Genome organization in the caliciviridae

Characterisation of Lagovirus europaeus GI-RHDVs (Rabbit Haemorrhagic Disease Viruses) in Terms of Their Pathogenicity and Immunogenicity

Rabbit haemorrhagic disease viruses (RHDV) belong to the family Caliciviridae, genus Lagovirus europaeus, genogroup GI, comprising four genotypes GI.1-GI.4, of which the genotypes GI.1 and GI.2 are pathogenic RHD viruses, while the genotypes GI.3 and GI.4 are non-pathogenic RCV (Rabbit calicivirus) viruses. Among the pathogenic genotypes GI.1 and GI.2 of RHD viruses, an antigenic variant of RHDV, named RHDVa-now GI.1a-RHDVa, was distinguished in 1996; and in 2010, a variant of RHDV-named RHDVb, later RHDV2 and now GI.2-RHDV2/b-was described; and recombinants of these viruses were registered. Pathogenic viruses of the genotype GI.1 were the cause of a disease described in 1984 in China in domestic (Oryctolagus (O.) cuniculus domesticus) and wild (O. cuniculus) rabbits, characterised by a very rapid course and a mortality rate of 90-100%, which spread in countries all over the world and which has been defined since 1989 as rabbit haemorrhagic disease. It is now accepted that GI.1-RHDV, including GI.1a-RHDVa, cause the predetermined primary haemorrhagic disease in domestic and wild rabbits, while GI.2-RHDV2/b cause it not only in rabbits, including domestic rabbits' young up to 4 weeks and rabbits immunised with rabbit haemorrhagic disease vaccine, but also in five various species of wild rabbits and seven different species of hares, as well as wild ruminants: mountain muskoxen and European badger. Among these viruses, haemagglutination-positive, doubtful and harmful viruses have been recorded and described and have been shown to form phylogenogroups, immunotypes, haematotypes and pathotypes, which, together with traits that alter and expand their infectious spectrum (rabbit, hare, wild ruminant, badger and various rabbit and hare species), are the determinants of their pathogenicity (infectivity) and immunogenicity and thus shape their virulence. These relationships are the aim of our consideration in this article.

Genomic Characterization and Molecular Evolution of Sapovirus in Children under 5 Years of Age

Sapovirus (SaV) is a type of gastroenteric virus that can cause acute gastroenteritis. It is highly contagious, particularly among children under the age of 5. In this study, a total of 712 stool samples from children under the age of 5 with acute gastroenteritis were collected. Out of these samples, 28 tested positive for SaV, resulting in a detection rate of 3.93% (28/712). Samples with Ct < 30 were collected for library construction and high-throughput sequencing, resulting in the acquisition of nine complete genomes. According to Blast, eight of them were identified as GI.1, while the remaining one was GI.6. The GI.6 strain sequence reported in our study represents the first submission of the GI.6 strain complete genome sequence from mainland China to the Genbank database, thus filling the data gap in our country. Sequence identity analysis revealed significant nucleotide variations between the two genotypes of SaV and their corresponding prototype strains. Phylogenetic and genetic evolution analyses showed no evidence of recombination events in the obtained sequences. Population dynamics analysis demonstrated potential competitive inhibition between two lineages of GI.1. Our study provides insights into the molecular epidemiological and genetic evolution characteristics of SaV prevalent in the Nantong region of China, laying the foundation for disease prevention and control, as well as pathogen tracing related to SaV in this area.

Complete Genome Sequence, Molecular Characterization and Phylogenetic Relationships of a Temminck's Stint Calicivirus: Evidence for a New Genus within Caliciviridae Family

Caliciviridae is a family of viral pathogens that naturally infects vertebrates, including humans, and causes a range of highly contagious infectious diseases. Caliciviruses are not well studied because of the lack of a universal approach to their cultivation; however, the development of molecular genetics and bioinformatics methods can shed light on their genetic architecture and evolutionary relationships. Here, we present and characterize the complete genome sequence of calicivirus isolated from a sandpiper-Temminck's stint (Calidris temminckii), preliminarily named Temminck's stint calicivirus (TsCV). Its genome is a linear, non-segmented, single-stranded (+sense) RNA with genome organization typical of avian caliciviruses. Comparative studies have shown significant divergence of the nucleotide sequence of the TsCV genome, as well as the amino acid sequence of the major capsid protein from all publicly available genomic and protein sequences, with the highest genome sequence similarity to unclassified Ruddy turnstone calicivirus A (43.68%) and the lowest pairwise divergence of the major capsid protein with unclassified goose calicivirus (57.44%). Phylogenetic analysis, as well as a comparative analysis of the homologous proteins, showed evidence of another separate genus within the Caliciviridae family-previously proposed, but not yet accepted by International Committee on Taxonomy of Viruses (ICTV)-the Sanovirus genus, which combines seven previously unclassified genomic sequences of avian caliciviruses, including the newly discovered TsCV, which we propose to consider as a separate species.

