Previous Article | Next Article ![]()
Applied and Environmental Microbiology, December 2004, p. 7185-7191, Vol. 70, No. 12
0099-2240/04/$08.00+0 DOI: 10.1128/AEM.70.12.7185-7191.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Laurent Gauthier,
*
Nathalie Zappulla,
Benjamin Dainat,
François Cousserans,
Marc Edouard Colin, and
Max Bergoin
Laboratoire de Pathologie Comparée des Invertébrés UMR5087, Université Montpellier II, Montpellier, France
Received 14 April 2004/ Accepted 12 July 2004
|
|
|---|
|
|
|---|
Two of these viruses, SBV and CBPV, produce clinical signs that are clearly identifiable by beekeepers, whereas the majority of the viruses are believed to cause persistent, usually inapparent infections in honey bees (3, 6, 17, 20), and their associations with colony mortality remain unexplained (10, 25).
The increased number of A. mellifera L. bee colony collapses in several European countries, especially France, during the last decade has resulted in great interest in honey bee toxicology and pathology. Two principal hypotheses might explain these collapses. One of these hypotheses is that the rapid dissemination of the ectoparasitic mite Varroa destructor (5) in A. mellifera L. colonies is the main cause. This mite is now widespread in Europe, North Africa, Asia, and the Americas, and colony death occurs in 2 years if the mite infestation is not controlled by acaricide treatments. The collapse of bee colonies severely infested by V. destructor has long been attributed to viruses and has been described as bee parasitic mite syndrome (29). Generally, two hypotheses are cited that could explain the deleterious effects of varroa parasitism on honey bee colony decline. First, the mite can act as a vector and can directly inject virus particles into the insect hemolymph (11); the detection of several bee viruses in varroa mites and the high virus load strongly support this hypothesis (2, 9, 14, 31). Second, the mite can also trigger virus replication by a simple mechanical effect, cuticle piercing, or by injection of external proteins into the insect hemolymph (18). The latter hypothesis is supported by several studies that demonstrated that there was reactivation of viruses already present in the insect following experimental inoculation (3, 17). However, the association between parasitic mite syndrome and viruses is still controversial (21). The mechanisms by which viruses could be reactivated in insects are not fully understood, and predisposing factors other than mite parasitism might play a role in this phenomenon; such factors include coinfection with bacteria or protozoans (7) and the effects of chemicals released into the environment (13, 15, 16). The latter factor is suspected to play a role in triggering viral disease outbreaks in mammals (32), as well as in insects (22).
The study of the impact of viral diseases on honey bee colonies suffers from a lack of data concerning bee virus prevalence, particularly in asymptomatic colonies. The great diversity of viruses isolated from honey bees, the lack of specific clinical signs, and the limited availability of tools for rapid and large-scale diagnoses are partially responsible for this situation. Complete or partial sequencing of several RNA viruses of the honey bee has allowed the recent development of highly sensitive methods for detection based on amplification by reverse transcription-PCR (RT-PCR) of specific viral sequences. Originally developed for diagnosis of human and animal RNA viruses, these methods have been successfully adapted to identify several RNA bee viruses, including SBV, KBV, CBPV, ABPV, BQCV, and DWV (12, 19, 28, 30, 31).
We report here the first survey, covering a 1-year period, of the prevalence and seasonal variations of six bee RNA viruses in French apiaries based on large-scale sampling of adult bees, brood, and varroa mites having different geographic origins. The samples were collected at three times (in the spring, summer, and autumn of 2002) from seemingly healthy colonies and were analyzed by using the RT-PCR technology for virus identification.
|
|
|---|
![]() View larger version (59K): [in a new window] |
FIG. 1. Distribution of the 36 apiaries where samples were collected in France. The reference numbers of the apiaries are indicated.
|
Varroa mites (100 mites per beehive) were collected at the end of August or beginning of September following colony treatment with an acaricide (Amitraz).
