Nosema apis is a microsporidian, a small, unicellular parasite recently reclassified as a fungus that mainly affects honey bees. It causes nosemosis, also called nosema, which is the most common and widespread of adult honey bee diseases. 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. Nosemosis is a listed disease with the Office International des Epizooties (OIE). Pathology Nosema apis is a single-celled parasite of the western honey bee (Apis mellifera). The species is of the class Microsporidia, which were previously thought to be protozoans, but are now classified as fungi or fungi-related. Microsporidia are intracellular parasites and they infect the epithelial cells of the midgut. N. apis has a resistant spore that withstands temperature extremes and dehydration. In 1996, a similar microsporidian parasite of the eastern honey bee (Apis cerana) was discovered in Asia, which was named Nosema ceranae. Little is known about the symptoms and the course of the disease. Chinese researchers found Nosema ceranae in spring 2005 in Taiwan for the first time, and it has now been seen on western honey bees. The new pathogen was discovered in 2005 in Spain and was observed to have a notably higher virulence than the western version. The disease caused by N. ceranae in western honey bees in Spain is related to heavier disease patterns deviating from the previously typical findings (unusually heavy intestine injuries in the bees, no diarrhea, preferential affliction of older collecting bees). Bees die far away from the dwellings, as when they leave they are too weak to return. This leads to collapse of the bee colony. Within a few years, a strongly increased propagation of Nosema was observed, and its occurrence was happening all year round due to the higher resistance of N. ceranae. A higher reinfection rate of the bee colonies is assumed, since the pathogen survives longer in the external environment. 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. Spanish researchers regard with alarm the insurgence of N. ceranae in Spain, which has now replaced N. apis. 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