Bee Health

American Foulbrood: How to Identify, Report, and Prevent It

By Scout Theory · May 2026 · 12 min read

American foulbrood (AFB) is the most serious bacterial disease of honey bees, and the three words every beekeeper dreads hearing from their state inspector. Caused by the spore-forming bacterium Paenibacillus larvae, AFB is highly contagious, nearly impossible to cure, and — in most states — legally reportable. A confirmed case typically means destroying the colony and burning the equipment.

The good news is that AFB is not common in well-managed apiaries. The bad news is that many beekeepers cannot recognize it until it is too late. This guide will teach you exactly what to look for, how to test, and — most importantly — how to prevent AFB from ever reaching your hives.

What AFB Looks Like

AFB attacks bee larvae after they have been capped. The visual signs appear on the surface of the brood comb:

Sunken, greasy cappings. Healthy capped brood has slightly convex, dry, tan cappings. AFB-infected cells have cappings that are concave (sunken inward), often darker in color, and have a wet, greasy appearance. Some cappings will have small irregular holes where workers have partially chewed through them.

Shotgun brood pattern. A healthy queen lays eggs in solid patterns with few empty cells. AFB creates a scattered, patchy pattern as workers remove some infected larvae but leave others. The mix of capped, open, and empty cells in random arrangement is called a "shotgun" or "pepperbox" pattern.

Foul smell. The name says it. Advanced AFB infections produce a distinctive, sour, rotting smell that is unlike anything else in a beehive. Some describe it as similar to rotting animal glue or sour milk. If you open a hive and your nose tells you something is wrong, take that seriously.

Ropy larvae. This is the definitive field test. Insert a toothpick or small twig into a sunken, discolored cell and slowly pull it out. If the contents stretch into a brown, mucus-like rope 1–2 inches long before breaking, that is the classic "ropiness" test for AFB. No other bee disease produces this ropy consistency.

Scale. Dead larvae that have dried down to the bottom of the cell form a hard, dark brown or black "scale" that is nearly impossible to remove. Hold the frame at an angle with the sun behind you and look along the bottom of cells — scales will catch the light. Scales contain billions of AFB spores and remain infectious for over 50 years.

AFB vs. EFB: Know the Difference

How to Test

The ropiness test is the standard field test and requires nothing but a toothpick. Insert it into a suspect cell, twirl gently, and pull slowly. If the contents stretch into a brown ropy string, presume AFB and contact your state apiary inspector immediately.

The Holst milk test is a simple confirmation test you can do at home. Collect a sample of suspect larvae (a few cells' worth) into a small container. Mix with an equal volume of milk (powdered or fresh). If the mixture clears within 10–20 minutes, AFB spores are present — their enzymes digest the milk proteins. A sample without AFB will remain cloudy.

Rapid lateral flow test kits (similar to COVID rapid tests) have been available from manufacturers like Vita. However, availability in the US has been inconsistent due to regulatory issues. Check with your local bee supply company for current availability, or order from the AFB test kits available on Amazon.

State lab testing is free in most states. Your state apiary inspector can collect samples and send them to the USDA Beltsville Bee Lab or your state diagnostic lab. This is the gold-standard confirmation.

What to Do If You Find AFB

Step 1: Close the hive immediately. Do not move frames, swap equipment, or use the same hive tool on other hives. AFB spores spread on equipment, hands, and frames.

Step 2: Contact your state apiary inspector. AFB is a reportable disease in most US states. Your inspector will confirm the diagnosis and guide you through the required steps. This is not optional — failing to report AFB can result in fines and puts every beekeeper in your area at risk.

Step 3: Follow your state's protocol. In most states, confirmed AFB colonies must be euthanized (killed by soapy water or gasoline) and all equipment — frames, boxes, foundation, everything that touched the infected hive — must be burned in a pit and buried. This is heartbreaking but necessary. AFB spores survive for 50+ years on equipment and will infect any colony that contacts them.

How to Prevent AFB

Never buy used equipment from unknown sources. Used frames and boxes can carry AFB spores for decades. If you buy used equipment, irradiate it (some state programs offer this) or scorch the inside of wooden boxes with a propane torch until the wood is lightly charred.

Never feed bees honey from unknown sources. Grocery store honey can contain AFB spores. Only feed your bees their own honey or sugar syrup.

Inspect your brood regularly. Check for healthy brood patterns at every inspection. The earlier you catch AFB, the less it has spread. See our guide to reading a brood frame.

Maintain strong colonies. Strong colonies are better at hygienic behavior — detecting and removing infected larvae before the disease establishes. Hygienic bee stock (VSH queens, for example) show some resistance to AFB through rapid removal of infected brood.

Register your hives. Most states require hive registration. This allows inspectors to track disease outbreaks and notify beekeepers in affected areas. Registration is usually free or under $10.