American Foulbrood (AFB): The Worst Larval Disease

Updated October 2026. This post is part of our series on honey bee diseases and parasites. We start with the worst of them all: American foulbrood, or AFB.

What causes AFB

AFB is caused by the spore-forming bacterium Paenibacillus larvae. Its spores survive for decades in honey, beeswax, soil and beekeeping equipment. It is the most dangerous and contagious honey bee disease.

AFB symptoms and diagnosis

AFB shows up in the capped brood, not the open brood, so inspect the capped brood closely.

This is a healthy brood comb. The brood area is uniform, there are no holes in the capped brood, the cappings are light (cappings on old combs are darker), and there is no unusual smell. The few open cells most likely hold pollen or honey.

Now compare the brood of a diseased colony:

These photos are from an infected colony. By the time the brood looks like this, the infection is advanced.

• Scattered brood. The brood area isn't uniform; open and sealed cells are mixed.
• Discolored pupae. Infected pupae turn yellow, then brown and finally black, die in the sealed cells, turn ropy and smell bad.
• Sunken cappings. Cappings over infected brood look darker, greasy and sunken.
• Holes in the cappings. The pupa under a punctured capping is dead.
• Toothpick test. The dead pupa is decayed and smells like sour wood glue.

Insert a toothpick into a cell through the hole in the capping and stir. With AFB, the dead larva sticks to the cell, doesn't come out in one piece, and stretches into a ropy thread. It's a simple and reliable field test.

• Tongue sticking out. The dead pupa's tongue often sticks up from the bottom of the cell.

• Scales. Dried remains stick to the bottom of the cells. Each one can hold millions of spores, and the spores stay alive for decades. Moving these combs into healthy colonies spreads the disease.

AFB vs. EFB: AFB kills the brood in sealed cells, and the dead pupa turns ropy and sticks to the cell. European foulbrood (EFB) affects younger larvae in open cells, and the dead larvae come out of the cells easily.

How AFB spreads

1. Robbing. Diseased colonies are usually weak and can't defend themselves, so other colonies rob them and carry contaminated honey home.
2. Beekeeping practices, such as:
   1. Uniting a diseased colony with a healthy one
   2. Moving combs of honey or pollen between colonies
   3. Using contaminated equipment (combs, hive bodies and tools)
   4. Feeding bees honey from an unknown or infected source
   5. Moving colonies, which spreads the disease from apiary to apiary and region to region

What to do if you suspect AFB

1. Contact your state apiary inspector right away and follow their instructions. AFB is a reportable disease in many states, and many states require infected colonies and their combs to be burned. Don't ignore it, and don't move any combs or equipment out of the hive in the meantime.
2. Don't treat with antibiotics on your own. In the US, antibiotics for honey bees require a veterinarian's prescription or Veterinary Feed Directive. Antibiotics only suppress AFB; they don't kill the spores or cure the colony. Talk to your inspector and a veterinarian before using any.
3. Clean your equipment the way your inspector tells you. AFB spores are very hard to kill, so follow your state apiary inspector's instructions for hive bodies, bottom boards, tools and other equipment, including your Apimaye hives.
4. Prevent it. Inspect your brood regularly, don't swap combs between colonies without checking them, and consider hygienic bee stock. Hygienic bees can detect dead larvae under the cappings, open the cells and remove them.

More in this series: Honey bee diseases and parasites · European foulbrood (EFB)