American foulbrood is the most destructive disease of bee brood, caused by the bacterium Paenibacillus larvae. The bacterium forms spores, and those spores remain viable for decades — in comb, in timber, in honey and on tools.

This durability sets the disease apart. In other diseases a colony may recover; here the real task is to stop the infection spreading.

How it spreads

Only very young larvae are susceptible, roughly the first three days. When a nurse bee feeds contaminated food the spore germinates in the larval gut, kills the larva and produces millions of new spores.

Routes between colonies:

• Robbing — honey from a weakening infected colony is carried away
• Sharing comb and equipment — the commonest route
• Hive tool and gloves
• Buying infected colonies or package bees
• Feeding honey from an unknown source

Signs

• Abnormal cappings: Sunken, dark, perforated and greasy-looking
• Irregular brood pattern: Gaps between filled cells
• Smell: A characteristic sharp and foul odour
• Scale: The dead larva dries and adheres firmly to the lower cell wall; it cannot be removed without scraping

The matchstick test

This is the most informative test that can be done in the field. A thin stick is inserted into a suspect dead larva and drawn out slowly.

If the remains stretch out like a thread for two or three centimetres, that is a strong indication of American foulbrood. In European foulbrood and sacbrood no such thread forms.

The test is not a definitive diagnosis; confirmation comes from a field kit or laboratory analysis.

Why antibiotics are not a solution

Antibiotics suppress the vegetative form of the bacterium but do not kill the spores. The result is:

• Symptoms disappear and the colony looks healthy
• Spores remain in the comb and the hive
• The disease returns when treatment stops
• Residues form in honey and wax
• The concealed infection spreads to the region through colonies that are sold

For these reasons antibiotic use in bees is restricted or prohibited in many countries.

What to do

• Close the hive and separate the equipment. Stop moving comb and sharing tools at once
• Report it. American foulbrood is notifiable in many countries. Inform the competent agricultural authority or your beekeeping association
• Wait for official instructions. The method to be applied differs by country; destruction of infected material is the common standard
• Disinfect the tool. Flaming is the most effective method

Prevention

• Do not buy colonies, comb or honey from unknown sources
• Clean the hive tool between hives
• Renew comb regularly
• Prevent robbing; leave no honey or comb in the open
• Keep a record of colony movements