Evaluation of Puumala virus IgG and IgM enzyme immunoassays based on recombinant baculovirus-expressed nucleocapsid protein for early nephropathia epidemica diagnosis

Hantavirus infection in central Sri Lanka - an unusual clinical presentation: a case report

Hantavirus infections are emerging zoonoses. In Asia, the hantavirus commonly manifests as haemorrhagic fever with renal syndrome (HFRS), apparent with fever, thrombocytopenia and acute kidney injury. There are a few cases with the atypical clinical course with cardiopulmonary symptoms in Asia including Sri Lanka. Here, we report a case of hantavirus infection with an atypical cardiopulmonary syndrome-like illness with serological evidence of the Puumala/Puumala-like virus.

Differential Cytokine Responses and the Clinical Severity of Adult and Pediatric Nephropathia Epidemica

Nephropathia epidemica (NE), caused by the hantavirus infection, is endemic in Tatarstan Russia. The majority of patients are adults, with infection rarely diagnosed in children. This limited number of pediatric NE cases means there is an inadequate understanding of disease pathogenesis in this age category. Here, we have analyzed clinical and laboratory data in adults and children with NE to establish whether and how the disease severity differs between the two age groups. Serum cytokines were analyzed in samples collected from 11 children and 129 adult NE patients during an outbreak in 2019. A kidney toxicity panel was also used to analyze urine samples from these patients. Additionally, serum and urine samples were analyzed from 11 control children and 26 control adults. Analysis of clinical and laboratory data revealed that NE was milder in children than in adults. A variation in serum cytokine activation could explain the differences in clinical presentation. Cytokines associated with activation of Th1 lymphocytes were prominent in adults, while they were obscured in sera from pediatric NE patients. In addition, a prolonged activation of kidney injury markers was found in adults with NE, whilst only a short-lasting activation of these markers was observed in children with NE. These findings support previous observations of age differences in NE severity, which should be considered when diagnosing the disease in children.

An HFman Probe-Based Multiplex Reverse Transcription Loop-Mediated Isothermal Amplification Assay for Simultaneous Detection of Hantaan and Seoul Viruses

Hantaviruses are zoonotic pathogens that are widely distributed worldwide. Hantaan virus (HTNV) and Seoul virus (SEOV) are two most common hantaviruses that infect humans and cause hemorrhagic fever with renal syndrome (HFRS). Rapid and sensitive detection of HTNV and SEOV are crucial for surveillance, clinical treatment and management of HFRS. This study aimed to develop a rapid HFman probe-based mulstiplex reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay to simultaneously detect HTNV and SEOV. A novel multiplex RT-LAMP assay was developed, and 46 serum samples obtained from clinically suspected patients were used for evaluation. The novel RT-LAMP assay can detect as low as 3 copies/reaction of hantaviruses with a detection limit of 41 and 73 copies per reaction for HTNV and SEOV, respectively. A clinical evaluation showed that the consistencies of the multiplex RT-LAMP with RT-qPCR assay were 100% and 97.8% for HTNV and SEOV, respectively. In view of the high prevalence of HTNV and SEOV in rural areas with high rodent density, a colorimetric visual determination method was also developed for point-of-care testing (POCT) for the diagnosis of the two viruses. The novel multiplex RT-LAMP assay is a sensitive, specific, and efficient method for simultaneously detecting HTNV and SEOV.

Abdominal CT findings in Puumala hantavirus-infected patients

BACKGROUND

Previous clinical studies have reported abdominal findings on ultrasonography or MRI in Puumala hantavirus-infected patients.

PURPOSE

To determine if abdominal computed tomography (CT) can lead to a diagnosis of Puumala virus infection in the presence of a suggestive clinical picture.

MATERIAL AND METHODS

CT findings were studied retrospectively in 30 patients who presented to the emergency department of two (Belgian) hospitals with serologically confirmed Puumala hantavirus infection.

RESULTS

The most frequent finding was perirenal fascial thickening (90%), followed by perirenal fat stranding (87%). Retroperitoneal fat stranding was found in 19 patients (64%) in the perivesical spaces along the fascia of the external iliac vessels with or without involvement of the presacral fat. Half of the patients had pelvic ascites, and pleural fluid was found in 7 of them. The right and left mean pole-to-pole kidney's lengths were respectively 125.7 mm and 127.8 mm in 28 patients.

CONCLUSION

Retroperitoneal fat stranding, perirenal fascial thickening and/or perirenal fat stranding were found in most patients with acute Puumala virus infection who have undergone CT. Although nonspecific, these findings may help to suggest Puumala hantavirus infection in the right clinical settings.

Puumala hantavirus: an imaging review

Puumala virus (PUUV) is the most common hantavirus in Europe. It is known to cause nephropathia epidemica, which is considered a mild type of hemorrhagic fever with renal syndrome. However, it does not only involve the kidneys and is rarely accompanied by symptomatic hemorrhage. We review the imaging abnormalities caused by PUUV infection, from head to pelvis, emphasizing the broad spectrum of possible findings and bringing further support to a previously suggested denomination "Hantavirus disease" that would encompass all clinical manifestations. Although non-specific, knowledge of radiological appearances is useful to support clinically suspected PUUV infection, before confirmation by serology.

Flash-Like Albuminuria in Acute Kidney Injury Caused by Puumala Hantavirus Infection

Transient proteinuria and acute kidney injury (AKI) are characteristics of Puumala virus (PUUV) infection. Albuminuria peaks around the fifth day and associates with AKI severity. To evaluate albuminuria disappearance rate, we quantified albumin excretion at different time points after the fever onset. The study included 141 consecutive patients hospitalized due to acute PUUV infection in Tampere University Hospital, Finland. Timed overnight albumin excretion (cU-Alb) was measured during the acute phase in 133 patients, once or twice during the convalescent phase within three months in 94 patients, and at six months in 36 patients. During hospitalization, 30% of the patients had moderately increased albuminuria (cU-Alb 20-200 μg/min), while 57% presented with severely increased albuminuria (cU-Alb >200 μg/min). Median cU-Alb was 311 μg/min (range 2.2-6460) ≤7 days after fever onset, 235 μg/min (range 6.8-5479) at 8-13 days and 2.8 μg/min (range 0.5-18.2) at 14-20 days. After that, only one of the measurements showed albuminuria (35.4 μg/min at day 44). At six months, the median cU-Alb was 2.0 μg/min (range 0.6-14.5). Albuminuria makes a flash-like appearance in PUUV infection and returns rapidly to normal levels within 2-3 weeks after fever onset. In the case of AKI, this is a unique phenomenon.

