Analysis of foot-and-mouth disease virus replication using strand-specific quantitative RT-PCR

A highly discriminatory RNA strand-specific assay to facilitate analysis of the role of cis-acting elements in foot-and-mouth disease virus replication

Foot-and-mouth-disease virus (FMDV), the aetiological agent responsible for foot-and-mouth disease (FMD), is a member of the genus Aphthovirus within the family Picornavirus. In common with all picornaviruses, replication of the single-stranded positive-sense RNA genome involves synthesis of a negative-sense complementary strand that serves as a template for the synthesis of multiple positive-sense progeny strands. We have previously employed FMDV replicons to examine viral RNA and protein elements essential to replication, but the factors affecting differential strand production remain unknown. Replicon-based systems require transfection of high levels of RNA, which can overload sensitive techniques such as quantitative PCR, preventing discrimination of specific strands. Here, we describe a method in which replicating RNA is labelled in vivo with 5-ethynyl uridine. The modified base is then linked to a biotin tag using click chemistry, facilitating purification of newly synthesised viral genomes or anti-genomes from input RNA. This selected RNA can then be amplified by strand-specific quantitative PCR, thus enabling investigation of the consequences of defined mutations on the relative synthesis of negative-sense intermediate and positive-strand progeny RNAs. We apply this new approach to investigate the consequence of mutation of viral cis-acting replication elements and provide direct evidence for their roles in negative-strand synthesis.

Infection and protection responses of deletion mutants of non-structural proteins of foot-and-mouth disease virus serotype Asia1 in guinea pigs

The development of a negative marker vaccine against the foot-and-mouth disease virus (FMDV) will enhance the capabilities to differentiate vaccinated from infected animals and move forward in the progressive control pathway for the control of FMD. Here, we report the development of mutant FMDV of Asia1 with partial deletion of non-structural proteins 3A and 3B and characterization of their infectivity and protection response in the guinea pig model. The deleted FMDV Asia1/IND/63/1972 mutants, pAsia^Δ3A and pAsia^Δ3A3B1 were constructed from the full-length infectious clone pAsia^WT, the viable virus was rescued, and the genetic stability of the mutants was confirmed by 20 monolayer passages in BHK21 cells. The mutant Asia1 viruses showed comparable growth pattern and infectivity with that of Asia^WT in the cell culture. However, the Asia^Δ3A3B1 virus showed smaller plaque and lower virus titer with reduced infectivity in the suckling mice. In guinea pigs, the Asia^Δ3A3B1 virus failed to induce the disease, whereas the Asia^Δ3A virus induced typical secondary lesions of FMD. Vaccination with inactivated Asia1 mutant viruses induced neutralizing antibody response that was significantly lower than that of the parent virus on day 28 post-vaccination (dpv) in guinea pigs (P < 0.05). Furthermore, challenging the vaccinated guinea pigs with the homologous vaccine strain of FMDV Asia1 conferred complete protection. It is concluded that the mutant Asia^Δ3A3B1 virus has the potential to replace the wild-type virus for use as a negative marker vaccine after assessing the vaccine worth attributes in suspension cell and protective efficacy study in cattle.Key points• Deletion mutant viruses of FMDV Asia1, developed by PCR-mediated mutagenesis of NSP 3A and 3B1, were genetically stable.• The growth kinetics and antigenic relatedness of the mutant viruses were comparable with that of the wild-type virus.• Vaccination of guinea pigs with the deletion mutant viruses conferred complete protection upon challenge with the homologous virus.

Design, development, and validation of a strand-specific RT-qPCR assay for GI and GII human Noroviruses

Human noroviruses (HuNoV) are the major cause of viral gastroenteritis worldwide. Similar to other positive-sense single-stranded RNA viruses, norovirus RNA replication requires the formation of a negative strand RNA intermediate. Methods for detecting and quantifying the viral positive or negative sense RNA in infected cells and tissues can be used as important tools in dissecting virus replication. In this study, we have established a sensitive and strand-specific Taqman-based quantitative polymerase chain reaction (qPCR) assay for both genogroups GI and GII HuNoV. This assay shows good reproducibility, has a broad dynamic range and is able to detect a diverse range of isolates. We used tagged primers containing a non-viral sequence for the reverse transcription (RT) reaction and targeted this tag in the succeeding qPCR reaction to achieve strand specificity. The specificity of the assay was confirmed by the detection of specific viral RNA strands in the presence of high levels of the opposing strands, in both RT and qPCR reactions. Finally, we further validated the assay in norovirus replicon-bearing cell lines and norovirus-infected human small intestinal organoids, in the presence or absence of small-molecule inhibitors. Overall, we have established a strand-specific qPCR assay that can be used as a reliable method to understand the molecular details of the human norovirus life cycle.

