Key points
- A bacterial brood disease that kills the colony and spreads very easily.
- Spores survive for decades in equipment, wax and honey.
- Warning signs: sunken, perforated cappings and ropy remains in the matchstick test.
- Notifiable in France and many other countries: report any suspicion at once.
American foulbrood (AFB) is the most serious bacterial disease of honey bee brood. It only affects larvae, but it kills the colony and spreads very easily between hives. It is harmless to people: the honey is safe to eat, but must never be fed to bees.
The agent: a spore-forming bacterium
Paenibacillus larvae produces extremely resistant spores that remain infectious for decades in equipment, wax and honey1, 4. A single dead larva contains millions of them, or more.
Young larvae become infected by swallowing spores with their food, mostly in their first hours of life. The bacterium multiplies in the gut, invades the larva and kills it, usually after capping, at the prepupal or pupal stage.
How it spreads
- Robbing of a sick or dead colony: the main route between apiaries.
- Swapping frames, supers or equipment between hives.
- Feeding honey of unknown origin.
- Buying unchecked colonies or second-hand equipment.
- Drifting bees between neighbouring hives.
Symptoms
- Spotty brood: empty, capped and open cells mixed together.
- Sunken, darker, greasy-looking cappings, often pierced with a small irregular hole.
- Light brown to chocolate-brown larval remains with a sticky consistency.
- At a late stage, a dry black scale firmly stuck to the lower side of the cell and hard to remove.
- Sometimes the pupal “tongue” stretched towards the top of the cell: a very typical sign when present.
- An unpleasant smell in advanced cases.
The rope test
Push a matchstick or twig into a suspect cell, twist it, then pull it out slowly. If the contents stretch into a brown, elastic thread longer than about one centimetre, American foulbrood is strongly suspected.
The test points the way; it does not confirm. Confirmation requires a laboratory analysis2.
Do not confuse with
| Disease | Main difference |
|---|---|
| European foulbrood | Mostly affects uncapped larvae, often twisted in the cell; pasty remains with little or no roping; scale easy to remove |
| Sacbrood | The larva becomes a fluid-filled sac with its head raised like a “gondola”; no rope |
| Chilled brood | Dead brood at the edges of the cluster, without signs of infection |
What to do if you suspect AFB
In France, AFB is a regulated disease. Any suspicion must be reported without delay to the departmental veterinary services (DDPP), or through your veterinarian or beekeepers’ health organisation5, 6. Most countries have a similar duty: contact your national bee inspectorate or veterinary authority.
While waiting for the authorities:
- Close or reduce the hive entrance to limit robbing.
- Do not move the hive or any frame.
- Do not harvest or swap any equipment.
- Disinfect hands, gloves and hive tools before touching other hives.
The measures are then decided by the authorities. Depending on the situation, they range from burning the affected colonies to shaking the bees onto new equipment and destroying the combs. A surveillance zone may be set up around the apiary, with a ban on moving hives and equipment5.
Antibiotics: why they are not a solution
In the European Union, no antibiotic is authorised for honey bees. In some countries, including the United States, antibiotics can be prescribed by a veterinarian. They suppress the symptoms but not the spores: the disease returns as soon as treatment stops, and residues can end up in the honey.
Prevention
- Check the brood systematically during spring and summer inspections.
- Only buy colonies and second-hand equipment from reliable sources.
- Never feed honey of unknown origin.
- Never leave dead hives open to robbing.
- Replace brood combs regularly.
- Disinfect reused equipment: scraping, then scorching with a blowtorch until the wood browns, is the usual method.
References
- Genersch E. (2010). American Foulbrood in honeybees and its causative agent, Paenibacillus larvae. Journal of Invertebrate Pathology 103: S10-S19. doi.org
- WOAH. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, chapter 3.2.2: American foulbrood of honey bees (version adopted 2016).
- WOAH. Terrestrial Animal Health Code, chapter 9.2: Infection of honey bees with Paenibacillus larvae. woah.org
- IZSVe, reference laboratory for honey bee health. American foulbrood factsheet. izsvenezie.com
- Prefecture of Pyrénées-Orientales (2024). Detection of American foulbrood (in French). pyrenees-orientales.gouv.fr
- Prefecture of Loire-Atlantique. Keeping bees: reporting regulated health hazards (in French). loire-atlantique.gouv.fr
- GDSA 30. Diseases, parasites and predators of colonies (in French). gdsa30.fr