Immunoblot detection of antibodies to Toscana virus

Reagents for detection of Rift Valley fever virus infection in sheep

Rift Valley fever virus (RVFV) causes Rift Valley fever (RVF), resulting in morbidity and mortality in humans and ruminants. Evidence of transboundary outbreaks means that RVFV remains a threat to human health and livestock industries in countries that are free from the disease. To enhance surveillance capability, methods for detection of RVFV are required. The generation of reagents suitable for the detection of RVFV antigen in formalin-fixed, paraffin-embedded tissues from infected animals have been developed and are described herein. Recombinant nucleoprotein (rNP) was expressed in Escherichia coli and purified using immobilized metal ion affinity chromatography. Purified rNP was used as an immunogen to produce anti-NP polyclonal antisera in rabbits for use in detection of RVFV NP in experimentally infected animals by immunohistochemistry. Antisera raised in rabbits against rNP were able to recognize viral NP antigen in fixed infected Vero cell pellets and sheep liver. Therefore, the methods and reagents described herein are useful in assays for detection of RVFV infections in animals, for research and surveillance purposes.

Comparison of enzyme-linked immunosorbent assay systems using rift valley fever virus nucleocapsid protein and inactivated virus as antigens

Background

Rift Valley Fever (RVF) is a mosquito-borne viral zoonosis. To detect RVF virus (RVFV) infection, indirect immunoglobulin G (IgG) and immunoglobulin M (IgM) enzyme linked immunosorbent assays (ELISAs) which utilize recombinant RVFV nucleocapsid (RVFV-N) protein as assay antigen, have reportedly been used, however, there is still a need to develop more sensitive and specific methods of detection.

Methods

RVFV-N protein was expressed in Escherichia coli (E. coli) and purified by histidine-tag based affinity chromatography. This recombinant RVFV-N (rRVFV-N) protein was then used as antigen to develop an IgG sandwich ELISA and IgM capture ELISAs for human sera. Ninety six serum samples collected from healthy volunteers during the RVF surveillance programme in Kenya in 2013, and 93 serum samples collected from RVF-suspected patients during the 2006–2007 RVF outbreak in Kenya were used respectively, to evaluate the newly established rRVFV-N protein-based IgG sandwich ELISA and IgM capture ELISA systems in comparison with the inactivated virus-based ELISA systems.

Results

rRVFV-N protein-based-IgG sandwich ELISA and IgM capture ELISA for human sera were established. Both the new ELISA systems were in 100% concordance with the inactivated virus-based ELISA systems, with a sensitivity and specificity of 100%.

Conclusions

Recombinant RVFV-N is a safe and affordable antigen for RVF diagnosis. Our rRVFV-N-based ELISA systems are safe and reliable tools for diagnosis of RVFV infection in humans and especially useful in large-scale epidemiological investigation and for application in developing countries.

Heartland Virus Epidemiology, Vector Association, and Disease Potential

First identified in two Missouri farmers exhibiting low white-blood-cell and platelet counts in 2009, Heartland virus (HRTV) is genetically closely related to severe fever with thrombocytopenia syndrome virus (SFTSV), a tick-borne phlebovirus producing similar symptoms in China, Korea, and Japan. Field isolations of HRTV from several life stages of unfed, host-seeking Amblyomma americanum, the lone star tick, implicated it as a putative vector capable of transstadial transmission. Laboratory vector competence assessments confirmed transstadial transmission of HRTV, demonstrated vertical infection, and showed co-feeding infection between A. americanum. A vertical infection rate of 33% from adult females to larvae in the laboratory was observed, while only one of 386 pools of molted nymphs (1930) reared from co-feeding larvae was positive for HRTV (maximum-likelihood estimate of infection rate = 0.52/1000). Over 35 human HRTV cases, all within the distribution range of A. americanum, have been documented. Serological testing of wildlife in areas near the index human cases, as well as in widely separated regions of the eastern United States where A. americanum occur, indicated many potential hosts such as raccoons and white-tailed deer. Attempts, however, to experimentally infect mice, rabbits, hamsters, chickens, raccoons, goats, and deer failed to produce detectable viremia. Immune-compromised mice and hamsters are the only susceptible models. Vertical infection augmented by co-feeding transmission could play a role in maintaining the virus in nature. A more complete assessment of the natural transmission cycle of HRTV coupled with serosurveys and enhanced HRTV disease surveillance are needed to better understand transmission dynamics and human health risks.

Vertebrate Host Susceptibility to Heartland Virus

Virus-infected Ag129 mice could be a useful model for identifying tick infection or virus transmission.

Heartland virus (HRTV) is a recently described phlebovirus initially isolated in 2009 from 2 humans who had leukopenia and thrombocytopenia. Serologic assessment of domestic and wild animal populations near the residence of 1 of these persons showed high exposure rates to raccoons, white-tailed deer, and horses. To our knowledge, no laboratory-based assessments of viremic potential of animals infected with HRTV have been performed. We experimentally inoculated several vertebrates (raccoons, goats, chickens, rabbits, hamsters, C57BL/6 mice, and interferon-α/β/γ receptor–deficient [Ag129]) mice with this virus. All animals showed immune responses against HRTV after primary or secondary exposure. However, neutralizing antibody responses were limited. Only Ag129 mice showed detectable viremia and associated illness and death, which were dose dependent. Ag129 mice also showed development of mean peak viral antibody titers >8 log₁₀ PFU/mL, hemorrhagic hepatic lesions, splenomegaly, and large amounts of HRTV antigen in mononuclear cells and hematopoietic cells in the spleen.

