On the contagion of foulbrood

American foulbrood is a serious and highly contagious brood disease. Incorrectly regarded by beekeepers as a shameful disease, many preconceived ideas about it persist. This is a good opportunity to revisit and clarify the subject.

Contagion of foulbrood – transmission pathways and beekeeping responsibility

American foulbrood is a severe and highly contagious brood disease caused by the bacterium Paenibacillus larvae. The pathogen exists in two forms: the vegetative bacillus, which multiplies inside larvae, and the spore, which is extremely resistant and can remain infectious for decades. Only a few dozen spores are sufficient to infect a healthy larva.

Larval infection occurs through ingestion of spore-contaminated food. Spores germinate in the larval gut, bacilli multiply rapidly, invade the body and kill the larva. As the dead larva dries out, each bacillus forms a spore, producing millions of infectious units. The bacillus enables multiplication, while the spore ensures persistence and spread.

A contaminated hive is not necessarily clinically diseased. Symptom development depends on spore load and colony strength. Strong colonies may tolerate low-level contamination, whereas stress factors such as Varroa infestation, food shortage or weakening can trigger disease expression. Foulbrood may therefore remain latent.

The main natural transmission route is robbing. In diseased colonies, brood and honey combs are heavily contaminated. When a weakened colony is robbed, robber bees carry large quantities of spore-laden honey back to their own hives. Transmission via flowers by foraging bees plays only a minor role.

Beekeeping practices are a major driver of disease spread. High-risk practices include:
– working between colonies without disinfecting tools,
– exchanging brood or honey frames,
– reusing equipment without inspection or disinfection,
– lack of quarantine for captured swarms,
– artificial splitting using potentially contaminated combs.

The most critical moment within an apiary is honey harvesting. Mixing frames from different hives during extraction can spread spores from a single infected colony to many others. Allowing bees to lick supers or feeding honey is strictly prohibited.

Antibiotics are not a solution. They kill vegetative bacilli but not spores, temporarily suppress symptoms and promote silent spread. American foulbrood must therefore be managed as a notifiable disease with strict sanitary measures.

In conclusion, foulbrood spreads primarily through robbing and improper beekeeping practices. Strict hygiene, disciplined equipment management and constant vigilance are the most effective preventive measures.
 

See also:

• Practical Guide: 2.1 American Foulbrood
• Practical Guide: 2.2 European Foulbrood
• Shook Swarm for Sanitation Purposes
• Practical Guide: 4.1 Hygiene
• Practical Guide: 4.8.3 Robbing