Viruses of commercialized insect pollinators

Journal of Invertebrate Pathology 147 (2017) 51–59
Contents lists available at ScienceDirect
Journal of Invertebrate Pathology
journal homepage: www.elsevier.com/locate/jip
Review article
Viruses of commercialized insect pollinators
Sebastian Gisder a, Elke Genersch a,b,⇑
a
b
Institute for Bee Research, Department of Molecular Microbiology and Bee Diseases, Friedrich-Engels-Str. 32, 16540 Hohen Neuendorf, Germany
Freie Universität Berlin, Fachbereich Veterinärmedizin, Institut für Mikrobiologie und Tierseuchen, Robert-von-Ostertag-Str. 7-13, 14163 Berlin, Germany
a r t i c l e
i n f o
Article history:
Received 13 June 2016
Revised 18 July 2016
Accepted 20 July 2016
Available online 3 August 2016
Keywords:
Honey bee viruses
Honey bee
Bumblebee
Mason bee
Stingless bee
a b s t r a c t
Managed insect pollinators are indispensable in modern agriculture. They are used worldwide not only in
the open field but also in greenhouses to enhance fruit set, seed production, and crop yield. Managed
honey bee (Apis mellifera, Apis cerana) colonies provide the majority of commercial pollination although
other members of the superfamily Apoidea are also exploited and commercialized as managed pollinators. In the recent past, it became more and more evident that viral diseases play a key role in devastating
honey bee colony losses and it was also recognized that many viruses originally thought to be honey bee
specific can also be detected in other pollinating insects. However, while research on viruses infecting
honey bees started more than 50 years ago and the knowledge on these viruses is growing ever since,
little is known on virus diseases of other pollinating bee species. Recent virus surveys suggested that
many of the viruses thought to be honey bee specific are actually circulating in the pollinator community
and that pollinator management and commercialization of pollinators provide ample opportunity for
viral diseases to spread. However, the direction of disease transmission is not always clear and the impact
of these viral diseases on the different hosts remains elusive in many cases. With our review we want to
provide an up-to-date overview on the viruses detected in different commercialized pollinators in order
to encourage research in the field of pollinator virology that goes beyond molecular detection of viruses.
A deeper understanding of this field of virology is urgently needed to be able to evaluate the impact of
viruses on pollinator health and the role of different pollinators in spreading viral diseases and to be able
to decide on appropriate measures to prevent virus-driven pollinator decline.
Ó 2016 Elsevier Inc. All rights reserved.
Contents
1.
2.
3.
4.
5.
6.
7.
8.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Deformed wing virus (DWV) is the most promiscuous honey bee virus . . . . . .
Black queen cell virus (BQCV) is wide spread in honey bees of the genus Apis.
Acute bee paralysis virus (ABPV) is another multi host virus . . . . . . . . . . . . . . .
Sacbrood virus (SBV) has a rather narrow host range . . . . . . . . . . . . . . . . . . . . .
Chronic bee paralysis virus (CBPV) and slow bee paralysis virus (SBPV) . . . . . .
Newly discovered viruses of unknown relevance . . . . . . . . . . . . . . . . . . . . . . . .
Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction
Reproduction and fruit set of plants depend on pollination
which can be mediated through water, wind, or animals. The latter
⇑ Corresponding author at: Institute for Bee Research, Department of Molecular
Microbiology and Bee Diseases, Friedrich-Engels-Str. 32, 16540 Hohen Neuendorf,
Germany.
E-mail address: elke.genersch@hu-berlin.de (E. Genersch).
http://dx.doi.org/10.1016/j.jip.2016.07.010
0022-2011/Ó 2016 Elsevier Inc. All rights reserved.
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are estimated to contribute to the sexual reproduction of 90% of
existent angiosperm species (Aizen et al., 2009; Kearns et al.,
1998) which also comprise all the crops directly or indirectly contributing to human nutrition. At least 35% of the global crop
production volume comes from crops that depend on animal
pollination (Klein et al., 2007) resulting in a global value of animal
pollination in agriculture of around €153 billion per year (Gallai
et al., 2009). Among the pollinating animals active on flowering
plants are bats, birds, and primarily insects like bees, flies, beetles,
52
S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
butterflies, and moths. Recently, an impressive worldwide study on
the impact of wild insect pollinators has been conducted for 41
crop systems at 600 different locations and revealed that flower
visitations by wild insect pollinators enhanced fruit set in all studied crop systems (Garibaldi et al., 2013). In contrast, honey bees
(Apis mellifera), although widely considered an indispensable pollinating insect species, pollinated less effectively and their flower
visitations had a positive effect in only 14% of the surveyed crops
(Garibaldi et al., 2013). Surprisingly, honey bees did neither maximize pollination nor were they able to fully substitute the lack of
wild insects. Therefore, the importance of pollination by wild
insects in agricultural ecosystems should not be underestimated.
