European Foulbrood in Honey Bees: Symptoms, Diagnosis, and Treatment

European foulbrood, often called EFB, is a bacterial disease of honey bee brood caused by Melissococcus plutonius. The bacteria contaminate larval food, then multiply in the larval gut and compete with the larva for nutrients. Many affected larvae die young, usually before the cell is capped.

This disease mainly affects developing larvae, not adult bees. Adult workers can still carry the bacteria and spread it within the hive or to other colonies through drifting, robbing, contaminated honey, or shared equipment. That is why one weak colony can become a wider apiary problem if it is not recognized early.

EFB is different from American foulbrood. It does not form the same long-lasting spores, and some colonies can recover if stress is reduced and brood rearing conditions improve. Still, it can cause major brood loss, poor colony growth, and in severe cases colony failure, so it deserves prompt attention from your vet and your apiary inspector.

European foulbrood is caused by the bacterium Melissococcus plutonius. Nurse bees spread the organism when they feed larvae contaminated brood food or clean infected cells. Adult bees may carry the bacteria without looking sick, which makes silent spread possible inside a colony.

Outbreaks are often linked to stress. EFB is more likely to show up when colonies are under nutritional pressure, when nectar flow is poor or inconsistent, when there are too few nurse bees for the amount of brood present, or during cool spring weather that slows normal brood care. In some colonies, signs improve later in the season as forage and nurse bee numbers recover.

The bacteria can also move between colonies through drifting bees, robbing, contaminated honey, and shared equipment. Buying used comb, feeding honey from outside sources, or moving frames between hives without strict biosecurity can all increase risk.

Diagnosis starts with a careful brood inspection. Your vet or apiary inspector will look for open larvae that are discolored, twisted, flaccid, or drying into scale, along with a patchy brood pattern. Because EFB can mimic several other brood problems, visual diagnosis is helpful but not always enough on its own.

Confirmation may involve a field diagnostic kit, microscopy, culture, or laboratory testing of brood comb or bee samples. In the United States, the USDA Bee Disease Diagnosis Service accepts samples from U.S. beekeepers and reports results to both the submitter and the appropriate apiary inspectors. Many states also help beekeepers connect with inspectors and diagnostic labs.

Do not start moving equipment or treating blindly if you are unsure what you are seeing. A confirmed diagnosis helps your vet decide whether conservative colony management is reasonable or whether veterinarian-directed antibiotics and stricter control steps are needed.

Prevention starts with strong colony management and biosecurity. Avoid feeding honey from unknown sources, limit sharing of frames and tools between colonies, and clean hive tools, gloves, and other frequently handled equipment. If you run multiple yards, keep equipment separated as much as practical and reduce drifting and robbing pressure.

Good nutrition matters. EFB often becomes visible when colonies are stressed by poor forage, cool weather, or an imbalance between nurse bees and brood. Supporting colonies during lean periods, avoiding unnecessary brood expansion in weak hives, and maintaining queens with good brood patterns can all help reduce risk.

Requeening with hygienic stock may also help because hygienic workers are better at removing diseased brood. There are no bee lines proven resistant to EFB, but hygienic behavior can lower disease impact. Regular brood inspections in spring and early summer give you the best chance of catching problems before they spread through the apiary.