Nosema: diagnosis first, then panic

Before we go further, a few words on what we are actually dealing with. Nosema disease, also called the spore disease, is one of the most common and most underrated diseases of the honey bee. It is a disorder of the gut of adult bees, caused by microsporidia, parasites that attack the epithelial cells of the midgut. A bee becomes infected by swallowing spores while cleaning soiled combs and the hive, and the spores multiply in its gut, shorten its life and weaken the whole colony. Nosema rarely kills spectacularly, overnight. Usually it grinds the apiary down quietly, and that is exactly why it is so insidious.

Nosema is at the same time one of those diseases beekeepers blame for everything they cannot otherwise explain. A poor spring, colonies dwindling, streaking on the front wall of the hive, and out comes the verdict that it must be nosema. Sometimes right, more often not. And since there is today no registered medicine for nosema in Poland or anywhere in the EU, all the more reason not to act on a hunch. Without a diagnosis you either treat something that is not there, or miss the real problem.

This post is in the same spirit as the earlier one on varroa treatment: check first, react second. I have laid it out properly, from biology, through diagnosis, to concrete prevention and the handling of equipment. I checked the facts repeatedly, because with a subject so overgrown with myths I would rather write less but be sure.

What it actually is

Nosema is a disease of the gut of adult bees, caused by microsporidia, that is single-celled, intracellular parasites. Three species have been described in the honey bee, but in practice two matter.

Nosema apis is the old species, known for over a hundred years and native to the honey bee. It is seasonal: it strikes mainly in late winter and early spring, after a long spell without a cleansing flight, and gives clear dysentery, that is brown faecal streaks on the hive and frames. It weakens colonies and shortens the life of foragers, but on its own it rarely kills the whole colony.

Nosema ceranae has its natural host in the Asian honey bee, Apis cerana, and was described scientifically in 1996. At some point it crossed over to the Western honey bee, probably through contact between the two species, though the exact moment and place of that jump are not certain. Today it is the one that dominates in most apiaries, especially in warmer regions, and the one that makes the biggest difference, because it harms in a different, insidious way, gnawing at the colony from the inside. The rest of this post is mostly about it.

Nosema neumanni is a third species, described in 2017 in Uganda. In a Polish apiary it is not a problem and not worth worrying about, but it is worth knowing that the subject is still developing.

There is also a naming matter. Some researchers propose moving these species into the genus Vairimorpha, so formally you will meet the name Vairimorpha ceranae. That change is contested, though. In 2024 a paper even appeared arguing that the revision was flawed and causes confusion, while other teams defend it. The dispute is a real one, which is why in practice the beekeeping world still simply says Nosema, and so do I.

Where ceranae came from

This is a topic overgrown with myths, so it is worth setting straight. As a natural infection in the honey bee, ceranae was described in Europe in 2006, in the work of Mariano Higes's team in Spain, and hence the impression that it is a fresh invader.

When archived, preserved bee samples were examined with molecular methods, however, it turned out it had been present in the honey bee much earlier. One of the oldest European traces is an Italian sample from 1993, and in the USA it was detected in samples from the mid-1990s. The oldest known trace of all comes from Brazil, from samples of Africanized bees reaching back to the late 1970s. So it did not so much "arrive" in 2006 as finally get noticed, once there were tools able to tell it apart from the old, well-known apis. Today ceranae is practically cosmopolitan, and in many regions, especially warmer ones, it has become the dominant species. It is worth remembering, though, that whether it truly displaced apis or was simply overlooked before is still under discussion.

What ceranae does to a single bee

To understand why this parasite is dangerous despite the lack of visible symptoms, you have to go down to the level of a single bee. A spore swallowed with soiled feed or water germinates in the midgut and injects its material into an epithelial cell. There it multiplies, using up the resources of the cell and of the whole bee.

The effects are multi-layered and described in many studies, though their strength varies between papers. The parasite causes an energetic and nutritional drain, so the infected bee is constantly hungry and wears out faster, and its life is shortened. Ceranae can also inhibit apoptosis, the natural death of infected gut cells, and so lives longer at the host's expense. On top of this comes a weakening of the bee's immunity, which opens the door to other pathogens, including viruses. Sick bees orient themselves worse in flight and struggle to return to the hive, and sometimes start foraging prematurely, before they mature, which further shortens their already weakened lives.