Calicivirus Non-structural Proteins: Potential Functions in Replication and Host Cell Manipulation

The Caliciviridae are a family of viruses with a single-stranded, non-segmented RNA genome of positive polarity. The ongoing discovery of caliciviruses has increased the number of genera in this family to 11 (Norovirus, Nebovirus, Sapovirus, Lagovirus, Vesivirus, Nacovirus, Bavovirus, Recovirus, Salovirus, Minovirus, and Valovirus). Caliciviruses infect a wide range of hosts that include fishes, amphibians, reptiles, birds, and marine and land mammals. All caliciviruses have a genome that encodes a major and a minor capsid protein, a genome-linked viral protein, and several non-structural proteins. Of these non-structural proteins, only the helicase, protease, and RNA-dependent RNA polymerase share clear sequence and structural similarities with proteins from other virus families. In addition, all caliciviruses express two or three non-structural proteins for which functions have not been clearly defined. The sequence diversity of these non-structural proteins and a multitude of processing strategies suggest that at least some have evolved independently, possibly to counteract innate and adaptive immune responses in a host-specific manner. Studying these proteins is often difficult as many caliciviruses cannot be grown in cell culture. Nevertheless, the study of recombinant proteins has revealed many of their properties, such as intracellular localization, capacity to oligomerize, and ability to interact with viral and/or cellular proteins; the release of non-structural proteins from transfected cells has also been investigated. Here, we will summarize these findings and discuss recent in silico studies that identified previously overlooked putative functional domains and structural features, including transmembrane domains that suggest the presence of viroporins.

Mutation in a Lordsdale norovirus epidemic strain as a potential indicator of transmission routes

An increase in norovirus outbreaks was reported internationally during 2002 and 2003 and was also observed in Oxfordshire (United Kingdom) hospitals. To understand their epidemiological relationships, viruses from 22 outbreaks (15 from one hospital) were subjected to nucleotide sequencing. The 3'-terminal 3,255 nt or complete genomes were determined for 49 viruses. All outbreaks were caused by a genogroup II norovirus related to the Lordsdale virus (GII 4), common in healthcare settings. The norovirus mutation rate was sufficiently high that the 3,255-nucleotide sequences allowed separate and potentially connected outbreaks to be identified, since all outbreaks with identical sequences were temporally or geographically linked. The high mutation rate was further indicated by four mutations and three microheterogeneities in 3,255 nucleotides during 17 days of norovirus shedding by an immunocompromised patient. The data suggested that multiple virus introductions from the community, occasional transmission among wards, and one instance of ongoing environmental contamination had occurred. The accumulation, or lack, of mutations within an outbreak was also used to indicate the predominant transmission route. In an outbreak where person-to-person spread was thought to predominate, six mutations were detected throughout the genome, whereas one mutation was detected when point source infection was suspected. This norovirus epidemic strain differed from its closest previously described relative by 11.4 to 13.6% in the outer P2 domain of the capsid, which also had a single-amino-acid insertion. Alterations to the capsid structure compared to previous noroviruses may explain the increased number of outbreaks during 2002 and 2003.

