Toroviridae: a taxonomic proposal*

Toroviridae: a proposed new family of enveloped RNA viruses

The proposed family Toroviridae is characterized by enveloped, peplomer-bearing particles containing an elongated tubular nucleocapsid of helical symmetry. The capsid may be bent into an open torus, conferring a biconcave disk or kidney shape on the virion (largest diameter 120-140 nm), or straight, resulting in a rod-shaped particle (dimensions 35 X 170 nm). Morphogenesis occurs by the budding of preformed tubular nucleocapsids through membranes of the Golgi system and of the rough endoplasmic reticulum. Berne virus, which is proposed as the family prototype, contains a single strand of infectious positive-sense RNA, of Mr about 7.0 X 10(6), which is polyadenylated. The RNA is surrounded by the major nucleocapsid phosphoprotein (about 20 kDa) which, in turn, is enveloped by a membrane containing a major 22 kDa protein and a 37 kDa phosphoprotein. The viral peplomers measuring about 20 nm in length, carry determinants for neutralization and haemagglutination; the peplomers are formed by an N-glycosylated protein in the 75 to 100 kDa range. Six (to seven) subgenomic polyadenylated RNAs have been identified in infected cells, with Mr values of 2.6, 1.2, (1.0), 0.55, 0.35, 0.27 and 0.22 X 10(6). Torovirus replication requires some synthetic activity of the host cell. All toroviruses identified so far cause enteric infections and are probably transmitted by the faecal-oral route. Serological relationships between the equine, bovine and human viruses have been demonstrated.

Bovine torovirus (Breda virus) revisited

Bovine torovirus (BoTV) is a pleomorphic virus with a spike-bearing envelope and a linear, non-segmented, positive-sense single-stranded RNA genome. This kidney-shaped virus is associated with diarrhea in calves and apparently has a worldwide distribution. This review provides details of the history and taxonomy of BoTV since its discovery in 1979. Information about virion morphology and architecture, antigenic and biological properties, viral genome, protein composition, thermal and chemical stability, and pH and proteolytic enzymes resistance is also summarized. A major focus of this review is to postulate a possible epidemiological cycle for BoTV, based on epidemiological data obtained in our studies and other published data, and progressing from the newborn calf to the adult animal. The distribution, host range, pathogenesis, disease and clinical signs (under experimental and natural exposure), pathology, diagnosis, prevention, treatment and control of BoTV infections are also described. In addition, a discussion of the zoonotic implications of torovirus-like particles detected in patients with gastroenteritis that resemble and cross-react with BoTV is presented. Hopefully, the findings described here will alert others to the existence of BoTV in cattle and its contribution to the diarrheal disease complex. This review also highlights the need for continual vigilance for potential zoonotic viruses belonging to the order Nidovirales, such as the SARS coronavirus.

Toroviruses of animals and humans: a review

Toroviruses are a group of enveloped positive-stranded RNA viruses that cause enteric, respiratory, and perhaps generalized infections in animals and humans. Their name refers to their unique morphological features: an elongated bacilliform core with two rounded ends is surrounded by a membrane that may either tightly adhere to or “shrink-wrap” it, without respecting the capsid's rod shape; in the first instance, straight or curved rhabdovirus-like particles are formed, whereas in the latter a biconcave disk results. Torovirus history is brief: the first representative, Berne virus (BEV), was isolated in Berne, Switzerland, in 1972 from a rectal swab taken from a horse with diarrhea 1 week before it died. BEV is the only equine torovirus isolate that replicates in cell culture; since most molecular data have been obtained with this isolate, BEV has been acknowledged as the torovirus prototype. Recognition of toroviruses as a new group of potentially pathogenic viruses came seven years after the discovery of BEV, when morphologically similar particles were discovered by electron microscopy (EM) in stool specimens from calves with severe diarrhea in a dairy herd in Breda, Iowa. Despite repeated attempts, BRV has not been adapted to the growth in cell or tissue culture, a problem which has hampered its biochemical, bio-physical, and molecular characterization. However, its pathogenesis and pathology have been studied in the experimentally infected gnotobiotic calves, showing that BRV infections may cause gastroenteritis. Recently, Vanopdenbosch et al. reported the isolation of a torovirus-like virus from the respiratory tract of calves with pneumonia, suggesting that both enterotropic and pneumotropic bovine toroviruses exist. Besides the established toroviruses of horses and cattle, torovirus-like particles (TVLPs) have been found by EM in different animal species; torovirus antibodies appear to be widespread in higher vertebrates, indicating that these viruses infect a broad range of animal hosts. The possibility of a torovirus infecting humans was first reported in 1984 and has become more likely in view of the recent data. This chapter is intended to update the information about toroviruses, and to describe the similarities and differences with the related coronaviruses.