Virulence beneath the fleece; a tale of foot-and-mouth disease virus pathogenesis in sheep

Foot-and-mouth disease virus (FMDV) is capable of infecting all cloven-hoofed domestic livestock species, including cattle, pigs, goats, and sheep. However, in contrast to cattle and pigs, the pathogenesis of FMDV in small ruminants has been incompletely elucidated. The objective of the current investigation was to characterize tissue- and cellular tropism of early and late stages of FMDV infection in sheep following three different routes of simulated natural virus exposure. Extensive post-mortem harvest of tissue samples at pre-determined time points during early infection (24 and 48 hours post infection) demonstrated that tissues specifically susceptible to primary FMDV infection included the paraepiglottic- and palatine tonsils, as well as the nasopharyngeal mucosa. Additionally, experimental aerosol inoculation of sheep led to substantial virus replication in the lungs at 24-48 hours post-inoculation. During persistent infection (35 days post infection), the paraepiglottic- and palatine tonsils were the only tissues from which infectious FMDV was recovered. This is strikingly different from cattle, in which persistent FMDV infection has consistently been located to the nasopharyngeal mucosa. Analysis of tissue sections by immunomicroscopy revealed a strict epithelial tropism during both early and late phases of infection as FMDV was consistently localized to cytokeratin-expressing epithelial cells. This study expands upon previous knowledge of FMDV pathogenesis in sheep by providing detailed information on the temporo-anatomic distribution of FMDV in ovine tissues. Findings are discussed in relation to similar investigations previously performed in cattle and pigs, highlighting similarities and differences in FMDV pathogenesis across natural host species.

New Viruses from the Ectoparasite Mite Varroa destructor Infesting Apis mellifera and Apis cerana

Varroa destructor is an ectoparasitic mite of Asian or Eastern honeybees Apis cerana(A. cerana) which has become a serious threat to European subspecies of Western honeybees Apis mellifera (A. mellifera) within the last century. V.destructor and its vectored honeybee viruses became serious threats for colony survival. This is a short period for pathogen- and host-populations to adapt. To look for possible variation in the composition of viral populations we performed RNA metagenomic analysis of the Western honeybee subspecies A. m. ligustica, A. m.syriaca, A. m. intermissa, and A. cerana and their respective V. destructor mites. The analysis revealed two novel viruses: Varroa orthomyxovirus-1 (VOV-1) in A. mellifera and V. destructor and a Hubei like-virga virus-14 homolog in V. destructor. VOV-1 was more prevalent in V. destructor than in A. mellifera and we found evidence for viral replication in both hosts. Interestingly, we found differences in viral loads of A. cerana and their V. destructor, A. m. intermissa, and its V. destructor showed partial similarity, while A. m.ligustica and A. m.syriaca and their varroa where very similar. Deformed wing virus exhibited 82.20%, 99.20%, 97.90%, and 0.76% of total viral reads in A. m. ligustica, A. m. syriaca, A. m. intermissa, and A. cerana, respectively. This is the first report of a complete segmented-single-stranded negative-sense RNA virus genome in honeybees and V. destructor mites.

Presence of Apis Rhabdovirus-1 in Populations of Pollinators and Their Parasites from Two Continents

