Dysentery vs Nosema in Bees: How to Tell Them Apart

Reviewed by Michael R. · Last Updated September 24, 2026 · How we review

Dysentery is a symptom and nosema is a disease, and they are not the same thing. Dysentery is bees soiling the inside and front of the hive with brown feces because they have held more waste and water than they can manage without a cleansing flight. Nosema is an infection of the bee’s midgut by a microscopic fungal parasite, Nosema apis or Nosema ceranae. The two can occur together, and N. apis is linked with dysentery, but either can happen without the other. You can’t tell which you have by looking at the stains. Only a microscope or a lab test can tell you.

That matters most in a northern winter. In eastern North Dakota and western Minnesota, bees can go months without a flying day, so spring streaking is common. This guide explains what actually causes dysentery, how the two nosema species differ, how to test, and what the current treatment situation is.

Key Takeaways

• Dysentery is a symptom, usually caused by long confinement, watery or poor winter stores, or syrup that is too dilute. Nosema is a parasite of the gut.
• Nosema apis is associated with dysentery and bees crawling at the entrance. Australia’s BeeAware notes that neither has been observed with Nosema ceranae, which Penn State Extension says is now the predominant species in the US.
• Fecal streaks cannot confirm or rule out nosema. The USDA’s diagnostic handbook says a positive diagnosis needs a microscope.
• The only nosema treatment sold for bees, Fumagilin-B, is labeled for Nosema apis. Its US label says it has not been found by FDA to be safe and effective. Research suggests it can backfire against N. ceranae.
• Most winter dysentery is prevented in the fall: thick syrup, dry stores, a well-sized cluster, and good moisture control.

Table of Contents

• Dysentery vs Nosema at a Glance
• What Causes Dysentery
• What Nosema Is: Apis vs Ceranae
• Does Nosema Cause Dysentery?
• How to Tell Which One You Have
• Treatment: What Fumagilin-B Can and Cannot Do
• Preventing Winter Dysentery in the Northern Plains
• Frequently Asked Questions

Dysentery vs Nosema at a Glance

The season patterns follow Penn State Extension’s nosema disease fact sheet by Robyn Underwood and Brenna Traver. The species contrasts follow Australia’s BeeAware nosema page.

What Causes Dysentery

Bees keep their hive clean by holding their feces until they can fly outside. Through a Dakota winter that can mean weeks or months of holding it. As winter bees eat stored honey, they pile up waste in the rectum along with water from their food. When the load gets too big, or their stores are too watery, they let go inside the hive or on the front of the boxes. The result is the familiar brown streaks and spots on the top bars, the inner cover, and around the entrance.

However, nosema disease is not the sole cause of dysentery. Long periods of confinement that prevent cleansing flights, consumption of too much moisture due to honey crystallization, or feeding sugar syrup that is too dilute can also cause dysentery (Alfonsus 1935).

Robyn Underwood and Brenna Traver, Penn State Extension, Nosema Disease of Honey Bees

Randy Oliver’s series on preventing dysentery treats it mainly as a water-balance problem. The winter cluster has to get rid of the water produced as bees burn honey. If it can’t, through respiration, brood rearing, or flights, the gut load builds. That is why dysentery tends to show up:

• after a long run of days too cold to fly, then clears quickly after the first good cleansing flight;
• when colonies wintered on syrup that was too thin, or on stores with too much water;
• when moisture condenses and drips back on the cluster. Our guide to reducing moisture in winter hives covers this;
• when late brood rearing or disturbance raises the cluster’s food and water intake.

Streaking that clears up after the first warm, sunny day and a mass cleansing flight is almost always plain confinement dysentery.

What Nosema Is: Apis vs Ceranae

Nosema disease is caused by single-celled, spore-forming parasites called microsporidia. Bees swallow the spores, which germinate in the midgut and multiply inside the cells lining it. New spores then pass out in the feces to infect other bees. Infected bees digest food poorly, age faster, and die younger. The colony shrinks and builds up slowly in spring. Taxonomists have proposed moving both species into the genus Vairimorpha (Tokarev and colleagues, 2020), so you may see Vairimorpha ceranae in newer papers. It is the same organism.

Nosema apis

This is the long-established species in European honey bees. Penn State Extension says infections typically peak in spring, with a smaller peak in fall, and that N. apis-infected bees are often found crawling in front of the hive. BeeAware describes dysentery as a common symptom of N. apis infection. The USDA handbook notes that the heaviest infections turn up in confined bees, such as winter bees and package bees.

Nosema ceranae

This species came from the Asian honey bee. Penn State Extension says it is now the predominant species found in the US, though whether it is actually displacing N. apis is still debated. Its effects are quieter: shortened worker lifespan, earlier foraging, and a slowly dwindling colony. BeeAware says the dysentery and crawling associated with N. apis have not been observed with N. ceranae. So a colony with a serious ceranae infection can have a spotless hive front.

Does Nosema Cause Dysentery?

This is where references disagree, and the old version of this page got muddled.

• Extension view. Penn State calls fecal streaking the quintessential symptom of nosema disease, while also saying nosema is not the only cause of dysentery.
• Skeptical view. BeeAware says dysentery is not actually caused by the fungus, though N. apis infection makes bees more susceptible to it. Randy Oliver, writing in the American Bee Journal, and Rusty Burlew of Honey Bee Suite go further. In “Repeat after me: Nosema does not cause dysentery”, Burlew argues that the two often occur together without one causing the other.

