Seoul hantavirus in Europe: first demonstration of the virus genome in wild Rattus norvegicus captured in France
Although rats (Rattus rattus or Rattus norvegicus) worldwide have been found to carry Seoul hantavirus, there are at present only a very few reports of confirmed human Seoul hantavirus infections outside Asia, where the virus, in certain areas, is responsible for approximately 25% of the human hantavirus infections. In Europe, no confirmed human infections outside laboratories have been described, and although rats occasionally have been found to be antibody positive, the viral genome has not been demonstrated in these animals. The present report describes the first confirmed finding of Seoul hantavirus in R. norvegicus captured in Europe.
Hantaviruses (genus Hantavirus, family Bunyaviridae) are enveloped viruses with tripartite RNA genome of negative polarity . Hantaviruses are the etiological agents of hemorrhagic fever with renal syndrome (HFRS) in Europe and Asia and of hantavirus pulmonary syndrome in the Americas. HFRS is characterized by fever and renal dysfunction, sometimes with hemorrhagic manifestations . At least five hantaviruses cause HFRS. Puumala hantavirus (PUUV), carried by Clethrionomys glareolus (bank vole) , is found in most of Europe and causes a milder form of HFRS known as nephropathia epidemica. Severe forms of HFRS are caused by Dobrava (DOBV) in Europe  and by Hantaan (HTNV) in Asia , with Apodemus flavicollis (yellow-necked mouse) and Apodemus agrarius (striped field mouse) as hosts, respectively. A. agrarius is so far the only Eurasian rodent known to carry two distinct hantaviruses, HTNV in Asia and Saaremaa virus (SAAV) in Europe [6, 7]; the latter one presumably causes a milder form of HFRS . Microtus arvalis (common vole) is the major host for Tula hantavirus (TULV) in Europe [8, 9, 10, 11]. Although considered nonpathogenic to humans, TULV was recently reported in connection with cases of HFRS and/or hantavirus disease [12, 13]. In the Americas, Sigmodontinae-associated hantaviruses cause hantavirus pulmonary syndrome . In the natural reservoir, hantaviruses establish a life-long, chronic infection, and the host develops a strong neutralizing antibody response against the virus .
Seoul hantavirus (SEOV), carried by Rattus rattus (black rat) and Rattus norvegicus (Norway, or brown, rat) , has been reported to cause milder disease as compared to HTNV and DOBV and is responsible for 25% of the HFRS cases in Asia . It has been suggested that, as Rattus species are cosmopolitan, SEOV has the potential to cause human disease worldwide. SEOV was found in Japan, South Korea, Egypt, the USA, Brazil [18, and the references therein], and, most recently, in Cambodia  and Indonesia . Although there have been occasional reports of SEOV antibody-positive rats worldwide, there are at present only a few reports of confirmed human SEOV infections outside Asia, i.e., in the USA  and Brazil . In Europe, only laboratory outbreaks caused by SEOV have been reported so far [23, and references therein]. We present here the first genetic evidence of SEOV in R. norvegicus in Europe.
Materials and methods
The rats examined all belong to the species R. norvegicus. They were obtained from five colonies dedicated to the study of efficacy of various anticoagulant rodenticides. From each colony or subgroup, ten rats (labeled 50–59, 60–69, 70–79, 80–89, 90–99; 50 in total) were sacrificed, and blood, lungs, and kidneys were removed. Each of the five subgroups has been caught in the wild: subgroups I (50–59), II (60–69), and IV (80–89) were originally captured near Lyon, France. Subgroups III (70–79) and V (90–99) were originally captured near Orleans, France. Whole blood was immediately centrifuged, and serum was kept at −20°C. Lung and kidney tissue was flash-frozen and kept at −80°C. All animals were adults; numbers 87, 88, 98, and 99 were females, while all others were males. Sample 89 was not available for testing. Initial screening was performed using enzyme immunoassay (EIA) tests, and an immunofluorescence assay was applied to confirm EIA findings. Animals that were positive by these techniques were subsequently submitted to immunoblot tests for demonstration of the hantavirus antigen and reverse transcription-polymerase chain reaction (RT-PCR) for demonstration of the hantavirus genome.