Reverse transcription-PCR assays for detection of bovine enteric caliciviruses (BEC) and analysis of the genetic relationships among BEC and human caliciviruses

Two genetically distinct bovine enteric caliciviruses (BECs) have been identified: the norovirus (NLV) Jena and Newbury Agent-2 (NA-2) BECs, which are genetically related to human noroviruses, and the Nebraska (NB) BECs, which is related to sapoviruses and lagoviruses but may also represent a new calicivirus genus. The prevalence of these two BEC genotypes in cattle is unknown. Although reverse transcription-PCR (RT-PCR) primers for human NLV recognize NLV-BECs, the genetic relationships between NLV from humans and the NLV-BECs commonly circulating in cattle is undefined. In the present study, veal calf fecal samples were assayed for enteric caliciviruses by using six RT-PCR primer sets designed for the detection of human NLVs or BECs. Caliciviruses genetically related to the NLV-BEC Jena and NA-2 strains or to the recently characterized NB BEC strain were identified in three of four and four of four sampled veal herds, respectively. Extended 3'-terminal genome sequences of two NLV-BECs, designated CV95-OH and CV186-OH, encoding the RNA-dependent RNA polymerase (RdRp; open reading frame 1 [ORF-1]), VP1 (ORF-2), and VP2 (ORF-3) genes were determined. Phylogenetic and sequence identity analyses of each genome region demonstrated these viruses to be most closely related to the NLV-BEC Jena and NA-2 strains. In initial testing, the human P289-P290 (P289/290) primer set was found to be the most sensitive for calicivirus detection. However, its failure to identify all positive fecal pools (as determined by other assays) led us to design two new primer sets, CBECU-F/R and NBU-F/R, for the sensitive and specific detection of NLV-BEC (NLV-BEC Jena and NA-2) and BEC-NB-like viruses, respectively. The RT-PCR assays with the new primers were compared against other primer sets, including P289/290. Composite results of the tests completed by using the new assays identified 72% (54 of 75) of veal calf fecal samples as positive, with 21 of 21 sequenced reaction products specific for the target RdRp gene. The same design strategy used for the new BEC assays may also be applicable to the design of similar assays for the detection of human caliciviruses (HuCVs). Our data support the genetic relationship between NLV-BECs and NLV-HuCVs but with the NLV-BECs comprising two clusters within a third NLV genogroup.

Characterization of an enteropathogenic bovine calicivirus representing a potentially new calicivirus genus

Bovine enteric caliciviruses (BEC) are associated with diarrhea in young calves. The BEC strains detected in Europe form a third genogroup within the genus "Norwalk-like viruses" (NLV) of the family Caliciviridae. In this report, we present sequence, clinical, and histological data characterizing a novel enteropathogenic BEC strain, NB, detected in fecal specimens from calves in the United States. The complete RNA genome of the NB virus is 7,453 bases long and is organized into two open reading frames (ORFs). ORF-1 is 2,210 amino acids long and encodes a large nonstructural polyprotein contiguous with the major capsid protein (VP1), similar to the lagoviruses and "Sapporo-like viruses" (SLV). The conserved calicivirus motifs were identified in the nonstructural proteins. ORF-2 is located at the 3' end of the genome and encodes a small basic protein (VP2) of 225 amino acids. The 5' and 3' untranslated regions are 74 and 67 bases long, respectively. Among caliciviruses, NB virus shows amino acid identities of 14.1 to 22.6% over the entire ORF-1 nonstructural-protein sequence with NLV, SLV, vesivirus, and lagovirus strains, while the overall sequence identity of the complete NB VP-1 with other caliciviruses is low, varying between 14.6 and 26.7%. Phylogenetic analysis of the complete VP1 protein, including strains from all four calicivirus genera, showed the closest grouping of NB virus to be with viruses in the genus Lagovirus, which cause liver infections and systemic hemorrhage in rabbits. In gnotobiotic calves, however, NB virus elicited only diarrhea and intestinal lesions that were most severe in the upper small intestine (duodenum and jejunum), similar to the NLV BEC strains. The tissues of major organs, including the lung, liver, kidney, and spleen, had no visible microscopic lesions.

Epidemiology of Norwalk-like human caliciviruses in hospital outbreaks of acute gastroenteritis in the Stockholm area in 1996

OBJECTIVES

Outbreaks of acute gastroenteritis associated with 'Norwalk-like viruses' (NLVs) cause significant health problems in hospitals. Hospital outbreaks in the Stockholm area in 1996 were investigated, in order to identify the magnitude of the problem, the mode of transmission, the effect of control measures and the genetic variability of outbreak strains. Determining the epidemiological and clinical significance involves a broad range of possibilities.

