Nosema ceranae is a microsporidian parasite affecting the gut of adult bees. Its original host is the Asian honey bee; it has since spread to the western honey bee and has become the dominant nosema species in many regions.

What makes it different

The classic picture of nosema — faecal marking, crawling bees, a colony failing in spring — belongs to Nosema apis. This species behaves differently, and that is why it went unrecognised for a long time.

• Usually no faecal marking. The most misleading difference. The absence of staining does not mean the absence of nosema
• Not confined to a season. It can be found year-round, whereas N. apis peaks in winter and early spring
• An insidious course. The colony weakens without visible signs

The damage

The parasite multiplies in the cells of the midgut and damages them. The consequences:

• Lifespan is shortened
• Bees may become foragers earlier than normal; the age structure of the colony is disturbed
• Foraging efficiency falls
• The brood food glands of nurse bees weaken
• If the queen is affected, laying declines and she may be superseded

Signs — and the difficulty

There is no distinguishing sign. What is seen is:

• A colony that does not build up although the queen is laying
• Foraging force declining without explanation
• A weak start in spring
• Failure to overwinter

All of these have other causes. Diagnosis requires microscopy; the two nosema species can only be separated by molecular analysis.

How it spreads

Spores are taken in with contaminated comb, water and food. Cleaning bees pick them up. Spores accumulate in old comb, which is why comb renewal matters.

What to do

Medicinal treatment is restricted in many countries, or no licensed product exists. The approach is management:

• Renew comb. The single most effective measure
• Provide clean water. Have your own source in the apiary
• Keep nutrition strong. Varied pollen supports resistance
• Reduce stress. Avoid excessive moving and disturbance
• Reduce moisture inside the hive
• Requeen where a colony weakens persistently
• Control Varroa. The two burdens compound each other

Practical position

If colonies weaken without explanation and no other cause is found, it is worth sending a sample for analysis. Guessing at nosema wastes both time and money.