European Foulbrood (EFB), one of the significant bacterial infections threatening honey bee colony health, often appears during brood-rearing periods. This condition, which can cause serious yield losses in beekeeping, targets young larvae in the hive. Early diagnosis and correct management strategies are vital for controlling the disease’s spread and maintaining colony strength. Although this infection is often confused with American Foulbrood (AFB), it shows distinct differences in its pathogen, symptoms, and control methods.
What is European Foulbrood?
European Foulbrood is a contagious disease caused by the bacterium Melissococcus plutonius, affecting the digestive system of honey bee larvae. It generally targets young larvae in open (unsealed) brood cells. Most deaths occur before the larval cells are capped. Hive hygiene, nectar flow, and general colony stress are factors that directly influence the disease’s severity. It can be more devastating in weak colonies and during periods of nutrient scarcity.
Characteristics of the Pathogen (Melissococcus plutonius)
The main pathogen of the disease, Melissococcus plutonius, is a Gram-positive bacterium but does not form spores. This non-spore-forming characteristic is its most fundamental difference from the American Foulbrood pathogen. The bacterium is ingested by young larvae (usually 1 to 4 days old) with infected food brought by forager bees. It settles in the larva’s midgut and multiplies rapidly. By competing for the larva’s food, it causes the larva to starve or its development to halt. The bacterium can survive in dried larval remains (scales) or honeycomb cells under harsh conditions for months, sometimes up to 2-3 years. These remains create a constant source of infection within the hive.
Key Differences from AFB
Although European Foulbrood (EFB) and American Foulbrood (AFB) have similar names, they are two diseases with very different outcomes for beekeeping. EFB is caused by Melissococcus plutonius and does not produce spores; AFB, however, originates from a spore-forming bacterium called Paenibacillus larvae. EFB generally affects open, unsealed larvae, while AFB is effective in sealed, capped brood cells. In EFB, the dead larva turns a yellowish-brown color, emits a sour or fishy odor, and has a watery consistency. It does not rope when a stick is inserted. In contrast, AFB turns the larva dark brown-black, has a distinct glue-like smell, and leaves a sticky residue that ropes 1-2 cm when the ‘matchstick test’ is applied. In EFB, the larval remnant (scale) is easily removed from the cell, whereas the AFB scale adheres tightly to the cell.
Disease Cycle
The disease cycle of European Foulbrood is based on the transmission from infected larvae to healthy larvae through hive dynamics. As nurse bees try to clean out infected or dead larvae, they contaminate their mouthparts with the bacterium (M. plutonius). They then transfer these bacteria to healthy larvae while feeding them. Young larvae ingest the bacterium, and the infection spreads rapidly in their intestines. This cycle accelerates, especially during stress periods when food resources diminish.
Death Dynamics in the Open Brood Stage
Infection usually occurs within the first 1-2 days of the larva’s life through feeding. The bacterium begins to multiply in the larva’s midgut. It enters into food competition with the larva for the consumed pollen and nectar. While a healthy larva normally prepares to enter the pupa stage on the 5th day, the infected larva starves due to this food competition. Death typically occurs when the larva is 4 to 5 days old, just before the cell is capped or very shortly after. The dead larva loses its typical ‘C’ shape in the honeycomb cell. It twists, changes position, and its color turns from pearly white to dull yellow, then brown.
Causes and Risk Factors
European Foulbrood is generally considered a ‘stress disease.’ The bacterium (M. plutonius) may be present in the hive at low levels but not cause noticeable illness. Triggering factors that weaken the colony are usually needed for an outbreak to occur. Nutritional deficiencies, sudden weather changes, and the presence of other pathogens are the main risk groups that increase the disease’s severity.
Nectar Dearth, Nutritional Deficiency, and Stress
The colony’s health is directly related to the amount of incoming fresh nectar and pollen. A sudden interruption in nectar flow, especially due to cold snaps or drought during the spring (early months of the season), puts the colony under severe nutritional stress. Nurse bees cannot adequately feed the existing larvae. This reduces the larvae’s ability to compete with the M. plutonius bacteria. The starving larva succumbs to the bacterial population in its gut. While healthy, well-fed colonies can often overcome this infection without showing symptoms, poor nutrition is the primary trigger for EFB outbreaks. Considering that a colony needs 20-30 kg of pollen annually for healthy brood development, brief interruptions in these resources increase the risk.
Varroa and Co-infections
The Varroa destructor mite is the main pest that suppresses the immune system of honey bee colonies. Varroa feeds on the bees’ hemolymph (blood fluid) while also transmitting various viruses (e.g., Deformed Wing Virus – DWV). A colony struggling with Varroa and viruses becomes much more vulnerable to opportunistic bacterial infections like EFB, as its immune system is weakened. It has been observed that EFB is more severe in hives with a heavy Varroa load (e.g., detection of more than 3 mites per 100 bees). Furthermore, after M. plutonius kills the larva, secondary bacteria (e.g., Paenibacillus alvei) often colonize the environment. These secondary infections can change the odor and appearance of the dead larva, making diagnosis difficult.
Transmission Routes (Feeding, Robbing, Equipment)
There are several main ways the infection spreads within the hive and apiary. The primary route is the cleaning and feeding activities of nurse bees. Bees cleaning diseased larvae ingest residues containing billions of bacteria and then transmit them when feeding healthy larvae. The second route of spread is robbing. When an infected but weakened hive is robbed by strong colonies, the robbing bees carry infected honey or residues back to their own hives. One of the most significant transmission vectors is the beekeeper. The use of shared equipment between hives, especially comb exchange, spreads the disease rapidly. If a hive tool, gloves, or other equipment used in an infected hive is used in a healthy hive without being disinfected, transmission is inevitable. The bacterium can remain active on equipment for months.