Evaluation of Real-Time RT-PCR for Diagnostic Use in Detection of Puumala Virus

Puumala virus (PUUV) is the most common cause of hantavirus infection in Europe, with thousands of cases occurring particularly in Northern, Central and Eastern Europe and Russia. It causes a mild form of hemorrhagic fever with renal syndrome also known as nephropathia epidemica (NE) with clinical picture ranging from mild to severe. Currently, the laboratory diagnosis of NE is mainly based on serology. Here, we evaluated a real-time one-step qRT-PCR (PUUV-qRT-PCR) for detection of PUUV with 238 consecutive diagnostic serum samples from patients with suspected PUUV infection. The PUUV-qRT-PCR was both specific and sensitive for PUUV RNA. The analytical sensitivity (limit of detection) was estimated to be four copies of PUUV per reaction. Altogether 28 out of 30 (93%) PUUV IgM positive samples were positive also for PUUV RNA. No false positives were detected and the specificity was thus 100%. Interestingly, one sample was found positive in PUUV-qRT-PCR prior to subsequent IgM and IgG seroconversion. PUUV-qRT-PCR could be used for diagnostics in the early phase of NE infection and might be helpful especially in the rare severe cases when the patient’s condition may deteriorate rapidly.

Hantavirus infections

Over the past few decades understanding and recognition of hantavirus infection has greatly improved worldwide, but both the amplitude and the magnitude of hantavirus outbreaks have been increasing. Several novel hantaviruses with unknown pathogenic potential have been identified in a variety of insectivore hosts. With the new hosts, new geographical distributions of hantaviruses have also been discovered and several new species were found in Africa. Hantavirus infection in humans can result in two clinical syndromes: haemorrhagic fever with renal syndrome (HFRS) and hantavirus cardiopulmonary syndrome (HCPS) caused by Old World and New World hantaviruses, respectively. The clinical presentation of HFRS varies from subclinical, mild, and moderate to severe, depending in part on the causative agent of the disease. In general, HFRS caused by Hantaan virus, Amur virus and Dobrava virus are more severe with mortality rates from 5 to 15%, whereas Seoul virus causes moderate and Puumala virus and Saaremaa virus cause mild forms of disease with mortality rates <1%. The central phenomena behind the pathogenesis of both HFRS and HCPS are increased vascular permeability and acute thrombocytopenia. The pathogenesis is likely to be a complex multifactorial process that includes contributions from immune responses, platelet dysfunction and the deregulation of endothelial cell barrier functions. Also a genetic predisposition, related to HLA type, seems to be important for the severity of the disease. As there is no effective treatment or vaccine approved for use in the USA and Europe, public awareness and precautionary measures are the only ways to minimize the risk of hantavirus disease.

Competitive Homogeneous Immunoassay for Rapid Serodiagnosis of Hantavirus Disease

In this study, we describe a competitive homogeneous immunoassay that makes use of Förster resonance energy transfer (FRET) in rapid detection of pathogen-specific antibodies. The assay principle is based on competition between a monoclonal antibody (MAb) and serum antibodies to a given antigen. In the assay, named competitive FRET immunoassay (CFRET-IA), the FRET signal is induced if MAb carrying a donor label binds to an acceptor-labeled antigen. Specific antibodies in serum compete for antigen binding, resulting in reduced FRET signal. The proof-of-principle for the assay was obtained using donor-labeled Puumala virus nucleocapsid protein (PUUV-N) and acceptor-labeled anti-PUUV-N MAb. The assay was evaluated by analyzing 329 clinical samples comprising 101 from individuals with acute PUUV infection, 42 from individuals with past infection, and 186 from individuals with PUUV-seronegative sera, and the results were compared to those of reference tests. The rapid serodiagnostic test we introduced herein performed with 100% sensitivity and 99% specificity for diagnosing acute hantavirus disease.

Indirect immunofluorescence assay for the simultaneous detection of antibodies against clinically important old and new world hantaviruses

In order to detect serum antibodies against clinically important Old and New World hantaviruses simultaneously, multiparametric indirect immunofluorescence assays (IFAs) based on biochip mosaics were developed. Each of the mosaic substrates consisted of cells infected with one of the virus types Hantaan (HTNV), Puumala (PUUV), Seoul (SEOV), Saaremaa (SAAV), Dobrava (DOBV), Sin Nombre (SNV) or Andes (ANDV). For assay evaluation, serum IgG and IgM antibodies were analyzed using 184 laboratory-confirmed hantavirus-positive sera collected at six diagnostic centers from patients actively or previously infected with the following hantavirus serotypes: PUUV (Finland, n=97); SEOV (China, n=5); DOBV (Romania, n=7); SNV (Canada, n=23); ANDV (Argentina and Chile, n=52). The control panel comprised 89 sera from healthy blood donors. According to the reference tests, all 184 patient samples were seropositive for hantavirus-specific IgG (n=177; 96%) and/or IgM (n=131; 72%), while all control samples were tested negative. In the multiparametric IFA applied in this study, 183 (99%) of the patient sera were IgG and 131 (71%) IgM positive (accordance with the reference tests: IgG, 96%; IgM, 93%). Overall IFA sensitivity for combined IgG and IgM analysis amounted to 100% for all serotypes, except for SNV (96%). Of the 89 control sera, 2 (2%) showed IgG reactivity against the HTNV substrate, but not against any other hantavirus. Due to the high cross-reactivity of hantaviral nucleocapsid proteins, endpoint titrations were conducted, allowing serotype determination in >90% of PUUV- and ANDV-infected patients. Thus, multiparametric IFA enables highly sensitive and specific serological diagnosis of hantavirus infections and can be used to differentiate PUUV and ANDV infection from infections with Murinae-borne hantaviruses (e.g. DOBV and SEOV).

