Nosema apis

Nosema apis is a microscopic fungal parasite (a microsporidian) that multiplies in the midgut of adult bees. Its classic picture is brown faecal staining, loss of bees and a colony that fails to build up in late winter and early spring. The symptoms are easily confused with other causes; a definitive diagnosis can only be made with a microscope.

What is it and how is it transmitted?

For a long time Nosema apis was thought to be the only nosema agent; today it is considered together with Nosema ceranae, and in many regions, including Türkiye, N. ceranae is found more often. In current classification both species have been moved to the genus Vairimorpha; beekeeping sources continue to use the old names.

Bees take in the resistant spores by mouth while cleaning combs contaminated with faeces, or with contaminated feed or water. The spore multiplies in the cells of the midgut, destroys them and passes into the faeces as millions of new spores. The consequences:

• The bees' lifespan is shortened; in winter and spring the colony dwindles faster than expected.
• The royal jelly glands of nurse bees weaken and brood care suffers.
• If the queen is infected, egg-laying declines; the colony may quietly supersede her.

How to recognise it

• Faecal staining: Dark brown stains on frame top bars, inside the hive, at the entrance and on the front wall.
• In front of the hive: Bees that cannot fly, crawling and trembling; dead bees on the floor.
• At colony level: A colony that does not grow in spring and weakens even though the queen is laying; early queen supersedure.
• In the gut: The midgut of a healthy bee is light brown to amber and semi-transparent; in heavy infections it looks swollen, cloudy and milky white.

The disease can also run without symptoms. The absence of faecal stains does not mean there is no nosema, and faecal stains do not always mean nosema.

How it differs from similar conditions

How to confirm it at the hive

• Take a sample of old forager bees (about 30 bees) from the entrance; young nurse bees can give misleading results.
• Kill the bees by freezing or in alcohol, label them and send them to a laboratory. The laboratory crushes the abdominal contents, looks for spores under the microscope and determines their density.
• As a preliminary field check, you can hold the tip of the abdomen of a few dead bees with tweezers, pull gently to draw out the gut and look at its colour; this only gives an indication and is not a diagnosis.

Triggering conditions

• Delayed cleansing flights due to long, cold and wet periods
• Damp, poorly ventilated hives
• Unsuitable winter stores: honeydew honeys such as pine honey and crystallised, poor-quality feed strain the gut
• Weak, stressed, frequently moved colonies
• Old, dark combs contaminated with faeces

What to do

1. First confirm the diagnosis: Send a sample to a laboratory; do not assume that every hive with stains has nosema.
2. Replace soiled combs: Gradually remove combs contaminated with faeces; this is the most effective measure.
3. Reduce humidity: Improve ventilation and keep the hive slightly tilted forward and raised off the ground.
4. Correct the winter feed: Instead of stores dominated by honeydew honey, feed suitable sugar syrup in autumn; in mid-winter use fondant (candy) if needed.
5. Strengthen the colony: Unite weak colonies, reduce the space to match the strength of the colony, and supplement pollen if it is insufficient.
6. Provide clean water and requeen colonies where the problem keeps recurring.

Treatment and medicine information

The antibiotic-derived active substance used against nosema in the past (fumagillin) is not authorised in the EU, and its effectiveness against N. ceranae is disputed. Do not use medicines on your own initiative or mixtures recommended on the internet; ask your veterinarian whether an authorised product exists in your country. The basis of treatment is the management measures above.

Prevention

• Renew part of the combs every year.
• Go into winter with strong, well-fed colonies headed by young queens.
• Set up the apiary in a sunny spot away from damp hollows.
• Do not give faecally soiled combs from a dead colony to another hive without cleaning them.