The genus Nosema belongs to the division Zygomycota and the class Microsporidia. It is a unicellular, obligatory intracellular parasite and to date only two species are known to parasitize honeybees: Nosema apis and Nosema ceranae. In the past Nosema apis was found in Apis mellifera whereas Nosema ceranae infected Apis cerana. However more recently both species have been found but in Apis mellifera (Paxton et al 2007). Both species infect the ventriculus (stomach) of the adult causing dysentery. They are transmitted horizontally for instance via spore ingestion, whilst workers are cleaning combs of infected faeces (Paxton et al 2007). Usually Nosema is not a big problem for the bees, but in certain condition (indoor wintering, rainy summer or long transit distances for foraging) bees are unable to defecate outside the hive as they would usually do because the hives are closed or simply because the rain they can’t go out. This can increase the level of parasite in the hive and the colony may be no longer able to manage the infection.

In the past some viruses (Black queen cell virus, Filamentous virus, and Bee virus Y ) were associated with Nosema (Bayley & Ball 1991), (Benjeddou et al 2001) but the mechanisms of any association remain unclear. One hypothesis is that Nosema could interfere with the antiviral resistance mechanisms (Bayley & Ball 1991).

Figure 1 shows two picture evidentiating  the different morphology of N. ceranae and N apis

(Beecraft, 2008) .

Fig 1: Nosema ceranae (left) and Nosema apis (right) with a single spore inset (x 400 and x 1000 magnification, respectively) (Beecraft, 2008)

Follow the link to FERA -NBU website to know more about Nosema or the chapter 2.2.4 of the O.I.E. Terrestrial manual (2008).