Now put this together into a picture of the colony: bees die faster and more often fail to return from the field. The colony does not drop dead overnight, it melts away, often with no heap of corpses in front of the entrance, because the foragers die far off. It is precisely this mechanism that is behind the empty or barely alive hives in spring, and it is easy to mistake it for robbing, the loss of the queen, or bad weather.

Symptoms: what you see in autumn, and what in spring

Nosema shows itself differently at different times of year, and it is worth knowing both of its faces, because it is they that most often prompt reaching for the microscope.

In autumn the disease acts quietly. The colony, instead of raising strong, healthy winter bees, weakens, develops more slowly and goes into winter depleted, with bees of shortened lifespan. With Nosema ceranae, dominant today, there is usually no dysentery or any other showy symptom, the colony is simply weaker than it should be, despite good conditions. And with a heavy infestation ceranae does not even wait for spring: it can empty a colony as early as late autumn, so that you look into the hive before winter and find a handful of bees or an empty space over untouched stores. It is a quiet signal, easy to miss, and one that usually comes back to bite in spring anyway.

In spring comes the reckoning, and it is this that makes the biggest impression on a beekeeper. The classic picture of Nosema apis is dysentery, brown faecal streaks on the front wall of the hive and on the frames, after a long winter without a cleansing flight, plus bees crawling in front of the hive, unable to fly, with a swollen abdomen. But the most dramatic, and most often mistaken for something else, is the sight when in spring you open the hive and find a handful of bees or an outright empty hive, despite the stores standing untouched. The colony did not die of hunger, nor did it lose its queen, it simply dwindled away through winter and early spring, because the sick, short-lived bees died faster than the queen could replace them. Such empty hives in spring, with stores and sometimes still a live queen on a remnant of bees, are one of the most typical and at the same time most painful consequences of nosema not caught in time.

And the key point, which flows from the biology of both species: the old rule "streaks on the hive equals nosema" is misleading today. With ceranae you can have a serious problem without a single streak, and the other way round, have streaks for a completely different reason, for example after honeydew stores, a long winter without a cleansing flight, or an ordinary overfull gut.

Diagnosis: the eye, the microscope, PCR

Since the symptoms mislead, you have to get down to specifics. Judging from the front wall of the hive is guesswork, because ceranae gives no dysentery, and dysentery need not mean nosema.

The microscope says much more. You grind up the abdomens or guts of a sample of bees, ideally thirty to sixty older, flying individuals, make a suspension and count the spores in a chamber under the microscope. A smaller sample raises the risk of missing the mark. This shows whether spores are there at all and how numerous, and so whether we are talking about a trace presence or a real problem. You do, however, need to know the limits of this method. First, the microscope does not reliably tell the species apart, because apis and ceranae spores are similar and ceranae is only slightly smaller. Second, it can miss a weak or early infection: in the Traver and Fell study, over half of the colonies that came out clean under the microscope turned out positive for ceranae in a sensitive PCR test. That is why a negative result under the microscope is not a full acquittal. Reliable species identification and detection of latent infections come only from PCR.

For everyday management decisions an assessment of whether spores are present and how numerous usually suffices, but if something does not add up and the microscope stays silent, it is worth handing a sample of bees to a bee disease lab. It is a few zloty and a few days, and it saves months of floundering.

There is one more thing to know: the spore count alone is a poor indicator of how serious things are. Studies show that bees without symptoms can carry high spore loads, and sick ones low. The threshold doing the rounds of apiaries, of the order of a million spores per bee, dates from the apis and cold-winter era, is not firmly established for ceranae, so treat it as a loose hint, not a verdict. What counts is the whole picture: the spore count plus what is actually happening with the colony. It is again the same logic as with varroa: numbers and trend, not a binary "there or not".

A silent killer or a harmless passenger

Here we come to the most contested point, because science does not speak with one voice. The Spanish school, led by Higes, described Nosema ceranae as a silent killer causing slow depopulation and colony collapse, especially in the warmer climate of the Iberian Peninsula. Meanwhile researchers in other parts of Europe, in the USA or in colder countries find the same parasite widely, but often with no clear harm at colony level. In many papers ceranae occurs with similar frequency in healthy and in declining colonies, so its role as a sole cause of collapse remains unresolved.

