American Foulbrood: How to Identify It and What to Do

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title: "American Foulbrood: How to Identify It and What to Do" category: "Hive Health & Disease" summary: "American Foulbrood is the most serious bacterial disease in beekeeping. It's also notifiable in most states. Here's how to identify it, confirm it, and what happens next." readTime: 7 difficulty: "beginner" season: "year-round" slug: "american-foulbrood" publishedAt: "2026-03-02" course: "beginner" module: "Hive Health & Disease" lessonOrder: 19

American Foulbrood (AFB) is caused by the spore-forming bacterium Paenibacillus larvae. It's the most destructive bacterial disease of honey bee brood, and unlike most hive problems, it cannot be resolved by management alone — the spores survive in wood and wax for decades and are highly infectious. In most U.S. states and many countries, AFB is a notifiable disease, meaning you're legally required to report it to your state apiarist.

This is not an article to alarm you — AFB is less common than many beekeepers fear. It is an article to make sure you can recognize it quickly, because early detection is the difference between treating one hive and losing an apiary.

How AFB Spreads

P. larvae spores are extraordinarily resilient. They remain viable in old comb, wooden equipment, and honey for 40 years or more. The disease spreads when:

• Bees from infected colonies rob healthy hives — bringing infected honey with them
• Beekeepers move contaminated equipment between hives (hive tools, frames, supers)
• Infected honey is fed to bees — never feed honey of unknown origin
• Swarms from infected colonies establish new hives carrying spores

Young larvae are infected when nurse bees feed them contaminated food. The bacteria multiply inside the larva, releasing billions of spores when the larva dies. A single infected larva contains enough spores to infect an entire colony.

How to Identify AFB

AFB affects capped brood, not eggs or young larvae. The infection progresses inside the capped cell, so by the time you see symptoms, the larva has already died.

What to look for:

• Sunken, discolored cappings. AFB cappings become darker (coffee-brown to black), sunken inward, and often punctured or perforated. Healthy cappings are tan, slightly domed, and intact.
• The ropiness test. Insert a toothpick or twig into a suspect capped cell and stir. Pull it out slowly. AFB-infected brood will rope out in a stringy, elastic thread — sometimes 2–3 cm long — before snapping back. This is the most reliable field test.
• Distinctive smell. AFB produces a strong, foul odor — described as rotting fish, old gym shoes, or fermenting glue. Not every case has a strong smell, especially in early infections, but a hive that smells clearly wrong warrants investigation.
• Dried scale. In advanced cases, the dead larva dries into a hard, dark scale lying flat on the lower wall of the cell. This scale is virtually impossible to remove without destroying the comb.

What AFB is not:

• Sacbrood causes similar discolored cappings but fails the ropiness test — the larva doesn't rope.
• European Foulbrood (EFB) affects younger larvae before capping, and the smell is sour rather than putrid.
• Chilled brood produces spotty capped brood but no odor and no ropiness.

When in doubt, do the toothpick test on multiple cells.

What to Do If You Suspect AFB

Do not share any equipment. If you suspect AFB, immediately stop moving frames, supers, or tools between that hive and any others. Use a dedicated hive tool for the suspect hive and do not bring it near other equipment without thorough disinfection.

Contact your state apiarist. AFB is notifiable in most U.S. states. Your state apiarist can inspect, confirm the diagnosis, and advise on legal requirements. Many states will provide a free inspection. Find your state apiarist through your state department of agriculture website.

Confirm with a field test kit. Vita Bee Health and Beelogics make lateral flow assay test strips (similar to a COVID test) that confirm AFB in the field within minutes. These are available through major bee supply companies and are worth keeping on hand.

Treatment and Disposal

In the United States, the only legal treatment for AFB is oxytetracycline (sold as Terramycin). However — critically — oxytetracycline does not kill spores. It suppresses the vegetative form of P. larvae and can prevent new infections, but it does not eliminate an active infection or render contaminated equipment safe. Oxytetracycline is a management tool, not a cure.

Burning equipment is the only method that reliably destroys AFB spores. In many states, a state inspector will order destruction of infected equipment. The bees, frames, comb, and sometimes the woodenware are burned on-site. It's a painful process — but equipment that isn't destroyed will be a source of infection for years.

Irradiation of woodenware (via a licensed gamma irradiation facility) is an alternative to burning that preserves boxes and other wooden equipment. It's more practical for commercial operations.

Prevention

• Never feed honey from unknown sources. This is the most common avoidable introduction route.
• Buy equipment from known sources. Used equipment can harbor spores.
• Requeen from hygienic stock. Hygienic bees detect and remove diseased larvae before the bacteria complete their cycle. Hygienic queens are available from breeders and are tested for hygienic behavior.
• Keep strong colonies. Strong colonies with low Varroa levels have better hygienic behavior and are more resistant to bacterial disease.
• Register your hives with your state apiarist. Registered beekeepers receive disease alerts when AFB is confirmed in their area.

Key Takeaways

• AFB cappings are sunken, dark, and often punctured. The ropiness test (toothpick pull) is the definitive field sign.
• The smell is strong and distinctive — putrid, not sour. If you notice an unusual smell, investigate immediately.
• AFB is notifiable in most states. Contact your state apiarist as soon as you suspect it.
• Do not move equipment from a suspect hive to other hives. Spores spread through contaminated equipment and honey.
• Prevention: never feed honey of unknown origin, buy equipment from reliable sources, and maintain hygienic breeding stock.