Bovine rhinitis viruses are common in U.S. cattle with bovine respiratory disease

Bovine rhinitis viruses (BRV) are established etiological agents of bovine respiratory disease complex however little research into their epidemiology and ecology has been published for several decades. In the U.S., only bovine rhinitis A virus 1 (BRAV1) has been identified while bovine rhinitis A virus 2 (BRAV2) and bovine rhinitis B virus (BRBV) were previously only identified in England and Japan, respectively. Metagenomic sequencing of a nasal swab from a bovine respiratory disease (BRD) diagnostic submission from Kansas identified contigs with approximately 90% nucleotide similarity to BRAV2 and BRBV. A combination of de novo and templated assemblies using reference genomes yielded near complete BRAV2 and BRBV genomes. The near complete genome of bovine rhinitis A virus 1 (BRAV1) was also determined from a historical isolate to enable further molecular epidemiological studies. A 5’-nuclease reverse transcription PCR assay targeting the 3D polymerase gene was designed and used to screen 204 archived BRD clinical specimens. Thirteen (6.4%) were positive. Metagenomic sequencing of six positive samples identified mixed BRAV1/BRAV2, BRAV1/BRBV and BRAV2/BRBV infections for five samples. One sample showed infection only with BRAV1. Seroprevalence studies using a cell culture adapted BRBV found immunofluorescence assay-reactive antibodies were common in the herds analyzed. Altogether, these results demonstrate that BRV infections are common in cattle with respiratory disease and that BRAV1, BRAV2 and BRBV co-circulate in U.S. cattle and have high similarity to viruses isolated more than 30 years ago from diverse locations.

Equine rhinitis A virus and its low pH empty particle: clues towards an aphthovirus entry mechanism?

Equine rhinitis A virus (ERAV) is closely related to foot-and-mouth disease virus (FMDV), belonging to the genus Aphthovirus of the Picornaviridae. How picornaviruses introduce their RNA genome into the cytoplasm of the host cell to initiate replication is unclear since they have no lipid envelope to facilitate fusion with cellular membranes. It has been thought that the dissociation of the FMDV particle into pentameric subunits at acidic pH is the mechanism for genome release during cell entry, but this raises the problem of how transfer across the endosome membrane of the genome might be facilitated. In contrast, most other picornaviruses form ‘altered’ particle intermediates (not reported for aphthoviruses) thought to induce membrane pores through which the genome can be transferred. Here we show that ERAV, like FMDV, dissociates into pentamers at mildly acidic pH but demonstrate that dissociation is preceded by the transient formation of empty 80S particles which have released their genome and may represent novel biologically relevant intermediates in the aphthovirus cell entry process. The crystal structures of the native ERAV virus and a low pH form have been determined via highly efficient crystallization and data collection strategies, required due to low virus yields. ERAV is closely similar to FMDV for VP2, VP3 and part of VP4 but VP1 diverges, to give a particle with a pitted surface, as seen in cardioviruses. The low pH particle has internal structure consistent with it representing a pre-dissociation cell entry intermediate. These results suggest a unified mechanism of picornavirus cell entry.

Picornaviruses are small animal viruses comprising an RNA genome protected by a roughly spherical protein shell with icosahedral symmetry. How the RNA is introduced into the cytoplasm of the host cell to initiate replication is unclear since they have no lipid envelope to facilitate fusion with cellular membranes. Instead, they become internalized into endocytic vesicles whence the viral genome must be delivered through the vesicle membrane, into the cytoplasm. In some picornaviruses (enteroviruses), genome delivery is proposed to be coordinated by an intact particle inducing pore formation in the membrane through which the genome can be transferred directly without exposure to the hostile vesicle environment. In contrast, other picornaviruses (aphthoviruses e.g. ERAV, FMDV) present a dilemma by appearing to simply fall apart in acidified vesicles. Here we show that acid treatment results in the formation of an intact but transient aphthovirus empty particle from which the genome has been released. We have determined the crystal structures of the ERAV particle at native and acidic pH. The acid induced structure is consistent with a destabilized particle en-route to disassembly. We propose that the entry process for this group of viruses involves externalisation of the RNA from a novel capsid intermediate and unifies in principle the entry process for all picornaviruses.

Recombination patterns in aphthoviruses mirror those found in other picornaviruses

Foot-and-mouth disease virus (FMDV) is thought to evolve largely through genetic drift driven by the inherently error-prone nature of its RNA polymerase. There is, however, increasing evidence that recombination is an important mechanism in the evolution of these and other related picornoviruses. Here, we use an extensive set of recombination detection methods to identify 86 unique potential recombination events among 125 publicly available FMDV complete genome sequences. The large number of events detected between members of different serotypes suggests that horizontal flow of sequences among the serotypes is relatively common and does not incur severe fitness costs. Interestingly, the distribution of recombination breakpoints was found to be largely nonrandom. Whereas there are clear breakpoint cold spots within the structural genes, two statistically significant hot spots precisely separate these from the nonstructural genes. Very similar breakpoint distributions were found for other picornovirus species in the genera Enterovirus and Teschovirus. Our results suggest that genome regions encoding the structural proteins of both FMDV and other picornaviruses are functionally interchangeable modules, supporting recent proposals that the structural and nonstructural coding regions of the picornaviruses are evolving largely independently of one another.

Sialic acid acts as a receptor for equine rhinitis A virus binding and infection

Equine rhinitis A virus (ERAV) is a member of the genus Aphthovirus, family Picornaviridae, and causes respiratory disease in horses worldwide. To characterize the putative receptor molecule(s) of the ERAV isolate 393/76 (ERAV.393/76) on the surface of Vero and other cells, an assay was developed to measure the binding of purified biotinylated ERAV.393/76 virions to cells by flow cytometry. Using this assay, the level of binding to different cell types correlated with the relative infectivity of ERAV in each cell type. In particular, equine fetal kidney cells, mouse fibroblast cells, rabbit kidney-13 and Crandell feline kidney cells bound virus at high levels and produced high virus yields (⩾107 TCID50 ml−1). Madin–Darby bovine kidney and baby hamster kidney cells showed little or no binding of virus, producing yields of ⩽101·8 TCID50 ml−1. Treatment of Vero and other cells with sodium periodate and the metabolic inhibitors tunicamycin, benzyl N-acetyl-α-d-galactosamide, d,l-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol and proteases indicated that part of the receptor-binding and entry complex for ERAV.393/76 is on N-linked carbohydrates and that the carbohydrate is likely to be present on a protein rather than a lipid backbone. The effect of carbohydrate-specific lectins and neuraminidases on ERAV.393/76 binding and infection of Vero and other cell types implicated α2,3-linked sialic acid residues on the carbohydrate complex in the binding and infection of ERAV.

Foot-and-mouth disease virus: cause of the recent crisis for the UK livestock industry

The recent outbreak of foot-and-mouth disease (FMD) in the United Kingdom is a stark reminder of the economic devastation that this disease can wreak. Tracing the origin of such an outbreak is an essential part of disease control. Modern molecular methods have been in place for a number of years to enable scientists to identify unambiguously the strain of virus responsible. However, tracing the precise origin of such a strain is not so straightforward because the virus can move rapidly around the world with legal and illegal trade in animals and animal products. This short review describes the virus, its control and epidemiology.

