# Nosema bombi
**Nosema bombi** is an obligate intracellular microsporidian parasite (a fungus-related single-celled pathogen) that infects bumble bees of the genus *Bombus*, reducing colony survival and reproduction. First described in 1914, it is now formally named *Vairimorpha bombi*, although much of the scientific literature still uses the original name.
| Key fact | Detail |
|---|---|
| Original description | Fantham & Porter, 1914, in *Annals of Tropical Medicine and Parasitology* 8: 623–638, named after the type host *Bombus agrorum*[1](https://www.tandfonline.com/doi/pdf/10.1080/03014223.1995.9518020) |
| Current name | *Vairimorpha bombi* (Fantham & A. Porter) Tokarev, Huang, Solter, Malysh, Becnel & Vossbrinck, 2020, *Journal of Invertebrate Pathology*[2](https://www.speciesfungorum.org/names/NamesRecord.asp?RecordId=522717) |
| Host range | Reported from more than 50 species of bumble bees; appears specific to the genus *Bombus*[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4818719/) |
| Fitness effects | Early colony infection reduced the functional fitness of males and young queens to zero in laboratory experiments[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705) |
| Field effect | None of 14 infected *B. terrestris* colonies produced sexual offspring, versus 4 of 14 control colonies[5](https://resjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.2008.00998.x) |
| Commercial link | Commercial hives had markedly higher frequencies of *N. bombi* than adjacent wild populations in Ireland[6](https://www.sciencedirect.com/science/article/pii/S0006320712004417) |
| North American history | Prevalence rose from low detectable frequency in the 1980s to significantly higher frequency in the mid- to late-1990s, with no conclusive genetic evidence of an exotic origin[7](https://pubmed.ncbi.nlm.nih.gov/27044096/) |
## Taxonomy and description
The first microsporidian reported from bumble bees was described by Fantham and Porter, who gave it the name *Nosema bombi* in reference to its type host, *Bombus agrorum*; the parasite was also seen in *B. terrestris* and other hosts[1](https://www.tandfonline.com/doi/pdf/10.1080/03014223.1995.9518020). A later re-examination using light and electron microscopy supported its validity as a species separate from *[Nosema apis](https://www.edgepedia.org/nosema-apis)*, the honey bee parasite[1](https://www.tandfonline.com/doi/pdf/10.1080/03014223.1995.9518020).
The classification changed in 2020, when Tokarev and colleagues placed the *Nosema* species that infect bees within a new genus, *Vairimorpha*; the current name is *Vairimorpha bombi*, and Species Fungorum records the generic name *Nosema* in this combination as no longer applying to an organism within the fungal clade[2](https://www.speciesfungorum.org/names/NamesRecord.asp?RecordId=522717) • [8](https://pmc.ncbi.nlm.nih.gov/articles/PMC7302437/). Microsporidia are unicellular intracellular parasites closely related to fungi, characterized by resistant spores and a unique coiled polar tubule used to inject sporoplasm into a host cell[9](https://www.cdc.gov/dpdx/microsporidiosis/). In general, microsporidians have two major life cycle stages, a spore stage and a vegetative stage[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705).
## Hosts and transmission
*N. bombi* infects different bumble bee species at a substantial, though variable, rate[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705). It has been found in species including *B. terrestris*, *B. lapidarius*, *B. haemorrhoidalis*, *B. montivagus*, and *B. breviceps*[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11768362/). Transmission usually occurs when bees consume spores from contaminated food or during mating[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11768362/). Because bumble bee colonies have an annual life cycle and only the queens survive the winter, the pathogen can continue to reproduce and spread after colonies have disappeared[10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11768362/).
## Effects on host fitness
**Laboratory experiments.** Otti and Schmid-Hempel experimentally infected colonies of *Bombus terrestris* with *N. bombi* spores at the start of worker production, with uninfected colonies as controls; 11 of 16 experimental colonies were successfully infected[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705). When infection occurred at an early stage of colony development, virtually all individuals were infected, spores were found in a number of tissues, and the functional fitness of males and young queens was reduced to zero; worker survival and the survival of infected males were also reduced[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705).
**Field experiments.** In a subsequent field study using wild-caught *B. terrestris* spring queens, colonies founded by queens carrying the infection were markedly reduced in size compared with colonies from uninfected queens; every one of the 14 infected colonies failed to yield sexual offspring, while male production was attained in four of the 14 control colonies[5](https://resjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.2008.00998.x). The authors concluded that the parasite's effects are amplified under natural conditions, and that by reducing host fitness so severely, *N. bombi* appears to decrease its own opportunities for transmission to the next host generation[4](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705) • [5](https://resjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.2008.00998.x).