Another triple-spanning envelope protein among intracellularly budding RNA viruses: the torovirus E protein

The nucleotide sequence of the Berne virus envelope (E) protein gene was determined and its 26.5K translation product was identified by in vitro transcription and translation. Computer analysis of the protein sequence revealed the characteristics of a class III membrane protein lacking a cleaved signal sequence but containing three successive transmembrane alpha-helices in the N-terminal half, much the same as the coronavirus membrane (M) protein. The disposition of the E protein in the membrane was studied by in vitro translation in the presence of microsomes and by subsequent proteinase K digestion. Only small portions of either end of the polypeptide were found to be exposed on opposite sides of the vesicle membranes. Experiments with a hybrid E protein (EM) containing the C-terminal tail of a coronavirus M protein, to which an anti-peptide serum was available, showed that this C-terminus was present at the cytoplasmic side of the membrane, which is another similarity to the coronavirus M protein. Immunofluorescence experiments indicated that the EM protein, expressed by a recombinant vaccinia virus, accumulated in intracellular membranes, predominantly those of the endoplasmic reticulum. The common features of the torovirus E and the coronavirus M protein support our hypothesis that an evolutionary relationship exists between these groups of intracellularly budding viruses.

Comparison of the genome organization of toro- and coronaviruses: evidence for two nonhomologous RNA recombination events during Berne virus evolution

Recently, toroviruses and coronaviruses have been found to be ancestrally related by divergence of their polymerase and envelope proteins from common ancestors. In addition, their genome organization and expression strategy, which involves the synthesis of a 3'-coterminal nested set of mRNAs, are comparable. Nucleotide sequence analysis of the genome of the torovirus prototype, Berne virus (BEV), has now revealed the results of two independent nonhomologous RNA recombinations during torovirus evolution. Berne virus open reading frame (ORF) 4 encodes a protein with significant sequence similarity (30-35% identical residues) to a part of the hemagglutinin esterase proteins of coronaviruses and influenza virus C. The sequence of the C-terminal part of the predicted BEV polymerase ORF1a product contains 31-36% identical amino acids when compared with the sequence of a nonstructural 30/32K coronavirus protein. The cluster of coronaviruses which contains this nonstructural gene expresses it not as a part of their polymerase, but by synthesizing an additional subgenomic mRNA.

Primary structure and post-translational processing of the Berne virus peplomer protein

The nucleotide sequence of the peplomer (P) protein gene of Berne virus (BEV), the torovirus prototype, was determined. The gene encodes an apoprotein of 1581 amino acids with an Mr of about 178K. The open reading frame was cloned behind the T7 RNA polymerase promoter and its translation product was identified as the BEV P protein precursor by in vivo expression and immunoprecipitation. The deduced amino acid sequence contains a number of domains which are typical for type I membrane glycoproteins: an N-terminal signal sequence, a putative C-terminal transmembrane anchor, and a cytoplasmic tail. Eighteen potential N-glycosylation sites, two heptad repeat domains, and a possible "trypsin-like" cleavage site were identified. The mature P protein consists of two subunits and their electrophoretic mobility upon endoglycosidase F treatment strongly suggests that the predicted cleavage site is functional in vivo. The heptad repeat domains are probably involved in the generation of an intra-chain coiled-coil secondary structure; similar inter-chain interactions can play a role in P protein oligomerization. Using a sucrose gradient assay the P protein was indeed shown to form dimers. The intra- and inter-chain coiled-coil interactions may stabilize the elongated BEV peplomers.

