Today, about 20 species of viral bee diseases are known that can affect insects of the genus Apis. Most of them are RNA viruses, and beekeepers can only recognize them through the symptoms they cause in bees. When a specific viral infection is suspected, obtaining an accurate diagnosis involves biochemical or genetic analyses in specialized laboratories.
Some bee viruses replicate in the nervous system, causing behavioural changes, as for example the group of paralysis viruses; Others multiply in the bees’ hemolymph or digestive system. Acute infection by some of these viruses can cause colony collapse and/or a complete loss of the entire colony.
Vertical and horizontal transmission
Indeed, viruses can be transferred vertically or horizontally. Vertical transmission refers to the transfer of a virus from parent to offspring. Horizontal transmission described the transmission of a virus between two members of an ecosystem that are not in a parent-offspring relationship One of the most important vectors for horizontal transmission is Varroa destructor. Alternatively, viruses can be transferred vertically, for example from adults (nursing bees) to brood or from queen to brood.
In general, viruses tend to be present in honeybee colonies without expressing symptoms. Chemical, biological, or nutritional stress can cause a weakened immune state, leading to the development of viral bee diseases. For example, high varroa mite parasitism can weaken the colony severely, leading to an expression of symptoms of different viral infections. In the case of the deformed-wing virus (DWV), viral replication in varroa mites appears to make the virus more virulent and therefore more likely to cause symptoms when it is transferred to bees.
There is no treatment for viral bee diseases available. Beekeepers can only avoid the triggers of the infection by maintaining low varroa infestation levels, a satisfactory level of sanitary conditions in the hive, and ensuring bees have enough food sources available. In the case of viruses transmitted by the varroa vector, infestation and infection go hand in hand; keeping a low level of parasites is the only way to control viral bee diseases at the same time.
Deformed Wing Virus (DWV)
This widespread virus can be found in bees of all developmental stages. In most cases the infection does not lead to the expression of symptoms. In case of high varroa mite infestation in the colony, the virus will replicate in mites as well and be transferred from varroa mites to bees. Replication inside the mite leads to a more virulent version of the DWV when it is transferred to bee pupae. The later emerging bees present deformed wings and swollen abdomens. Thus, they are unable to fly, and they have a reduced live expectancy.
As mentioned before, a sustainable varroa control strategy is essential to minimize the effect of this virus.
Sacbrood Virus (SBV)
Larvae can be found with their heads up in brood cells. If extracted from their cells they show a sac-like appearance which also contains clear fluid. The affected brood eventually dies and turns into a dry brown-black scab inside the cell. The shape is reminiscent of a gondola or boat. The oral infection through nurse bees affects the fat body, muscles and then trachea. Affected larvae do not moult, and the exoskeleton does not dissolve. Although they do not show any visible symptoms, adult bees can be infested, leading to a shorter life expectancy.
Black Queen-Cell Virus (BQCV)
Like SBV only affects queen cells and is one of the most frequent causes of mortality among queen larvae. Affected queen cells can be identified by a typical dark brown to black colouring on the walls. The larvae inside appear yellowish and can later turn dark. While the virus is asymptomatic in adult bees, brood could be infected.
It’s a gut pathogen, often associated with N. apis, that increases susceptibility in workers showing unclear symptoms like disorientation, increasing drifting and the possibility of spreading the infection among other hives.
Viral bee diseases causing paralysis.
Acute bee paralysis virus (ABPV), Israeli acute paralysis virus (IAPV) and Kashmir bee paralysis virus (KBV) belong to the same group, and they share similar characteristics. These viruses affect the bees’ nervous system, causing paralysis of the extremities and eventually death of the affected individual within just a few days. The virus can infect larvae as well as adults and multiplies in the bees’ haemolymph. It can also be transmitted by varroa mites.
Acute Bee Paralysis Virus (ABPV)
Although in most cases the virus is present in small numbers causing no obvious symptoms, under some circumstances could be particularly virulent. When the virus reaches the nervous system is when bees become disoriented (increased drifting), trembling, paralysis and die within a short time. Varroa can act as a vector for ABPV. The virus also reduces winter bees’ ability to survive until spring.
Israeli Acute Paralysis Virus (IAPV)
This virus can be found in the digestive system of bees as well as in their nervous system and hypopharyngeal glands. Infected bees show shivering wings, paralysis, and will eventually die. Varroa mites can act as a vector for the virus. Once transmitted, it quickly multiplies in the bees’ haemolymph, Leading to deadly outcomes in bee brood and adult bees.
Kashmir Bee Virus (KBV)
First described in the Eastern bee Apis cerana, this virus kills adult bees within a short time span after infection. Varroa also acts as a vector of KBV. Black, hairless, and dead bees can be observed inside and outside affected hives.
Chronic Bee Paralysis Virus (CBPV)
Horizontal transmission happens through direct contact between infected and healthy bees. The virus gets into the bees’ haemolymph through wounds in the exoskeleton and by intake of faecal matter (viral diarrhoea). The virus spreads faster in densely populated hives.
In summer, trembling bees can be observed in affected apiaries. Many of them present a swollen abdomen. Diarrhoea spots can be found inside the hive, and hairless black bees expelled from the hive can be seen outside.
In many cases, clinical signs disappear after a few days, in severe cases a high mortality of adult bees can compromise the colony.
Others
Cloudy Wing Virus (CWV)
Closely linked to varroa mite infestation. The bee´s wings lose their transparency and take on a greyish appearance. Causes early death of adult workers.
Slow Bee Paralysis Virus (SBPV)
Mainly an adult bee disease. It’s asymptomatic when the infection is oral and causes severe disease when varroa acts as a vector. 10-12 days post-infection affected bees shows paralysis in the first two pair of legs and after that die.
