European foulbrood (EFB) is caused by the Melissococcus plutoniusbacteria that affects honeybee larvae. Unlike American foulbrood, Melissococcus plutonius does not form spores. EFB is highly contagious and bacteria can remain viable for years in honey, wax or equipment. 

EFB is a problem for beekeepers throughout the world, with the United Kingdom in particular struggling to contain it (EFB is the widest-spread bacterial brood disease in the UK). Although it has yet to spread as far as New Zealand, it is found throughout eastern Australia. It is likely that without strong preventative measures, it will continue to spread to the few remaining areas in the world that are as yet unaffected.

Weak or stressed colonies are particularly susceptible to infection by EFB.

EFB affects larvae and can cause a significant reduction in the capacity of the hive and if unchecked can lead to the entire colony dying. Hives can be infected with EFB but show no visible signs. Stressful conditions such as hive movement, climatic conditions or poor nutrition can trigger EFB symptoms.  

The larvae first become infected when they take in food that is contaminated with the Melissococcus pluton bacterium.

They can also be infected by:

• robbing – a colony with EFB is susceptible to robbing because it will be weakened, increasing the chances that the infection will be spread to other hives
• beekeeping equipment that has been contaminated with EFB and not properly cleaned
• transferring combs from infected hives to previously uninfected hives.

Once the bacterium is inside the larvae it grows within their gut, consuming most of the food they take in. This normally results in starvation and the death of the larvae. If a larva survives the diseases and pupates, it will then leave more of the bacteria through its faeces, which can spread the infection further within the hive. If the larvae dies, it dries to a dark scale which also spreads the infection.

EFB-infected dead larva with yellow gut visible

Because worker bees often remove diseased larvae, EFB can be hard to detect. Most larvae will die before capping but some will die after — which can lead to a misdiagnosis of American foulbrood. EFB-infected larvae will not ‘rope’ when tested with a matchstick as happens with AFB.

Beekeepers should look out for the following signs of infection in their hives:

• Diseased larvae change colour from white to a yellowish-brown
• Dead larvae may be watery in consistency
• There may be a sour odour present
• The brood pattern will become uneven, with capped and uncapped cells mixed together.

Patchy brood pattern with discoloured larvae, caused by EFB infection

The best way to protect your hives is to stop them getting infected in the first place. While it is almost impossible to fully protect a hive these steps should significantly reduce the risk:

• Maintain the highest possible levels of hive hygiene by keeping all beekeeping equipment clean. Try not to introduce unknown bees or equipment to the hive to reduce the risk of infection spreading.
• Requeen the hive with a young queen regularly — a younger, healthier queen will be better for the hive. Breed from bees that have already proven to be resistant to disease if possible.
• Take every precaution when moving bees to minimise stress. Stress is a key risk factor for EFB.
• Ensure bees have access to good supplies of pollen and nectar and using artificial feeding methods when necessary, as this too will help keep stress at a minimum.

EFB is a notifiable disease in some areas. Check with your local Department of Agriculture what the regulations and permitted treatment options are in your region.

If you have multiple colonies and more than 10 per cent of them show signs of the disease, every colony should be treated.

Antibiotic treatment is an option, but this can lead to antibiotic residues in the honey. You may have to discard the first honey harvest after treating a hive with antibiotics. EFB can be treated without antibiotics through good colony management. In cases of severe infection, destruction of infected hives may be advised.