Viral Diseases in Honey Bees

Honey bee colonies are under threat from numerous viral pathogens. These agents, especially when combined with the Varroa mite, cause colonies to weaken rapidly and collapse. Common bee viral diseases, such as Deformed Wing Virus (DWV) and Acute Bee Paralysis Virus (ABPV), affect both larvae and adult bees, posing serious risks. Understanding and managing the dynamics of these bee viral diseases is vitally important.

The Most Common Agents (DWV, BQCV, SBV, ABPV, CBPV)

Although more than twenty different viruses that threaten honey bee health have been identified, five stand out globally due to both their prevalence and their devastating impact on colonies. These bee viral diseases often exist as “latent” (covert) infections. The virus is present in the colony at low levels without causing obvious symptoms. However, when the colony encounters a stress factor (poor nutrition, pesticide exposure, or climatic challenges) or, more importantly, is activated by a vector (especially the Varroa mite), these hidden infections quickly turn into “overt” (symptomatic) disease.

Deformed Wing Virus (DWV) is undoubtedly the most well-known and destructive bee virus worldwide. It gets its name from the fact that infected bees emerge from the pupal stage with wings that fail to develop properly, remaining wrinkled, small, and non-functional. Without Varroa mites, DWV usually remains at low levels in the colony and does not cause serious problems. However, when transmitted by the mite, the viral load (viremia) increases logarithmically, and the bee’s immune system is severely suppressed. This situation shortens the bee’s lifespan by 40% to 50% and leads to young worker bees being seen crawling around the hive entrance, unable to fly.

Black Queen Cell Virus (BQCV), as its name suggests, primarily targets queen bee larvae and pupae. The infected queen pupa dies, its tissues liquefy, and the cell wall it is in characteristically turns from dark brown to black. Worker bee larvae can also be affected by this virus, but the disease is usually noticed during queen rearing activities or when the colony’s attempt to replace its queen fails. BQCV often shows a synergistic effect with Nosema (a microsporidian parasite) infections, severely undermining the colony’s reproductive capacity and continuity.

Sacbrood Virus (SBV) is another common and significant viral disease affecting larvae. The virus prevents the infected larva from completing its final molt (ecdysis). The larva accumulates fluid within its old skin, and its body increasingly resembles a fluid-filled sac, or “sacbrood.” The dead larva is typically found in the comb cell curled in a characteristic ‘L’ shape, with a yellowish-brown color. Unlike American or European Foulbrood, these larvae can be easily and wholly removed from the cell with tweezers. The disease is transmitted to larvae by young nurse bees via contaminated brood food (royal jelly).

Acute Bee Paralysis Virus (ABPV) causes rapidly progressing paralysis and death within hours in adult bees. The Varroa mite is also the primary and most dangerous vector for this virus. While it rarely causes problems in the absence of mites, it becomes one of the fastest-killing bee viral diseases when injected directly into the hemolymph (bee blood) by the mite. Infected bees are usually found outside the hive, on the flight board, or at the bottom of the hive, paralyzed after a characteristic trembling fit.

Chronic Bee Paralysis Virus (CBPV) differs from the others in its transmission mechanism; it can cause serious outbreaks largely independent of Varroa. It spreads through contact (bees rubbing against each other in dense populations) and contaminated food. It has two typical symptom syndromes. In the Type 1 syndrome, bees show abnormal trembling, abdominal swelling, and inability to fly. The Type 2 syndrome is more common; bees lose their body hair, darken, and take on a shiny, “greasy” appearance. These “hairless-black” bees are often mistaken for intruders by guard bees, attacked, and denied entry to the hive.

The Varroa–Virus Interaction and Colony Risk

The greatest threat to colony health in modern beekeeping is not a single virus or parasite alone, but the deadly and complex interaction between the Varroa destructor mite and viruses. Varroa is an obligate external parasite that feeds on the fat bodies and hemolymph of honey bees (both adults and developing pupae). This feeding action drives the colony toward a rapidly escalating destruction in two different ways.

First, as the mite feeds, it secretes chemicals that directly suppress the bee’s immune system. It reduces the bee’s ability to produce natural defense proteins against viral infections (immunosuppression). Second, and more importantly, Varroa acts as a biological vector, a “dirty syringe.” It picks up low-level (latent) viruses in the hive and directly injects them into other bees it feeds on, or especially into vulnerable pupae.