Efficient accommodation of recombinant, foot-and-mouth disease virus RGD peptides to cell-surface integrins

The engineering of either complete virus cell-binding proteins or derived ligand peptides generates promising nonviral vectors for cell targeting and gene therapy. In this work, we have explored the molecular interaction between a recombinant, integrin-binding foot-and-mouth disease virus RGD peptide displayed on the surface of a carrier protein and its receptors on the cell surface. By increasing the number of viral segments, cell binding to recombinant proteins was significantly improved. This fact resulted in a dramatic growth stimulation of virus-sensitive BHK(21) cells but not virus-resistant HeLa cells in protein-coated wells. Surprisingly, growth stimulation was not observed in vitronectin-coated plates, suggesting that integrins other than alpha(v)beta(3) could be involved in binding of the recombinant peptide, maybe as coreceptors. On the other hand, both free and cell-linked integrins did not modify the enzymatic activity of RGD-based enzymatic sensors that contrarily, were activated by the induced fit of anti-RGD antibodies. Those findings are discussed in the context of a proper mimicry of the unusually complex architecture of this cell-binding site as engineered in multifunctional proteins.

Inhibition of L-deleted foot-and-mouth disease virus replication by alpha/beta interferon involves double-stranded RNA-dependent protein kinase

We previously demonstrated that the ability of foot-and-mouth disease virus (FMDV) to form plaques in cell culture is associated with the suppression of alpha/beta interferon (IFN-alpha/beta). In the present study, we used Escherichia coli-expressed porcine and bovine IFN-alpha or -beta individually to demonstrate that each was equally effective in inhibiting FMDV replication. The block in FMDV replication appeared to be at the level of protein translation, suggesting a role for double-stranded RNA-dependent protein kinase (PKR). In support of these findings, treatment of porcine and bovine cells with 2-aminopurine, an inhibitor of PKR, increased the yield of virus 8.8- and 11.2-fold, respectively, compared to that in untreated infected cells. In addition, results of FMDV infection in mouse embryonic fibroblast cells derived from gene knockout mice lacking the gene for RNase L(-/-) or PKR(-/-) or both indicated an important role for PKR in the inhibition of FMDV replication.

The foot-and-mouth epidemic in Great Britain: pattern of spread and impact of interventions

We present an analysis of the current foot-and-mouth disease epidemic in Great Britain over the first 2 months of the spread of the virus. The net transmission potential of the pathogen and the increasing impact of control measures are estimated over the course of the epidemic to date. These results are used to parameterize a mathematical model of disease transmission that captures the differing spatial contact patterns between farms before and after the imposition of movement restrictions. The model is used to make predictions of future incidence and to simulate the impact of additional control strategies. Hastening the slaughter of animals with suspected infection is predicted to slow the epidemic, but more drastic action, such as "ring" culling or vaccination around infection foci, is necessary for more rapid control. Culling is predicted to be more effective than vaccination.

Role of the cytoplasmic domain of the beta-subunit of integrin alpha(v)beta6 in infection by foot-and-mouth disease virus

Field isolates of foot-and-mouth disease virus (FMDV) are believed to use RGD-dependent integrins as cellular receptors in vivo. Using SW480 cell transfectants, we have recently established that one such integrin, alpha(v)beta6, functions as a receptor for FMDV. This integrin was shown to function as a receptor for virus attachment. However, it was not known if the alpha(v)beta6 receptor itself participated in the events that follow virus binding to the host cell. In the present study, we investigated the effects of various deletion mutations in the beta6 cytoplasmic domain on infection. Our results show that although loss of the beta6 cytoplasmic domain has little effect on virus binding, this domain is essential for infection, indicating a critical role in postattachment events. The importance of endosomal acidification in alpha(v)beta6-mediated infection was confirmed by experiments showing that infection could be blocked by concanamycin A, a specific inhibitor of the vacuolar ATPase.

A single amino acid substitution in nonstructural protein 3A can mediate adaptation of foot-and-mouth disease virus to the guinea pig

The genetic changes selected during the adaptation of a clonal population of foot-and-mouth disease virus (FMDV) to the guinea pig have been analyzed. FMDV clone C-S8c1 was adapted to the guinea pig by serial passage in the animals until secondary lesions were observed. Analysis of the virus directly recovered from the lesions developed by the animals revealed the selection of variants with two amino acid substitutions in nonstructural proteins, I(248)-->T in 2C and Q(44)-->R in 3A. On further passages, an additional mutation, L(147)-->P, was selected in an important antigenic site located in the G-H loop of capsid protein VP1. The amino acid substitution Q(44)-->R in 3A, either alone or in combination with the replacement I(248)-->T in 2C, was sufficient to give FMDV the ability to produce lesions. This was shown by using infectious transcripts which generated chimeric viruses with the relevant amino acid substitutions. Clinical symptoms produced by the artificial chimeras were similar to those produced by the naturally adapted virus. These results obtained with FMDV imply that one or very few replacements in nonstructural viral proteins, which should be within reach of the mutant spectra of replicating viral quasispecies, may result in adaptation of a virus to a new animal host.

Development of a novel real-time RT-PCR assay for quantitation of foot-and-mouth disease virus in diverse porcine tissues

Pigs are more difficult to immunise and more variable in their response to foot-and-mouth disease (FMD) than other livestock species. This has important consequences for FMD control during both prophylactic vaccination programmes in endemic situations and when emergency vaccination may be used as an adjunct to stamping out during outbreaks in countries normally free from the disease. The rapid and effective control of FMD in pigs is especially important in regions of high pig density since infected pigs have the potential to generate plumes of airborne virus and spread infection beyond the immediate control area. Increased knowledge of the kinetics of FMDV replication in pigs, especially in their respiratory tracts, could create opportunities for strategies to improve FMD vaccines for pigs. A fluorogenic TaqMan RT-PCR assay specific for the IRES (internal ribosome entry site) sequence of the O(1) Kaufbeuren/Lausanne strain of FMDV has been developed for this purpose. The assay had a sensitivity of 0.1 TCID(50)/ml, and a dynamic range of at least eight orders of magnitude. It was found that an established method of quantitation, relative to a housekeeping gene, exhibited two significant shortcomings when applied to a set of 16 anatomically highly diverse solid tissues: (i) tissue differences in housekeeping gene expression caused errors of up to 60-fold in estimated FMDV concentrations; and (ii) variability in total RNA yields caused unpredictable saturation of RT reactions, which in turn caused errors of up to 250-fold in the estimated FMDV concentration. A novel quantitation strategy, designated the C(t)(min) method, was developed to overcome these problems. The C(t)(min) method was based on the the RT-PCR examination of a dilution series of spectrophotometrically quantitated total RNA, spanning the optimum for the RT-PCR system used. The new method was influenced minimally by any tissue-specific RT-PCR inhibitors and was used to determine FMDV concentrations in tissues from four experimentally infected pigs. The results suggest that the lungs play a less important role in the replication of FMDV in pigs than was thought previously.