Screening for SEOV IgG antibodies was performed by applying a standard immunofluorescence assay technique (Progen Biotechnik, Heidelberg, Germany). In brief, 10-well slides coated with SEOV-infected cells (strain R22) were rehydrated, and to each well 25 μl of a 1/64 dilution of rat serum was added. After appropriate incubation, slides were washed and 25 μl of anti-rat IgG FITC-labeled conjugate was added to each well. After incubation and washing, the slides were examined under a fluorescent microscope. Samples showing a positive reaction at a 1/64 dilution were further diluted and tested until the endpoint titer was obtained.
Screening was performed by applying a standard EIA technique (C. de Carvalho-Nicacio and Å. Lundkvist, unpublished), using native irradiation-inactivated viral antigens (SEOV strain R22VP30, kindly provided by Dr T. Avsic-Zupanc) and recombinant N-antigens (PUUV strain Kazan, and DOBV strain Slovenia, kindly provided by Dr Å. Lundkvist), for detection of IgG antibodies. In brief, 96-well microtiter plates were coated with antigen and incubated overnight at 4°C. Unsaturated binding sites were blocked, after which rodent sera diluted at 1:200 was incubated at 37°C for 1 h. Peroxidase-(Sigma, St Louis, MO, USA) or alkaline-phosphatase-labeled (Jackson, West Grove, PA, USA) anti-rat IgG antisera and, where appropriate, anti-mouse IgG antisera, were incubated for 1 h. Subsequently, 3,3′,5,5′ tetramethylbenzidine (TMB) or p-nitrophenyl substrates (Sigma) were added, and, after color development, the reaction was stopped with 2 M H2SO4 (for TMB substrate). The optical density (OD) was determined at 450 nm or 405 nm against a reference wavelength of 620 nm.
Eight lung tissue samples were submitted to immunoblotting. Tissue samples 50 and 67 were used as negative controls, while the remaining six (56, 83, 87, 90, 92, and 98) were SEOV antibody positive in the EIA.
Immunoblotting was performed essentially as described earlier . Briefly, lung tissue samples were homogenized in Laemmli sample buffer, separated by electrophoresis in 10% sodium dodecyl sulfate-polyacrylamide gels, and immunoblotted with rabbit polyclonal antibodies raised against DOBV (courtesy of Dr Å. Lundkvist), which cross-react with SEOV antigen. Mouse anti-rabbit antibodies conjugated with horseradish peroxidase (Dako, Glostrup, Denmark) were used as secondary antibodies.
RT-PCR and sequencing
RNA was extracted from the eight lung tissue samples (numbers 50, 56, 67, 83, 87, 90, 92, and 98) with the TriPure reagent (Boehringer Mannheim, Germany) according to the manufacturer’s instructions. Tissue samples 50 and 67, negative by both EIA and immunoblot, belonged to subgroups I and II and were intended as negative controls. Tissue samples 56, 83, 87, 90, 92, and 98 (from subgroups I, IV, and V) were antibody positive, except for sample 92, which was positive by EIA and negative by Western blot.
RNA was then subjected to a nested RT-PCR that yielded a 324-bp PCR amplicon from the M segment (nucleotides 1979–2302, primers excluded) or a 320-bp amplicon  from the S-segment (nucleotides 509–782, primers excluded). RT-PCR on the M segment was performed with primers 5′GTGGACTCTTCTTCTCATTATTG3′ and 5′TGGGCAATCTGGGGGGTTGCA3′. For nested PCR, two additional primers, 5′TGGGC(A/G)GCAAGTGCAGCAGA3′ and 5′GCATTTTGCAGTGTGCCATGG3′, were used. PCR amplicons were gel-purified using the QIAquick Gel Extraction Kit (Qiagen, Valencia, CA, USA) and sequenced automatically using either ABI PRISM Dye Terminator or ABI PRISM M13F and M13R Dye Primer sequencing kits (Applied Biosystems, Foster City, CA, USA).
