Emerging and re-emerging viruses of the honey bee (Apis mellifera L.)

Abstract

Until the late 1980s, specific viral infections of the honey bee were generally considered harmless in all countries. Then, with the worldwide introduction of the ectoparasite mite Varroa destructor, beekeepers encountered increasing difficulties in maintaining their colonies. Epidemiological surveys and laboratory experiments have demonstrated that the newly acquired virulence of several viruses belonging to the family Dicistroviridae (acute bee paralysis virus, Kashmir bee virus and Israeli acute paralysis virus) in Europe and the USA had been observed in relation with V. destructor acting as a disseminator of these viruses between and within bee colonies and as an activator of virus multiplication in the infected individuals: bee larvae and adults. Equal emphasis is given to deformed wing virus (DWV) belonging to the Iflaviridae. Overt outbreaks of DWV infections have been shown to be linked to the ability of V. destructor to act not only as a mechanical vector of DWV but also as a biological vector. Its replication in mites prior to its vectoring into pupae seemed to be necessary and sufficient for the induction of a overt infection in pupae developing in non-viable bees with deformed wings. DWV in V. destructor infested colonies is now considered as one of the key players of the final collapse. Various approaches for combating bee viral diseases are described: they include selection of tolerant bees, RNA interference and prevention of new pathogen introduction. None of these approaches are expected to lead to enhanced bee-health in the short term.

Figure 1.

Non-viable, adult bee (Apis mellifera) exhibiting deformed wings as clinical symptom of an overt DWV infection. A V. destructor individual is still clinging to one of the legs (picture taken by Michael Traynor). (A color version of this figure is available at www.vetres.org.)

Figure 2.

Transmission routes for DWV at the individual bee level within colonies. The vectorial transmission of DWV has been experimentally proven [52, 133]. Evidence for additional horizontal routes between nurse bees and larvae through larval food has also been provided [132]. The best analyzed transmission route for DWV is the vectorial transmission through the ectoparasitic mite V. destructor. DWV is transmitted by mites to parasitized pupae and during the phoretic phase to adult bees. The mites in turn acquire DWV when feeding on infected pupae and adult bees. Non-viable bees exhibiting deformed wings as the most prominent clinical symptom only occur when DWV transmission to pupae via V. destructor initiated an overt infection.