However, in agriculture, managed insect pollinators have an
undoubted advantage over wild insect pollinators: They are movable to wherever necessary so that field-grown and greenhousegrown crops alike can be stocked with high densities of the best
suited pollinator species. Widely used as commercial pollinators
are different social and solitary bee species belonging to the family
Apidae. The polylectic honey bees (Apis mellifera, Apis cerana) are
generalist pollinators managed by humans since several thousands
of years, mostly for honey production. Nowadays, honey bees are
used for commercial pollination of crops in the open field worldwide. Actually, A. mellifera is the dominantly used honey bee species and the demand for honey bee pollination is increasing due
to the increase in the area cultivated with pollinator dependent
crops (Aizen et al., 2008). Accordingly, data from the Food and
Agriculture Organization of the United Nations (FAO; http://faostat3.fao.org/home/E) confirm a continuous increase in the global
number of managed A. mellifera colonies over the last 55 years
(Aizen et al., 2008; Aizen and Harder, 2009; Moritz and Erler,
2016; vanEngelsdorp and Meixner, 2010). In Central and South
America, the eusocial Melipona bees, which are a genus of stingless
bees, play an important role in honey production. They are the
dominant insect pollinators in the open field in the neotropic ecozone (Sommeijer, 1999). In greenhouses, managed bumblebees
(Bombus spp.) are the preferred pollinators worldwide. Their commercial production started in 1987 and it is an ever growing business due to the still increasing demand for pollination of various
crops cultivated in greenhouses (Velthuis and van Doorn, 2006;
Zhang et al., 2015). The alfalfa leafcutting bee (Megachile rotundata)
is the most intensively managed solitary bee and used for commercial pollination in e.g., alfalfa or blueberry fields (Pitts-Singer and
Cane, 2011). Different species of the solitary mason bee (Osmia
spp.) are being developed as manageable alternative crop pollinators especially for orchards (Bosch and Kemp, 2002; Sedivy and
Dorn, 2014).
In the recent past, devastating losses of managed honey bee
colonies (Apis mellifera, Apis cerana) have been reported from all
parts of the world. Due to the indispensable role of honey bees
for commercial crop pollination these losses caused and still cause
great concern among bee keepers, farmers and scientists alike
because they might pose a serious threat to human food production. Although these losses are a complex phenomenon, there is a
general consensus that both parasitic mites (Varroa destructor,
Acarapis woodi, Tropilaelaps spp.) and honey bee viruses play an
important role (Anderson and Roberts, 2013; Bacandritsos et al.,
2010; Baker and Schroeder, 2008; Berenyi et al., 2006; Cox-Foster
et al., 2007; Forsgren et al., 2009; Gauthier et al., 2007; Genersch
et al., 2010; Kojima et al., 2011; Li et al., 2012, 2014; Meixner
et al., 2014; Nielsen et al., 2008; Runckel et al., 2011; Soroker
et al., 2011; Tentcheva et al., 2004; vanEngelsdorp et al., 2009).
Recently, managed Melipona scutellaris colonies in Brazil have
experienced dramatic losses which were reported to be linked to
virus infections (Ueira-Vieira et al., 2015). Alarming declines in
bumble bee populations have also been reported and related to
the microsporidian pathogen Nosema bombi (Cameron et al.,
2011), reduced genetic diversity (Cameron et al., 2011), climatic
factors (Kerr et al., 2015), and virus infections putatively spilled
over from managed honey bee colonies (Fürst et al., 2014). Hence,
virus infections are often implicated in losses of managed bee
species.
It has recently been shown that RNA viruses are widespread in
hymenopteran pollinators and transmitted not only between
hymenopteran taxa (Ravoet et al., 2014; Singh et al., 2010) but also
to non-hymenopteran arthropods associated with honey bee apiaries (Levitt et al., 2013). Therefore, viruses which were originally
thought to be honey bee specific are much more widespread and
transmission of RNA viruses between managed bees and wild bees
is a common phenomenon (McMahon et al., 2015) although the
directionality of virus transmission remains elusive in most cases
(Tehel et al., 2016). It is, therefore, not surprising that honey bee
viruses are frequently found in flower visiting insects (Mazzei
et al., 2014) and especially insect pollinators and that the host
range of honey bee viruses will increase with an increasing number
of studies on non-Apis pollinators.
The geographic range of managed Apis mellifera colonies is
nearly worldwide as is the range of the accompanying honey bee
viruses. Until now, more than 20 viruses have been detected in
honey bees and most of them are positive- and single-stranded
RNA viruses of the families Dicistroviridae and Iflaviridae (Chen
and Siede, 2007; Li et al., 2014; Runckel et al., 2011). These viruses
might form so-called mutant clouds existing as variant swarms or
clades rather than as a species defined by a consensus sequence
(Domingo and Holland, 1997; Lauring and Andino, 2010). This quasispecies nature of RNA viruses has to be taken into account when
evaluating bee virus detection studies performed via PCR-based
methods: primers designed for a specific virus will always only
detect those viruses of the mutant cloud whose genome sequences
are not (yet) mutated in the region of the primer binding sites.
