Vairimorpha (formerly Nosema) disease is caused by the microsporidia parasites, V. ceranae and V. apis, which attack the gut tissue of honey bees. This parasite is one of the most common gut infections in honey bees, and many beekeepers rely on Fumagilin-B ® as treatment. Understanding what fumagillin is and how it functions can allow beekeepers to make informed decisions for managing this disease.

The mode of action for Nosema treatment Fumagillin

Fumagillin has been used by beekeepers since the 1950s to treat V. apis, and it is also successful in controlling V. ceranae ¹. Fumagillin is isolated from the fungus Aspergillus fumigatus and is an effective compound that works against microsporidia ². However, fumagillin on its own is unstable in water and breaks down quickly³. For this reason, commercial products (e.g., Fumagilin-B ®) are formulated as fumagillin dicyclohexylamine. Dicyclohexylamine (DCH) is a salt that increases the stability of the fumagillin compound, so it can be mixed into sugar syrup ³. The DCH portion is not the active ingredient against Vairimorpha, it simply protects fumagillin long enough for bees to consume it.

Figure 1: Fumagilin-B ® Product (Country Fields ©)

Once ingested by bees, fumagillin acts on the parasite’s growing stage, when it replicates inside the midgut ⁷. It works by binding to methionine aminopeptidase-2 (MetAP2), which is an enzyme microsporidia needed to process new proteins ⁴. Fumagillin binds to MetAP2 through a covalent bond, interfering with the parasite's ability to function normally ⁴. Without a working MetAp2 enzyme, the parasite cannot grow in the gut, and spore production drops.

Because fumagillin targets the parasites’ active stage, it does not kill spores directly. Instead, it reduces the number of new spores being produced inside the bee and over time, this lowers the overall spore load in the colony. Fumagillin is most effective when the colony is active, and bees are consuming the treated syrup, which allows the product to reach the midgut where the parasite grows.

Seasonal timing plays an important role in how well fumagillin works. In the Maritimes, colonies are most responsive to treatment in the spring when they are building up their colony and actively taking syrup. This is also the period when spore levels tend to be highest in Canada ^5,6. Treating during this time helps reduce the number of new spores produced inside the bees.

Some beekeepers apply fumagillin in the fall to help reduce infection heading into winter. Fall treatment can lower spore loads before overwintering, but fumagillin does not eliminate spores already present on the comb or in the hive environment ⁷. Research shows that fumagillin’s effect on spore loads is temporary because it only suppresses the actively growing stages of the parasite, once medicated syrup is no longer being consumed, spore levels may rise again if contamination persists. Because Vairimorpha spores are environmentally resistant and can persist on comb and hive surfaces, reinfection can still occur after being treated. Regular monitoring helps confirm whether treatment has been effective and if additional management is needed.

Fumagillin remains a useful tool for managing Vairimorpha spores when applied according to label directions, it can reduce spore production inside infected bees, but it does not kill spores already present in the environment of the hive. Understanding how fumagillin works, spore dynamics and why monitoring matters helps beekeepers make informed decisions about treating bees. 

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:

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References:

1. Higes, M., Nozal, M.J., Alvaro, A., Barrios, L., Meana, A., Martín-Hernández, R., Bernal, J.L. and Bernal, J., 2011. The stability and effectiveness of fumagillin in controlling Nosema ceranae(Microsporidia) infection in honey bees (Apis mellifera) under laboratory and field conditions. Apidologie, 42(3), pp.364-377.
2. Van den Heever, J.P., Thompson, T.S., Curtis, J.M., Ibrahim, A. and Pernal, S.F., 2014. Fumagillin: an overview of recent scientific advances and their significance for apiculture. Journal of agricultural and food chemistry, 62(13), pp.2728-2737.
3. Food and Agriculture Organization of the United Nations & World Health Organization. 2024. Fumagillin dicyclohexylamine: Residue Monograph prepared by the meeting of the Joint FAO/WHO Expert Committee on Food Additives (JECFA), 98th Meeting 2024. FAO JECFA Monographs, 33. Retrieved May 6^th , 2026, from https://www.fao.org/fileadmin/user_upload/vetdrug/docs/1_dd405a226d1d1d0c1921a14367b6c989.pdf
4. Guruceaga, X., Perez-Cuesta, U., Abad-Diaz de Cerio, A., Gonzalez, O., Alonso, R.M., Hernando, F.L., Ramirez-Garcia, A. and Rementeria, A., 2019. Fumagillin, a mycotoxin of Aspergillus fumigatus: biosynthesis, biological activities, detection, and applications. Toxins, 12(1), p.7.
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., 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.
7. Pernal, S.F. and Clay, H. 2013. Honey Bee Diseases & Pests, Third Edition. Canadian Association of Professional Apiculturists, Beaverlodge, AB, Canada, 68 pp.