Deformed wing disease
The deformed wing disease virus (DWV) is one of the most widespread viruses in Apis mellifera and one of the least virulent. In the absence of facilitating factors, it causes only a covert infection, that is, an infection without clinical signs.
General information
The deformed wing disease virus (DWV) is one of the most widespread viruses in Apis mellifera and one of the least virulent. In the absence of facilitating factors, it causes only a covert infection, that is, an infection without clinical signs. Its worldwide distribution (except in Oceania) and the damage it causes to colonies are linked to its close association with the mite Varroa destructor. Its existence remained unsuspected until apiaries became infested with varroa; nevertheless, it must already have been part of the spectrum of viruses capable of infecting Apis mellifera, maintaining a balanced relationship with its host without affecting its health. In association with the mite, it is widely implicated in the aetiology of winter losses.
Susceptibility
All castes and all developmental stages (egg, larvae, pupae, adults) can carry DWV, but pupae are the most susceptible to infection. Their development is disrupted, resulting in malformations or death. Hygienic strains and strong colonies are the least susceptible.
Transmission
The virus is transmitted:
- orally between bees: through trophallaxis or feeding of larvae
- vertically: from queens and drones to their offspring
- by varroa mites to adults and especially to pupae; this mode of transmission triggers the disease
Symptoms
The disease manifests mainly at the end of summer and in autumn. It is characterised by the emergence of bees with malformed wings (reduced, crumpled, shrivelled), a shortened abdomen, and sometimes abnormalities in body coloration. The lifespan of these workers, which are functionally ineffective in any case, is very short, but the lifespan of infected bees even without obvious symptoms may also be reduced. Mortality may be observed at the pupal stage. The virus is also suspected of causing impairment of certain learning abilities and of olfaction in workers, which would disrupt their behaviour and reduce their foraging performance. The combined effects of these pathogenic actions lead to a weakening of the colony that may progress to collapse, particularly at the beginning of winter, when the cluster becomes too small to withstand the cold.
Prognosis
It is very poor for weak colonies that are heavily infested with varroa mites.
Diagnosis
Diagnosis is relatively straightforward when, at the end of summer or in autumn, bees with malformed wings are observed, often in association with colony weakening. It can be confirmed by laboratory analysis of a sample of symptomatic bees.
Management
To eliminate the clinical form and prevent winter mortality, the following measures are required:
- early and effective treatment against varroa
- overwintering of strong colonies
- selection of hygienic strains
It is not uncommon to observe bees with malformations for several weeks after the application of acaricide treatment; however, this does not necessarily call its effectiveness into question. The virus continues to exert its damaging effects for some time after the varroa population has been brought under control.
See also:
- Varroa destructor – biology and health impact
- Major honey bee viruses
- Removal of drone brood for varroa control