The viral ecology of bee communities is complex, where viruses are readily shared among co-foraging bee species. Additionally, in honey bees (Apis mellifera), many viruses are transmitted - and their impacts exacerbated - by the parasitic Varroa destructor mite. Thus far, the viruses found to be shared across bee species and transmitted by V. destructor mites are positive-sense single-stranded RNA viruses. Recently, a negative-sense RNA enveloped virus, Apis rhabdovirus-1 (ARV-1), was found in A. mellifera honey bees in Africa, Europe, and islands in the Pacific. Here, we describe the identification - using a metagenomics approach - of ARV-1 in two bee species (A. mellifera and Bombus impatiens) and in V. destructor mites from populations collected in the United States and Israel. We confirmed the presence of ARV-1 in pools of A. mellifera, B. impatiens, and V. destructor from Israeli and U.S. populations by RT-PCR and found that it can reach high titers in individual honey bees and mites (107-108 viral genomic copies per individual). To estimate the prevalence of ARV-1 in honey bee populations, we screened 104 honey bee colonies across Israel, with 21 testing ARV-1-positive. Tagged-primer-mediated RT-PCR analysis detected the presence of the positive-sense ARV-1 RNA in A. mellifera and V. destructor, indicating that ARV-1 replicates in both hosts. This is the first report of the presence of ARV-1 in B. impatiens and of the replication of a rhabdovirus in A. mellifera and V. destructor. Our data suggest that Varroa mites could act as an ARV-1 vector; however, the presence of ARV-1 in B. impatiens (which are not parasitized by Varroa) suggests that it may not require the mite for transmission and ARV-1 may be shared among co-foraging bee species. Given that ARV-1 is found in non-Apis bee species, and because "ARV" is used for the Adelaide River virus, we propose that this virus should be called bee rhabdovirus 1 and abbreviated BRV-1. These results greatly expand our understanding of the diversity of viruses that can infect bee communities, though further analysis is required to determine how infection with this virus impacts these different hosts.

Dengue virus replicates and accumulates in Aedes aegypti salivary glands

Dengue virus (DENV) is an RNA virus transmitted among humans by mosquito vectors, mainly Aedes aegypti. DENV transmission requires viral dissemination from the mosquito midgut to the salivary glands. During this process the virus undergoes several population bottlenecks, which are stochastic reductions in population size that restrict intra-host viral genetic diversity and limit the efficiency of natural selection. Despite the implications for virus transmission and evolution, DENV replication in salivary glands has not been directly demonstrated. Here, we used a strand-specific quantitative RT-PCR assay to demonstrate that negative-strand DENV RNA is produced in Ae. aegypti salivary glands, providing conclusive evidence that viral replication occurs in this tissue. Furthermore, we showed that the concentration of DENV genomic RNA in salivary glands increases significantly over time, indicating that active replication likely replenishes DENV genetic diversity prior to transmission. These findings improve our understanding of the biological determinants of DENV fitness and evolution.

Biological function of Foot-and-mouth disease virus non-structural proteins and non-coding elements

Foot-and-mouth disease virus (FMDV) represses host translation machinery, blocks protein secretion, and cleaves cellular proteins associated with signal transduction and the innate immune response to infection. Non-structural proteins (NSPs) and non-coding elements (NCEs) of FMDV play a critical role in these biological processes. The FMDV virion consists of capsid and nucleic acid. The virus genome is a positive single stranded RNA and encodes a single long open reading frame (ORF) flanked by a long structured 5ʹ-untranslated region (5ʹ-UTR) and a short 3ʹ-UTR. The ORF is translated into a polypeptide chain and processed into four structural proteins (VP1, VP2, VP3, and VP4), 10 NSPs (L^pro, 2A, 2B, 2C, 3A, 3B_1–3, 3C^pro, and 3D^pol), and some cleavage intermediates. In the past decade, an increasing number of studies have begun to focus on the molecular pathogenesis of FMDV NSPs and NCEs. This review collected recent research progress on the biological functions of these NSPs and NCEs on the replication and host cellular regulation of FMDV to understand the molecular mechanism of host–FMDV interactions and provide perspectives for antiviral strategy and development of novel vaccines.

Partial deletion of stem-loop 2 in the 3' untranslated region of foot-and-mouth disease virus identifies a region that is dispensable for virus replication

The 3' untranslated region (3' UTR) of the foot-and-mouth disease virus (FMDV) genome plays an essential role in virus replication, but the properties of the 3' UTR are not completely defined. In order to determine the role of different regions of the 3' UTR in FMDV replication, we conducted site-directed mutagenesis of the 3' UTR of FMDV serotype O IND R2/1975 using a cDNA clone. Through independent serial deletions in various regions of the 3' UTR, we demonstrated that deletion of nucleotides between the stem-loop (SL) structures and in the beginning and the end regions of the SL2 structure could be lethal for FMDV replication. However, a block deletion of 20 nucleotides (nt 60 to 79) in the middle of SL2 did not affect the viability of FMDV in cultured cells. Characterisation of the deletion mutant virus (O(R2/1975-Δ3'UTR 60-79)) revealed no significant difference in growth kinetics or RNA replication ability compared to the parental virus. However, the mutant virus produced slightly larger plaques when compared to the parental virus. This is the first description of a dispensable 20-nucleotide region in SL2 of the FMDV 3' UTR.

