About European foulbrood

European foulbrood (EFB) is caused by a non-spore-forming bacterium, Melissococcus plutonius, and is a serious brood disease of honey bees. EFB infects the digestive track of honey bee larvae, with larvae younger than three days old being most susceptible. Once infected, most larvae die at four to five days old. EFB does not affect adult honey bees. EFB is often associated with poor nutrition or a lack of available forage for the colony. EFB is also thought to be problematic when colonies are under stress, such as during hive movement or poor weather conditions. EFB is highly contagious and the disease will contaminate beekeeping equipment, bees and honey, and will weaken, and in severe cases, kill a honey bee colony. While EFB is most commonly found in early spring, it may appear at any time of the year in honey bee colonies. Some antibiotics have been effective in the treatment of EFB. Beekeepers can take steps to mitigate an infection from establishing itself and manage an infection if present in their beekeeping operation.

EFB was prevalent several decades ago in Ontario, followed by a period when EFB was relatively rare and largely addressed through antibiotics. However, within the last decade EFB has re-emerged and is much more virulent in many regions of North America, including Ontario, and has resulted in the death of colonies in jurisdictions outside of Ontario. In general, pest and disease statuses change over time as they evolve and as such pests and diseases may become more or less common or more or less virulent. At present, EFB is considered a serious disease in Ontario that has increased in prevalence.

Transmission and life cycle

Bees on wax comb

The disease begins with nurse bees picking up EFB from infected colonies or from brood food that is contaminated with the bacterium Melissococcus plutonius. Contaminated food is subsequently fed to developing honey bee brood.

European foulbrood infected larva twisted in a brood cell.

The bacteria multiply in the gut of the honey bee larvae, eventually competing with the larvae for food. Infected larvae often die before capping of the brood cell (three to five days old).

Dead and darkened larvae with prominent, white tracheal network, curled at the end of the cell.

The dead larvae change from a healthy pearl white colour to yellow and then to brown. The larvae then become fluid-like before drying out to become a rubbery scale.

Bees on wax comb

Nurse bees remove the dead or dying larvae and contaminate their mouthparts with EFB. The bacteria is inadvertently fed to other developing brood in the colony. The cylce repeats with more honey bee brood being infected with EFB. Some infected larvae may survive to become adults, but will spread the bacteria in their feces resulting in further infection in the colony.

A colony of honey bees

As the colony weakens, it may be targeted by other honey bee colonies and robbed of its honey stores. Honey containing EFB is taken back to the healthy honey bee colony and the infection is spread to honey bee brood in the new colonies.

Signs and symptoms

It is important for beekeepers to familiarize themselves with healthy brood conditions and types of brood disease as symptoms of EFB are found in the brood nest and used brood comb of the honey bee colony. There are several symptoms that may indicate an EFB infection, but these will vary depending on the stage of infection.

Abnormal, discoloured larvae

Healthy larvae are always pearl white (Figure 1). Anytime a larva dies in the cell it becomes discoloured and darkens. Having some larvae death in a colony can be normal, therefore seeing discoloured larvae is not always specific to EFB. Brood infected with EFB will turn from white, to yellow, and eventually become brown in colour.

Healthy, pearl white and c-shaped larvae in brood cells.

Figure 1. Healthy, pearl white and c-shaped larvae in brood cells.

The midgut of larvae infected with EFB may be chalky white, as opposed to a healthy midgut, which would be yellow-orange. As well, the trachea (breathing tubes) can appear noticeably white as the EFB infected larvae darkens in the larval stage (Figure 2).

Dead and darkened larvae with prominent, white tracheal network, curled at the end of the cell.

Figure 2. Dead and darkened larvae with prominent, white tracheal network, curled at the end of the cell.

Infected larvae may also appear to be twisted or stretched in the cell (Figure 3), unlike the traditional “c” shape of a healthy larvae (Figure 1).

European foulbrood infected larva twisted in a brood cell.

Figure 3. EFB infected larva twisted in a brood cell.

Watery, rubbery or dehydrated scale that adheres to the wall of the wax cell

Depending on the stage of the infection, the physical consistency of EFB infected larvae may vary from melted to watery to rotten or become a rubbery or dehydrated scale that lightly adheres to the wall of the wax cell and can be easily removed from the frame (Figure 4).

Dead larvae that has become a rubbery scale adhered to the wall of a wax cell.

Figure 4. Dead larvae that has become a rubbery scale adhered to the wall of a wax cell.

Sour smell

Honey bee diseases and issues may be associated with an odour (for example, American foulbrood (AFB) can have a distinct fishy or rotten smell). In some cases of EFB, a sour odour may be noticed. This attribute may be somewhat subjective as there are some people who can readily detect and identify the smell where others cannot.

