What Is Cloudy Wing Virus? A Beekeeper’s Guide

A beekeeper inspecting for diseases like Cloudy Wing Virus

Cloudy wing virus (CWV) is an unclassified viral infection that affects honey bees (Apis mellifera). This little-studied disease produces unproductive bees that typically die within a few days of contraction.

CWV is commonly diagnosed in hives with varroa mite infestations. Although researchers don’t have conclusive research, they hypothesize that these mites are a vector for the infection, similar to Nosemosis and Deformed Wing Virus.

What are the symptoms of Cloudy Wing Virus?

As the name suggests, cloudy-wing virus may result in honey bees with opaque wings. But this symptom doesn’t always present, making it an unreliable method of diagnosis.  

Beekeepers will find cloudy wing virus challenging to diagnose. It shares symptoms with other common viral bee diseases like chronic bee paralysis virus. For conclusive results, laboratory testing is recommended. Signs of CWV include:

  • Bees crawling or paralyzed in front of the hive.
  • Cloudy, opaque wings.
  • Foul-smelling dysentery.
  • Large numbers of dead bees outside the hive.
  • Deformed, shriveled wings.

How is CWV transmitted?

Little is known about how Cloudy wing virus is transmitted. Scientists have observed higher infection rates in crowded hives due to bad weather. Similar results were found in overcrowded lab cages.

This suggests short-range airborne virus or transmission through physical contact. Viral particles may get ingested or enter the bee through minor wounds to the body.

Severe Cloudy wing virus infestations have been found in sealed brood. This supports the hypothesis of an oral transmission route. Infected nurse bees can transmit the virus to their larvae via food. Source.

CWV can be transmitted horizontally or vertically:

Horizontal transmission: A virus is spread amongst bees from the same generation. It is commonly transmitted by body contact, oral transfer, or vectors like mites.

Vertical transmission: A viral infection gets passed from generation to generation. It may occur when drones pass the virus to queens during mating or when the queen lays infected eggs.


Cloudy wing virus can be confidently diagnosed by laboratory testing. Beekeepers should follow these steps to get the problem diagnosed:

  • Select ten sick-looking bees still alive from near the hive’s entrance.
  • Place them in a paper envelope or bag labeled with your name and hive number. Avoid containers or plastic bags.
  • Freeze the bag for 24 hours, then send it to a local testing office.

Recommended reading:


Beekeepers can help prevent CWV and similar honey bee viruses by maintaining healthy colonies.

  • Control varroa mites: The Varroa destructor may be an effective vector of Cloudy wing virus in honey bees. The beekeeper should consider eradication methods if mite levels exceed 3 per 100 bees. Learn more about eradicating varroa mites here.
  • Reduce stress: Stress impacts a colony’s ability to fight off illness. Beekeepers should keep hive disturbances to a minimum and avoid opening hives in cold weather.
  • Keep bees nourished: During nectar dearth and food shortages, provide sugar syrup or pollen substitutes to keep the hive strong and healthy.
  • Consider bee breeds: Some bee stocks have excellent housekeeping behavior and have better resistance to disease and pests. For example, Russian bees instinctively suppress varroa mite populations, reducing viral spread.
  • Hygienic beekeeping practices: Maintain clean hive tools and equipment to reduce spread; never move frames from sick to healthy ones or reuse equipment from dead hives; space out colonies to reduce drifting behavior.
  • Reduce chemicals: Restrict hive treatments like miticides, which may reduce bee virus tolerance.

Keep a watchful eye on other bee pests and diseases.

Treatment options

There are currently no reliable treatments available for Cloudy wing virus. Existing options for eradicating the infection may also harm or kill the colony.

Beekeepers can help control the disease by following recommended beekeeping practices and implementing sanitary measures. Requeening and supplemental feeding will also help hives fight the disease.

5 facts about Cloudy wing virus

  1. It is an icosahedral viral infection that is fatal and may lead to colony collapse.
  2. The pathogen was initially considered an adult bee disease but has since been discovered to impact bee brood.
  3. Although the virus is common, it lacks research. There is much to learn about the disease’s transmission, its effect on the colony, and treatment methods.
  4. CWV affects bees throughout most beekeeping countries worldwide.
  5. There appears to be a close link between CWV and Varroa mites.

Further reading:

The epidemiology of cloudy wing virus infections in honey bee colonies in the UK. Read the research>

If you enjoyed this article, also read our guides to sacbrood, Black queen cell virus, and Israeli acute paralysis virus.

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