Managing pests and disease is necessary in order to maintain healthy honey bee colonies. This presents one of the biggest challenges to maintaining vigorous bee hives and successful over wintering. Therefore, competent beekeepers are aware specifically of the damage caused by the varroa mite but may be less aware of viral diseases of honey bees. Also there may be limited awareness of how these mites and viruses synergistically harm our bees. Varroa mites are a means of spread, or a vector, for a range of viral diseases and over the next few weeks we will provide some information on a few. We will start this series by looking at Deformed Wing Virus.
Deformed Wing Virus: A Persistent Pathogen of Honey Bees
There are more than 20 viruses which can infect honey bees
but one of the most common and easily identifiable is the Deformed Wing Virus
(DWV). So, unlike other viruses which
may be less visible, the symptoms of DWV are obvious to the observant
beekeeper. The body malformations
resulting from DWV infection are shown in the figure. This RNA virus, base on the nucleic acids
which make up its genome, is ubiquitous.
One study, indicated that DWV was found in 100% of varroa mites in countries
where testing was conducted (Tantillo et al, 2015). The virus can be transmitted horizontally via
the varroa mite. The strong association
between varroa infestation and deformed wing virus is such that the
malformation of the wings was originally thought to be caused by the loss of
hemolymph due to varroa infestation.
Later molecular diagnostic techniques disproved this theory but DVW and
varroa still go hand-in-hand.
The DWV exists as two well known strains, the DWV-A and
DWV-B, as well as a recently discovered third or C strain. As with all viral strains, evolutionary
pressure causes changes in the dominant strain.
It is thought that DWV-B is currently dominant with DWV-C perhaps
emerging. This will of course be
determined by the host, individual or colony, susceptibility to the different
variants.
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Figure 1. Honey bee with wing malformations due to Deformed
Wind Virus. Photo: Traynor, M. |
The implications for foraging ability of DWV infected bees
are obvious but there are perhaps more subtle damaging effects. It is suggested that DWV alone cannot bring
down a colony but will, if combined with varroa infestation, devastate a hive. Viruses can exist as a covert-persistent
infection or acute-overt infection. The
first type will reflect low levels of infection, show a slow replication rate
and little or no outward symptoms. The acute-overt
infection manifests itself in obvious clinical symptoms and ill health of
individual bees and the entire colony.
In the case of the DWV, the transition from covert-persistent to
overt-acute may involve the varroa mite!
What is known is that Varroa destructor is a vector
for the DWV. The combination of an DWV
infection and weakening effects of a high varroa load is thought to result in
the devastation overt-acute infection. So,
although DVW can transmit vertically, for example from drones to queens or from
queens to eggs, we know that varroa mites are a horizontal vector for DVW
transmission. It is also known that
honey bees can maintain a covert -persistent DVW infection but a combined
varroa mite infestation may act as a trigger leading to more damaging effects.
There is considerable research currently ongoing to better
understand DVW. For example, does the virus
replicate in the mite is a question currently being explored. What is obvious is that malformed wings of the
honey bees will result in these newly emerged workers quickly dying and contributing
nothing to the overall colony. There
also may be other effects on the bees which are as yet unknown. We can be assured that there is a strong
association between DVW and varroa mites and that this is a significant disease
of the honey bee which can be managed through control of varroa mite
populations.
Tantillo,G., M. Bottaro, A. Di Pinto, V. Martella, P. Di
Pinto, V. Terio (2015) Virus infections of honeybees Apis Mellifera, Italian
journal of food safety., 4.
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