Spotty brood pattern

A healthy, productive colony will produce a solid brood pattern with few empty cells, whereas unhealthy or diseased colonies may have a brood pattern with many missed cells (Figure 5). This characteristic is not always specific to EFB since a spotty brood pattern is common to many colony issues including other brood diseases or a poor or weak queen.

A solid brood pattern on a frame of honey bee brood compared to a spotty brood pattern on a frame of honey bee brood. A white plastic stencil is placed on the wax comb.

Figure 5. A solid brood pattern on a frame of honey bee brood (left) versus a spotty brood pattern on a frame of honey bee brood (right). A white plastic stencil has been placed on the wax comb for demonstration purposes. Photo credit: Shelley Hoover, University of Lethbridge.

Sunken or perforated wax cappings

Wax cappings are normally slightly convex and light brown in colour. In the presence of an EFB infection, wax cappings will be slightly concave and could be perforated (Figure 6). Perforations of brood cell cappings that appear irregular and are located at the margin of the cappings are more likely caused by a brood disease. These symptoms can also be present due to an AFB infection, so more distinguishable characteristics (such as watery larvae) are needed to accurately diagnose EFB.

Perforated cell cappings.

Figure 6. Perforated cell cappings.

Watery dead larvae

EFB infected larvae will become gooey or have a watery consistency (Figure 7) and may stretch out of the cell to a length of 1.5 cm or less. This is distinguishable from AFB, which will rope out of a cell to a length of 2.5 cm or more. While these characteristics will help distinguish between EFB and AFB, these symptoms are dependent on the stage of the infection and will not always be present earlier or later in the infection cycle.

Various stages and appearance of European foulbrood-infected larvae.

Figure 7. Various stages and appearance of EFB infected larvae. Photo credit: Ali Panasiuk, Alberta Agriculture, Forestry and Rural Economic Development.

Prevalence and spread

While EFB is most commonly found in early spring, it may appear at any time of the year in honey bee colonies. Since EFB is highly contagious, it can spread to neighbouring apiaries if not well-managed and can survive in honey bee colonies and on used beekeeping equipment.

Spread of EFB through honey bee activity

Drifting

EFB can spread within a bee yard when honey bees from an infected colony mistakenly enter another colony after returning home from foraging.

Robbing

Robbing occurs when honey bees from one colony collect or rob the honey stores of another colony. It is a natural behaviour of honey bees. In extreme circumstances, where there is a shortage of nectar or weak and/or dead colonies in the surrounding environment, robbing can become quite intense. Robbing can result in the spread of EFB within a bee yard or between different bee yards (up to 8 km away). Robbing is particularly a concern during a nectar shortage.

Spread of EFB through beekeeper activity

Exchanging equipment between colonies

Moving frames of brood between different colonies in a bee yard to equalize colonies or to boost weak colonies is a common and necessary beekeeping practice. However, EFB may be spread from an infected colony to an uninfected colony when this is done. Beekeepers must be mindful of this practice's risk and should be vigilant for any signs and symptoms of the disease.

Packages

Although not likely to spread EFB, honey bee packages are a potential source of infection. Packages therefore require a federal and/or provincial import permit.

Purchasing infected honey bee colonies or equipment

Selling and purchasing honey bees or used equipment is a common practice in the beekeeping industry. This practice does risk introducing EFB into an operation. It is important that selling beekeepers are diligent in inspecting the brood of their colonies to detect EFB. It is equally important that purchasing beekeepers purchase from permitted sources. Selling beekeepers must have a valid permit issued by the Ministry of Agriculture, Food and Rural Affairs’ (OMAFRA) Apiary Program prior to any sales of bees or used equipment taking place. Permits are issued by the ministry’s Apiary Program only after the inspection requirements are met, which helps protect the health of honey bees, particularly from pests and diseases.

Swarms

Swarms may be a source of EFB if the swarm originated from an infected colony. Swarms always pose a risk of transferring pests and diseases since there is no way to be certain of the swarm's origin. However, since a swarm has no brood there is an opportunity to take management steps to minimize EFB risk.

Used beekeeping equipment

Beekeeping equipment that is not properly stored and is in a location accessible to honey bees is a possible source of infection. In most cases, the beekeeper may not be aware the equipment is infected.

Mitigate an infection or outbreak

Beekeepers can take steps to mitigate an EFB infection from establishing itself in their beekeeping operation.