The samples were collected by beekeepers and immediately sent to the laboratory by express mail. The pupae were removed from their puparia by using toothpicks upon reception, and the samples were frozen at 20°C.
Sample preparation and PCR analysis.
The frozen samples were crushed in a mortar in the presence of liquid nitrogen and were homogenized in 10 mM Tris-400 mM NaCl buffer (pH 7.5). After homogenization, the samples were centrifuged for 10 min at 5,000 x g to clarify them. An aliquot of supernatant was used for extraction of total RNA with a Nucleospin RNA-II kit (Macherey-Nagel) used according to the supplier's recommendations. Total RNA was resuspended in water and quantified by spectrophotometry. An average of 2 µg of total RNA was retrotranscribed at 25°C for 10 min and at 50°C for 1 h with a Thermoscript RT-PCR kit (Invitrogen) by using random hexamers. A passive RNA reference (5 x 107 copies of tobacco mosaic virus strain INRA, which was gift from B. Alliot, ENSAM Montpellier) was introduced into each sample during RNA preparation to monitor the efficiency of RNA purification and cDNA synthesis and to reveal the presence of PCR inhibitors in the samples.
PCR assays were done as follows. Five microliters of cDNA (1/10 dilution with water) was mixed with 5 µl of 10x buffer (100 mM Tris-HCl [pH 9], 500 mM KCl, 15 mM MgCl2, 1% Triton X-100, 2 mg of bovine serum albumin per ml), 1 µl of a solution containing each deoxynucleoside triphosphate at a concentration of 10 mM, each primer at a concentration of 0.4 µM, and 1 U of Taq polymerase (Q-Biogen); the final volume of the mixture was 50 µl. The mixture was heated for 2 min at 95°C, and this was followed by 35 amplification cycles (30 s at 95°C, 30 s at 56°C, and 1 min at 72°C) and then by 7 min at 72°C to complete the polymerization. PCR products were analyzed by 1.5% agarose gel electrophoresis. The primers used in the assays are shown in Table 1. The SBV, BQCV, and ABPV primers were designed from viral genomic sequences deposited in the GenBank database under accession numbers AF092924, AF183905, and AF150629, respectively, and the corresponding PCR amplicons were sequenced to ensure their specificity.
|
View this table: [in a new window] |
TABLE 1. Primers used for PCR diagnosis
|
Nucleotide sequence accession numbers.
The sequences of some isolates were deposited in the GenBank database under accession numbers AY669845, AY669846, AY669847, AY669848, AY669849, AY669850, AY669851, AY669852, and AY669853.
|
|
|---|
|
View this table: [in a new window] |
TABLE 2. Seasonal variation of DWV, SBV, ABPV, BQCV, CBPV, and KBV in bee and Varroa samples from 36 apiaries
|
|
View this table: [in a new window] |
TABLE 3. Frequencies of infected apiaries during 2002
|
![]() View larger version (26K): [in a new window] |
FIG. 2. Frequencies of bee virus infections in samples of adult bees (A) and pupae (B) collected from 360 colonies. The data are expressed as percentages. The detection of DWV, SBV, CBPV, ABPV, BQCV, or KBV in the spring, summer, and autumn in the same colony is referred to as stable. Viral infections that were detected once or twice during the year in the same colony are referred to as variable. Not detected refers to colonies in which virus was not detected at any time of the year.
|
![]() View larger version (24K): [in a new window] |
FIG. 3. Percentages of apiaries in which 0 to 10 colonies were infected by DWV, SBV, BQCV, or ABPV at the adult or larval stage.
