Lesson 58: American Foulbrood

The most serious brood disease: how to recognize it, test for it, and what to do

American foulbrood (AFB) is the most serious bacterial disease of honey bee brood. It is extremely contagious, its spores survive for decades in equipment and honey, and it usually kills the colony. There is no cure. Every beekeeper needs to recognize it, and to act fast if they suspect it.

What causes it

AFB is caused by the bacterium Paenibacillus larvae. It spreads only through its spores:

• Nurse bees feed spore-contaminated food to young larvae. The bacteria multiply and kill the larva after its cell is capped, at the end of the larval stage or early pupal stage.
• The dead larva dries into a scale packed with spores. A single scale can contain hundreds of millions to a billion spores, and very few spores can start a new infection.
• Spores stay alive for decades, 70 years or more by one estimate, in comb, hive parts and honey. They can't be killed by antibiotics.
• AFB spreads between colonies through robbing, drifting bees, contaminated equipment, hive tools, beekeepers' hands, and feeding honey from other hives. Adult bees carry the spores without showing signs.

The signs

Learn these, and look for them at every inspection (lesson 34):

• A spotty, irregular brood pattern ("pepper-box" brood).
• Cappings that are sunken (concave), darkened, greasy-looking, and perforated with ragged holes.
• Dead larvae or pupae that turn from pearly white to coffee-brown or chocolate-brown and melt into a gooey mass on the floor of the cell.
• "Pupal tongue": sometimes the dead pupa's tongue sticks up to the top of the cell. It's rare, but unique to AFB.
• Scale: in later stages the remains dry into a dark brown-to-black scale lying flat on the lower side of the cell, stuck so tightly it can't be removed without damaging the comb.
• A foul odor in heavily infected colonies, often compared to a chicken house. It isn't always present, and not everyone can recognize it.
• The colony dwindles and collapses as more brood dies.

Healthy brood cappings can have holes too: cells are capped from the edge in, so an unfinished capping has a small, centered hole with smooth edges. Diseased cappings have ragged, off-center holes, and are sunken and dark.

Field tests

Use more than one test to confirm.

1. The rope test (the classic field test). Pick a cell with brown, gooey (not dried) remains. Push a matchstick, toothpick or twig into it, stir, and slowly pull it out. - AFB: the brown mass stretches out in a sticky "rope", about ½ to 1 inch or more, before snapping back. - Not AFB: the material doesn't rope, or breaks off right away.

2. The black-light test. AFB scales glow greenish-blue under an ultraviolet (black) light.

3. The milk test (Holst milk test). Mix a pinch of powdered milk in water, split it into two small clear containers, and add brown larval remains to one. Keep both warm. If the one with the sample turns clear within about 15 minutes to an hour while the other stays milky, that suggests AFB.

4. Field test kits. Commercial lateral-flow test kits for AFB (and separate ones for EFB) are sold by bee suppliers.

5. A laboratory test. Send a comb sample to a lab (lesson 40). If you're not sure, get an expert to look: a state apiary inspector or extension specialist.

How it looks: Healthy brood compared with American and European foulbrood.

AFB: spotty brood with sunken, darkened, perforated cappings.

AFB up close: sunken cappings and dead brood.

Another comb with American foulbrood.

If you suspect AFB

Act immediately. Don't delay.

1. Close the hive and reduce its entrance to limit robbing.
2. Don't move any frames, honey or equipment from that hive to other hives. Clean your hive tool and hands, and use disposable gloves.
3. Get it confirmed. Contact your state apiary inspector (see the box below) or an extension specialist. In many states AFB must be reported.
4. Follow the inspector's instructions. They depend on your state's rules and the time of year, but typically:
5. All frames (brood and honey) from an AFB colony must be destroyed. Where allowed, equipment is burned in a pit and the ashes buried; otherwise it is double-bagged and sent to a landfill.
6. Boxes, lids and bottoms are destroyed, or sterilized by approved methods (scorching and hot-wax dipping, or irradiation facilities).
7. The adult bees may have to be destroyed, or in some states, early in the season, shaken onto new equipment ("shook swarm") under supervision.
8. Watch your other colonies closely for the next year.

