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Symptoms of European Foulbrood + Pictures

A hive with severe EFB.

European foulbrood (EFB) is a destructive brood disease found on every continent where bees live. It is the dominant bacterial brood disease in the United Kingdom and is a growing problem for beekeepers in the rest of Europe, the United States, Canada, and Australia.   

What is European foulbrood?

EFB is a disease that infects larvae at any stage of development. Melissococcus plutonius bacteria is distributed by contaminated food and multiply in the host’s gut. The larvae soon starve to death as the bacteria feed on any food consumed.

What are the symptoms of European foulbrood?

To identify European foulbrood, beekeepers should look for clinical signs such as a spotty brood pattern and sunken capping. Diseased larvae often turn yellow or brown and curl upwards in a C-shape. Other symptoms include a sour, rotting aroma and dead larvae with a rubbery, melted, watery appearance.

Spotty brood pattern

Spotted brood pattern is also known as shotgun pattern. As larvae die in an infected colony, an irregular brood pattern will result. This spotty formation looks a lot different from healthy brood, which has a uniform shape and color.

Shotgun brood results from a wide range of diseases and hive issues. It should never be used on its own for the diagnosis of EFB.

Shotgun brood caused by with EFB.
Spotty pattern is an indication that the brood may not be healthy.

Sunken capping

While healthy brood capping appears full, it will look sunken in a foulbrood-infected hive. The caps may also have a punctured appearance.

Cappings that appear deflated are also signs of other diseases like American foulbrood. It’s never a good sign to discover sunken cells, but it doesn’t guarantee the hive has EFB.

A closeup image showing sunken cappings and also dead brood transformed into scale.
The caps appear sunken of concave.

Unhealthy looking larvae

Larvae infected by EFB will transform from pearly white to a yellow shade and then brown. They may also deflate, melt, or look dehydrated.

Sick or deceased larvae often twist and curl up as they die. During inspections, look for young C-shaped larvae close to their cell walls. This symptom is common at the 3-5 day stage of development.

Closeup of larval gut.
Larval gut due to brood infected with European foulbrood.

Visible tracheae

A sick larva may have a diseased trachea that is easier to see through its protective outer layer. Its body is deflating, making the tracheal tube more prominent.

Unpleasant smell

Hives with European foulbrood sometimes give off a foul odor. Beekeepers often describe smelling rotting fish or a slightly sour fragrance. However, an unpleasant aroma isn’t always present, so a neutral smell isn’t confirmation that the hive is healthy.

How to test for European foulbrood

Matchstick test

A matchstick, toothpick, or small stick is excellent for testing the consistency of larvae. It offers clues into what the hive is suffering from but doesn’t provide conclusive answers.

  1. Gently push the tester into a dead larva or any suspicious-looking cell with discolored or sunken capping.
  2. Slowly pull the matchstick out and look for ropey larval goo. A string under 1.5cm strongly signals that you’re dealing with European foulbrood.

The brood may still contain EFB if the larva doesn’t rope out. This test usually only works in old infections. If Paenibacillus alvei are also present, the ropiness will extend further.

Top down shot displaying brood with severe EFB disease.
The advanced stages of EFB.

Diagnosis kits

Confirming that a colony has EFB is tricky, so diagnostic kits will help. They are easy to use, accurate, and help beekeepers to avoid taking incorrect action against a similar bacteria or virus.

Diagnosis kits only test for one disease, so one designed for American foulbrood will not provide a positive result for EFB.

Steps for using a diagnosis kit:

When using a testing kit for EFB, it is best to start by labeling the bottle with the tested colony and current date. This step makes keeping track of multiple hives much easier.

  1. Remove the extraction bottle’s lid and drop a potentially infected larva into it. The bottle should already contain extraction fluid; if it doesn’t, add it now.
  2. Seal the bottle’s lid and give it a hard shake for around 20 seconds.
  3. Take off the lid and take a liquid sample using the pipette that comes with the kit.
  4. Squeeze the end of the pipette and release three drops of fluid onto the sample well. Allow three seconds before checking the result.

What is the difference between European and American foulbrood?

American and European foulbrood often get mixed up as they have similar symptoms. However, larvae infected by EFB usually die before the brood is capped; AFB-infected larvae typically survive until capping.

Testing the ropiness of brood with a matchstick is another helpful way of determining which type of foulbrood you’re dealing with. A shorter rope often signifies EFB, while longer ones are more likely to be AFB. 

Be sure to check out all the symptoms of American foulbrood here.

Birdseye view of brood inflicted with EFB.
Cells infected with EFB.

Commonly asked questions

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. Healthy larvae should be C-shaped, plump, and pearly white.

When should I inspect for EFB?

The best time to check for EFB is in spring and fall. During these seasons, diseases like foulbrood are most common. 

How does EFB impact a hive?

European foulbrood is a disease caused by the bacterium Melissococcus pluton. It can severely reduce the hive’s productivity and may lead to the death of an entire colony.

Worker bees feed infected food to their larvae which causes the bacteria to spread rapidly. Once inside the gut, the bacterium feeds on the larva’s food, causing the undeveloped bee to starve. 

Image attribution: “Courtesy The Animal and Plant Health Agency (APHA), Crown Copyright”. Sourced from BeeBase Gallery.

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