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 for the spread of American foulbrood. Such apiaries or honeycomb 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 that differ in terms of their virulence, which also influences the symptoms and detection by beekeepers or bee experts. Two of the five known ERIC genotypes have been detected in Austria in research projects to date.

In the case of the ERIC I genotype, a significant proportion of the diseased larvae reach the capping stage and only die afterwards, resulting in the mass formation of spores. Typical signs are capped cells with stringy masses and stagnant cells. The course of the disease in the colony is usually rapid.

If the ERIC II genotype is present, a large proportion of diseased larvae die before capping and the cells with dead brood are cleared out by the bees. This leads to a patchy brood nest. As this is an unspecific symptom that can also be caused by other brood diseases or damage, there is a risk that the disease will not be recognised for some time. Only a small proportion of the diseased larvae survive until capping and, to a lesser extent, capped brood with the typical symptoms is formed.

Therapy

An outbreak of the disease should be combated quickly in order to prevent the possible spread of the pathogen by bees flying away or by predation.

The method of choice for sanitising an outbreak of American foulbrood is generally the swarm sweeping method in combination with extensive cleaning and disinfection measures. Every apiary with diseased colonies must be completely subjected to the sweep swarm method. Sanitisation should be carried out as soon as possible after the disease has been diagnosed. Possible period of sanitation, which should be adapted to the respective climatic and weather conditions: March to September (time of the first expansion until the end of feeding). Ideally, all hives with diseased colonies within a restricted area (restricted area with a 3-km radius) should be sanitised within a short period of time.

• Sweep off the colony
• Plan a starvation phase with or without cellar confinement
• Place artificial swarm on centre walls
• Sufficient liquid feeding (also stimulates the cleaning instinct)
• Melt down the combs
• Scrape off wax and propolis residues from the inside of the hive or clean with a steam jet
• Wash the equipment (hive chisels, feeding utensils) and hives with 3 % hot caustic soda solution; rinse with clear water - beware, caustic soda is corrosive; protective clothing (goggles, gloves) is therefore essential
• Flame hive chisels and hives with a gas burner
• Clean and, if necessary, flame the comb cabinet and apiary; clean non-flammable parts with lye

The sweeping swarm procedure can generally be carried out on all colonies. If the individual colonies are too small, collective sweep swarms can also be formed using the better queen. It makes sense to kill off diseased colonies if they are already very weak or if the time of year (overwintering period between October and March) does not permit the formation of sweep swarms. All brood combs should be burnt, all other combs (including all storage combs) should be cut out and sent for wax processing with sufficient heat treatment of the wax. If American foulbrood is diagnosed during a mass harvest, the approval of the district administrative authority should be obtained for a step-by-step procedure: Immediately kill or sweep away the clinically diseased colonies of a stand (place these sweep swarms on the stand). After the end of the honey harvest, sweep off all the colonies in the apiary (possibly cage the queen for three weeks so that the colonies are brood-free after the honey harvest).

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