Viral capsid mobility: a dynamic conduit for inactivation

Mass spectrometry and fluorescent probes have provided direct evidence that alkylating agents permeate the protein capsid of naked viruses and chemically inactivate the nucleic acid. N-acetyl-aziridine and a fluorescent alkylating agent, dansyl sulfonate aziridine, inactivated three different viruses, flock house virus, human rhinovirus-14, and foot and mouth disease virus. Mass spectral studies as well as fluorescent probes showed that alkylation of the genome was the mechanism of inactivation. Because particle integrity was not affected by selective alkylation (as shown by electron microscopy and sucrose gradient experiments), it was reasoned that the dynamic nature of the viral capsid acts as a conduit to the interior of the particle. Potential applications include fluorescent labeling for imaging viral genomes in living cells, the sterilization of blood products, vaccine development, and viral inactivation in vivo.

Emergence in Asia of foot-and-mouth disease viruses with altered host range: characterization of alterations in the 3A protein

In 1997, an epizootic in Taiwan, Province of China, was caused by a type O foot-and-mouth disease virus which infected pigs but not cattle. The virus had an altered 3A protein, which harbored a 10-amino-acid deletion and a series of substitutions. Here we show that this deletion is present in the earliest type O virus examined from the region (from 1970), whereas substitutions surrounding the deletion accumulated over the last 29 years. Analyses of the growth of these viruses in bovine cells suggest that changes in the genome in addition to the deletion, per se, are responsible for the porcinophilic properties of current Asian viruses in this lineage.

The ability of integrin alpha(v)beta(3) To function as a receptor for foot-and-mouth disease virus is not dependent on the presence of complete subunit cytoplasmic domains

The integrin alpha(v)beta(3) has been shown to function as one of the integrin receptors on cultured cells for foot-and-mouth disease virus (FMDV), and high-efficiency utilization of the bovine homolog of this integrin is dependent on the cysteine-rich repeat region of the bovine beta(3) subunit. In this study we have examined the role of the cytoplasmic domains of the alpha(v) and beta(3) subunits in FMDV infection. We have found that truncations or extensions of these domains of either subunit, including deletions removing almost all of the cytoplasmic domains, had little or no effect on the ability of the integrin to function as a receptor for FMDV. The lysosomotropic agent monensin inhibited viral replication in cells transfected with either intact or cytoplasmic domain-truncated alpha(v)beta(3). In addition, viral replication in transfected cells was inhibited by an alpha(v)beta(3) function-blocking antibody but not by function-blocking antibodies to three other RGD-directed integrins, suggesting that these integrins are not involved in the infectious process. These results indicate that alterations to the cytoplasmic domains of either subunit, which lead to the inability of the integrin receptor to function normally, do not abolish the ability of the integrin to bind and internalize this viral ligand.

A novel protein-RNA binding assay: functional interactions of the foot-and-mouth disease virus internal ribosome entry site with cellular proteins

Translation initiation on foot-and-mouth disease virus (FMDV) RNA occurs by a cap-independent mechanism directed by a highly structured element (approximately 435 nt) termed an internal ribosome entry site (IRES). A functional assay to identify proteins that bind to the FMDV IRES and are necessary for FMDV IRES-mediated translation initiation has been developed. In vitro-transcribed polyadenylated RNAs corresponding to the whole or part of the FMDV IRES were immobilized on oligo-dT Dynabeads and used to deplete rabbit reticulocyte lysate (RRL) of IRES-binding proteins. Translation initiation factors eIF4G, eIF4A, and eIF4B bound to the 3' domain of the FMDV IRES. Depletion of eIF4G from RRL by this region of the FMDV IRES correlated with the loss of translational capacity of the RRL for capped, uncapped, and FMDV IRES-dependent mRNAs. However, this depleted RRL still supported hepatitis C virus IRES-directed translation. Poly (rC) binding protein-2 bound to the central domain of the FMDV IRES, but depletion of RRL with this IRES domain had no effect on FMDV IRES-directed translation initiation.

Internal ribosomal entry site-mediated translation initiation in equine rhinitis A virus: similarities to and differences from that of foot-and-mouth disease virus

Equine rhinitis A virus (ERAV) has recently been classified as an aphthovirus, a genus otherwise comprised of the different serotypes of Foot-and-mouth disease virus (FMDV). FMDV initiates translation via a type II internal ribosomal entry site (IRES) and utilizes two in-frame AUG codons to produce the leader proteinases Lab and Lb. Here we show that the ERAV 5' nontranslated region also possesses the core structures of a type II IRES. The functional activity of this region was characterized by transfection of bicistronic plasmids into BHK-21 cells. In this system the core type II structures, stem-loops D to L, in addition to a stem-loop (termed M) downstream of the first putative initiation codon, are required for translation of the second reporter gene. In FMDV, translation of Lb is more efficient than that of Lab despite the downstream location of the Lb AUG codon. The ERAV genome also has putative initiation sites in positions similar to those utilized in FMDV, except that in ERAV these are present as two AUG pairs (AUGAUG). Using the bicistronic expression system, we detected initiation from both AUG pairs, although in contrast to FMDV, the first site is strongly favored over the second. Mutational analysis of the AUG codons indicated that AUG2 is the major initiation site, although AUG1 can be accessed, albeit inefficiently, in the absence of AUG2. Further mutational analysis indicated that codons downstream of AUG2 appear to be accessed by a mechanism other than leaky scanning. Furthermore, we present preliminary evidence that it is possible for ribosomes to access downstream of the two AUG pairs. This study reveals important differences in IRES function between aphthoviruses.

High-efficiency utilization of the bovine integrin alpha(v)beta(3) as a receptor for foot-and-mouth disease virus is dependent on the bovine beta(3) subunit

We have previously reported that Foot-and-mouth disease virus (FMDV), which is virulent for cattle and swine, can utilize the integrin alpha(v)beta(3) as a receptor on cultured cells. Since those studies were performed with the human integrin, we have molecularly cloned the bovine homolog of the integrin alpha(v)beta(3) and have compared the two receptors for utilization by FMDV. Both the alpha(v) and beta(3) subunits of the bovine integrin have high degrees of amino acid sequence similarity to their corresponding human subunits in the ectodomains (96%) and essentially identical transmembrane and cytoplasmic domains. Within the putative ligand-binding domains, the bovine and human alpha(v) subunits have a 98.8% amino acid sequence similarity while there is only a 93% similarity between the beta(3) subunits of these two species. COS cell cultures, which are not susceptible to FMDV infection, become susceptible if cotransfected with alpha(v) and beta(3) subunit cDNAs from a bovine or human source. Cultures cotransfected with the bovine alpha(v)beta(3) subunit cDNAs and infected with FMDV synthesize greater amounts of viral proteins than do infected cultures cotransfected with the human integrin subunits. Cells cotransfected with a bovine alpha(v) subunit and a human beta(3) subunit synthesize viral proteins at levels equivalent to those in cells expressing both human subunits. However, cells cotransfected with the human alpha(v) and the bovine beta(3) subunits synthesize amounts of viral proteins equivalent to those in cells expressing both bovine subunits, indicating that the bovine beta(3) subunit is responsible for the increased effectiveness of this receptor. By engineering chimeric bovine-human beta(3) subunits, we have shown that this increase in receptor efficiency is due to sequences encoding the C-terminal one-third of the subunit ectodomain, which contains a highly structured cysteine-rich repeat region. We postulate that amino acid sequence differences within this region may be responsible for structural differences between the human and bovine beta(3) subunit, leading to more efficient utilization of the bovine receptor by this bovine pathogen.

