Development of reverse transcription-PCR (oligonucleotide probing) enzyme-linked immunosorbent assays for diagnosis and preliminary typing of foot-and-mouth disease: a new system using simple and aqueous-phase hybridization

Combining Multiple Assays Improves Detection and Serotyping of Foot-and-Mouth Disease Virus. A Practical Example with Field Samples from East Africa

Multiple serotypes and topotypes of foot-and-mouth disease virus (FMDV) circulate in endemic areas, posing considerable impacts locally. In addition, introductions into new areas are of great concern. Indeed, in recent years, multiple FMDV outbreaks, caused by topotypes that have escaped from their original areas, have been recorded in various parts of the world. In both cases, rapid and accurate diagnosis, including the identification of the serotype and topotype causing the given outbreaks, plays an important role in the implementation of the most effective and appropriate measures to control the spread of the disease. In the present study, we describe the performance of a range of diagnostic and typing tools for FMDV on a panel of vesicular samples collected in northern Tanzania (East Africa, EA) during 2012–2018. Specifically, we tested these samples with a real-time RT-PCR targeting 3D sequence for pan-FMDV detection; an FMDV monoclonal antibody-based antigen (Ag) detection and serotyping ELISA kit; virus isolation (VI) on LFBKαVβ6 cell line; and a panel of four topotype-specific real-time RT-PCRs, specifically tailored for circulating strains in EA. The 3D real-time RT-PCR showed the highest diagnostic sensitivity, but it lacked typing capacity. Ag-ELISA detected and typed FMDV in 71% of sample homogenates, while VI combined with Ag-ELISA for typing showed an efficiency of 82%. The panel of topotype-specific real-time RT-PCRs identified and typed FMDV in 93% of samples. However, the SAT1 real-time RT-PCR had the highest (20%) failure rate. Briefly, topotype-specific real-time RT-PCRs had the highest serotyping capacity for EA FMDVs, although four assays were required, while the Ag-ELISA, which was less sensitive, was the most user-friendly, hence suitable for any laboratory level. In conclusion, when the four compared tests were used in combination, both the diagnostic and serotyping performances approached 100%.

Development and evaluation of tailored specific real-time RT-PCR assays for detection of foot-and-mouth disease virus serotypes circulating in East Africa

Rapid, reliable and accurate diagnostic methods provide essential support to programmes that monitor and control foot-and-mouth disease (FMD). While pan-specific molecular tests for FMD virus (FMDV) detection are well established and widely used in endemic and FMD-free countries, current serotyping methods mainly rely either on antigen detection ELISAs or nucleotide sequencing approaches. This report describes the development of a panel of serotype-specific real-time RT-PCR assays (rRT-PCR) tailored to detect FMDV lineages currently circulating in East Africa. These assays target sequences within the VP1-coding region that share high intra-lineage identity, but do not cross-react with FMD viruses from other serotypes that circulate in the region. These serotype-specific assays operate with the same thermal profile as the pan-diagnostic tests making it possible to run them in parallel to produce CT values comparable to the pan-diagnostic test detecting the 3D-coding region. These assays were evaluated alongside the established pan-specific molecular test using field samples and virus isolates collected from Tanzania, Kenya and Ethiopia that had been previously characterised by nucleotide sequencing. Samples (n=71) representing serotype A (topotype AFRICA, lineage G-I), serotype O (topotypes EA-2 and EA-4), serotype SAT 1 (topotype I (NWZ)) and serotype SAT2 (topotype IV) were correctly identified with these rRT-PCR assays. Furthermore, FMDV RNA from samples that did not contain infectious virus could still be serotyped using these assays. These serotype-specific real-time RT-PCR assays can detect and characterise FMDVs currently circulating in East Africa and hence improve disease control in this region.