Surface-activated microtiter-plate microarray for simultaneous CRP quantification and viral antibody detection

Microarrays are widely used in high-throughput DNA and RNA hybridization tests and recently adopted to protein and small molecule interaction studies in basic research and diagnostics. Parallel detection of serum antibodies and antigens has several potential applications in epidemiologic research, vaccine development, and in the diagnosis of allergies, autoimmunity, and infectious diseases. This study demonstrates an immobilization method for immunoassay-based microarray in conventional 96-well polystyrene plates for a serologic diagnostic method combined with quantitative C-reactive protein (CRP) assay. A synthetic peptide (HIV-1), a recombinant protein (Puumala hantavirus nucleocapsid), and purified virus preparations (Sindbis and adenoviruses) were used as antigens for virus-specific antibody detection and monoclonal anti-CRP antibody for antigen detection. The microarray was based on conventional enzyme immunoassays and densitometry from photographed results. Peptide and recombinant antigens functioned well, while whole virus antigens gave discrepant results in 1 out of 23 samples from the reference method, tested with human sera with various antibody responses. The CRP results were in concordance in the concentration range 0.5-150 mg/L with 2 commercially available CRP assays: ReaScan rapid test (R(2) = 0.9975) and Cobas 6000 analyzer (R(2) =0.9595). The results indicate that microtiter plates provide a promising platform for further development of microarrays for parallel antibody and antigen detection.

A Case Report of Haemorrhagic Fever with Renal Syndrome

A 42-year-old air-conditioning technician presented with abdominal pain and influenza like illness eventually developed thrombocytopenia and haemoconcentration with renal impairment shortly after admission. The patient required a section of haemodialysis for acute oliguric renal failure with fluid overload. He developed diuresis and his clinical condition was improved with supportive treatment. Hantaan virus infection with haemorrhagic fever with renal syndrome was confirmed by serologic test.

Clinical assessment and improved diagnosis of bocavirus-induced wheezing in children, Finland

Accurate diagnosis of respiratory infections requires serologic analysis and PCR of serum.

Human bocavirus (HBoV) is a widespread respiratory virus. To improve diagnostic methods, we conducted immunoglobulin (Ig) G and IgM enzyme immunoassays with recombinant virus–like particles of HBoV as antigen. Acute-phase and follow-up serum samples from 258 wheezing children and single serum samples from 115 healthy adults in Finland were examined. Our assays had a sensitivity of 97% and a specificity of 99.5%. Of adults, 96% had immunity; none had an acute infection. Of 48 children with serologically diagnosed acute HBoV infections, 45 were viremic and 35 had virus in nasopharyngeal aspirates (NPAs). Of 39 HBoV NPA PCR–positive children co-infected with another virus, 64% had a serologically verified HBoV infection. HBoV caused illness of longer duration than rhinovirus and of equal severity to that of respiratory syncytial virus. Among children with bronchiolitis, >25% had acute HBoV infections. Accurate HBoV diagnosis requires serologic analysis or PCR of serum; PCR of NPAs alone is insufficient.

Case-control study on Puumala virus infection: smoking is a risk factor

Puumala hantavirus (PUUV) is apparently transmitted to humans by inhalation of aerosolized secretions of carrier rodents (bank voles). The means of transmission and the associated risk factors are poorly defined. An epidemiological study during the peak of an epidemic season in Finland was conducted based on 282 acute clinical PUUV infections and 204 controls without PUUV infection or immunity. The main risk factors adjusted by age, sex and living environment were cigarette smoking [odds ratio (OR) 3.6, 95% confidence interval (CI) 2.1-5.9, P<0.0001] and buildings with holes allowing rodents to enter (OR 3.3, 95% CI 2.0-5.6); these results were similar in two subsets. Further, use of rodent traps (OR3.5, 95% CI 2.2-5.7) and handling firewood (OR 2.7, 95% CI 1.6-4.4) were associated with a risk. The risk attributed to smoking also remained high using simulated population controls with average smoking habits. The results suggest that hantavirus transmission occurs by inhalation mainly indoors and is dependent on the condition of the respiratory tract.

Serological microarray for detection of HSV-1, HSV-2, VZV, and CMV antibodies

The seroprevalence of human herpesviruses is high and reactivations occur frequently. A microarray was designed and tested for the detection of IgG and IgM antibodies for Puumala hantavirus (PUUV) and IgG antibodies against four herpesviruses. Initially, a microarray platform was set up using an unrelated in-house antigen, PUUV recombinant nucleocapsid protein, to optimize the protocol for the detection of antibodies. Detection of the four herpesviruses was set up in a microarray using the recombinant proteins of herpes simplex virus (HSV) glycoprotein G1 and G2, varicella-zoster virus (VZV) glycoprotein E, and cytomegalovirus (CMV) pp150 phosphoprotein. The results of the PUUV panel were in good agreement with the PUUV IgG immunofluorescent assay and IgM enzyme immunoassay (EIA). Seropositive and negative clinical reference panels were tested for herpesviruses by the serological microarray, and the results were compared to those of individual EIAs used for standard diagnostic purposes. The serologic microarray for HSV, VZV and CMV antibody detection gave good specificities for IgG. However, sensitivities of the assay varied depending on the herpesvirus detected. The serological microarray showed potential for screening purposes. The microarray based analyses were easy to perform, and HSV-1, HSV-2, VZV, and CMV antibodies could be detected on the same microarray.

How to diagnose hantavirus infections and detect them in rodents and insectivores

Hantaviruses are carried by rodents and insectivores in which they cause persistent and generally asymptomatic infections. Several hantaviruses can infect humans and many of them cause either haemorrhagic fever with renal syndrome (HFRS) in Eurasia or hantavirus cardiopulmonary syndrome (HCPS) in the Americas. In humans hantavirus infections are diagnosed using IgM-capture tests but also by RT-PCR detection of viral RNA. For detection of hantavirus infections in rodents and insectivores, serology followed by immunoblotting of, for example, lung tissue, and RT-PCR detection of viral RNA may be used, and if of interest followed by sequencing and virus isolation. For sero/genotyping of hantavirus infections in humans and carrier animals neutralisation tests/RNA sequencing are required. Hantaviruses are prime examples of emerging and re-emerging infections and it seems likely that many new hantaviruses will be detected in the near future.

Development of an ELISA to detect Sin Nombre virus-specific IgM from deer mice (Peromyscus maniculatus)

Peromyscus maniculatus (deer mouse) is the primary reservoir for Sin Nombre virus (SNV). Although the presence of IgG antibodies is often used as a marker of infection, it provides little information on active infections in a population but usually is an indicator of past infections. The presence of IgM antibodies is a much better marker for determining whether active infections are present in a population. A mu-capture SNV-specific IgM enzyme linked immunosorbent assay (ELISA) was developed. From live-trap and release studies a total of 68 rodent sera were studied for the presence of Sin Nombre virus-specific IgG and IgM antibodies. In these studies, IgM responses were detected in a number of animals. In some cases early SNV infection was determined through the presence of anti-SNV IgM before IgG antibodies could be detected. From the set of animals analyzed, it was concluded that the IgM response against SNV can persist anywhere from 1 to up to over 2 months, with a median of less than 1 month. Most importantly, it was demonstrated that anti-Sin Nombre virus IgM is an important tool for detection of early infections in rodents and should be considered as a key diagnostic tool.