Development of vaccines against Crimean-Congo haemorrhagic fever virus

Crimean-Congo haemorrhagic fever virus (CCHFV) is a deadly human pathogen of the utmost seriousness being highly lethal causing devastating disease symptoms that result in intense and prolonged suffering to those infected. During the past 40years, this virus has repeatedly caused sporadic outbreaks responsible for relatively low numbers of human casualties, but with an alarming fatality rate of up to 80% in clinically infected patients. CCHFV is transmitted to humans by Hyalomma ticks and contact with the blood of viremic livestock, additionally cases of human-to-human transmission are not uncommon in nosocomial settings. The incidence of CCHF closely matches the geographical range of permissive ticks, which are widespread throughout Africa, Asia, the Middle East and Europe. As such, CCHFV is the most widespread tick-borne virus on earth. It is a concern that recent data shows the geographic distribution of Hyalomma ticks is expanding. Migratory birds are also disseminating Hyalomma ticks into more northerly parts of Europe thus potentially exposing naïve human populations to CCHFV. The virus has been imported into the UK on two occasions in the last five years with the first fatal case being confirmed in 2012. A licensed vaccine to CCHF is not available. In this review, we discuss the background and complications surrounding this limitation and examine the current status and recent advances in the development of vaccines against CCHFV.

Development and Characterization of Monoclonal Antibodies Directed Against the Nucleoprotein of Heartland Virus

Heartland virus (HRTV), a phlebovirus first isolated from two Missouri farmers in 2009, has been proposed to be transmitted to humans by the bite of infected Amblyomma americanum ticks. It is closely related to severe fever with thrombocytopenia syndrome virus (SFTSV) from China, another previously unrecognized phlebovirus that has subsequently been associated with hundreds of cases of severe disease in humans. To expand diagnostic capacity to detect HRTV infections, 20 hybridoma clones secreting anti-HRTV murine monoclonal antibodies (MAbs) were developed using splenocytes from HRTV-inoculated AG129 alpha/beta and gamma interferon receptor-deficient mice. Nine of these MAbs were characterized herein for inclusion in future HRTV diagnostic assay development. All of the MAbs developed were found to be non-neutralizing and reactive to linear epitopes on HRTV nucleocapsid protein. MAb 2AF11 was found to be cross-reactive with SFTSV.

Fine epitope mapping of the central immunodominant region of nucleoprotein from Crimean-Congo hemorrhagic fever virus (CCHFV)

Crimean-Congo hemorrhagic fever (CCHF), a severe viral disease known to have occurred in over 30 countries and distinct regions, is caused by the tick-borne CCHF virus (CCHFV). Nucleocapsid protein (NP), which is encoded by the S gene, is the primary antigen detectable in infected cells. The goal of the present study was to map the minimal motifs of B-cell epitopes (BCEs) on NP. Five precise BCEs (E1, 247FDEAKK252; E2a, 254VEAL257; E2b, 258NGYLNKH264; E3, 267EVDKA271; and E4, 274DSMITN279) identified through the use of rabbit antiserum, and one BCE (E5, 258NGYL261) recognized using a mouse monoclonal antibody, were confirmed to be within the central region of NP and were partially represented among the predicted epitopes. Notably, the five BCEs identified using the rabbit sera were able to react with positive serum mixtures from five sheep which had been infected naturally with CCHFV. The multiple sequence alignment (MSA) revealed high conservation of the identified BCEs among ten CCHFV strains from different areas. Interestingly, the identified BCEs with only one residue variation can apparently be recognized by the positive sera of sheep naturally infected with CCHFV. Computer-generated three-dimensional structural models indicated that all the antigenic motifs are located on the surface of the NP stalk domain. This report represents the first identification and mapping of the minimal BCEs of CCHFV-NP along with an analysis of their primary and structural properties. Our identification of the minimal linear BCEs of CCHFV-NP may provide fundamental data for developing rapid diagnostic reagents and illuminating the pathogenic mechanism of CCHFV.

Generation and characterization of monoclonal antibodies against Rift Valley fever virus nucleoprotein

Due to the unpredictable and explosive nature of Rift Valley fever (RVF) outbreaks, rapid and accurate diagnostic assays for low-resource settings are urgently needed. To improve existing diagnostic assays, monoclonal antibodies (MAbs) specific for the nucleocapsid protein of RVF virus (RVFV) were produced and characterized. Four IgG2a MAbs showed specific binding to denatured nucleocapsid protein, both from a recombinant source and from inactivated RVFV, in Western blot analysis and in an enzyme-linked immunosorbent assay (ELISA). Cross-reactivity with genetically related and non-related arboviruses including Bunyamwera and Calovo viruses (Bunyaviridae family), West Nile and Dengue-2 viruses (Flaviviridae family), and Sindbis and Chikungunya viruses (Togaviridae family) was not detected. These MAbs represent a useful tool for the development of rapid diagnostic assays for early recognition of RVF.

