disease worse.

Useful kit

  • Fine tweezers and matchsticks or sticks.
  • A head torch to see the bottom of the cells.
  • Varroa counting equipment.
  • A box and paper to send a piece of comb to the laboratory.

Checklist

  1. Sac-shaped larva, no rope, no smell: probable sacbrood.
  2. Doubt about foulbrood: sample and test.
  3. Low stores: supportive feeding.
  4. Weak colony: strengthen or unite.
  5. Varroa: count, treat if needed.
  6. Recurrence the following year: requeen.

The most common mistakes

  • Diagnosing sacbrood without a matchstick test, and missing foulbrood.
  • Conversely, destroying a sacbrood colony that would have recovered on its own.
  • Moving affected brood combs into healthy colonies.
  • Keeping a queen whose colony shows symptoms every year.
  • Forgetting varroa, which makes every viral disease worse.

Frequently asked questions

Should an affected colony be destroyed? No, unless it is very heavily affected and does not recover13.

Is honey from an affected colony safe to eat? Yes: SBV is a bee virus and harmless to humans. Avoid feeding that honey to other colonies, however.

Does Thai sacbrood virus threaten Europe? These strains have been described mainly in Asia. Their ability to affect Apis mellifera in China nonetheless justifies vigilance over bee imports6.

Does a positive PCR on adult bees mean the colony is sick? Not necessarily: the virus is present in many healthy colonies. A high viral load in larvae showing symptoms is what confirms the disease2, 11.

References

  1. Chen Y.P., Siede R. (2007). Honey bee viruses. Advances in Virus Research 70: 33-80. doi.org
  2. 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). doi.org
  3. FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
  4. Bailey L. (1969). The multiplication and spread of sacbrood virus of bees. Annals of Applied Biology 63(3): 483-491. doi.org
  5. Bailey L., Fernando E.F.W. (1972). Effects of sacbrood virus on adult honey-bees. Annals of Applied Biology 72(1): 27-35. doi.org
  6. Wei R., Cao L., Feng Y., Chen Y., Chen G., Zheng H. (2022). Sacbrood virus: a growing threat to honeybees and wild pollinators. Viruses 14(9): 1871. doi.org
  7. Bailey L., Carpenter J.M., Woods R.D. (1982). A strain of sacbrood virus from Apis cerana. Journal of Invertebrate Pathology 39(2): 264-265. doi.org
  8. Verma L.R., Rana B.S., Verma S. (1990). Observations on Apis cerana colonies surviving from Thai sacbrood virus infestation. Apidologie 21(3): 169-174. doi.org
  9. Ball B.V. (1999). Sacbrood. In: Colin M.E., Ball B.V., Kilani M. (eds), Bee disease diagnosis. Options Méditerranéennes, Series B, No. 25: 91-97. CIHEAM, Zaragoza. ressources.ciheam.org
  10. Grabensteiner E. et al. (2001). Sacbrood virus of the honeybee (Apis mellifera): rapid identification and phylogenetic analysis using reverse transcription-PCR. Clinical and Diagnostic Laboratory Immunology 8(1): 93-104. doi.org
  11. Roy C., Vidal-Naquet N., Provost B. (2015). A severe sacbrood virus outbreak in a honeybee (Apis mellifera L.) colony: a case report. Veterinární medicína 60(6): 330-335. doi.org
  12. Kwon M., Jung C., Kil E.-J. (2023). Metagenomic analysis of viromes in honey bee colonies (Apis mellifera; Hymenoptera: Apidae) after mass disappearance in Korea. Frontiers in Cellular and Infection Microbiology 13: 1124596. doi.org
  13. AGES (Austrian Agency for Health and Food Safety). Sacbrood (pathogen factsheet). ages.at
  14. WOAH (World Organisation for Animal Health). Diseases of bees (bee diseases listed in the Terrestrial Code). woah.org

Updated : October 8, 2026
Written by : Apisanitas editorial team
Reviewed by : Review in progress