Filamentous Bee Virus

Filamentous virus (AmFV) affects adult honey bees and is one of the few DNA viruses known in the honey bee. It usually runs a mild course, produces no symptom that can be recognised by eye in the hive and is often found together with Nosema. There is no medicine and none is needed; keeping the colony strong and controlling Nosema and varroa is enough.

What is it?

Its scientific name is Apis mellifera filamentous virus (AmFV). It gets this name because it carries a folded, thread-like (filamentous) structure inside oval, enveloped particles. While most honey bee viruses are RNA viruses, this one is a large DNA virus.

The virus affects the adult bee, not the larva. It multiplies in the bee's fat body and ovaries and enters the blood (haemolymph). In heavy infection the bee's haemolymph, normally clear, becomes milky white and cloudy; this is due to the sheer number of virus particles.

It is widespread in many parts of the world. Its frequency rises especially in spring and early summer.

How does it spread?

• Orally: Food sharing between bees and contaminated food are the most likely route.
• From queen to brood: The presence of the virus in the queen's ovaries and in eggs suggests transmission by this route.
• Varroa: The virus has also been detected in mites; their role as carriers is not fully clear.

How to recognise it

There is no symptom specific to filamentous virus in the hive. Affected bees look normal from the outside; in heavy infection their lifespan may be shortened and the colony may weaken slightly. Milky haemolymph is only noticed when the bee is examined in the laboratory.

So do not attribute symptoms such as larval death, flight problems or inactivity to this virus. With such symptoms other causes are far more likely:

Relationship with Nosema

Filamentous virus is frequently found in the same bees as Nosema apis, and the intensity of both agents rises in the same season. It therefore makes sense for the laboratory to look for Nosema first in colonies that weaken in spring; once Nosema is under control, the virus also becomes less important.

How to confirm it in the hive

There is no field test. A definitive diagnosis is made in the laboratory by microscopic examination of the bee's haemolymph and by molecular tests. In cases of unexplained weakening, send a sample of adult bees via the official veterinary service / local agriculture authority; ask them how the sample should be stored.

What to do

No special intervention is needed for a colony in which filamentous virus has been detected. What needs to be done is to keep the colony resilient against diseases:

1. Control Nosema. For colonies that weaken in spring, ask the laboratory for a Nosema examination; replace old and soiled combs.
2. Keep varroa low. Count regularly and treat with an authorised medicine when the threshold is exceeded.
3. Strengthen nutrition. Make sure there is enough honey and pollen in spring; give syrup and a protein supplement if necessary.
4. Review the queen. Replace an old or poorly laying queen.

Treatment and medicines

There is no medicine against this virus and none is needed. The effect of products sold as "virus medicine" has not been proven; antibiotics have no effect on viruses and leave residues in honey.

Prevention

• Strong, well-fed colonies; a young, productive queen
• Replacing brood combs every few years
• Regular varroa monitoring and timely treatment
• Vigilance against Nosema in late winter and spring: avoid damp, poorly ventilated hives and the stress of bees being confined for long periods