Establishment and validation of two duplex one-step real-time RT-PCR assays for diagnosis of foot-and-mouth disease

Two duplex one-step TaqMan-based RT-PCR protocols for detection of foot-and-mouth disease virus (FMDV) were established and validated. Each RT-PCR test consists of a ready-to-use master mix for simultaneous detection of the well established 3D or IRES FMDV targets and incorporates the host β-actin mRNA as an internal control target, in a single-tube assay. The two real-time RT-PCR 3D/β-actin and IRES/β-actin tests are highly sensitive and able to detect up to 7TCID50/ml of FMDV and 10 copies/1μl of viral RNA. In field epithelium samples, the diagnostic sensitivity was 100% (95% CI; 91-100%) for the 3D/β-actin test and 97% (95% CI; 87-100%) for the IRES/β-actin test. The diagnostic specificity was 100% (95% CI; 95-100%) for both RT-PCRs. In addition, the two protocols proved to be robust, showing inter-assay coefficients of variation ranging from 1.94% to 6.73% for the IRES target and from 2.33% to 5.42% for the 3D target for different RNA extractions and different RT-PCR conditions. The internally controlled one-step real-time RT-PCR protocols described in this study provide a rapid, effective and reliable method for the detection of FMDV and thus may improve the routine diagnosis for foot-and-mouth disease.

Development and Characterization of Probe-Based Real Time Quantitative RT-PCR Assays for Detection and Serotyping of Foot-And-Mouth Disease Viruses Circulating in West Eurasia

Rapid and accurate diagnosis of foot-and-mouth disease (FMD) and virus serotyping are of paramount importance for control of this disease in endemic areas where vaccination is practiced. Ideally this virus characterization should be achieved without the need for virus amplification in cell culture. Due to the heterogeneity of FMD viruses (FMDVs) in different parts of the world, region specific diagnostic tests are required. In this study, hydrolysable probe-based real time reverse transcription quantitative polymerase chain reaction (RT-qPCR) assays were developed for specific detection and serotyping of the FMDVs currently circulating in West Eurasia. These assays were evaluated, in parallel with pan-FMDV diagnostic assays and earlier serotype-specific assays, using field samples originating from Pakistan and Afghanistan containing FMD viruses belonging to different sublineages of O-PanAsia, A-Iran05 and Asia-1 (Group-II and Group-VII (Sindh-08)). In addition, field samples from Iran and Bulgaria, containing FMDVs belonging to the O-PanAsia^ANT-10 sublineage were also tested. Each of the three primer/probe sets was designed to be specific for just one of the serotypes O, A and Asia-1 of FMDV and detected the RNA from the target viruses with cycle threshold (C_T) values comparable with those obtained with the serotype-independent pan-FMDV diagnostic assays. No cross-reactivity was observed in these assays between the heterotypic viruses circulating in the region. The assays reported here have higher diagnostic sensitivity (100% each for serotypes O and Asia-1, and 92% [95% CI = 81.4–100%] for serotype A positive samples) and specificity (100% each for serotypes O, A and Asia-1 positive samples) for the viruses currently circulating in West Eurasia compared to the serotyping assays reported earlier. Comparisons of the sequences of the primers and probes used in these assays and the corresponding regions of the circulating viruses provided explanations for the poor recognition of some of the viruses by the earlier assays. These new assays should help in the early detection and typing of serotype O, A and Asia-1 FMDVs circulating in West Eurasia to enable improved disease control.

Development of a Multiplex Real-Time PCR Assay for the Detection of Treponema pallidum, HCV, HIV-1, and HBV

Treponema pallidum, hepatitis C virus (HCV), human immunodeficiency virus (HIV)-1, and hepatitis B virus (HBV) are major causes of sexually transmitted diseases passed through blood contact. The development of a sensitive and efficient method for detection is critical for early diagnosis and for large-scale screening of blood specimens in China. This study aims to establish an assay to detect these pathogens in clinical serum specimens. We established a TaqMan-locked nucleic acid (LNA) real-time polymerase chain reaction (PCR) assay for rapid, sensitive, specific, quantitative, and simultaneous detection and identification. The copy numbers of standards of these 4 pathogens were quantified. Standard curves were generated by determining the mean cycle threshold values versus 10-fold serial dilutions of standards over a range of 10(6) to 10(1) copies/μL, with the lowest detection limit of the assay being 10(1) copies/μL. The assay was applied to 328 clinical specimens and compared with enzyme-linked immunosorbent assay (ELISA) and commercial nucleic acid testing (NAT) methods. The assay identified 39 T. pallidum-, 96 HCV-, 13 HIV-1-, 123 HBV-, 5 HBV/HCV-, 1 T. pallidum/HBV-, 1 HIV-1/HCV-, and 1 HIV-1/T. pallidum-positive specimens. The high sensitivity of the assay confers strong potential for its use as a highly reliable, cost-effective, and useful molecular diagnostic tool for large-scale screening of clinical specimens. This assay will assist in the study of the pathogenesis and epidemiology of sexually transmitted blood diseases.

