Nosemosis

Nosemosis is a gut disease of adult honey bees. It is caused by two microscopic parasites that multiply inside the cells of the gut wall, destroy the digestive lining and shorten the lives of workers. At colony level, this means slow spring build-up, a smaller honey crop and, in severe cases, dwindling or winter loss.

The classic picture, combs stained with dysentery at the end of winter, is no longer enough since Nosema ceranae arrived in Europe: this second species often causes silent weakening, without dysentery. Only a microscope examination tells you whether a colony is infected, and only a molecular test tells you which species is involved.

The agents: two reclassified microsporidia

The agents of nosemosis are microsporidia, single-celled parasites related to fungi that can only multiply inside their host’s cells. Outside the bee, they survive as resistant spores, visible under the microscope.

Two species infect the western honey bee Apis mellifera:

• Nosema apis, the long-established species in Europe;
• Nosema ceranae, originally described from the Asian honey bee Apis cerana, then detected in honey bees worldwide during the 2000s¹.

In 2020, a revision based on molecular phylogeny showed that both species are closer to the genus Vairimorpha than to Nosema in the strict sense. They are therefore now called Vairimorpha apis and Vairimorpha ceranae². The old names are still widely used, including in regulations: this factsheet uses both.

The cycle in the midgut

The whole cycle takes place in the digestive tract of the adult bee. Brood is not infected.

Ingestion. A bee swallows spores with contaminated water or food (honey, pollen, royal jelly), during food sharing or while cleaning soiled surfaces⁷.

Germination. Once in the midgut (ventriculus), the spore fires a long hollow filament, the polar tube, which pierces a cell of the gut wall. The infective contents of the spore, the sporoplasm, are injected into that cell⁷.

Multiplication. Inside the cell, the parasite multiplies and forms new spores, which infect neighbouring cells or are released into the gut⁷.

Excretion. Spores leave the bee in its faeces, normally during flight. During long confinement or dysentery, bees defecate inside the hive: combs and stores become contaminated, and the bees that clean them become infected in turn.

Spread between colonies. Robbing, drifting, exchanging combs and food, contaminated equipment; drones may also spread the parasite⁷. The infective dose is low: published estimates are in the order of a hundred spores per bee⁷.

N. apis and N. ceranae: two different diseases

The two species behave differently, and are sometimes referred to as type A (N. apis) and type C (N. ceranae) nosemosis⁷.

N. apis also shows a small autumn peak. N. ceranae is detected all year round, with the highest levels in Spain from late summer to spring⁷.

In Spain, colonies naturally infected with N. ceranae were followed until death: a long symptomless phase, compensatory egg-laying in winter, a false recovery in spring, then sudden depopulation, with no dysentery⁶. Elsewhere, N. ceranae is often found in colonies that do not collapse: its role in colony losses is still debated^{1, 7}.

Why it matters: effects on the bee

A hungry bee

By destroying midgut cells, the parasite reduces nutrient absorption. Infected foragers are hungrier, consume more sugar and have disrupted carbohydrate metabolism: this is known as energetic stress⁷.

Nurse bees that age too fast

The switch from nursing to foraging is regulated by the balance between vitellogenin, a storage protein in young bees, and juvenile hormone, which rises in foragers. In bees infected with N. ceranae, this balance is reversed: vitellogenin drops and juvenile hormone rises. Infected bees start foraging earlier and live shorter lives than healthy bees⁹. Infected nurses also produce glandular secretions that are poorer in protein⁷.

For the colony, these effects add up: fewer effective nurses, foragers that die young, a population that fails to build up in spring and winter bees that live shorter lives (see Winter losses).

Interactions with pesticides and viruses

In the laboratory, several studies have shown a synergistic effect between Nosema and neonicotinoid insecticides: together they increase bee mortality more than either factor alone⁷. Other work links pollen contaminated with fungicides or acaricides to a higher risk of infection, but results vary between substances⁷.

