Nosemosis: what it is, how to detect it and how to act

Nosema is a fungus that parasitises and destroys the inner wall of the honeybee’s intestine, preventing it from digesting and absorbing nutrients. The bee becomes malnourished, weakens and may die. Its spores are present in around 70 % of Spanish apiaries without causing harm: it is a stress factor —a cold snap, poor location, varroa infestation or poisoning— that triggers mortality, and then it can devastate a colony in less than a week.

In this guide

• the parasite’s life cycle;
• its actual presence in apiaries;
• the triggering factors;
• the symptoms and how to diagnose them, including field autopsy;
• hygiene and prophylaxis.

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Nosema is a microorganism belonging to the fungal group that parasitises the cells of the honeybee’s inner intestinal wall, destroying it. That wall produces gastric juices that aid digestion and absorbs nutrients for transfer to the haemolymph. When destroyed, the honeybees cannot digest or assimilate: they become malnourished, weaken and, depending on concurrent factors, may die.

Until 2004 it was believed that two geographically separated species existed, one western (Nosema apis) and one Asian (Nosema ceranae). Molecular PCR techniques have demonstrated that both species are now distributed worldwide.

Nosema is a microorganism belonging to the group of fungi that parasitises the inner wall cells of the bee’s intestine, destroying them. This wall is responsible for producing gastric juices that assist in food digestion and for absorbing nutrients to transfer them into the capillaries of the circulatory system throughout the body, the haemolymph. Its destruction therefore prevents bees from digesting food and absorbing nutrients. This places them in a state of malnutrition, weakens them, and ultimately, depending on a series of factors, may lead to death.

Until 2004, two species of this microorganism were believed to infect bees: one western, Nosema apis, and one eastern, Asian, Nosema ceranae. Molecular analytical techniques, such as PCR, have since revealed that both species are now distributed worldwide.

The life cycle of Nosema

Nosema produces spores, which are small capsules with a protective outer coat enclosing a reproductive core; this allows them to survive adverse conditions.

When a spore is ingested by a bee, it passes into the gut. There, it is attacked by gastric juices, which gradually weaken its protective coat. By the end of the intestine, the coat is severely weakened, allowing the core to exit and enter an intestinal cell, where it takes control of its vital systems. The epithelial cell is forced to alter its function and dedicate itself to producing new Nosema spores. Eventually, the cell dies and releases a large number of spores into the intestine, which may infect new intestinal cells or exit the bee in faeces, thereby infecting other bees.

Normally affects worker bees, but may also infect the queen, and if it reaches her ovaries, may cause sterility. Infection can spread from hive to hive through robbing, drifting, or drones.

Presence

Both Nosema species are widespread, with the Asian species, Nosema ceranae, currently the most common. According to PCR analyses conducted in Spain, spores are found in around 70% of colonies, although they generally cause loss of vigour and mortality only when a stress factor is also present, such as a cold snap, poisoning by pesticides or acaricide residues, poor nutrition, an unsuitable apiary location, poor beekeeping practices or Varroa.  

Triggering factors

As previously mentioned, the presence of stress factors may trigger mortality due to Nosema. It is therefore essential to avoid them.

An extremely important factor is the apiary location. Nosema is favoured by sites that are poorly exposed to sunlight, shaded during rainy seasons or persistent fog. Or by areas experiencing frequent sharp drops in temperature during late autumn or early spring.

Under these conditions, Nosema can reproduce explosively, causing significant damage and mortality within less than a week.

Symptoms. Field and laboratory diagnosis

Initially, a useful sign of the presence of Nosema is that the abdomen of a significant proportion of workers does not extend clearly beyond the wings; in extreme cases it is shorter than the wings, giving the bees a fly-like appearance. This is caused by malnutrition, which forces the bees to use up all the fat reserves beneath the 5th dorsal abdominal segment (the 3rd visible segment, since the first two are tucked inside the thorax), shortening the abdomen.

However, this symptom may also indicate poor nutrition due to Varroa infestation, which also consumes the fat reserves of its hosts; or poor nutrition due to lack of stores, honey and/or pollen.

We can, however, distinguish these causes. In the case of Varroa, there will also be scattered brood, bees with deformed wings… but there will be food stores in the combs, especially honey. If malnutrition is due to lack of stores, this will be easily observable during hive inspection.