A soluble and heat-resistant form of hantavirus nucleocapsid protein for the serodiagnosis of HFRS

Hemorrhagic fever with renal syndrome (HFRS) is a rodent-borne viral zoonosis characterized by fever, hemorrhagic manifestations, and renal disorder. The causative agent of HFRS has been identified as a hantavirus. Hantavirus nucleocapsid proteins have recently been shown to be immunodominant antigens in HFRS, inducing an early and long-lasting immune response, and their amino termini are sensitive tools for the detection of specific antibodies in HFRS patient sera. Previous work has demonstrated that the introduction of the acidic tail of alpha-synuclein (ATS) into heat-labile proteins protects them from heat-induced aggregation. In this study, the ATS peptide was introduced into the N-terminal antigenic portion of the nucleocapsid proteins (amino acid residues 1-70) of the Hantaan virus (HTNV-delta N) and Seoul virus (SEOV-delta N). The recombinant ATS-HTNV-delta N and ATS-SEOV-delta N fusion proteins were heat-resistant, and the proteins purified by heat treatment were immunoreactive to sera from patients with HFRS. Compared with sera from patients with leptospirosis and scrub typhus, sera from patients with HFRS showed much higher reactivity in ATS-HTNV-delta N- or ATS-SEOV-delta N-based IgG ELISAs. Immunoblotting analysis revealed that only sera from patients with HFRS specifically recognized the ATS-HTNV-delta N and ATS-SEOV-delta N, indicating that the ATS-HTNV-delta N and ATS-SEOV-delta N were highly purified species without any other immunoreactive proteins as contaminants. These data demonstrate that the ATS-HTNV-delta N and ATS-SEOV-delta N fusion proteins offer a safe and inexpensive source of pure and specific antigen for large-scale diagnosis and seroepidemiological studies of HFRS, and that ATS-fusion technology can also be utilized to solubilize other antigens that could be used for large-scale diagnosis and seroepidemiological studies of other diseases.

Puumala hantavirus viremia diagnosed by real-time reverse transcriptase PCR using samples from patients with hemorrhagic fever and renal syndrome

Puumala virus (PUUV) is the endemic hantavirus in northern Sweden and causes nephropathia epidemica (NE), a milder form of hemorrhagic fever with renal syndrome. There is a need for fast and reliable diagnostics to differentiate the disease from other infections. By aligning virus RNA sequences isolated from 11 different bank voles and one human patient, we designed a real-time reverse transcriptase (RT) PCR method for detection of PUUV RNA. The real-time RT-PCR assay showed linearity from 20 to 2 x 10(6) virus copies with a correlation coefficient above 0.98 to 0.99 for all experiments. The detection threshold for PUUV cDNA was two copies per reaction. A two-step qualitative RT-PCR to detect PUUV RNA showed 100% concordance with the real-time RT-PCR assay. PUUV RNA viremia was detected in 33 of 34 PUUV immunoglobulin M (IgM)-positive patients with typical clinical NE disease from the region of endemicity. One PUUV IgM-negative sample had PUUV RNA, and 4 days later, the patient was IgM positive. Of samples with indeterminate IgM, 43% were PUUV RNA positive. The kinetics of antibody titers and PUUV viremia were studied, and five of six NE patients displayed a decrease in PUUV viremia a few days after disease outbreak coupled with an increase in PUUV IgM and IgG. In one patient with continuously high PUUV RNA levels but low IgM and no IgG response, the infection was lethal. These findings demonstrated that real-time RT-PCR is a useful method for diagnosis of PUUV viremia and for detecting PUUV RNA at early time points, before the appearance of IgM antibodies.

Mild course of Puumala nephropathy in children in an area with sporadic occurrence Hantavirus infection

The first three children with Puumala virus nephropathy diagnosis in the Czech Republic are reported on. A boy and two girls were admitted with symptoms of interstitial nephritis. The medical history in all children revealed flu-like symptoms. All patients were mildly pyrexial and had elevated erythrocytes sedimentation rate, C-reactive protein and low hemoglobin levels. Serum creatinine levels were elevated and proteinuria exceeded 700 mg/L in all children. Tubular proteinuria, glycosuria, high urinary N-acetyl-beta-D-glucosaminidase levels and alpha-1-microglobulin levels confirmed the tubular lesion. Renal biopsies revealed a uniform pattern and showed non-purulent interstitial nephritis in all patients. Puumala virus antigen antibodies were detected in the plasma. All patients were treated with steroids and urine abnormalities and renal function returned to normal within 4 weeks. Hantavirus infection should be considered as one of possible causes of interstitial nephritis with decreased GFR in children even in areas with a low incidence of this infection.

Development of novel immunoglobulin G (IgG), IgA, and IgM enzyme immunoassays based on recombinant Puumala and Dobrava hantavirus nucleocapsid proteins

Human infections with Asian and European hantaviruses can result in hemorrhagic fever with renal syndromes of differing severities characterized by renal dysfunction and sometimes by pulmonary symptoms. For the serological detection of human infections by hantaviruses relevant for Europe, we developed monoclonal antibody capture immunoglobulin G (IgG) and IgA enzyme-linked immunosorbent assays (ELISAs) based on yeast-expressed nucleocapsid proteins of Puumala and Dobrava hantaviruses. Moreover, for diagnosis of acute infections, mu-capture IgM ELISAs were established with nucleocapsid proteins expressed in Drosophila melanogaster Schneider S2 cells. The cutoff values of the ELISAs were determined by investigation of up to 500 human anti-hantavirus-negative serum samples. The specificities of the Puumala and Dobrava virus-specific IgM, IgA, and IgG ELISAs were found to be 100%. The sensitivities of these ELISAs were determined to be 100% with panels of characterized anti-Puumala or anti-Dobrava virus-positive human serum samples. In most cases, Puumala and Dobrava virus infections could be differentiated by ELISA reactivity alone, i.e., endpoint titration with homologous and heterologous antigens.

