American Foulbrood (AFB) is one of the most destructive and dangerous bacterial diseases for the apiculture industry on a global scale. This disease directly targets the brood-rearing activities of honey bee colonies, meaning the developing larvae. Its agent, a bacterium named Paenibacillus larvae, causes the larvae to rot and die within their honeycomb cells. The most feared aspect of the disease is the extraordinary resistance of the spores produced by the bacterium to environmental conditions. These spores can remain viable and infectious in hive equipment and soil for decades. Its high contagiousness and the lack of an effective treatment often require the complete destruction of infected colonies, leading to serious economic losses.
American Foulbrood and its Agent: Paenibacillus larvae
The sole culprit behind the disease is Paenibacillus larvae, a Gram-positive bacterium capable of forming endospores. This microorganism spends its life cycle in the digestive system of honey bee larvae. The bacterium’s real strength lies not in its active (vegetative) form, but in its endospores, which can survive for decades in harsh conditions. These spores are the primary reason for the disease’s persistence and the difficulties in its control.
Life Cycle and Sporulation Dynamics
The infection cycle begins when very young larvae (usually less than 48 hours old) consume food (royal jelly or a honey/pollen mixture) contaminated with spores. The gut environment of young larvae is conducive for the spores to germinate (transition to the vegetative state). The spores germinate in the gut, the bacteria begin to multiply rapidly, and they invade the larval tissue. The bacteria consume the larvae’s food and secrete lethal toxins. The infected larva typically dies just after the cell is sealed by nurse bees. After the larva dies and its food source is depleted, the millions of vegetative bacteria enter the sporulation phase again. Each deceased larval remnant can contain approximately 2.5 billion new spores. These spores are then transported to other parts of the hive and other larvae by worker bees attempting to clean the cell.
P. larvae and Spore Resistance
Paenibacillus larvae spores are among the most durable biological structures known. This characteristic is fundamental to why the disease is so persistent in apiaries and why its eradication is so difficult. The spores show incredible resistance to high temperatures (even brief boiling), low temperatures, disinfectants, ultraviolet light, and extreme drought. It has been scientifically documented that they can remain viable and infectious in contaminated old combs, hive materials, or forgotten honey for 30 to 40 years, and according to some sources, even longer.
Strain Differences and Virulence
There are multiple genetically different strains (subtypes) of the P. larvae bacterium. These strains are generally classified using molecular methods such as ERIC (Enterobacterial Repetitive Intergenic Consensus) types (e.g., ERIC I, ERIC II). The most important difference between these strains is their virulence, meaning the severity and speed with which they cause disease. For example, some strains (like ERIC I) may be more aggressive, capable of collapsing a colony much faster, and may show more typical clinical symptoms. Other strains may progress more slowly or cause atypical symptoms, making diagnosis difficult. This genetic diversity is a significant factor in explaining why the disease behaves differently in various geographic regions or apiaries.
Disease Spread and Transmission Routes
The spread of American Foulbrood occurs via its highly infectious spores. This process is rapidly accelerated both by the natural behaviors of bees (robbing, drifting) and by the beekeeper’s well-intentioned but flawed practices (equipment transfer). The slightest carelessness can lead to the infection of the entire apiary.
Equipment and Foundation-Borne Transmission
The beekeeper themself is the number one vector for the disease between hives and apiaries. A hive tool, bee brush, smoker, gloves, or frame grips used in an infected hive can easily transfer millions of spores to healthy hives. In particular, transferring frames (with brood or honey) between hives is one of the riskiest practices. The same risk applies to contaminated but uncleaned old hives, supers, and even insufficiently sterilized foundation. Buying second-hand beekeeping equipment can mean importing spores from an apiary of unknown origin.
Spread via Robbing and Swarming
A colony weakened or abandoned due to the disease becomes a robbing target for healthy, strong colonies in the vicinity. Robber bees enter these weak hives, steal honey heavily contaminated with spores, and carry it back to their own hives. When they use this honey to feed their own brood, the infection cycle begins in the new hive. This is the most common and rapid natural way for the disease to spread among hives in an apiary. Similarly, natural swarms emerging from an infected colony can carry the disease kilometers away to a new area.
Symptoms and Diagnosis
Detecting American Foulbrood depends on the correct interpretation of symptoms. The beekeeper must know very well what a healthy brood comb should look like. The disease presents different symptoms in its early and advanced stages. While field tests can create strong suspicion, a definitive diagnosis always requires laboratory confirmation.