Viruses belonging to the same quasispecies but carrying a mutation in one or both of the primer binding sites will remain undetected even if they actually represent the majority of the mutant
cloud present in the specific host. Hence, commonly used RT-PCR
protocols for the species-specific detection of most of the honey
bee viruses are prone to produce false negative results. Therefore,
the distribution of viruses, which were originally thought to
be honey bee specific, in different taxa might be heavily
underestimated.
In the following part of our review we will focus on virus
infections in the above mentioned managed pollinator bee species.
We are aware that our selection of bee species represents only a
fraction of the relevant species of insect pollinators. However, the
number of studies on pathogen occurrence, infectivity and
virulence relates to the importance of the insect species. Therefore,
little or close to nothing is known about viruses in non-managed
bee species. Most data are available on viruses in Apis species;
Bombus species are also covered quite well; few data exist for
not dominantly managed species like Melipona and Osmia. For
these species, mostly PCR detection of viruses is reported but
pathogenicity or virulence of detected viruses for the putative host
species remains elusive. No viruses have been reported for
Megachile so far (Table 1).
2. Deformed wing virus (DWV) is the most promiscuous honey
bee virus
For DWV (Iflaviridae), one of the most abundant honey bee
viruses, it is already accepted that it exists as a quasispecies
(Martin et al., 2012; Mordecai et al., 2016) and forms the DWV/
VDV-1 (deformed wing virus/Varroa destructor virus-1) clade
(de Miranda and Genersch, 2010). Virulence of representatives of
S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
53
Table 1
Detection and proof of infection or replication of honey bee viruses in managed and wild pollinator insects. (See below-mentioned references for further information.)
Note: (+), molecular detection of viral genomes; (++), proof of viral replication; ( ), not yet detected; (Ø), not yet analyzed; DWV, deformed wing virus; VDV-1, Varroa
destructor virus-1; ABPV, acute bee paralysis virus; SBV, sacbrood virus: CSBV, Chinese sacbrood virus: IAPV, Israeli acute paralysis virus; CBPV, chronic bee paralysis virus;
KBV, Kashmir bee virus; SBPV, slow bee paralysis virus; LSV, Lake Sinai virus strain 1 and strain 2; AmFV: Apis mellifera filamentous virus; ALPV, aphid lethal paralysis virus;
BSRV, Big Sioux River virus; BeeMLV, Bee Macula-like virus; A. mellifera, Apis mellifera; A. cerana, Apis cerana, B. agrorum, Bombus agrorum; B. atratus, Bombus atratus; B.
hortorum, Bombus hortorum; B. huntii, Bombus huntii; B. ignitus, Bombus ignitus; B. impatiens, Bombus impatiens; B. lapidarius, Bombus lapidarius; B. lucorum, Bombus lucorum; B.
pascuorum, Bombus pascuorum; B. pratorum, Bombus pratorum; B. ruderarius, Bombus ruderarius; B. ternarius, Bombus ternarius; B. terrestris: Bombus terrestris, B. vagans, Bombus
vagans; M. scutellaris, Melipona scutellaris; O. cornuta, Osmia cornuta; O. bicornis, Osmia bicornis; managed and commercialized species are highlighted by an asterisk.
the DWV-clade to both honey bee individuaĺs and colonies is
clearly depending on the ectoparasitic mite Varroa destructor acting as biological vector of DWV (Gisder et al., 2009; Möckel et al.,
2011; Schöning et al., 2012; Yue and Genersch, 2005) and it has
even been proposed that DWV and V. destructor are ‘‘linked in a
mutualistic symbiosis” (Di Prisco et al., 2016). Moreover, it has
been proposed that V. destructor not only influences the diversity
of the DWV-clade within honey bee colonies (Mordecai et al.,
2016) but also at the population level (Martin et al., 2012). However, despite this intricate relationship between DWV and the
mite, DWV has not been introduced into the honey bee population
by the mite but also existed and still exists in bee populations that
never had any or still have no contact with the mite (Mondet et al.,
2014; Shutler et al., 2014; Singh et al., 2010; Wilfert et al., 2016;
Yue and Genersch, 2005). In the absence of V. destructor, DWV
transmission occurs vertically via eggs and drone sperm and
horizontally via larval food, trophallaxis, or cannibalism of miteparasitized, heavily DWV-infected pupae by adult bees engaged
in hygienic behavior (Ravoet et al., 2015b; Schöning et al., 2012;
Yue et al., 2007). These transmission routes result in covert infections, hence, in infected animals not exhibiting any symptoms
(Hails et al., 2008; Möckel et al., 2011). In contrast, overt DWV
infections are dependent on the mite acting as biological DWV vector while parasitizing pupae. Overt infections most often result in
pupal death, the development of non-viable adult bees with
malformed appendages or adult bees emerging as healthy looking
individuals but suffering from an infection of the brain and nervous
system, as recently reviewed by (de Miranda and Genersch, 2010;
McMenamin and Genersch, 2015). The triangular relationship
between honey bees, DWV, and Varroa mites is very complex
and most aspects of it still remain elusive. Over the past decade,
many studies, some of them being contradictory, have been
published trying to understand this relationship at individual host,
cellular, and molecular level. For instance, it had been demonstrated that mite infestation induces an immunosuppression in
the parasitized honey bee pupa which in turn activates DWV
replication (Yang and Cox-Foster, 2005). Hence, DWV would be
dependent on the mite’s immunosuppressive activity for full
reproductive capacity in pupae. However, other studies did not
support a general immunosuppressive effect of mite infestation
but rather showed that with increasing numbers of mites per pupa
the immune response may also be activated (Gregory et al., 2005;
Navajas et al., 2008; Zhang et al., 2010). Unfortunately, the impact
of an increased immune response upon mite infestation on DWV
replication and virulence in pupae was not analyzed in these
studies. Most recently, it was suggested that DWV induces
immunosuppression in infected pupae and in turn promotes mite
reproduction (Di Prisco et al., 2016). In this scenario, the mite
would be dependent on DWV’s immunosuppressive activity for full
reproductive capacity. To fully understand and verify this proposed
‘‘loop of reciprocal stimulation of the two symbionts” (Di Prisco
et al., 2016), much more research is needed. However, this example
might already show that reviewing and evaluating all the data
available on the interaction between honey bees, DWV, and varroa
mites goes beyond the scope of this review and rather deserves a
review of its own.