Rift Valley fever virus structural and nonstructural proteins: recombinant protein expression and immunoreactivity against antisera from sheep

The Rift Valley fever virus (RVFV) encodes the structural proteins nucleoprotein (N), aminoterminal glycoprotein (Gn), carboxyterminal glycoprotein (Gc), and L protein, 78-kD, and the nonstructural proteins NSm and NSs. Using the baculovirus system, we expressed the full-length coding sequence of N, NSs, NSm, Gc, and the ectodomain of the coding sequence of the Gn glycoprotein derived from the virulent strain of RVFV ZH548. Western blot analysis using anti-His antibodies and monoclonal antibodies against Gn and N confirmed expression of the recombinant proteins, and in vitro biochemical analysis showed that the two glycoproteins, Gn and Gc, were expressed in glycosylated form. Immunoreactivity profiles of the recombinant proteins in western blot and in indirect enzyme-linked immunosorbent assay against a panel of antisera obtained from vaccinated or wild type (RVFV)-challenged sheep confirmed the results obtained with anti-His antibodies and demonstrated the suitability of the baculo-expressed antigens for diagnostic assays. In addition, these recombinant proteins could be valuable for the development of diagnostic methods that differentiate infected from vaccinated animals (DIVA).

Diagnostic tools for Toscana virus infection

Toscana virus (TOSV; Phlebovirus, Bunyaviridae) is an important etiological agent of acute meningitis and meningoencephalitis in Mediterranean countries. Laboratory diagnosis has been carried out in serological studies using ELISA, immunofluorescence and/or neutralization tests that are not influenced by the virus viability; however, in the acute phase of the infection, nucleic acid amplification techniques are the methods of choice to diagnose viral meningitis from cerebrospinal fluid samples. Molecular methods are rapid and sensitive and, unlike traditional methods, such as virus isolation by cell culture, they are not influenced by the viability of the virus in the clinical specimen; however, the RNA integrity is crucial for the success of these methods. Real-time PCR is the most important molecular method used in laboratories worldwide, since it is less time-consuming and it reduces the risk of contamination. Therefore, a sensitive real-time PCR has been developed for diagnosis of suspected cases of TOSV infection either autochthonous and/or imported, since a new lineage of TOSV, divergent from the Italian prototype, has recently been reported in Spain.

Preparation and evaluation of recombinant severe fever with thrombocytopenia syndrome virus nucleocapsid protein for detection of total antibodies in human and animal sera by double-antigen sandwich enzyme-linked immunosorbent assay

The recent emergence of the human infection confirmed to be caused by severe fever with thrombocytopenia syndrome virus (SFTSV) in China is of global concern. Safe diagnostic immunoreagents for determination of human and animal seroprevalence in epidemiological investigations are urgently needed. This paper describes the cloning and expression of the nucleocapsid (N) protein of SFTSV. An N-protein-based double-antigen sandwich enzyme-linked immunosorbent assay (ELISA) system was set up to detect the total antibodies in human and animal sera. We reasoned that as the double-antigen sandwich ELISA detected total antibodies with a higher sensitivity than traditional indirect ELISA, it could be used to detect SFTSV-specific antibodies from different animal species. The serum neutralization test was used to validate the performance of this ELISA system. All human and animal sera that tested positive in the neutralization test were also positive in the sandwich ELISA, and there was a high correlation between serum neutralizing titers and ELISA readings. Cross-reactivity was evaluated, and the system was found to be highly specific to SFTSV; all hantavirus- and dengue virus-confirmed patient samples were negative. SFTSV-confirmed human and animal sera from both Anhui and Hubei Provinces in China reacted with N protein in this ELISA, suggesting no major antigenic variation between geographically disparate virus isolates and the suitability of this assay in nationwide application. ELISA results showed that 3.6% of the human serum samples and 47.7% of the animal field serum samples were positive for SFTSV antibodies, indicating that SFTSV has circulated widely in China. This assay, which is simple to operate, poses no biohazard risk, does not require sophisticated equipment, and can be used in disease surveillance programs, particularly in the screening of large numbers of samples from various animal species.

Competitive ELISA for the detection of antibodies to Rift Valley fever virus in goats and cattle

Rift Valley fever virus (RVFV) is one of the important emerging viral diseases of serious impact in public health and animal hygiene both in human and animal industries. In this study, we developed a monoclonal antibody-based competitive ELISA for the detection of antibodies to RVFV in goats and cattle. The recombinant N protein of RVFV was expressed in E. coli with a six-histidine tag, and the purified N protein was used for detecting antigen with a competitive monoclonal antibody against RVFV antibodies. The competitive ELISA (C-ELISA) could detect antibodies at 9-11 days after inoculation in goats and cattle with a sensitivity of 94.7% (virus neutralization titer >32) and specificity of 99.7%, respectively. In addition, the C-ELISA did not show any cross-reactivity with positive sera against arboviruses such as Akabane, Aino, Chuzan, Ibaraki and bovine ephemeral fever virus, which are prevalent viral agents in ruminant animals throughout Southeast Asia. The results of the present study indicate that the C-ELISA is a simple, rapid and convenient serodiagnostic method for RVFV in goats and cattle.

Performance of various commercial assays for the detection of Toscana virus antibodies

INTRODUCTION

Sandfly fever virus (SFV) serotypes sandfly fever Naples virus, sandfly fever Sicilian virus, and sandfly fever Cyprus virus cause febrile diseases, whereas Toscana virus (TOSV) is responsible for aseptic meningoencephalitis. Diagnosis and surveillance of TOSV depend heavily on virus serology, and various commercial assays utilizing various antigen sources and formats have been available. The aim of this study was to perform comparative evaluation of commercially available serological assays for anti-TOSV immunoglobulins.