This situation allows the virus to completely bypass its normal transmission routes. When viruses are normally ingested (via trophallaxis or food), they must overcome the defense barriers in the bee’s midgut wall, which reduces the likelihood of infection. However, Varroa bypasses these defense barriers, delivering the virus directly into the bee’s bloodstream (hemolymph). This vector-mediated transmission dramatically increases the replication rate of viruses, especially DWV and ABPV. Research shows that the viral load injected by a mite can reach levels 1,000 to 10,000 times higher than through normal transmission. This extremely high viral load causes the bee to become ill and die quickly. It has been observed that in colonies with high Varroa infestations, more virulent forms of viruses (e.g., the DWV-B variant) are selected for and become dominant. The main reason behind colony losses and sudden colony collapses is almost always this uncontrolled Varroa-virus complex.

Clinical Symptoms (Worker/Larva) and Colony-Level Effects

The clinical symptoms of bee viral diseases show distinct differences depending on the agent, the viral load, and the life stage it targets (larva, pupa, or adult). Careful observation of these symptoms in field conditions is necessary for proper management.

Symptoms at the Larva and Pupa Level:
In a healthy colony, the brood area is compact and uniform. Viral infections disrupt this pattern.

• Spotty Brood Pattern: “Perforated brood” or “spotty brood” is the most common finding. Among the capped pupa cells, there are numerous empty, skipped, or dead larvae-containing cells. This is the first sign of a problem in brood development.
• SBV (Sacbrood): The larva dies in the comb cell, its color turning from a healthy pearly white to a dull yellow, then brown. Its body takes the form of a fluid-filled sac (sacbrood) as it fails to molt. It does not smell and is easily removed.
• BQCV (Black Queen Cell): The queen pupa dies, and the cell walls and pupal remains turn a distinct black. This directly affects the colony’s ability to replace its queen.
• General Impact: Brood mortality reduces the colony’s capacity to produce a new generation of worker bees. This leads to a rapid aging of the population and imbalances the nurse-to-forager bee ratio.

Symptoms at the Worker (Adult) Bee Level:
Symptoms in adult bees are often more pronounced and provide clues about the type of disease.

• DWV (Deformed Wing): This is the clearest and most frequently encountered clinical sign. Bees are born with small, wrinkled, crumpled, and non-functional wings. These bees cannot fly; they crawl helplessly at the hive entrance or on the comb surface. Their abdomens may also be smaller than normal.
• CBPV (Chronic Paralysis): Bees characteristically lose their body hairs, their chitin layer darkens, and they take on a shiny, “oily” appearance. Trembling (paralysis) is seen due to neurological damage in the flight muscles. These black bees pile up at the hive entrance or are harassed by other bees at the entrance.
• ABPV (Acute Paralysis): Characterized by sudden paralysis and very rapid death (usually within 24-48 hours). Bees are found suddenly dead on the hive flight board or inside the hive.

Colony-Level Effects:
When individual symptoms accumulate, the colony’s overall health and productivity rapidly decline. The hive population visibly decreases. Brood rearing slows down or stops completely. The amount of pollen and nectar entering the hive drops, which directly affects honey yield and winter stores. The colony becomes extremely vulnerable to other secondary diseases and stress. Colonies that enter the wintering period in the autumn with a high viral load are very unlikely to survive until spring.

Diagnostic Approaches (PCR/ELISA) and Sample Collection

Although clinical symptoms (deformed wings, hairless black bees, sacbrood) raise strong suspicions about bee viral diseases, many viruses can cause similar symptoms, or infections may remain latent (covert). Laboratory analysis is required for a definitive diagnosis and to determine which agent is dominant. Today, two main molecular methods are widely used: PCR and ELISA.

PCR (Polymerase Chain Reaction): This is the most sensitive and specific method, considered the gold standard in viral diagnostics. PCR detects the virus’s genetic material (RNA for most bee viruses). It can amplify (multiply) even very small amounts of viral RNA millions of times, making it detectable. This method is critical for determining exactly which virus or viruses (mixed infections are common) are active in the colony. An advanced version, Quantitative PCR (qPCR or RT-qPCR), can precisely measure not only the presence of the virus but also the amount of virus in the bee’s body (viral load). A high viral load is directly correlated with disease severity and the risk of colony collapse.

ELISA (Enzyme-Linked Immunosorbent Assay): This serological method looks for the virus’s specific proteins (antigens) or the antibodies produced by the bee’s immune system against the virus, rather than the genetic material. It is generally faster and less costly than PCR, but its sensitivity (especially at low viral loads) may be lower. It is often preferred for large-scale field screening programs or for quickly confirming the presence of a specific virus.