Caspases are not involved in the cleavage of translation initiation factor eIF4GI during picornavirus infection

Infection of cells by many picornaviruses results in the rapid inhibition of cellular protein synthesis due to cleavage of the translation initiation factor eIF4G. The poliovirus (PV) 2A and foot-and-mouth disease virus (FMDV) L proteases are each sufficient to mediate this cleavage, but the cleavage mechanism may be indirect, involving an unidentified cellular protease(s). eIF4G is also targetted for cleavage by caspase-3 during apoptosis. Here, it is shown that caspase inhibitors do not inhibit the cleavage of eIF4GI during PV or FMDV infection. Similarly, in transient-expression studies, the cleavage of eIF4GI induced by PV 2A or FMDV L was unaffected by these inhibitors. Furthermore, the cleavage of eIF4GI was observed in PV-infected MCF-7 cells lacking caspase-3. These data, and the fact that induction of apoptosis yields different eIF4GI cleavage fragments, indicate that caspases do not have a major role in the cleavage of eIF4GI during PV or FMDV infection.

Foot-and-mouth disease virus is a ligand for the high-affinity binding conformation of integrin alpha5beta1: influence of the leucine residue within the RGDL motif on selectivity of integrin binding

Field isolates of foot-and-mouth disease virus (FMDV) use RGD-dependent integrins as receptors for internalization, whereas strains that are adapted for growth in cultured cell lines appear to be able to use alternative receptors like heparan sulphate proteoglycans (HSPG). The ligand-binding potential of integrins is regulated by changes in the conformation of their ectodomains and the ligand-binding state would be expected to be an important determinant of tropism for viruses that use integrins as cellular receptors. Currently, alphavbeta3 is the only integrin that has been shown to act as a receptor for FMDV. In this study, a solid-phase receptor-binding assay has been used to characterize the binding of FMDV to purified preparations of the human integrin alpha5beta1, in the absence of HSPG and other RGD-binding integrins. In this assay, binding of FMDV resembled authentic ligand binding to alpha5beta1 in its dependence on divalent cations and specific inhibition by RGD peptides. Most importantly, binding was found to be critically dependent on the conformation of the integrin, as virus bound only after induction of the high-affinity ligand-binding state. In addition, the identity of the amino acid residue immediately following the RGD motif is shown to influence differentially the ability of FMDV to bind integrins alpha5beta1 and alphavbeta3 and evidence is provided that alpha5beta1 might be an important FMDV receptor in vivo.

Memory in viral quasispecies

Biological adaptive systems share some common features: variation among their constituent elements and continuity of core information. Some of them, such as the immune system, are endowed with memory of past events. In this study we provide direct evidence that evolving viral quasispecies possess a molecular memory in the form of minority components that populate their mutant spectra. The experiments have involved foot-and-mouth disease virus populations with known evolutionary histories. The composition and behavior of the viral population in response to a selective constraint were influenced by past evolutionary history in a way that could not be predicted from examination of consensus nucleotide sequences of the viral populations. The molecular memory of the viral quasispecies influenced both the nature and the intensity of the response of the virus to a selective constraint.

Cell recognition by foot-and-mouth disease virus that lacks the RGD integrin-binding motif: flexibility in aphthovirus receptor usage

Cell surface molecules that can act as virus receptors may exert an important selective pressure on RNA viral quasispecies. Large population passages of foot-and-mouth disease virus (FMDV) in cell culture select for mutant viruses that render dispensable a highly conserved Arg-Gly-Asp (RGD) motif responsible for integrin receptor recognition. Here, we provide evidence that viability of recombinant FMDVs including a Asp-143-->Gly change at the RGD motif was conditioned by a number of capsid substitutions selected upon FMDV evolution in cell culture. Multiply passaged FMDVs acquired the ability to infect human K-562 cells, which do not express integrin alpha(v)beta(3). In contrast to previously described cell culture-adapted FMDVs, the RGD-independent infection did not require binding to the surface glycosaminoglycan heparan sulfate (HS). Viruses which do not bind HS and lack the RGD integrin-binding motif replicate efficiently in BHK-21 cells. Interestingly, FMDV mutants selected from the quasispecies for the inability to bind heparin regained sensitivity to inhibition by a synthetic peptide that represents the G-H loop of VP1. Thus, a single amino acid replacement leading to loss of HS recognition can shift preferential receptor usage of FMDV from HS to integrin. These results indicate at least three different mechanisms for cell recognition by FMDV and suggest a potential for this virus to use multiple, alternative receptors for entry even into the same cell type.

Ability of foot-and-mouth disease virus to form plaques in cell culture is associated with suppression of alpha/beta interferon

A genetic variant of foot-and-mouth disease virus lacking the leader proteinase coding region (A12-LLV2) is attenuated in both cattle and swine and, in contrast to wild-type virus (A12-IC), does not spread from the initial site of infection after aerosol exposure of bovines. We have identified secondary cells from susceptible animals, i.e., bovine, ovine, and porcine animals, in which infection with A12-LLV2, in contrast to A12-IC infection, does not produce plaques; this result indicates that this virus cannot spread from the site of initial infection to neighboring cells. Nevertheless, A12-LLV2 can infect these cells, but cytopathic effects and virus yields are significantly reduced compared to those seen with A12-IC infection. Reverse transcription-PCR analysis demonstrates that both A12-LLV2 and A12-IC induce the production of alpha/beta interferon (IFN-alpha/beta) mRNA in host cells. However, only supernatants from A12-LLV2-infected cells have significant antiviral activity. The antiviral activity in supernatants from A12-LLV2-infected embryonic bovine kidney cells is IFN-alpha/beta specific, as assayed with mouse embryonic fibroblast cells with or without IFN-alpha/beta receptors. The results obtained with cell cultures demonstrate that the ability of A12-IC to form plaques is associated with the suppression of IFN-alpha/beta expression and suggest a role for this host factor in the inability of A12-LLV2 to spread and cause disease in susceptible animals.