MATERIALS AND METHODS

A collection of 120 sera from healthy blood donors from an endemic region, previously identified to be reactive for antibodies against various SFV serotypes by indirect immunofluorescence test (IIFT), was reevaluated for IgG/IgM via IIFT, enzyme-linked immunosorbent assay, and an immunoblot assay manufactured by Euroimmun, Diesse, and Mikrogen, respectively. Virus neutralization test (VNT) was performed for 99 sera using standard TOSV, sandfly fever Sicilian virus, and sandfly fever Naples virus strains.

RESULTS

A total of 89 samples (74.2%) were reactive for TOSV IgG in at least one of the commercial assays, and 31 samples (31.3%) were reactive in VNT for various SFV serotypes. Average percentage agreements among commercial assays and between VNT and the commercial assays were noted as 57.8% and 62.6%, respectively. No significant correlation between assay results and VNT titers was observed. SFV IgM antibodies were detected in a total of eight samples (6.7%) via IIFT, which were nonreactive in enzyme-linked immunosorbent assay and VNT.

DISCUSSION

Commercial diagnostic immunoassays displayed slight to fair agreement for TOSV IgG as assessed via kappa and percentage agreement values. The results could only be confirmed via virus neutralization in a portion of the samples, and overall agreement between the commercial assays and VNT was slight. Commercial assays such as immunoblot can be used in addition to VNT for confirmation of TOSV exposure.

Sandfly fever virus activity in central/northern Anatolia, Turkey: first report of Toscana virus infections

Sandfly fever viruses (SFVs) cause febrile diseases as well as aseptic meningitis/encephalitis and include serotypes sandfly fever Sicilian virus (SFSV), sandfly fever Naples virus (SFNV) and Toscana virus (TOSV). Infections are endemic in the Mediterranean basin and data on SFV activity in Turkey are limited. In this study, sera from 1533 blood donors from the Ankara, Konya, Eskisehir and Zonguldak provinces of Turkey were evaluated for SFV exposure by indirect immunofluorescence test (IIFT) and confirmed by virus neutralization test (VNT). One hundred and two patients with central nervous system (CNS) infections of unknown aetiology were also tested via IIFT and real-time reverse-transcription PCR for SFV/TOSV. Rate of overall IgG reactivity in IIFT was 32.9% (505/1533) among blood donors. TOSV exposure was confirmed by VNT in all study regions. Exposure to the recently-identified serotype sandfly fever Turkish virus, as evaluated by VNT, was revealed in Konya and Ankara. SFNV exposure was identified in Konya and SFSV was observed to be present in all regions except Zonguldak. TOSV RNA was detected in 15.7% (16/102) and was accompanied by TOSV IgM in 25% (4/16) of the patients. Partial L and S sequences suggested that TOSV circulating in Turkey can be grouped into TOSV genotype A strains. Exposure to TOSV and other SFV serotypes was revealed in blood donors and CNS infections by TOSV were identified for the first time in Turkey. Infections are observed to be endemic in central Anatolia and should be considered as aetiologic agents in cases/outbreaks of fever and meningoencephalitis.

Rift Valley fever virus(Bunyaviridae: Phlebovirus): an update on pathogenesis, molecular epidemiology, vectors, diagnostics and prevention

Rift Valley fever(RVF) virus is an arbovirus in the Bunyaviridae family that, from phylogenetic analysis, appears to have first emerged in the mid-19th century and was only identified at the beginning of the 1930's in the Rift Valley region of Kenya. Despite being an arbovirus with a relatively simple but temporally and geographically stable genome, this zoonotic virus has already demonstrated a real capacity for emerging in new territories, as exemplified by the outbreaks in Egypt (1977), Western Africa (1988) and the Arabian Peninsula (2000), or for re-emerging after long periods of silence as observed very recently in Kenya and South Africa. The presence of competent vectors in countries previously free of RVF, the high viral titres in viraemic animals and the global changes in climate, travel and trade all contribute to make this virus a threat that must not be neglected as the consequences of RVF are dramatic, both for human and animal health. In this review, we present the latest advances in RVF virus research. In spite of this renewed interest, aspects of the epidemiology of RVF virus are still not fully understood and safe, effective vaccines are still not freely available for protecting humans and livestock against the dramatic consequences of this virus.

Phlebovirus meningoencephalis complicated by Pseudomonas aeruginosa pneumonia: a case report

In June 2004 an 8-year-old boy was admitted to a hospital in Thessaloniki, Greece, because of high fever, tachypnea, hypotonia, diarrhea, and tonoclonic convulsions. Phlebovirus infection was diagnosed by IgG seroconversion to Toscana virus. As IgM antibodies were not detected, it is suggested that this was an acute infection caused by a phlebovirus virus distinct from Toscana virus. Complication by a hospital-acquired Pseudomonas aeruginosa pneumonia resulted in 2 months of hospitalization. Slight ataxia was still present on discharge.