Interactions with viruses are complex. Co-infection with N. ceranae and black queen cell virus (BQCV) increases mortality; conversely, prior infection with N. ceranae appears to hinder deformed wing virus in some experiments⁷. These results come mostly from cage trials and do not predict what happens in a given colony.

Recognising nosemosis

These signs suggest nosemosis but are not enough to confirm it:

• brownish faecal staining on combs, top bars, floor and hive front, especially in late winter (suggestive of N. apis);
• bees crawling in front of the hive, unable to fly, with swollen abdomens;
• a colony that builds up poorly in spring although the queen is laying;
• a strong colony that dwindles for no visible reason, sometimes after an apparent recovery in spring (suggestive of N. ceranae).

Do not confuse with

Diagnosis

Sampling

The age of the bees matters. Foragers, the oldest bees, carry the most spores; house bees can give a falsely reassuring picture^{6, 7}. Standard protocols therefore recommend sampling returning foragers at the hive entrance^{8, 13}:

1. Pick a flying day and avoid early morning.
2. Briefly block the entrance, let returning foragers gather and collect them in a jar.
3. In cold or rainy weather, sample from outer combs instead.
4. Take at least 30 bees: 30 examined individually, or a pool of about 60 for a composite analysis¹³.
5. Freeze the sample, label it (apiary, hive, date) and send it promptly if it is going to a laboratory.

Microscope examination step by step

This is the basic method, possible with a microscope magnifying 400 times^{3, 8}.

1. Separate the abdomens from the bees.
2. Crush the abdomens in a mortar with water, about 1 ml of water per bee, until the suspension is uniform¹³.
3. Place a drop in a counting chamber (Neubauer-type haemocytometer) and let the spores settle.
4. Examine at × 400. Spores appear as small, oval, bright, regular grains, distinct from debris and pollen grains.
5. Count the spores in the chamber squares; a factor based on chamber volume and dilution gives the average number of spores per bee⁸.

A pooled sample gives an average load; examining bees one by one gives the proportion of infected bees, a better reflection of severity^{6, 8}.

PCR to tell the species apart

Under the microscope, the spores of the two species are too similar for reliable identification⁷. The species is identified by PCR, which detects the parasite’s DNA. A common method amplifies, in a single reaction, a fragment specific to each species, of different sizes, which also reveals mixed infections^{8, 13}. Real-time PCR can also estimate the amount of parasite.

In France, these tests are offered by specialised laboratories; the Anses national reference laboratory produces reference materials for microscopy and PCR⁴.

Regulation

In France, Nosema apis disease was long a notifiable contagious disease, then classified as a category 1 health hazard. It is not listed under the EU Animal Health Law. According to the national reference laboratory’s 2025 report, it is still on the national list of provisionally regulated diseases (annex II of the order of 3 May 2022), and it has become very rare in France⁴. N. ceranae disease is not regulated.

This provisional status may change: if a test is positive for N. apis, check with the local veterinary services (DDPP) or your beekeeping health organisation. Rules differ from country to country.

No authorised medicine in the EU

Fumagillin, an antibiotic, was long the only specific treatment for nosemosis. Its use in bees is prohibited in the European Union¹¹, where it is not authorised, partly because of concerns about side effects⁷. In addition, at low concentrations, as the product breaks down, it may actually favour N. ceranae multiplication¹².

There is therefore no authorised veterinary medicine against nosemosis in the EU. Feed supplements sold “against Nosema” are not medicines and their effectiveness is poorly demonstrated.

Prevention and management

Control rests on three levers: reducing the number of spores in the hive, avoiding the stresses that favour infection, and keeping colonies strong⁵.

Renew combs. Soiled combs are a reservoir of spores: brood combs are replaced regularly and the most contaminated ones are melted down.

Disinfect equipment. Equipment from sick or dead colonies is cleaned before reuse: scraping and scorching boxes, fumigating stored combs with acetic acid (corrosive, handle with protection)¹⁰. See Disinfecting equipment.

Winter strong colonies, with a young queen and good-quality stores: indigestible food favours dysentery, which spreads spores.

