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Brood diseases

European foulbrood

Melissococcus plutonius

Severity : High Mainly spring and early summerReview in progress
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Key points

  • A bacterial disease of young larvae, which usually die before capping.
  • Outbreaks mostly occur when the colony is short of nurse bees: cold spring, dearth, fast brood expansion.
  • Slumped larvae, yellowish then brown, with little or no roping; scale that comes away easily.
  • A lateral flow field test exists; only a laboratory test gives definitive confirmation.
  • No antibiotic is authorised for honey bees in the EU: act on the colony, the combs and the queen.

European foulbrood (EFB) is a bacterial disease of open brood caused by Melissococcus plutonius. Found almost everywhere honey bees are kept, it rarely kills a strong colony on its own, but it can hold a colony back for a whole season, recur and spread through an entire apiary1, 2.

Despite the name, it has almost nothing in common with American foulbrood, apart from killing larvae. This page explains how the bacterium works, why outbreaks happen in spring, how to recognise the disease, get it confirmed and bring a colony back on its feet without antibiotics.

The agent: a bacterium of the larval gut

Melissococcus plutonius is an elongated, lance-shaped coccus (described as lanceolate). It does not form spores, which sets it apart from Paenibacillus larvae, the agent of American foulbrood. Even so, it survives for a long time outside the larva: in soiled cells, on comb walls, in wax and in carrier adult bees that show no signs1, 2.

How larvae become infected

Young larvae swallow the bacterium with the food given by nurse bees; M. plutonius then multiplies in the midgut. The classic mechanism, described as far back as Bailey’s work, is competition for food: the bacterium takes part of what the larva receives, and the underfed larva starves to death1, 5. Some strains also carry virulence factors, including a putative toxin, melissotoxin A, encoded on a plasmid9.

Larvae usually die at around four to five days old, one or two days before capping or shortly after, and always before pupation2. Infected larvae that survive leave bacteria-laden faeces at the bottom of the cell when they pupate, contaminating the next generations1, 2.

Secondary bacteria

Dead larvae are quickly invaded by other bacteria; the most often cited are Enterococcus faecalis, Paenibacillus alvei and Achromobacter eurydice2, 5. They are responsible for the sour smell and pasty look of advanced cases.

Bailey regarded P. alvei as a mere saprophyte of dead larvae5, and in experimental infections, adding E. faecalis or P. alvei did not increase larval mortality8, 9. These bacteria accompany the disease and complicate microscopic diagnosis, but the cause remains M. plutonius.

More and less virulent strains

M. plutonius strains fall into three clonal complexes, known as CC3, CC12 and CC13. In England and Wales, CC3 is by far the most common. The complexes there differ in pathogenicity, and infections with the more aggressive variants more often end in destruction of the colony6.

In the laboratory, a CC12 strain proved the most virulent, a CC3 strain intermediate and a CC13 strain almost harmless7. So-called atypical strains, reported in Asia, Europe and the Americas, belong to CC12. They are associated with more serious, harder to clear, recurrent outbreaks2. A note of caution, though: across sixteen European isolates, virulence did not depend on genetic type but on the strain’s growth and the presence of the toxin-carrying plasmid9. In practice, two outbreaks can follow very different courses.

Why it matters

EFB is often described as a passing problem that clears up once the flow begins: that is a risky bet.

  • It robs the colony of young bees in spring: the main flow can be lost.
  • It spreads through the apiary by drifting, robbing, swapping combs and tools. Neighbouring colonies that look healthy may be carriers1.
  • It recurs from soiled combs, often the following spring.
  • Some strains destroy colonies, especially when the outbreak is not dealt with early6.

The stress triggers

Outbreaks mostly occur when there are not enough nurse bees to feed the brood. WOAH notes that incidence is higher when colonies are under stress, during periods of rapid growth2. The situations most often cited are:

  • a cold or wet spring that keeps foragers in while the queen is laying heavily;
  • a pollen or nectar dearth, or the sudden end of a flow;
  • rapid expansion of the brood nest, with too many larvae for too few nurses;
  • a colony weakened by varroa, nosema or a failing queen.

Correcting these stresses is the first control measure.

Recognising European foulbrood

Signs on the comb

  • Patchy, pepper-pot brood, with dead larvae among healthy ones and empty cells.
  • Displaced larvae: instead of lying in a glistening white “C”, they are twisted, stretched along the cell wall or slumped, as if melted12.
  • Colour: dull white, then yellowish, then brown.
  • Consistency: pasty or granular, with little or no roping.
  • Scale: the larva dries into a brown, rubbery scale that comes away easily from the cell.
  • Smell: sour, vinegary or putrid in marked cases; absent in mild ones.
  • A few perforated cappings when larvae die just after capping.
Photo needed — Close-up of open brood with yellowish larvae twisted or slumped against the cell wall, next to healthy white C-shaped larvae.

Do not confuse with

The main pitfall is American foulbrood, which is regulated. In the matchstick test (push a twig into the larva and pull it out slowly), AFB stretches into a sticky thread; EFB does not rope, or only slightly, although a larva invaded by secondary bacteria may rope a little2, 3.