Laboratory safe detection of nucleocapsid protein of Rift Valley fever virus in human and animal specimens by a sandwich ELISA

A safe laboratory procedure, based on a sandwich ELISA (sAg-ELISA), was developed and evaluated for the detection of nucleocapsid protein (NP) of Rift Valley fever virus (RVFV) in specimens inactivated at 56 degrees C for 1h in the presence of 0.5% Tween-20 (v/v) before testing. Polyclonal capture and detection immune sera were generated respectively in sheep and rabbits immunized with recombinant NP antigen. The assay was highly repeatable and specific; it detected strains of RVFV from the entire distributional range of the disease, isolated over a period of 53 years; no cross-reactivity with genetically related African phleboviruses or other members of the family Bunyaviridae was observed. In specimens spiked with RVFV, including human and animal sera, homogenates of liver and spleen tissues of domestic ruminants, and Anopheles mosquito homogenates, the sAg-ELISA detection limit ranged from log(10)10(2.2) to 10(3.2) TCID(50)/reaction volume. The ELISA detected NP antigen in spiked bovine and sheep liver homogenates up to at least 8 days of incubation at 37 degrees C whereas infectious virus could not be detected at 48h incubation in these adverse conditions. Compared to virus isolation from sera from RVF patients and sheep infected experimentally, the ELISA had 67.7% and 70% sensitivity, and 97.97% and 100% specificity, respectively. The assay was 100% accurate when testing tissues of various organs from mice infected experimentally and buffalo foetuses infected naturally. The assay was able to detect NP antigen in infective culture supernatants 16-24h before cytopathic effects were observed microscopically and as early as 8h after inoculation with 10(5.8) TCID(50)/ml of RVFV. This ability renders the assay for rapid identification of the virus when its primary isolation is attempted in vitro. As a highly specific, safe and simple assay format, the sAg-ELISA represents a valuable diagnostic tool for use in less equipped laboratories in Africa, and for routine differential diagnosis of viral hemorrhagic fevers.

Recombinant nucleocapsid-based ELISA for detection of IgG antibody to Rift Valley fever virus in African buffalo

Wild ruminants are thought to serve as natural hosts for Rift Valley fever virus (RVFV) but the role of these animals as reservoirs for RVFV during inter-epidemic periods and as amplifiers during epidemics is not well understood. An indirect enzyme-linked immunoassay (I-ELISA) based on the recombinant nucleocapsid protein (rNp) of RVFV was validated for the detection of specific IgG antibodies in African buffalo. Data sets derived from testing buffalo sera from Kenya (n=405) and South Africa (n=618) were dichotomised according to the results of a virus neutralisation test. The assay characteristic performance was analysed using threshold values optimised by the two-graph receiver operating characteristics (TG-ROC) analysis, and by mean plus two, as well as by mean plus three standard deviations derived from I-ELISA PP values in uninfected animals. Among 1023 buffalo sera tested, 77 (7.5%) had detectable virus neutralising antibodies. The assay had high intra- and inter-plate repeatability in routine runs. At a cut-off optimised by the TG-ROC at 95% accuracy level, the diagnostic sensitivity of the I-ELISA was 98.7% and diagnostic specificity 99.36% while estimates for the Youden's index (J) and efficiency (Ef) were 0.98 and 99.31%. When cut-off values determined by traditional statistical approaches were used, the diagnostic sensitivity was 100% but estimates of J, Ef and other combined measures of diagnostic accuracy were lower compared to those based on cut-off value derived from the TG-ROC. Results of the study indicate that the I-ELISA based on the rNp would be useful for seroepidemiological studies of RVFV infections in African buffalo.

Rapid detection of important human pathogenic Phleboviruses

BACKGROUND

Rapid diagnostics are not available for several human pathogens in the genus Phlebovirus of the Bunyaviridae.

OBJECTIVES

To develop RT-PCR assays for Sandfly Fever Sicilian virus (SFSV), Sandfly Fever Naples virus (SFNV), Toscana virus (TOSV) and Rift Valley Fever virus (RVFV).

STUDY DESIGN

RNA standards were generated and used to test the performance of the assays.

RESULTS

A detection limit of 10-100 RNA molecules was determined for the SFSV, TOSV and RVFV assays. The sensitivity of the SFNV assay was not determined. The TOSV and the RVFV assays detected recent isolates from Spain and Africa, respectively.

CONCLUSION

The assays should help to improve surveillance of pathogenic Phleboviruses.

Validation of an indirect ELISA based on a recombinant nucleocapsid protein of Rift Valley fever virus for the detection of IgG antibody in humans

An indirect enzyme-linked immunoassay (I-ELISA) based on the recombinant nucleocapsid protein (rNp) of Rift Valley fever virus was validated for the detection of specific IgG antibody in human sera. Validation data sets derived from testing sera collected in Africa (n=2967) were categorized according to the results of a virus neutralisation test. The assay had high intra- and inter-plate repeatability in routine runs. No detectable cross-reactions between IgG antibodies generated from mice experimentally infected with viruses representing genus Phlebovirus, Nairovirus, Orthobunyavirus and Bhanja virus of the family Bunyaviridae were observed. At a cut-off optimised by the two-graph receiver operating characteristics analysis at 95% accuracy level, the diagnostic sensitivity of the I-ELISA was 99.72% and diagnostic specificity 99.62% while estimates for the Youden's index (J) and efficiency (Ef) were 0.993 and 99.62%. When cut-off values determined by mean plus two and by mean plus three standard deviations derived from I-ELISA readings in an uninfected reference population were used, the diagnostic sensitivity was 100% but estimates of Y, Ef and other combined measures of diagnostic accuracy were lower. The I-ELISA based on rNp is highly sensitive, specific and robust and can be applied for diagnosis of infection of Rift Valley fever and disease-surveillance studies in humans.