The most likely explanation is that ceranae rarely acts alone. Only in combination with other burdens, such as varroa and the viruses it carries, poor forage, pesticides or stress, does it turn from a passenger into a real threat. The practical conclusion is simple and consistent with the rest of this post: do not panic at the mere sight of spores, but do not dismiss ceranae when it goes hand in hand with real symptoms and other problems.

Ceranae and temperature, or careful with simple conclusions

This is a passage where it is easy to overstate, so I will put it carefully. In laboratory conditions ceranae multiplies better at higher temperatures, and its spores germinate poorly in the cold. In the work of Gisder and colleagues from 2010, at around four degrees about eighty per cent of Nosema apis spores germinated, but only about ten per cent of ceranae spores. Ceranae spores are also considered sensitive to frost. All this fits the picture in which ceranae reaps the biggest harvest in a warm climate.

It has to be said honestly, though, that field data are sometimes contradictory. There are studies in which it was cooler conditions that went together with higher nosema intensity, so the simple equation "the warmer, the worse" is an overreach. That is why I treat the link with climate as probable but unresolved. One thing is practical and well confirmed: since ceranae spores do not tolerate frost, freezing empty combs and equipment really does reduce their numbers, and I will come back to that under disinfection.

No medicine, so what to actually do

Let us start with a hard fact: in Poland and across the EU there is no registered medicine for nosema. Fumagillin, the only agent historically used, has no authorised form in the EU, among other reasons because of toxicity to humans and residues in honey. Preparations based on it are authorised in the USA or Canada, but not here. On top of that its efficacy against Nosema ceranae is doubtful: the work of Huang and colleagues from 2013 showed that at the low, declining concentrations after treatment ceranae can multiply more strongly than in an untreated colony, meaning that at the wrong concentration the agent revs the parasite up.

I also have to address honestly a practice that is whispered about: some beekeepers reach for fumagillin preparations meant for birds, for example for pigeons, and use them on bees second-hand. I understand the temptation, but I will say plainly that it is a risky road. First, it is off-label use of an agent in food-producing animals, so legally and in terms of residues in honey you are on thin ice. Second, the dosing from a pigeon product is in no way established for bees. Third, the efficacy against today's ceranae is uncertain. Better to put your energy into management, which works and leaves no problems in the jar.

So what is left and really helps?

Requeening and strengthening the colony. A strong, vigorously developing colony with a young queen copes with nosema far better than a weakened one with a flagging queen. Replacing the queen can turn the trend around, because a fresh injection of brood and energy dilutes the problem.

Good wintering conditions and good stores. Nosema is favoured by everything that burdens the gut and lengthens the spell without a cleansing flight: poor, honeydew stores, moisture in the hive, a weak, stressed colony. Everything I wrote about good wintering, that is healthy bees, good clean stores, dry ventilation and colony strength, is at the same time prevention of nosema.

Keeping varroa in check. Since ceranae does the most harm in a duo with varroa and viruses, controlling varroa is indirectly also a fight against nosema.

What about natural support

This is the place for things that give promising, though not miraculous, results in studies, mostly under cage and laboratory conditions. Worth knowing, but to be treated as support, not a cure.

Herbal additives and oils in the feed. Thymol given in syrup to infected bees lowered spore counts and improved their condition and immunity. Similar, encouraging results were noted for carvacrol from oregano, thyme, savory or resveratrol. There is an important caveat, though: thymol given to healthy bees can harm them, so it is not about pouring it in prophylactically blind, but about supporting a colony that really has a problem.

It is worth distinguishing two situations. Strong, harsh additives like thymol you leave for colonies that genuinely have a problem. Mild herbal and tannin preparations, on the other hand, can be given prophylactically in the feed, precisely to support the bee's gut in periods when nosema pressure rises, that is in autumn before winter and in spring for the start. It is a sensible part of management, as long as you treat it as prevention, not as an emergency cure.