Strand-Specific Quantitative Reverse Transcription-Polymerase Chain Reaction Assay for Measurement of Arenavirus Genomic and Antigenomic RNAs

Arenaviruses are bi-segmented, single-stranded RNA viruses that cause significant human disease. The manner in which they regulate the replication of their genome is not well-understood. This is partly due to the absence of a highly sensitive assay to measure individual species of arenavirus replicative RNAs. To overcome this obstacle, we designed a quantitative reverse transcription (RT)-PCR assay for selective quantitation of each of the lymphocytic choriomeningitis virus (LCMV) genomic or antigenomic RNAs. During the course of assay design, we identified a nonspecific priming phenomenon whereby, in the absence of an RT primer, cDNAs complementary to each of the LCMV replicative RNA species are generated during RT. We successfully circumvented this nonspecific priming event through the use of biotinylated primers in the RT reaction, which permitted affinity purification of primer-specific cDNAs using streptavidin-coated magnetic beads. As proof of principle, we used the assay to map the dynamics of LCMV replication at acute and persistent time points and to determine the quantities of genomic and antigenomic RNAs that are incorporated into LCMV particles. This assay can be adapted to measure total S or L segment-derived viral RNAs and therefore represents a highly sensitive diagnostic platform to screen for LCMV infection in rodent and human tissue samples and can also be used to quantify virus-cell attachment.

Development of a strand-specific real-time qRT-PCR for the accurate detection and quantitation of West Nile virus RNA

Studying the tropism and replication kinetics of West Nile virus (WNV) in different cell types in vitro and in tissues in animal models is important for understanding its pathogenesis. As detection of the negative strand viral RNA is a more reliable indicator of active replication for single-stranded positive-sense RNA viruses, the specificity of qRT-PCR assays currently used for the detection of WNV positive and negative strand RNA was reassessed. It was shown that self- and falsely-primed cDNA was generated during the reverse transcription step in an assay employing unmodified primers and several reverse transcriptases. As a result, a qRT-PCR assay using the thermostable rTth in combination with tagged primers was developed, which greatly improved strand specificity by circumventing the events of self- and false-priming. The reliability of the assay was then addressed in vitro using BV-2 microglia cells as well as in C57/BL6 mice. It was possible to follow the kinetics of positive and negative-strand RNA synthesis both in vitro and in vivo; however, the sensitivity of the assay will need to be optimized in order to detect and quantify negative-strand RNA synthesis in the very early stages of infection. Overall, the strand-specific qRT-PCR assay developed in this study is an effective tool to quantify WNV RNA, reassess viral replication, and study tropism of WNV in the context of WNV pathogenesis.

Development of a strand specific real-time RT-qPCR assay for the detection and quantitation of murine norovirus RNA

Murine norovirus (MNV), currently the only norovirus that efficiently replicates in cell culture, is often used as a model system to understand the molecular mechanisms of norovirus replication. MNV is a single stranded positive sense RNA virus of the Caliciviridae family. Replication of MNV involves the synthesis of both full length genomic and sub-genomic RNAs. The replication of these RNAs involves the synthesis of negative strand intermediates. To understand the molecular mechanism of RNA replication and the role of viral and host factors in virus replication, it is necessary to quantify accurately both positive and negative sense RNA molecules of the viral RNA during replication. Increasingly, strand specific reverse transcription-quantitative PCR (RT-qPCR) is becoming the method of choice for this kind of quantitation. Many strategies have been developed to avoid the false priming property of reverse transcriptase and to amplify specifically one strand in the presence of excess opposite strand. In this report, a SYBR based, real time RT-qPCR assay was developed to detect and quantify specifically the negative and the positive sense RNAs of MNV genomic RNA. This assay is based on using a tagged RT primer containing a non-viral sequence at the 5' end of the viral strand specific sequence. This non-viral sequence is then used to amplify selectively the strand specific cDNA at the PCR stage. This assay can be used for a range of MNV strains including MNV-1 and 3, as these are now widely accepted for use in molecular studies. The specificity of this assay was determined by its ability to quantify one strand in the presence of up to 10(6) copies of competitor opposite sense RNA. Using this assay, the production of both strands of MNV-1 RNA was monitored during viral single step growth curve.