METHODS

Ten hospitals, representing 66% of the hospitals in the Stockholm area, participated in the study, which included 211 wards. Of these, 18 were selected as control. A standardized protocol that included personal contacts was administered. Outbreak wards were visited between 5 and 10 times. Wards that had reported outbreaks in 1996 were prospectively followed through 1999 by personal contacts, and the available data from 1991 on outbreak reports were collected. A total of 253 stool samples from outbreaks in 1996 were analyzed by electron microscopy (EM) for the presence of NLVs. Positive samples were confirmed by the reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

In total, 4 326 patients and 1 119 staff were exposed on the 43 wards that reported 54 outbreaks. The mean attack rate was 13% for patients and 21% for staff. The number of outbreaks in 1996 outnumbered the reported outbreaks in the preceding years (4-70%) and later years (35-40%). Admission to 24 (56%) of the outbreak wards was stopped. The mean duration of illness for patients was 35 hours and for staff, 30 hours. The main symptoms were diarrhoea (80%) and vomiting (68%). Genotyping revealed that the majority of the hospital outbreaks in the Stockholm area in 1996 were caused by a single NLV strain.

CONCLUSIONS

The study confirmed that outbreaks of NLV are an increasing public-health problem in hospitals. The risk of being affected by an outbreak was significantly greater on wards that had reported outbreaks in the previous year. It was not obvious which measures had helped to shorten the outbreaks to any appreciable extent. Different managements must therefore be carefully interpreted and adapted to the prevailing circumstances. Genotyping of strains is an important tool of getting a better insight into transmission routes and the mechanism behind the appearance of epidemic strains.

Organization and expression of calicivirus genes

The application of molecular techniques to the characterization of caliciviruses has resulted in an extensive database of sequence information. This information has led to the identification of 4 distinct genera. The human enteric caliciviruses have been assigned to 2 of these genera. This division is reflected not only in sequence diversity but in a fundamental difference in genome organization. Complete genome sequences are now available for 5 enteric caliciviruses and demonstrate that human and animal enteric caliciviruses are phylogenetically closely related. Currently, there is no cell culture system for the human viruses; therefore, studies have relied on heterologous expression and in vitro systems. These studies have shown that in both human and animal viruses the viral nonstructural proteins are produced from a polyprotein precursor that is cleaved by a single viral protease. The purpose of this article is to provide an overview of the current knowledge of genome structure and gene expression in the enteric caliciviruses.

Molecular characterization of a bovine enteric calicivirus: relationship to the Norwalk-like viruses

Jena virus (JV) is a noncultivatable bovine enteric calicivirus associated with diarrhea in calves and was first described in Jena, Germany. The virus was serially passaged 11 times in colostrum-deprived newborn calves and caused diarrheal disease symptoms at each passage. The complete JV genome sequence was determined by using cDNA made from partially purified virus obtained from a single stool sample. JV has a positive-sense single-stranded RNA genome which is 7,338 nucleotides in length, excluding the poly(A) tail. JV genome organization is similar to that of the human Norwalk-like viruses (NLVs), with three separate open reading frames (ORFs) and a 24-nucleotide sequence motif located at the 5' terminus of the genome and at the start of ORF 2. The polyprotein (ORF 1) consists of 1,680 amino acids and has the characteristic 2C helicase, 3C protease, and 3D RNA polymerase motifs also found in the NLVs. However, comparison of the N-terminal 100 amino acids of the JV polyprotein with those of the group 1 and group 2 NLVs showed a considerable divergence in sequence. The capsid protein (ORF 2) at 519 amino acids is smaller than that of all other caliciviruses. JV ORF 2 was translated in vitro to produce a 55-kDa protein that reacted with postinfection serum but not preinfection serum. Phylogenetic studies based on partial RNA polymerase sequences indicate that within the Caliciviridae JV is most closely related to the group 1 NLVs.

Characterization of a recombinant human calicivirus capsid protein expressed in mammalian cells

The capsid protein of the Hawaii strain of human calicivirus was expressed in the transient MVA/bacteriophage T7 polymerase hybrid expression system in order to examine its processing in mammalian cells. Selected amino acid modifications (an insertion, deletion, and substitution) at the predicted amino terminus of the capsid protein as well as the presence or absence of the ORF3 gene were examined for their effect on capsid expression. The protein was expressed efficiently in cell lines derived from three different species, with most of the expressed protein remaining localized within the cells. There was no evidence for N-linked glycosylation or myristylation of the 57 kDa capsid protein. Hawaii virus-like particles (HV VLPs), efficiently produced in the baculovirus expression system, were not observed in this expression system under the conditions in this study.