Development of DNA vaccines for foot-and-mouth disease, evaluation of vaccines encoding replicating and non-replicating nucleic acids in swine

We have developed naked DNA vaccine candidates for foot-and-mouth disease (FMD), an important disease of domestic animals. The virus that causes this disease, FMDV, is a member of the picornavirus family, which includes many important human pathogens, such as poliovirus, hepatitis A virus, and rhinovirus. Picornaviruses are characterized by a small (7-9000 nucleotide) RNA genome that encodes capsid proteins, processing proteinases, and enzymes required for RNA replication. We have developed two different types of DNA vaccines for FMD. The first DNA vaccine, pP12X3C, encodes the viral capsid gene (P1) and the processing proteinase (3C). Cells transfected with this DNA produce processed viral antigen, and animals inoculated with this DNA using a gene gun produced detectable antiviral immune responses. Mouse inoculations with this plasmid, and with a derivative containing a mutation in the 3C proteinase, indicated that capsid assembly was essential for induction of neutralizing antibody responses. The second DNA vaccine candidate, pWRMHX, encodes the entire FMDV genome, including the RNA-dependent RNA polymerase, permitting the plasmid-encoded viral genomes to undergo amplification in susceptible cells. pWRMHX encodes a mutation at the cell binding site, preventing the replicated genomes from causing disease. Swine inoculated with this vaccine candidate produce viral particles lacking the cell binding site, and neutralizing antibodies that recognize the virus. Comparison of the immune responses elicited by pP12X3C and pWRMHX in swine indicate that the plasmid encoding the replicating genome stimulated a stronger immune response, and swine inoculated with pWRMHX by the intramuscular, intradermal, or gene gun routes were partially protected from a highly virulent FMD challenge.

Evidence for the role of His-142 of protein 1C in the acid-induced disassembly of foot-and-mouth disease virus capsids

Foot-and-mouth disease virus (FMDV) capsids are inherently labile under mildly acidic conditions, dissociating to pentamers at pH values in the region of 6.5, with the release of protein 1A and the viral RNA. This acid-induced disassembly is thought to be required for the entry of the virus genome into the host cell. Previous work has highlighted a histidine-alpha-helix charge-dipole interaction at the twofold axes of symmetry between pentamers and has suggested that this interaction plays a role in acid-induced disassembly. The validity of this theory has now been tested by converting the implicated residue, His-142 of protein 1C, to Arg, Phe and Asp. The effects of such changes were studied by using a previously described vaccinia virus expression system, in which synthesis and processing of FMDV capsid proteins results in the self-assembly of capsids. In agreement with the histidine-alpha-helix charge-dipole theory, assembly in the arginine mutant was found to be greatly reduced, while capsids of the aspartic acid mutant were considerably more stable under acidic conditions than the wild-type. Aberrant but acid-stable complexes were obtained in the phenylalanine mutant.

Emergency vaccination of sheep against foot-and-mouth disease: protection against disease and reduction in contact transmission

The ability of several emergency FMD vaccine formulations to elicit early protective immunity in sheep was examined. All vaccine formulations were shown to protect sheep against airborne challenge with homologous FMDV within 4 days of vaccination. Protection was associated in part with the induction of serum antibody responses but was also demonstrated in the absence of any detectable antibody response at the time of challenge. Aqueous Al(OH)3/saponin vaccine formulations and oil emulsion vaccines based on Montanide ISA 206 adjuvant reduced virus replication and the numbers of animals subclinically infected up to 28 days post-challenge, when compared with non-vaccinated animals, consequently limiting transmission of the disease or infection to in-contact susceptible animals.

Multiple molecular pathways for fitness recovery of an RNA virus debilitated by operation of Muller's ratchet

Repeated bottleneck passages of RNA viruses result in fitness losses due to accumulation of deleterious mutations. We have analysed the molecular events underlying fitness recovery of a highly debilitated foot- and-mouth disease virus (FMDV) clone, upon serial passage in BHK-21 cells. The debilitated clone included an unusual, internal polyadenylate extension preceding the second functional AUG initiation codon, and a number of additional mutations scattered throughout the genome. Comparison of entire genomic nucleotide sequences in the course of passaging documented that loss of the internal polyadenylate was the first event in the process of fitness recovery. Further increases of fitness were associated with very few true reversions and with the accumulation of additional mutations affecting non-coding and coding regions. Remarkably, four biological subclones of the same debilitated FMDV clone gained fitness through three separate molecular pathways regarding correction of the internal polyadenylate: (i) a true reversion to yield the wild-type sequence at the second functional AUG; (ii) a shortening of the internal polyadenylate tract; or (iii) a deletion of 69 residues spanning the site of the polyadenylate extension. The results document that an RNA virus can find multiple pathways to reach alternative high fitness peaks on the fitness landscape.

[Foot-and-mouth disease of cattle is not a zoonosis]

In 1997 there was an outbreak of foot-and-mouth disease (FMD) among cattle in Turkey. People visiting that country were warned against importing animal products into the Netherlands. This had nothing to do with hazards to human health, as FMD virus is not a zoonotic virus, but with the risk of spread of the disease to livestock in the Netherlands, notably to cattle and pigs. A disease with similar clinical symptoms in pigs is swine vesicular disease (SVD), which is not a zoonosis either. FMD virus is an aphtovirus, SVD virus is an enterovirus. Hand-foot-and-mouth disease in humans is caused by other enteroviruses, i.e. Coxsackie virus and enterovirus 71.

An RNA virus can adapt to the multiplicity of infection

RNA viruses evolve as complex distributions of mutants termed viral quasispecies. For this reason it is relevant to explore those environmental parameters that favour the selective advantage of some viral subpopulations over others. In the present study we provide direct evidence that the relative fitness of two competing viral subpopulations may depend on the multiplicity of infection (m.o.i.). Two closely related subpopulations of foot-and-mouth disease virus (FMDV) of serotype C, which differed in their history of cytolytic passages in BHK-21 cells, were subjected to growth-competition experiments in BHK-21 cells. One of the populations, termed S, was found to have a selective advantage over the other population, termed L, only when the competition passages were carried out at low m.o.i. In contrast, both populations, L and S, coexisted during serial passages carried out at high m.o.i. No differences between S and L were detected in assays of inhibition of infectivity by synthetic peptides, in cell binding-competition experiments, or in virulence for BHK-21 cells. However, FMDV S displayed increased heparin binding compared with L, and L higher virulence for Chinese hamster ovary (CHO) cells than S. These results with FMDV suggest that small differences in the interaction of the virus with the host cell may contribute to an m.o.i.-dependent selective advantage of one viral subpopulation over a closely related subpopulation. Therefore, different viral mutants from quasispecies replicating in vivo may be selected depending on the number of variant viruses relative to the number of susceptible cells.

Rapid selection in modified BHK-21 cells of a foot-and-mouth disease virus variant showing alterations in cell tropism

With persistent foot-and-mouth disease virus (FMDV) in BHK-21 cells, there is coevolution of the cells and the resident virus; the virulence of the virus for the parental BHK-21 cells is gradually increased, and the cells become partially resistant to FMDV. Here we report that variants of FMDV C3Arg/85 were selected in a single infection of partially resistant BHK-21 cells (termed BHK-Rb cells). Indirect immunofluorescence showed that the BHK-Rb cell population was heterogeneous with regard to susceptibility to C3Arg/85 infection. Infection of BHK-Rb cells with C3Arg/85 resulted in an early phase of partial cytopathology which was followed at 6 to 10 days postinfection by the shedding of mutant FMDVs, termed C3-Rb. The selected C3-Rb variants showed increased virulence for BHK-21 cells, were able to overcome the resistance of modified BHK-21 cells to infection, and had acquired the ability to bind heparin and to infect wild-type Chinese hamster ovary (CHO) cells. A comparison of the genomic sequences of the parental and modified viruses revealed only two amino acid differences, located at the surface of the particle, at the fivefold axis of the viral capsid (Asp-9-->Ala in VP3 and either Gly-110-->Arg or His-108-->Arg in VP1). The same phenotypic and genotypic modifications occurred in a highly reproducible manner; they were seen in a number of independent infections of BHK-Rb cells with viral preparation C3Arg/85 or with clones derived from it. Neither amino acid substitutions in other structural or nonstructural proteins nor nucleotide substitutions in regulatory regions were found. These results prove that infection of partially permissive cells can promote the rapid selection of virus variants that show alterations in cell tropism and are highly virulent for the same cells.

