Honey Bee Viruses
By: Clarence Collison
Viruses are obligate intracellular infectious agents that rely on their host machinery for multiplication (i.e., transcription, translation and replication). By doing so, they can cause significant damage to their hosts and express a variety of symptoms. In honey bees, viral infections have been reported to affect diverse traits, such as morphology, physiology and behavior; in severe cases, viruses may lead to increased mortality at the individual as well as colony levels (Beaurepaire et al., 2020). For example, wing deformities and shortened abdomens are characteristic symptoms of deformed wing virus infection (De Miranda and Genersch, 2010). However, some bee viruses have been recently discovered that show no apparent symptoms, or only mild symptoms, in infected honey bees (Levin et al., 2016; Remant et al., 2017; Levin et al., 2019; Beaurepaire et al., 2020).
Viruses with RNA genomes predominate in honey bees. These viruses typically consist of small icosahedral particles (17, 30 or 35 nm) that contain a positive-sense, single-stranded RNA genome (De Miranda et al., 2013; Bailey, 1971; Bailey and Ball, 1991). After attaching to and penetrating into their host cells, these viruses generally replicate by injecting RNA directly into the cytoplasm. The host machinery (e.g., ribosomes) will then transcribe and generate new viral proteins, that will be assembled as a viral particle, released from the infected cell and will start infecting new cells. More recently, RNA viruses with a negative-sense, single-stranded genome, which are generally more difficult to propagate in vivo, have also been identified in Apis mellifera (Levin et al., 2016; Remant et al., 2017; Levin et al., 2019). Additionally, a few viruses with a DNA genome have been identified in honey bees. These DNA viruses differ from RNA viruses in the way they replicate. Once within their host cells, DNA viruses are transported to the host nucleus to be transcribed and translated (Beaurepaire et al., 2020).
At least 24 honey bee-associated viruses have been reported (Tantillo et al., 2015) including seven viruses that are widespread. These are Acute bee paralysis virus (ABPV), Deformed wing virus (DWV), Sacbrood virus (SBV), Black queen cell virus (BQCV), Israeli acute paralysis virus (IAPV), Chronic bee paralysis virus (CBPV) and Kashmir bee virus (KBV) (Tantillo et al., 2015; Chen and Siede, 2007; Guo et al., 2020).
RNA viruses impact honey bee health and contribute to elevated colony loss rates worldwide. Deformed wing virus (DWV) and the closely related Varroa destructor virus-1 (VDV1), are the most widespread honey bee viruses. VDV1 is known to cause high rates of overwintering colony losses in Europe, however, it was unknown in the United States (U.S.). Using next generation sequencing, Ryabov et al. (2017) identified VDV1 in honey bee pupae in the U.S. They tested 603 apiaries in the U.S. in 2016 and found that VDV1 was present in 66.0% of them, making it the second most prevalent virus after DWV, which was present in 89.4% of the colonies. VDV1 had the highest load in infected bees compared to other tested viruses, with DWV second. Analysis of 75 colonies sourced in 2010 revealed that VDV1 was present in only two colonies (2.7%), suggesting its recent spread. They also detected newly emerged recombinants between the U.S. strains of VDV1 and DWV. The presence of these recombinants poses additional risk, because similar VDV1-DWV recombinants constitute the most virulent honey bee viruses in the UK.
The honey bee is commonly infected by multiple viruses. Carrillo-Tripp et al. (2016) developed an experimental system for the study of such mixed viral infections in newly emerged honey bees and in the cell line AmE-711, derived from honey bee embryos. When inoculating a mixture of iflavirids [sacbrood bee virus (SBV), deformed wing virus (DWV)] and dicistrovirids; Israeli acute paralysis virus (IAPV), black queen cell virus (BQCV)] in both live bee and cell culture assays, IAPV replicated to higher levels than other viruses despite the fact that SBV was the major component of the inoculum mixture. When a different virus mix composed mainly of the dicistrovirid Kashmir bee virus (KBV) was tested in cell culture, the outcome was a rapid increase in KBV but not IAPV. They also sequenced the complete genome of an isolate of DWV that covertly infects the AmE-711 cell line and found that this virus does not prevent IAPV and KBV from accumulating to high levels and causing cytopathic effects. These results indicate that different mechanisms of virus-host interaction affect virus dynamics, including complex virus-virus interactions, superinfections, specific virus saturation limits in cells and virus specialization for different cell types (Carrillo-Tripp et al., 2016).
Kashmir bee virus (KBV) is a potentially lethal virus of honey bees that has recently come to prominence as one of several viruses closely associated with colony collapse because of infestation with varroa mites (Ball and Bailey, 1997). Like most honey bee viruses, KBV is thought to persist as an in-apparent infection within the bee community, until stress or an alternative vector (such as varroa) causes it to become epidemic and lethal. The geographical and host origins of KBV are obscure. It was discovered in 1974 as a contaminant in preparations of Apis iridescent virus from the Asian hive bee (Apis cerana) that multiplied to high titers when injected or fed to adult Apis mellifera bees (Bailey and Woods, 1977). Although it was suspected that KBV originated in A. cerana and SE Asia, the detection of KBV, or its serological relatives, in natural populations of A. mellifera from around the world (Ball and Bailey, 1997; Allen and Ball, 1995), as well as A. cerana from India (Bailey and Woods, 1977; Bailey et al., 1979), bumble bees (Bombus spp.) from New Zealand and European wasps (Vespula germanica) from Australia (Anderson, 1991) has made this difficult to prove (De Miranda et al., 2004).