Among North American bees suffering chronic infections, researchers have observed smaller colonies, males whose sperm counts are lowered, poorer survival through hibernation and success in establishing colonies, a decline in the number of reproductive females, and females with diminished mating ability[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4818719/). Higher prevalences have been reported in several North American species with apparently declining populations, including *B. occidentalis* and *B. pensylvanicus*, although cause and effect has not been established[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC4818719/) • [10](https://pmc.ncbi.nlm.nih.gov/articles/PMC11768362/).
## Commercial bees and pathogen spillover
Commercially reared bumble bees used for greenhouse pollination often carry higher pathogen levels than wild bees, and *N. bombi* is one of the parasites found to be far more abundant in commercial than in wild bumble bees[11](https://www.sciencedirect.com/science/article/pii/S0006320705004994). In Ireland, commercial *B. terrestris* hives had markedly higher frequencies of the gut parasites *Crithidia* spp. and *N. bombi* compared to adjacent wild populations, and the probability of infection declined with distance from greenhouses up to 10 km in a host sex- and pathogen-specific manner[6](https://www.sciencedirect.com/science/article/pii/S0006320712004417). In Canada, bumble bees collected near commercial greenhouses were more frequently infected by the flower-transmitted parasites *C. bombi* and *N. bombi* than bees collected away from greenhouses[11](https://www.sciencedirect.com/science/article/pii/S0006320705004994).
Pathogen (*N. bombi*) outbreaks have apparently decimated commercial *B. occidentalis* rearing, resulting in the widespread use of *B. impatiens* throughout North America[12](https://pmc.ncbi.nlm.nih.gov/articles/PMC2464710/). Proposed responses include national best management practices for users of commercial pollinators combined with international legislation regulating the sanitation of commercial hives in production facilities[6](https://www.sciencedirect.com/science/article/pii/S0006320712004417).
## The invasive pathogen hypothesis in North America
Shrinking or disappearing bumble bee populations in the United States have been ascribed principally to a hypothesized invasive, virulent strain of *N. bombi* introduced from Europe in the early 1990s via commercial colony development, the "European Nosema invasion hypothesis"[13](https://pmc.ncbi.nlm.nih.gov/articles/PMC4843438/). To test this hypothesis, researchers extracted *Nosema* DNA from museum specimens gathered between 1980 and 2011, and discovered that the pathogen rose markedly in prevalence, moving from barely detectable levels in the 1980s to a much greater frequency by the mid- to late-1990s, matching the era when a large-scale epidemic of *N. bombi* was reported in commercially reared bumble bee stocks in North America[7](https://pubmed.ncbi.nlm.nih.gov/27044096/). However, genetic analysis of global *Nosema* populations found no conclusive evidence of an exotic origin: the widespread strain found in declining United States bumble bees was present in the United States before commercial colony trade[7](https://pubmed.ncbi.nlm.nih.gov/27044096/). *N. bombi* has consequently been described as part of the natural pathogen load in North American bumble bee populations[14](https://www.nrcs.usda.gov/sites/default/files/2024-02/NEPP%20Biological%20Assessment%2C%20Opinion%2C%20Conference%20Report%2C%20and%20Partnership%20Agreement.pdf).
A separate line of evidence concerns pesticides: greater usage of the fungicide chlorothalonil was the best predictor of *N. bombi* prevalence in four declining US bumble bee species, suggesting an interaction between pathogen prevalence, fungicide use, and bumble bee declines[15](https://pubmed.ncbi.nlm.nih.gov/29142119/).
## Prevalence in wild populations
Prevalence varies widely by region and species. In West Siberia, prevalence of *N. bombi* infection varied from 4 to 20% depending on the region[16](https://pubmed.ncbi.nlm.nih.gov/26063531/). In northern Thailand, a survey of 280 female bumble bees found *N. bombi* in 38 individuals (13.57%), with infection rates higher in *B. breviceps* than in other species, and *N. bombi* more prevalent than *N. ceranae* across the three species sampled[17](https://pmc.ncbi.nlm.nih.gov/articles/PMC6405097/). In a broad survey of plant-pollinator communities, 0.3% of individual bees were positive for *N. bombi*, with *Bombus bimaculatus* showing the highest prevalence among well-sampled species at 1.8%[18](https://pmc.ncbi.nlm.nih.gov/articles/PMC7529964/). In Slovenia, PCR screening of bumble bee workers detected *N. bombi* in 16.3% of individuals[19](https://pubmed.ncbi.nlm.nih.gov/34358034/).