Viral zoonoses and food of animal origin: caliciviruses and human disease

Caliciviruses are important veterinary and human pathogens. The viruses gain their name from characteristic cup-shaped structures seen on the virion surface by negative stain electron microscopy. In humans caliciviruses are a major cause of diarrhoeal disease. There are two fundamentally different genome structures amongst human caliciviruses. The Norwalk-like or small round structured viruses (SRSVs) are viruses that have an amorphous structure when viewed by EM, they have a genome composed of 3 major open reading frames (ORFs). These viruses cause epidemic gastroenteritis amongst all age groups. In contrast, the 'classic' human caliciviruses (HuCVs) display the typical calicivirus surface structure and have their capsid ORF fused to and contiguous with the non structural proteins forming one giant polyprotein. HuCVs are predominantly associated with paediatric infections and are only a minor cause of disease in humans. Spread of disease for both SRSVs and HuCVs is usually by faecal oral transmission. SRSVs are a major cause of foodborne gastroenteritis especially linked to the consumption of sewage-contaminated shellfish. However, there is no evidence that these viruses replicate in shellfish or that they originate from an animal source.

The role of human caliciviruses in epidemic gastroenteritis

Members of the Caliciviridae family of small, positive-sense RNA viruses exhibit a broad host range. The Norwalk and Norwalk-like caliciviruses in this family are major etiologic agents of epidemic gastroenteritis in humans. This illness characteristically lasts 24-48 h and often occurs in group settings such as families, schools, institutions, or communities. The spread of the human caliciviruses is considered to be predominantly by person-to-person contact via the fecal-oral-route. However, the ingestion of calicivirus-contaminated food or water can result in large-scale common-source outbreaks. Many basic features concerning the biology and replication of the human caliciviruses are not known because they have not yet been grown in cell culture and the virus does not appear to replicate in animal models other than the chimpanzee. Sequence analysis of RT-PCR-generated DNA fragments derived from serotypically distinct reference strains (such as the Norwalk, Hawaii, and Snow Mountain viruses) and other circulating strains associated with gastroenteritis has provided evidence for marked genetic diversity among these viruses. Moreover, analysis of the antigenic relationships among these viruses using paired sera from individuals infected with well-characterized reference strains or from animals immunized with recombinant "virus-like particles" (VLPs) suggests that several serotypes of these viruses are circulating worldwide. The availability of molecular techniques for the detection of these fastidious viruses has enabled epidemiologic studies that have strengthened the association of human caliciviruses with acute gastroenteritis and has demonstrated a potential role for antigenic diversity in the natural history of these pathogens.

Expression and self-assembly of recombinant capsid protein from the antigenically distinct Hawaii human calicivirus

The Norwalk and Hawaii viruses are antigenically distinct members of the family Caliciviridae and are considered to be important etiologic agents of epidemic gastroenteritis, with most studies focusing on the role of Norwalk virus. To further investigate the importance of Hawaii virus, Hawaii virus-like particles (VLPs) were produced by expression of its capsid protein in the baculovirus system and these VLPs were used as the antigen in an enzyme-linked immunosorbent assay that was efficient in the detection of a serologic response to Hawaii virus. The ready availability of Hawaii VLPs should enable larger-scale epidemiological studies to further elucidate the importance of this agent.

Sensitive detection of human Caliciviridae by RT-PCR

A semi-nested reverse transcriptase-polymerase chain reaction (RT-PCR) was developed for the detection of human Caliciviridae. The method was evaluated on faecal samples from patients with gastroenteritis sent to the Norwegian National Institute of Public Health for routine diagnosis by direct electron microscopy (EM). Of 166 samples, 49 were found to contain Caliciviridae by EM, while 7 samples contained other viruses. A total of 74 samples was positive by PCR, including all the samples with EM detectable Caliciviridae, while specimens containing other agents were negative. Phylogenetic analysis of RNA sequences from 14 Norwegian samples indicated that the viruses present in Norway are evenly distributed when compared to sequences of human Caliciviridae from other countries. The PCR primers should therefore be useful for samples from other regions. The phylogenetic analysis did not cluster viruses with a calici-like morphology, but mingled them with sequences from Norwalk-like viruses, indicating that the two morphological types do not represent separate genogroups.

