Why is European Foulbrood Treatable?

Antibiotics, Probiotics and Gut Microflora in the Honey bee Colony

European foulbrood (“EFB”) is a treatable condition as the bacterium Melissococcus plutonius does not form spores like Paenibacillus larvae, the agent of American foulbrood. To confirm EFB diagnosis, samples can be sent to a laboratory for cell culture and microscopy testing, or through field test kits.

Life Cycle of European Foulbrood

Larvae can be infected under 24 hours of age through food and generally die at 4–5 days old. Once inside the larval gut, the M. plutonius grows and compete for nutrients, depriving the larvae of nutrition. The infected larva shifts in the brood cell and dies twisted around the walls or stretched out lengthways. The colour of the larvae changes from pearly white to yellow, then brown and finally when they decompose, grayish black.

Field testing for EFB

If a commercial test kit is unused, the beekeeper can look for visual symptoms in infected colonies:

• Larvae appear twisted or coiled in the cell, often lying unnaturally on their sides rather than in the typical ‘C’ shape.

• Infected larvae may appear yellowish or brown and can eventually become mushy and disintegrate into a gooey mass.

• Presence of irregular brood pattern or discoloured cappings – Cells with infected larvae may be partially or completely uncapped by nurse bees.

• There is a smell associated with EFB, but it is not as putrid as AFB.

European Foulbrood Treatments

Several treatment options exist, from chemical treatments (ideal for a large number of hives) to colony management practices.

Using Oxytetracycline

For commercial beekeepers, antibiotic treatments are recommended. In Australia and many parts of the world, oxytetracycline hydrochloride (OTC) is used. The powder is mixed with sugar and applied in between brood frames to avoid larval irritation from direct contact. The adult bees consume the sugar-OTC mix, and some is added to the brood food for larval consumption.

Some of the treatment may end up being stored as nectar and ripened into honey. This is a greater issue with wet treatments (i.e. OTC applied in sugar syrup), as regional maximum residue limits were exceeded. As such the dry powder approach was introduced to reduce the likelihood of OTC in honey.

How does Oxytetracycline work on M. plutonius?

In simple terms, it prevents protein synthesis and stops bacterial growth. Rather than dying, M. plutonius goes into stasis, and the symptoms stop. This gives the colony a break, enabling it to build its strength until it naturally overcomes the infection.

More technically, OTC belongs to the tetracycline drug class. Tetracyclines are broad-spectrum, bacteriostatic antibiotics that inhibit protein synthesis by preventing tRNA attachment to the A site of the ribosome (reviewed in Chopra & Roberts, 2001).

Bacterial Resistance to Oxytetracyclines

A more niche point: resistance to OTC and other tetracyclines is caused by the transfer of antimicrobial resistance genes via bacterial mechanisms of genetic exchange (Plasmids, horizontal & vertical transmission). These genes lead to resistance development by producing efflux shuttle proteins, ribosomal modification proteins, and antibiotic destruction.

Resistance to OTC can occur when it is stored in honey – Bacillus cereus strains extracted from honey in treated hives were found to carry tetracycline resistance genes (Lopez et al., 2008). If M. plutonius were to end up way in the honey stores, it may interact with this non-pathogenic strain and receive tetracycline resistant genes, making treatments difficult in outbreaks.

The Shook Swarm Method

The shook swarm method re-homes bees into a new hive with new frames. Brood frames from an infected hive are shaken over the new hive box, and the old frames destroyed. The bees (and queen) start building and gathering from scratch in a hive free from contamination.

A combination of the shook swarm method and oxytetracycline was more effective in preventing recurrence than either method alone (Waite et al., 2003). But further research showed that the shook swarm alone is more effective, as the antibiotic suppressed symptoms; a colony may be deemed cured, only to have the infection return or spread to other honey bee colonies (Thompson et al., 2006).

Treating EFB via Colony Management

The following is applicable to most honey bee diseases, and overlaps with the prevention techniques:

• Regularly replace old brood comb and frames to break the disease cycle.
• Clean and sterilise hive tools and equipment regularly.
• Keep strong colonies, especially in spring when the brood population rapidly increases compared to adults. Provide supplemental feeding if required.
• Requeen hives with a disease resistant stock.
• Where possible, space hives a few hundred meters apart to prevent robbing and drifting

Gut Bacteria and Probiotics

Research on probiotics as a disease treatment is ongoing. Evans and Armstrong found that the bee gut bacteria can out-compete with pathogens like P. larvae & M. plutonius, reducing the severity of infections or protecting colonies altogether (Evans & Armstong, 2006).

The Bacillus bacteria benefit honey bees by aiding in pollen fermentation, food protection, and disease prevention within their gut, as well as in production of antimicrobial peptides and immune system stimulation. For example, Sabate & colleagues found that a B. subtilis strain improved colony performance by inhibiting the growth of P. larvae and Ascosphaera apis (the causative agent of chalkbrood disease; Sabate et al., 2009).

Lactic acid bacteria are common in the honey bee intestinal tract and hive environment, with the two most abundant species being Lactobacillus plantarum and lactobacillus kunkeei¹(Vasquez et al., 2012). They are beneficial as they produce antibacterial metabolites – organic acids and bacteriocins. Iorizzo and colleauges investigated four strains from L. plantarum and L. Kuneei on P. larvae and M. plutonius, and found that they inhibited the attachment of the pathogenic bacteria to the larval gut walls, preventing them from establishing an infection (Iorizzo et al., 2022).

Other Lactobacillus, Enterococcus and Bifidobacterium, and Bacillus species have demonstrated antibacterial effects against AFB and EFB; the interested reader is referred to the literature for further information.

