Nosema apis

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Nosema apis
Scientific classification
Kingdom: Fungi
Phylum: Zygomycota
Class: Dihaplophasea Microsporidia
Order: Dissociodihaplophasida
Family: Nosematidae
Genus: Nosema
Species: N. apis
Binomial name
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 adult honey bee diseases.[citation needed] 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) .

Contents

[edit] 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.[1] 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 Western honey bees. Shortly afterwards Spanish bee researcher (Higes et al.) reported that the new pathogen was also 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, 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 such as PCR.

Spanish researchers regard with alarm the insurgence of Nosema ceranae in Spain, obviously a replacement of Nosema apis since nearly only eastern specimens were found. 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 in France (since end of the 90's) and Germany (2002/2003).

In the first samples of German reference laboratories in the winter 2005/2006 the new type of pathogen 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 (its origin has not been conclusively determined) 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.

[edit] Symptoms

The symptoms of the Nosema are relatively nonspecific. This makes it easy to confuse with other diseases of the honeybee. It arises mostly in the spring after periods of bad weather, although it may also be a winter disease that is only noticed in the spring when beekeepers first inspect their hives. The female worker bees are most strongly afflicted, less so the drones. The queen bee is rarely infected since afflicted bees rarely participate in feeding the queen. The most notable symptom is dysentery. This appears as yellow stripes on the outside of the hive and in severe cases, inside the hive. Bees may also be unable to fly (“crawling”) due to disjointed wings.

Further symptoms include increased girth of the abdomen, missing sting reflex and early supersedure of the queen. If the queen is infected, its ovaries degenerate and ovum production drops due to atrophy of the ova, after which it is likely to be superseded. The disease pattern described by Higes et al.. in Spain for Nosema ceranae is slightly different than with Nosema apis. The changes in the digestive system were substantially more serious than with Nosema apis, related to particularly heavy and spacious cell lesions. On the other hand, classical symptoms were missing from Nosema ceranae, such as diarrhea, crawling, large numbers of dead bees in the apiary etc. Bees tend to die away from the apiary, which causes a reduction in food gathered and can eventually lead to colony collapse. Also Ritter (CVUA Freiburg) reports symptoms can arise throughout the year from Nosema ceranae, in contrast to Nosema apis. In the winter some colonies died within a short time and the bees lay dead in the box (in Spain, hives usually remained empty). Whether these features are related to the new form of Nosema, cannot be conclusively proven.

[edit] 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).

[edit] 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.

[edit] 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).

[edit] 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 for an extensive disease outbreak.

The spores are sensitive to chemicals acetic acid, Formalin and physical radiation: ultrasonic and gamma radiation. A natural treatment product 'Protofil', made of plant extractions, vitamins and microelements, was presented at the International Federation of Beekeepers' Association - Apimondia 2004 as a preventitive remedy.

Heat treatment in 49 C for 24 hours can be used to kill the spores on contaminated equipment.

[edit] 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) 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.

[edit] See also

Fungi portal

[edit] References

  1. ^ Liu YJ, Hodson MC, Hall BD. Loss of the flagellum happened only once in the fungal lineage: phylogenetic structure of kingdom Fungi inferred from RNA polymerase II subunit genes. BMC Evol Biol. 2006 Sep 29;6:74.
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