KBV is serologically and biologically closely related to acute bee paralysis virus (ABPV) (Allen and Ball, 1995; Anderson, 1991). Like KBV it was discovered as a contaminant, during transmission studies of chronic bee paralysis virus (Bailey et al., 1963) and is extremely lethal to adults and larvae, both by injection and in larger doses by feeding (Bailey et al., 1963). It is common in seemingly normal, healthy colonies and has been heavily implicated in varroa induced colony losses, primarily in Europe in the 1980s (Ball, 1985; Allen et al., 1986; Ball and Allen, 1988; Bailey and Ball, 1991). Varroa can transfer ABPV among adults and pupae with 50–80% efficiency, depending on the sensitivity of the detection method used. This efficiency drops with successive transfers and there is no noticeable latent period between acquisition and transmission, which suggests that there is no virus replication in the mite (De Miranda et al., 2004).
Lake Sinai virus (LSV1) and LSV2 were discovered in honey bee samples obtained from a migratory commercial beekeeping operation with sites near Lake Sinai, South Dakota. These viruses were the most abundant pathogens detected in a 10-month honey bee pathogen monitoring study carried out in the U.S. in 2008-2009. In that sample cohort, LSV2 was the most abundant virus with peak levels in April and January, whereas LSV1 infections peaked in July (Runckel et al., 2011). While the pathogenicity of LSVs is not well understood, LSV1 and LSV2 loads were higher in Colony Collapse Disorder (CCD) affected colonies, as compared to unaffected colonies (Cornman et al., 2012). Since the discovery of LSV1 and LSV2, the LSV group has been expanded to include LSV3 (Cornman et al., 2012), LSV-Navarra (Granberg et al., 2013), LSV4, LSV5 and several LSVs discovered in Belgium (Ravoet et al., 2013; Ravoet et al., 2015). LSVs have been detected in the U.S., Spain, Belgium and Turkey (Cornman et al., 2012; Runckel et al., 2011; Granberg et al., 2013; Ravoet et al., 2013; Ravoet et al., 2015; Tozkar et al., 2015), as well as in multiple bee species (Ravoet et al., 2014; Daughenbaugh et al., 2015).
Slow bee paralysis virus (SBPV) is characterized by the paralysis of the front two pairs of legs of adult bees, a few days before dying, after inoculation by injection (Bailey and Woods, 1974). The virus is associated with and transmitted by Varroa destructor (Bailey and Ball, 1991). Despite this association, SBPV is rarely detected in bee colonies (Bailey and Ball, 1991; De Miranda et al., 2010). SBPV can also be detected in larvae and pupae but produces no symptoms in these (De Miranda et al., 2013).
Cloudy Wing Virus (CWV) symptoms consist of opaque wings of severely infected adult bees, with lower titers resulting in asymptomatic infected bees (Bailey et al., 1980; Bailey and Ball, 1991; Carreck et al., 2010). It cannot be propagated in larvae or pupae. It has an unpredictable incidence, no regular associations with other pathogens or pests. Like chronic bee paralysis satellite virus it has a small particle and very small genome, but they are serologically unrelated and their single capsid proteins are of different size (Bailey et al., 1980; De Miranda et al., 2013).
Knowledge of the spreading mechanism of honey bee pathogens within the hive is crucial to understanding bee disease dynamics. The aim of Ribière et al., (2007) was to assess the presence of infectious chronic bee paralysis virus (CBPV) in bee excreta and evaluate its possible role as an indirect route of infection. Samples of paralyzed bees were (i) produced by experimental inoculation with purified virus and (ii) collected from hives exhibiting chronic paralysis. CBPV in bee heads or feces (crude or absorbed onto paper) was detected by reverse transcription-PCR. CBPV infectivity was assessed by intrathoracic inoculation of bees with virus extracted from feces and by placement of naive bees in cages previously occupied by contaminated individuals. CBPV RNA was systematically detected in the feces of naturally and experimentally infected bees and on the paper sheets that had been used to cover the floors of units containing bees artificially infected with CBPV or the floor of one naturally infected colony. Both intrathoracic inoculation of bees with virus extracted from feces and placement of bees in contaminated cages provoked overt disease in naive bees, thereby proving that the excreted virus was infectious and that this indirect route of infection could lead to overt chronic paralysis. This is the first experimental confirmation that infectious CBPV particles excreted in the feces of infected bees can infect naive bees and provoke overt disease by mere confinement of naive bees in a soiled environment (Ribière et al., 2007).
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Clarence Collison is an Emeritus Professor of Entomology and Department Head Emeritus of Entomology and Plant Pathology at Mississippi State University, Mississippi State, MS.