|
2 = 31.03, and P < 0.001; for summer adults versus autumn adults, df = 1,
2 = 9.91, and P < 0.001; for spring pupae versus summer pupae, df = 1,
2 = 8.94, and P < 0.001; for summer pupae versus autumn pupae, df = 1,
2 = 3.17, and P < 0.07). In adults, the SBV frequencies in colonies were 44, 64, and 26% in the spring, summer, and autumn, respectively. In pupae, the values were 13, 18, and 3%, respectively. The same distribution was observed for BQCV infections in adults, as the frequencies were 49, 58, and 36% in the spring, summer, and autumn, respectively (for spring adults versus summer adults, df = 1,
2 = 21.56, and P < 0.001; for summer adults versus autumn adults, df = 1,
2 = 21.48, and P < 0.001). In contrast, the DWV infections increased during the year both in adults and in pupae. The recorded DWV frequencies in the spring, summer, and autumn were 56, 66, and 85%, respectively, for adults (for spring adults versus summer adults, df = 1,
2 = 34.88, and P < 0.001; for summer adults versus autumn adults, df = 1,
2 = 20.55, and P < 0.001) and 16, 38, and 54%, respectively, for pupae (for spring pupae versus summer pupae, df = 1,
2 = 16.01, and P < 0.001; for summer pupae versus autumn pupae, df = 1,
2 = 26.83, and P < 0.001). Similarly, ABPV infections were prominently found during the summer in adult bees (for summer adults versus autumn adults, df = 1,
2 = 11.42, and P < 0.001). Conversely, KBV infections tended to regress in autumn, while CBPV infections were detected throughout the year but at low frequencies.
![]() View larger version (30K): [in a new window] |
FIG. 4. Seasonal variation of viral infection rates in honey bee colonies.
|
![]() View larger version (9K): [in a new window] |
FIG. 5. Prevalence of viral infections in V. destructor samples.
|
|
|
|---|
Here, we describe the first survey of the prevalence of six honey bee viruses in seemingly healthy bee colonies randomly chosen from 36 apiaries, in which the RT-PCR technique was used. The samples came from different geographic areas in France, including plains with intensive industrial crops, valleys, and mountains.
DWV.
Of the six viruses identified by PCR in this study, DWV was by far the most frequently detected in colonies, both in bee samples (adults and brood) and in varroa mites. For adults and pupae, the frequency of DWV-infected colonies increased from spring to autumn. The seasonal variations in DWV incidence were much more pronounced for pupae than for adult bees. In mites, DWV was found in 98% of the samples analyzed. These data confirm the putative role of varroa in the transmission of this virus (14, 26, 31). Interestingly, a large number of DWV-positive colonies were detected both for adult bees and for pupae or larvae throughout the year. According to Bailey and Ball (8), DWV is thought to be responsible for wing deformities when infection occurs during the white-eyed pupation stage of bee development. However, the mechanisms that lead to these symptoms are not clearly understood, and usually only a few bees in a colony severely infested by V. destructor display such deformities. DWV has been cited as potentially responsible for bee colony collapse (25), but other studies have shown that this virus might be considered poorly pathogenic (14, 31).
SBV.
The sacbrood virus was also found in the majority of bee colonies and was found more frequently in adults than in pupae, but unlike DWV, the SBV frequencies were much higher during the spring and summer. Similar SBV frequencies were previously found in healthy workers and pupae in Australia by an indirect SBV detection method (3). Likewise, Bailey and coworkers found that 33% of apparently healthy colonies examined in Great Britain were infected, with few larvae exhibiting sacbrood clinical signs (6). This latter work also showed that the occurrence of sacbrood was greater in the spring and in the summer. The seasonal variation in sacbrood virus frequencies observed both in adult bees and in pupae might reflect a difference in bee susceptibility to the virus or changes in the environment, such as the quality of the pollen consumed by larvae. Bees could also develop a kind of molecular defense mechanism to reduce virus multiplication. However, we cannot exclude the putative role of V. destructor in SBV propagation as the chi-square test showed that there was a positive correlation between the prevalence of SBV in varroa samples and the presence of SBV in adults (df = 1,
2 = 0.26, P = 0.61) and pupae (df = 1,
2 = 1.57, P = 0.21) in samples collected in the summer.
BQCV.