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S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
Within the scope of this review, however, is the multi host
character of DWV and its infectivity for other pollinating insects.
DWV is not only the most prevalent virus in the A. mellifera populations worldwide, but it also has the broadest host range among
the viruses originally thought to be specific for A. mellifera (Table 1).
In Asian countries, DWV is prevalent in A. cerana colonies irrespectively whether they are managed, non-managed, isolated without
any prior contact to A. mellifera or feral colonies (Ai et al., 2012;
Forsgren et al., 2015; Wang et al., 2015; Zhang et al., 2012). DWV
could also be detected in the non-managed honey bee species A.
dorsata and A. florea (Zhang et al., 2012). So far, there are no reports
on clinical symptoms (e.g., crippled wings) associated with DWV
infections in these honey bee species. Since DWV infections were
not restricted to honey bee species that had been in contact to A.
mellifera colonies, a possible spill-over from A. mellifera colonies
is an unlikely reason for the presence of DWV in other Apis species.
A more likely explanation is that DWV is a multi-host-pathogen
which has been considered honey bee specific in the past just for
the lack of data on other host species.
DWV has also been detected in several species of bumblebees
although the prevalence found in the various studies differed considerably. In a case study from Columbia, 19 samples of Bombus
atratus worker bees were screened for the presence of several
honey bee viruses and DWV was present in 100% of the samples
(Gamboa et al., 2015). In contrast, in Bombus impatiens from Mexico, the prevalence of RNA viruses generally was very low and out
of 120 samples only three (2.5%) were positive for DWV (SachmanRuiz et al., 2015). In a large scale field study in Pennsylvania (USA),
analysis of DWV replication was included and revealed active DWV
infections in the bumble bee species B. impatiens and B. vagans
(Levitt et al., 2013). Such active infections might even lead to overt
or lethal infections in bumblebees as the occurrence of crippled
wings in B. terrestris and B. pascuorum could already be related to
DWV-infections (Genersch et al., 2006) and experimental infection
of B. terrestris resulted in a high mortality (50%) of the infected animals 15 days post infection (Graystock et al., 2016b). Honey bees
have been discussed as the source of the described overt bumblebee DWV infections (Genersch et al., 2006). Moreover, DWV
spill-over from A. mellifera colonies has been shown to be one, if
not the main route of DWV infection for bumblebees in the field
(Fürst et al., 2014; McMahon et al., 2015). This finding even culminated in the media statement that honey bees may be the ‘‘Typhoid
Marys” for bumblebees and the pollinator world.
In the course of a broad study looking for honey bee viruses in
solitary bees, DWV could only be detected in Osmia bicornis
(Ravoet et al., 2014). However, the host range of DWV is not
restricted to members of the superfamily Apoidea because DWV
has also been detected in adults of the genus Vespula (superfamily
Vespoidea) (Levitt et al., 2013). The question, whether these insects
were actually infected or only carried the viruses in their intestine
because they consumed contaminated nectar or pollen, could not
be answered because total RNA was isolated from whole body
extracts and viral replication was not analyzed.
The host range of the DWV clade is even not restricted to the
class Insecta because its replication in V. destructor mites (Gisder
et al., 2009; Ongus et al., 2004; Yue and Genersch, 2005) proves
that it can also actively infect members of the class Arachnida.
Initially, the identified sequence differences between VDV-1, originally detected as a virus replicating in the mite, and the deposited
genome sequences of DWV isolated from honey bees led to the
assumption that VDV-1 represents a virus species of its own
(Ongus et al., 2004). However, further analyses of VDV-1 infectivity
and pathology in honey bees (Zioni et al., 2011) rather suggested
that VDV-1 is one of the many variants of the DWV mutant cloud.
