American Foulbrood: Signs, Rope Test, Treatment & Vaccine

American Foulbrood: Signs, the Ropiness Test, and What to Do (Including the New Vaccine)

Most things that go wrong in a hive can be fixed with a stronger colony, a better queen, or a mite treatment. American foulbrood (AFB) is the exception. It is caused by a spore-forming bacterium, Paenibacillus larvae, whose spores stay viable in wax and wood for decades, and in most US states a confirmed case means the colony and its frames are burned. Every beekeeper needs to be able to recognize it, and to know what it is not.

The short version

Sunken, dark, greasy, perforated cappings; a foul (not sour) smell; dead larvae that rope out ½–1 inch on a matchstick; hard black scales glued to the cell floor. If you see that, stop, close the hive, and call your state apiary inspector. AFB is reportable and antibiotics only mask it. Don't confuse it with varroa damage: spotty brood with chewed cappings and no rope is far more often mites, which you can fix with a mite wash and an oxalic acid vaporizer.

What AFB Looks Like

Sign	What you'll see
Cappings	Sunken, darkened, greasy-looking, with pinholes ("pepper-box" pattern). Brood pattern spotty.
Dead larvae	Die after capping. Turn coffee-brown, then melt into a gooey mass on the cell floor.
Ropiness test	Stir a dead larva with a matchstick and pull out slowly. AFB material stretches ½ inch to an inch (2.5 cm) then snaps back. Ropy stage lasts about three weeks after death.
Scale	After about a month the larva dries to a hard, black, brittle scale glued to the lower cell wall. It cannot be removed without tearing the cell. (EFB scale is rubbery and lifts out.)
Pupal tongue	Sometimes a dead pupa leaves a thread stretched from the cell floor to the roof.
Smell	Distinctly foul, often compared to a chicken house or glue pot. Sour smell points to EFB instead.

A positive rope test is a presumptive positive. Confirm with a Vita lateral-flow AFB test kit or a sample to your state lab.

Why It's Different From Every Other Disease

Each dead larva produces roughly a billion spores. A few dozen spores are enough to infect a healthy larva. And the spores last: seventy years or more in old equipment, indefinitely on wooden ware, decades in wax and honey. There is no such thing as "letting it clear up." A colony can collapse within three weeks of the first visible cells, and robbing bees carry the spores home to every hive in range.

That is why AFB is on the USDA APHIS National List of Reportable Animal Diseases and why many states require burning. It is also why antibiotics are the wrong tool. Oxytetracycline, tylosin and lincomycin suppress the active bacteria but do nothing to the spores; the disease returns when the drug stops. Since January 2017, those antibiotics also require a veterinary prescription or Veterinary Feed Directive from a licensed vet; they are no longer over the counter.

What To Do

Stop. Don't move frames, don't swap boxes, don't feed the honey to other colonies.
Confirm. Rope test, then a test kit or your state apiary inspector. Most states inspect for free.
Follow your state's rule. In most cases: kill the colony in the evening, burn bees, frames and comb in a pit about 18 inches deep, scorch or burn the boxes, bury the ash. Never keep a "hospital yard."
Check the rest of the yard. Inspect every colony, and again in a month. Reduce entrances to limit robbing.
Never buy used comb you can't vouch for. That is how most hobbyists get it.

The AFB Vaccine

In January 2023 the USDA issued a conditional license to Dalan Animal Health for the first honey bee vaccine: killed P. larvae cells mixed into queen candy. Workers pass it to the queen in royal jelly, and fragments end up in her ovaries so the larvae she lays are primed against the bacterium. It was rolled out first to commercial operations, and the company completed the efficacy trials for full licensure in 2025. It is a prevention tool, not a treatment, and it does not change the reporting or burn rules if you find active AFB.

Don't Mistake Mites for Foulbrood

The most common "AFB scare" we hear about is actually parasitic mite syndrome: spotty brood, sunken and chewed-open cappings, larvae dead in cells. The differences: PMS larvae don't rope, there's no foul smell, you'll often see bees with deformed wings, and a mite wash comes back high. That one you can fix with oxalic acid vaporization on the right schedule. Always wash before you panic.

Tools We Use (and Recommend)

InstantVap Oxalic Acid Vaporizer: the cordless, battery-powered oxalic acid vaporizer we use in our own yards to keep mite loads low, which is the single best defense against every problem on this page. See the Lite, Compact, Original and Turbo. Lorob Bees is the primary US seller, with stateside customer service and a warranty we stand behind.
Mite Monitoring: the Easy-Check mite wash. How to do a wash in five minutes.
Oxalic Acid: EPA-registered EZ-OX.

Happy Beekeeping from the Lorob Bees Team

Written by Rob Pollock

Rob keeps bees in Baltimore, Maryland, and owns Lorob Bees. He built and repaired his own corded oxalic acid vaporizers for years before becoming the primary US seller of the InstantVap, and he still picks up the phone when one needs service. Oxalic acid vaporization is the thing he cares most about in beekeeping, because it's the treatment that keeps hives alive without breeding resistant mites. More about Lorob Bees.

Keep reading: Explore all Lorob Bees beekeeping guides — varroa, oxalic acid vaporization, pests and diseases, all in one place.

Sources

Honey Bee Health Coalition, Identifying and Mitigating Foulbrood in Honey Bee Colonies, 2019
Penn State Extension, Honey Bee Diseases: American Foulbrood
USDA APHIS, American Foulbrood of Honey Bees Case Definition, 2023
AVMA, Honeybee vaccine receives conditional license from USDA, 2023
FDA, Using Medically Important Antimicrobials in Bees: Q&A, 2024

General beekeeping information, not a substitute for the product label. Always read and follow the label of any product you use in a hive.