Cloning and expression of Rift Valley fever virus nucleocapsid (N) protein and evaluation of a N-protein based indirect ELISA for the detection of specific IgG and IgM antibodies in domestic ruminants

Serodiagnosis of Rift Valley fever (RVF) currently relies on the use of live or inactivated whole virus as antigens. The recombinant nucleocapsid (N) protein of RVF virus was tested for diagnostic applicability in an indirect enzyme-linked immunosorbent assay (I-ELISA), using sera from experimentally infected sheep (n=128), vaccinated sheep (n=240), and field-collected sera from sheep (n=251), goats (n=362) and cattle (n=100). The N-protein based I-ELISA performed at least as good as VN and HI tests. In goat the diagnostic sensitivity (D-Sn) and specificity (D-Sp) of the I-ELISA was 100% when using the anti-species IgG conjugate. Using protein G as a detection system, the D-Sn and D-Sp in goats were 99.4% and 99.5%, in sheep field sera both 100%, in cattle 100% and 98.3%, respectively. The I-ELISA based on recombinant N-protein has the potential to complement the traditional assays for serodiagnosis of RVF. Advantages of the N-protein are its safety, stability and cost-effectiveness in use and production.

Preparation and evaluation of a recombinant Rift Valley fever virus N protein for the detection of IgG and IgM antibodies in humans and animals by indirect ELISA

This paper describes the cloning, sequencing and bacterial expression of the N protein of the Rift Valley fever virus (RVFV) ZIM688/78 isolate and its evaluation in indirect ELISAs (I-ELISA) for the detection of IgM and IgG antibodies in human and sheep sera. Sera used for the evaluation were from 106 laboratory workers immunised with an inactivated RVF vaccine, 16 RVF patients, 168 serial bleeds from 8 sheep experimentally infected with wild type RVFV and 210 serial bleeds from 10 sheep vaccinated with the live attenuated Smithburn RVFV strain. All human and animal sera that tested positive in the virus neutralisation test were also positive in the IgG I-ELISA. There was a high correlation (R2=0.8571) between virus neutralising titres and IgG I-ELISA readings in human vaccinees. In experimentally infected sheep IgG antibodies were detected from day 4 to 5 post-infection onwards and IgM antibodies from day 3 to 4. The IgG I-ELISA was more sensitive than virus neutralisation and haemagglutination-inhibition tests in detecting the early immune response in experimentally infected sheep. The I-ELISAs demonstrated that the IgG and IgM response to the Smithburn vaccine strain was slower and the levels of antibodies induced markedly lower than to wild type RVFV infection.

Emerging infectious diseases: TheBunyaviridae

The Bunyaviridae are a large group of viruses that infect a diversity of arthropod vectors and animal hosts. They have a worldwide distribution and can be the cause of human illness ranging from mild asymptomatic infection to hemorrhagic fever and fatal encephalitis. The growth of the human population, the expansion of agricultural and economic development, climatic changes, and the speed and frequency of global transportation all favor the emergence of bunyaviruses and other arthropod borne viruses. International monitoring of the Bunyaviridae and a greater understanding of their ecology and biology are needed to prepare for future outbreaks.

[Meningitis by Toscana virus in Spain: description of 17 cases]

BACKGROUND AND OBJECTIVE

We aimed to analyze the clinical and epidemiological data from the first series of patients with meningitis by Toscana virus in Spain.

PATIENTS AND METHOD

We analyzed a total of 724 cerebrospinal fluid (CSF) samples from patients with suspicion of aseptic meningitis for virus isolation in cell culture. The clinical records of patients in whom Toscana virus was isolated were analyzed.

RESULTS

Toscana virus was isolated in CSF in 17 patients (7% of all viral isolates). The first case was diagnosed in June 1988 and the last one in August 2002. The mean age was 27 years (range: 10-64 years). Most patients were based in rural area (n = 11, 64.7%). Most common symptoms were headache (holocranial or focal) present in all patients and moderate fever observed in 76.5% of them with a mean duration of 48 h (range: 18 h-5 days). Nuchal rigidity was present in 9 patients (53%). All cases were seen between June and October, and predominantly in August (53%). The outcome was favorable in all cases, and the mean time of duration of the disease was 7 days (range: 3-10 days).

CONCLUSIONS

Toscana virus must be taken into account among those agents responsible of lymphocytic meningitis in Spain.

Unusual presentation of life-threatening Toscana virus meningoencephalitis

This case report describes a brother and a sister with severe meningoencephalitis caused by Toscana virus (TOSv). The clinical presentation was characterized by stiff neck, deep coma, maculopapular rash, diffuse lymphadenopathy, hepatosplenomegaly, renal involvement, tendency to bleeding, and diffuse intravascular coagulation. The boy had epididymo-orchitis. Recovery with neurologic sequelae as hydrocephalus was observed. Microbiological diagnosis was obtained by serological tests and reverse transcriptase-polymerase chain reaction. Sequencing of polymerase chain reaction products from the S and M segments was carried out. TOSv may be a causative agent in severe meningoencephalitis.