Sample Collection: The accuracy of the lab result depends entirely on correct sample collection. For a general colony health screening, approximately 50 to 100 adult worker bees should be randomly collected from different parts of the hive (both brood and honey frames). If a brood disease like SBV is suspected, a sample of the comb containing symptomatic larvae should be taken. To prevent the degradation of sensitive genetic material like RNA, samples must be stored in a cold chain at -20°C or lower, or in RNA preservative fluids, until they reach the laboratory.

Types of Bee Viral Diseases

Bee viral diseases are classified according to the life stage they target (brood or adult) and their transmission mechanisms. Sacbrood Virus (SBV) targets the brood, while Chronic Bee Paralysis Virus (CBPV) destroys the existing workforce. Transmission routes can be vertical (queen to offspring) or horizontal (contact, equipment, Varroa). Their widespread nature increases the importance of biosecurity and management protocols.

Distinction Between Adult-Focused and Brood-Focused Viruses

Bee viruses have evolved and specialized to target different demographic structures (age groups) of the colony. Understanding this distinction is fundamental to predicting the disease’s progression rate and potential damage within the colony. Different functions of the colony are disrupted by different groups of viruses.

Brood-Focused Viruses (Larva and Pupa):
Viruses in this group target the colony’s future: brood rearing and population replacement.

• Sacbrood Virus (SBV): The classic and best-known larval disease. The virus is fed to young larvae (usually 3 to 5 days old) by nurse bees via contaminated brood food. The virus multiplies in the larva’s digestive system and disrupts molting hormones, stopping the larva’s development. This directly prevents colony population replacement.
• Black Queen Cell Virus (BQCV): Primarily affects queen pupae. It destroys the colony’s ability to produce or, in emergencies, replace its most valuable individual, the queen, thereby directly threatening the colony’s long-term survival.

Adult-Focused Viruses:
These viruses target the colony’s current workforce—the foragers, guards, and nurse bees.

• Chronic Bee Paralysis Virus (CBPV): Affects the nervous system of adult bees (especially the brain and thoracic ganglia). It causes paralysis, trembling, and hair loss, destroying the bee’s ability to fly and navigate. The loss of forager bees rapidly reduces the colony’s capacity to collect pollen and nectar.
• Acute Bee Paralysis Virus (ABPV) / Israeli Acute Paralysis Virus (IAVP): These viruses, especially when transmitted by Varroa, lead to very rapid systemic infections and death in adult bees. They cause sudden and massive losses in the worker bee population, collapsing the colony’s defense and maintenance capabilities.

Those Affecting Both Stages (The Most Devastating Group):
Deformed Wing Virus (DWV) is the most dangerous example in this category. The virus is transmitted by Varroa during the pupal stage and replicates rapidly during this developmental phase. It disrupts pupal development (wing deformation). The symptoms, however, appear in the adult bee stage. Therefore, DWV both sabotages brood development (pupa deaths) and renders the adult workforce that manages to emerge non-functional (unable to fly). This dual destructive effect places DWV among the most common and harmful bee viral diseases.

Transmission Routes: Vertical, Horizontal, Equipment

The spread of bee viruses within a colony and between apiaries occurs through both biological and mechanical means. A thorough understanding of these transmission routes forms the basis for effective biosecurity measures. bee viral diseases can spread quickly and silently through these pathways.

Vertical Transmission (Parent to Offspring):
This is the transmission of the disease through the genetic line, i.e., from generation to generation, and is the most difficult to control.

• Transovarial Transmission: An infected queen bee can pass the virus directly into her ovaries and, consequently, into the eggs she lays. Larvae born this way start life already infected.
• Venereal Transmission: Infected drones (male bees) can transmit the virus to the queen bee during mating via virus-laden sperm. The queen then stores this and passes it to her offspring.

Horizontal Transmission (Individual to Individual within the Colony):
This is the most common and continuous transmission cycle within the colony.

• Trophallaxis (Food Exchange): As a social behavior, bees efficiently transmit viruses to each other while transferring food from mouth to mouth (trophallaxis). The feeding of larvae with brood food by nurse bees is the main transmission route for viruses like SBV.
• Contaminated Food and Water: The virus can be acquired through virus-contaminated pollen or nectar brought into the hive, or from shared water sources in the apiary.
• Fecal-Oral Route: In cases of poor hygiene, some viruses, like CBPV, can contaminate fecal residues, which then mix with food and infect other bees.

Transmission via Vector and Equipment (Apiary Spread):
This is often the most dangerous, fastest, and outbreak-causing route of spread.