Choose the site. A dry, sunny apiary sheltered from the wind lets bees make cleansing flights whenever the weather allows.

Limit spread. No exchange of combs or food from a suspect colony, reduced entrances against robbing, never honey of unknown origin.

Control varroa and provide varied pollen sources.

Manage affected colonies. A weak colony can be moved onto new combs or united with a strong one; a dying colony is removed rather than left to be robbed.

Most common mistakes

• Diagnosing nosemosis from dysentery without a test, or ruling it out because there is no dysentery.
• Sampling house bees instead of foragers, which underestimates infection.
• Reusing combs from a dead colony without melting them down or disinfecting them.
• Buying a product presented as a treatment: no medicine is authorised in the EU.
• Neglecting varroa, which worsens weakening.

Frequently asked questions

Can a colony recover on its own? With N. apis, infection often declines once flying resumes. With N. ceranae, it is more persistent. A strong colony on clean combs has a better chance of pulling through.

Is nosemosis dangerous for people or for honey? These microsporidia do not affect people. Honey from an affected colony should not, however, be fed to other colonies, as it may contain spores.

Should every colony be tested? Not routinely. Testing is useful for colonies that build up poorly, after unexplained losses or before uniting colonies.

What should be done with equipment from a colony that died of nosemosis? Melt down soiled combs, scrape and scorch boxes and floor before any reuse. See also Apiary biosecurity.

References

1. Fries I. (2010). Nosema ceranae in European honey bees (Apis mellifera). Journal of Invertebrate Pathology 103: S73-S79. doi.org
2. Tokarev Y.S. et al. (2020). A formal redefinition of the genera Nosema and Vairimorpha (Microsporidia: Nosematidae) and reassignment of species based on molecular phylogenetics. Journal of Invertebrate Pathology 169: 107279. doi.org
3. WOAH. Terrestrial Manual, chapter 3.2.4: Nosemosis of honey bees.
4. Anses, Sophia Antipolis Laboratory (2025). Activity report of the national reference laboratory for bee health (in French). anses.fr
5. FAO, IZSLT, Apimondia, CAAS (2021). Good beekeeping practices for sustainable apiculture. FAO Animal Production and Health Guidelines No. 25.
6. Higes M., Martín-Hernández R., Botías C. et al. (2008). How natural infection by Nosema ceranae causes honeybee colony collapse. Environmental Microbiology 10: 2659-2669. doi.org
7. Martín-Hernández R., Bartolomé C., Chejanovsky N., Le Conte Y. et al. (2018). Nosema ceranae in Apis mellifera: a 12 years postdetection perspective. Environmental Microbiology 20(4): 1302-1329. doi.org
8. Fries I., Chauzat M.-P., Chen Y.-P. et al. (2013). Standard methods for Nosema research. The COLOSS BEEBOOK, Vol. II. Journal of Apicultural Research 52(1): 1-28. doi.org
9. Goblirsch M., Huang Z.Y., Spivak M. (2013). Physiological and behavioral changes in honey bees (Apis mellifera) induced by Nosema ceranae infection. PLoS ONE 8(3): e58165. doi.org
10. Jones B. (2021). Advances in Nosema research. BeeFarmer, December 2021 (National Bee Unit). nationalbeeunit.com
11. Botías C., Martín-Hernández R., Meana A., Higes M. (2013). Screening alternative therapies to control Nosemosis type C in honey bee (Apis mellifera iberiensis) colonies. Research in Veterinary Science 95(3): 1041-1045. doi.org
12. Huang W.-F., Solter L.F., Yau P.M., Imai B.S. (2013). Nosema ceranae escapes fumagillin control in honey bees. PLoS Pathogens 9(3): e1003185. doi.org
13. SOLATINA network / CYTED. Standardisation of Nosema spp. diagnostic methods: sampling, counting and PCR (technical document, in Spanish). cyted.org
14. USGS Bee Inventory and Monitoring Lab. Apis mellifera, worker side view (public domain). usgs.gov

Updated : October 8, 2026
Written by : Apisanitas editorial team
Reviewed by : Review in progress