In vivo analysis of the stability and fitness of variants recovered from foot-and-mouth disease virus quasispecies

We have analysed the ability to infect pigs of two foot-and-mouth disease virus (FMDV) variants isolated at low frequencies from virus populations (quasispecies) generated in pigs on infection with a parental virus, C-S8c1. A monoclonal antibody-resistant mutant (MARM21), and a variant isolated at early times post-infection (S-3T1), each exhibiting a unique amino acid substitution in VP1, were able to cause disease in pigs, both by direct inoculation or by contact transmission. The symptoms developed were similar to those produced by C-S8c1 or the related virus C-S15c1. The VP1 sequence of viral RNA directly recovered from lesions of infected animals confirmed the stability of the variant genotypes. Pigs infected with S-3T1 consistently showed an advance of 12 to 24 h in the emergence of fever and lesions when compared to animals infected with C-S8c1 or the remaining variants, an observation consistent with its early isolation. The ability of FMDV variants to compete in vivo with C-S8c1 was investigated in co-infection experiments. Analysis of the proportion of each of the competitors in lesions of co-infected pigs revealed that none of the variants was completely overgrown by the parent. However, co-infection with C-S8c1 and MARM21 resulted in lesions in which C-S8c1 was predominant, indicating a selective disadvantage of this variant in swine. In contrast, lesions from swine co-infected with C-S8c1 and S-3T1 contained similar proportions of the two viruses. These results document fitness variations in vivo among components of the mutant spectrum of FMDV quasispecies.

[Molecular basis of changes in biological properties of foot and mouth disease virus of subtype A22]

Primary structure of capsid proteins and RNA polymerase of three closely related strains of foot and mouth disease virus (FMDV), subtype A22, differing by biological properties (the initial epitheliotropic strain A22 550 and its derivatives: thermoresistant myotropic A22 550/4 and thermosensitive attenuated A22 645) are compared by nucleic acid sequencing and analysis of the amino acid sequencing. The study revealed 1 substitute in VPI and 8 in RNA polymerase in the myotropic variant and 1 substitute in VP2, 2 in VP3, 13 in VP1, and 3 in RNA polymerase. Alteration of A22 550/4 tropism is probably due to a single substitution Gly 145-->Thr in the RGD site of capsid protein VP1. Analysis of the origin and biological properties of the attenuated strain A22 645 and the results of studies of the primary structure of proteins permit us to hypothesize that attenuation is polygenic, caused by adaptation to a heterologous host (continuous porcine cell culture), and can be expressed by changes in the structure of virus antireceptor providing its binding to cell receptors. Sites responsible for the reproduction of A22 FMDV at certain temperatures are presumably located in RNA polymerase.

Inhibition of foot and mouth disease virus (FMDV) uncoating by a plant-derived peptide isolated from Melia azedarach L leaves

Meliacine (MA), a peptide isolated from leaves of the high plant Melia azedarach L inhibited the multiplication of foot and mouth disease virus (FMDV) in BHK-21 cells. In this report, we establish that the MA-inhibitable process takes place within the first hour of the viral reproductive cycle. MA had no virucidal effect and did not affect adsorption and penetration of the virus in cells. In experiments with neutral red-labeled virus, it was found that MA significantly suppressed the development of photoresistance of the virus in infected cells. In untreated cultures nearly all virus which adsorbed to cells was uncoated within 1 h at 37 degrees C, whereas in treated cultures, even after 3 h only 3% of the virus was uncoated. Labeling of BHK-21 cells with acridine orange showed that MA affects the pH of intracellular acidic vesicles. Therefore, it is concluded that MA prevents the process of uncoating of FMDV in BHK-21 cells by inhibiting vacuolar acidification.

Foot-and-mouth disease virus virulent for cattle utilizes the integrin alpha(v)beta3 as its receptor

Adsorption and plaque formation of foot-and-mouth disease virus (FMDV) serotype A12 are inhibited by antibodies to the integrin alpha(v)beta3 (A. Berinstein et al., J. Virol. 69:2664-2666, 1995). A human cell line, K562, which does not normally express alpha(v)beta3 cannot replicate this serotype unless cells are transfected with cDNAs encoding this integrin (K562-alpha(v)beta3 cells). In contrast, we found that a tissue culture-propagated FMDV, type O1BFS, was able to replicate in nontransfected K562 cells, and replication was not inhibited by antibodies to the endogenously expressed integrin alpha5beta1. A recent report indicating that cell surface heparan sulfate (HS) was required for efficient infection of type O1 (T. Jackson et al., J. Virol. 70:5282-5287, 1996) led us to examine the role of HS and alpha(v)beta3 in FMDV infection. We transfected normal CHO cells, which express HS but not alpha(v)beta3, and two HS-deficient CHO cell lines with cDNAs encoding human alpha(v)beta3, producing a panel of cells that expressed one or both receptors. In these cells, type A12 replication was dependent on expression of alpha(v)beta3, whereas type O1BFS replicated to high titer in normal CHO cells but could not replicate in HS-deficient cells even when they expressed alpha(v)beta3. We have also analyzed two genetically engineered variants of type O1Campos, vCRM4, which has greatly reduced virulence in cattle and can bind to heparin-Sepharose columns, and vCRM8, which is highly virulent in cattle and cannot bind to heparin-Sepharose. vCRM4 replicated in wild-type K562 cells and normal, nontransfected CHO (HS+ alpha(v)beta3-) cells, whereas vCRM8 replicated only in K562 and CHO cells transfected with alpha(v)beta3 cDNAs. A similar result was also obtained in assays using a vCRM4 virus with an engineered RGD-->KGE mutation. These results indicate that virulent FMDV utilizes the alpha(v)beta3 integrin as a primary receptor for infection and that adaptation of type O1 virus to cell culture results in the ability of the virus to utilize HS as a receptor and a concomitant loss of virulence.

Dissecting the roles of VP0 cleavage and RNA packaging in picornavirus capsid stabilization: the structure of empty capsids of foot-and-mouth disease virus

Empty capsids of foot-and-mouth disease virus (FMDV) type A22 Iraq 24/64, whose structure has been solved by X-ray crystallography, are unusual for picornaviruses since they contain VP2 and VP4, the cleavage products of the protein precursor VP0. Both the N terminus of VP1 and the C terminus of VP4, which pack together close to the icosahedral threefold symmetry axis where three pentamers associate, are more disordered in the empty capsid than they are in the RNA-containing virus. The ordering of these termini in the presence of RNA strengthens interactions within a single protomer and between protomers belonging to different pentamers. The disorder in the FMDV empty capsid forms a subset of that seen in the poliovirus empty capsid, which has VP0 intact. Thus, VP0 cleavage confers stability on the picornavirus capsid over and above that attributable to RNA encapsidation. In both FMDV and poliovirus empty capsids, the internal disordering uncovers a conserved histidine which has been proposed to be involved in the cleavage of VP0. A comparison of the putative active sites in FMDV and poliovirus suggests a structural explanation for the sequence specificity of the cleavage reaction.

