Nosema is a microscopic fungal parasite that infects the gut of adult honey bees, causing a disease called nosemosis. It’s one of the most common and damaging honey bee diseases worldwide, responsible for shortened worker lifespans, reduced honey production, and significant colony losses, particularly over winter. Two species affect honey bees: Nosema apis, known since the early 1900s, and Nosema ceranae, first identified in 2005 and now spreading rapidly across every beekeeping continent.
How Nosema Infects a Bee
Nosema belongs to a group of organisms called microsporidia, which despite their tiny size are classified within the kingdom of fungi. The parasite exists as a tough, oval spore small enough that millions can fit inside a single bee’s gut. Nosema apis spores measure roughly 4 to 6 micrometers long, while Nosema ceranae spores are slightly smaller at 3.3 to 5.5 micrometers.
The infection begins when a bee swallows spores, typically while cleaning contaminated comb or sharing food with nestmates. Once a spore reaches the midgut, it germinates and deploys a coiled structure called a polar tube, essentially a microscopic harpoon that pierces a gut lining cell and injects the parasite’s contents directly inside. The host cell doesn’t recognize the intruder. Within about 16 hours of entering the cell, Nosema ceranae is already settling into the host’s machinery.
Inside the cell, the parasite goes through several rounds of division, producing new forms that eventually mature into fresh spores. The entire cycle from initial infection to new spore production takes roughly 96 hours. When the host cell is packed full, it ruptures, releasing spores into the gut. Some of those spores immediately infect neighboring cells, while others pass out of the bee in its feces, ready to infect the next bee that encounters them.
How It Spreads Through a Colony
Under normal conditions, honey bees defecate during flight outside the hive, which limits contamination inside. But Nosema-infected bees often develop dysentery, losing the ability to hold their waste until they can fly. This leads to fecal spotting on combs, frames, and hive walls. Spores can remain viable for months in dried fecal spots on brood combs.
The worst contamination typically happens near the end of winter, when bees are confined to the hive and combs become soiled with infected excreta. As the colony ramps up brood rearing in spring, house bees clean those soiled combs and ingest the spores. This is why infection levels spike in mid-spring and remain high through summer before declining in fall. Research from both the U.S. and Western Canada confirms this seasonal pattern for both Nosema species.
Nosema also spreads through the air inside and around hives. Studies have detected spore DNA on air sampling tapes placed near colonies, with Nosema apis present at roughly six times the concentration of Nosema ceranae in airborne samples.
Nosema Apis vs. Nosema Ceranae
Nosema apis was the only species known to infect European honey bees for nearly a century. It causes the “classic” form of nosemosis, with visible dysentery, crawling bees unable to fly near the hive entrance, and clear seasonal peaks in spring. Colonies that survive winter with heavy Nosema apis infections often fail to build up properly in spring.
Nosema ceranae originally parasitized Asian honey bees but jumped to European honey bees and was first reported in 2005. It has since spread throughout Europe, North and South America, and is now considered to be replacing Nosema apis in many regions. Ceranae is often described as more insidious because infected colonies may not show the obvious dysentery and crawling that apis causes, making it harder to detect without laboratory testing. Under experimental conditions, Nosema ceranae has proven highly pathogenic at the colony level, producing significant reductions in colony size, brood rearing, and honey production.
What Nosema Does to a Colony
The damage from Nosema goes well beyond a few sick bees. Infected workers have shorter lifespans, which means the colony’s workforce shrinks at the same time it needs to be growing. Fewer workers means less foraging, less brood care, and less honey stored. Multiple studies have linked Nosema infection to increased winter mortality and reduced overall colony strength.
Queens are not immune. Research has shown that Nosema ceranae infection alters a queen’s fertility markers and her pheromone profile. Infected queens produce significantly different levels of the mandibular pheromones that signal their presence and health to the colony. These changes may explain why Nosema-infected colonies often supersede (replace) their queens at higher rates than healthy colonies. A colony that repeatedly replaces its queen loses valuable brood-rearing time and genetic continuity.
Diagnosing Nosema
You can’t see Nosema with the naked eye. Dysentery (streaks of brown feces on the hive exterior) is a clue, but it’s not definitive since other conditions cause dysentery too, and Nosema ceranae often produces no visible symptoms at all.
The standard diagnostic method involves crushing a sample of bees in water and examining the liquid under a microscope at 400x magnification. Nosema spores appear as bright, rice-shaped objects. A hemocytometer (a gridded counting slide) is used to estimate the number of spores per bee. Many beekeeping experts and extension services consider a count of 1 million or more spores per bee to be the threshold requiring management action. Counts below that level indicate the colony is carrying the parasite but may manage it on its own. Distinguishing between the two species requires more advanced techniques like PCR testing, since the spores look similar under a standard microscope.
Treatment and Management
Fumagillin, an antibiotic derived from a fungus, has been the primary chemical treatment for Nosema for decades. It inhibits both species, and systematic reviews of field trials show no evidence that resistance has developed. The traditional approach uses a spring dose of around 100 mg of fumagillin base per colony (mixed into sugar syrup) or a fall dose of 200 mg. However, research suggests these standard doses provide benefits but may be too low to fully control the disease on their own. Studies testing higher fall doses in the range of 400 to 600 mg per colony showed significantly better Nosema control the following spring compared to the standard 200 mg dose.
Fumagillin is banned in many countries, including throughout the European Union, due to concerns about residues in honey. This has driven interest in plant-based alternatives. Extracts from sugar apple, basil, guava, and java plum at 2% concentration have shown the ability to inhibit Nosema spore development in colony trials. After 90 days of treatment, spore levels in colonies treated with these plant extracts were comparable to those treated with fumagillin. While promising, these alternatives are still in the research phase.
Beyond chemical treatment, management practices play a major role. Replacing old, dark brood combs removes a reservoir of spores that accumulate in wax and fecal deposits over years. Ensuring colonies have adequate nutrition, especially protein from pollen or supplements, helps bees mount a stronger immune response. Good ventilation reduces moisture buildup that can worsen dysentery during winter confinement. Requeening with hygienic stock and keeping colonies strong heading into fall are among the most practical defenses a beekeeper has against Nosema becoming a colony-level problem.