A grounded synthesis of the most-cited open-access papers on honey bee microsporidia. Every claim is traceable to a cited study; curated overview, not exhaustive.
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Nosema — obligate intracellular microsporidian parasites that infect the midgut epithelium of adult honey bees — are among the most prevalent bee pathogens worldwide. Two species affect honey bees: Nosema apis, long known from Apis mellifera, and Nosema ceranae, originally a parasite of the Eastern honey bee Apis cerana that has crossed into and now largely dominates Western honey bee populations (Chen 2009). (The genus has recently been reclassified within Vairimorpha.)
A striking epidemiological shift has occurred: N. ceranae appears to be replacing N. apis globally in A. mellifera, implying a competitive advantage (Milbrath 2015). The two coexist asymmetrically, each able to cross-infect the other's original host, and long-term surveillance documents the changing prevalence of the two species over time (Chen 2009; Gisder 2017).
Both species damage the midgut and impose physiological costs on individual workers, with comparative studies measuring their relative virulence and effects on host survival in each host species (Sinpoo 2018; Milbrath 2015). Gut proteomic analyses of infected bees reveal how infection disrupts digestive and metabolic function along the intestine (Houdelet 2021).
A central puzzle is that Nosema's impact varies widely between regions and studies, from severe to negligible — part of why management is difficult (Holt 2016). Some of this variability reflects interactions: Nosema co-parasitises bees alongside Varroa, and the combined burden depends on the bees' defence level (Bahreini 2015). Nosema also interacts synergistically with viruses (see the multi-stressor model).
Nosema is best understood as a context-dependent stressor: prevalent and capable of harm, but with an impact that depends on species, region, host condition and co-infection. Management is correspondingly nuanced — historically reliant on the antibiotic fumagillin, and complicated by the species shift and inconsistent virulence (Holt 2016).
Chen et al., Journal of Invertebrate Pathology 2009 · 88 citations — Documents cross-infection and the spread of N. ceranae into A. mellifera.
Milbrath et al., Journal of Invertebrate Pathology 2015 · 55 citations — Evidence that N. ceranae is competitively replacing N. apis.
Sinpoo et al., Journal of Insect Physiology 2018 · 51 citations — Comparative effects on host survival and physiology.
Gisder et al., Frontiers in Cellular and Infection Microbiology 2017 · 40 citations — A decade-scale view of the species shift.
Holt et al., Journal of Economic Entomology 2016 · 29 citations — Why mixed virulence makes management hard.
Added 2026-06-23 from a scan of recent, lightly-cited papers — see Research Frontier for the full review and caveats. These are recent single studies; treat as leads, not settled fact.
Curated synthesis of representative and most-cited studies — not exhaustive. Explore via search. Related: Pests overview · Multi-stressor model.