Plasmid DNA encoding replicating foot-and-mouth disease virus genomes induces antiviral immune responses in swine

DNA vaccine candidates for foot-and-mouth disease (FMD) were engineered to produce FMD virus (FMDV) particles that were noninfectious in cell culture or animals. The prototype plasmid, pWRM, contains a cytomegalovirus immediate-early promoter-driven genome-length type A12 cDNA followed by the bovine growth hormone polyadenylation site. BHK cells transfected with this plasmid produced virus, but the specific infectivity of pWRM was much lower than that achieved with in vitro-generated RNA genomes. To improve the infectivity of the plasmid, a cDNA encoding the hepatitis delta virus ribozyme was added to the 3' end of the FMDV cDNA. The resulting plasmid, pWRMH, exhibited slightly increased infectivity in cell culture and produced virus when inoculated into suckling mice. A third plasmid, pWRMHX, was created by removal of the sequences encoding the cell binding site found in capsid protein VP1 of pWRMH. Although cells transfected with pWRMHX produced viral capsids, this plasmid was not lethal in suckling mice, indicating that particles lacking the cell binding site were not able to initiate secondary infectious cycles. Swine inoculated with pWRMHX did not show any signs of disease and produced neutralizing antibodies to FMDV, and 20% of the vaccinated animals were protected from challenge. A derivative of pWRMHX, pWRMHX-pol-, harboring a mutation designed to inactivate the viral polymerase was much less immunogenic, indicating that immunogenicity of pWRMHX resulted, in part, from amplification of the viral genome in the animal.

Foot-and-mouth disease virus and poliovirus particles contain proteins of the replication complex

Nonstructural proteins 2C, 3CD, 3C, and 3D, and the cellular protein actin, are present in highly purified preparations of foot-and-mouth disease virus (FMDV) and poliovirus. They remain bound in variable amounts to the RNAs when the RNAs are extracted from the viruses with phenol or phenol-sodium dodecyl sulfate (SDS) and, for FMDV, when the RNA is released from the particles by a lowering of the pH below 7. RNA prepared by these methods is rapidly degraded at 37 degrees C, particularly in the presence of NH4+ ions, but hydrolysis can be prevented by antibody against Escherichia coli-expressed 3D, indicating that it is the RNA polymerase that has nuclease activity. In contrast, virion RNA from which the nonstructural proteins and actin have been removed by extraction with guanidine thiocyanate-phenol-chloroform or proteinase K-phenol is stable at 37 degrees C, although its specific infectivity is lower than that of the RNA extracted with phenol or phenol-SDS. The possible implications of the close association of replication complex proteins with the RNA in virus particles are discussed.

Simulated airborne spread of Aujeszky's disease and foot-and-mouth disease

The atmospheric dispersion of virus was simulated using a computer model which had been developed for predicting the dispersion of toxic gases from chemical engineering plants. The results were compared with data from four outbreaks in which virus was believed to have been transported by air: two outbreaks of foot-and-mouth disease in the United Kingdom in 1967 and outbreaks of Aujeszky's disease in Yorkshire in 1981 to 1982 and Indiana in 1988. There was relatively good agreement with most of these data. The paper shows that the model could be useful in an emergency because the risk of virus spread could be predicted in real time.

Evolution subverting essentiality: dispensability of the cell attachment Arg-Gly-Asp motif in multiply passaged foot-and-mouth disease virus

Aphthoviruses use a conserved Arg-Gly-Asp triplet for attachment to host cells and this motif is believed to be essential for virus viability. Here we report that this triplet-which is also a widespread motif involved in cell-to-cell adhesion-can become dispensable upon short-term evolution of the virus harboring it. Foot-and-mouth disease virus (FMDV), which was multiply passaged in cell culture, showed an altered repertoire of antigenic variants resistant to a neutralizing monoclonal antibody. The altered repertoire includes variants with substitutions at the Arg-Gly-Asp motif. Mutants lacking this sequence replicated normally in cell culture and were indistinguishable from the parental virus. Studies with individual FMDV clones indicate that amino acid replacements on the capsid surface located around the loop harboring the Arg-Gly-Asp triplet may mediate in the dispensability of this motif. The results show that FMDV quasispecies evolving in a constant biological environment have the capability of rendering totally dispensable a receptor recognition motif previously invariant, and to ensure an alternative pathway for normal viral replication. Thus, variability of highly conserved motifs, even those that viruses have adapted from functional cellular motifs, can contribute to phenotypic flexibility of RNA viruses in nature.

Contamination of animal products: the minimum pathogen dose required to initiate infection

When an animal product contains a low level of contamination (perhaps less than the minimum infective dose of a pathogen as determined experimentally), the theoretical probability remains that if a large number of animals are exposed to that product, at least one animal in the group will become infected. Such an infected animal could start an outbreak of the disease. These aspects, therefore, should be considered when risk assessments are performed. Foot and mouth disease virus in milk is used as an example.

Characterization of synthetic foot-and-mouth disease virus provirions separates acid-mediated disassembly from infectivity

One of the final steps in the maturation of foot-and-mouth disease virus (FMDV) is cleavage of the VP0 protein to produce VP4 and VP2. The mechanism of this cleavage is unknown, but it is thought to function in stabilizing the virus particle and priming it for infecting cells. To investigate the cleavage process and to understand its role in virion maturation, we engineered synthetic FMDV RNAs with mutations at Ala-85 (A85) and Asp-86 (D86) of VP0, which border the cleavage site. BHK cells transfected with synthetic RNAs containing substitutions at position 85 (A85N or A85H) or at position 86 (D86N) yielded particles indistinguishable from wild-type (WT) virus in sedimentation and electrophoretic profiles. Viruses derived from these transfected cells were infectious and maintained their mutant sequences upon passage. However, BHK cells transfected with synthetic RNAs encoding Phe and Lys at these positions (A85F/D86K) or a Cys at position 86 (D86C) produced noninfectious provirions with uncleaved VP0 molecules. Despite their lack of infectivity, the A85F/D86K provirions displayed cell binding and acid sensitivity similar to those of WT virus. However, acid breakdown products of the A85F/D86K provirions differed in hydrophobicity from the comparable WT virion products, which lack VP4. Taken together, these studies are consistent with a role for soluble VP4 molecules in release of the viral genome from the endosomal compartment of susceptible cells.