The DWV mutant cloud had already been shown to contain variants able to replicate in mites and these variants could be related
to increased DWV virulence in bees (Gisder et al., 2009; Schöning
et al., 2012; Yue and Genersch, 2005).
3. Black queen cell virus (BQCV) is wide spread in honey bees of
the genus Apis
BQCV belongs to the genus Cripavirus within the family Dicistroviridae (Mayo, 2002a,b). This virus was first described in 1977 when
it was isolated from A. mellifera queen pupae and pre-pupae which
were found decomposed in patchily black colored cells (Bailey and
Woods, 1977). Case studies confirmed BQCV to be the most common
cause of death in queen larvae (Anderson, 1993; Benjeddou et al.,
2001) and suggested an association between weakening of colonies
and BQCV infection (Daughenbaugh et al., 2015). BQCV is now considered one of the most common and prevalent virus infections in A.
mellifera worldwide (Antunez et al., 2006; Berenyi et al., 2006; de
Graaf et al., 2008; Higes et al., 2007; Hornitzky, 1987; Mookhploy
et al., 2015; Singh et al., 2010; Tentcheva et al., 2004; Topolska
et al., 1995; Zhang et al., 2012) (Table 1).
In China, Thailand, and Japan, BQCV was detected in the native
honey bees Apis cerana indica, Apis cerana japonica, Apis dorsata
(giant honey bee), and Apis florea (dwarf honey bee) and also in
non-native Apis mellifera colonies (Mookhploy et al., 2015; Zhang
et al., 2012) (Table 1). Phylogenetic analyses based on the partial
capsid protein coding region gave contradictory results in respect
to whether isolates cluster according to geography or according
to Apis species. BQCV virus isolates from Thailand and Japan
revealed a geographical clustering rather than a clustering according to Apis species. In this study, isolates from one geographical
region derived from different Apis species were more closely
related than isolates from one Apis species originating from different regions (Mookhploy et al., 2015). These results are in accordance with another phylogenetic study from Japan which
showed close genetic relationship of BQCV isolates from A. mellifera
and A. cerana japonica (Kojima et al., 2011). They also support the
notion that the partial capsid protein gene sequences of BQCV in
A. mellifera are suitable to differentiate virus isolates from different
geographic origins (Reddy et al., 2013; Tapaszti et al., 2009) and
extends this finding to other Apis species. The studies in Japan
and Thailand suggest that transmission of BQCV between different
Apis species is determined by sharing the same habitat and visiting
the same flowers and include the possibility of virus spillover from
managed A. mellifera colonies to native bee species. However, a
phylogenetic tree from China showed clustering according to
honey bee species (A. mellifera, A. florea, A. dorsata) rather than
according to region (Zhang et al., 2007) suggesting virus adaptation
to different hosts. Unfortunately, in the latter study the genomic
region used for phylogeny is not clearly given making it impossible
to fully evaluate this study and its impact.
The host range of BQCV is not restricted to Apis species but also
includes Bombus species (Table 1). In the course of several field surveys, BQCV genomic RNA was detected by molecular methods in
different Bombus species (Choi et al., 2015, 2010; Gamboa et al.,
2015; McMahon et al., 2015; Reynaldi et al., 2013; Singh et al.,
2010) (Table 1). While in these field studies only presence of viral
RNA but no viral replication was analyzed, another experimental
study detected BQCV in different tissues of bumblebees (B. huntii)
caught in the wild or reared in the lab and showed that viral replication was restricted to the gut (Peng et al., 2011). Surprisingly,
BQCV infections were more widespread in the body of fieldcollected bumblebees than in the body of lab-reared individuals
suggesting that the virus is rather taken up during the natural
foraging activity of bumblebees and less so during the mass rearing
process (Peng et al., 2011). These findings and the fact that
BQCV from US populations of bumblebees and honey bees were
S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
phylogenetically closely related (Peng et al., 2011) argue for an oral
infection route via virus contaminated food. A prerequisite for this
would be that BQCV is circulating in an ecosystem inhabited by
both honey bees and bumblebees. Infected honey bee colonies
may be the source of BQCV contaminating shared nectar and pollen
resources (McMahon et al., 2015).
4. Acute bee paralysis virus (ABPV) is another multi host virus
ABPV (Dicistroviridae) is a member of a complex of three closely
related viruses comprising also Kashmir bee virus (KBV) and Israeli
acute paralysis virus (IAPV), which is, hence, named AKI-complex
(de Miranda et al., 2010a). Phylogenetic analyses clearly revealed
genomic and evolutionary differences between the sequenced representatives of these three viruses (de Miranda et al., 2010a, 2004;
Maori et al., 2007). However, considering that ABPV, KBV, and IAPV
are single- and plus-stranded RNA viruses which exist as quasispecies and form mutant clouds, it still remains to be determined
whether these three viruses really represent separate species or
are just distant variants of the same mutant cloud. All three viruses
have in common that they are highly virulent and lethal within a
couple of days when injected into pupae or adult A. mellifera bees
(Bailey et al., 1963; Maori et al., 2007).
