European Foulbrood Disease: Understanding Its Impact and Management in Apiculture

Of all the diseases that can silently undermine a honeybee colony, European Foulbrood (EFB) ranks among the most widespread. Found on every continent where Apis mellifera is managed, this bacterial disease targets larvae at their most vulnerable stage – before they are even capped. While it is generally considered less devastating than American Foulbrood (AFB), a poorly managed EFB outbreak can seriously weaken a colony and, in severe cases, destroy it. Understanding how EFB works, what it looks like, and how to control it is essential knowledge for any beekeeper.

What causes European foulbrood? 🔗

EFB is caused by Melissococcus plutonius, a Gram-positive, non-spore-forming bacterium that colonizes the gut of young honeybee larvae. Unlike the spore-forming Paenibacillus larvae responsible for AFB, M. plutonius does not produce spores – and this distinction matters greatly. Because it does not form spores, European foulbrood can be treated, whereas American foulbrood cannot.

The infection pathway is straightforward but insidious. Larvae become infected through oral ingestion when nurse bees – themselves asymptomatic – contaminate the brood food they provide. The pathogen then colonizes the larval gut, directly competing with the host for nutrients, and ultimately causing the larva to starve. Recent research has added another layer of concern, suggesting that M. plutonius may also breach the larval gut wall to cause a deeper septic infection.

It is also worth noting that the presence of M. plutonius alone does not guarantee visible disease. Many colonies carry the bacterium without showing clinical symptoms, and outbreaks are often triggered by other stressors affecting the colony. These stressors include hive movement, cool and wet weather, and poor nutrition.

How EFB differs from AFB 🔗

Beekeepers sometimes confuse EFB and AFB because both affect brood and create a characteristic foul odour. However, there are key differences worth knowing.

Around 90% of EFB-infected larvae die before capping and appear coiled or twisted in their cells – in contrast to AFB, where most infected larvae die after capping. Colonies infected with EFB tend to have a sour smell, while those with AFB smell sulfurous. A practical field test is the “rope test”: inserting a matchstick or twig into an affected cell and slowly withdrawing it. AFB-infected brood typically draws out a ropy thread of 3-5 cm, compared to a much shorter thread – up to about 1.5 cm – for EFB. Additionally, the dried scales left by EFB are rubbery and can be removed from the cell relatively easily, while AFB scales are brittle and firmly stuck.

Symptoms to look for 🔗

Recognizing EFB early can make the difference between a containable problem and a collapsing colony. Healthy larvae appear plump, pearly white, and lie in a characteristic “C” shape at the back of the cell. Diseased larvae, in contrast, appear melted, shrunken, or deflated, and may twist or curl upward in the uncapped cell.

As the disease progresses, the colour change follows a predictable sequence. Dead larvae shift from a healthy pearly white to yellow and then to brown, eventually becoming a semi-fluid mass before drying out into a rubbery scale that adheres loosely to the cell wall. The brood pattern takes on a “shotgun” appearance – an irregular, patchy layout of capped and empty cells – as infected larvae are removed and others survive.

EFB is highly contagious and can remain viable for several years in honey, wax, and equipment. Hives may carry the infection asymptomatically for extended periods, with sudden flare-ups occurring when the colony comes under stress.

Secondary infections 🔗

In advanced cases, cells may also be infected with Paenibacillus alvei, a secondary bacterial invader associated with EFB. This secondary infection can produce extra ropiness, making the brood resemble AFB more closely and complicating diagnosis. When there is uncertainty, laboratory testing or a field diagnostic kit – such as the Vita EFB Test Kit – can provide a quick, reliable confirmation.

How EFB spreads through and between colonies 🔗

Inside the hive, the cycle of infection perpetuates itself efficiently. Even after nurse bees remove dead or infected larvae, the bacteria can remain in brood cells or in the feces left behind. Nurse bees then pick up and transfer the pathogen to healthy larvae when feeding and cleaning brood.

Between colonies, EFB travels primarily through robbing and drifting. When a diseased colony weakens and dies, robber bees from neighbouring colonies collect the EFB-contaminated honey and nectar, bringing the bacteria back to healthy hives. Beekeepers themselves can also be unintentional vectors – moving frames between colonies or feeding shared honey and pollen without checking for infection can spread the disease rapidly across an apiary.

Seasonal patterns 🔗

EFB is seasonal in nature, occurring most often in early spring. During this period, forager recruitment increases while the nurse bee population is still growing, leaving fewer bees to feed and tend the developing brood. This nutritional stress on larvae makes them more susceptible to infection. Often, once the ratio of nurse bees to larvae stabilizes during a good nectar flow, symptoms can diminish on their own in otherwise strong colonies.

Management and control of EFB 🔗

There is no single magic solution for EFB. Effective management typically combines colony-level interventions that reduce pathogen load, strengthen the colony’s own defenses, and – where legally permitted – selective antibiotic treatment.

