Symptoms of American Foulbrood [Diagnosis Guide]

A beekeeper inspecting brood for disease

American foulbrood is a highly infectious brood disease caused by Paenibacillus larva , a spore-forming bacterium. Its spores live on hives and beekeeping equipment for decades, reinfecting new colonies as they arrive.

Beekeepers should take foulbrood seriously. While the disease is hard to eradicate, identifying early symptoms will help control its spread. This guide will explain the symptoms of American foulbrood so that you can treat the problem sooner rather than later.

What are the clinical signs of American foulbrood?

Diagnosing American foulbrood can be tricky. The symptoms vary depending on how long the hive has had the disease. Over time, it goes through stages. Few larvae are affected by the disease at the beginning.

1. Foul odor

Beekeepers often report a foul smell coming from hives with foulbrood. It can build in intensity as the disease becomes more severe. Sometimes, the unpleasant aroma overwhelms the senses before opening the hive.

Not all hives infected with American foulbrood give off a bad odor. Some hives smell bad but don’t have the disease making identification more challenging.

2. Pupal tongue

Some infected larvae may reach the pupa stage before dying. If it gets the chance to pupate, the beginnings of a proboscis will often develop. This pupal tongue will retain its form and stretch from the scale to the other side of the cell.

Closeup of pupal tongue inside brood cells
Pupal tongue signifies AFB.

Bees inflicted by American foulbrood won’t reach a fully formed bee. If you see a visible tongue and the bee’s body, AFB is not the culprit. Varroa mite is possible reason for this situation. Check out how to test for varroa mite here.

3. Caramel color

Honey bee larvae inflicted by foulbrood will transform from pearly white to a caramel shade. This results from bacteria breaking down the developing bee and is only a temporary characteristic.

The caramel appearance is a vital sign the colony has American foulbrood. Most disease-ridden larvae turn yellow or gray. 

Open brood cells showing beige larvae that are infected by foulbrood.
Notice how the brood has changed color?

4. Spotted brood pattern

Spotted brood pattern is also known as shotgun pattern. Most larvae will die in a heavily infected colony, resulting in an irregular brood pattern. A spotty formation is in stark contrast to regular, healthy brood with a uniform color and shape.   

A top down image of brood that is spotted.
Spotted or shotgun brood pattern.

Factors like a poor queen, environmental factors, and other diseases can also cause spotted brood. Non-regular brood is another clue that AFB may be present but isn’t a certainty.  

5. Sunken cappings

While healthy brood cells look full, infected ones look sunken. That’s because infected larvae usually die after the nurse bee caps the cell.

Bird's eye view of sunken cappings caused by American foulbrood
Capping with sunken brood cells.

Cappings that appear deflated are also signs of other diseases like European foulbrood. It’s not a good sign to discover sunken cells, but it doesn’t guarantee the hive has AFB.

You may also want to check out the symptoms of European foulbrood.

6. Dark cappings with holes

Cappings will appear darker that contain dead larvae. They may also have an unpleasant greasy or moist appearance. Beekeepers should look for jagged holes in the cappings. They won’t typically be in the center of the cell.

Keep in mind that sometimes healthy brood has holes in the cappings. A quick check underneath will determine whether the larvae look a healthy white or dead.

7. Larval scale

Thriving larvae are pearly white. Once infected, they darken and transform into goo. Recently deceased larvae will appear wet and have a caramel shade. As time passes, they turn into a hard, dry larval scale that gets stuck to the lower cell wall.

Closeup of American foulbrood scale inside cells of a hive.
Foulbrood scale is black, hard, and easy to identify.

Dried black American foulbrood scale is easy for beekeepers to spot. Hold a frame, ensuring the top bar faces toward you. The cell walls facing toward you will have the black scale.


How to prepare for an AFB inspection

Hive inspections should never transfer spores and bacteria to new healthy colonies. Transmission is possible on equipment like hive tools and smokers. Thoroughly wash your tools after each visit to a suspect hive.

  • Remove propolis using rubbing alcohol and a scrubber.
  • Wash equipment in a solution of bleach and water.
  • Sterilizing tools over a flame may not remove all the disease.

Beekeepers should also follow best practice by wearing nitrile gloves. Latex ones will work as a second-best option but avoid using leather gloves as they can’t be cleaned. Carefully dispose of gloves after working on the hive.

Keep your hands clean by washing them thoroughly with soapy water. Take care to remove any propolis or wax. If you don’t have access to water near the hive, use hand sanitizer or rubbing alcohol.

Watch how to identify AFB

What field tests can I run for American foulbrood?

Beekeepers have a range of field tests that they can run to diagnose American foulbrood. Popular methods include the match stick test, Holst milk test, and commercial diagnostic kits.

While a positive result means the hive has foulbrood, a negative reading could indicate the disease was missed. In this case, the beekeeper will have to make a judgment call on what to do with the colony.

1. Match stick test

Use for: active infections

This is a simple test that requires a matchstick, toothpick, or small stick that is capable of piercing through capping. Gently push the tester into any suspicious-looking cell with discolored or sunken capping. Slowly pull the matchstick out and look for ropey larval goo. If you see a string over ¾” (2cm), you’ve got a positive result for foulbrood.

The brood may still contain AFB if the larva doesn’t rope out. This test only works if the larva is sufficiently decayed but not too old.

Two shots demonstrating the matchstick test to test for signs of AFB.
Check if larvae forms a rope when a stick is removed.

2. Holst milk test

Use for: active infections or scales

American foulbrood enzymes break down milk protein. Beekeepers can add suspect larvae to a milk solution to test for AFB. The disease’s presence will make the milk clear brown, similar to iced tea. Other diseases found in beehives won’t have the same reaction, so you can be confident it’s AFB with a positive result.    

3. Diagnostic kit

Beekeepers can buy commercial testing kits that are reliable and easy to use. They typically look like a pregnancy test, with a line that indicates a positive result.

Commonly asked questions

What does foulbrood look like?

Tell-tale signs of American foulbrood are sunken sealed brood and larvae that is tan or coffee colored instead of white. You may also notice spotty brood pattern or an unpleasant rotting smell coming from the hive.  

When are good times to inspect brood?

Beekeepers should schedule brood inspections in early spring and during peak honey production. It is also best practice to check hives in the fall as beekeepers prepare for winter. Outside these times, inspections may also be a good plan when honey supers are removed.

What does healthy brood look like?

Healthy brood has a uniform pattern, with eggs typically laid by the queen in regular patterns or concentric circles. The bright caps should have a convex shape.

Closeup of healthy brood in a hive
Healthy larvae have a pearly white appearance.

Summing up

American foulbrood isn’t the same threat to the beekeeping industry that it was back in the 1920s. But the pathogen is still present and is extremely hard to eradicate.

Knowing how to recognize the American foulbrood symptoms and effectively deal with it is essential knowledge for beekeepers. It is highly contagious, so early detection may stop your colony from spreading the disease to other hives.

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