Nosema apis

 Nosema apis is a microsporidian, a small, unicellular parasite that mainly affects honey bees. It causes nosemosis, also called nosema, which is the most widespread of the adult honey bee diseases. The dormant stage of nosema is a long lived spore which is resistant to temperature extremes and dehydration. Nosema apis spores cannot be killed by freezing contaminated comb. Nosemosis is a listed disease with the Office International des Epizooties (OIE).

Pathogene
Until recently Nosema apis has been considered to be single-celled protozoan pathogen of the western honey bee (Apis mellifera). Nosema apis is an unicellular parasite of the class of microsporidian (Microsporidia), that are now considered to the fungi. Nosema apis has a resistant spore, which 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 (Huang et al.) found Nosema ceranae in spring 2005 in Taiwan for the first time also on the western honey bee (Apis mellifera). Shortly afterwards Spanish bee researcher (Higes et al.) reported, that the new pathogene was also discovered in 2005 in Spain and has according to their realisation 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, which die faraway the dwellings, and conditioning „dead head flights“ and collapsing of the bee colonies). Furthermore it was observed within fewer years very strongly increased propagation of Nosema and their occurrence at unusual seasons (all-year-season), which is due to obviously the higher resistance of Nosema ceranae. Therefore also a higher reinfection rate of the bee colonies is assumed, since the pathogene 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 (PCR).

As alarmingly the researchers regard the circumstance, that Nosema ceranae in Spain obviously win through against Nosema apis (nearly only eastern specimens were found). Therefore they bring an emerging of this pathogene in relation to the substantial bee dying observed in Spain since autumn 2004. They assume that also in other European countries similar findings could be placed, as there were also reported from France (since end of the 90's) and Germany (2002/2003) about increased and so far conclusively not cleared up colony losses.

In the first samples of German reference laboratories in the winter 2005/2006 the new type of pathogene was also proved in Germany in eight of ten examined bee hives (CVUA Freiburg), whereby the distribution varies from federal state to federal state. The bees with the classical pathogene Nosema apis came from Thuringia and Bavaria, whereas Nosema ceranae prevailed in Baden-Wuerttemberg, Bavaria and North Rhine-Westphalia. Meanwhile there were also reported from Switzerland (July 2006) and from several regions of Italy (September 2006) from Nosema ceranae findings at bee colonies with increased mortality.

The German scientists (Ritter, CVUA Freiburg) however ask themselves, whether „the eastern “pathogene (the origin is not really clarified) could not be already longer present in Europe and was not differentiated so far from Nosema apis. Possibly the current disease processes are more extreme with the rise of a Nosema affliction, because the colonies are weakened by the Varroa mite and other factors altogether and therefore more susceptible. There are however signs for the fact that the process of the Nosema has changed and the disease arises now contrary to the classical form during the whole year.

The investigation of 131 bee colonies from Bavaria, clinically remarkable in the majority, within the framework of a dissertation (Zohni, July 2006) supports the thesis of a causal participation of bee viruses, which were transferred by arthropods (for instance the Varroa mite), at the periodically arising mass losses. Since only comparatively few of these colonies were afflicted with microsporidians to attribute (provided evidence in 14,5% of the cases affliction of microsporidian spores, in each case half of these findings were to Nosema apis and/or Nosema ceranae), a correlation between microsporidian affliction and virus infection against it could not be determined. The question, whether the colonies dying rather on „the new“ version of the Nosema, which (possibly) possesses a higher pathogenicity, or due to virus diseases connected with varroa affliction, is internationally controversially continued to discuss therefore among scientists and beekeepers.

See also: Nosema ceranae - a new threat to Apis mellifera honey bees

Symptoms
The symptoms of the Nosema are relatively nonspecific, whereby confusions with other diseases of the honeybee can happen. It arises mostly in the spring after bad weather periods. Most strongly the female workers are afflicted, less the drones. Since afflicted bees hardly participate in feeding the bee queen, it is likewise more rarely infected. Most important symptom is dysentery (“diarrhea”), which shows itself through yellow strips outside of the colony. In addition flight inability (“crawler”) can occur due to disjointed wings. Further symptoms are increase of girth the abdomen, missing sting reflex and earlier supersedure of the queen. The honey production and life expectancy of the bees sink due to the reduced utilization of pollen. In the case of a disease of the queen it comes to a degeneration of its ovaries and its ovum production drops due to atrophie of the ovums. Thereby a supersedure of the queen is released in the colony. The disease patterns described by Higes et al. in Spain and related to the affliction Nosema ceranae exhibits some distinctions. The changes in the digestive system, observed in afflicted bees, were substantially more seriously than known injuries with Nosema apis, which are related with a particularly heavy and spacious cell lesions. On the other hand classical symptoms were missing such as diarrhea, crawler, remarkable case of dead in proximity of the apiary etc. as reason for the continuous reduction of the number of bees of the colony the afflicition of the field bees are among other things supposed, which do not return any longer and far far away from the apiary die. Thereby caused less and less fodder is brought in, what can lead together with the remaining consequences of the bee decrease up to the colony collapse. Also Ritter (CVUA Freiburg) reports of changes in the disease process of the Nosema, so in contrary to the classical creeping form the crawler and losses arose during the whole year. In the winter some colonies died within a short time and the bees lay dead in the box (differently than in Spain, where those hives remained usually simply bee empty). Whether these features are related to the new form of the Nosema, currently cannot be clarified conclusively.