Strengthening colony hygiene behavior 🔗

In strong colonies, nurse bees will remove abnormally hungry or infected larvae, effectively controlling EFB before it escalates. Strong colonies can contain the infection entirely as long as they are able to remove dead and infected larvae in a timely manner. Keeping colonies well-fed, reducing unnecessary stress, and ensuring adequate nurse bee populations are therefore the first line of defense.

Removing infected combs 🔗

When clinical symptoms are present, removing contaminated combs is a critical step. The shook swarm method is a widely used technique for this purpose. It involves shaking all the bees off the combs in EFB-infected hives, caging the queen separately, placing the bees on fresh, clean equipment and foundation, and destroying the contaminated combs. Although some bacteria remain on the adult bees transferred to the new colony, the operating principle is that the pathogen load is reduced below the threshold needed to sustain disease by the time new brood is present.

Research has shown that shook swarm combined with oxytetracycline treatment can reduce the recurrence of EFB at the colony level. Studies recommend the total shook swarm (TSS) over the partial version where possible, as the full method is more effective at preventing the persistence of M. plutonius in storage combs.

Requeening to enhance colony resistance 🔗

Requeening is another important management tool. Introducing a new queen can add more hygienic behavior to the colony and creates a natural break in the brood cycle, giving nurse bees time to remove dead larvae and polish the cells before new eggs are laid. Queens from lines selected for hygienic behavior – the tendency to detect and remove diseased brood – are particularly valuable in apiaries where EFB is a recurring problem.

Antibiotic treatment: benefits and risks 🔗

When colony management alone is insufficient to control an active outbreak, antibiotics may be considered. In North America, oxytetracycline (OTC) is approved to combat EFB, while tylosin and lincomycin are also registered for use against related foulbrood diseases. These drugs work by reducing bacterial loads in the hive, allowing larvae to survive infection and the colony to recover.

However, antibiotic use in beekeeping carries significant risks that must be carefully weighed.

Risk of honey contamination 🔗

In the USA, antibiotics used for treating bees must be discontinued with sufficient time prior to honey flow to prevent residues in the honey, as there are no authorized residue limits for these antibiotics in honey. In the European Union, honeybees are classified as food-producing animals, and in principle only medicinal products that do not result in residues in honey can be authorized – effectively meaning a zero-tolerance policy on antibiotic residues in EU honey.

Conventional antibiotic application methods – such as mixing antibiotics into sucrose solution and dripping it over combs – can contaminate the wood of the hive, comb structure, and stored honey. A 2024 study found that treating with oxytetracycline resulted in weaker colonies, subclinical disease cases, and elevated antibiotic residues in the honey yield. Notably, two months after antibiotic administration, residue levels dropped below the maximum limit set by EU legislation, suggesting that a sufficient withdrawal period is critical.

Antibiotic resistance 🔗

There are growing reports of oxytetracycline-resistant M. plutonius isolates, including a confirmed case from British Columbia, Canada. This underscores the importance of using antibiotics judiciously and exploring non-chemical management options wherever possible.

Emerging alternatives 🔗

Researchers are actively investigating alternatives to antibiotics for managing EFB, including the development of probiotics and targeted microbiota management techniques designed to enhance the overall resilience of bee populations against this disease. While these approaches are not yet widely available to commercial beekeepers, they represent a promising direction for sustainable apiculture.

Prevention: the beekeeper’s best tool 🔗

Preventing EFB from entering or persisting in an apiary requires consistent hygiene and vigilance. Any equipment – hive tools, smokers, bee suits – should be sterilized immediately if EFB is suspected, before moving on to other colonies. Beekeepers should avoid sharing frames, feeding honey of unknown origin, or interchanging equipment between apiaries without inspection. Regular brood inspections, especially in early spring, give beekeepers the best chance of catching the disease before it spreads.

It is equally important to recognize that EFB is associated with nectar dearths, high Varroa mite populations, and other colony stressors – meaning that good overall colony management, including Varroa control and adequate nutrition, is part of EFB prevention as much as it is general beekeeping best practice.

What do you think? Given that strong colonies can often suppress EFB on their own, how should beekeepers balance colony strength management against the reflex to reach for antibiotic treatments? And as antibiotic resistance in M. plutonius becomes increasingly documented, what role do you think non-chemical interventions like requeening and the shook swarm method should play as the first line of response?

References

1. https://ask.ifas.ufl.edu/publication/IN1272
2. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2024.1495010/full
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC11694448/
4. https://beeaware.org.au/archive-pest/european-foulbrood/
5. https://txbeeinspection.tamu.edu/european-foulbrood/
6. https://bee-health.extension.org/shook-swarm-and-otc-antibiotics-for-european-foulbrood-control/
7. https://www.nationalbeeunit.com/assets/PDFs/3_Resources_for_beekeepers/articles_reports/BBKA_news/August_BBKA_News_p275_Ben_Jones-5.pdf
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC10819488/
9. https://www.nature.com/articles/s41598-022-09796-4
10. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/melissococcus-plutonius