American foulbrood (AFB) is the most destructive bacterial disease of honeybee colonies. Caused by the spore-forming bacterium Paenibacillus larvae, it infects and kills bee larvae inside their cells, then reduces them to a dark, gluey mass. Unlike many bee diseases, AFB is extremely difficult to eliminate because its spores can survive for decades in equipment, wax, and honey, making reinfection a persistent threat.
How the Infection Works
The cycle begins when adult bees unknowingly feed contaminated food to young larvae. Spores enter the larval gut, germinate, and the resulting bacteria multiply rapidly inside the midgut. Eventually the bacteria breach the gut wall, killing the larva. The dead larva then decomposes into a brownish, ropy mass, which produces billions of new spores. Nurse bees that attempt to clean out the infected cell spread those spores to other larvae, and the cycle accelerates.
Only larvae are vulnerable to infection. Adult bees carry and distribute spores without becoming sick themselves, which is part of what makes AFB so insidious. A colony can harbor enormous numbers of spores with relatively few visibly diseased larvae, passing routine inspections while silently spreading the pathogen.
Recognizing AFB in the Hive
AFB affects sealed brood, so the visible signs appear on capped cells. The most telling indicators include:
- Uneven “pepper pot” brood pattern: healthy and empty cells scattered randomly, rather than the solid pattern of a thriving queen.
- Sunken, greasy, or darkened cappings: cell caps that look wet or discolored instead of dry and slightly convex.
- Perforated cappings: small holes chewed by adult bees that detected something wrong underneath.
- The ropiness test: if you push a matchstick or toothpick into a suspect cell and slowly pull it out, the larval remains of an AFB-infected cell will stretch into a brown, stringy rope, often 2 to 3 centimeters long. This is the classic field test.
- Hard, dark scales: once the dead larva dries out completely, it forms a black scale glued tightly to the lower cell wall. These scales are very difficult to remove and are packed with spores.
A foul smell sometimes accompanies advanced infections, though this isn’t always present. If you suspect AFB, lateral flow test kits designed for field use can confirm the diagnosis in minutes. These kits report about 92% sensitivity and 99% specificity, meaning they catch most true infections and rarely give false positives. A negative result on a kit with strong visual signs, however, warrants a follow-up lab test.
How AFB Spreads Between Colonies
Spores move between colonies through several routes. Robbing is one of the most common: when a colony weakened by AFB collapses or becomes easy to raid, nearby bees steal the contaminated honey and carry spores home. Drifting bees that enter the wrong hive can deliver spores the same way. Swarms from infected colonies bring the pathogen to new locations.
Beekeepers themselves are a major vector. Moving frames, supers, or tools between hives without proper sanitation spreads spores efficiently. Buying used equipment or packaged bees from infected operations is another well-documented pathway. A USDA study analyzing nearly 4,800 brood samples submitted between 2015 and 2022 found that about 8.6% tested positive for AFB across the United States, confirming it remains a widespread concern even in well-managed operations.
Why Burning Is the Standard Response
In most jurisdictions, AFB is a notifiable disease. Beekeepers are legally required to report confirmed cases to their state or regional apiary inspector, often within 24 hours. The standard response in many countries, and the approach recommended by most inspectors, is to destroy the infected colony and its equipment by burning.
The burning procedure is straightforward but deliberate. You dig a pit at least 30 centimeters deep, place all diseased frames, bees, and hive components into the pit, burn everything completely, and then bury the ash. The pit prevents molten wax and honey from flowing out where other bees could access it. This sounds extreme, but the reason is simple: P. larvae spores are extraordinarily resilient. They resist heat, desiccation, and most common disinfectants. Scraping or cleaning contaminated woodenware does not reliably eliminate them.
The one alternative to burning equipment is gamma irradiation at a dose of 10 kilograys, which has been shown to inactivate AFB spores effectively. Some beekeeping regions offer irradiation services, allowing beekeepers to salvage expensive hive boxes and frames. This isn’t available everywhere, though, and the cost may not be practical for small operations.
The Role of Antibiotics
Antibiotics can suppress AFB symptoms but cannot cure the disease. They kill the actively growing bacteria but have no effect on spores, which remain viable in the comb indefinitely. When antibiotic treatment stops, the spores germinate and the infection returns.
In the United States, one approved treatment involves dusting 200 milligrams of tylosin tartrate mixed with powdered sugar over the top bars of the brood chamber once per week for three consecutive weeks. This controls visible disease and can buy time, but it masks the problem rather than solving it. A colony that appears healthy while on antibiotics can still be loaded with spores, spreading infection to neighboring hives. For this reason, many countries, including Australia and New Zealand, prohibit antibiotic use for AFB entirely and mandate destruction of infected colonies instead.
Even where antibiotics are legal, experienced beekeepers and inspectors generally view them as a stopgap. Relying on repeated treatments creates a cycle of suppression and re-emergence, increases the risk of antibiotic-resistant strains, and raises concerns about residues in honey.
Preventing AFB in Your Apiary
Prevention centers on breaking the spore transmission cycle. Regular brood inspections, at least twice during the active season, are the first line of defense. Catching AFB early, before a colony collapses and gets robbed out, limits how far spores spread. Learn what healthy brood looks like so that irregular patterns, discolored cappings, or off-smelling cells immediately stand out.
Keep your equipment clean and avoid sharing frames or tools between hives without sterilization. Be cautious about purchasing used equipment, especially drawn comb, from unknown sources. If you buy bees, ask about the seller’s disease history and inspection records. Reducing the number of colonies in a single apiary location can also lower transmission pressure, since drifting and robbing happen more frequently when hives are closely packed.
Some beekeepers keep sentinel hives, essentially bait hives near known problem areas, to catch swarms that might carry AFB before they establish feral colonies that become ongoing reservoirs. While this takes extra effort, it can protect an entire area over time by reducing the environmental spore load.