A number of management activities influence the health, production and population of honey bee colonies. Management practices can and will vary depending on the focus of the beekeeping operation (such as honey production, queen and nucleus production, and pollination), but should incorporate basic biosecurity and Essential practices for beekeepers in Ontario (best management practices or BMPs). Good biosecurity practices and basic colony BMPs have been found to be the most effective and practical means of mitigating EFB infections and include:

  • familiarizing yourself with the symptoms of EFB and other pathogens of honey bees
  • examining your colonies regularly to ensure that they are healthy and disease free, keeping an eye out for the symptoms of brood diseases
  • conducting a thorough inspection of the brood nest for brood diseases at least twice a year, in the spring and fall, and before treatment with drugs (such as oxytetracycline for AFB) which can mask some symptoms of EFB
  • properly store used beekeeping equipment, promptly manage dead colonies in a bee yard, and have equipment inspected before a sale or transfer of ownership

Best management and biosecurity practices when infection is found

  • Work with an apiary inspector on steps to address EFB where it is confirmed.
  • Do not extract honey from an infected colony.
  • For a mild infection, promptly destroy infected frames of bees. Replace destroyed frames in mildly infected colonies with new or used uninfected frames.
  • For a severe infection, promptly destroy the entire infected colony and associated equipment (for example, bottom board and other woodenware that cannot be scorched).
  • Retain all colonies at the EFB infected bee yard until an OMAFRA apiary inspector determines no infected colonies are present.
  • Consider treating symptomatic, mild infected colonies (after infected frames are removed and destroyed) and all non-symptomatic colonies remaining in the yard with an antibiotic (this will depend on the time of year and conditions).
  • Provide sugar syrup (2 sugar:1 water) and pollen patties if colonies appear to be under nutritional stress.
  • Monitor the health status of the yard that was infected with EFB on a more frequent basis and be vigilant for EFB in your other bee yards or in the surrounding area.

Refer to general best management and biosecurity practices for Ontario beekeepers.

Treatment

Antibiotics may resolve an active EFB infection in a colony but are not a guarantee that the infection will end. Beekeepers should be aware that EFB infections can still take place even when antibiotics have been used. Antibiotics are not a replacement for the destruction or removal of infected materials or proper biosecurity practices.

Beekeepers require a prescription from a veterinarian to access antibiotics for their honey bees. For general information on antimicrobial use in agriculture visit the Antimicrobial resistance in agriculture webpage. For more specific information on antibiotics for beekeeping in Ontario refer to the Ontario Beekeepers’ Association’s Antibiotic Access Resources for Beekeepers.

Consult the Treatment options for honey bee pests and diseases in Ontario for recommended monitoring methods, treatment methods and timing of treatments for honey bee pests and disease.

If you suspect infection

Finding an EFB infection as early as possible and taking immediate action is critical to preventing its spread within an operation and area.

If signs or symptoms of an EFB infection are observed in a colony (alive or dead), the beekeeper is to immediately contact their local apiary inspector. It is a requirement of the Ontario Bees Act that beekeepers report this disease. An apiary inspector will inspect the colonies to determine if EFB is present.

If you find infected colonies

If EFB is determined to be present on inspection, all colonies and associated equipment in the yard are to be inspected by an apiary inspector to determine each colony’s EFB status. The inspector will require the beekeeper to take action(s) on the EFB infected colonies, associated equipment and the bee yard within a specified timeframe. Depending on the severity of the infection, beekeepers can, voluntarily or by inspector Order, take one or all of these actions:

  • destroy parts of infected colonies (for example, infected frames) or entire infected colonies depending on the severity of the infection
  • treat symptomatic mildly infected colonies (after infected frames are removed and destroyed) and all non-symptomatic colonies remaining in the yard with an antibiotic (this will depend on the time of year and conditions)
  • retain all colonies and associated equipment at the bee yard until an OMAFRA apiary inspector determines no infected colonies and equipment are present

Treatment of non-symptomatic colonies may prevent these colonies from being infected but is not a guarantee that they will not also develop an EFB infection. Once EFB is present in a yard, regular monitoring of the other colonies for signs and symptoms of an EFB infection is necessary so they too can be managed in a timely manner to stop the EFB cycle.

In Ontario, OMAFRA’s Apiary Program maintains a standard practice that non-symptomatic, live colonies from EFB infected yards must be retained in the yard (that is, not moved from that location), via retainment Order, until an apiary inspector completes a re-inspection a minimum of seven days (recommended 14 days) from the date that EFB was detected. This is done to allow the beekeeper time to control the infection and allow for re-inspection of colonies and equipment in the EFB infected yard. After re-inspection, the retainment Order is lifted if the bee yard is assessed to have no EFB present.

Honey may be salvaged from a yard with an EFB infection prior to reinspection if the following conditions are met:

  • honey supers are returned to the same bee yard where they originated. Beekeepers should label these supers so that they may be returned to the colony they originated from and honey can be harvested separately from other uninfected yards.
  • destruction of EFB infected frames and/or entire colony(s) is complete.

A colony is considered to be infected with EFB if there are one or more honey bee larvae (fresh or old) within the colony that display signs or symptoms of the disease. All associated equipment that is or was in contact with this colony has a higher risk of being infected with EFB.

