Nosema in Honey Bees: Symptoms, Diagnosis & Treatment

Nosema in Honey Bees: Symptoms, Why Dysentery Isn't Diagnosis, and What the Research Actually Says

Nosema is the disease beekeepers diagnose most confidently and correctly least often. Brown streaks on the front of the hive in March, a colony that dwindled over winter, and the verdict is in. Except that the streaks usually aren't nosema, the dominant species in the US rarely causes them, and the biggest study ever done on the question found that nosema barely moved the needle on winter loss compared to something else.

The short version

Nosema is a gut microsporidian with two species: N. apis (cold-climate, spring dysentery) and N. ceranae (now dominant, mostly silent, shortens worker life). You cannot diagnose it by eye; it takes a microscope. A 15-year study of 3,502 colonies found nosema's effect on winter loss was statistically real but biologically trivial, and that varroa was the major cause. Get the mite count down with a wash and an oxalic acid vaporizer first; that fixes most "nosema" colonies.

Two Species, Two Diseases

	Nosema apis	Nosema (Vairimorpha) ceranae
Where / when	Cold climates, peaks in late winter and spring	Everywhere, year-round; now the dominant species in the US
Visible signs	Dysentery streaks, crawling bees, swollen abdomens, poor spring buildup	Usually none. Early foraging, shorter worker life, slow dwindling, queen supersedure
Effect	Reduced honey yield and forager lifespan	Suppresses immunity; interacts with viruses, pesticides and poor nutrition

Why "Streaks = Nosema" Is a Myth

Dysentery is bees defecating in or on the hive because they couldn't hold it. Penn State lists the common causes: long confinement, crystallized honey, overly dilute syrup, and fermenting stores. N. apis can add to that, but Randy Oliver, who has counted spores in thousands of samples, reports no particular relationship between nosema and dysentery beyond the fact that dysentery spreads spores around. And N. ceranae, the one your bees almost certainly have if they have any, typically produces no dysentery at all. Streaks tell you the bees were stressed. They don't tell you why.

How Nosema Is Actually Diagnosed

Under a microscope at 400x. Crush a sample of at least 50 older bees from an outer comb in about 1 mL of water per bee, put a drop on a hemocytometer and count the rice-shaped spores. The traditional action threshold is around 1 million spores per bee, though Oliver argues prevalence matters more: fewer than 3 infected bees out of 10 showed no link to wintering trouble, more than half infected is a problem. Telling apis from ceranae takes PCR; the USDA-ARS Beltsville bee lab will do it. Most state apiary programs and many bee clubs have a scope. If nobody has counted spores, nobody has diagnosed nosema.

Fumagillin: Where It Stands

Fumagilin-B was the only registered nosema drug, and its Canadian maker shut down in 2018 when the active ingredient's supplier lost authorization. Vita Bee Health restarted production, and it is again sold over the counter in the US and is not covered by the veterinary feed rules. The caveats are real: it kills the active stage, not spores; it can't be used during a honey flow; it is banned in the EU; and a PLoS Pathogens study found N. ceranae can rebound to higher levels as fumagillin wears off. Most extension services now treat it as a targeted tool for a confirmed heavy infection, not a routine spring feed additive.

What the Big Study Found

A 2023 paper in Communications Biology analyzed 3,502 colonies over 15 years in Germany. N. ceranae was statistically associated with winter loss, but the effect size was below 0.1, which the authors called "significant, but not biologically relevant." The only place nosema mattered on its own was in the 149 colonies that were nearly mite-free. Their conclusion, in the title of the paper's discussion: Varroa destructor is the major cause of colony winter losses.

That matches what we see. The dwindling, streaked, slow-to-build colony in March is almost always a colony whose winter bees were damaged by mites and deformed wing virus the previous August. Nosema rides along on a weakened immune system; it rarely drives.

What To Do

Wash for mites first. A mite wash takes five minutes and answers the question that matters most.
Treat mites on the count. Oxalic acid vaporization, on the schedule for the season. No resistance, works with supers on.
Fix the stress. Good ventilation, dry stores, enough pollen going into fall, a young queen, no old fermenting honey.
Then, if you still suspect nosema, count spores. Treat with fumagillin only on a confirmed heavy infection, per label.

Tools We Use (and Recommend)

InstantVap Oxalic Acid Vaporizer: the cordless, battery-powered oxalic acid vaporizer we use in our own yards to keep mite loads low, which is the single best defense against every problem on this page. See the Lite, Compact, Original and Turbo. Lorob Bees is the primary US seller, with stateside customer service and a warranty we stand behind.
Mite Monitoring: the Easy-Check mite wash. How to do a wash in five minutes.
Oxalic Acid: EPA-registered EZ-OX.

Happy Beekeeping from the Lorob Bees Team

Written by Rob Pollock

Rob keeps bees in Baltimore, Maryland, and owns Lorob Bees. He built and repaired his own corded oxalic acid vaporizers for years before becoming the primary US seller of the InstantVap, and he still picks up the phone when one needs service. Oxalic acid vaporization is the thing he cares most about in beekeeping, because it's the treatment that keeps hives alive without breeding resistant mites. More about Lorob Bees.

Keep reading: Explore all Lorob Bees beekeeping guides — varroa, oxalic acid vaporization, pests and diseases, all in one place.

Sources

Penn State Extension, Nosema Disease of Honey Bees
Gray et al. / Genersch group, Significant, but not biologically relevant: Nosema ceranae infections and winter losses, Communications Biology, 2023
Randy Oliver, Scientific Beekeeping, Sick Bees Part 13: Simple Microscopy of Nosema
Huang W.-F. et al., Nosema ceranae Escapes Fumagillin Control in Honey Bees, PLoS Pathogens, 2013
American Bee Journal, Life Without Fumagilin, 2018

General beekeeping information, not a substitute for the product label. Always read and follow the label of any product you use in a hive.