Oak-bark preparations, that is Nozevit and similar. A separate category, popular in Poland, are ready preparations based on plant polyphenols, above all oak-bark extract, with Nozevit at the front. They work through tannins, which bind proteins and form a protective layer on the gut epithelium, making it harder for the parasite to settle in. I will say honestly what it is and what it is not: it is not a medicine and will not cure advanced nosema, and the maker does not promise that either, because studies show a reduction in spore numbers rather than their removal. It does, however, have two real advantages: it leaves no residues in honey and has no withdrawal period, so it suits routine prevention. It is given in syrup, roughly of the order of a few millilitres per litre, added to winter feeding or to stimulative feeding, and more often with a heavier infestation, and is sometimes applied as a spray on the bees. I treat such preparations as part of prevention and support for the colony, not as a bottle that will sort things out by itself.

Acidified feed. Nosema is a gut parasite, and some beekeepers give lightly acidified feed, for example syrup with a small addition of acid, counting on a less favourable environment for the parasite. Here a caution, though, the same one as with wintering: do not invert sugar with acid while hot, because you will produce HMF, which poisons bees. If you do tinker with acidifying, do it cold, cautiously and not for the winter stores, only for the stimulative spring feed that the bee will quickly use up.

Formic acid alongside varroa. Formic acid used against varroa acidifies the nest and sometimes, as a side effect, does not favour nosema. This is not its registered or certain indication, and do not treat it as a medicine for nosema, but it is one of those side effects of good management that play in your favour.

What to do with equipment and combs

Since there is no pill, equipment hygiene is one of the pillars of the fight against nosema, and in Polish practice it is a well-worn subject. Spores gather in old combs, on frames, in the hive and on small equipment, and sick bees foul the nest with droppings. Between colonies the parasite also travels on drifting and robbing bees, and on equipment carried from hive to hive, so cutting the colony off from this reservoir is key.

Comb replacement. This is the most important and best-documented measure, above all with Nosema apis, and Polish guidance speaks of replacing as much as half the combs a year. With ceranae the efficacy of the classic methods is less well proven, but fresh, light wax works in the colony's favour anyway. Old, dark combs, especially older than two or three years, go to melting down, not back into the hive.

Disinfecting empty combs. The classic Polish method is fumigating stored combs with the vapours of glacial acetic acid, which destroys nosema spores and, incidentally, the stages of the wax moth. You do it in a sealed box or hive, away from people, and you have to air it well afterwards, because the vapours are caustic. Some beekeepers are moving away from this method as a nuisance and simply rely on melting down old wax. Frost is a free ally too: ceranae spores do not tolerate low temperatures, so freezing empty combs in winter really does reduce their numbers.

Disinfecting hives and frames. Wooden hives and frames are worth scorching with the flame of a gas burner, especially the inside of the hive, before a new colony moves in. Formalin at a dilution of the order of three per cent is also used for disinfection against nosema. Polystyrene hives are scraped and washed with hot water and detergent, and the outer walls, once cleaned, repainted.

Moving onto clean equipment in heavier cases. Transferring a colony onto disinfected, fresh combs is a more radical version of wax replacement, useful when the infection is strong.

Small equipment, hands and clothing. Wash the hive tool, bee brush and other tools after work with hot water, with a little spirit or a ready disinfectant. Launder apiary clothing periodically in hot water and dry it in the sun. These are small things, but it is precisely on them that spores travel from hive to hive.

The apiary ground. The spots under the entrances, where bees take their cleansing flights and leave droppings, are worth sprinkling once a year with quicklime or watering with milk of lime.

Pulling it into one sentence: since there is no medicine, then clean, frequently replaced wax, disinfected equipment and strong colonies are your medicine.

When to really worry about it

Nosema, especially ceranae, is common in apiaries, and a trace presence of spores does not mean a catastrophe straight away. It becomes a problem only when the intensity is high and goes hand in hand with real symptoms: poor spring development, colonies melting away, worse harvests despite good conditions.

And the other way round, if colonies develop well and give honey, and you are winding yourself up about nosema only because you saw a few streaks on the hive after a long winter, it is probably panic over nothing. Look into the stores, check the conditions, perhaps hand in a sample for testing, but do not make a mountain out of a molehill.