A Mediterranean arbovirus: the Toscana virus

Toscana virus (Bunyaviridae family, Phlebovirus genus) is a sandfly fever virus responsible for human neurological infections. Sandfly viruses are transmitted by insect vectors (Phlebotomus species) and the infection is present in climatic areas that allow the life cycle of the vector. The arthropode-borne Toscana virus is the etiologic agent of meningitis, meningoencephalitis, and encephalitis. The frequency of this neuropathic infection increases in the summer months, peaking in August in the endemic Mediterranean areas (Italy, Portugal, Spain, and Cyprus). Infection diagnosis is carried out by molecular assays and immunoenzymatic tests, which are rapid and sensitive. Recent studies have investigated the antigenic properties of the viral proteins (nucleoprotein N and surface glycoproteins G1 and G2), to better understand their immunogentic role.

Epidemiological, clinical and laboratory aspects of sandfly fever

PURPOSE OF REVIEW

Sandfly fever viruses are still a significant health problem in many regions of the world, such as Africa, the Mediterranean basin, the Middle East, and Central Asia. This review provides an update on the advances in knowledge about epidemiological, clinical, and laboratory aspects of infections caused by Toscana, Sicilian and Naples viruses.

RECENT FINDINGS

Diagnosis of Toscana virus infection has been facilitated by new molecular methods and by immunoenzymatic tests based on the recombinant nucleoprotein. Gene analysis has allowed identification of circulating Toscana variants possibly involved in the protean pathomorphism and extreme variability of the clinical picture. New attention has been addressed to the antigenic properties of the viral proteins (the nucleoprotein N and the surface glycoproteins G1 and G2), in order to understand their immunogenetic role. High genetic divergence within the serocomplexes belonging to each of the Sicilian and the Naples viruses has suggested that infection with one genotype may not completely immunize against infection with all other genotypes in a given serocomplex. These findings could serve as a basis for vaccine development and may account for reports of multiple episodes of sandfly fever in the same host. Recently, the performance of analysis models based on weather data and reported vector surveys has allowed the prediction of the risk of acquiring sandfly infection in the endemic geographic areas.

SUMMARY

Recent developments include a better knowledge of the epidemiological, clinical, and laboratory aspects of sandfly infection, while the search for effective drugs and vaccines is still in progress.

Detection and identification of Toscana and other phleboviruses by RT-nested-PCR assays with degenerated primers

Phleboviruses are a large and widespread group of viruses that are transmitted by arthropods. Toscana virus is one of the principal agents that causes meningitis in humans during the summer in Italy and, possibly, in other Mediterranean countries. Rift Valley Fever virus can cause serious illness in both animals and humans, leading to high morbidity and mortality, and is considered to be a potential agent for epizootics and human epidemics. Since information on this group of viruses is still scant, reliable laboratory tools for diagnosis and epidemiological surveillance must be developed, in order to ascertain their real impact on Public Health. Sequence data obtained from Spanish isolates of Toscana virus and other phleboviruses confirmed that natural genome variability may hamper the diagnosis of these agents by molecular methods, so this must be borne in mind when developing reliable assays. In view of the above, a novel and useful protocol has been developed for the detection and specific identification of every member of the phlebovirus genus present in a sample, including Toscana virus, based on a generic RT-nested-PCR, followed by sequencing of the amplified fragment. A change in this method also allowed specific direct detection and identification of wild isolates of Toscana virus of different geographical origin, using newly designed primers. Testing clinical samples with these assays confirmed the role of Toscana virus as an agent that causes acute aseptic meningitis in the central region of Spain.

Acute meningitis due to Toscana virus infection among patients from both the Spanish Mediterranean region and the region of Madrid

Toscana virus (TOSV) is a member of the genus Phlebovirus that is transmitted to humans by two different species of sand fly and causes acute aseptic meningitis (AAM) and meningoencephalitis in Central Italy. Fifteen cases of AAM due to TOSV have been found at the Spanish province of Granada, but no data regarding the presence of TOSV-related disease in other regions of Spain have been still reported. A collection of 88 serum and 53 cerebrospinal fluid (CSF) samples taken from 81 selected patients with AAM of unknown aetiology, residing at Madrid or at the southern Mediterranean coast of Spain, was retrospectively studied for presence of TOSV-specific antibodies from both IgG and IgM classes. Anti-TOSV IgG was also investigated in 457 serum samples from healthy individuals, aged 2-60 years, residing at the south of the Region of Madrid. Specific IgM in serum and/or intrathecally produced anti-TOSV IgG were detected in seven patients, three residents from the Mediterranean region and the remainder four from the Region of Madrid. The overall prevalence of anti-TOSV among the healthy population studied was 5%. These results confirm the role of TOSV as an agent causing AAM in the Spanish Mediterranean coast, extend these findings to the central region of the country and suggest that TOSV might be producing infection and neurological disease in every area of Spain harbouring significant populations of the viral vectors.

Human antibody response to Toscana virus glycoproteins expressed by recombinant baculovirus

The arthropod-borne Toscana virus has been associated with acute neurological disease in humans. In this study, the viral envelope glycoproteins were expressed in soluble form in a baculovirus system. The recombinant sGN and sGC proteins were used as viral antigens in a Western blot assay to analyze the specific immune response in sera from patients with recognized virus-associated aseptic meningitis. The anti-glycoprotein and the anti-nucleoprotein N IgG responses were compared by an immunoassay based on the recombinant proteins. In this system, all the sera showed a high reactivity to the N protein, but they differed in the response to the glycoproteins.

