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Journal of Clinical Microbiology, February 2005, p. 808-812, Vol. 43, No. 2
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.2.808-812.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Identification of Genetic Evidence for Dobrava Virus Spillover in Rodents by Nested Reverse Transcription (RT)-PCR and TaqMan RT-PCR
Manfred Weidmann,1*
P. Schmidt,1
M. Vackova,2
K. Krivanec,3
P. Munclinger,4 and
F. T. Hufert1
Unit for Microbiological Preparedness and Emerging Infectious Diseases, University Hospital, Freiburg, Germany,1
Purkyne Military Medical Academy, Department of Epidemiology, Hradec Kralove,2
Central Military Institute of Health, Ceske Budejovice,3
Charles University Prague, Biodiversity Research Group, Department of Zoology, Prague, Czech Republic4
Received 12 May 2004/
Returned for modification 23 July 2004/
Accepted 6 September 2004

ABSTRACT
A survey of 158 rodents caught in the Czech Republic identified
Dobrava virus sequences closely related to that of the Dobrava
virus type strain in
Apodemus sylvaticus and
Mus musculus rodents.
The identity of
A. sylvaticus was unequivocally confirmed by
random amplified polymorphic DNA analysis. The data seem to
indicate hantavirus spillover from
Apodemus flavicollis to other
rodents.

INTRODUCTION
Hantaviruses are subdivided into the Eurasian and the American
groups. The distinction manifests itself in serological and
genetic differences as well as in the clinical picture of the
diseases elicited in humans. The up to 20 individual hantavirus
types have been observed to be strictly coevolving with the
distinct rodent species that carry them. Prominent hantaviruses
in western and central Europe are Dobrava virus (DOBV), Puumala
virus (PUUV), and Tula virus (TULV) (
6,
16,
21).
DOBV was originally isolated in Slovenia from the rodent Apodemus flavicollis (DOBV-Af) (3) and is a Eurasian hantavirus that predominantly causes disease in humans in southeastern Europe. DOBV-Af has also been isolated in Greece, Albania, and Bosnia-Herzegovina (1, 10, 14). Serological studies have indicated the presence of DOBV-Af beyond the geographical limits of the Balkans in Slovakia (22) and recently in the Czech Republic (15, 25). The isolation of a Dobrava-like virus from the rodent Apodemus agrarius (DOBV-Aa) was reported in Estonia (11), Russia (17), and Slovakia (23), supplemented by a report on genetic evidence of this DOBV-Aa type from rodent samples in Hungary (20). Whether this new type is a type of its own, as comparative serological and genetic analyses suggest, is still a matter of debate (7, 12, 18). The distribution of the two rodent host species overlaps in Europe, but rodent carriers of both DOBV types coexist only in the border region of southeastern and central Europe (2, 23). In order to investigate the occurrence of hantaviruses in a rodent population from the Czech Republic, we analyzed the kidneys, lungs, and spleens of 157 rodents and 1 shrew (A. flavicollis, n = 77; A. sylvaticus, n = 34; Mus musculus, n = 2; Clethrionomys glareolus, n = 41; Microtus arvalis, n = 2; Microtus agrestis, n = 1; Sorex araneus, n = 1) caught at the military training area of Boletice, close to Ceské Budejovice in the south of the Czech Republic, from May to November 1999.