A bee yard is considered to be infected with EFB when one or more colonies in the yard are infected with EFB.

Destruction of infected colonies, associated equipment and materials

The level of response to an EFB infection is dictated by the severity of the infection found in either the colony or the bee yard. In severe cases, immediate destruction of EFB infected full colonies (bees) and associated equipment (hive box, lid, frames and other materials from infected colonies) is necessary to prevent the spread of the disease to other nearby colonies or bee yards. In colonies with a low infection severity, beekeepers must remove and destroy infected frames from the EFB infected colony.

Where full colonies must be destroyed, materials to be destroyed are:

  • biologicals — all honey bees (larvae, adults), wax comb and the honey crop (honey inside frames).
  • equipment — all frames (including brood and honey frames), foundation, honey supers and bottom boards within the colony. The only materials that may be disinfected and not destroyed are hive bodies or boxes, queen excluders (if metal) and telescoping lids.

Skipping steps or salvaging materials or equipment that are associated with an EFB infected colony increases the likelihood and opportunity that the EFB infection will persist and spread, resulting in more colonies and equipment becoming infected.

How to destroy infected colonies, associated equipment and materials

Destruction by fire is the most common method used, and is the method specified in the Bees Act, for destruction of colonies with serious infections of EFB.

Approval for another destruction method may be considered by the Provincial Apiarist in circumstances where there are large amounts of plastic hive equipment (such as full plastic frames versus wooden frames with plastic foundation) or where burn restrictions or fire bans are in place. Contact the Provincial Apiarist in writing if consideration for an alternative destruction or disposal method is sought. Other methods (such as burial or irradiation) must be approved by the Provincial Apiarist.

Whatever the approved method, the beekeeper is ultimately responsible for the destruction of EFB infected colonies, associated equipment and materials when required.

1. Preparing colonies for destruction

  • Seal bees inside hive.
    • Completely block the entrance to the hive and fill any cracks in the hive.
    • Complete this activity at a time when the bees are not flying (such as evenings or during rainy weather) and are settled (hives should not be sealed/destroyed immediately after inspecting or disturbing the colony). This practice ensures the bees are all at home in the hive and prevents bees from the EFB infected colony relocating to neighbouring colonies and spreading the disease.

2. Immobilizing and killing bees

  • Use diesel fuel to immobilize and kill the colony.
    • Remove the lid from the hive when the bees are not flying and sprinkle diesel fuel over the colony.
    • It may be necessary to split the chambers and add diesel fuel to the lower chamber(s) as well. The amount of diesel fuel needed depends on the size of the hive. It is estimated that:
      • 300-500 mL of diesel fuel is needed for a one-to-two story hive
      • 1 L of diesel fuel is needed for a three-to-four story hive
    • Securely replace the lid on the hive.
    • After 10 minutes, check to see if all the adult bees are immobilized. If not, repeat the above process to wet the remaining adult bees.

3. Destruction of infected colonies, associated equipment and materials by fire

  • Obtain a fire permit from the proper authority, be compliant with the law, including all local by-laws, and employ safe burning practices at all times. Note that:
    • Diesel fuel is not added as a fire accelerant. Beeswax is extremely flammable and there is sufficient wax in the combs to fuel the fire.
    • Hives burn vigorously and flames can reach two to three times the height of the stack.
    • Boxes of frames soaked with diesel fuel may explode if put into the fire whole.
    • It may take two to three hours to burn a hive and more to burn three to four hives.
  • Dig a hole large enough to contain the materials to be burned.
    • A hole of 1 m (3 ft) in diameter and at least 30 cm (1 ft) deep is suggested for one hive. Expand the hole accordingly for multiple hives.
  • Ensure the hole is deep enough so that any infected material that may not be completely destroyed by fire can be buried so as not to be accessible or found by foraging bees.
  • Slope the bottom of the hole to provide a pooling location for unburned honey so that it does not choke out the fire.
  • Place the hive(s) and materials to be destroyed about 3 m (9 ft) from the hole.
    • To avoid dropping dead bees or honey on the ground, move the hives as a complete unit. Alternatively, individual hive boxes can be moved one by one in an upturned hive lid as this will prevent bees or honey from falling on the ground.
  • Start the fire in the empty hole.
  • Once the fire is well established, it works best to destroy affected material in the following order:
    • Frames
      • Place frames with honey around the edge of the fire so not to douse the fire
      • Once the honey has run out of the frames, push the frames into the fire
    • Bottom boards
    • Lids
    • All parts of the hive that have diesel fuel on them
  • Once the fire has burned down to embers, cover the embers and any material that has not fully burnt with the soil from the hole.
  • If parts of the hive, that do not have diesel fuel on them, are to be salvaged (such as boxes, telescoping lids and metal queen excluders) they must be well flamed (with a propane torch) before used again.