ABPV has been detected in V. destructor parasitizing A. mellifera
colonies (Bakonyi et al., 2002) and there is statistical evidence that
ABPV infections contribute to colony losses (Bakonyi et al., 2002;
Berenyi et al., 2006; Berthoud et al., 2010; Francis et al., 2013;
Genersch et al., 2010; Nordstrom et al., 1999). So far, it has not
been shown that ABPV can replicate in V. destructor. It has been
suggested that the high virulence of ABPV towards honey bees
prevented this virus to develop an intricate association with
V. destructor in a similar manner as DWV did because ABPV transmitted by V. destructor to pupae results in pupal death. Hence, this
association is a dead end road for both, virus and mite (de Miranda
et al., 2010a).
While ABPV or, more general, members of the AKI-complex are
widespread not only in weak or collapsing (Bakonyi et al., 2002;
Berenyi et al., 2006; Genersch et al., 2010) but also in healthy A.
mellifera colonies (Tentcheva et al., 2004), these viruses are rather
rarely found in Apis cerana colonies (Forsgren et al., 2015) (Table 1).
In Asia, increased infections rates were detected in managed
A. cerana colonies and in A. cerana colonies in the vicinity of managed A. mellifera colonies suggesting a spillover effect from the
non-native A. mellifera to the native A. cerana bees (Forsgren
et al., 2015).
Shortly after its discovery it became evident that the host range
of ABPV goes beyond A. mellifera because it was also detected as
causing inapparent infections in bumblebees from the field
(Table 1). Recent studies confirmed these findings of ABPV infections in field samples of Bombus spp. (Bailey and Gibbs, 1964;
Gamboa et al., 2015; Sachman-Ruiz et al., 2015; Singh et al.,
2010) and even demonstrated that ABPV was significantly more
prevalent in bumblebee than honey bee foragers (McMahon
et al., 2015). Therefore, ABPV infections in bumblebees may not
represent a recent phenomenon or classify as emerging infectious
disease (EID) in bumblebees and honey bee colonies may not
necessarily be the source of infection. The other members of the
AKI-complex, KBV and IAPV, could also be detected in bumblebees
(Sachman-Ruiz et al., 2015; Singh et al., 2010) and there is evidence
for IAPV that the infection is spreading from infected honey bee
colonies into non-Apis hymenopteran pollinators (Singh et al., 2010).
Infection experiments conducted with several Bombus species
revealed that adults of all tested species were susceptible towards
injection of virus particles and developed symptoms of fatal paralysis (Bailey and Gibbs, 1964). Recent experimental studies revealed
55
that also IAPV is highly virulent for B. terrestris when injected into
adult bumblebees: Injection of as little as 20 viral particles caused
100% mortality within six days (Niu et al., 2016). In contrast, oral
infection of B. terrestris workers with KBV or IAPV did not result
in increased mortality. However, micro-colonies established by
infected workers exhibited significantly delayed oviposition by
the pseudo-queen (only KBV) and reduced drone production
(KBV and IAPV) (Meeus et al., 2014). Oral transmission is the most
likely natural infection route for bumblebees with members of the
AKI-complex. Therefore, immediate symptoms like increased
worker mortality only seen after virus injection are less likely in
the field than sublethal effects on the reproductive success of
bumblebee colonies. Further studies are needed to evaluate the
overall impact of these virus infections for bumblebees.
The host range of ABPV/KBV/IAPV might not be restricted to
Apis and Bombus species: a molecular screening of bees from M.
scutellaris colonies in Brazil for seven common honey bee viruses
revealed that from the AKI-complex only ABPV was present and
could be found in 100% of the sampled colonies (Table 1)
(Ueira-Vieira et al., 2015). Again, A. mellifera has been discussed
as possible source of infection because pollen and honey produced
by A. mellifera colonies and presumably contaminated with viral
particles are used in meliponiculture as a supplemental food
source (Ueira-Vieira et al., 2015). ABPV genome detection statistically correlated with the occurrence of dead stingless bees in the
surveyed meliponary thus suggesting that ABPV infections might
be involved in the observed decrease in the stingless bee population and increase in the number of dead stingless bees in individual
colonies (Ueira-Vieira et al., 2015). However, ABPV replication in
Melipona bees was not analyzed and, therefore, further studies
are necessary to determine ABPV pathogenicity and virulence for
stingless bees.
5. Sacbrood virus (SBV) has a rather narrow host range
SBV (family Iflaviridae) is one of the most common viruses in A.
mellifera and A. cerana (Table 1 and references therein) and the etiological agent of the brood disease sacbrood. SBV is transmitted to
larvae by covertly infected nurse bees (Bailey, 1969). Following
infection by virus contaminated larval food, the larvae fail to
pupate (Bailey et al., 1964; Chen and Siede, 2007) and acquire a
sac-like appearance which can be readily diagnosed in the field.
