American Foulbrood: the Bad Brood is Back!

Avoiding the Potential Perils of Used Beekeeping Equipment

American Foulbrood: the Bad Brood is Back!

Reading Time: 5 minutes

“American Foulbrood is a bacterial apiary disease that spreads between hives.”

Attendees of the Nevada State Beekeepers Conference filtered back to their seats after lunch, still laughing at jokes and conversing with new friends about their apiary projects. Dr. Meghan Milbrath of Michigan State University stood at the podium, the microphone boosting her voice over the chatter.

“And it has the potential to wipe out the entire industry.”

The room fell silent.

Now with the room’s full attention, Dr. Milbrath described a disease that plagued beekeepers in the early 20th century but had mostly been eradicated. It was back.

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It can be spread hive-to-hive by other bees through robbing and swarming but has no alternative hosts such as wild bees. Spores aren’t designed to be carried by the wind so, though it’s possible, it isn’t known to occur. Most transmission occurs because of poor hygiene among beekeepers. Sharing supers, feeding honey frames from other hives, etc. While the risk of spreading the disease on clothing is a lot lower, Dr. Milbrath says it’s theoretically possible. Leather gloves are almost impossible to sanitize.

Dr. Milbrath described a common scenario where people discover their grandfather’s old hives within a barn and decide to take up beekeeping, though grandpa isn’t there to tell them that he stopped keeping bees because American Foulbrood had killed them all off. Ignorant of spores’ potential to last at least decades within the wood grain, the prospective beekeeper sets up his hives.

When a disease hasn’t been a problem for a long time, people forget how to handle and prevent it.

“American Foul Brood Comb” by Shawn Caza is licensed under CC BY-NC-SA 2.0

Caused by bacterium Paenibacillus larvae, American Foulbrood (AFB) is unrelated to European Foulbrood (Melissococcus plutonius) and much more devastating. While European Foulbrood has been found to be related to stress, these rules do not apply to AFB so all hives are “fair game.” AFB spores persist for decades within equipment, wax, comb, and pollen. Though they have been proven to last at least 80 years, studies have only existed since about 1920, so there is no known extent for how long they can actually survive.

American Foulbrood symptoms include a spotted brood pattern, meaning living cells alternate with empty or dark/dead cells. Cappings sink because larvae die after cells are capped; those cappings may also have holes in them. Larvae, normally translucent white, turn a warm caramel color  — a symptom exclusive to American Foulbrood, with no other cause. Empty cells may contain the pupal tongue, another symptom only found with AFB, because this body part is hardy and disintegrates later. A characteristic smell accompanies AFB, though not all people can detect or recognize it. Black larval scales stick in frames.

When a disease hasn’t been a problem for a long time, people forget how to handle and prevent it.

Though American Foulbrood poses no risk to humans, as few as 10 spores can infect larvae 0-10 days old. Nurse bees provide spore-infected food to larvae, where the pathogen desporulates and reproduces mid-gut. This produces antimicrobial peptides which kill off good bacteria, then it produces toxins which breach the larval epithelium and kills within 12 days. Bacteria then overtake the larva, turning it into a stinky “goo,” hence the “foulbrood” name. Once food (the dead larva) runs out, bacteria then turn back into spores and the larval sludge becomes a black scale-like deposit which can contain millions of spores.

For prevention and detection, keep a beehive inspection checklist that includes “foul odor ” as an indicator of AFB

If you suspect American Foulbrood, field testing such as the matchstick test and the Holst milk test can help diagnose. The matchstick test involves inserting a toothpick or coffee stirrer into cells and slowly drawing them out to look for sludge. Because the same enzymes that break down larvae also break down milk proteins, beekeepers conduct the Holst test by diluting skim milk 1:4 with water then adding sludge/deposits. If it is American Foulbrood, water loses its cloudiness and looks like iced tea. Dr. Milbrath warns that old, used beekeeping equipment does not have active enzymes, so the milk test will not work, but spores may still exist. Another commercially available test called “ELISA” resembles a pregnancy test and is very accurate; any indication of a line confirms the presence of AFB. Samples can be sent to the USDA lab in Beltsville, Maryland, where a free test can confirm field results and inform you of possible resistance to antibiotics. Sending samples also helps the USDA keep track of the disease.

No matter which treatment method you choose, frames always need to be burned and buried.

Some states require beekeepers to then destroy infected hives by burning and burying them. If the state allows leeway, beekeepers must then decide whether to treat or destroy. This becomes complicated because antibiotics only destroy live bacteria but have no effect on spores. Terramycin (oxytetracycline) leaves the hive sooner; though antibiotic resistance isn’t likely, it has been seen. Tylan (tylosin) stays longer in the hive, but so far researchers have not seen resistance to it. Also, because of the Veterinary Feed Initiative, acquisition of these antibiotics involves a working relationship with a veterinarian, which may be difficult to obtain on short notice. Dr. Milbrath suggests creating that relationship when you start keeping bees. Factor it into the cost of beekeeping. Veterinarians may be unwilling to prescribe the medication since their training involves little to nothing about bees. Antibiotics can exist in hives and honey for a long time and they also destroy crucial gut bacteria in bees.

The “shook swarm” method of treatment involves shaking bees into new, clean hives with all-new frames, administering antibiotics and feeding the bees, then burning old hives.

Treat all colonies in the yard with antibiotics, regardless of symptoms, and operate the yard like a quarantine area. Do not move equipment until antibiotics are done and no signs of the disease remain. And ask yourself: what is the potential of 10 remaining spores to be fed to any new larvae?

Uncapped brood within a healthy hive.

Treating infected bee boxes involves scorching them then dipping in hot wax (at least 160C/320F) for at least 10 minutes. But, with a goal of stopping the active infection and preventing reinfection from spores, many state and provincial inspectors may require that you burn everything involved with a contaminated hive. Dig a hole, burn everything within the hole, and bury the ashes. Digging the hole prevents infected honey and wax from melting and pouring all over the ground.

No matter which treatment method you choose, frames always need to be burned and buried.

While American Foulbrood has not reached proportions it did in the early 20th century, and though some states have fewer instances than others, knowledge and proper care of used beekeeping equipment are key factors in ensuring it doesn’t spread and take down a crucial branch of agriculture and pollination.


Apiary Inspectors of America ( provides a list of state and provincial inspectors

Honey Bee Veterinary Consortium: ( “is made up of students and professionals from all segments of veterinary medicine and animal science who care about bees and beekeeping.”

Northern Bee Network ( is an organization designed to support beekeepers in the Northern States by promoting collaboration between beekeepers and providing resources for more sustainable beekeeping.

Dr. Meghan Milbrath provides valuable information on her website:

How to send AFB samples to the Bee Research Laboratory in Beltsville, Maryland:

Photos: “fb2” and “American Foul Brood Comb” by Shawn Caza is licensed under CC BY-NC-SA 2.0 

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