Feature European foulbrood American foulbrood
Stage affected Open larvae, before capping Capped brood, prepupae
Age at death Around 4 to 5 days After capping
Position Twisted, slumped, displaced Lying flat at the bottom of the cell
Colour Dull white, yellowish then brown Coffee-coloured, then dark brown
Matchstick test Little or no roping Clearly ropy, elastic
Scale Rubbery, easy to remove Sticks hard, difficult to remove
Cappings Rarely affected Sunken, darkened, perforated
Spores No (except secondary bacteria) Yes, highly resistant
Status in France Not notifiable Any suspicion must be reported

Late-season varroa can also produce brood that looks very much like EFB2. See also sacbrood and chalkbrood. If in doubt, treat the colony as a suspected AFB case until the result is in: closed to robbing, with no combs or equipment moved.

Diagnosis

The lateral flow field test

A lateral flow test specific to M. plutonius gives a result in about ten minutes from a crushed larva2. During its validation in the United Kingdom, it gave a correct diagnosis for 96% of samples tested in the apiary, with about 1% false positives; British bee inspectors have used it routinely since10.

The test is done on a diseased larva; a negative result does not prove the disease is absent.

Laboratory tests

  1. Sampling: cut out a piece of comb with as many diseased larvae as possible and no honey. Wrap it in paper, not plastic.
  2. Sending: in France, go through your vet, your GDS (local animal health group) or an approved laboratory, stating the suspicion.
  3. Microscopy: a stained smear shows characteristic lanceolate cocci. This is the first-line test, but E. faecalis and A. eurydice can be mistaken for it2.
  4. PCR: detects M. plutonius DNA and is used to confirm microscopy. In France, the Anses reference laboratory uses real-time PCR14.
  5. Culture: tricky, on special media and in a low-oxygen atmosphere. It allows the strain to be isolated and typed, mainly for research2, 13.

Regulations

In France, EFB is not notifiable, unlike American foulbrood, which is a listed disease under EU animal health law. Management is up to the beekeeper, with support from their vet and GDS; Anses provides reference diagnosis14.

In the United Kingdom, it is a notifiable disease: any suspicion must be reported to the inspection services (the National Bee Unit in England and Wales). The inspector confirms and decides what happens next. Destruction is required for weak colonies (fewer than six frames of bees), heavily affected ones (more than 25% of open brood visibly diseased) or those relapsing within twelve months; the others may be treated by shook swarm12.

Antibiotics. No antibiotic is authorised for honey bees in the European Union4. Oxytetracycline, permitted in some non-EU countries, was long the usual treatment in England; a British study found fewer recurrences with a shook swarm plus antibiotic than with the antibiotic alone11, because the antibiotic does not remove the bacterium from the combs. Rules change: check those in your country.

What to do

Basic measures, in every case

  1. Isolate the colony: reduced entrance, no swapping of combs or supers, tools disinfected between hives.
  2. Correct the stress: feed if stores are low or the flow stops, and provide pollen if the colony is short of it.
  3. Remove the worst-affected combs and destroy them rather than recycling them.
  4. Reduce the space so that the bees cover the remaining brood well.
  5. Requeen if the colony relapses, shows poor hygienic behaviour or the queen is old.
  6. Inspect the whole apiary, colony by colony, as the disease is rarely isolated.

The shook swarm, step by step

A shook swarm transfers the adult bees onto new frames, leaving behind all the brood and all the contaminated comb. It is the standard method in countries that regulate the disease11, 12. It suits a colony that is still strong and only lightly affected, between spring and midsummer: too early, the bees will not draw comb; too late, the colony will not raise enough winter bees12.

  1. Prepare a clean brood box (new or disinfected) filled with frames of foundation, plus a floor, crown board, feeder and bags to take away the old combs. Choose a mild day.
  2. Move the diseased hive to one side and put the clean hive on its exact spot.
  3. Find the queen and cage her, or place her directly in the clean hive. A queen excluder under the brood box for a few days prevents absconding.
  4. Shake each frame sharply over the clean hive, then the old box, and put each comb straight into a closed bag.
  5. Close the hive, with a reduced entrance, and feed syrup so the bees draw comb quickly.
  6. Destroy the old brood combs so that the wax does not go back into beekeeping use. Disinfect the old box, floor and crown board (see disinfecting equipment).
  7. Remove the queen excluder once laying has resumed, then monitor the colony over the following weeks and seasons, as the disease can return12.

Where several colonies in an apiary are affected, the British services recommend considering a shook swarm of the whole apiary12.

Photo needed — Shook swarm in progress: bees shaken from a brood comb over a clean hive fitted with foundation, old combs going into a bag.

In practice: what beekeepers actually do

EFB is most often discovered during the first spring inspections. Here is how beekeepers generally respond to an affected colony.

First steps, on the day it is found

  • Confirm first: matchstick test on several larvae, then a lateral flow test if one is to hand, or a sample sent to the laboratory.
  • Close the hive to robbing: reduced entrance, no removed comb left in the open.
  • Go round the apiary the same day, finishing with the diseased colony.