Symptoms and Diagnosis
The detection of European Foulbrood relies primarily on the careful inspection of the brood area on the combs. The observed symptoms usually provide strong clues to the disease’s presence, but laboratory analysis may be necessary for a definitive diagnosis. Early detection is critical for successful control, as EFB can sometimes be confused with other brood diseases.
Visual Symptoms (Color, Odor, Scattered Brood Pattern)
The most distinct visual sign of EFB is a scattered brood pattern. While brood in a healthy hive appears solid and regular (without empty cells), an EFB-affected hive will have many empty, skipped cells or dead open larvae interspersed among healthy sealed brood. Infected larvae lose their healthy pearly white color; they first become dull, then yellowish, and finally a light brown. The larvae lose their normal ‘C’ position within the cell, appearing twisted, shrunken, or slumped to the bottom of the cell. Depending on the severity of the disease, a sour, vinegar-like, or fishy odor may come from the hive. When the dead larva dries, it forms a rubbery, non-adherent scale (remnant) that is easily removed. This scale is not sticky like in AFB and does not rope in the matchstick test.
Laboratory Confirmation (Culture/PCR)
While visual symptoms (especially the sour odor and scattered open brood) are sufficient to suspect EFB, a definitive diagnosis is important. It can be particularly confused with conditions like Sacbrood or chilled brood. For a precise diagnosis, a piece of comb containing suspicious larvae (approximately 10 cm x 10 cm) is cut out, or dead larvae are sent to a laboratory. In the laboratory, the bacterium’s presence is confirmed by culturing it (microbiological planting) or by the faster, more sensitive Polymerase Chain Reaction (PCR) test, which directly searches for the bacterium’s DNA. PCR can detect even low concentrations of the bacteria, clearly confirming the disease’s presence.
Treatment / Control Methods
The management of European Foulbrood allows for more flexible approaches compared to American Foulbrood because the EFB pathogen does not form spores. Control varies depending on the severity of the disease, seasonal conditions, and colony strength. Approaches range from mechanical cleaning (shook swarm) to chemical intervention. The main goal is to reduce the bacterial load within the hive and strengthen the colony’s defense mechanisms.
Artificial Swarm/Shook Swarm and Comb-to-Comb Exchange
One of the most effective and lasting solutions is the ‘artificial swarm’ or ‘Shook Swarm’ technique. In this method, all adult bees from the infected hive are collected and transferred to a completely new or sterilized hive with new foundation (undrawn comb). All of the old, infected combs are destroyed (melted down). This process completely rids the hive of the combs that are the source of the bacteria. The approximately 2-3 day period while the bees draw out the new comb and the queen resumes laying provides a critical ‘brood break’ to interrupt the disease cycle. After the procedure, the bees are usually fed 1:1 sugar syrup to encourage comb building. In mild cases, replacing only the 2-3 most infected frames with healthy combs may be sufficient.
Colony Isolation and Equipment Disinfection
As soon as the disease is detected, the infected hives must be separated from other healthy hives in the apiary. Isolation involves moving the infected colonies (e.g., to a different area in the apiary) or reducing their entrances to prevent robbing. All beekeeper interventions should start with the healthy hives and move toward the infected ones, with the sick hives being inspected last. All used equipment (hive tool, brush, gloves) must be meticulously disinfected when moving between hives. The most effective method for metal tools like hive tools is sterilization by scorching with a flame (blowtorch). Hive bodies should also be scorched with a flame until the inner surfaces turn light brown.
Antibiotic Application (Oxytetracycline, etc.)
Antibiotic applications are sometimes used in the treatment of European Foulbrood. The most commonly used active ingredient is oxytetracycline. This chemical is usually mixed with powdered sugar and dusted over the frames in the brood area or dissolved in syrup and fed to the colony. The antibiotic stops the bacteria from multiplying and allows healthy larvae to develop. During this time, the nurse bees also clean out the infected larvae. However, antibiotic use does not clean the contaminated combs, which are the source of the disease; it only suppresses the infection. Incorrect or unnecessary use can leave residues in honey and lead to bacterial resistance. Therefore, antibiotic applications must be completed at least 4-6 weeks before the main nectar flow and should only be done upon authorized recommendation after a laboratory diagnosis.
Queen Replacement and Strong Colony Management
Since EFB often appears in weak and stressed colonies, strengthening the colony is a fundamental part of the treatment. Queen replacement is a key part of this strategy. A young, healthy queen (ideally from a strain with hygienic behavior traits) has a high laying capacity (1,500-2,000 eggs per day). This intensive egg-laying rapidly increases the colony’s population and encourages cleaning behavior in nurse bees. A strong colony identifies and removes infected larvae from the hive faster than healthy ones (hygienic behavior). Sometimes, interrupting the brood cycle by temporarily caging the queen (7-10 days) can also help break the disease cycle, similar to the shook swarm method.
Prevention / Prophylaxis Strategies
The most effective way to combat European Foulbrood is to prevent the disease from occurring. Preventive measures are less costly and more sustainable than treatment methods. Encouraging strong colonies, implementing strict hygiene rules in the apiary, and minimizing environmental stress factors are the keys to keeping EFB out of the apiary.
Hygiene Protocol and Equipment Sanitation
Sanitation in apiary management is the first line of defense in preventing EFB. Comb (frame) exchange should not be done between hives, especially between weak colonies or those of unknown health status. Used hive materials or equipment must be disinfected (e.g., by scorching with a flame or using appropriate disinfectant solutions) before being given to a new colony. Old, darkened combs act as a reservoir for bacteria and pathogens. Therefore, it is critical to regularly replace old combs in the hives (e.g., 20-30% annually) with new ones (comb rotation). New colonies or swarms brought into the apiary should be kept in quarantine for a period and checked for health before being integrated into the main apiary.