A replication analysis of foot-and-mouth disease virus in swine lymphoid tissue might indicate a putative carrier stage in pigs

Foot-and-mouth disease virus (FMVD), one of the most contagious viruses of cloven-hoofed animals, may cause a prolonged, asymptomatic but persistent infection in ruminants, named the "carrier state". However, it remains an open question whether this carrier state occurs in pigs. Here we present quantitative analyses of the duration of FMDV RNA and infectivity in lymphoid and epithelial tissues in experimentally infected pigs with FMDV C-S8c1. The data indicated that although FMDV RNA remained in blood until day 14 post-infection (pi), viremia was cleared by day 7 pi. However, all tissues tested were positive for FMDV until day 14-17 pi. Interestingly, the specific infectivity of FMDV in these tissues was in some cases even higher than the FMDV C-S8c1. We therefore propose that a "pseudopersistent state" may occur in pigs in which virus replicates in lymphoid tissues for a prolonged period of time, thereby representing a potential source of virus.

Recovery of infectious foot-and-mouth disease virus from full-length genomic cDNA clones using an RNA polymerase I system

The prototypic foot-and-mouth disease virus (FMDV) was shown more than a century ago to be the first filterable agent capable of causing FMD, and it has served as an important model for studying basic principles of Aphthovirus molecular biology. However, the complex structure and antigenic diversity of FMDV have posed a major obstacle to the attempts at manipulating the infectious virus by reverse genetic techniques. Here, we report the recovery of infectious FMDV from cDNAs based on an efficient in vivo RNA polymerase I (polI) transcription system. Intracellular transcription of the full-length viral genome from polI-based vectors resulted in efficient formation of infectious virus displaying a genetic marker. Compared with wild-type virus, an abundance of genomic mRNA and elevated expression levels of viral antigens were indicative of the hyperfunction throughout the life-cycle of this cDNA-derived virus at transcription, replication, and translation levels. The technology described here could be an extremely valuable molecular biology tool for studying FMDV complex infectious characteristics. It is an operating platform for studying FMDV functional genomics, molecular mechanism of pathogenicity and variation, and lays a solid foundation for the development of viral chimeras toward the prospect of a genetically engineered vaccine.

New mathematical approaches to quantify human infectious viruses from environmental media using integrated cell culture-qPCR

Quantifying infectious viruses by cell culture depends on visualizing cytopathic effect, or for integrated cell culture-PCR, attaining confidence a PCR-positive signal is the result of virus growth and not inoculum carryover. This study developed mathematical methods to calculate infectious virus numbers based on viral growth kinetics in cell culture. Poliovirus was inoculated into BGM cell monolayers at 10 concentrations from 0.001 to 1000 PFU/ml. Copy numbers of negative-strand RNA, a marker of infectivity for single-stranded positive RNA viruses, were measured over time by qRT-PCR. Growth data were analyzed by two approaches. First, data were fit with a continuous function to estimate directly the initial virus number, expressed as genomic copies. Such estimates correlated with actual inoculum numbers across all concentrations (R(2)=0.62, n=17). Second, the length of lag phase appeared to vary inversely with inoculum titers; hence, standard curves to predict inoculum virus numbers were derived based on three definitions of lag time: (1) time of first detection of (-)RNA, (2) second derivative maximum of the fitted continuous function, and (3) time when the fitted curve crossed a threshold (-)RNA concentration. All three proxies yielded standard curves with R(2)=0.69-0.90 (n=17). The primary advantage of these growth kinetics approaches is being able to quantify virions that are unambiguously infectious, a particular advantage for viruses that do not produce CPE.

Development and validation of a duplex quantitative real-time RT-PCR assay for simultaneous detection and quantitation of foot-and-mouth disease viral positive-stranded RNAs and negative-stranded RNAs

A simplified, cost-effective, two-step duplex quantitative real-time RT-PCR assay was shown to detect and quantify foot-and-mouth disease virus positive-stranded RNAs and negative-stranded RNAs simultaneously for improved investigation of the state of virus infection and replication. The primers and Taqman probes were selected from the coding regions of 2B gene and 3D gene respectively, which have the least variations among serotypes. Cells infected acutely, tissue samples and single cell samples were used for evaluation of the assay. At the early stages of virus infection in vitro, the replication level reached a peak at 9h.p.i. and the negative strands were detectable until 3h.p.i. The kinetics of ratios of positive strands to negative strands (+RNA/-RNA) in vivo in the liver, kidney and spleen were similar, which demonstrated that the replication dynamics were similar in the three organs. 55 single cell samples out of 187 were positive by both positive strands qPCR and negative strands qPCR, the ratios (+RNA/-RNA) ranged from 15.6 to 1463.4 which showed considerable difference among single cell samples, indicating that active viral replication differs greatly in single cells. A duplex quantitative real-time RT-PCR was validated as effective and reliable.