The most common strategies

1. Strong colony, mild signs. Most beekeepers remove the worst-affected combs, reduce the space, feed if the flow is late and check at every visit.

2. Strong colony, clear signs or relapse. A shook swarm is the most widely used solution, often combined with requeening.

3. Weak or heavily affected colony. Many beekeepers prefer to destroy it, as it remains a source of infection.

4. Apiary where the disease comes back every year. Systematic comb replacement, requeening of susceptible colonies, selection for hygienic behaviour, sometimes a shook swarm of the whole apiary.

Equipment to have

  • Lateral flow tests for European and American foulbrood.
  • Clean brood boxes, frames of foundation, feeders and syrup.
  • Queen cages and queen excluders.
  • Disinfection kit: blowtorch for woodware, a suitable solution for tools.

Prevention checklist

  1. Replace brood combs regularly, starting with dark, old combs.
  2. Do not weaken a colony at the wrong time: measured splits in spring, adequate stores coming out of winter.
  3. Keep varroa under control so as not to go into spring with fragile colonies.
  4. Limit comb swaps between colonies and between apiaries, especially brood combs.
  5. Buy colonies and queens from reliable sources, and inspect the brood of any colony you buy.
  6. Disinfect tools between hives at risky times (see apiary biosecurity).

The most common mistakes

  • Waiting for the flow to “sort it out” without removing affected combs: soiled cells set up the relapse.
  • Recycling diseased combs into other colonies, or leaving them out to be robbed.
  • Shaking a colony that is too weak, or too late in the season, then losing it in winter.
  • Confusing it with American foulbrood, or vice versa, for want of a test or analysis.
  • Using an antibiotic, illegal in the EU and masking the disease without eliminating it.
  • Forgetting the causes: food, varroa, queen. Unless the stress is corrected, the disease returns.

Frequently asked questions

Does European foulbrood go away on its own? Sometimes, when a strong flow restores the balance between nurse bees and brood. But the bacterium stays in the combs and in carrier bees: a relapse the following year is common unless the combs are replaced.

Is honey from an affected colony safe to eat? EFB is a bee disease and poses no danger to humans. However, never feed this honey to other colonies, or leave supers or combs out to be cleaned by bees: that would pass the bacterium from one hive to another.

Does the disease have to be reported in France? No, it is not compulsory. However, if there is any doubt about American foulbrood, which is notifiable, have a sample tested.

References

  1. Forsgren E. (2010). European foulbrood in honey bees. Journal of Invertebrate Pathology 103: S5-S9. doi.org
  2. WOAH. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, chapter 3.2.3: European foulbrood of honey bees (infection of honey bees with Melissococcus plutonius), version adopted 2023. woah.org
  3. Forsgren E., Budge G.E., Charrière J.-D., Hornitzky M.A.Z. (2013). Standard methods for European foulbrood research. The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(1).
  4. FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
  5. Bailey L. (1963). The pathogenicity for honey-bee larvae of microorganisms associated with European foulbrood. Journal of Insect Pathology 5(2): 198-205.
  6. Budge G.E., Shirley M.D.F., Jones B., Quill E., Tomkies V., Feil E.J., Brown M.A., Haynes E.G. (2014). Molecular epidemiology and population structure of the honey bee brood pathogen Melissococcus plutonius. The ISME Journal 8(8): 1588-1597. doi.org
  7. Nakamura K. et al. (2016). Virulence differences among Melissococcus plutonius strains with different genetic backgrounds in Apis mellifera larvae under an improved experimental condition. Scientific Reports 6: 33329. doi.org
  8. Lewkowski O., Erler S. (2019). Virulence of Melissococcus plutonius and secondary invaders associated with European foulbrood disease of the honey bee. MicrobiologyOpen 8(3). doi.org
  9. Grossar D., Kilchenmann V., Forsgren E., Charrière J.-D., Gauthier L., Chapuisat M., Dietemann V. (2020). Putative determinants of virulence in Melissococcus plutonius, the bacterial agent causing European foulbrood in honey bees. Virulence 11(1): 554-567.
  10. Tomkies V., Flint J., Johnson G., Waite R., Wilkins S., Danks C., Watkins M., Cuthbertson A.G.S., Carpana E., Marris G., Budge G., Brown M.A. (2009). Development and validation of a novel field test kit for European foulbrood. Apidologie 40(1): 63-72. doi.org
  11. Waite R.J., Brown M.A., Thompson H.M., Bew M.H. (2003). Controlling European foulbrood with the shook swarm method and oxytetracycline in the UK. Apidologie 34(6): 569-575. doi.org
  12. National Bee Unit (APHA, United Kingdom). Fact sheet 11: Control of European foulbrood. nationalbeeunit.com
  13. Budge G.E. et al. (2025). Standard methods for European foulbrood research 2.0. Journal of Apicultural Research 64(2): 403-442. doi.org
  14. Anses, Sophia Antipolis Laboratory (2025). Activity report of the French national reference laboratory for bee health. anses.fr

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