American Foulbrood in Beehives

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Beekeepers face many challenges but one of the most frustrating is American Foulbrood (AFB). This contagious disease can wipe out the whole apiary. No beekeeper wants to find it in their hives. But, it is important to understand what it is, how to recognize it and what to do if you suspect it to be a problem in your bees.

Beekeeper using light to inspect frame for American Foulbrood image.

Many challenges to honey bee health harm the colony in small ways. Sometimes these bee diseases cause harm in ways we don’t see. Shortened lives or failure to thrive may be symptoms. But, serious infectious diseases may harm not only your colonies but those of your neighbor as well.

What is American Foulbrood Disease?

American Foulbrood (also called AFB) is a bacterial disease caused by a spore forming bacteria (Paenibacillus larvae). It is a common widespread disease that affects honey bee brood.  

Only young developing bees are killed by this disease. Infected adults show no symptoms but can be carriers that spread the disease to others.

For quite a while, the infected colony may not show a decline in population. Things go along and from outward appearances nothing is seems wrong.

However, American Foulbrood usually results in death of the colony. The survival time varies among hives. It may be months or even next year before the colony crashes.

During all this time, infected adults could be spreading the disease to other colonies in the area.

What AFB Looks Like in the Hive

Most brood that die as a result of AFB are in the late larval or capped brood stage of development. Death often occurs after the cell is capped and the larva has spun the pupal cocoon. 

Once the larva is dead, the capping sinks inward toward the bottom of the cell. Instead of protruding slightly above the surface of the comb, the capping is slightly concave.

The previously white bee larva changes from white to caramel to very dark brown. If death occurs in the pupal stage, you may see the tongue protruding from the dead larva-but do not rely on this for a diagnosis.

At the mid-stage of decay, the contents of the brood cell are ropey. Insert a matchstick into the cell, stir and pull out the contents. If the brood goop stretches out ¾” (2cm) or more, you may have AFB.

The decaying larva will continue to dry out and become a brown scale that is very difficult to remove from the honeycomb cell.

Colonies with AFB often have a brood pattern that is patchy. This is due to infected and healthy brood being mixed on the comb.

How American Foulbrood is Spread

This honey bee disease is very contagious. Inside the hive, infected nurse bees unknowingly feed foulbrood spores to developing young. Once the spores reach the larval intestines, they become active.  

Germination and rapid multiplication continue in the gut of the bee larvae. Death usually results about the time the cell is capped.  

As dead larvae break down into a gooey glue-like mass. New spores begin to form.

As more and more brood is diseased, comb and honey in the hive become contaminated. When colony population drops as a result of fewer bees living to maturity, robber bees may attack the hive. 

They take the stored honey and the disease back to their own hive. When house bees clean out cells with dead brood, they are exposed to more spores and facilitate transmission of the disease throughout the hive.

AFB can also spread through honey bee swarming. The swarm issues from a colony that is still rather robust but has AFB spores in the population. 

In this case, the bacteria is present and ready to become a problem at the new hive location.

Beekeepers spread AFB too. Hive tools or other pieces of equipment can harbor disease spores. (If you go to visit a beekeeper friend, don’t take your hive tool. Use theirs. 🙂

Another way we spread it in our own apiary is by moving frames from one hive to another. This is a common practice in colony management. Whether sharing hive resources or making hive splits – all spread the disease.

Because the spores of AFB are so long lived and can be present in honey, beekeepers should never feed honey from an unknown source to bees. Even pasteurized honey can still contain foulbrood spores – just don’t do it.

Honeycomb infected with american foulbrood showing sunken caps, pinholes and scale image.

American Foulbrood Symptoms

How do you know if you have AFB in your colonies? The most fool proof way to find out is to send a sample in for testing if you suspect the problem. 

Various labs across the country are available for testing of bees and comb. Samples can be sent to the USDA-ARS laboratory for diagnosis-but this takes time.

Depending on the regulations in your state, you may be required to call in your state apiary inspector for assistance. Some states have strict regulations regarding dealing with this dangerous condition.

But, there are several conditions that cause similar symptoms. It not always easy to diagnosis AFB.  The field condition of each hive will vary a bit depending on what phase of the disease is present.  

These are a few situations that require further inquiry.

  • Rotting meat smell – dead animal
  • Sunken cell cappings with off-center pinholes
  • Dead larval contents rope out up to ¾  inch 2 cm
  • Caramel color brood – may have pupal tongue extended
  • Brown larval scale in cells that are hard to remove
  • Spotty brood pattern – shot gun brood pattern

Colonies with an American Foulbrood infestation have a distinctive rotting meat smell.  Personally, I think all dead brood stinks so I would not rely on this as my sole indicator.

Many sunken cells of capped brood with off-center holes in the cap are a possible indicator.  As the dead larva shrink, the capping is pulled down into the cell.