About antibiotics

• Antibiotics can't cure AFB. They don't kill spores, and they can't save infected larvae. They only mask the symptoms, which return when treatment stops.
• Since 2017, antibiotics for bees are available only with a veterinarian's order (a Veterinary Feed Directive or prescription).
• Specialists advise using antibiotics only together with destroying infected frames, and never to keep sick colonies alive. Overuse has already produced antibiotic-resistant AFB. Beekeepers should never keep "hospital yards" of AFB colonies; they're reservoirs that spread disease for miles.

Prevention

• Don't buy used equipment or colonies of unknown history (lesson 28). Scales are easy to miss.
• Never feed honey from other sources (lesson 39).
• Clean hive tools (isopropyl alcohol) and hands between colonies and yards; avoid leather gloves, which can hold spores.
• Prevent robbing and drifting: reduce entrances on weak colonies; arrange hives in a horseshoe or irregular pattern rather than a straight row; paint hives different colors.
• Keep hygienic stock (lesson 15) and replace old comb on a three-year cycle (lesson 53).
• Inspect brood regularly, so problems are caught early.

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📍 Georgia

This insert is shown to customers in Georgia.

AFB is regulated by the Georgia Department of Agriculture (GDA). Georgia's bee law gives the Commissioner of Agriculture full power to deal with American and European foulbrood, and to require destruction, treatment or disinfection of infected or exposed bees and equipment (lesson 4). Infected colonies are normally burned by state inspectors.

If you suspect AFB in Georgia, contact the GDA Apiary Program (see its Apiary Program: Honeybees web page for current contacts) to report it and arrange an inspection.

Compensation: when bees or equipment are ordered destroyed, a Georgia resident beekeeper can be paid 50% of the appraised value, up to $25 per colony, when funds are available.

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What to remember

• AFB = the worst brood disease. Spores last decades; no cure; usually fatal.
• Signs: spotty brood; sunken, dark, greasy, perforated cappings; brown, melted larvae; ropy remains; flat, black scale stuck to the cell floor; pupal tongue; foul odor.
• Test: rope test (½–1 inch rope), black light, milk test, test kit, lab; get an expert to confirm.
• Suspect it? Close the hive, move nothing, call the state inspector. All frames from an AFB colony are destroyed.
• Antibiotics don't cure it and need a vet's order. Prevention is everything.

Quick check

1. What makes AFB so hard to get rid of?
2. Describe a positive rope test.
3. You think you've found AFB. What are your first three steps?

Show answers

1. Its spores survive for decades in comb, equipment and honey, and antibiotics can't kill them. 2. Stirring the brown remains with a stick and pulling it out slowly draws out a sticky rope about ½ to 1 inch long. 3. Close the hive and reduce the entrance; don't move any frames, honey or equipment and clean your tools and hands; contact the state apiary inspector to confirm and follow their instructions.

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Adapted from: UGA Bee Program, Honey Bee Disorders: Bacterial Diseases; Penn State Extension, Honey Bee Diseases: American Foulbrood (updated 2025); Honey Bee Health Coalition, Identifying and Mitigating Foulbrood in Honey Bee Colonies (2nd ed., 2022); Shimanuki & Knox, Diagnosis of Honey Bee Diseases (USDA AH-690, rev. 2000); UF/IFAS Extension, Identification and Treatment of European Foulbrood (IN1272), Table 1; Georgia Department of Agriculture, Georgia Bee Law (O.C.G.A. §§ 2-14-40 to 2-14-47).

Photos: AFB comb, Tanarus, CC BY-SA 3.0 · AFB comb, Pollinator, CC BY-SA 3.0 (all via Wikimedia Commons). Diagram: Forge Guides.

Adapted from university Extension sources. Experienced Georgia beekeepers are reviewing these lessons. See something that doesn't match your experience? Tell us.