Corticosteroids combined with continuous veno-venous hemodiafiltration for treatment of hantavirus pulmonary syndrome caused by Puumala virus infection

Reported here are two cases of hantavirus pulmonary syndrome caused by Puumala virus infection, which rapidly resolved after initiation of corticosteroid treatment combined with continuous veno-venous hemodiafiltration. These cases emphasize the role of the inflammatory response in the pathogenesis of hantavirus pulmonary syndrome.

Serological assays for the detection of human andes hantavirus infections based on its yeast-expressed nucleocapsid protein

BACKGROUND

The objective of the study was to develop and evaluate IgM and IgG ELISAs and an IgG Western blot test for the serological detection of human infections with Andes virus (ANDV), the major cause of hantavirus cardiopulmonary syndrome (HCPS) in South America.

METHODS

The entire nucleocapsid (N) protein-encoding sequence of ANDV (strain AH-1) was cloned and expressed in the yeast Saccharomyces cerevisiae. The polyhistidine-tagged recombinant N (rN) protein of ANDV was purified by nickel-chelation chromatography and characterized by its reactivity with different N-specific monoclonal antibodies. To detect an antibody response directed against ANDV in humans, indirect IgM and IgG ELISAs and an IgG Western blot test based on ANDV rN antigen were developed. The evaluation of the tests was performed using a negative serum panel and 63 blinded sera from Argentina and Chile, containing acute-phase and convalescent sera from HCPS patients.

RESULTS

The specificities and sensitivities for the IgM and IgG ELISAs were demonstrated to be very high. The IgG ELISA data were confirmed by the IgG Western blot assay based on the same rN antigen. Almost all anti-ANDV-positive sera reacted to higher endpoint titers with N protein of ANDV than with those of Sin Nombre, Laguna Negra or Puumala virus. The cross-reactivity of anti-ANDV-N IgG-positive sera to rN proteins of other hantaviruses was found to be increased with time after the onset of HCPS.

CONCLUSION

The high sensitivity of the novel assays should facilitate early diagnosis of ANDV infections and might contribute to a successful treatment of HCPS patients.

Development and evaluation of serological assays for detection of human hantavirus infections caused by Sin Nombre virus

BACKGROUND

The hantavirus cardiopulmonary syndrome (HCPS) was first recognized in 1993 after a cluster of acute respiratory distress syndrome deaths in the southwestern of the United States. The major causative agent of HCPS in North America is the Sin Nombre virus (SNV) carried by the deer mouse Peromyscus maniculatus. The first HCPS case imported to Europe was reported in 2002.

OBJECTIVES

The objective of the study was to develop and evaluate ELISA and Western blot tests for the serological detection of human infections caused by SNV including those imported to Europe.

STUDY DESIGN

A polyhistidine (His)-tagged recombinant nucleocapsid (rN) protein of SNV was expressed in Saccharomyces cerevisiae and purified by nickel chelation chromatography. On the basis of the purified SNV rN protein mu-capture and indirect IgM and IgG ELISAs and an IgG Western blot were developed. The evaluation of the tests was performed using a negative serum panel and a blinded serum panel from the US containing acute-phase sera from HCPS patients.

RESULTS

Based upon the results obtained using a panel of negative control sera the specificity for SNV mu-capture and indirect IgM and IgG ELISAs were found to be 100%. All 33 sera from SNV-infected HCPS patients included in the blinded panel were detected by the SNV mu-capture and indirect IgM ELISAs. Twenty-nine out of the 33 SNV-IgM positive sera reacted also in the SNV-IgG ELISA. An SNV-IgG Western blot confirmed the data of the SNV-IgG ELISA. Although the majority of anti-SNV positive sera cross-reacted with rN proteins of Puumala virus and Dobrava virus, the lacking reactivity of a few sera with these heterologous rN antigens in the corresponding IgM and IgG ELISAs demonstrates the value of virus-specific test formats for acute-phase sera.

CONCLUSIONS

The novel SNV ELISA and Western blot tests represent a useful tool for the serological detection of SNV infections.

Identification of Dobrava, Hantaan, Seoul, and Puumala viruses by one-step real-time RT-PCR

We developed four assays for specifically identifying Dobrava (DOB), Hantaan (HTN), Puumala (PUU), and Seoul (SEO) viruses. The assays are based on the real-time one-step reverse transcriptase polymerase chain reaction (RT-PCR) with the small segment used as the target sequence. The detection limits of DOB, HTN, PUU, and SEO assays were 25, 25, 25, and 12.5 plaque-forming units, respectively. The assays were evaluated in blinded experiments, each with 100 samples that contained Andes, Black Creek Canal, Crimean-Congo hemorrhagic fever, Rift Valley fever and Sin Nombre viruses in addition to DOB, HTN, PUU and SEO viruses. The sensitivity levels of the DOB, HTN, PUU, and SEO assays were 98%, 96%, 92% and 94%, respectively. The specificity of DOB, HTN and SEO assays was 100% and the specificity of the PUU assay was 98%. Because of the high levels of sensitivity, specificity, and reproducibility, we believe that these assays can be useful for diagnosing and differentiating these four Old-World hantaviruses.

Thrombocytopenia and acute renal failure in Puumala hantavirus infections

Low platelet counts are a novel predictive marker suitable for risk-adapted patient management.

Nephropathia epidemica, caused by Puumala virus (PUUV) infection, is a form of hemorrhagic fever with renal syndrome of variable severity. Early prognostic markers for the severity of renal failure have not been established. We evaluated clinical and laboratory parameters of 15 consecutive patients with acute PUUV infection, which is endemic in the Alb-Danube region, South Germany. Severe renal failure (serum creatinine >620 µmol/L) was observed in seven patients; four required hemodialysis treatment. Low platelet count (<60 x 10⁹/L), but not leukocyte count, C-reactive protein, or other parameters obtained at the initial evaluation, was significantly associated with subsequent severe renal failure (p = 0.004). Maximum serum creatinine was preceded by platelet count nadirs by a median of 4 days. Thrombocytopenia <60 x 10⁹/L appears predictive of a severe course of acute renal failure in nephropathia epidemica, with potential value for risk-adapted clinical disease management.