Either way, the practical answer is the same. Dysentery is a weak signal for N. apis and tells you nothing about N. ceranae. Heavy N. apis infections do often come with dysentery, especially in confined bees, so a colony that streaks, dwindles, and has crawling bees in spring is worth testing. A colony that streaks once and recovers after a flight usually is not.

How to Tell Which One You Have

However, positive diagnosis can be made only by microscopic examination of suspect bees or their fecal material for the presence of N. apis spores (appendix A).

H. Shimanuki and D. A. Knox, USDA Agricultural Research Service, Diagnosis of Honey Bee Diseases (Agriculture Handbook 690)

Step 1: Collect the right bees

Nosema lives in older bees, so take foragers from the entrance or from the outer frames, not young nurse bees.

Step 2: Look under a microscope

The USDA handbook describes a quick check: grind the abdomens of 10 or more bees in water and look for the large, oval spores under the microscope, at about 400x. Our step-by-step guide to testing for nosema spores at home covers equipment and counting. Penn State notes that a threshold of about 1 million spores per bee is widely used for deciding to act, but that the figure is anecdotal.

Step 3: Send it out to learn the species

A microscope finds spores but cannot reliably tell apis from ceranae. That takes PCR. In the Red River Valley, the National Agricultural Genotyping Center in Fargo runs a honey bee pathogen panel that separates Nosema apis from Nosema ceranae. The North Dakota Department of Agriculture points beekeepers to it. Our guide to laboratory testing for bee diseases covers other labs.

Treatment: What Fumagilin-B Can and Cannot Do

There is no treatment for dysentery itself. You fix the cause. For nosema, the only product sold for bees is fumagillin, marketed as Fumagilin-B. Its availability has been shaky. The previous manufacturer, Medivet, closed in 2018 and supplies ran out for a time. Vita Bee Health later brought production back in Canada. The current US label, on the National Library of Medicine’s DailyMed (label information updated July 2024), says:

• the drug “has not been found by FDA to be safe and effective,” and the labeling has not been approved by FDA;
• it is indicated as an aid in preventing nosema, and the product description refers to Nosema apis;
• it is fed in syrup in spring or fall, and treatments must be finished at least 4 weeks before the main honey flow to avoid residues in honey.

There is also a real concern about N. ceranae. A 2013 study in PLoS Pathogens by Huang and colleagues found that as fumagillin broke down to lower levels, N. ceranae spore production rose by up to 100% above untreated infected bees. The authors concluded that the usual way of applying it “may exacerbate N. ceranae infection rather than suppress it.” So test first, and learn the species if you can, before you buy a treatment. Don’t medicate a colony because its hive front is streaked. For the wider decision around new colonies, see should you medicate package bees.

Management helps with both species: replace old, fouled brood comb on a rotation, keep colonies strong and well fed on pollen, and control Varroa. Our guide to how often to replace old brood comb sets out a schedule.

Preventing Winter Dysentery in the Northern Plains

• Feed thick syrup early. Fall syrup should be heavy (2:1 sugar to water by weight is the usual target) and fed early enough for the bees to evaporate and cap it before cold weather. Penn State lists syrup that is too dilute as a cause of dysentery.
• Control condensation. Moisture dripping on the cluster adds to the water load. Compare a moisture board with a quilt box, and consider an upper entrance.
• Keep the cluster the right size. Go into winter with a strong colony in a box it can cover, not a weak one rattling around in a lot of empty space.
• Keep flight paths open. Clear snow from entrances so bees can take a flight on the first mild, sunny day.
• Don’t disturb them. Opening a winter cluster raises its food intake and water load.
• Check after the first flight. If streaking continues after good flying weather, or you see crawling bees and a dwindling cluster, test for nosema.

Beekeepers who winter colonies in a controlled-temperature building face a different version of the same problem: long confinement with no flights at all. See indoor overwintering for how that is managed.

Frequently Asked Questions

Is dysentery the same as nosema?

No. Dysentery is bees soiling the hive with feces. Nosema is a gut parasite. Nosema apis can come with dysentery, but most winter dysentery comes from long confinement and a heavy water load. Nosema ceranae infections are not associated with dysentery.

Can you tell nosema from dysentery by the fecal streaks?

No. The streaks look the same whatever the cause. The USDA handbook says positive diagnosis needs microscopic examination of bees or their feces for spores.

What does nosema ceranae look like in a hive?

Often like nothing obvious. Colonies dwindle, build up slowly, and lose foragers in the field, but without the dysentery and crawling bees seen with Nosema apis.

Should I feed Fumagilin-B if my bees have dysentery?

Not on the strength of dysentery alone. Test first. The US label says it has not been found by FDA to be safe and effective, it is labeled for Nosema apis, and research suggests it may worsen Nosema ceranae infections as it breaks down.

Will a colony with dysentery recover?

Usually, if the cause is confinement. Streaking typically stops after a few good cleansing flights. Watch for dwindling, crawling bees, or streaking that continues in good flying weather, and test those colonies.

How can I prevent dysentery in winter?

Feed thick syrup early in fall so it is capped before cold weather. Control condensation, winter a strong cluster in a box it can cover, keep entrances clear of snow, and avoid opening the hive in the cold.

The Bottom Line

Brown streaks on a hive in March tell you the bees were confined too long or their stores were too wet. They don’t tell you the colony has nosema. Nosema apis can cause dysentery, Nosema ceranae usually doesn’t, and only a microscope or PCR test will tell you what you have. Prevent the dysentery in the fall, test before you medicate, and use Fumagilin-B strictly by its label.