Pathogen
Until recently Nosema apis had been considered to be a single-celled protozoan pathogen of the Western honey bee (Apis mellifera). Nosema apis is a unicellular parasite of the class Microsporidia, which are now classified as fungi or fungi-related. Nosema 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. There is little 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. Spanish bee researchers (Higes et al.) reported that 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 Nosema 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). It was observed within a few years that there had been a strongly increased propagation of Nosema and their occurrence was happening all year round due to the higher resistance of Nosema 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 Nosema ceranae in Spain, which has now replaced Nosema 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 conjecture 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.
In the samples of examined in German laboratories in the winter of 2005/2006, the new pathogen was present in eight of ten examined bee hives examined (CVUA Freiburg), with the distribution varying from state to state. The bees with the classical pathogen Nosema apis came from Thuringia and Bavaria, whereas Nosema ceranae prevailed in Baden-Wuerttemberg, Bavaria and North Rhine-Westphalia. There were also reports from Switzerland (July 2006) and from several regions of Italy (September 2006) where Nosema ceranae were found in bee colonies with increased mortality.
German scientists do not know whether Nosema ceranae was already present in Europe and simply had not yet been differentiated from Nosema apis. It is possible the current disease processes 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. There are however signs that the disease process of Nosema has changed, and that the disease arises now all year round.
The investigation of 131 bee colonies from Bavaria within the framework of a dissertation supports the thesis of a causal participation between bee viruses, which were transferred by arthropods (for instance the Varroa mite), and the periodically arising mass losses of life in the hives. Since only comparatively few of these colonies were afflicted with microsporidians (evidence showed 14.5% of the cases were afflicted with microsporidian spores, with half of the cases by Nosema apis and/or Nosema ceranae), a correlation between microsporidian affliction and virus infection could not be determined. 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 internationally controversially continued to be discussed among scientists and beekeepers.
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