Nosema

The Invisible Gut Disease

Nosema is one of the most widespread and economically damaging diseases of managed honey bee colonies worldwide. Unlike American Foulbrood (which produces visible symptoms in brood) or varroa mites (which can be seen on bees), Nosema infection is internal and invisible without microscopic examination. An infected bee looks normal from the outside while its midgut lining is being destroyed by millions of multiplying spores.

Two Species

Nosema apis has been known to beekeepers for over a century. It causes classic symptoms: dysentery (fecal streaking on the hive exterior and frames), population decline in late winter and early spring, reduced honey production, and shortened worker lifespan. Nosema ceranae, originally a parasite of the Asian honey bee (Apis cerana), crossed to European honey bees and has become the dominant species in most regions. N. ceranae is more insidious: it causes fewer obvious symptoms but can be lethal at high spore loads, producing colony decline without the telltale dysentery.

Transmission and Impact

Bees ingest spores when cleaning contaminated comb, grooming infected nestmates, or consuming contaminated food stores. Inside the midgut, spores germinate and inject their contents into gut lining cells, hijacking the cell machinery to produce millions of new spores. The damaged gut absorbs nutrients poorly, effectively starving the bee from the inside. Infected bees forage less efficiently, live shorter lives, and contribute fewer resources to the colony.

Management

The antibiotic fumagillin was the historical treatment for Nosema, but its registration has been withdrawn in many countries due to concerns about residues and efficacy against N. ceranae. Current best practices focus on cultural management: maintaining strong colonies, replacing old comb regularly, ensuring adequate nutrition, and reducing stress. Some research suggests thymol-based supplements and probiotic treatments may help, but no universally accepted pharmacological treatment currently exists.