Nosema apis is a microsporidian, a small, unicellular parasite recently reclassified as a fungus that mainly affects honey bees.

[1] The dormant stage of N. apis is a long-lived spore which is resistant to temperature extremes and dehydration, and cannot be killed by freezing the contaminated comb.

The two pathogen types cannot be differentiated with usual routine investigations, but can be distinguished only with the assistance of molecular-genetic methods such as polymerase chain reaction.

Because of this newly emergent parasite, the pathogen is assumed to be related to the substantial bee mortality observed in Spain since autumn 2004.

They conjectured a similar cause of increased bee colony losses reported in other European countries, such as those experienced in France since end of the 1990s and in Germany in 2002 and 2003.

Cases were also reported from Switzerland (July 2006) and from several regions of Italy (September 2006) where N. ceranae was found in bee colonies with increased mortality.

The current disease processes possibly are more extreme when a Nosema affliction occurs because the colonies are already weakened by the Varroa mite or other factors that make them more susceptible.

The question of whether the colonies were dying rather from the "new" version of Nosema, which (possibly) possesses a higher pathogenicity, or due to virus diseases connected with Varroa affliction, is controversially continuing to be discussed internationally among scientists and beekeepers.

Bees tend to die away from the apiary, which causes a reduction in food gathered and can eventually lead to colony collapse.

[6] In the winter, some colonies died within a short time and the bees lay dead in the box (in Spain, hives usually remained empty).

[8][9] Spores germinate quickly after entering the insect, and the epithelial cells of the ventriculus are infected when the vegetative stage is introduced by way of the hollow polar filament.

Nosema, if left untreated, can cause queen supersedure, winter kills, reduced honey yields, and dwindling populations.

Diagnosis is dependent on microscopic examination of the ventricular (midgut) content and/or fecal matter or on PCR analysis of infected tissue.

No specific outward sign of disease may be present, although in dissections, the ventriculus often appears whitish and swollen in late stages of infection.

[citation needed] Though the spores of N. apis and N. ceranae have slight morphological and ultrastructural differences, they cannot be reliably differentiated via light microscopy.