## Diagnosis
A PCR-based method using ribosomal RNA gene fragments was developed for specific and sensitive diagnosis of *N. bombi*. The primer pair Nbombi-SSU-Jf1/Jr1 could distinguish *N. bombi* from the related honey bee microsporidia *N. apis* and *N. ceranae*, and testing 99 bumble bees showed PCR diagnosis with a combination of two primer pairs to be more accurate than light microscopy, with detection of all developmental stages[20](https://pubmed.ncbi.nlm.nih.gov/16376373/).
## References
1. [Fantham & Porter (1914, republished account). Nosema bombi, a microsporidian pathogen of the bumble bee Bombus terrestris (L.), New Zealand Journal of Zoology](https://www.tandfonline.com/doi/pdf/10.1080/03014223.1995.9518020)
2. [Species Fungorum name record: Nosema bombi Fantham & A. Porter (current name Vairimorpha bombi)](https://www.speciesfungorum.org/names/NamesRecord.asp?RecordId=522717)
3. [Stentiford et al. Microsporidia – Emergent Pathogens in the Global Food Chain, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4818719/)
4. [Otti & Schmid-Hempel (2007). Nosema bombi: A pollinator parasite with detrimental fitness effects. Journal of Invertebrate Pathology 96: 118–124](https://www.sciencedirect.com/science/article/abs/pii/S0022201107000705)
5. [Otti & Schmid-Hempel (2008). A field experiment on the effect of Nosema bombi in colonies of the bumblebee Bombus terrestris. Ecological Entomology 33: 577–582](https://resjournals.onlinelibrary.wiley.com/doi/10.1111/j.1365-2311.2008.00998.x)
6. [Murray et al. (2013). Pathogen prevalence in commercially reared bumble bees and evidence of spillover in conspecific populations. Biological Conservation 159: 269–276](https://www.sciencedirect.com/science/article/pii/S0006320712004417)
7. [Cameron et al. (2016). Test of the invasive pathogen hypothesis of bumble bee decline in North America. PNAS](https://pubmed.ncbi.nlm.nih.gov/27044096/)
8. [A growing pandemic: A review of Nosema parasites in globally distributed domesticated and native bees, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7302437/)
9. [CDC DPDx – Microsporidiosis](https://www.cdc.gov/dpdx/microsporidiosis/)
10. [The Role of Pathogens in Bumblebee Decline: A Review, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC11768362/)
11. [Otterstatter & Thomson (2006). Plight of the bumble bee: Pathogen spillover from commercial to wild populations. Biological Conservation 129: 461–467](https://www.sciencedirect.com/science/article/pii/S0006320705004994)
12. [Otterstatter & Thomson (2008). Does Pathogen Spillover from Commercially Reared Bumble Bees Threaten Wild Pollinators? PLoS ONE, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC2464710/)
13. [Test of the invasive pathogen hypothesis of bumble bee decline in North America, PMC full text](https://pmc.ncbi.nlm.nih.gov/articles/PMC4843438/)
14. [USDA NRCS Region 5 Pollinator Biological Assessment](https://www.nrcs.usda.gov/sites/default/files/2024-02/NEPP%20Biological%20Assessment%2C%20Opinion%2C%20Conference%20Report%2C%20and%20Partnership%20Agreement.pdf)
15. [McArt et al. (2017). Landscape predictors of pathogen prevalence and range contractions in US bumblebees (PubMed)](https://pubmed.ncbi.nlm.nih.gov/29142119/)
16. [Distribution and diversity of Nosema bombi in natural populations of bumblebees from West Siberia (PubMed)](https://pubmed.ncbi.nlm.nih.gov/26063531/)
17. [Prevalence of infection by the microsporidian Nosema spp. in native bumblebees in northern Thailand, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC6405097/)
18. [Dominant bee species and floral abundance drive parasite temporal dynamics in plant-pollinator communities, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7529964/)
19. [The Pathogens Spillover and Incidence Correlation in Bumblebees and Honeybees in Slovenia (PubMed)](https://pubmed.ncbi.nlm.nih.gov/34358034/)
20. [Specific and sensitive detection of Nosema bombi in bumble bees by PCR of partial rRNA gene sequences (PubMed)](https://pubmed.ncbi.nlm.nih.gov/16376373/)
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*Topic: Encyclopedia › Life and health › Animals › Invertebrates › Arthropods › Insects › Bees, wasps, and ants › Aculeata: bees, wasps, and ants › Bees (Anthophila) and apiculture › Bee ecology and life histories › Bumblebees (Bombus) › Bumblebee parasites, predators, and diseases*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 11, 2026; Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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