Polyprotein processing in Southampton virus: identification of 3C-like protease cleavage sites by in vitro mutagenesis

A genomic clone of the small, round-structured virus Southampton virus (SV) was constructed from a set of overlapping PCR amplicons. Sequence analysis confirmed the absence of mutations and accurate ligation of the PCR products. The SV cDNA was cloned into a vector for in vitro production of RNA and subsequent translation by rabbit reticulocyte lysate. Two polypeptides corresponding to the N-terminal and C-terminal regions of the viral polyprotein were expressed in Escherichia coli and used to produce murine antisera for detection of translation products. Three major translation products of 113, 48, and 41 kDa were identified in a coupled transcription-translation system. The large 113-kDa protein reacted with antisera raised against the C-terminal region of the polyprotein and represents a precursor of the viral RNA polymerase. The 48-kDa protein detected in vitro reacted specifically with antisera raised against the polyprotein N terminus, showing that translation was initiated in SV at the three tandem in-frame AUG codons at the 5' end of the genome. A series of nested 3' deletions of the large open reading frame encoding the viral polyprotein was used to define the translation initiation site and genomic location of the viral protease. The results are consistent with a model in which translation of the viral genome is initiated at one of the three in-frame AUG codons starting at nucleotide position 5 and in which active viral protease is produced following translation of a region located between NheI (nucleotide 3052) and SphI (nucleotide 4056), resulting in rapid cleavage of a large precursor protein. Abolition of the viral 3C-like protease activity by site-directed mutagenesis of the putative active-site cysteine (Cys-1238) resulted in production of a large protein of approximately 200 kDa which reacted with both N-terminal and C-terminal antisera. Two potential polyprotein cleavage sites containing the preferred picornaviral QG recognition site were identified on either side of the putative 2C-like helicase region of the polyprotein. Proteolysis at these positions would give rise to products with relative molecular masses identical to those of the products detected in the rabbit reticulocyte system. Site-directed mutagenesis was used to introduce a single base change which resulted in the substitution of glutamine residues with proline residues at amino acids 399 and 762. These mutations completely abolished cleavage of the polyprotein at these positions and gave rise to alternative products with molecular masses which matched the predicted sizes for a single cleavage at either Q-399 or Q-762. These data indicate that the small, round-structured virus Southampton virus produces a 3C-like protease which has two primary cleavage sites at positions 399 and 762. Proteolytic cleavage at these positions releases the putative viral 2C-like helicase.

Comparison of the replication of positive-stranded RNA viruses of plants and animals

It is clear from the experimental data that there are some similarities in RNA replication for all eukaryotic positive-stranded RNA viruses—that is, the mechanism of polymerization of the nucleotides is probably similar for all. It is noteworthy that all mechanisms appear to utilize host membranes as a site of replication. Membranes appear to function not only as a way of compartmentalizing virus RNA replication but also appear to have a central role in the organization and functioning of the replication complex, and further studies in this area are needed. Within virus supergroups, similarities are evident between animal and plant viruses—for example, in the nature and arrangements of replication genes and in sequence similarities of functional domains. However, it is also clear that there has been considerable divergence, even within supergroups. For example, the animal alpha-viruses have evolved to encode proteinases which play a central controlling function in the replication cycle, whereas this is not common in the plant alpha-like viruses and even when it occurs, as in the tymoviruses, the strategies that have evolved appear to be significantly different. Some of the divergence could be host-dependent and the increasing interest in the role of host proteins in replication should be fruitful in revealing how different systems have evolved. Finally, there are virus supergroups that appear to have no close relatives between animals and plants, such as the animal coronavirus-like supergroup and the plant carmo-like supergroup.

Historical background and classification of caliciviruses and astroviruses

Infections caused by caliciviruses, i.e., vesicular exanthema virus of swine were recognised as a major cause of economic loss in the 1930s. However, it was not until the application of electronmicroscopy in the 1970s that caliciviruses and astroviruses were recognised and proven to be a cause of diarrhoea and vomiting. The following review briefly describes the steps which have led to the development of diagnostic tests and enabled the characterization of several members of the Caliciviridae and Astroviridae. In the past five years this has culminated in the sequencing of their genomes and the expression of viral proteins. This in turn has led to the development of improved diagnostic tests e.g., RT-PCR and enzyme immunoassays, and may pave the way towards producing effective vaccines in the future.