Perhaps the near future will shift into favouring probiotics in our sugar syrup feeds. However we must ensure to use specific strains meant for honey bee colonies, as generic probiotics alter the gut flora and impair immune function (Andrearczyk et al., 2014, Ptaszynska et al., 2016).

Melissococcus plutonius Microbiology

The non-technical reader is welcome to skip this section

M. plutonius is a Gram-positive, coccus that can sometimes appear rod-like. The bacterial cells occur as single forms, in pairs or in chains of various lengths (Floyd et al., 2020). M. plutonius is anaerobic and needs carbon dioxide for growth, which can be provided by secondary infections.

Other Bacteria Associated with EFB Infections

EFB infections can be made worse by co-infection with another bacterial species that are non-pathogenic alone. For example, Achromobacter euridice is commonly found alongside M. plutonius, and is thought to help the initial infection by consuming oxygen (Bailey 1957). Bailey also reported that when A. euridice is introduced along with M. plutonius, EFB symptoms were more readily apparent than M. Plutonius infection alone (Bailey, 1956). A. euridice is found in pollen, and is transferred to larvae by the nurse bees during feeding.

Enterococcus faecalis is another species found in larvae infected by M. plutonius. The bacteria itself can be pathogenic at high concentrations doses (eg: treatment under laboratory conditions) and becomes an issue in chronically diseased colonies (Bailey, 1963). Paenibacillus alvei is bacteria that instead of growing in the larval gut, establishes itself on the larval remains following death from EFB; its spores multiplying faster than other saprophytic bacteria (“those that digest organic matter”; Bailey, 1963).

Transmission of European foulbrood

As with other honey bee diseases, EFB can spread from sources like contaminated equipment, flowers, drifting bees or returning robber bees. The infected adult bees transfer bacteria to the nurse bees during adult-to-adult feeding (trophallaxis) or by depositing contaminated forage into the combs.

Adult honey bees do not show signs of infection, and act as carriers of the bacterial disease. Larval infection occurs during feeding by nurse bees. This is because feeding is done mouth-to-mouth; the brood food kept in the honey bee’s stomach and is mixed with its contents (including the beneficial bacteria). M. plutnious spores out-compete the other microbes, and establish themselves in the hindgut where they germinate.

Why Does EFB Grow in the Larval Gut?

It is thought that M. plutonius grows in the honey bee brood due to a lack of gut microbes. In adult bees, the gut bacteria protect them from infections, whereas young larvae do not have the same microorganisms inside them.

For adults, Lactic Acid Bacteria (LAB), Bifidobacteria and Bacillus species, are the major groups (Daisley et al., 2020). The benefits of Bacillus species lie in their ability to produce enzymes, antibiotic-like substances and increase gene expression of immune proteins (AMPs, apidaecin and hymenoptaecin; Kwong et al., 2017).

These effects are not localised to the gut, and impact the overall bee health. The absence of these beneficial microorganisms in the developing larvae increases the risk of infections from dietary sources, like EFB. Extending the analogy, it’s one reason why larval feeding is tightly controlled by the colony – they require a special diet for normal development, unlike adult bees who can feed on whatever flower forage they find.

Can Small Hive Beetles carry EFB?

Analysis of small hive beetle abdomens showed that they carried M. plutnious, amongst other pathogens (Huang et al., 2019). The adult beetles that feed on contaminated combs pick up M. plutonius and carry it in their gut (as opposed to the bacteria dying after ingestion).

The spread of M. plutonius between colonies by small hive beetles can be thought of as follows:

1. The adult beetle (or larva) feeds on infected pollen and nectar.
2. The beetle (or larva) leaves the hive (whether naturally or by eviction).
3. The beetle enters new hive OR the larva pupates in the soil and the infected adult beetle emerges to seek a honey bee hive.
4. As the adult beetle defecates inside the hive, its faeces contain M. plutonius (amongst other pathogens).
5. The worker bee cleans the droppings, and becomes infected through direct contact.

This is all theoretical – I do not know of any studies showing small hive beetle-mediated EFB cases.

Preventing European Foulbrood

The advice is generic since many honey bee diseases have the same underlying causes: stressed bees or infected equipment. If you avoid these, you avoid most diseases and can maintain relatively healthy hives. Diving deeper:

• Ensure colonies are strong and healthy. Strong colonies with good genetics are more resistant to diseases.
• Provide adequate nutrition, especially during times of nectar dearth. Supplement with sugar syrup and pollen patties if natural forage is insufficient.
• Regularly inspect the brood pattern for signs of EFB, including discoloured and twisted larvae, sunken and discoloured cappings, and for those with excellent noses, an unpleasant smell.
• Practice good hive hygiene. Regularly clean and sterilise tools and equipment.
  • If EFB is detected, remove and destroy affected brood combs to reduce the spread of bacteria.
• Prevent robbing, which can spread EFB from an infected hive to healthy ones. Use entrance reducers to manage hive traffic. If possible, situate hives a few hundred meters apart to reduce the chance of drifting.
• Consider requeening your hives from stock that shows resistance to EFB or bees containing disease-resistant traits.
• Feed bees with clean, uncontaminated sugar syrup and ensure feeding equipment is sterile. Avoid feeding honey from unknown sources as it can contain EFB spores.
• Quarantine new colonies before introducing them to your apiary. Monitor them for signs of EFB (and any other honey bee pests and disease).
• If treatment is warranted, then antibiotics like oxytetracycline can be used. This does not cure EFB and only suppresses symptoms until the colony is healthy enough to overcome it.
  • Also refer to the shook swarm method mentioned above.

Comparing European Foulbrood with American Foulbrood

We conclude with a table. For further reading on AFB, refer to the article on American foulbrood.

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Footnotes

1. These are their former names. Their current names are Lactiplantibacillus plantarum and Apilactobacillus kunkeei (Iorizzo et al., 2020) ↩︎