MATERIALS AND METHODS
A total of 10 to 50 mg of tissue of each organ was homogenized
in a FastPrep machine with the FastRNA Green kit (Qbiogene,
Hilden, Germany) to perform total RNA extraction. This closed
system avoids the generation of contaminating aerosols. We screened
the extracted RNA for DOBV-Af RNA by a one-step TaqMan reverse
transcription (RT)-PCR and for hantavirus RNA with the degenerate
primers for nested PCR published by Scharninghausen et al. (
20).
For the DOBV TaqMan RT-PCR, we used 500 nM concentrations of
primers DOBFP (5'-TGGCTTGACCTCCCGTG-3') and DOBRP (5'-CAAGCGCTCCTTGTCTTTGA-3')
and 200 nM probe DOBP (5'-ATCTCCAACGTCTTTGACCAAAGGCCC-3') tagged
with 6-carboxyfluorescein (FAM)-6-carboxytetramethylrhodamine
(TAMRA). To detect synthetic RNA we used the SRDOB TaqMan RT-PCR
with 500 nM concentrations of primers SRDOBFP (5'-TAGTGCTCTTTAGGGAGTCTACTAC-3')
and SRDOBRP (5'-CAAGCGCTCCTTGTCTTTGA-3') and 200 nM probe SRDOB
P (5'-AAAGCCGAATTCTGCAGATATCCAT-3') tagged with FAM and TAMRA.
The SRDOB amplicon spans the extreme 5' end of the DOBV S-segment
RNA transcript. The probe and the reverse primer bind in the
trace plasmid sequence stemming from pCRII into which the S
segment was ligated. We performed the amplification on a LightCycler
instrument using an RNA Master Hybridization Probes kit (Roche,
Mannheim, Germany) and a temperature profile of RT at 60°C
for 20 min, activation at 95°C for 2 min, and 45 cycles
of PCR at 95°C for 15 s and 60°C for 1 min. The efficiencies
of the TaqMan RT-PCRs were calculated as 10
1/slope
1. The nested PCR was done with a touchdown amplification profile
adapted according to Sachadyn et al. (
19): first round, RT at
53°C for 30 min; PCR with 20 cycles of 95°C for 1 min,
65°C for 1 min, and 72°C for 1 min; and completion of
extension at 72°C for 10 min with primers M9 (5'-TGRYCNAGYTGTATYCCCATWGATTG-3')
and M4 (5'-ATGAARGCNGAWGARNTNACMCCNGG-3 [nucleotide abbreviations
according to the IUB code]); second round, activation at 95°C
for 10 min; PCR with 6 cycles of 95°C for 1 min, 65°C
for 1 min, and 72°C for 1 min, followed by 3 cycles each
of the same profile with hybridization at 66, 64, 66, 64, 62,
60, 62, 60, 59, 60, and 59°C and completion of the extension
at 72°C for 10 min with primers M6 (5'-AGYCCWGTNATGRGWGTRATTGG-3')
and M8 (5'-GAKGCCATRATNGTRTTYCKCATRTCCTG-3'). The buffer conditions
for the first round of RT-PCR were as described previously (
26),
the second round of PCR was done with AmpliTaq Gold, 1 µM
primers, and 500 µM deoxynucleoside triphosphates. Three
negative controls were included in each cycling step. Random
amplified polymorphic DNA (RAPD) analysis for differentiation
of the rodent species was done as described previously (
4).
For phylogenetic analysis, alignment was done with the ClustalW
program in the Megalign software module of the Lasergene package
(DNAstar) by using the default settings. To plot the trees,
we used the Phylip software package 3.6.seqboot module for bootstrapping
(200 bootstraps, 55-bp sequence blocks) and the dnapars module
to build the most parsimonious tree. The tree was drawn with
the TreeEdit program (version 1.0a10). The tree in Fig.
2 was
initially rooted to the sequence of Hantaan virus (HTNV) type
strain 76/118. The DOBV sequence most closely related to it
was then used to root the tree in Fig.
2.
To plot divergence against geographical distance, divergence
values were extracted from the alignment obtained with the ClustalW
program, and the coordinates of the isolation sites were determined
(
http://wayhoo.com). Polar coordinate distances between the
various isolation sites and the isolation site in Slovenia were
calculated (
http://jan.ucc.nau.edu/
cvm/latlongdist.html).
Nucleotide sequence accession numbers.
The sequences obtained from A. sylvaticus and M. musculus have been submitted to GenBank and given the accession numbers AJ578081 and AJ578082, respectively.

