American foulbrood of bumblebees
American foulbrood is a serious bacterial disease affecting the brood—the larvae and pupae developing in the nest. It is caused by a spore-forming bacterium, Paenibacillus larvae, capable of surviving for very long periods in the environmen...
What is it?
American foulbrood is a serious bacterial disease affecting the brood—the larvae and pupae developing in the nest. It is caused by a spore-forming bacterium, Paenibacillus larvae, capable of surviving for very long periods in the environment.
Although the name refers to honeybees, this disease or closely related forms can also affect the brood of bumblebees (Bombus sp.), particularly in bumblebee colonies used for pollination in greenhouses. It severely weakens or destroys affected colonies.
It is a notifiable disease for honeybees in many countries; its management in bumblebees follows similar principles of health precaution.
Key point
A serious and highly contagious disease within an apiary with no reliable cure; its management depends on strict sanitary measures.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The main sign is brood with abnormal appearance: cells or cocoons are irregular, sunken, and discolored (brownish to blackish) instead of the light color typical of healthy larvae.
An unpleasant odour, sometimes described as glue-like or putrid, may come from the affected nest.
The colony shows signs of overall weakness: fewer adult individuals, reduced activity, and difficulty maintaining the nest in good condition.
At an advanced stage, the contents of affected cells become viscous and ropy, a fairly characteristic sign of this disease in honeybees and likely similar in bumblebees.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
The disease is caused by a bacterium of the genus Paenibacillus, capable of forming highly resistant spores. These spores contaminate larval food; young larvae ingest them, the bacterium then multiplies in their intestine, and eventually kills them before or shortly after cell capping (sealing of the cell).
Spores released by dead larvae then contaminate the rest of the nest and can persist for a very long time in materials and equipment.
Bumblebee colonies bred in large numbers for commercial pollination in high-density conditions are at greater risk.
Reusing breeding equipment without thorough disinfection, introducing colonies or queens from uncontrolled sources, and transfers between apiaries increase the risk of contamination.
Stress, poor-quality diet, or general decline in colony strength can promote disease expression.
Transmission
Transmission occurs through highly resistant bacterial spores present in contaminated larval food. It can spread within a colony, between colonies via shared materials, or when introducing new queens or colonies that have not been health-checked.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
Diagnosis is based on examination of the brood by a trained professional, supplemented if necessary by laboratory analysis to identify the causative bacterium (culture, specific testing). Such testing is carried out by specialized veterinary or beekeeping laboratories.
A veterinarian specializing in pollinator insect disease or an official beekeeping service should be contacted as soon as suspicion arises, since this disease is often subject to specific health regulations.
Treatment
No reliable, recommended treatment exists for this disease due to the extreme resistance of bacterial spores. Management depends essentially on sanitary measures: destruction of affected colonies and disinfection or disposal of contaminated equipment, according to recommendations from the competent health authorities.
The use of antibiotics is not a recommended solution: it can mask symptoms without eliminating spores, promoting silent spread of the disease.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
Prevention relies first and foremost on strict hygiene in the apiary: regular disinfection of equipment, health screening of colonies before introduction, and careful monitoring of brood appearance.
It is essential to source bumblebees from certified producers with documented health records, particularly for greenhouse pollination purposes.
Any suspected colony must be isolated promptly to prevent spread to other colonies at the same site.
Possible complications
Without prompt action, the colony can collapse entirely. Bacterial spores persistently contaminate the environment and equipment, exposing subsequent colonies to a high risk of reinfection.
When to see a veterinarian
Brood showing abnormal colour or texture (brownish, sunken, sticky)
Unusual and unpleasant odour coming from the nest
Rapid and unexplained weakening of the colony
Abnormal mortality of larvae or pupae
Uncertainty about the health status of a newly introduced colony
Prognosis
The outlook for an affected colony is guarded to poor: the disease is usually fatal to the nest if not managed early through elimination and disinfection measures. The outlook for the operation depends mainly on how quickly the disease is detected and how strictly sanitary measures are applied.
Frequently asked questions
Can American foulbrood of bumblebees contaminate honeybees?
The risk of cross-transmission between bumblebees and honeybees cannot be completely ruled out, particularly through shared equipment or flowers. As a precaution, it is recommended to keep bumblebee colonies separate from apiaries and to observe strict hygiene protocols.
Can a bumblebee colony affected by disease be treated with antibiotics?
No, this is not recommended. Antibiotics do not destroy the bacterial spores responsible and can mask the symptoms, which delays detection and promotes the spread of the disease.
How to tell if a bumblebee colony you've purchased is healthy? You should check several indicators: **The queen:** She should be visibly present and active. Look for her moving around the nest regularly. An inactive or absent queen is a bad sign. **Worker activity:** The colony should show constant movement and buzzing. Workers should be attending to the brood (larvae and pupae) and building comb. **Brood pattern:** Examine the comb for a good brood pattern—cells should be well-populated with eggs, larvae, and capped pupae. Scattered or sparse brood suggests problems. **Food stores:** There should be visible pollen and nectar stored in cells. This indicates the colony can sustain itself. **Absence of disease:** Look for no signs of mold, fungus, or unusual odors. Dead bees shouldn't be accumulating inside. **Population size:** The colony should match what was promised at purchase. Count the visible workers—a typical healthy colony has dozens to hundreds depending on species and development stage. **Container condition:** The nest box should be clean and properly ventilated. Check that it's not waterlogged or excessively dry. **Behavior:** Bees should seem calm or only mildly defensive. Excessive aggression or lethargy can indicate stress or illness. Request a health certificate from the supplier if available, and inspect the colony within 24 hours of arrival while conditions are still fresh.
It is recommended to source supplies solely from certified producers who apply rigorous health monitoring and can guarantee the absence of brood diseases.
Is this disease dangerous for humans?
No, it only affects insects and poses no risk to people or other animals.
References
WOAH — Organisation mondiale de la santé animale (ex-OIE)
FAO — Organisation des Nations unies pour l'alimentation et l'agriculture
Content reviewed by the TunisieVet editorial team · Updated 25 August 2026
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