All forms of life are attacked by viruses and honey bees are no exception. A virus particle is simply a small piece of genetic material, usually RNA (ribonucleic acid), enclosed in a protein shell. Viruses are not able live independently and need to live and replicate as part of their hosts cells. When the host cell dies it releases millions of virus particles ready to infect other cells.
There is a wide variety of virus types, each type being host specific or with a narrow host range. Very different virus particles often look similar, even under an electron microscope, and are only distinguishable using special methods of identification. The viruses of honey bees are very common and of many different types. With the global spread of Varroa and colony collapse disorder research interest has grown about the effects these organisms on honey bees.
Modes of infection are by ingestion, inhalation via the tracheal tubes and direct injection into the body fluid. The number of virus particles transferred by inhalation and ingestion is normally low and bees have a considerable degree of resistance to viruses under normal circumstances. It is the direct injection infection route that most threatens honey bee survival and productivity. Damage to the integument (skin) of the honey bee larva by breeding Varroa mites is a leading cause of this mode of infection.
Consequently, before the widespread infestation of colonies by Varroa destructor honey bee viruses were generally an insignificant problem. A characteristic feature of honey bee viruses is that, in most cases, they live, multiply and spread within their host without causing any obvious ill effect. However, the introduction of Varroa destructor changed that and now the effect of viruses on a Varroa damaged colony may be one of the most significant problems of beekeeping in many parts of the world.
There are at least 20 honey bee viruses already identified and there will undoubtedly be more. Sacbrood is an example of a virus causing obvious symptoms. The sacbrood virus interferes with the shedding of larval skins during development and prevents pupation. In Asia the Thai sacbrood virus is a significant problem for Apis cerana.
Many viruses are associated with other diseases and problems. For instance, Nosema is associated with black queen cell virus, bee virus Y and filamentous virus. Varroa is associated with all of the chronic, acute and slow paralysis viruses, as well as cloudy and deformed wing viruses. CCD (colony collapse disorder) has been shown to have a relationship with Israeli acute paralysis virus (IAPV). In each case the effect of the virus is thought to act to increase the severity of the disease.
There is no cure for viruses and the only means of reducing them is to control the agent which is causing their spread. Breeding for resistance will assist in controlling the problem and encouraging native and locally adapted bees will probably be of benefit. Moving bees about, and keeping large numbers of bees in one place, exacerbate infection.
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