Nosema Disease: The Silent Gut Infection Weakening Your Bees

If you've lost colonies over winter and couldn't figure out why, there's a good chance Nosema was involved. This microsporidian gut parasite infects adult bees worldwide and is implicated in colony losses ranging from gradual decline to sudden spring "dwindling." The challenge? Infected colonies often look normal until it's too late.

What Is Nosema?

Nosema is a fungal gut parasite (technically a microsporidian) that infects the midgut lining of adult honey bees. When bees ingest Nosema spores—typically through contaminated food, water, or by cleaning infected comb—the spores germinate in the gut and begin reproducing inside the cells lining the digestive tract.

The infection damages the bee's ability to digest food and absorb nutrients. Infected bees:

• Live significantly shorter lives (often 50% reduced lifespan)
• Cannot properly digest protein, leading to malnutrition
• Have reduced hypopharyngeal gland function (critical for producing royal jelly)
• Show impaired foraging ability and orientation
• In queens: reduced egg-laying and premature supersedure

Because the infection is internal and symptoms are often subtle, heavily infected colonies can appear "normal" during inspections while actually declining rapidly.

Two Species: N. apis vs N. ceranae

There are two Nosema species that infect Western honey bees, and they behave quite differently:

The important takeaway: N. ceranae is now the dominant species in most of the world, and it's harder to detect because infected bees often don't show the classic dysentery symptoms associated with traditional Nosema disease. Colonies can carry heavy infections while looking superficially healthy.

Signs & Symptoms

Nosema symptoms range from obvious to nearly invisible. Here's what to watch for:

Classic Symptoms (More Common with N. apis)

Subtle Symptoms (Common with N. ceranae)

How to Diagnose Nosema

Visual symptoms are suggestive but not diagnostic. The only way to confirm Nosema infection is to look for spores under a microscope or send samples to a diagnostic lab.

DIY Microscopic Examination

If you have access to a compound microscope (400x magnification), you can check for Nosema spores yourself:

1. Collect sample: Capture 20-30 older forager bees from the hive entrance (foragers have highest spore loads).
2. Remove abdomens: Pull off the abdomens and place them in a ziplock bag or small container.
3. Crush and dilute: Add 1 mL of water per bee abdomen. Crush thoroughly to create a suspension.
4. Prepare slide: Place a drop of the suspension on a microscope slide, add coverslip.
5. Examine at 400x: Look for oval, rice-grain-shaped spores. They're highly refractive (appear bright/shiny).

Lab Testing

If you don't have a microscope, several options exist:

• USDA Bee Research Labs — Free Nosema testing for beekeepers. Send samples to the Beltsville, MD lab. Results in 1-2 weeks.
• State Apiarist — Many states offer free or low-cost disease testing through the state agriculture department.
• University Extension — Some land-grant universities provide bee disease diagnostics.
• Private Labs — Companies like Vita Bee Health or various veterinary labs offer paid testing with faster turnaround.

Treatment Options

Treatment options for Nosema are limited compared to other bee diseases. Here's the current landscape:

Fumagilin-B (Fumagillin)

Fumagillin is the only antibiotic approved for treating Nosema in honey bees. It's been the standard treatment for decades.

Alternative Approaches

Given Fumagilin-B's uncertain future, many beekeepers focus on management-based approaches:

• Thymol-based products — Some research suggests thymol (found in products like ApiGuard) may have some activity against Nosema, though it's not approved for this use.
• Oxalic acid — Primarily a Varroa treatment, some studies show modest effects on Nosema, but evidence is inconsistent.
• Probiotics — Experimental research on bee gut probiotics shows promise but no commercial products are proven effective yet.
• Essential oils — Various essential oils have been tested; results are mixed and nothing is reliably effective at field scale.