Hantaviruses: Immunology, Treatment, and Prevention

Hantaviruses are rodent-borne bunyaviruses that are associated with two main clinical diseases in humans: hemorrhagic fever with renal syndrome and hantavirus pulmonary syndrome. It has been suggested that host-related immune mechanisms rather than direct viral cytopathology may be responsible for the principal abnormality (vascular dysfunction) in these syndromes. This review summarizes the current knowledge on hantaviral host immune responses, immune abnormalities, laboratory diagnosis, and antiviral therapy as well as the current approaches in vaccine development.

Detection of Puumala Hantavirus Antibody with ELISA Using a Recombinant Truncated Nucleocapsid Protein Expressed inEscherichia coli

A truncated recombinant nucleocapsid protein (rNp118) consisting of the first 118 amino-terminal amino acids (AA) of the Puumala hantavirus (PUUV) nucleocapsid protein expressed in Escherichia coli, was evaluated for its antigenicity and reliability as serodiagnostic antigen in an enzyme-linked immunosorbent assay (ELISA) for detection of PUUV antibodies in human sera. The PUUV nucleocapsid protein has been shown to contain several B-cell epitopes, mapped within the first 118 amino-terminal AA. This finding makes the rNp118 an interesting recombinant protein to use as serodiagnostic antigen. The sensitivity of this new PUUV-rNp118 ELISA, was compared with those of a commercially available PUUV ELISA assay and an home-made ELISA based on a recombinant whole nucleocapsid protein of PUUV. Eighty-six human serum samples clinically suspected for PUUV-induced nephropathia epidemica, and previously screened with the reference assays, were tested. The sensitivity of the new assay was compared with that of the reference assays and an excellent correlation between the assays was found. Sera found to be negative by other methods were also negative in our assay. The ELISA based on rNp118 represents an alternative and valid test for detection of antibodies to PUUV in human sera.

Plasma Exchange--Associated Immunoglobulin M--Negative Hantavirus Disease after a Camping Holiday in Southern France

A 37-year-old Belgian patient presented with acute nephropathia epidemica (NE) shortly after a camping holiday in southern France. Unusual symptoms were initial noncardiogenic lung involvement, followed by severe acute renal failure, acute acalculous cholecystitis, presence of immunoblasts in the bone marrow, and hemolytic anemia, presenting as hemolytic uremic syndrome. Positive immunoglobulin (Ig) A and rising IgG titers against Puumala hantavirus (PUUV) were detected, but IgM remained negative on days 8 and 20. The results of reverse-transcriptase-polymerase chain reaction performed on day 8 were positive for PUUV. This is the first report of an iatrogenically IgM-negative hantavirus case due to the selective removal of heavy-weight molecules during plasma exchange via the centrifugation technique. This is also the first report of proven NE from the Mediterranean part of France.

Immune responses to Puumala virus infection and the pathogenesis of nephropathia epidemica

Puumala virus, causative agent of a mild form of hemorrhagic fever with renal syndrome, also known as nephropathia epidemica, induces long-lasting humoral and cellular immunity in patients. The virus itself is not cytopathic, and the immune responses to the virus may be involved in teh pathogenesis of the disease.

Occurrence of renal and pulmonary syndrome in a region of northeast Germany where Tula hantavirus circulates

Hantavirus species Tula (TULV) is carried by European common voles (Microtus spp.). Its pathogenic potential for humans is unknown. In a rural region of northeast Germany, a 43-year-old man became ill with fever, renal syndrome, and pneumonia. Typing of late acute- and convalescent-phase sera by focus reduction neutralization assay revealed the presence of neutralizing antibodies against TULV. Moreover, we detected TULV genetic material in Microtus arvalis animals that were trapped at places only a few kilometers from the home village of the patient. Phylogenetic analysis of completely sequenced genomic S segments from three virus strains grouped them within a third genetic lineage of the TULV species. This is the first case of hemorrhagic fever with renal syndrome and pulmonary involvement which can be associated with TULV infection.

Diagnostic rapid tests for acute hantavirus infections: specific tests for Hantaan, Dobrava and Puumala viruses versus a hantavirus combination test

Hantaviruses infecting humans in Eurasia include Hantaan, Seoul, Puumala and the closely related Dobrava and Saaremaa viruses. These viruses are causative agents of hemorrhagic fever with renal syndrome (HFRS), which is recognized as a severe health care problem in several countries. Diagnostics of hantavirus infections relies on serology, performed principally with enzyme immunoassay (EIA) or immunofluorescence assay (IFA). We developed four 5-min immunochromatographic IgM-antibody tests for diagnostics of acute Puumala, Dobrava and Hantaan virus infections and a similar combination test to detect all Eurasian pathogenic hantavirus infections. We evaluated the assays using 100 fingertip blood samples collected randomly from Finnish volunteers, 28 confirmed hantavirus IgM-negative sera, and 77 sera from patients with acute infections of various hantaviruses. The specificities and sensitivities of the Puumala-, Dobrava- and Hantaan virus -specific tests varied from 96 to 100%, whereas, the combination test showed 96% specificity and 80 to 93% sensitivity. Cross-reactions were observed commonly between the Dobrava and the Hantaan virus tests, but only rarely between the Puumala and the Hantaan virus, or the Puumala and the Dobrava virus, tests. Altogether, the rapid tests showed less cross-reactivity than the respective EIA tests. According to the results, the performance of these tests meets well the requirements for diagnostic use. Nevertheless, the specific one-antigen tests were markedly more sensitive than the combination test. However, if optimized, a combination test would be suitable for regions where several hantaviruses circulate.

Hantavirus immunology

Two clinical syndromes are associated with hantavirus infection in humans: hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). Autopsy findings typically reveal a common feature of increased permeability in microvascular beds, suggesting vascular endothelium is a prime target for virus infection. Endothelial cells are susceptible to hantavirus infection; however, virus does not cause cytopathic effects, to explain increased endothelium permeability. Therefore, immune mechanisms were suggested to play a crucial role in hantavirus pathogenesis. In this review, we summarize data on hantavirus-induced immune disturbances and discuss their implication in capillary leakage caused by hantavirus infection.

Comparison of a new immunochromatographic rapid test with a commercial EIA for the detection of Puumala virus specific IgM antibodies

BACKGROUND

Hantaviruses are associated with two human diseases: haemorrhagic fever with renal syndrome (HFRS) and Hantavirus pulmonary syndrome (HPS). Puumala virus (PUUV), which is one of the Hantaviruses, is a causative agent of nephropathia epidemica (NE), a mild form of HFRS.