The polymerase gene of corona- and toroviruses: evidence for an evolutionary relationship

In this paper we demonstrate that the organization of the polymerase gene of toroviruses and coronaviruses is similar. The polymerase gene of both virus families consists of at least two large ORFs (1a and 1b). Four domains of conserved amino acid sequences have been identified in nearly identical positions in the 3' ORF of the pol gene of toroviruses and coronaviruses. The most 3' conserved domain which is still unique for these viruses encodes a 33-kDA protein in MHV-A59, which is cleaved from a precursor protein. Expression of ORF1b of the pol gene of both virus families occurs by ribosomal frameshifting. A predicted stem-loop structure and pseudoknot are conserved in the ORF1a/ORF1b overlap of toro- and coronaviruses. On the basis of these results we postulate that toro- and coronaviruses are ancestrally more related to each other than to other families of positive stranded RNA viruses.

Seroepidemiology of Breda virus in cattle using ELISA

Two direct blocking enzyme linked immunosorbent assays (ELISA) for the detection of antibodies to Breda virus in sera of cattle were compared. An ELISA with consecutive addition of antigen and test serum to an antibody-coated plate gave higher positive: negative absorbence ratios than an ELISA in which antigen and test serum were added simultaneously. Sera collected from breeding and fattening herds in The Netherlands (n = 1313) and the F.R.G. (n = 716) were tested, and antibodies to Breda virus were demonstrated in 94% of adult cattle. Ninety percent of newborn calves had high levels of maternal antibodies, which waned until the age of 3 months. Active seroconversion occurred between 7 and 24 months in most animals.

Prevalence of neutralising antibodies to Berne virus in animals and humans in Vellore, South India. Brief report

In Southern India the prevalence of neutralising antibody to Berne virus was high in sera obtained from cattle (49%), horses (38%), and sheep (36%). Neutralising antibody was not detected in sera from humans and monkeys.

Studies on bovine Breda virus

Diarrheic feces from 21 calves were examined by electron microscopy and 16 contained particles morphologically similar to those of Breda virus. The particles were spherical or elongated, 60-270 nm in greatest dimension and had surface spikes 9-13 nm long. Convalescent serum from a human patient with Breda virus-associated diarrhea reacted with one of the bovine viruses by immune electron microscopy, suggesting a serological resemblance between human and bovine Breda-like viruses. Immune electron-microscopy and immunofluorescence demonstrated that isolates of bovine Breda virus from the U.S.A. were related to the French virus. One of the viruses had a density in sucrose solution of 1.16, similar to the value for Berne virus.

Detection of Breda virus antigen and antibody in humans and animals by enzyme immunoassay

Enzyme immunoassays were developed for the detection of Breda virus antibody and antigen. Cattle sera collected in the United Kingdom were found to have a high prevalence of antibody (55%) to Breda virus when examined in a competitive enzyme-linked immunosorbent assay. A low prevalence of antibody was found in pigs (2.2%), and no antibody was found in sheep or goat sera. No antibody to either Breda virus or Berne virus was detected in human sera collected from veterinarians and farm workers. Only 1 of 430 human fecal specimens (0.2%) contained Breda virus antigen detectable by enzyme-linked immunosorbent assay.

Resistance of Berne virus to physical and chemical treatment

Thermal inactivation of Berne virus proceeded at a linear rate between 31 degrees and 43 degrees C. Storage at temperatures lower than -20 degrees C preserved the infectivity, while at 4 degrees C appreciable loss occurred between 92 and 185 days. Freeze-drying or desiccation at 22 degrees C caused only insignificant loss of infectivity. Virus preparations were not affected by pH values between 2.5 and 10.3. Inactivation by UV occurred more rapidly than with herpes, toga and rhabdoviruses. Berne virus infectivity was sensitive to pronase and B. subtilis proteinase. It was not inactivated by trypsin and chymotrypsin treatment, which resulted in enhancement of infectivity; low concentrations of pronase (less than 10 micrograms ml-1) had a similar effect on Berne virus. Neither phospholipase C or RNase, alone or in combination, nor sodium deoxycholate (0.1%) inactivated the virus; in contrast, Triton X-100 (0.1%; 1.0%) caused rapid inactivation with a constant level of residual infectivity.