Development of tailored real-time RT-PCR assays for the detection and differentiation of serotype O, A and Asia-1 foot-and-mouth disease virus lineages circulating in the Middle East

Rapid and accurate diagnosis is essential for effective control of foot-and-mouth disease (FMD). In countries where FMD is endemic, identification of the serotypes of the causative virus strains is important for vaccine selection and tracing the source of outbreaks. In this study, real-time reverse transcription polymerase chain reaction (rRT-PCR) assays using primer/probe sets designed from the VP1 coding region of the virus genomes were developed for the specific detection of serotype O, A and Asia-1 FMD viruses (FMDVs) circulating in the Middle East. These assays were evaluated using representative field samples of serotype O strains belonging exclusively to the PanAsia-2 lineage, serotype A strains of the Iran-05 lineage and serotype Asia-1 viruses from three relevant sub-groups. When RNA extracted from archival and contemporary field strains was tested using one- or two-step rRT-PCR assays, all three primer/probe sets detected the RNA from homotypic viruses and no cross-reactivity was observed with heterotypic viruses. Similar results were obtained using both single- and multiplex assay formats. Using plasmid standards, the minimum detection level of these tests was found to be lower than two copies. The results illustrate the potential of tailored rRT-PCR tools for the detection and categorization of viruses circulating in the Middle East belonging to distinct subgroups of serotypes O, A and Asia-1. These assays can also overcome the problem of serotyping samples which are found positive by the generic rRT-PCR diagnostic assays but negative by virus isolation and antigen-detection ELISA which would otherwise have to be serotyped by nucleotide sequencing. A similar approach could be used to develop serotyping assays for FMDV strains circulating in other regions of the world.

Development and evaluation of a real-time reverse transcription-loop-mediated isothermal amplification assay for rapid serotyping of foot-and-mouth disease virus

A one-step, real-time reverse transcription-loop-mediated isothermal amplification assay (RT-LAMP) for rapid detection and serotyping of Indian foot-and-mouth disease virus (FMDV) is described. The RT-LAMP assay was found to be 10(3)-10(5) fold more sensitive in comparison with RT-PCR, with a detection limit ranging from 10(-3) to 10(-5) TCID(50) of virus samples of all three serotypes. The RT-LAMP assay and qRT-PCR could detect 100 percent of clinical samples of three serotypes, whereas the RT-PCR detected 69.7% of type O, 58.1% of type A and 60.0% of Asia 1 samples. The qRT-PCR has the same sensitivity as the RT-LAMP. The assay conditions with absence of cross reactivity within the three serotypes of FMDV and FMDV negative samples were established. The RT-LAMP assay could detect 100% of samples stored in FTA(®) cards. In comparison with the performance of the RT-PCR; the RT-LAMP appears to be more sensitive, rapid and specific, with the potential for use as a point-of-care (POC) test, especially in developing countries. The use of FTA(®) cards for the preservation of RNA samples coupled with the RT-LAMP assay for the identification of serotypes may help in achieving improved FMDV serotype identification both in the field and in the laboratory.

An overview on ELISA techniques for FMD

Background

FMD is one of the major causes of economic loss of cloven-hoofed animals in the world today. The assessment of dominant genotype/lineage and prevalent trends and confirmation the presence of infection or vaccination not only provides scientific basis and first-hand information for appropriate control measure but also for disease eradication and regaining FMD free status following an outbreak. Although different biological and serological approaches are still applied to study this disease, ELISA test based on the distinct format, antigen type and specific antibody reinforce its predominance in different research areas of FMD, and this may replace the traditional methods in the near future. This review gives comprehensive insight on ELISA currently available for typing, antigenic analysis, vaccination status differentiation and surveillance vaccine purity and content at all stages of manufacture in FMDV. Besides, some viewpoint about the recent advances and trends of ELISA reagent for FMD are described here.

Methods

More than 100 studies regarding ELISA method available for FMD diagnosis, antigenic analysis and monitor were thoroughly reviewed. We investigated previous sagacious results of these tests on their sensitivity, specificity.

Results

We found that in all ELISA formats for FMD, antibody-trapping and competitive ELISAs have high specificity and RT-PCR (oligoprobing) ELISA has extra sensitivity. A panel of monoclonal antibodies to different sites or monoclonal antibody in combination of antiserum is the most suitable combination of antibodies in ELISA for FMD. Even though from its beginning, 3ABC is proven to be best performance in many studies, no single NSP can differentiate infected from vaccinated animals with complete confidence. Meanwhile, recombinant antigens and peptide derived from FMDV NPs, and NSPs have been developed for use as an alternative to the inactivated virus antigen for security.