Curiosities and controversies

A medicine that can make things worse. The effect described by Huang, mentioned above, is surprising enough to be worth repeating: fumagillin, at the declining concentration after treatment, can rev ceranae up more than in a colony not treated at all. It is one of the reasons why the chase after it is questioned today even where it is legal.

It was here before we noticed it. Archived samples showed ceranae in the honey bee years, even decades, before its official description in 2006: in Europe they reach back to 1993, and the oldest known traces, from Brazil, to the late 1970s. It is a good lesson in humility: no detection does not mean no presence, and new tools can change the picture of a disease.

A parasite that fears frost. In a world where most diseases are favoured by everything, ceranae is unusual, because it is its own spores that tolerate low temperatures poorly. It is a rare case in which a harsh winter is sometimes the beekeeper's ally, while a mild, warm winter rather does not help them.

The nosema that does not soil. The classic picture of nosema is dysentery on the hive, but today's dominant ceranae usually gives none. That is why it is most dangerous not when you can see it, but when you cannot.

The most common mistakes

Diagnosing from streaks on the hive. Dysentery is not proof of nosema, and its absence is not proof of health. Certainty comes only from the microscope, and the species from PCR.

Trusting a single negative microscope result. The microscope misses part of the latent ceranae infections. If something does not add up and the microscope stays silent, order a PCR.

Diagnosing from a single spore count. Symptomless bees can carry high loads, and sick ones low. What counts is the count plus the condition and trend of the colony.

Tinkering with bird preparations. Fumagillin from pigeon products is off-label use, a risk of residues in honey and uncertain efficacy against ceranae. Better to back management.

Skimping on comb replacement and disinfection. Old, dirty combs and undisinfected equipment are a store and a carrier of spores. Without fresh wax and clean equipment every other action is half-measures.

Sources and further reading

• Higes M., Martin-Hernandez R., Meana A. (2010). Nosema ceranae in Europe: an emergent type C nosemosis. Apidologie. The different, less symptomatic course of nosema caused by ceranae.

• Fries I. (2010). Nosema ceranae in European honey bees (Apis mellifera). Journal of Invertebrate Pathology. Biology, epidemiology and diagnosis, including archived samples, the host-switch hypothesis from Apis cerana, and the limits of microscopy.

• Fries I. (1988). Comb replacement and Nosema disease in honey bee colonies. Apidologie 19: 343-354. The classic confirmation of the role of comb replacement.

• Gisder S. et al. (2010). Appl. Environ. Microbiol. The effect of temperature on spore germination: approx. 80 per cent for apis versus approx. 10 per cent for ceranae at 4 C.

• Field studies linking nosema intensity with temperature, including ones in which higher intensity went together with cooler conditions (incl. Retschnig et al. 2017). The basis for caution over conclusions about climate.

• Traver B.E., Fell R.D. (2011). The limits of microscopy: over half of the colonies negative under the microscope were positive by PCR for ceranae.

• Huang W.F. et al. (2013). Nosema ceranae escapes fumagillin control in honey bees. PLoS Pathogens. The rebound of ceranae after a drop in fumagillin concentration.

• Chemurot M. et al. (2017). Nosema neumanni n. sp. in Apis mellifera in Uganda. European Journal of Protistology. The basis for the mention of the third species.

• Teixeira et al. (2013). Nosema ceranae in archived samples of Africanized bees in Brazil reaching back to the late 1970s. The basis for the information on the oldest traces.

• Tokarev Y.S. et al. (2020) and the 2024 polemic on the flawed revision of the genera Nosema and Vairimorpha. Background to the naming dispute.

• Work on thymol and resveratrol in limiting Nosema ceranae, with the caveat about thymol's harm to healthy bees (incl. Insects 2022, PLOS One 2020).

• Materials on plant-polyphenol preparations based on oak bark (Nozevit, Apivita) and studies on reducing Nosema spore numbers (incl. the University of Zagreb). The basis for the section on tannin support, with the caveat that it is support, not a cure.

• The National Veterinary Research Institute in Pulawy (Department of Bee Diseases), regional beekeeping associations and the monthly Pszczelarstwo. Polish guidance on equipment disinfection, fumigating combs with acetic acid, flame scorching, formalin and wax replacement.