OBJECTIVE

a new 5 min rapid test, POC PUUMALA (Erilab Ltd, Finland), for detecting IgM antibodies to PUUV was evaluated and compared with the commercially available Hantavirus (Puumala) IgM ELISA test (Progen, Germany). Discrepant test results between the two tests were confirmed by a mu-capture reference EIA.

STUDY DESIGN

Two hundred and thirty five serum samples, which had earlier been analyzed with the Progen IgM ELISA, were assayed with the POC PUUMALA rapid test. Five persons, without knowing the Progen IgM ELISA test results, interpreted independently the rapid test results. In addition, a panel of 48 serum samples was analyzed in parallel with the rapid test and the Progen IgM ELISA by one technician in daily routine diagnostics in a clinical microbiology laboratory.

RESULTS

the agreement between the results of the five interpreters was 95%, and the congruence of the results between individual readers and commercial ELISA test varied from 93 to 96%. Diagnostic efficacy of the rapid test varied between 98 and 99% compared with 96% of the Progen IgM ELISA. The POC PUUMALA rapid test showed higher or similar sensitivity compared with the Progen IgM ELISA, whereas both the tests had similar levels of specificity.

CONCLUSIONS

the analytical performance of the POC PUUMALA rapid test was found to be as good or even slightly better than the analytical performance of the Progen IgM ELISA. In addition, the rapid and straightforward procedure makes the POC PUUMALA a feasible tool for the diagnosis of the acute PUUV infection.

Hantavirus infections and their prevention

Hantaviruses are rodent-borne bunyaviruses which cause haemorrhagic fever with renal syndrome and Hantavirus pulmonary syndrome in humans. This review covers the host interactions of the viruses, including the rodent reservoirs, the clinical outcome of human infections as well as the pathogenesis and laboratory diagnosis of infections. The current stage in prophylaxis and therapy of hantaviral diseases is described and different approaches in vaccine development are discussed.

New immunochromatographic rapid test for diagnosis of acute Puumala virus infection

A new immunochromatographic rapid test, POC PUUMALA (Erilab Ltd., Kuopio, Finland), for detection of acute-phase Puumala virus (PUUV) infection was developed based on a highly purified baculovirus-expressed PUUV nucleocapsid protein antigen and lateral immunodiffusion techniques. After addition of sample (5 microl of serum, plasma, or fingertip blood) and buffer, PUUV-specific immunoglobulin M (IgM) antibodies, if present, together with the gold-conjugated anti-human IgM, formed a specific colored line in 5 min. The sensitivity and specificity of the test were evaluated with 200 serum samples and 30 fingertip blood samples. The reference method for the serum samples was a micro-capture enzyme immunoassay (EIA) for IgM and an immunofluorescence assay (IFA) for IgG antibodies. The analytical sensitivity and specificity of the rapid test were 100 and 99%, respectively, for unfrozen serum samples (n = 103; 12 PUUV IgM-positive samples). When freeze-thawed serum samples were used, the sensitivity and specificity were each 97.1% (n = 70; 35 PUUV IgM-positive samples). The specificity of the test was 96.2% for 27 serum samples with nonspecific IgM antibodies or rheumatoid factor (RF). The fingertip blood samples (n = 30) were negative, but they gave clear positive results when spiked with IgM-positive sera (n = 20). The results were in good agreement with the standard diagnostic methods. The rapid performance, the lack of need for refined laboratory equipment, and the high specificity with fresh serum and fingertip blood samples indicate that the developed POC PUUMALA rapid test is a useful tool for fast diagnosis of acute PUUV infection.

Diagnosis of Oropouche virus infection using a recombinant nucleocapsid protein-based enzyme immunoassay

Oropouche (ORO) virus is an emerging infectious agent that has caused numerous outbreaks of an acute febrile (dengue-like) illness among humans in Brazil, Peru, and Panama. Diagnosis of ORO virus infection is based mainly on serology. Two different antigens, hamster serum antigen (HSA) and Vero cell lysate antigen (VCLA), are currently used in enzyme immunoassays (EIAs) in Brazil and Peru, respectively, to investigate the epidemiology of ORO virus infection. Both antigens involve use of infectious virus, and for this reason their use is restricted. Consequently, the frequency and distribution of ORO virus infection are largely unexplored in other countries of South America. This report describes the use of a bacterially expressed recombinant nucleocapsid (rN) protein of ORO virus in EIAs for the diagnosis of ORO virus infection. The data revealed that the purified rN protein is comparable to the authentic viral N protein in its antigenic characteristics and is highly sensitive and specific in EIAs. Among 183 serum samples tested, a high degree of concordance was found between rN protein-based EIA and HSA- and VCLA-based EIAs for the detection of both ORO virus-specific immunoglobulin M (IgM) and IgG antibodies. The high sensitivity, specificity, and safety of the rN protein-based EIA make it a useful diagnostic technique that can be widely used to detect ORO virus infection in South America.

Evaluation of a pan-reactive hantavirus enzyme immunoassay and of a hantavirus immunoblot for the diagnosis of nephropathia epidemica

BACKGROUND

Nephropathia epidemica (NE) caused by the hantavirus serotype Puumala (PUUV) is endemic in large parts of Europe. The prognosis of this disease is usually good. However, a rapid serological diagnosis is important to differentiate NE from potentially more severe renal conditions.

OBJECTIVE

To evaluate the diagnostic usefulness of a novel pan-reactive hantavirus enzyme immunoassay (EIA) and of a novel hantavirus immunoblot (IB).

STUDY DESIGN

Three groups of serum samples were tested with both assays: 79 samples from 43 patients with acute NE, 27 samples from healthy adults, and 29 tricky samples from patients with autoantibodies, with acute Epstein-Barr virus (EBV) or cytomegalovirus (CMV) infections, and from pregnant women.

RESULTS

With the EIA, all but two of the early samples of the NE patients and all of the follow-up samples were positive for hantavirus IgG. All control samples were negative. The IgM EIA was positive in 42 of the 43 primary NE samples. Weak IgM EIA reactions were observed for some of the serum samples from patients with acute EBV and CMV infections. Optimal sensitivity and specificity values for the EIA were achieved when both the IgG and the IgM results were considered for the diagnosis of acute NE. All of the early NE samples reacted with the hantavirus nucleocapsid proteins in the IgG IB and all but one of these samples in the IgM IB. Cross reactions between the PUUV and the Hantaan antigens were very common. Several of the control samples did show borderline or positive bands, but these were mostly bands against only one hantavirus antigen in either the IgG or the IgM IB. The presence of at least three hantavirus bands (PUUV or HTNV) in the IgG and IgM assays was highly predictive of acute NE.

