Nosemosis is a fungal disease of the adult honey bee that leads to weakening of colonies and diarrhea symptoms.

Nosema species have been recorded on all continents where the Western honeybee occurs, they are very widespread.

Diseased bees excrete spores in the bee feces. These remain infectious for over a year and can be found on honeycombs and hive surfaces as well as in the food stores.

Workers, drones and queens can be affected by the infection. The spores are ingested orally with the food

Nosema apis leads to symptoms mainly in spring, which are expressed in the decline of the bee population, sometimes severe bee mortality and fecal stains ("chain of dots") on combs and flight board.

Nosema ceranae, on the other hand, leads to year-round occurrence of flightless bees ("crawlers"), decline in bee population, lower honey production, colony collapse.

In Austria, no approved control agents are available against the two pathogens of nosemosis

Selection of a suitable location (sheltered from the wind, sunny) with a sufficient supply of nectar and pollen; hygiene measures; timely and sufficient feeding with suitable winter food, if possible do not leave forest and melezitose honey in the colony.

Nosemosis is widespread in Austria's honey bee colonies. The occurrence of nosemosis is not notifiable. For this reason, the frequency of occurrence is only recorded randomly in the course of research projects. As part of the "Future Bee" project, samples from 210 bee colonies from autumn 2015 and 150 bee colonies from spring 2016 were examined as part of the posthoc study. In autumn, 20.0 % (95 % confidence interval: 15.0-25.8 %) of the samples were positive, in spring 26.3 % of the samples (95 % confidence interval: 19.8-33.4 %). The species identification revealed that all positive samples were infected with Nosema ceranae and that two samples were also infected with Nosema apis.

Nosemosis is an intestinal disease of the adult honey bee, which can be caused by the indigenous pathogen Nosema apis or the introduced pathogen Nosema ceranae. The genus Nosema is counted among the microsporidia and assigned to the fungi. Infection occurs through oral ingestion of spores, which germinate in the midgut. Subsequently, the intestinal wall is attacked and destroyed. Nosema apis leads to weakening of the colonies and diarrhea symptoms with a seasonal focus in spring ("spring consumption"). Infection with Nosema ceranae, on the other hand, leads to symptoms such as flightless bees and weakening of colonies throughout the year.

In Austria, both Nosema species are found in bee colonies. Nosema apis is native to Europe. Nosema ceranae was introduced and could be traced back to 2003 on archived bee samples from Austria. Since 2010, Nosema ceranae has been almost exclusively detectable. In individual cases, both pathogens can be detected simultaneously in one bee colony or one bee.

Workers, drones and queens can be affected by the infection. The spores are ingested orally with the food and germinate in the midgut. Subsequently, the released pathogens infect the cells of the intestinal wall, where they multiply. At the end of the multiplication cycle, the host cells are destroyed and the spores (permanent stages of the pathogen) are released in the intestine. After release, the spores either re-germinate and invade new intestinal cells, or are excreted in the honey bee's feces. Spores in bee feces remain contagious for over a year.

Transmission in the colony: The disease leads to defecation in the colony; subsequently, bees come into contact with spore-containing feces and fall ill. In this way, honeycombs, food supplies and other surfaces in the colony can also become contaminated.

Transmission from colonyto colony: The transmission from colony to colony occurs through flight and predation of infected bees, in the case of boiled water troughs, as well as through beekeeping activities (rehanging of combs, uniting of weak or sick colonies with healthy colonies, use of spore-contaminated equipment and hives or feeding of supplies from Nosema-infected colonies).

Infection depends on temperature conditions and infection pressure. In Nosema apis, infection with 100 spores can lead to destruction of all cells of the intestinal wall within two weeks, rapidly leading to limitation of intestinal function. In experimental infection, the peak of infection was detected histologically 15 days after infection in Nosema apis and 10 days after infection in Nosema ceranae.

Symptomatology

Nosema apis causes symptoms mainly in spring, which are manifested by a decline in bee population, sometimes severe bee mortality and fecal stains ("chain of dots") on combs and flight board.

Nosema ceranae, on the other hand, leads to the appearance of flightless bees ("crawlers") throughout the year, the decline of the bee population, lower honey production, colony collapse or an unexpected brood strike in cold months.

Effects on the individual bee:

• Destruction of the intestinal cells
• Disruption of protein metabolism (pollen can no longer be utilized)
• Glandular reduction (forage sap and enzyme production are reduced due to disturbed food intake)
• In case of infestation in autumn: reduced fat body formation (winter bees are not fully formed)
• Reduction of life span up to 30 % (pollen can no longer be fully utilized)
• Reduced resistance to other infections (e.g. some viroses, malpighamoeba)
• Infected queens: Ovaries regress, egg production decreases

Effect on the entire colony:

• Increased loss of flight bees; flight bees die outside the hive.
• Young bees start to collect prematurely to compensate for the loss of the collectors, thus limited brood care and shortened life span of the adult bees.
• Disturbance of the balance in the colony (division of tasks)
• Rapid weakening of the colony (if not enough brood is available to replace the loss of adult bees)
• Death of the colony especially in case of infected queen (winter loss, queenless colony)
• Increasing risk of development of brood diseases (e.g. lime brood)

Factors that promote disease outbreak:

• Long winter period without opportunity for cleaning flights (risk of bees depositing spore-contaminated feces in hive).
• Early start of brood care during subsequent cold weather periods
• Too early and generous expansion
• Poor pollen supply
• Unsuitable location (damp, shady in spring)
• Flight stop (cold weather, misplaced or too small flight hole)
• Stress of bees due to disturbances (frequent opening and working of colonies in spring)
• lack of bees
• Lack of food (especially in the summer after the bees have been ejected, during the rearing of the winter bees) or unsuitable winter food

Prevention

• Choice of a suitable location: protected from the wind, sunny, not in a cold air pool and with a sufficient supply of nectar and pollen (to promote brood turnover)
• Sanitation measures: Melt down combs of diseased colonies, melt down older combs, remove dead bees from the ground, and set up a clean bee trough with no fecal input
• Winter food: feed with suitable winter food in time and sufficiently, if possible do not leave forest and melezitose honey in the colony.
• Operation: adapt the operation to the life cycle of the bee colony and to the conditions of the hive, avoid massing colonies at one location, do not winter weak or sick colonies and do not unite sick colonies with healthy colonies.

In Austria, there are no approved control agents against the two pathogens of nosemosis. If symptoms of nosemosis are visible, keep the colonies close together, feed them to promote bee turnover and melt down any combs that are covered with bees. Reposition the colony with a young, vital queen. Kill severely diseased, weakened colonies.