American foulbrood is an infectious, serious disease of bee brood caused by the bacterium Paenibacillus larvae, which requires extensive control and sanitation measures if it occurs. In its reproductive form, P. larvae is an all-round flagellated rod, while in its permanent form it is a very resistant spore that represents the infectious stage of the pathogen and can remain infectious for more than 40 years.

Worldwide

Symptomatic brood from diseased bee colonies contains a large number of spores. During cell cleaning, the cleaning bees and subsequently the surfaces in the colony, the stored food and the other bees are contaminated. Even commercial honey, especially from countries where American foulbrood is more widespread, can contain spores of P. larvae. As the pathogen can only infect bee larvae and is harmless to humans, spore-contaminated honey can therefore be consumed without risk.

Young bee larvae aged one to two days can be infected by ingesting spores in their food. The spores develop into rod-shaped bacteria in the midgut after just 24 hours. These multiply en masse one to four days after infection. They then succeed in overcoming the intestinal epithelium in some places by migrating between the intestinal epithelial cells or in destroyed epithelial areas and thus enter the abdominal cavity in large numbers. When this occurs, the larva dies, the larval body decomposes into a stringy mass and the vegetative bacteria transform into spores. To a lesser extent, sporulation already occurs in earlier phases of the disease.

Transmission in the colony

In the course of normal colony activity (cleaning, foraging, social foraging), foulbrood spores can be transmitted in the diseased or even subclinically infected colony. All parts of the colony and the hive can be contaminated with spores. Spores can be found in diseased brood, in brood trays, on combs, in stored food and honey, in pollen, on bees (hair coat), on hive surfaces, in bee droppings and in muck.

Transmission from colony to colony

Transmission occurs on the one hand by the bees themselves through robbing (honey robbing from weak colonies - also known as "silent" robbing) and the flight of bees and drones. On the other hand, transmission also occurs through beekeeping activities, such as swapping honeycombs between diseased and healthy colonies, forming colonies from diseased colonies, using contaminated equipment for different colonies and hives, "licking out" contaminated honeycombs and uncapping wax and feeding contaminated (e.g. external or foreign) honey or pollen.

It often takes weeks to months between infection and outbreak (= first visible signs of disease), depending on the condition of the bee colony, infection pressure and bacterial genotype.

American foulbrood leads to the death of the brood with both unspecific symptoms, such as a patchy brood nest or capped brood cells that have been left standing, and typical clinical symptoms:

• Brood cell with sunken, perforated, possibly moist, shiny cell cap
• The larva in the brood cell has transformed into a light to dark brown, stringy mass
• Firmly attached, dark scabs in the lower groove of the brood cell as dried brood residue
• Possibly characteristic odour of the slimy mass

The colonies are usually sanitised using the sweeping swarm method, accompanied by extensive cleaning and disinfection measures. In rare cases, killing the colonies and destroying the hive is indicated. No medication is authorised in Austria to combat American foulbrood.

Consistent comb renewal is recommended as a preventative measure. Ideally, separate tools should be used for each apiary to prevent the spread of pathogens.

• Regularly carry out a forage or muck inspection for early detection of possible spore contamination; if spores are detected, report this and take cleaning and disinfection measures to prevent the outbreak of disease
• Do not bring any foreign colonies, combs and equipment onto the stand without checking them
• Only buy colonies after a forage ring test with negative results and a brood check at the stand of origin
• Disinfect used hives and equipment before use
• Do not feed foreign honey or pollen
• Do not set up apiaries near sources of danger (neglected apiaries, foreign honey processing companies, landfill sites)
• Allow foreign swarms to starve in the swarm box until the first bees fall down so that the food supply in the honey bladder is used up
  

American foulbrood is widespread in Austria, with the number of reported outbreaks varying greatly from year to year. The genotypes ERIC I and ERIC II occur.

Unattended, neglected apiaries can be a potential source of the spread of American foulbrood. Such apiaries, or comb material stored in a way that is freely accessible to bees, are often only discovered during official inspections of the 3-km exclusion zone.

Genotypes

There are different strains or genotypes of P. larvae, which differ in terms of their virulence, a factor that also influences the symptoms and detection by beekeepers or bee experts. Research projects have so far identified two of the five known ERIC genotypes in Austria. 

In the presence of the ERIC I genotype, a significant proportion of the diseased larvae reach the capping stage and only die afterwards, leading to the mass formation of spores. Typical signs include capped cells containing stringy masses and cells that have not been capped. The course of the disease within the colony is usually rapid.

In the presence of the ERIC II genotype, a large proportion of diseased larvae die before capping, and the cells containing dead brood are cleared away by the bees. This leads to a patchy brood nest. As this is a non-specific symptom that can also be caused by other brood diseases or damage, there is a risk that the disease will go unrecognised for some time. Only a small proportion of the affected larvae survive until capping, and there is a lesser degree of formation of capped brood with the typical symptoms.

Treatment

An outbreak of the disease should be tackled promptly to prevent further spread of the pathogen through the flight of bees or by robbing. 

To eradicate an outbreak of American foulbrood, the swarm removal method combined with extensive cleaning and disinfection measures is generally the method of choice. Every apiary containing affected colonies must be subjected to the swarm removal method in its entirety (affected and unaffected colonies). Eradication should be carried out as soon as possible after the disease has been diagnosed. Possible timeframe for decontamination, to be adapted to the respective climatic and weather conditions: March to September (from the start of the first expansion until the end of feeding). Ideally, all apiaries with diseased colonies within a restricted zone (a 3-km radius exclusion zone) should be decontaminated within a short period of time.

• Removing the colony
• Plan for a starvation period with or without confinement
• Place artificial swarm on foundation sheets
• Provide sufficient liquid feed (also stimulates the cleaning instinct)
• Melting down the combs
• Clean the hives and equipment used for colony treatment by scraping off wax and propolis, then, if necessary, clean with lye (3–5% sodium hydroxide solution or 3% sodium carbonate solution) or a special cleaning agent for beekeeping, and rinse thoroughly with water
• Disinfect hives and equipment by flame-sterilising or using approved disinfectants. According to information from the ‘Team Biocides/Federal Environment Agency’, these are currently Halades 01 and Virkon S (this list is not exhaustive and is subject to change)
• Clean the honeycomb cabinet and apiary and, if necessary, flame-sterilise them; clean parts that cannot be flame-sterilised with lye

It is advisable to destroy diseased colonies if they are already weak or if the season (wintering period between October and March) does not allow for the formation of a swarm. All brood combs must be burnt; all other combs (including all honey combs) must be cut out and sent for wax processing with sufficient heat treatment of the wax. If American foulbrood is diagnosed during a mass nectar flow, approval should be sought from the district administrative authority for a phased approach: immediately destroy or remove the clinically diseased colonies from a apiary (place these swarms on the apiary site). After the foraging season has ended, remove all colonies from the apiary (the queen may be caged for three weeks to ensure the colonies are brood-free after the foraging season).

Any suspicion of American foulbrood must be reported to the competent district administrative authority (district authority, magistrate). This authority will instruct the official veterinarian (ATA) or an expert to check the colonies for clinical symptoms. If symptoms of the disease are present, it is recommended that a brood sample with suspected AFB brood or brood with unclear symptoms be sent to the official testing centre (National Reference Laboratory for Bee Health, AGES) for confirmation or clarification.

A two-stage diagnostic procedure is used for American foulbrood: examination of the brood combs for clinical symptoms and the microbiological culture procedure. Further diagnostic procedures (PCR, ERIC typing) are only carried out as part of research activities.

Sample submission

Piece of brood comb (approx. 15 x 15 cm) or whole comb with diseased brood