CONCLUSION

Both assays were highly sensitive for the diagnosis of acute NE. However, the specificity of the IB IgM was only 76%. The specificity of both the IB and the EIA can be increased by modifications of the result interpretation.

Expression of immunogenic Puumala virus nucleocapsid protein in transgenic tobacco and potato plants

Transgenic plants, expressing recombinant proteins, are suitable alternatives for the production of relevant immunogens. In the present study, the expression of Puumala virus nucleocapsid protein in tobacco and potato plants (Nicotiana tabacum and Solanum tuberosum) and its immunogenicity was investigated. After infection of leaf discs of SR1 tobacco and tuber discs of potato cv. "Desiree" with the Agrobacterium strain LBA4404 (pAL4404, pBinAR-PUU-S) containing the 1302 bp cDNA sequence of S-RNA segment of a Puumala virus, transgenic tobacco and potato plants expressed the Puumala virus nucleocapsid protein under control of the cauliflower 35S promoter. The recombinant proteins were found to be identical to the authentic Puumala virus nucleocapsid protein as analyzed by immunoblotting. Expression of the nucleocapsid protein was investigated over four plant generations (P to F4) and found to be stable (1 ng/3 microg dried leaf tissue). Transgenic tobacco plants were smaller compared to controls. The transformed potato plants were morphologically similar to control plants and produced tubers as the control potatoes. The S-antigen was expressed at a level of 1 ng protein/5 microg and 1 ng protein/4 microg dried leaf and root tissues, respectively, and remained stable in the first generation of vegetatively propagated potato plants. The immunogenicity of the Puumala virus nucleocapsid protein expressed in Nicotiana tabacum and Solanum tuberosum was investigated in New Zealand white rabbits. They were immunized with leaf extracts from transgenic tobacco and potato plants, and the serum recognized Puumala virus nucleocapsid protein. Transgenic plants expressing hantaviral proteins can thus be used for the development of cost-effective diagnostic systems and for alternative vaccination strategies.

Evaluation of two commercially available immunoassays for the detection of hantavirus antibodies in serum samples

BACKGROUND

hantaviruses are members of the family Bunyaviridae and the spectrum of clinical symptoms in humans may vary from sub-clinical to severe haemorrhagic fever with renal syndrome (HFRS) or pulmonary syndrome (HPS). Several serotypes have been described from which at least five are pathogenic to humans. Each serotype has a different animal reservoir and geographical distribution. In the acute phase of the disease the clinical diagnosis may be confirmed by serology or by polymerase-chain reaction (PCR).

OBJECTIVE

to evaluate two commercially available immunoassays using sera from hantavirus suspected and non-hantavirus patients: an enzyme immunoassay (EIA) developed by MRL Diagnostics, for the detection of immunoglobulins M (IgM) and G (IgG) against several hantavirus serotypes and an indirect immunofluorescence assay (IFA) from Progen, based on slides coated with Hantaan virus (HNTV) and Puumala virus (PUUV), infected cells.

STUDY DESIGN

a total of 145 serum samples were used for this study. The serum panel included serum samples from patients suspected of mild (n=91), severe (n=10) HFRS and patients with other viral infections (n=44).

RESULTS

the agreement between the MRL EIA and the Progen IFA for the detection of IgM and IgG serum antibodies ranged from 87 to 91%, respectively. In the non-hantavirus group one out of 44 samples was positive by the Progen HNTV IgM IFA, none in the Progen PUUV IFA and two samples in the MRL IgM EIA, resulting in specificities of 98, 100 and 95%, respectively. The sensitivities and specificities of the MRL EIAs compared to the Progen overall PUUV and HNTV IFAs were 90 and 91% for IgM, respectively, and 96% for IgG in both immunoassays.

CONCLUSIONS

the MRL EIA proved to be relatively sensitive and specific assay for the serological diagnosis of mild and severe HFRS.

Diagnostic potential of puumala virus nucleocapsid protein expressed in Drosophila melanogaster cells

Puumala virus (PUU) nucleocapsid protein (N) was expressed in insect cells by using the Drosophila Expression System (DES; Invitrogen BV, Groningen, The Netherlands). Stable transfectants were established by hygromycin B selection and showed continuous expression of the recombinant protein (DES-PUU-N) for at least 5 months. The antigenic property of DES-PUU-N was shown to be identical to that of native PUU N when examined with a panel of hantavirus-specific monoclonal antibodies. Enzyme-linked immunosorbent assays (ELISAs) for detection of human immunoglobulin M (IgM) and IgG antibodies were established by using DES-PUU-N as antigen and were compared to assays based on native N. The ELISAs were evaluated for patient diagnosis and seroepidemiological purposes with panels of sera collected from patients with hemorrhagic fever with renal syndrome (HFRS) and from healthy blood donors. Equally high sensitivities and specificities for detection of PUU-specific IgM in acute-phase HFRS patient sera were obtained by the ELISA based on DES-PUU-N and the assay based on the native antigen. For detection of PUU-specific IgG, the ELISA based on monoclonal antibody-captured DES-PUU-N antigen showed optimal sensitivity and specificity.

Dobrava virus infection: serological diagnosis and cross-reactions to other hantaviruses

Recent data have shown that Dobrava (DOB) hantavirus is the cause of severe haemorrhagic fever with renal syndrome (HFRS) in central and eastern Europe. To determine whether serological assays need to be based on the homologous viral antigen rather than on closely related hantavirus antigens, acute and convalescent sera from patients with HFRS collected in former Yugoslavia were examined for IgM and IgG to three hantavirus antigens; DOB, Hantaan (HTN) and Puumala (PUU). Focus reduction neutralization test was included for comparison and confirmation of the enzyme-linked immunosorbent assay (ELISA) results. Although the results showed that the cross-reactivity was high between these three antigens during the acute phase of the disease, one of 155 patients serum samples reacted only in the DOB antigen-based IgM assay. The evaluation of IgG reactivities revealed that a DOB antigen-based IgG ELISA has to be used in sero-epidemiological studies; 7.1% (11/155) of the acute phase/early convalescent sera and 12.5% (2/16) of the late convalescent sera, respectively, reacted only with the homologous DOB antigen.