Early Clinical Symptoms
The first sign of the disease is usually an irregularity in the brood area. In a healthy comb, the brood area has a dense, uninterrupted pattern with few or no empty cells. When AFB begins, numerous empty, skipped, or re-laid cells appear among the sealed (capped) cells. This is known as a “spotty brood” or “perforated brood” appearance. Infected larvae in uncapped cells (open brood) also change color. Unlike the bright, pearly white color of healthy larvae, infected larvae first become dull, then turn yellowish, and finally a brown hue.
Advanced Findings and the “Rope Test”
As the disease progresses, the larva inside the sealed cell dies completely and begins to rot. The larva’s internal organs and tissues liquefy, turning into a sticky, slimy, brown mass. At this stage, the most classic field test for the disease, the “rope test” (or matchstick test), can be applied. A matchstick or a small twig is inserted into a suspicious-looking sealed cell, the sticky mass inside is stirred, and it is slowly withdrawn. If the larval remnant stretches out as a continuous, slimy thread 2 to 5 centimeters (sometimes more), this is a very strong indicator of American Foulbrood. This sticky fluid may also emit a characteristic, sharp “glue pot” odor as it decays.
Visual Inspection: Sunken Cappings and Perforations
As the larva liquefies and rots inside the cell, its mass decreases, and it slowly begins to dry. This causes the wax capping over the cell to lose its tension and sink inward (taking on a concave appearance). The color of the capping also becomes darker than normal, with a “greasy” or wet look. Often, worker bees responsible for hive hygiene notice the problem and try to perforate the capping to clean out the infected cell. Therefore, small, randomly opened holes on the comb and sunken, darkened cappings are typical visual signs of the disease. Over time, the completely dried larval remnant adheres tightly to the bottom wall of the cell, becoming a black, brittle “scale.” It is impossible for the bees to clean this scale.
Laboratory Diagnosis: Culture and PCR
Although visual symptoms and a positive rope test create strong suspicion for the beekeeper, a definitive laboratory diagnosis is essential for an official action plan (like destruction or quarantine). The two most common methods are culture and PCR. The culture method involves inoculating special media with samples taken from the suspicious comb to grow the P. larvae bacterium. This is considered the “gold standard” as it proves the bacterium is alive and active. PCR (Polymerase Chain Reaction) is a modern molecular method that provides much faster results. PCR detects the DNA of the bacterial spores. Its greatest advantage is its ability to detect the presence of spores (subclinical infection) in honey or comb even before disease symptoms (clinical signs) appear.
Sample Collection and Packaging Standards
Correctly taking the sample to be sent to the laboratory is critical for the reliability of the test result. Generally, it is requested that a piece of brood comb measuring approximately 10×10 cm and showing suspicious symptoms (sunken cappings, holes, dead larvae) be cut. This sample must absolutely not be placed in a plastic bag or an airtight plastic container. The resulting moisture can cause saprophytic molds and fungi to grow rapidly, making the detection of P. larvae difficult or impossible. The most correct method is to wrap the sample in a paper bag or paper sack and transport it to the lab in a cardboard box to prevent it from being crushed.
Which Test to Request and When
If an active outbreak is suspected (rope test is positive, odor is present, spotty brood is visible), a “culture” test should be requested to confirm the suspicion. This definitively proves the presence of the disease. However, to screen the general health status of an apiary, to check second-hand equipment before buying, or to determine if spores are present in honey from a suspicious region (i.e., when no clinical signs are present), the “PCR” test is preferred. PCR is much more sensitive and is ideal for detecting the mere presence of spores.
Treatment and Control Methods
Because the spores of American Foulbrood cannot be destroyed, there is no proven, permanent “treatment” for this disease. Some chemicals (which may be legal in some countries) only temporarily suppress the vegetative form of the bacterium, but the disease inevitably recurs because the spores remain. Therefore, control strategies focus on stopping the spread of the disease and completely eliminating the source of the spores. In many countries, AFB is a notifiable disease.
Quarantine and Hive Isolation
The moment AFB is definitively diagnosed in an apiary, that apiary must immediately be placed under official quarantine. The confirmed infected hives and all hives that may have had direct contact with them must be clear-ly marked. The flight entrances of infected hives should be closed or securely reduced, especially in the evening (to completely prevent robbing). Under no circumstances should any bees, queen bees, comb, honey, pollen, or any beekeeping equipment be removed from the quarantined apiary.
Equipment Disinfection Protocols
The beekeeper must pay extreme attention to their own hygiene to prevent the spread of the disease. After touching a suspicious hive, all tools must be meticulously disinfected before moving to a healthy hive. Metal tools like hive tools should either be scorched (heated with a flame) or soaked in a recommended disinfectant solution (e.g., chlorine). The use of porous materials that are difficult to clean, such as shared brushes, must be stopped immediately. Gloves should be changed frequently or disinfected.
