Nosema: The Silent Stressor in the Hive Part 2: Viral Load, Colony Collapse, and What We Might Be Missing

In the mid-2000s, beekeepers across the U.S. began reporting something strange. Colonies weren’t just dying—they were vanishing. One day a hive looked strong, and the next, it was nearly empty. The phenomenon was eventually labeled Colony Collapse Disorder (CCD), and while the exact cause is still debated, one thing has become increasingly clear: CCD isn’t caused by a single factor—it’s the result of multiple stressors piling up.

A Quiet Catalyst for Collapse

We now know that Nosema doesn’t always kill bees directly. What it does is weaken them. It damages the midgut lining, shortens lifespans, suppresses immune function, and disrupts digestion. Infected bees often can’t absorb enough nutrients—even when food is plentiful.

But the real danger is what happens next.

Recent research shows that colonies with high Nosema loads often have elevated viral loads as well. Deformed Wing Virus (DWV), Black Queen Cell Virus (BQCV), Sacbrood Virus (SBV), and others tend to spike when Nosema is present. That’s because a weakened immune system gives viruses the perfect opportunity to replicate—and spread.

In one study, bees infected with Nosema ceranae were significantly more likely to carry multiple viruses at once, and those viruses tended to reach higher concentrations. The link between Nosema and viral outbreaks is now so well-established that some researchers consider Nosema a “gateway” pathogen—not always lethal on its own, but dangerous because of what it allows in.

Connecting the Dots: Nosema and CCD

Colony Collapse Disorder is defined by a very specific set of symptoms: rapid loss of adult bees, an abandoned queen with brood still present, and few (if any) dead bees inside or outside the hive. It doesn’t look like a typical crash.

While CCD cases have declined since their peak, CCD-like events still happen—especially in colonies under heavy stress. Nosema, especially N. ceranae, has been repeatedly found at high levels in CCD-affected colonies. Whether it’s a trigger or just a contributing factor is still being studied, but the pattern is hard to ignore:

     •           High Nosema = High viral load

     •           High viral load = immune breakdown

     •           Immune breakdown = colony collapse

Chart: Nosema & Virus Loads (April 2025)

Higher Nosema levels are linked with higher viral counts—especially DWV and Satellite Virus, as seen in sample P91. Data from BVS Inc.

The challenge is that Nosema infections don’t always show symptoms. You won’t see bees with deformed wings or piles of dead workers. Often, all you see is a hive that fails to thrive—or quietly disappears.

And Nosema might be one of the biggest—and most overlooked—pieces of that puzzle.

So What Can You Do?

The first step is awareness. If your bees aren’t building up as expected—or if a colony crashes without warning—test for Nosema. Knowing your spore counts gives you the power to act before viral loads spiral out of control.

In Part 3, we’ll walk through what you can actually do about it—covering different options, and practical management techniques to help you keep Nosema under control and your colonies thriving.