Although sacbrood disease is lethal for infected larvae, colonies
showing symptoms of sacbrood rarely collapse and, hence, SBV
poses a moderate threat to managed honey bees. Several strains
of SBV have been isolated worldwide. Recently described strains
are Korea sacbrood virus, KSBV (Reddy et al., 2016), Chinese sacbrood virus, CSBV (Zhang et al., 2013) and Thai sacbrood virus,
TSBV (Rana et al., 2011). Phylogenetic analysis revealed the existence of an European serotype, a New Guinea serotype, and an
Asian serotype as well as several subtypes within each serotype
(Roberts and Anderson, 2014).
Although SBV has also been detected in V. destructor (Mondet
et al., 2014; Shen et al., 2005), no replication of SBV in V. destructor
has been demonstrated so far suggesting that the mite does neither
serve as host nor as biological vector for SBV. Instead, the mite
might serve as mechanical SBV vector after acquiring virus particles through parasitizing SBV infected pupae or adult bees (Shen
et al., 2005). In honey bee colonies parasitized by V. destructor, viral
infections by DWV, KBV, and SBV had a significant higher prevalence compared to honey bee colonies which were free of mites
suggesting a vectorial transmission of SBV (Mondet et al., 2014).
Only few reports on SBV detection in non-Apis-bees exist
despite the fact that nearly all studies on virus detection in insect
pollinators included SBV. This indicates a rather narrow host range
56
S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
for SBV (Table 1). SBV viral RNA could be detected in several Bombus species (Gamboa et al., 2015; McMahon et al., 2015; Reynaldi
et al., 2013; Singh et al., 2010) and in the solitary bee Andrena vaga
(Ravoet et al., 2014) (Table 1). Pathogenicity and virulence of SBV
for these putative hosts cannot be finally evaluated because neither viral replication nor symptoms of sacbrood disease were analyzed in these studies.
6. Chronic bee paralysis virus (CBPV) and slow bee paralysis
virus (SBPV)
CBPV is an unclassified virus phylogenetically placed between
the Nodaviridae family of insect and fish viruses and the plant virus
family Tombusviridae (Ribiere et al., 2010, 2002). CBPV is a rather
unusual honey bee virus in that it is an enveloped virus with a genome consisting of two single-, plus-stranded RNA molecules. CBPV
was first isolated in 1963 from diseased bees showing symptoms of
paralysis like incapacity to fly, ataxia, and trembling (Bailey et al.,
1963). CBPV shows neurotropism in Apis mellifera (Olivier et al.,
2008) explaining the neurological symptoms in infected bees.
CBPV is worldwide distributed in Apis mellifera populations
(Ribiere et al., 2010) but was also found in Apis cerana (Wang
et al., 2015), in Bombus terrestris (Choi et al., 2010), and Bombus
impatiens (Sachman-Ruiz et al., 2015). In addition, CBPV is able to
infect ants (Formica rufa, Camponotus vagus) and V. destructor
(Celle et al., 2008) and it has been discussed that ants represent
a virus reservoir and the mite serves as virus vector facilitating
CBPV spreading in the honey bee population.
SBPV belongs to the virus family Iflaviridae (de Miranda et al.,
2010b) and has been described as etiological agent of a rather slow
form of paralysis of adult bees: upon injection of SBPV, adult bees
died within approximately 12 days and showed paralysis of the
anterior two pairs of legs one or two days before death (Bailey
and Woods, 1974). Using serological detection methods, SBPV
was implicated in V. destructor induced colony losses (Carreck
et al., 2010). Molecular detection of SBPV became feasible only
recently with the publication of the genomic sequence of SBPV
(de Miranda et al., 2010b). Hence, many virus surveys in honey
bee populations do not include analysis of SBPV. However, analyzing samples from different European countries for SBPV using
newly developed RT-PCR protocols revealed a low natural prevalence of SBPV in the European honey bee population (de Miranda
et al., 2010b).
A recent virus detection study in wild bumblebees collected in
the UK (McMahon et al., 2015) and in Belgium (Parmentier et al.,
2016) revealed that SBPV infections can also occur in bumblebees
albeit with a low prevalence. Despite the low detection rate, SBPV
was slightly more prevalent in bumblebees than in honey bees
(McMahon et al., 2015) questioning the direction of SBPV transmission between managed and wild pollinators (Parmentier et al.,
2016).
7. Newly discovered viruses of unknown relevance
Reports on devastating honey bee colony losses and on declines
in bumblebee populations caused an increased scientific interest in
the pathogens, including viruses, presumably involved in these
phenomena. In order to detect more than the known and usually
taken seven to eight honey bee viruses, next generation sequencing
methods were recently introduced for bee virus detection studies.
