Horizontal transmission of deformed wing virus: pathological consequences in adult bees (Apis mellifera) depend on the transmission route

Abstract

Recent reports on a steady decline of honeybee colonies in several parts of the world caused great concern. There is a consensus that pathogens are among the key players in this alarming demise of the most important commercial pollinator. One of the pathogens heavily implicated in colony losses is deformed wing virus (DWV). Overt DWV infections manifested as deformed-wing syndrome started to become a threat to honeybees only in the wake of the ectoparasitic mite Varroa destructor, which horizontally transmits DWV. However, a direct causal link between the virus and the symptom 'wing deformity' has not been established yet. To evaluate the impact of different horizontal transmission routes, and especially the role of the mite in the development of overt DWV infections, we performed laboratory infection assays with pupae and adult bees. We could demonstrate that pupae injected with DWV dose-dependently developed overt infections characterized by deformed wings in adult bees, suggesting that DWV, if transmitted to pupae by the mite, is the causative agent of the deformed-wing syndrome. The OID 50 (overt infection dosage) was approximately 2500 genome equivalents. Injecting more than 1¾10 7 DWV genome equivalents into adult bees also resulted in overt infections while the same viral dosage fed to adult bees only resulted in covert infections. Therefore, both infection of adult bees through DWVtransmitting phoretic mites and infection of nurse bees through their cannibalizing DWV-infected pupae might represent possible horizontal transmission routes of DWV. INTRODUCTIONManaged honeybees are indispensable for profitable and sustainable agriculture and for many non-agricultural ecosystems because of the extraordinary pollination service they provide. Hence, reports on a steady decline in honeybee populations in Europe, USA and Japan in the recent past (for a review see vanEngelsdorp & Meixner, 2010) caused great concern and several studies were initiated to identify the underlying cause. From these studies it became more and more evident that the key players in most of the 'inexplicable' colony losses are pathogens (Ratnieks & Carreck, 2010). Especially, the ectoparasitic mite Varroa destructor in concert with certain pathogenic bee viruses played a major role in the observed colony collapses (Cox-Foster et al., 2007;Genersch et al., 2010; Guzmán-Novoa et al., 2010;Highfield et al., 2009;Siede et al., 2008;vanEngelsdorp, 2008;vanEngelsdorp et al., 2009).One of the viruses heavily implicated in colony losses is Deformed wing virus (DWV), a plus-stranded RNA virus belonging to the genus Iflaviridae (Lanzi et al., 2006). DWV normally causes covert infections in honeybees (de Miranda & Fries, 2008;Hails et al., 2008;Yue et al., 2007). Overt DWV infections characterized by the occurrence of visible disease symptoms (deformed wings, bloated and shortened abdomen, and miscolouring) are associated with V. destructor infestation (Ball & Allen, 1988;Bowen-Walker et al., 1999;Martin, 2001;Martin et al., 1998; Santillán-Galicia et...

Evidence of Varroa-mediated deformed wing virus spillover in Hawaii

“…One of the proposed routes of DWV transmission involves ingestion of contaminated hive products such as, pollen and honey, and/or consumption of larvae, pupae, or adult bees (Chen et al, 2006;Genersch et al, 2006;Möckel et al, 2010;Singh et al, 2010). Insects that rob colony resources, or those that feed directly on live or dead bees, may take in viral particles with the food they ingest.…”

“…The pathogenicity of DWV and the relationship between the virus, the mite, and the honey bee continue to be the focus of much research in honey bee pathology (Di Prisco et al, 2016;Möckel et al, 2010;Ryabov et al, 2014). The known DWV strains (Type A, B, and C), and recombinants thereof, may be linked to differences in DWV virulence in honey bee colonies (Martin et al, 2012;McMahon et al, 2016;Ryabov et al, 2014;Zioni et al, 2011), however, there is no evidence that strains may be specifically linked to wing deformities on bees, rather it appears that viral loads of DWV play a significant role in the expression of this phenotype in honey bees (Brettell et al, 2017).…”

Israeli acute paralysis virus associated paralysis symptoms, viral tissue distribution and Dicer-2 induction in bumblebee workers (Bombus terrestris)

“…FISH is a powerful method to determine DNA or RNA molecules in multiple types of samples. In honeybees, in situ hybridization has been applied to detect viruses such as DWV, IAPV and Kakugo virus (KV) and bacterial diseases in both adult and larval tissues (Chen et al, 2014;Fujiyuki et al, 2009;Martinson et al, 2012;Möckel, et al, 2011;Shah et al, 2009;Yue et al, 2008). In the present study, we localized IAPV in different parts of the brains of B. terrestris workers using paraffin-embedded tissue sections.…”

“…FISH was performed as described previously with several modifications (Müller et al, 2009;Möckel, et al, 2011;Yue et al, 2008). SP+ (n=10), SPÀ (n=10) and PBS-injected (n=6) bees were collected at 3 days p.i.…”

Covert deformed wing virus infections have long-term deleterious effects on honeybee foraging and survival

“…To reduce the likelihood of horizontal transmission from the DWV infected to the group of uninfected bees [48], we also added a double-stranded RNA treatment to our DWV-negative lysate [49,50] to try to keep those bees DWVfree, and added a control GFP-dsRNA treatment in the DWV positive lysate to control for the possible effects of foreign dsRNA injection. The amount of DWV injected was estimated at 1.2 Â 10 4 -4.6 Â 10 5 DWV copies per bee, and was aimed at mimicking infection loads reported for bees with covert infections (1.4 Â 10 3 -2.4 Â 10 9 copies per bee, [51]).…”