Risks of introducing foot and mouth disease through the importation of beef from South America

The safety of beef with respect to foot and mouth disease (FMD) is determined by the level of risk which the exporting region poses through disease prevalence, the reliability of the surveillance system of the region, the efficacy of the prevention and control measures, the efficiency of the Veterinary Services and the support of the private sector. The South American continent has been regionalised in accordance with these criteria. Today there are approximately 90 million cattle in a territory of over 5 million km2 comprising regions classified as having a very low to low level risk for FMD with regard to the export of animals and animal products. Another 50 million cattle live in regions classified as posing a moderate risk. These risk categories reflect varying levels of risk. The harvest of beef in the meat-exporting regions of South America includes a series of risk mitigation measures, from the origin of the source herd to the final packing of the beef. These measures reduce the unrestricted risk estimate by almost six orders of magnitude. Therefore, the final risk of FMD for the global trade of beef originating from the low risk regions in South America is extremely small.

Risks of spreading foot and mouth disease through milk and dairy products

A review of epidemics of foot and mouth disease (FMD) has highlighted the important role which raw (untreated) milk can play in the spread of the disease in a country which is normally free of FMD and whose cattle are not routinely vaccinated. The greatest hazard is likely to be in the early stages of an outbreak, before disease control measures have been implemented. The spread of FMD through milk can be prevented by the effective application of control measures combined with 'codes of practice' for the treatment of potentially infected milk. The author considers the probable mechanisms of transmission of FMD by milk and dairy products. These mechanisms are based on the quantities of virus excreted in milk, the survival of the virus under various management and manufacturing conditions and the minimum doses required to initiate infection in susceptible animals by different routes. The key points for consideration when making a risk assessment of the importation of milk and dairy products are also discussed.

Baculovirus expressed 2C of foot-and-mouth disease virus has the potential for differentiating convalescent from vaccinated animals

Determining whether animals have been infected with foot-and-mouth disease virus or vaccinated is important because infected animals frequently become carriers of the virus, shed it intermittently and thus may be the source of new outbreaks of the disease. We had shown previously that the sera of convalescent animals contain antibodies to 2C, a highly conserved non-structural protein, whereas the sera of vaccinated animals do not. This is explained by observation that 2C is retained on the membranes of cells used for growing the virus for vaccine production. In contrast, the non-structural protein 3D, which is released into the medium, is not removed by centrifugation or filtration during vaccine production and therefore stimulates an immune response in both vaccinated and convalescent cattle. In this study we produced 2C and 3D in insect cells infected with recombinant baculoviruses. As demonstrated by serology and electron microscopy, 2C is also retained on the membranes of the insect cells. Both expressed proteins react with sera of convalescent animals, indicating that they are conformationally similar, but the 2C does not react with sera from vaccinated animals. The baculovirus expressed 2C appears to be a suitable antigen for the development of a reliable diagnostic test.

Point mutations within the betaG-betaH loop of foot-and-mouth disease virus O1K affect virus attachment to target cells

The amino acid sequence Arg-Gly-Asp (RGD) is a highly conserved region located on the P1D protein of most sero- and subtypes of foot-and-mouth disease virus (FMDV)and participates in binding of FMDV to their target cells. In order to analyze the role of the RGD sequence in FMDV infection of cells in more detail, 13 mutations within or near the RGD sequence of virus type O1Kaufbeuren were designed by using a full-length cDNA plasmid. Transfection of baby hamster kidney cells (BHK-21) with in vitro-transcribed cRNAs containing mutations bordering the RGD sequence led to the production of infectious virus in most cases. In contrast, almost all of the mutants containing changes within the RGD sequence produced noninfectious viral particles indistinguishable from wild-type virus by electron microscopy. In order to demonstrate that these noninfectious progeny from the RGD mutants were defective only in their cell adsorption, the respective cRNAs were cotransfected together with a cRNA expressing the wild-type P1 protein. The resulting virus particles were able to infect BHK-21 cells. These results demonstrate the important role of the RGD sequence in FMDV binding to cells but also emphasize the influence of other amino acids in the bordering region.

Infectivity assays of foot-and-mouth disease virus: contact transmission between cattle and buffalo (Bubalus bubalis) in the early stages of infection

No differences were observed between cattle and Indian buffalo (Bubalus bubalis) in terms of temperature, viraemia or virus replication in the pharyngeal area, during the acute phase of foot-and-mouth disease. Like cattle, the Indian buffalo became infected and excreted virus before any clinical signs of foot-and-mouth disease developed. The disease was transmitted from cattle to buffalo and vice versa, during the acute stage of infection, as if the animals had been of the same species, presumably because of their close phylogenetic relationship. There were more tongue lesions in the cattle than in the buffalo. Foot lesions in the buffalo at first had a scaley appearance, but later became vesicular. Anti-virus infection associated antigen and neutralising antibodies were synthesised at the same time in both species and reached similar titres in the same period. Persistent infection in the buffalo during the first 35 days after infection was similar to that in the cattle.

Propagation of an attenuated virus by design: engineering a novel receptor for a noninfectious foot-and-mouth disease virus

To gain entry into cells, viruses utilize a variety of different cell-surface molecules. Foot-and-mouth disease virus (FMDV) binds to cell-surface integrin molecules via an arginine-glycine-aspartic acid (RGD) sequence in capsid protein VP1. Binding to this particular cell-surface molecule influences FMDV tropism, and virus/receptor interactions appear to be responsible, in part, for selection of antigenic variants. To study early events of virus-cell interaction, we engineered an alternative and novel receptor for FMDV. Specifically, we generated a new receptor by fusing a virus-binding, single-chain antibody (scAb) to intracellular adhesion molecule 1 (ICAM1). Cells that are normally not susceptible to FMDV infection became susceptible after being transfected with DNA encoding the scAb/ICAM1 protein. An escape mutant (B2PD.3), derived with the mAb used to generate the genetically engineered receptor, was restricted for growth on the scAb/ICAM1 cells, but a variant of B2PD.3 selected by propagation on scAb/ICAM1 cells grew well on these cells. This variant partially regained wild-type sequence in the epitope recognized by the mAb and also regained the ability to be neutralize by the mAb. Moreover, RGD-deleted virions that are noninfectious in animals and other cell types grew to high titers and were able to form plaques on scAb/ ICAM1 cells. These studies demonstrate the first production of a totally synthetic cell-surface receptor for a virus. This novel approach will be useful for studying virus reception and for the development of safer vaccines against viral pathogens of animals and humans.

Long-term, large-population passage of aphthovirus can generate and amplify defective noninterfering particles deleted in the leader protease gene

During serial undiluted passage of a clonal population of foot-and-mouth disease virus (FMDV C-S8c1) in BHK-21 cells, two species of defective RNA were generated and selected. Sequence analysis revealed that they included deletions within the L-coding region, and retained the correct reading frame for viral protein synthesis. These deleted RNAs directed the synthesis of capsid protein VP1, were packaged in particles sedimenting with standard virus, required homologous infectious helper virus in order to produce viral particles, but did not interfere with the replication of helper virus. While detection of defective particles in FMDV required more than 100 serial passages, once produced, these defective RNAs could be stably maintained upon further passages in the FMDV C-S8c1 quasispecies. Furthermore, a high fitness, monoclonal-antibody-resistant virus was able to replace the standard virus and support the amplification of the deleted particles. This is the first description of naturally occurring, defective particles of FMDV.