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.

Detection of Toscana virus-specific immunoglobulins G and M by an enzyme-linked immunosorbent assay based on recombinant viral nucleoprotein

An enzyme-linked immunosorbent assay (ELISA) based on the recombinant Toscana virus nucleoprotein (rN) has been developed. Its sensitivity and specificity for the detection of virus-specific immunoglobulins G and M in human sera were similar to those of the ELISA that is based on an antigen extracted from infected mouse brain and that is routinely used for serodiagnosis.

Laboratory diagnosis of Toscana virus infection by enzyme immunoassay with recombinant viral nucleoprotein

A recombinant enzyme immunoassay (rEIA) to detect serum immunoglobulin M (IgM) and IgG to Toscana virus (TOSV) was developed with the aim of establishing a simple and easily available assay for diagnosing acute and/or previous infections. The rEIA, based on the recombinant nucleoprotein of TOSV expressed in Escherichia coli, was evaluated with 97 serum samples collected in an area where TOSV is endemic and compared to an analogous assay based on cell-derived TOSV. Discordant results were resolved by immunoblotting (IB). Twenty-two of these samples, obtained from subjects hospitalized during the summer season with meningitis of suspected TOSV etiology, were further characterized by indirect immunofluorescence and IB, and detection of specific TOSV RNA sequences in the cerebrospinal fluid of these patients was attempted by nested PCR. The results indicated that rEIA was able to diagnose acute TOSV infection by detection of specific serum IgM in all of the subjects with TOSV meningitis confirmed by nested PCR or serology. The overall sensitivity and specificity of rEIA were both 100% for IgM detection and 100 and 96.6%, respectively, for IgG detection. Thus, rEIA appears to be a simple and reliable laboratory test for the diagnosis of acute TOSV infection and for the assessment of immune status.

Humoral response in Toscana virus acute neurologic disease investigated by viral-protein-specific immunoassays

The Toscana virus (family Bunyaviridae, genus Phlebovirus) is the only sandfly-transmitted virus that demonstrates neurotropic activity. Clinical cases ranging from aseptic meningitis to meningoencephalitis caused by Toscana virus are yearly observed in central Italy during the summer, and several cases have been reported among tourists returning from zones of endemicity (Italy, Portugal, Spain, and Cyprus). In Toscana virus patients, immunoglobulin M (IgM) antibodies, usually present at the onset of symptoms, can reveal elevated titers by enzyme-linked immunosorbent assay and can persist for at least 1 year. IgG antibodies can be absent at the onset of symptoms: titers rise in convalescent sera and persist for many years. At least five proteins have been identified in Toscana virus-infected cells: nucleoprotein N, glycoproteins G1 and G2, a large protein (L) assumed to be a component of the polymerase, and two nonstructural proteins, NSm and NSs. We report results of a study on the antibody response to individual viral proteins in patients with Toscana virus-associated acute neurologic disease. Immunoblotting and semiquantitative radioimmunoprecipitation assay (RIPA) allow identification of nucleoprotein N as the major antigen responsible for both IgM and IgG responses. Antibodies to proteins other than nucleoprotein N are detected only by RIPA. Antibodies to glycoproteins are detected in about one-third of patients, and whereas their presence always predicts neutralization, some serum samples with neutralizing activity have undetectable levels of antibodies to G1-G2. Antibodies to nonstructural proteins NSm and NSs are also identified. The results obtained raise some questions about antigenic variability and relevant neutralization epitopes of Toscana virus.

A recombinant Toscana virus nucleoprotein in a diagnostic immunoblot test system

Sandfly fever, a vector-borne disease endemic in the Mediterranean region, is caused by Toscana virus (TOS). The disease is increasingly important as a travel-related infection. Serological diagnosis is currently dependent on viral antigens derived from TOS-infected cell cultures. In this study, we report the cloning and expression of the TOS nucleoprotein (N) in Escherichia coli and evaluation of the recombinant (r) TOS N protein as an antigen for immunoblot assays. The TOS N gene was amplified by reverse-transcriptase polymerase chain reaction and cloned into the bacterial expression vector pTrcHis-A. Sera with known TOS antibody status were used to evaluate the immunoblot assay. The expressed rTOS N protein was purified and used as antigen for immunoblots. By recombinant immunoblot, the TOS antibody status (IgM and/or IgG) of the test panel was correctly identified. No cross-reactivity was detected. The rTOS N protein is useful as an antigen for immunoblot assays, and will enable more laboratories to perform TOS antibody diagnosis.

Diagnostic potential of Toscana virus N protein expressed in Escherichia coli

The nucleocapsid (N) protein of the Toscana (TOS) virus was expressed in Escherichia coli by using a pET15b vector. The recombinant protein was purified by affinity chromatography and was characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and enzyme immunoassay (EIA). The recombinant antigen was reactive with positive human sera, and the reactivity correlated very well (r = 0.9) with that of a whole-virus antigen when tested by EIA with 30 TOS virus-positive and 30 TOS virus-negative serum samples. The results demonstrate that the recombinant N protein can be easily produced in a procaryotic system and used for diagnostic assays for TOS virus immunity.