Apiculture Factsheet #230
Kashmir Bee Virus
Kashmir Bee Virus (KBV) was diagnosed in the Fraser Valley in the
spring of 2004, in a commercial operation whose honeybee colonies
had declined and subsequently died. It has also been recently found
in colonies that appear healthy.
The virus had previously been diagnosed in British Columbia in
the early 1980s, in honeybee stock originally imported from
Australia and New Zealand. At that time, well before the Varroa mite
arrived, no symptoms had been observed and therefore, KBV was not a
In the late 1980s, researchers in Europe and the US reported that
KBV was a highly contagious and virulent pathogen in the presence of
Varroa mites. Over the years, little information became available
about KBV and its distribution and impact, since colony losses were
mostly attributed to mite parasitism or other causes without proof.
KBV and other viral honeybee agents may not have received much
attention because disease symptoms were generally not as definitive
as others, while laboratory analysis required expertise and
The precise distribution of KBV in North America is not known.
KBV was detected in 1995 in a small US survey of seven states. All
states were positive: California, Florida, Maine, Minnesota,
New York, Texas, and Washington. It has been suggested that the
virus is endemically present throughout
most of North America’s honeybee population.
KBV Origin and Classification
The Kashmir Bee Virus is a natural disease of the eastern
honeybee Apis cerana. After the introduction of the western
honeybee Apis mellifera into the distribution range of A.
cerana in southern Asia, the virus made a “species-jump” and
began to parasitize its new host.
KBV of honeybees is a RNA-virus in the newly established family
Dicistroviridae. RNA viruses are very small (in comparison to DNA-viruses)
and associated with the mitochondria of host cells.
KBV’s virulence is made possible because of its association with
the Varroa mite. Mites carry the viruses externally and internally.
By piercing the honeybee’s cuticle, they transfer viral particles
into the host’s tissue. The bee’s pupal stage appears most
susceptible to infection. In the confined space of the capped pupal
cell, viruses are not only transferred from mite to bee, but also
from mite to mite. After several mite generations in the honeybee
colony, the majority of mites will be KBV carriers. As more bees
become infected, the transfer of food, grooming and other physical
contact between bees facilitates the further spread of the virus.
It is not known whether the honey bee tracheal mite (Acarapis
woodi) and Nosema (Nosema apis) are viral vectors as
well. Since these pathogens cause tissue damage in the tracheal
tubes and epithelial cell wall of the midgut of adult bees
respectively, they may play a role in the introduction of viral
agents into the host’s body cavity.
Expression of Virulence
At this time, the causes that trigger virulence are unclear.
Throughout the 1980s, the presence of KBV in BC colonies was an
academic curiosity, as the virus remained non-virulent or in a
latent state. Even after the introduction of Varroa in British
Columbia in 1990, there were no reports of viral infestations. In
recent years, occasional incidents of colony losses that could not
be attributed to common honeybee diseases may have involved KBV.
Some studies have shown that high mite levels result in high
virulence. This correlation can be readily accepted as large numbers
of KBV-carrying mites contribute to the rapid spread of the virus
throughout the bee population, while stress caused by mite
parasitism increases the honeybee’s susceptibility to viral
infection. However, surveys carried out by BC Ministry of
Agriculture indicate that KBV
virulence also occurs when Varroa infestation levels are low or
There is no prescribed set of symptoms confirming KBV in the
field. Definitive diagnosis is carried out in the laboratory through
Polymerase Chain Reaction (PCR) analysis. However, beekeepers may
observe various symptoms that may point to viral infection. These
- Weakening of the colony without any apparent presence of brood
diseases and mites.
- Increasing numbers of dead or dying bees on the inner cover,
landing board or in front of the hive. Dying bees may be trembling
and display uncoordinated movement.
- Affected bees are partly or completely hairless where the
upper surface of the thorax is especially dark.
- Older adult bees have a greasy or oily appearance while
recently emerged bees may appear opaque as if pigmentation of the
tissue had not been completed prior to emergence.
Sample Collection and Diagnosis
For definitive identification, whole adult bees must be analyzed
in the laboratory. Adult bees are highly perishable and need to be
preserved as best as possible after collection. The following
collection method is recommended:
- Collect live bees from the hive entrance or in front of the
hive. Select bees that appear hairless and greasy.
- Collect 10 live adult bees by squeezing their head-thorax and
place in a paper lunch bag or paper envelope. Do not
place bees in a plastic bag, wrapping or container.
- Mark the sample bag or envelope with your name and
corresponding hive number or apiary and place in a freezer for 24
- Submit sample bag(s) to the Apiculture office for
- KBV diagnosis of a sample of bees collected from one colony
does not mean the viral infestation is limited to that single
colony. Due to the highly contagious nature of KBV, the entire
apiary is suspect and should be considered KBV positive. As a
result, a single composite apiary sample can also be collected,
with a number of colonies contributing bees to the sample.
There is no product available for KBV control. Most viral
infections become evident when bees have been stressed due to other
diseases, weather conditions or management practices. Some bee
stocks have shown higher susceptibility to viral infection than
others; this can be remedied by replacing the queen with a queen
from another source.
To minimize the impact of KBV and other viral infections:
- Reduce stress to bees by applying good management practices.
- Provide plenty of food stores, especially pollen.
- Keep mite levels low through frequent monitoring and applying
mite control products when necessary.
- Apply hygienic management practices in the apiary.
For more information, contact the Apiculture office in Abbotsford
at 604-556-3129 / 604-556-3152.