In a case study, 20 migratory honey bee colonies were analyzed for
the common seven honey bee viruses (CBPV, IAPV, DWV, ABPV,
BQCV, SBV, KBV) using the Arthropod Pathogen Microarray
(APM) which is capable of detecting over 200 arthropod associated
viruses, microbes, and metazoan parasites. In addition, an ultra-
deep sequencing approach was applied and led to the discovery
of four distinct viruses which were either novel (Lake Sinai virus
strain 1 and strain 2 (LSV1 and 2; presumably Noraviridae) and
Big Sioux River virus (BSRV; Dicistroviridae)) or had not been associated with honey bees so far (aphid lethal paralysis virus (ALPV;
Dicistroviridae)), (Runckel et al., 2011) (Table 1). ALPV and BSRV
could be detected in almost every sample during the 10 month
sampling period. ALPV and LSV were shown to replicate in honey
bees (Ravoet et al., 2015a, 2013; Runckel et al., 2011) and prevalence of LSV-1 and LSV-2 was further investigated in six colonies
which were monitored over a three month period. The results from
this small sample cohort suggested an association between weakening of colonies and infections with LSV1, LSV2, and BQCV
(Daughenbaugh et al., 2015). However, there was no statistically
significant evidence that LSV strains contribute to colony mortality
(Daughenbaugh et al., 2015). In the meantime, a number of additional LSV strains has been described (Cornman et al., 2012;
Granberg et al., 2013; Ravoet et al., 2015a, 2013) underlining the
major strain diversity that appears to exist and co-circulate for
many RNA-bee viruses. LSV was also detected in mason bees
although viral replication could only be demonstrated in Osmia
cornuta but not in Osmia bicornis (Ravoet et al., 2015a). In V.
destructor, only the positive sense LSV genome could be detected
but not the negative-strand intermediate only occurring during
viral replication suggesting the absence of replicating virus
(Ravoet et al., 2015a). Hence, V. destructor might serve as mechanical vector in honey bees but is not an alternative host for LSV. Oral
transmission routes for LSV through contaminated pollen seem
likely considering that LSV was found in pollen pellets (Ravoet
et al., 2015a). In addition, the known Apis mellifera filamentous
virus (AmFV; an unclassified DNA virus) (Clark, 1978; Gauthier
et al., 2015) and a novel virus now called Bee macula-like virus
(BeeMLV, Tymoviridae) (de Miranda et al., 2015) were recently discovered in honey bees (Ravoet et al., 2013) and subsequently also
detected in bumblebees and mason bees (Ravoet et al., 2014)
(Table 1). While the well-known honey bee viruses are fairly well
characterized and experimental studies on their pathogenicity
and virulence at least in honey bees have been performed, it still
remains elusive whether the viruses recently discovered in
managed and wild bees induce individual (or colony) mortality
or at least affect the fitness of the infected bee species. Much more
research is necessary to fully understand the impact of viral pathogens on the pollinator community.
8. Outlook
There is an ever increasing demand in the agricultural sector
not only for honey bees but also for alternative non-honey bee pollinators which can be used as managed commercial pollinators
under special circumstances (e.g., in greenhouses) or for specific
crops (e.g., blueberry, alfalfa). However, mass rearing and commercialization of these insect species is not without problems. Mass
rearing of any organism poses the risk of increased disease transmission followed by a higher prevalence of pathogens in the mass
reared individuals compared to feral individuals (Colla et al., 2006;
Eilenberg et al., 2015). Outbreaks of epidemics not observed in the
wild may occur in lab reared or mass reared populations resulting
in the collapse – or near collapse – of these populations (Eilenberg
et al., 2015). Therefore, special care must be taken to prevent
disease transmission and outbreaks in insect production facilities.
In the era of globalization, commercialization of pollinators led to
their worldwide trade and their importation into countries far
beyond their natural geographic range. This already happened to
the Western honey bee, A. mellifera, which is native to Europe,
Africa and the Middle East (Han et al., 2012) but has been
S. Gisder, E. Genersch / Journal of Invertebrate Pathology 147 (2017) 51–59
introduced into all regions of the world by beekeeping activities.
Likewise, since the commercialization of bumblebees in the late
eighties of the 20th century, mainly B. terrestris but also other Bombus species have been imported for greenhouse pollination into
many countries where they are not the native bumblebee species.
Spillover of diseases from and introduction of new pathogens
(emerging infectious diseases, EID) through these non-native pollinators into the native ecosystems might pose a serious threat to
these systems and might lead to losses and declines of native wild
pollinator species (Goulson, 2003; Goulson and Hughes, 2015;
Graystock et al., 2016a). Therefore, a broad knowledge on the
pathogens naturally circulating in diverse ecosystems is important
to evaluate whether or not pathogens are really newly introduced
into a specific host population or are just detected for the first time
because they were surveyed for the first time. Likewise, comprehensive knowledge on all pathogens associated with certain pollinators is vital to take appropriate measures in order to prevent that
these pathogens become worldwide distributed once their hosts
are worldwide distributed (Goulson and Hughes, 2015).
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