All About Nosema

An in-depth account of this unpleasant diseasethe-new-you


Pam Gregory, MSc, NDB

updated July 2017

Nosema is a disease that affects adult honey bees. It substantially reduces a bees’ life span by infecting the mid-gut and preventing proper food digestion. There are two forms of the pathogen; Nosema apis and Nosema ceranae. They are both highly specialised, single-celled parasites belonging to the taxonomic class Microsporidia. Nosema was initially classified as a protozoan but is now considered to be a fungus. Nosema apis (Zander) was first described in 1909 but in 2005 Spanish bee researchers found Nosema ceranae (Fries) as a new pathogen in Europe. Nosema ceranae originated on the Asian Honeybee and shows just how good people have become at globalising bee diseases. N. ceranae is now widespread in the UK and seems to be a much more virulent form of the disease than N. apis.


Adult honey bees are infected by the Nosema parasite in its spore form (resting stage). This normally happens when the house bees are cleaning up contaminated combs but can also be transmitted by food sharing (trophallaxis) and sometimes grooming. Once inside the gut the spores germinate developing a hollow filamentous tube from one end of the cell. This penetrates the digestive cells of the gut wall and injects the contents of the spore (technically called the sporoplasm) into them. This growth (vegetative) stage of the parasite multiplies very quickly forming huge quantities of new spores after about 5 days or so. This seriously interferes with the bee’s ability to digest food particularly pollen. The host cell then splits spilling the spores into the gut. There can be hundreds of thousands, if not millions, of spores in a heavily infected bee. The spores are then excreted in the faeces along with the other gut contents.

The spores are spread around by cell cleaning especially in spring when colonies are expanding rapidly. Bees cannot bear to be dirty so house bees clean the hive by licking and nibbling and in doing this they inadvertently ingest the Nosema spores. It can also be spread about in the bee’s faeces if this gets on the comb. Normally, bees do their ‘squittering’ on cleansing flights, but if the weather (or other conditions such as travelling) are such that the bees are unable to get out of the hive then the combs may become contaminated with faeces. Careless squashing of bees when replacing hive parts can also help to spread Nosema.

Nosema apis spores are shaped like a rice grain and measure approximately 5–7 μm long × 3–4 μm wide while Nosema ceranae spores are a little smaller and have a very slightly different shape. The spores are relatively long lived, can withstand dehydration and extremes of temperature and can persist on the comb for a year or more (even if it is not in the hive) triggering further infection the following winter. Transferring or reusing combs from infected colonies or using contaminated stored comb that hasn’t been fumigated all spread nosema between colonies.

Effects on the bee

Nosema shows very few visual signs to alert beekeepers to the disease even when the spore level is high enough to cause significant problems. Sometimes bees may have disjointed (or K) wings or be found crawling round aimlessly outside the hive. This can however, also be indicative of acarine and some viral diseases too so is not at all conclusive -only a sign that something is wrong.

Inside the bee, pathological changes in the gut cells include altered digestion and a reduction in RNA synthesis. Damage to the hypopharyngeal (brood food) glands of infected nurse bees (presumably as result of poor protein uptake in the gut) accelerates behavioural development in the bees leading to premature aging (I know how they feel!) which can shorten a bees lifespan by 50% or more. The ventriculus, (or mid gut) which is normally brown, can become white and very fragile. Winter bees have less protein in their fat bodies and haemolymph so will be less able to feed the new larvae in the spring should they live long enough to survive the winter.

Young queens can get also Nosema and those that do are normally superseded very quickly, maybe within a month. However, queens are much less likely to get Nosema because they don’t clean combs so can only be infected from ingesting contaminated food. Infected queens lay fewer eggs and new queen cells are less likely to hatch (sometimes being infected with Black Queen Cell virus) leaving the colony open to queenlessness. Similarly, drones can become infected although they rarely do.

Nosema apis does not affect larvae and young bees on hatching are always free of infection only becoming infected if they come into contact with contaminated comb or food as part of their hive duties. However, with N. ceranae, feeding larvae with contaminated pollen brought back by infected foragers prevents the removal of infection during the summer.

Differences between N.apis and N. ceranae

There is a strong seasonal relationship between the annual development of the colony and the number of Nosema apis spores. Once the weather improves the bees are able to defaecate outside the hive removing infective spores. In addition, infected bees die away from the hive without transmitting their infection so spore levels reduce naturally in the summer. This is not the case with Nosema ceranae which can cause problems for beekeepers all year round. Although the build-up of spore numbers is slower in N. ceranae, their continual increase throughout the year means much higher infection levels can be reached even in the summer months. This leads to a constant loss of infected foraging bees and reduces the food brought into the hive. This is probably the main practical difference between the two types of Nosema. If the bees are not thriving it is worth checking them for Nosema.

Effects on the colony

The effect of Nosema on a colony really depends on the species of Nosema present, the time of year and the number of infected bees. Since the life span of the winter bees is shortened, colonies are more likely to die over the winter. Those that make it through may build up very slowly in the spring, or even dwindle and die out depending on the severity of the infection. To prevent Nosema being a problem in the spring, check for it in the autumn.