Destruction of Bees: Safe Burning Steps
The destruction of infected colonies and contaminated materials is the safest and most definitive way to eliminate the spore source. This procedure should be carried out in accordance with legal procedures and usually at dusk (when all forager bees have returned to the hive). For a safe burning process, a pit at least 45-50 cm deep should be dug in a windless area, away from hives and flammable materials. A strong fire is built in the pit. First, a substance like sulfur can be applied to the hive entrance to kill the bees, or the bees are thrown into the fire. Then, all infected materials (frames, combs, wax, and all dead bees) are thrown into this fire and burned until they are completely ash. After the process is complete, the ashes in the pit must be immediately covered with soil.
Disinfectant Agents
The extraordinary resistance of P. larvae spores renders ordinary cleaning agents (detergent, alcohol) completely ineffective. Therefore, disinfection requires specific methods capable of killing the spores. Only certain
strong chemicals or physical applications (like high heat) can destroy these durable structures. Disinfection is a critical control step for non-porous equipment that is salvaged (not burned).
Disinfection with Chlorine-Based Solutions
Sodium hypochlorite (the active ingredient in household bleach) has a proven effect on spores. Metal tools or durable plastic materials can be disinfected by soaking them in a solution containing 1% active chlorine (e.g., 1 part bleach to 9 parts water) for at least 20 minutes. Wooden materials like hive bodies can be scrubbed with this solution, but the porous nature of wood does not guarantee the chemical will reach every spore. After this process, all equipment must be rinsed with plenty of water and allowed to air dry completely before coming into contact with bees.
Heat Treatment and Surface Sterilization with a Blowtorch
Heat is the most reliable physical method for killing spores. The most common application for wooden hive bodies and lids is surface sterilization using a blowtorch (propane torch). The goal is to scorch all internal surfaces of the hive (including corners) with the flame. The process should continue until the wood surface is lightly charred, meaning it turns a dark brown color. This intense heat kills the spores on the surface and in the near-surface pores. Other industrial methods include sterilizing equipment with pressurized steam (autoclave) or radiation (gamma irradiation).
Prevention Methods
The most effective, sustainable, and economical way to combat American Foulbrood is to prevent the disease from ever entering the apiary. Protective beekeeping (prophylaxis) is fundamentally based on good hygiene practices, careful material management, and “barrier management” principles. Once the disease enters an apiary, its control is both very costly and very destructive.
Hive Hygiene and Equipment Management
Strong and healthy colonies are always more successful at overcoming diseases. Beekeepers should regularly remove old and darkened brood combs from the system to reduce the pathogen load within the hive. As a general rule, it is recommended to replace at least 20% to 30% of the combs in the brood area annually. Swapping combs between hives (even if they appear healthy) must be strictly avoided. Each hive, or at least each apiary section, should have its own dedicated equipment (hive tool, brush) that is never mixed.
Foundation Sterilization and Material Control
Contaminated beeswax and the foundation made from it pose a serious risk in spreading the disease. If beekeepers are rendering their own wax to make new foundation, they must ensure this wax is free of spores. Foundation should only be purchased from reliable, certified sources. Industrial-level wax sterilization is typically done using methods like high-pressure steam or gamma irradiation, which neutralize P. larvae spores without degrading the wax structure.
Economic Impacts
An American Foulbrood outbreak is not just a biological disaster; it is also a serious economic threat to the beekeeping sector. The losses caused by the disease are not limited to the value of the destroyed colonies; the indirect effects of control measures and trade restrictions cause much broader economic damage.
Direct Cost of Colony Loss
Each colony destroyed due to the disease represents a direct capital loss for the beekeeper. This loss includes not only the value of the bees and hive materials (boxes, frames) but also the lost income from valuable products that colony would have produced that season and perhaps the next (honey, pollen, royal jelly, or propolis). Especially in large-scale commercial beekeeping, the destruction of tens or hundreds of colonies can completely eliminate the sustainability and profitability of the operation.
Impact of Quarantine and Trade Restrictions on the Sector
When an AFB outbreak is detected in a region, official authorities usually declare a quarantine over a wide area. This situation prevents migratory beekeepers from moving their hives to honey flow regions or pollination areas. This means a direct loss of income. Furthermore, the sale of queen bees, package bees, nucs, or colonies from the quarantine zone is completely prohibited. These trade restrictions negatively affect not only the infected apiaries but also the clean apiaries in the same area that are caught in the quarantine, leading to a sector-wide stagnation and loss of markets.