What you see	Nosema	Varroa	Lack of reserves
Short abdomen, fly-like appearance	Yes	Yes	Yes
Scattered brood	No	Yes	No
Worker bees with deformed wings	No	Yes	No
Honey reserves in the combs	Yes, they exist	Yes, they exist	No, they do not exist
Brown liquid diarrhoea	Only N. apis	No	No
Low activity at the hive entrance	Yes	Yes	Yes

Another symptom of Nosema is reduced bee activity, both at the hive entrance and inside the hive.

Nosema apis also causes very watery, more or less dark-brown diarrhoea, visible as streaks at the hive entrance and on the frames. This symptom does not occur with Nosema ceranae. 

After a cold snap, if mortality is seen at the hive entrance and half of the dead bees have a ‘short’ abdomen, Nosema may be suspected as the cause.  

To increase certainty, carry out a field post-mortem examination using at least 20 older bees (which will carry more Nosema). If there is no activity at the hive entrance, collect them from inside the hive, from the last occupied frame. If there is activity at the hive entrance, close it for a while so older bees gather there and can be collected.

Kill the bees by crushing their heads. Hold the head with one hand, and with the other, pinch the last ring with the thumbnail against the index finger, twist half a turn (about 180°), and gently pull. In healthy bees, the rectum (partly filled with digested pollen) will emerge first, followed by the small intestine, then the large intestine, and usually the crop (honey stomach) will break off (rarely it will emerge intact, which would be next). In bees infected with Nosema, only the rectum and large intestine emerge; the digestive tract ruptures at the junction between the small and large intestine, as its wall has been destroyed by infection. This test can only be performed on fresh bees. If frozen bees are used, ice crystals will have ruptured the tissues, altering consistency. The same applies if they have been dead for some time and have begun to decompose.

The large intestine is amber in colour, variable; colour is not a symptom of disease. It may contain boluses of pollen in digestion, of varying colours. Superficial white spots are not symptoms of Nosema; they are usually fragments of respiratory sacs.

This diagnosis has a reliability of 80 %. The definitive diagnosis is laboratory-based. It requires crushing the abdomens of a sample of 20 older bees with a few drops of water, and preparing a slide with a drop of the resulting liquid, stained with methylene blue. The spores are rice-grain-shaped, visible at 400 x magnification, measuring approximately 4×2 microns, and appear refractive, unstained, against the blue background of the preparation.

Hygiene and prophylaxis

When Nosema appears, hygienic measures must be applied to affected hive equipment, and prophylactic actions to prevent recurrence.

Main hygienic measures should include:

• Removing dead bees from in front of the hive entrance, burying or burning them.
• Disinfect boxes from affected hives with a blowtorch, or with 1 part bleach diluted in 5 parts water plus a squirt of washing-up liquid.
• Do not transfer frames from these colonies to others. Disinfect the frames and any containers or other equipment, or burn them. The wax can be recycled: the spores do not survive at 49 °C for 24 hours.

Prophylactic measures, especially in early spring and autumn, and particularly in established apiaries or hives where it has previously occurred, should include:

• Avoiding wet, marshy, or poorly sunlit sites (e.g. in the middle of closed woodlands, or near damp riverbanks).
• Ensuring proper nutrition, especially by providing pollen supplements (e.g. Ultra-bee).
• Act quickly when symptoms appear: move to a more suitable site and correct feeding. No veterinary medication is registered against nosema in Spain, so resolution depends on management. There are nutritional supplements based on plant extracts, such as Nozevit or Concentrado N, but they are supplements, not medications: they do not treat or cure the disease and do not replace correction of location and nutrition.

Source

Written exclusively for Latiendadelapicultor.com by PAJUELO Apicultural Consultants, leading authorities in apicultural research and bee health

Frequently asked questions

Continue reading

• How to inspect a hive, where these symptoms are detected.
• How to choose the site, which here is half the treatment.
• Disinfection of apiary equipment, with correct dosages and timing.
• Protein supplementation, to correct nutrition.

Informational content. In case of abnormal mortality in the apiary, contact your ADS or official veterinary services and send a sample to a laboratory for a definitive diagnosis.