Multiple nucleotide and amino acid sequence alignments were prepared manually using the SeqApp 1.9a169 sequence editing program. To infer phylogenies, the PHYLIP program package (version 3.5c, http://evolution.genetics.washington.edu/phylip.html) was used. One thousand bootstrap replicates (Seqboot program) were fed to the distance matrices algorithm (Dnadist, with Kimura’s two-parameter option), and distance matrices were analyzed with the neighbor-joining tree-fitting algorithm (Neighbor); the bootstrap support values were calculated with the Consensus program (part of the PHYLIP package). For comparison, the hantavirus sequences listed below were used.
Puumala virus (PUUV), strain Sotkamo (GeneBank accession no. X61034); Tula virus (TULV), strain Moravia/02v (Z69993); Dobrava virus (DOBV), strain Dobrava (L33685); Saaremaa virus (SAAV), strain Saaremaa/160v (AJ009774); Da Bie Shan virus (DBSV), strain NC167 (AB027115); Thailand virus (THAIV) (L08756); Hantaan virus (HTNV), strains HV114 (L08753), A9 (AF035831), 76-118 (M14627), HoJo (D00376), and Lee (D00377); and Seoul virus (SEOV), strains Gou3 (AB027521), Baltimore (U00151), Brazil (U00460), IR461 (AF458104), J12 (AB027082), HB55 (AF035832), L99 (AF035833), NM39 (AB027080), Wan (AB027081), Henan (AB027083), Shanxi (AB027084), Z37 (AF187081), c3 (AB027088), Hebei4 (AB027089), CD10 (AB027092), SD227 (AB027091), Tchoupitoulas (U00473), 80-39 (S47716), Hubei1 (S72343), SR11 (M34882), KI-88-15 (D17594), KI-83-262 (D17592), KI-85-1 (D17593), KI-262 (U00466), B1 (X53861), Jakarta137 (AJ620583), Egypt (U00463), Girard Point (U00464), Houston (U00465), HN71-L (AB027085), Guang199 (AB027086), and Beijing-Rn (AB027087).
PUUV, strain Sotkamo (X61035); TULV, strain Moravia/02v (Z69991); DOBV, strain Dobrava (L41916); SAAV, strain Saaremaa/160v (AJ009773); HTNV, strains A16 (AB027099), A9 (AF329390), 76-118 (M14626), Maaji (AF321095), Solovey/AP63/1999 (AB071184), and Solovey/AP61/1999 (AB071183); DBSV, strains NC167 (AB027523), AH211 (AF288647), and AH09 (AF285264); and SEOV, strains Gou3 (AF184988), Gou3v9 (AB027522), Cambodia (Camb)117 (AJ427511), Camb96 (AJ427512), Camb174 (AJ427513), Hb8610 (AF288643), L99 (AF288299), R22 (AF288295), Z37 (AF187082), IR461 (AF329388), Tchoupitoulas (AF329389), SR11 (M34881), zy27 (AF406965), Pf26 (AY006465), Camb175 (AJ427505), Camb41 (AJ427501), Camb32 (AJ427508), Camb58 (AJ427510), and Camb180 (AJ427506).
Screening of serum and tissue samples for the presence of hantavirus markers
All rats from subgroups II and III were SEOV antibody negative by EIA; one rat from subgroup I (no. 56) was antibody positive. Of the rats from subgroups IV and V, 15 of 19 (78.9%) were SEOV antibody positive. As expected, cross-reactions with PUUV and DOBV antigens were noted. Immunofluorescence assay results confirmed the EIA results for 100% of the samples.
Eight lung tissue samples were submitted to immunoblotting with DOBV antigen. Cross-reactivity of this test with SEOV antibodies was apparently sufficient (data not shown). Tissue samples no. 50 and no. 67 were taken from antibody-negative rats and therefore served as negative controls and were found negative by the immunoblot as well. Of six samples taken from antibody-positive rats (56, 83, 87, 90, 92, and 98) all except no. 92 were N-antigen-positive (with no. 87 producing a very weak band).
Simultaneously, 320-bp amplicons from the hantavirus S segment were recovered from sample nos. 83, 90, and 92. Nucleotide sequences of all three amplicons (nucleotides 509–782 of the S segment, primers excluded) were identical and showed the highest level of similarity (98.2–99.6%) to the recently described wild-type SEOV strains 032, 041, 058, 175, and 180 from Cambodia, all carried by R. norvegicus . Deduced N-protein sequences of all these SEOV strains were identical. On the phylogenetic tree calculated for the S-segment sequences, the French strain was located within the genetic lineage formed by the Cambodian strains 032, 041, 058, 175, and 180 (Fig. 1b). This lineage was moderately supported (bootstrap support value of 64% for 1,000 replicates).