BQCV was very prevalent in colonies, especially in the adult bee population, in which the frequency reached 58% in the summer. In pupae, however, BQCV infections were scarcely detected; the maximum frequency was 2% in the summer. As shown in Table 2, adults in 24 apiaries were found to be positive for this virus throughout the year, which is an indication that BQCV infections (probably inapparent infections) can be maintained throughout the year. It was reported previously that this virus is found in close association with the protozoan Nosema apis, a parasite of the honey bee, and that in laboratory conditions per os infection of adult bees with BQCV is totally dependent on the presence of the parasite (7). The prevalence of N. apis infections in the samples which we analyzed (data not shown) may confirm these observations. Dissemination of BQCV by varroa mites appears to be improbable as this virus was never detected in any of the mite samples that we examined.
CBPV.
Likewise, all the varroa samples that we analyzed were negative for CBPV. These results suggest that the contribution of mites to the dissemination of this virus in the colony, if any, is small. Our data showed that CBPV was found only in adult bees, and the maximum frequency in colonies was only 4%, while CBPV infections recorded throughout the year occurred in 28% of the apiaries involved in our study (Table 2). The sporadic detection of CBPV during the year suggests that this virus might persist at undetectable levels in healthy colonies. Even if CBPV was shown to replicate in larvae after experimental inoculation (8), no CBPV could be detected in larvae or pupae in the samples that we examined. Taken together, our findings for CBPV prevalence are in agreement with previous observations showing that outbreaks of severe disease are erratic and exhibit no seasonal pattern (6, 28) and that CBPV transmission might result preferentially from contact between adults of the same colony (8).
ABPV.
ABPV is known to persist as inapparent infections in seemingly healthy bee colonies (6), but its presence was found to be correlated with the mortality of colonies infested with V. destructor (1). Twenty of the 195 varroa samples that we examined were positive for ABPV. The incidence of the virus in colonies was higher in the summer and autumn. Compared to the population dynamics of V. destructor, this suggests that the mite has a role in spreading this virus. However, the virus was also present in bee samples from apiaries where no ABPV-positive varroa mites were detected, suggesting that ABPV transmission may occur by contact between individual bees. The presence throughout the year of ABPV-positive adult bees in 21 of the 36 apiaries (Table 2) indicates that persistent ABPV infections can occur in honey bee colonies.
KBV.
KBV has been found frequently in Australia and in the United States but had never been identified in France previously. Here, we report the presence of KBV in 6 of 36 apiaries, and despite its lower prevalence, the distribution pattern of KBV appears to be slightly different than that of ABPV as the viral infections were shown to regress in autumn.
The data presented here clearly show that virus infections in apiaries are quite common. Furthermore, multiple infections were the rule in all the apiaries, and no significant geographical localization could be established for a given virus. These results provide clear evidence that bee virus infections occur persistently in bee colonies, despite the lack of clinical signs. One of the consequences of this is that outbreaks of virus disease leading to colony collapse probably result from external factors that provide favorable conditions (i) for the dissemination of the virus between individuals of the same colony and between different colonies and (ii) for the spread and replication of a virus from primary infection sites (e.g., gut epithelial cells) to critical targets (e.g., the nervous system) in bees. Although the lack of virus detection for apiary 429 (Ouessant Island) can be related to the geographical isolation of this apiary, it is worth noting that this finding correlates very well with the lack of varroa mites on the island. The high frequencies of viruses in mites suggest that V. destructor probably contributes efficiently to the outbreak of bee virus diseases, acting both as a vector and as an activator of virus replication. The latter phenomenon is a very challenging field for understanding the relationships among viruses, honey bee colonies, and V. destructor better.
This work was supported by a grant from the EEC and the French Ministère de l'Agriculture et de la Pêche (Règlement CE no. 1221/97, convention no. 2207).
D.T. and L.G. contributed equally to this work. ![]()
|
|
|---|
This article has been cited by other articles:
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Copyright © 2009 by the American Society for Microbiology. For an alternate route to Journals.ASM.org, visit: http://intl-journals.asm.org | More Info»