• Varroa Vectoring: The Varroa mite is the primary biological vector carrying viruses from bee to bee. This is the most aggressive and deadly form of horizontal transmission for bee viral diseases.
• Beekeeping Equipment: The beekeeper’s hive tool, smoker, brush, or especially unwashed gloves can mechanically transfer the virus from one hive to another.
• Comb Transfer: Giving a brood or honey frame from a diseased colony (even one not showing symptoms) to a healthy colony for “support” is the most certain and common beekeeper error that directly transfers the disease.
• Robbing and Drifting: The robbing of weak colonies (due to viruses or other reasons) by strong colonies causes the virus to spread rapidly to other hives in the apiary. Similarly, bees that lose their orientation (drifting) and enter the wrong hive can also carry the virus.

Widespread Prevalence and Field Findings

Globally, honey bee populations are vital for agriculture. This high density of colonies and migratory beekeeping practices create an ideal environment for the spread of bee viral diseases. Recent academic research and field surveys in various regions have clearly demonstrated the prevalence of bee viruses. These findings highlight the significant impact these pathogens have on apiculture worldwide.

Molecular (RT-qPCR based) screenings have confirmed that pathogens such as Deformed Wing Virus (DWV) and Black Queen Cell Virus (BQCV), as well as Acute Bee Paralysis Virus (ABPV), Chronic Bee Paralysis Virus (CBPV), and Sacbrood Virus (SBV), are widespread in apiaries. The incidence of these viruses varies significantly based on geographical regions, beekeeping practices, and especially Varroa management habits.

Field findings strongly support that a large portion of sudden colony losses and winter die-offs, parallel to global trends, are caused by the synergistic and deadly interaction of the Varroa mite with these viruses (especially DWV). In apiaries where Varroa control is insufficient, mistimed, or uses ineffective chemicals, sudden colony collapses linked to DWV and ABPV and weak spring development are reported much more frequently. While it is important for beekeepers to recognize clinical symptoms (deformed wings, hairless black bees), many infections proceed without symptoms (latent), causing the risk to be overlooked and the disease to spread silently.

Control: Biosecurity and Management Protocols

As of today, there is no direct-acting, approved, or field-usable antiviral (virus-killing) chemical treatment for bee viral diseases. Therefore, control relies on indirect methods that prevent the replication and spread of viruses—namely, an integrated approach consisting of prevention, vector control, and good beekeeping practices. Successful management of bee viral diseases depends strictly on this multi-pronged strategy.

Core Strategy: Integrated Varroa Control
The most critical, fundamental, and only effective way to combat viruses is the effective control of the Varroa mite, the main vector. When the Varroa population is kept below the acceptable threshold (e.g., 2%-3% infestation rate) throughout the year, the rate of virus transmission via injection from bee to bee and the viral load in the brood decrease dramatically. Timely (especially post-honey flow autumn treatment), rotational (to prevent resistance), and legal, licensed chemical Varroa control ensures that colonies enter winter with a low viral load and in good health.

Biosecurity Protocols:
Biosecurity is a set of strict practices that prevent the entry of disease into the apiary and its spread from one hive to another within the apiary.

1. Equipment Disinfection: When working between hives, tools like hive tools and gloves should be quickly disinfected with at least a 1% sodium hypochlorite (bleach) solution or a blowtorch flame (if the tool is not flammable).
2. No Frame Transfers: Brood, honey, or pollen frames should never be transferred from weak or sick colonies to healthy colonies for “support.” This is a direct transfer of the virus.
3. Quarantine: Newly purchased colonies or queens should be observed in an isolated (quarantine) area for at least 2-3 weeks and checked for Varroa before being introduced to the main apiary.
4. Prevent Robbing and Drifting: Entrances of weak colonies should be narrowed, and no honey or syrup residue should ever be left out in the apiary. Hives should be painted different colors or placed facing different directions to prevent bee drifting.

Good Management Practices (Colony Resilience):
Keeping the colony’s general health resilience high prevents latent viral infections from turning into active disease.

• Working with Strong Colonies: The apiary should always contain strong colonies with young populations. Colonies that are constantly weak or failing to thrive (a potential source of disease) should be combined or culled.
• Regular Comb Renewal: Old, darkened combs that potentially harbor pathogens (especially in the brood nest) should be regularly replaced (at least 20%-30% annually) and melted down. Viruses can remain viable in beeswax for a long time.
• Strategic Feeding: To keep the bees’ immune systems strong, high-quality protein (pollen or its equivalent feed) and carbohydrate (syrup) feeding should be provided, especially during dearth or stressful periods when nectar flow stops.
• Genetic Selection: The use of queen lines that are more resistant to diseases, especially those exhibiting hygienic behavior (quickly detecting and removing diseased brood) or Varroa Sensitive Hygiene (VSH), should be encouraged.