RESULTS AND DISCUSSION
The DOBV TaqMan RT-PCR assay was tested with a synthetic RNA
standard transcribed from a DOBV S segment ligated into pCRII
(Invitrogen, Breda, The Netherlands). This assay had a sensitivity
of detection of 10 molecules (standard range, 10
7 to 10 molecules;
correlation coefficient, 0.99; efficiency, 0.84). We also established
a second TaqMan RT-PCR to specifically detect the extreme 5'
end of the DOBV RNA transcript with a downstream primer and
probe that bind to the trace plasmid sequence originating from
between the T7 promoter and the 5' end of the cloned DOBV S
fragment (standard range, 10
5 to 10
2; correlation coefficient,
0.96; efficiency, 0.94). It had a sensitivity of detection of
100 molecules (Fig.
1, to panels). We tested the nested PCR
with the synthetic RNA standard and found that we could increase
the sensitivity of detection from 10
3 to 10 molecules by using
the touchdown protocol instead of the protocol originally used
for this PCR (Fig.
1, top panels). The TaqMan RT-PCR detected
DOBV RNA in 7 of 474 rodent tissue samples. The nested PCR yielded
a 389-bp band for 2 of 474 rodent tissue samples. These two
samples also tested positive by the TaqMan RT-PCR (Table
1).
The DOBV TaqMan assay additionally detected DOBV-Af in one
A. flavicollis-positive sample and in four
C. glareolus-positive
samples. The DOBV TaqMan assay is specific for DOBV-Af and does
not cross amplify DOBV-Aa, PUUV, or TULV sequences (data not
shown). Several measures were adopted to avoid false-positive
PCR signals. We did not culture DOBV-Af in a biosafety level
3 laboratory at the time of tissue preparation. To exclude false-positive
results due to our synthetic DOBV-Af RNA standard, we tested
all positive samples by the SRDOB TaqMan RT-PCR. All DOBV-positive
samples tested negative with primer SRDOB (data not shown).
We included one negative control for each TaqMan RT-PCR and
three negative controls for each step of the nested PCR. The
amplicons from the two nested PCRs were ligated into pCRII by
TA cloning (Invitrogen). We sequenced the recombinant plasmids
of two independent clones each and aligned the consensus sequence
of the sequences obtained to other hantavirus sequences in GenBank.
Both sequences turned out to be closely related to DOBV-Af.
One sequence originated from the kidney of a captured
A. sylvaticus mouse and the other was amplified from the spleen of a captured
M. musculus mouse. To confirm the identity of
A. sylvaticus,
whose phenotype can easily be mistaken for
A. flavicollis in
some parts of central Europe, we used a RAPD system to identify
the samples from which the DOBV-Af sequence had been amplified
(
4). The RAPD analysis confirmed the identity of the mouse species
implicated (Fig.
1, bottom panel), making this the first confirmed
report of a DOBV-Af sequence identified in a captured
A. sylvaticus mouse and a captured
M. musculus rat. Excluding the primer binding
sites, we aligned 319 bp of the sequences obtained to all published
DOBV sequences that cover that region of the S fragment and
to the respective sequence of HTNV type strain 76/118. In a
weighted phylogram rooted to the DOBV sequence most closely
related to the HTNV type strain, the new sequences group with
the original DOBV-Af isolate and other DOBV-Af sequences (Fig.
2). The DOBV-Aa sequences group outside of the DOBV-Af clade.
We also found in the GenBank data bank two entries (GenBank
accession numbers
AF442622 and
AF442623) for sequences from
A. sylvaticus isolates that as yet have not been published in
a paper. These sequences and the one that we derived from
A. sylvaticus group together in the DOBV-Af clade. The sequence
that we have derived from
M. musculus also grouped into this
clade. In the DOBV-Af clade there seems to be an obvious correlation
between the geographic distance from Slovenia and the percent
identities of the sequences to the DOBV-Af type strain. These
range from 99.1% for the closest sequence from the Czech Republic
to 90.4% for the farthest sequence from Russia (Fig.
3A). In
fact, the geographical distances of the sites of isolation of
the DOBV-Af strains to the site of isolation of the DOBV-Af
type strain plotted versus sequence divergence results in a
correlation (
r2) of 0.91 (