Of the eight genetic lineages seen on the M tree (Fig. 1a), only three (nos. 1, 5, and 7) can be recognized on the S tree, most probably due to the fact that the S-segment sequences were not reported for all SEOV strains presented on the M tree. Instead, there are two additional genetic lineages: no. 9, formed by two Chinese strains of unknown origin, zy27 and pf26, and no. 10, which includes three strains from Cambodia carried by either R. rattus or unclassified Rattus species .
We present here the first confirmed finding of the SEOV genome in R. norvegicus captured in Europe. Earlier findings of SEOV antibodies in humans were based on the results of highly cross-reactive immunofluorescence assays, which are also known to be prone to false-positive reactions, and have never been confirmed by reliable methods such as the focus reduction neutralization test. SEOV-caused laboratory outbreaks have been reported in Belgium, France, the Netherlands, and the UK (reviewed by Lee ), and the strain of SEOV virus from the English outbreak has been genetically characterized , but the SEOV genome was never found in the local wild rats. Neither was there clear evidence of SEOV-related HFRS cases in Europe outside the laboratories.
Very recently, we became aware of the data of Lundkvist et al. on the presence of SEOV-neutralizing antibodies in convalescent serum of a French HFRS patient (Å. Lundkvist, personal communication). Combined with our data, these observations show that SEOV is present in France and has reached humans outside the laboratories. To evaluate the seroprevalence of SEOV-neutralizing antibodies in France, a large-scale seroepidemiological study should be conducted.
It is of interest that the newly described SEOV strain from France is most closely related to the Indonesian wild-type strain Jakarta137 (Fig. 1a) and the Cambodian wild-type strains 32, 41, 58, 175, and 180 (Fig. 1b), all associated with R. norvegicus, but it is quite distinct from Cambodian wild-type strains 96, 117, and 174 carried by R. rattus or unidentified Rattus species . Unfortunately, neither S-segment sequences from the Indonesian SEOV strains nor M-segment sequences from the Cambodian strains were recovered; therefore, analysis of their phylogenetic relationships has yet to be performed. Close relationships between the French SEOV strain, on one hand, and the Indonesian and Cambodian strains, on the other, may reflect major trade routes or could be related to the historical connections between these regions and France.
Throughout history, R. norvegicus has been one of the most harmful mammal species to mankind. Rat-borne diseases have probably taken more human lives than all the wars in human history combined, and control attempts directed against Norway rats invariably fail. In the UK, R. norvegicus was found to harbor no less than 13 zoonotic agents and ten nonzoonotic parasite species .
Norway rats originated on the plains of Asia, probably in what is now northern China and Mongolia, where wild rats today still live in burrows [27, 28]. They arrived in Europe most probably during the 16th century . Along its journey, R. norvegicus developed human-dependent behavior: Norway rats easily access human habitation, and it is therefore important for modern public health systems to maintain surveillance of the species. Asia excluded, SEOV does not seem to be a major public health threat, which is in contradiction with its worldwide spread. A possible explanation could be that, in a given region, testing of human sera for SEOV is not done on the same scale as for the “locally” most plausible serotype. When, however, testing for the presence of SEOV antibodies in human sera is included in the test panel, a certain level of seropositivity is invariably encountered.
Data from this study add SEOV to the existing list of European hantavirus pathogens that currently includes PUUV, DOBV, TULV, and SAAV. This important addition should be taken into consideration for improving both diagnostics and clinical awareness of physicians.
This study was made possible by grants STE/R and T WB11 from the Belgian Ministry of Defence, and grants RFA915 and 202012 from the Academy of Finland. The collection of rats was carried out during a preliminary research program on PUUV infection in bank voles (AER 2002), a program supported by the Veterinary School of Lyon in collaboration with Claude Bernard University, Lyon, and the Pasteur Institute, Paris. The experiments described comply with current European Union laws and regulations.
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