Nosema ceranae

Discover the intriguing journey of a beekeeper’s quest for answers about Nosema ceranae. In March, they noticed their colonies weren’t thriving despite Varroa treatment. Follow the investigation into these struggling hives, uncovering a silent threat – Nosema ceranae. Explore the diagnosis, treatment recommendations, and the broader implications for beekeeping in a changing world.

What happened?


In March, a beekeeper contacts their local bee inspector office to ask for an inspection. In one of his apiaries, he found several colonies that are not progressing as they should. They were treated against varroa mites (Varroa destructor) a few weeks ago and, although the beekeeper cannot detect mites when visually inspecting these hives, the population build-up does not increase like it does in other colonies. The vet / bee inspector visits within 48 hours to inspect the hives and take samples.


The beekeeper owns a commercial operation with 2000 hives. The colonies spent the past autumn in a mountainous area, the queens were replaced in the previous year, all replacements came from the same operation. The honey harvest that year was on a low to medium level compared with other years.Now the colonies are in a riverside area, in an environment that combines low grass and some abandoned citrus crops, near the railway line. The climate is Mediterranean, with an early and warm spring and there is abundant nectar and pollen sources available. The hives have been in the same place for several weeks already as the relatively high temperatures in late winter led to an earlier beginning of the season.The hives were fed glucose in winter and sucrose syrup in the spring.The beekeeper applied a treatment against Varroa with fluvalinate 3 weeks before the visit, he has only done a visual inspection of parasitism.Two neighbouring apiaries of other beekeepers are located 1.5 and 2 km away from this apiary.As a prophylactic measure, the beekeeper scorches the hives that remain empty with a torch, he has not implemented any other hygienic measures despite having attended hygiene courses that the bee inspector office organized in previous years.

The Inspection:

During the inspection, the veterinarian observes an apiary of about 80-100 hives placed in a row. Buzzing from the bee colonies appears to be reduced considering the morning time. No damage is observed on the boxes, and they are all closed correctly. There are no diarrhoea spots. Worker bee mortality is not observed at the entrances. 6 boxes are opened as a sample, representative of the apiary. Focusing especially on hives that show less vitality, the veterinarian / inspector verifies that no more than 4 frames of bees and 2 to 2.5 frames of brood are present. There is a moderate input of nectar and very low reserves of pollen and honey in the adjoining frames. A normal rate of drones, eggs and young larvae is present in the colonies, but in very small amounts. No symptoms can be observed in the brood, the caps are intact and there is no suspicious odour. An alcohol wash of nurse bees yields a low infestation rate, of 1%. No signs of viral diseases are observed either. Samples of adult bees are taken from the honey boxes and from the entrance. The general impression is that of little-developed colonies, clearly behind with respect to the rest of the operation. Samples are taken from each of the hives that were inspected and kept frozen until they are sent to a bee pathology laboratory.


The laboratory confirms a medium-high infestation by Nosema ceranae in the samples that were sent.


Even though nosemosis causes collapse and mortality, especially during the winter, spring is also a time when strong symptoms of the disease may appear.In this case, in the absence of any other sign of infectious or parasitic disease, and observing a clear lack of vigour as well as slow population build-up, the presumptive diagnosis is nosemosis type C.


Since treatment with the antibiotic fumagillin is prohibited in Europe, a product to support the bee gut health, based on seaweed, thymol and citronella is proposed by the veterinarian / inspector. Additionally, it is recommended to move the hives, especially the most affected ones, to locations with a much more abundant and varied food offer to favour an immune response.


The hives remained in the same apiary throughout the spring and moved to a mountainous area later in summer. Almost all the colonies survived, some, according to the beekeeper, had to be merged because they did not respond to the feed supplement. Some of the treated colonies demonstrated a good honey production while most produced a moderate or low amount of honey.  


The entry of Nosema ceranae in beekeeping has been progressive, to a certain extent silent, because it was often confused with other pathogenic or toxicological agents. The disease was not always clearly detectable because colony collapse occurs due to many different factors. But a pilot program concerned with the loss of honey bee colonies in Spain has recently found that the presence of Nosema spp has increased, and today it affects more than 80% of Spanish colonies in one way or another. The severity of the Symptoms depends on various factors such as the age of the queen, the nutritional status of the colony, occurrence of other pathogens, and of course the level of bee parasitism (varroa, viruses, …). It is, without a doubt, a pathogen which we must keep an eye on, as numerous studies have demonstrated its destructive effects on bee colonies.

At the same time, it is essential to monitor the nutritional status of the hive since it plays a fundamental role for the immune response of the colony. It is also convenient to keep quarantine apiaries to be able to isolate those colonies that, as a precaution, must be separated from the rest.