2.1 American foulbrood

American foulbrood is a bacterial disease. Under optimal conditions, the bacteria multiply by cell division. When conditions become unfavourable, spores are formed that remain infectious for decades. These highly infectious spores are distributed within the colony in the brood nest through the cleaning and maintenance activities of worker bees. Infection by spores occurs via the royal jelly on which larvae feed during the first 48 hours of their life. Adult bees are not infected but can act as carriers of the pathogen. A completely dried larva forming a crust may contain more than 2 billion spores.

Practical Guide (BGD / SSA) – Summary

Factsheet: 2.1 American foulbrood

• Infectious agent: American foulbrood is caused by the spore-forming bacterium Paenibacillus larvae. The spores are extremely durable and remain infectious for many years. Adult bees can be carriers without becoming ill, whereas infected larvae eventually die.:contentReference[oaicite:7]{index=7}
• Clinical signs: Dead larvae turn into a brownish, sticky mass; cappings become sunken or perforated; when drying, the material forms hard scales that are difficult to remove.:contentReference[oaicite:8]{index=8}
• Transmission mechanisms: Spores are transmitted via contaminated food, tools, honey, frames, or the bees themselves (robbing, drifting), leading to rapid spread between colonies.:contentReference[oaicite:9]{index=9}
• Obligations: American foulbrood is a notifiable epizootic disease; any suspicion must be reported immediately to the apiary inspector. Quarantine zones may be established to limit its spread.:contentReference[oaicite:10]{index=10}
• Initial measures: Systematically inspect suspicious combs, isolate or remove potentially contaminated material, and reduce interactions between apiaries.:contentReference[oaicite:11]{index=11}
• Control measures: Clinically infected colonies must be eliminated appropriately; contaminated material must be treated or destroyed to prevent reinfestation.:contentReference[oaicite:12]{index=12}

► PDF of the official factsheet (FR)

► Information page “American and European foulbrood” (abeilles.ch)

Summary based on factsheet 2.1 (V 2208). Last verification: 01/2026.

See also:

• Factsheet: 2.2 European foulbrood
• On the contagion of foulbrood
• Factsheet: 4.7.2 Eliminate colonies
• Bee health guide