P = 0.0008). Without the new sequences
r2 value is reduced to 0.85 (
P = 0.026) (Fig.
3B). A similar
correlation has been observed and substantially analyzed for
the tick-borne encephalitis virus cline in the Northern Hemisphere
(
27).
In a study of the 1989 hemorrhagic fever with renal syndrome
outbreak in Yugoslavia, 544 trapped rodents captured at various
sites were studied (
5). The investigators found hantavirus antigen-positive
lung tissues (by immunofluorescence assay) and hantavirus antibody-positive
sera (by immunofluorescence assay and enzyme-linked immunosorbent
assay) in six rodent species (
A. flavicollis, 46.5% [88 positive
animals of 189 animals captured];
A. agrarius, 15.6% [10 of
64];
A. sylvaticus, 19.1% [28 of 146];
C. glareolus, 26.5% [36
of 63];
M. musculus, 48.3% [14 of 29]; and
Rattus norvegicus,
66.6% [14 of 21]). In recent work (
9), the lung tissues of
A. flavicollis,
A. sylvaticus, and
C. glareolus rodents in Croatia
have also been found to be hantavirus antigen positive. In Belgium
a survey to study hantaviruses in wild, trapped rodents found
C. glareolus,
A. sylvaticus, and
Mus arvalis rodents that were
seropositive for PUUV by immunoglobulin G enzyme-linked immunosorbent
assay and focus reduction neutralization test (
8). Genetic information
for TULV, which is carried by
M. arvalis in central Europe,
was amplified from the lung tissue of a
Pitymys subterraneus rodent in Serbia (
24).
Here we present the first genetic evidence of DOBV-Af in A. sylvaticus and M. musculus. This seems to confirm the findings from the earlier study by Gligic et al. (5), who indicated that these species were hantavirus positive by serological assays. In the Belgian study, A. sylvaticus was found to be PUUV seropositive. The investigators concluded that they had documented a spillover of PUUV from C. glareolus to A. sylvaticus, which share the same biotope. We seem to have observed a similar event for DOBV-Af spillover from A. flavicollis to A. sylvaticus, M. musculus, and C. glareolus, although spillover to C. glareolus has not yet been confirmed by sequencing data. The relatedness of the new DOBV-Af sequences to the DODV-Af type strain sequence argues for this possibility, especially since they also fit well into the observed geographic distribution of the DOBV-Af sequences. In fact, they are not deviant isolated lineages of their own but are most likely spillover strains from the local DOBV-Af stains usually present in A. flavicollis. Due to antigen cross-reactivity, which gives false-positive results, investigators may have been careful to interpret the antigen-positive results for rodent tissues as evidence for hantavirus infection in the past. However, in the light of the studies cited above and the data presented here, the view on the ecology of hantavirus transmission in rodent populations in central Europe may have to change. Spillover events seem to be far more common than expected. To study these spillover events in more detail, it will be necessary to examine the dynamics of the major host rodent populations. High population densities of host rodents, which fluctuate in 3- to 4-year cycles, have been directly linked to high hantavirus infection rates in the rodent population (13). Spillover from the rodent host populations to other rodents of the same biotopes should most likely occur at these time points. Future trapping programs should take these cycles into account.

ACKNOWLEDGMENTS
This project was supported by grants InSanI 0598-V4301 and InSanI
030-V4304 of the Bundesministerium für Verteidigung, Bonn,
Germany, and grant 0021620828 MSMT of the Ministry of Education,
Youth and Sport of the Czech Republic.

FOOTNOTES
* Corresponding author. Mailing address: Unit for Microbiological Preparedness and Emerging Infectious Diseases, University Hospital, Stefan-Meier-Str. 19, 79014 Freiburg, Germany. Phone 49 761 203 5436. Fax: 49 761 203 5434. E-mail:
manfred.weidmann{at}uniklinik-freiburg.de.


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Journal of Clinical Microbiology, February 2005, p. 808-812, Vol. 43, No. 2
0095-1137/05/$08.00+0 doi:10.1128/JCM.43.2.808-812.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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