Nosema ceranae

Nosema ceranae is a spore-transmitted microsporidian that infects the midgut of the adult bee. Unlike Nosema apis, it often causes no diarrhoea stains and can be found in every season of the year; the colony weakens silently. It can only be diagnosed with a microscope; the most effective measures are comb renewal, good nutrition and varroa control.

What is it?

Its original host is the Asian honey bee (Apis cerana). It was noticed in the honey bee (Apis mellifera) in the mid-2000s; examination of older samples showed that it had been present before. Today it is found almost everywhere in the world and in many regions, including Türkiye, it is more widespread than N. apis. In current scientific classification it is also referred to as Vairimorpha ceranae.

The bee takes in the spore orally. The spore enters the gut cells and multiplies, destroying the cells, and millions of new spores are released with the faeces. An infected bee gets less benefit from its food, starts foraging early and lives shorter.

How it differs from Nosema apis

The practical upshot: no faecal stains in the hive does not mean no nosema. Infection can be heavy even in a colony that looks clean.

How to recognise it

The symptoms are not specific; be suspicious if the following occur together:

• Unexplained population decline progressing over weeks
• The colony failing to grow and honey yields falling even though the brood area is reasonable
• Fewer foragers; a small number of crawling or dead bees in front of the hive
• Silent supersedure or early loss of the queen
• A colony that looked strong in autumn failing to survive the winter

How does it spread?

• Comb, honey and pollen contaminated with spores; moving used frames between hives
• Bees cleaning inside the hive and sharing food
• Robbing and drifting bees
• Dirty, stagnant shared water sources
• Trade in colonies, package bees and queens

How to confirm it in the hive

1. Sample the right bees. The spore load is highest in older bees; if possible, take the sample from foragers returning to the entrance. Young bees in the brood area carry fewer spores and can make the result look better than it is. Usually about 30 bees are enough; ask the laboratory in advance how many it wants.
2. Microscopic examination: The bees' abdomens are crushed with a little water and examined for spores at 400× magnification. Beekeepers who have a microscope can learn this method.
3. Species identification: A molecular test is carried out in the laboratory to distinguish N. ceranae from N. apis.

The external appearance of the gut (a swollen, whitish midgut) is more pronounced with N. apis; with N. ceranae it is often not a reliable sign.

Its effect in combination with other stresses

The effect of N. ceranae on the colony varies with the region and conditions. Some studies have linked it to heavy colony losses, others have found a more limited effect. The consensus is this: the damage increases markedly when it is combined with varroa, viruses, poor nutrition and pesticide exposure. If a colony is weakening for no apparent reason, look at all these factors together instead of searching for a single cause.

What to do

1. Replace old, dark combs regularly. Spores accumulate in the comb; renewing part of the combs every year is the most effective structural measure.
2. Keep nutrition strong. Varied and sufficient pollen sources and timely feeding during a nectar dearth increase the colony's resilience.
3. Keep varroa under control. This is the most important step in reducing the overall stress.
4. Provide clean water. Have a clean water source near the apiary that is running or frequently refreshed.
5. Help a persistently weak colony recover. Requeening and transferring the colony onto clean comb help. Uniting a heavily infected colony with a healthy one can carry the disease over; clarify the diagnosis before uniting.
6. Clean empty combs. Empty combs without honey from diseased colonies can be disinfected with acetic acid fumes. The acid is corrosive; it is applied with protective equipment, in the open air and away from bees.
7. Do not move comb and frames between colonies. In particular, do not give material from diseased colonies to healthy ones.

Medicinal treatment

Fumagillin, used against nosema in the past, is not authorised in many countries, including the European Union, and its effectiveness against N. ceranae is disputed. Evidence for the effectiveness of the herbal "nosema support" products on the market is limited. Do not use medicines on your own; consult your veterinarian about the authorised options in your country and the restrictions relating to the honey harvest.