A novel single-cell quantitative real-time RT-PCR method for quantifying foot-and-mouth disease viral RNA

Foot-and-mouth disease virus is a positive-sense, single-stranded RNA virus with a negative strand as its replication intermediate, which can cause severe acute infection in sensitive cell lines. To investigate better the actual state of virus infection, there is a need to measure the amount of FMDV RNA in a single acutely infected cell rather than in a large number of cells. Therefore, in the present study, a strand-specific single-cell quantitative real-time RT-PCR was developed to analyze the RNA or FMDV. This new method uses two techniques in concert with each other: a technique for isolating single cells with micromanipulators, which is coupled to an assay for detecting viral RNA by real-time RT-PCR. In the assay of acute infection, 185 of 224 (82.6%) single-cell samples were positive and contained viral genome copies ranging from several to thousands, and up to 1,000,000 copies. However, not all cells were infected and there were differences in the number of viral RNA copies between cells. A single-cell quantitative RT-PCR was validated to be feasible and effective.

Understanding the molecular epidemiology of foot-and-mouth-disease virus

The use of molecular epidemiology is an important tool in understanding and consequently controlling FMDV. In this review I will present basic information about the disease, needed to perform molecular epidemiology. I will give a short introduction to the history and impact of foot-and-mouth disease, clinical picture, infection route, subclinical and persistent infections, general aspects of the transmission of FMDV, serotype-specific epidemiological characteristics, field epidemiology of FMDV, evolution and molecular epidemiology of FMDV. This is followed by two chapters describing the molecular epidemiology of foot-and-mouth disease in global surveillance and molecular epidemiology of foot-and-mouth disease in outbreak investigation.

Development of a Taqman RT-PCR assay for the detection and quantification of negatively stranded RNA of human enteroviruses: evidence for false-priming and improvement by tagged RT-PCR

Human enteroviruses are among the most common viruses infecting humans. These viruses are known to be able to infect a wide range of tissues and are believed to establish persistent infections. Enteroviruses are positive-sense single-stranded RNA viruses whose replication involves the synthesis of negative strand intermediates. Therefore, the specific detection of negatively stranded viral RNA in tissues or cells is a reliable marker of active enteroviral replication. The present report presents the development of a real-time RT-PCR allowing the specific detection and quantification of negatively stranded viral RNA. Since it was known that specific amplification of single-stranded RNA can be made difficult by false-priming events leading to false-positive or overestimated results, the assay was developed by using a tagged RT primer. This tagged RT-PCR was shown to be able to amplify specifically negative RNA of enteroviruses grown in cell cultures by preventing the amplification of cDNAs generated by false-priming.

Foot-and-mouth disease virus infection in fetal lambs: tissue tropism and cytokine response

Foot-and-mouth disease virus (FMDV) can cause transplacental infection and death in fetal lambs. This study investigates the pathogenesis of FMDV infection in ovine fetuses using in-situ hybridization (ISH) to detect viral transcripts in tissue and real-time reverse transcriptase polymerase chain reaction (RT-PCR) assays to quantify the fetal cytokine response to infection. FMDV ribonucleic acid (RNA) was localized mainly to the heart and skeletal muscles of fetuses and was only occasionally expressed in the lingual epithelium, demonstrating that FMDV has a different tissue tropism in the fetus compared with that in adult sheep. There was early expression of genes encoding anti-viral cytokines (IFN-alpha and IFN-beta) in fetuses at 2 and 4 days post-infection (dpi), followed by a marked rise in the transcription of pro-inflammatory cytokine genes (IFN-gamma, TNF-alpha and IL-1alpha) from 7 to 18 dpi, particularly in the heart. The degree of cytokine mRNA expression correlated with fetal infection and was likely to be a factor in fetal death. In contrast, cytokine gene expression in infected neonatal lambs was much less and mainly occurred between 2 and 4 dpi. This study identifies two key factors in the pathogenicity of FMDV in fetal lambs: viral tropism for cardiac and skeletal muscles, and a marked cytokine response following infection.