Transmission
Newly emerged bees are always free from infection. Spores must be swallowed by a bee for the infection to be initiated. Spores germinate quickly after entering the ventriculus, and the epithelial cells of the ventriculus are infected when the vegetative stage is introduced by way of the hollow polar filament. Once inside a cell, the vegetative stage increases in size and multiplies, effecting an apparent concurrent reduction of RNA synthesis in the host cell. In 6-10 days the infected host epithelial cell becomes filled with new spores. Epithelial cells are normally shed into the ventriculus where they burst - releasing digestive enzymes. When infected cells are shed similarly, they release infective spores when they burst (30-50 million).

Effects on the hive
Nosema spores are spread to other colony members through fecal matter. The disease impairs the digestion of pollen thereby shortening the life of the bee. A greater proportion of worker bees become infected than drones or queens, probably due to the comb cleaning activities of young bees in which drones and queens do not participate. Nosema infected bees do not attend or feed the queen to the same extent as healthy bees, which helps the queen to escape infection. When the queen becomes infected her ovaries degenerate and her egg laying capacity is reduced due to atrophy of the oocytes. Queens that become infected by the parasite during the brood rearing season are superseded by the bees.

The seasonal trend of typical infections exhibits low levels during summer, a small peak in autumn, and a slow rise of infection during winter. In the spring the level of infection increases rapidly as brood rearing starts and while flight possibilities are still limited. Colonies in Northern climates are more seriously affected than colonies in the South because of the increased amount of time bees are confined in the hive. Nosema, if left untreated, can cause queen supersedure, winter kills, reduced honey yields and dwindling populations. It is more common during times of confinement like winter and spring.

Diagnosis
Diagnosis is dependent on microscopic examination of the ventricular content and/or fecal matter. No specific outward sign of disease is present, although in dissections the ventriculus often appears whitish and swollen in a late stage of infection. The disease is easily detected in samples of whole bees macerated in water. The fluid is examined under light microscope at 250-500 x magnification where the characteristic Nosema spores can be observed. Thereby the two pathogene types Nosema apis and Nosema ceranae can be only very difficulty differentiated, or not ( it requires genetical investigations with PCR).

Treatment
Treatment with the antibiotic Fumidil B (prepared from Aspergillus fumigatus, the causative agent of Stone Brood) inhibits the spores reproducing in the ventriculus, but does not kill the spores. A disinfection of the honeycombs and utensils is recommended. The spores are sensitive opposite acetic acid, Formalin, ultrasonic and gamma radiation. Also a biological fight by breeding folder formation is possible. A prevention is possible by care measures such as Alter parentall care. The International Federation of Beekeepers' Association (Apimondia recommend the natural product 'Protofil' as prevention remedy. Heat treatment in 49 C for 24 hours can be used to kill the spores.

Nosema in other animals
Members of the genus Nosema also afflict other species of insects, e.g. Nosema vespula (European wasps), Nosema oulemae (cereal leaf beetle), Nosema trichoplusiae (moth Trichoplusia ni), Nosema furnacalis (Ostrinia furnacalis), Nosema necatrix (cutworm moth Mythimna unipuncta), Nosema bombycis (silk moth) and Nosema pyrrhocoridis (Firebug). The mark craze (Pébrine disease), caused by Nosema bombycis, is one of the most important parasitic diseases of the silk moth.

Other pathogens similar to Nosema sp. cause diseases in mammals. In veterinary practice the term “Nosema” is sometimes used to refer to Encephalitozoonosis, a disease common in rabbits, in which the brain is infected with the intracellular parasitic microsporidian Encephalitozoon cuniculi. The term arises from the fact that encephalitozoon (other kinds afflict humans) were formerly classified as members of the genus Nosema.