Prevention Strategies

Since treatment options are limited, prevention and management are your best tools. Here's what works:

1. Good Nutrition

Well-fed bees resist Nosema better than nutritionally stressed bees. Ensure:

• Adequate pollen/protein sources (natural forage or supplemental patties)
• Sufficient honey stores going into winter
• Supplemental feeding when natural forage is scarce

2. Reduce Moisture & Improve Ventilation

Nosema spores spread through fecal contamination, which increases when bees defecate inside the hive (due to confinement and moisture). Good ventilation reduces humidity and allows cleansing flights.

• Ensure upper ventilation (notched inner cover, ventilation box, or similar)
• Use screened bottom boards in mild climates
• Position hives in sunny locations with morning sun exposure
• Tilt hives slightly forward so moisture drains out the entrance

3. Young Queens

Colonies with young, vigorous queens build up faster and replace infected workers more quickly. Requeen every 1-2 years, especially if you've had Nosema problems.

4. Comb Rotation

Old, dark brood comb can harbor Nosema spores. Rotate out old comb (3-5 year cycle) and replace with new foundation. This is good practice for multiple disease and pest reasons.

5. Don't Share Equipment from Infected Colonies

Nosema spores can survive on comb and equipment. If a colony dies with suspected Nosema, don't redistribute that equipment to healthy colonies without cleaning. Spores can be killed by:

• Acetic acid fumigation (80% concentration, professional use)
• Heat treatment (120°F / 49°C for 24 hours)
• Freezing (doesn't kill spores but prevents spread of live bees carrying infection)
• Irradiation (commercial operations only)

6. Control Varroa Mites

Research shows that Varroa mite infestation increases Nosema severity. Bees weakened by Varroa (and the viruses mites vector) are more susceptible to Nosema infection and less able to recover. Effective mite management indirectly helps with Nosema control.

Understanding the Disease Lifecycle

Knowing how Nosema spreads helps you interrupt the cycle:

1. 1

   Spore Ingestion

   Bee ingests spores while cleaning contaminated comb, consuming contaminated food/water, or during trophallaxis (food sharing) with infected bees.

2. 2

   Spore Germination

   In the midgut, spores evert a polar filament that penetrates gut epithelial cells, injecting the parasite's contents.

3. 3

   Reproduction

   The parasite multiplies inside gut cells, eventually destroying them. One spore can produce millions of new spores within 10-14 days.

4. 4

   Spore Release

   Mature spores are released when gut cells rupture. They're shed in feces, contaminating the hive environment.

5. 5

   Environmental Contamination

   Spores accumulate on comb, in honey stores, and on hive surfaces. House bees cleaning cells become infected, continuing the cycle.

Key insight: The cycle accelerates when bees are confined (winter, rainy periods) because they can't take cleansing flights and feces accumulates inside the hive. Spring "outbreaks" are often the culmination of winter-long spore buildup.

When to Take Action

Not every Nosema-positive colony needs intervention. Here's a decision framework:

Spore Count Thresholds

If you're doing microscopic analysis, here are general guidelines (spores per bee):

• < 1 million spores/bee: Light infection, likely manageable
• 1-10 million spores/bee: Moderate infection, take preventive action
• > 10 million spores/bee: Heavy infection, treatment recommended if available

Most labs report results in these terms. Keep in mind that spore counts fluctuate seasonally—spring samples typically show higher counts than fall samples.

Living with Nosema: A Practical Approach

Nosema is endemic—it's everywhere, and most colonies carry some level of infection. The goal isn't to eliminate it entirely (that's unrealistic) but to keep infection levels low enough that colonies thrive despite it.

Here's the practical approach for most beekeepers:

1. 1 Test periodically. Send samples to your state apiarist or USDA lab, especially if colonies are underperforming without obvious cause.
2. 2 Focus on prevention. Good nutrition, ventilation, young queens, and comb rotation do more than any treatment.
3. 3 Control Varroa aggressively. Mite-weakened bees are Nosema's best friend.
4. 4 Treat with Fumagilin-B if available and warranted. Fall treatment before winter confinement is most effective timing.
5. 5 Don't panic. Nosema is common, and most colonies manage it fine with good management. It's one factor among many affecting colony health.