With AFB, you may see brood that has died in the pupal stage with extended tongues but that does not always happen.

The most common way to investigate possible cases of AFB is with the ropey test. Do the liquid contents of the dead brood cell stretch out like a rope? If so, this certainly requires further testing.

Difficult for the unexperienced eye to discern, brood killed by AFB will dry to a hard brown scale that if very difficult to remove from the comb.

Larva killed by American Foulbrood ropes out more than 1 inch image.

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Beekeeper Responsibility for AFB

Whether your state has guidelines on what to do if you suspect AFB or not, every beekeeper has a responsibility to be a good citizen and protect the all hives in their region. 

In addition to some field tests to try, beekeepers can purchase a commercial AFB test kit to give quick results on whether or not your hive is likely infected. 

This might be something to consider if you are worried but not quite sure if you need to send in a sample.

Any type of disease pathogen – like nosema, a pest problem or virus can spread from one colony to another – even to nearby apiaries. That is why it is important to routinely inspection your colonies.

Beekeeper checks hive for disease.

American Foul Brood Treatment:

There is no cure for AFB.  Because AFB is so very contagious, some states have a burn only policy. The entire colony, including bees and equipment must be burned to prevent spread of this devastating disease.

Other states will allow beekeepers to have the colony tested for the exact strain of AFB and possibly treat the hive with antibiotics such as -Terramycin (oxytetracycline hydrochloride). 

In some cases, the bees themselves are treated and “shook” into new equipment with no existing comb or woodenware.

There are 2 antibiotic treatments used for AFB: Terramycin and Tylan.  Recommendation regarding their use varies from time to time.  I suggest checking with your state apiarist to get the latest recommendation

As of 2017, beekeepers need a veterinarian prescription for these antibiotics. Keep in mind, antibiotics are used to control the vegetative form of the disease. 

They do not destroy the spores that remain in the hive. Any bee yard that has had an outbreak of AFB must be monitored closely for additional outbreaks.

Spore Form of the Disease

Because AFB has 2 forms: vegetative (active) and spore (dormant) it is very difficult to eradicate.

The spore form of AFB is very stable and can live on equipment for decades. Researchers are looking for a way to prevent the spore form from becoming active.

Foulbrood Diseases: AFB vs EFB

A little word of caution here because I do not want to cause any beekeepers to freak out.  AFB is a serious contagious honey bee disease.  However, many things can cause dead brood in your hive.

One is another brood disease called European Foulbrood (EFB). This is another bacterial disease that affects brood but it does not form spores. EFB can be treated and often clears up on its own when foraging conditions improve.

American Foulbrood Prevention

In this world of raising bees that we can not control, completely preventing anything is likely a dream. However, there are things you can do to limit the chance of infection in your colonies.

  • Avoid bringing comb from outside sources into your apiary
  • Never share tools – hive tools, gloves etc between apiaries
  • Reduce drifting and robbing opportunities in the bee yard
  • Inspect hives routinely – test suspected cases
  • Do not feed bees honey from other sources

Until a better treatment or preventive option is available, be aware of the danger of buying used beekeeping equipment – especially brood frames and honeycomb. 

Anytime you bring comb into your apiary, even when buying bee nucs, you do have an element of risk.

In some regions, beekeepers have access to sterilization equipment that will allow continued use of beekeeping equipment that may have been infected. Contact your local beekeeping association of state inspector to see if this is an option.

AFB is a bigger problem in some areas of the country than others. It is also a bigger problem in some years than others. But it can occur anywhere that honey bees live. 


Is American Foulbrood dangerous to humans?

AFB is not harmful to humans because it is not zoonotic. This means it has not made the jump from animal to human.

How common is American Foulbrood?

American Foulbrood is one of the most common and widespread bee diseases worldwide.

What is AFB (American Foulbrood)?

This is a disease of honey bees caused by a highly contagious bacteria. One of the symptoms is dead brood (young bees) that have a very bad odor.

How does American Foulbrood affect colonies?

The disease weakens the colony because young die before reaching adulthood. Over time, this lowers colony population drastically until the colony collapses.

How can I identify American Foulbrood?

The definitive way to detect AFB in a hive is by using a lab test. Suspected colonies may be screened for some of the visual symptoms: sunken perforated cappings on brood cells, dead brood twisted in the cell that are very ropy, very bad odor.

A Final Word

Given the way bees are moved around the country for pollination efforts and the sale of package bees – every beekeeper must be concerned. This risk of AFB is the very reason that many states have strict laws regarding beehive registration and inspections by apicultural authorities. 

A bee yard with American Foulbrood is a risk not only to itself but also to any beehives within flying distance. Learning how to recognize and diagnose problems is a great step in the right direction.

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