Nosema levels will rise under any circumstances that remove opportunities for cleansing flights or force bees close together for long periods. Anything that confines the bees to the hive will exacerbate any Nosema present (especially if they are stressed by other problems especially dysentery). Travelling long distances with bees, bees sold as ‘packages’ and some management activities, such as queen rearing, will all serve to exacerbate Nosema. Over the winter nosema levels rise relentlessly. Consequently, a light infection of Nosema in the autumn will become a heavy one by the spring. There is also a positive correlation of Nosema with poor summer weather, which is likely to be especially significant for N. cerana. Bees are happiest in reliably hot summers when infected bees spend less time in contact with each other and cleansing flights and bees dying away from the hive removes infection.


Although Nosema and dysentery are often associated, bees can have dysentery without having Nosema and vice-versa. Dysentery is a separate condition caused by an excessive amount of water in a bee’s body normally as a result of consuming food with a high water content which puts pressure on the ability of the bee’s gut to retain faeces. It is exacerbated by prolonged confinement during winter and early spring when there are fewer opportunities for the bees to defecate outside the hive. Bees with Nosema are more prone to dysentery because the changed water balance in the gut means their rectal contents build up more rapidly than normal. Sometimes faeces can be seen splattered about the frames and brood comb and around the hive entrance especially if Nosema ceranae is present.


Amoeba can also occur with Nosema. Amoeba is caused by a protozoan (called Malpighamoeba mellifica) that lives in the Malpighian tubules (the bee’s equivalent to our kidneys). It can occur in several forms but is most familiar as a round cyst. It passes round the colony in the same way as Nosema. Not much is known about Amoeba, but it is rare, has no outward symptoms and probably has little effect so, in practise, can currently be ignored. An affected colony can be transferred onto clean comb if necessary. Like Nosema, it can only be identified by microscopic examination.


The presence of Nosema (but not which type it is) can be diagnosed using a microscope with X 400 magnification. It is a very simple technique if you have the right microscope. About 30 bees are collected and their abdomens mashed up in a pestle and mortar with a few drops of water. A single drop of the resulting ‘soup’ is put onto a microscope slide and covered with a coverslip. Under the microscope look for little rice shaped grains. Your local association may well have a microscopist who can do this examination for you as it is otherwise quite an expensive test. Although the spores of N. apis and N. ceranae have slight morphological differences, there is not much discernable

difference under a light microscope and the only reliable way to distinguish them is under an electron microscope or using the fancily named Polymerase Chain Reaction (PCR) analysis – neither of which is available to the average beekeeper.


The antibiotic fumagillin (Fumidil B®) is no longer a legal treatment for Nosema. Currently (2017) there are no other authorised treatments.

You can gain some temporary respite by uniting ‘sick’ colonies to form a large enough unit to survive in the short term. However, this enlarged colony will need shaking onto clean combs (and some luck) if it is to survive in the long term.


Spores of both nosema species can remain viable on the combs (whether in or out of the hive) for a year or more and the disease is easily spread through the use of contaminated combs. To clear persistent spores from an infected colony brood combs need to be replaced. There are several ways to do this which have varying efficiency. The standard advice is to replace two or three combs each year with clean (fumigated) comb or foundation. Comb replacement doesn’t act as a treatment but will help to reduce spore numbers. However, for colonies infected with Nosema you will need to prevent the disease coming back the following year. To be properly effective this will require replacing all the combs in one go using either a Shook Swarm or a Bailey frame change. Shook swarms in particular have been shown to give colonies new vigour, probably because all the horrid pathogens in the comb are completely removed.

Where combs are to be reused they should be fumigated first. Stored supers and brood frames can be fumigated with 80% acetic acid which kills both Nosema spores and EFB bacteria and no doubt some of the fungal pathogens too. Stack supers, sealed as tightly as possible, with an acetic acid pad over every two supers and leave for a week to 10 days. Use Vaseline to protect metal parts from the corrosive effects of acetic acid. Air the frames well to clear any remaining fumes before using them.

Nosema spores are susceptible to a number of commonly used disinfectants. Tools should be routinely cleaned with a solution of washing soda while out of circulation hive boxes and other parts should be washed and flamed where possible before bringing them back into use.

Caution. Use protective equipment when handling acetic acid or commonly used disinfectants. Be aware that acetic acid has a corrosive effect on metal hive parts

Related viruses

Black queen cell virus, Kashmir bee virus and Virus Y are all associated with nosema while Virus X is associated with amoeba. It is not clear what the relationship between viruses and nosema actually is but it is thought that damage to the gut lining made by injecting the nosema sporoplasm makes it easier for viruses to the enter the cells. It may also work the other way round; so that bees infected by virus become more susceptible to nosema.

Genetic resistance

No genetic resistance to the parasite has been unequivocally demonstrated although it is not unrealistic to expectation that bees should be more resistant to the original parasite, Nosema apis,

than the more recently introduced N. ceranae. However, the bees defence mechanisms [protecting them from Nosema is linked to seasonal weather patterns so it is difficult to assess the effect of the different pathology of N. ceranae or the effects of continuing cool, wet summers.

Further reading

Bailey, L and Ball, B.1991. Honey Bee Pathology; 2nd ed. Academic Press, London, UK. Morse, R. ed; Honey Bee Pests, Predators and Diseases. Cornell. US Aston, D and Bucknell, B. 2010. Keeping Healthy Honey Bees. Northern Bee Books. HIGES M., MARTÍN R. & MEANA A. (2006). Nosema ceranae, a new microsporidian parasite in honeybees in Europe. J. Invertebr. Pathol. 92, 93–95. NBU (CSL/National Bee Unit) Fact sheets (

Randy Oliver – Scientific beekeeper website –


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