Understanding Vairimorpha Disease in Honey Bees

Honey bees depend on a healthy gut to digest food, absorb nutrients and maintain a strong immune system. When parasites invade this part of the digestive system, they can disrupt these processes and weaken the entire colony. One of the most common gut parasites found in honey bees is Vairimorpha (formerly Nosema), a microsporidian that infects honey bees and harms their colonies.

Understanding Vairimorpha in Honey Bees

Vairimorpha disease is caused by two spore-forming species, Vairimorpha ceranae and Vairimorpha apis. These parasites infect the epithelial cells of the honey bees’ ventriculus (midgut), where digestion and nutrient absorption take place ¹. As these cells become damaged, bees struggle to process food efficiently, which can shorten worker lifespan and weaken colony productivity ¹. Because the infection is internal, honey bees do not show outward symptoms specific to this disease, making this disease easy to overlook without testing. 

Figure 1: Vairimorpha spp. spore (arrow) under a compound microscope 100x

Even without obvious clinical signs, Vairimorpha infections can influence colony performance. Reduced nutrient absorption can weaken foraging ability, lower honey yield and slow spring build-up ¹. Classic symptoms such as fecal staining have been associated with V. apis ². The dominant species in Canada, V. ceranae, does not have distinct symptoms ². This makes visual inspection alone not reliable for detecting this disease and shows the importance of routine monitoring. 

The microsporidian spreads easily within a colony because the spores are environmentally resistant and spread through fecal-oral transmission. Bees can ingest the spores while cleaning contaminated comb, grooming or by trophallaxis ². Foragers can also pick up the spores from contaminated water sources ². Once inside the gut, the spores infect the midgut and multiply, allowing infection to spread quickly. Since this disease is transmitted through normal social behaviours, even strong colonies can become infected without showing obvious signs.

For decades, these pathogens were referred to as Nosema apis and Nosema ceranae. However, recent genomic work has shown both species are more closely related to the genus Vairimorpha, clustering with Vairimorpha necatrix, rather than the species Nosema bombycis in the genus Nosema ³. Although the scientific names have changed, the disease is still commonly referred to as Nosema or Nosemosis in the apiculture industry. It is important to note that this taxonomic update does not change how the disease is diagnosed or managed. 

Across Canada, research shows a clear shift toward V. ceranae being the dominant species. In a study on Nova Scotia colonies, all positive Vairimorpha samples contained V. ceranae, with V. apis not detected at all ⁴. Similar reports in other provinces, such as Ontario and Alberta, found that in polymerase chain reaction (PCR) surveys, V. ceranae was more common than V. apis ⁵.

Diagnosing Vairimorpha infections relies on microscopy, which allows beekeepers to quantify the spores. These spores represent the mature, environmentally resistant stage of the parasite, so spore counts provide an indirect measure of infection intensity. However, because the spores of V. ceranae and V. apis are difficult to distinguish, species-level identification requires polymerase chain reaction (PCR) to detect genetic differences between the microsporidia ².

Seasonal monitoring is essential because Vairimorpha levels fluctuate throughout the year. In Nova Scotia, McCallum et al. (2020) have shown that spore loads spike in May, then drop through the summer, with a smaller increase in spore loads in the fall ³. In Ontario, Emsen et al. (2020) have shown that infection intensity and prevalence of V. ceranae are highest in spring and summer seasons, and slightly lower in the fall ⁷. These seasonal trends emphasize the value of spring and fall testing for providing meaningful information on spore counts. 

Understanding the treatment of this disease is essential, like how fumagillin works and when it is most effective, when making management decisions. Next week’s blog will explore how Fumagillin interacts with the parasite inside the bee and how effective this is. 

 Written by Kaitlyn Newton, ATTTA Seasonal Apiculturist

Connecting with ATTTA Specialists

If you’d like to connect with ATTTA specialists or learn more about our program, you can:

visit our website at https://www.perennia.ca/portfolio-items/honey-bees/

Email attta@perennia.ca

References:

1. Sammataro, D. and Avitabile, A. 2021. A Beekeeper’s Handbook: Fifth Edition. Cornell University Press. 
2. Pernal, S.F. and Clay, H. 2013. Honey Bee Diseases & Pests, Third Edition. Canadian Association of Professional Apiculturists, Beaverlodge, AB, Canada, 68 pp. 
3. McCallum, R., Olmstead, S., Shaw, J. and Glasgow, K., 2020. Evaluating efficacy of Fumagilin-B® against nosemosis and tracking seasonal trends of Nosema spp. in Nova Scotia honey bee colonies. Journal of Apicultural Science, 64(2), pp.277-286.
4. Bojko, J., Becnel, J., Bessette, E., Edwards, S., Gao, J., Huang, W.F., Katanić, N., Khalaf, A., Li, T., Snow, J.W. and Solter, L.F., 2025. Nosema or Vairimorpha: genomic/proteomic support to a complex socio-economic issue rooted in taxonomic change. Journal of Invertebrate Pathology, 212, p.108376. 
5. McCallum, R., Olmstead, S., Shaw, J. and Glasgow, K., 2020. Evaluating efficacy of Fumagilin-B® against nosemosis and tracking seasonal trends of Nosema spp. in Nova Scotia honey bee colonies. Journal of Apicultural Science, 64(2), pp.277-286.
6. Emsen, B., Guzman-Novoa, E., Hamiduzzaman, M.M., Eccles, L., Lacey, B., Ruiz-Pérez, R.A. and Nasr, M., 2016. Higher prevalence and levels of Nosema ceranae than Nosema apis infections in Canadian honey bee colonies. Parasitology research, 115(1), pp.175-181.
7. Emsen, B., De la Mora, A., Lacey, B., Eccles, L., Kelly, P.G., Medina-Flores, C.A., Petukhova, T., Morfin, N. and Guzman-Novoa, E., 2020. Seasonality of Nosema ceranae infections and their relationship with honey bee populations, food stores, and survivorship in a North American region. Veterinary sciences, 7(3), p.131.