Conclusions

There is a need of target protein, which accurately determines the susceptible animal status based on the simple, fast and reliable routine laboratory test. A further alternative based on virus-like particle (VLP, also called empty capsids) in combination of high throughput antibody technique (Phage antibody library/antibody microarray) may be the powerful ELISA diagnostic reagents in future.

A Brief Review on Diagnosis of Foot-and-Mouth Disease of Livestock: Conventional to Molecular Tools

Foot-and-mouth disease (FMD) is one of the highly contagious diseases of domestic animals. Effective control of this disease needs sensitive, specific, and quick diagnostic tools at each tier of control strategy. In this paper we have outlined various diagnostic approaches from old to new generation in a nutshell. Presently FMD diagnosis is being carried out using techniques such as Virus Isolation (VI), Sandwich-ELISA (S-ELISA), Liquid-Phase Blocking ELISA (LPBE), Multiplex-PCR (m-PCR), and indirect ELISA (DIVA), and real time-PCR can be used for detection of antibody against nonstructural proteins. Nucleotide sequencing for serotyping, microarray as well as recombinant antigen-based detection, biosensor, phage display, and nucleic-acid-based diagnostic are on the way for rapid and specific detection of FMDV. Various pen side tests, namely, lateral flow, RT-LAMP, Immunostrip tests, and so forth. are also developed for detection of the virus in field condition.

Development and comparison of genome detection assays for the diagnosis of foot-and-mouth disease suspected clinical samples

Detection of foot-and-mouth disease virus (FMDV) from clinical specimens by conventional sandwich enzyme-linked immunosorbent assay (ELISA) and virus isolation in cell culture is often compromised owing to limited sensitivity and inactivation during transit, respectively. A RT-PCR (oligoprobing) ELISA in both solid and aqueous phase hybridization formats targeting an across serotype conserved site at 3C-3D region was developed and its effectiveness was compared with that of the known targets at the IRES region. A non-isotopic RNA dot hybridization assay with colorimetric detection targeting both the IRES and the 3D region were also validated, which is capable of handling high throughput samples with ease. RT-PCR (oligoprobing) ELISA and dot hybridization assay showed 1000- and 10-fold greater sensitivity than the sandwich ELISA, respectively. Robustness of these diagnostic methods was explored by examining on sandwich ELISA-negative clinical samples. Both the assays developed in the present study were able to detect viral genomes in samples undetectable by conventional ELISA, thereby demonstrating 'proof of sensitivity'. Although the potential of these assays for providing definitive diagnosis in carrier hosts and in species where clinical disease is inapparent remains to be examined, nevertheless these assays can be adapted for comprehensive surveillance of foot-and-mouth disease in India.

Reverse transcription-PCR-enzyme-linked immunosorbent assay for rapid detection and differentiation of alphavirus infections

Due to the lack of a rapid, simple, and inexpensive assay for detecting alphavirus infections, we combined a reverse transcription-PCR with an enzyme-linked immunosorbent assay (RT-PCR-ELISA) to identify human pathogenic alphaviruses that are endemic in the New World. By combining the sensitivity of PCR, the detection simplicity of ELISA, and the specificities of DNA probes, this method rapidly detected and differentiated closely related species and subtypes of several medically important alphaviruses. After an amplification using RT-PCR with primers targeting conserved sequences in the nonstructural protein 1 gene, sequence-specific, biotin-labeled probes targeted against Venezuelan, eastern, and western equine encephalitis or Mayaro virus genes were used for the detection of amplicons using ELISA. The assay is simple, fast, and easy to perform in an ordinary diagnostic laboratory or clinical setting. Nucleic acid derived from cell cultures infected with several alphaviruses, clinical specimens, and mosquito pools as well as frozen and paraffin-embedded animal tissues were detected and identified within 6 to 7 h in a sensitive and specific manner.

A novel genetic lineage differentiating RT-PCR as a useful tool in molecular epidemiology of foot-and-mouth disease in India

Comparison of nucleotide sequences at the VP1 coding region of foot-and-mouth disease serotype Asia1 viruses from India has revealed two genetic lineages with emergence of a genetically divergent group in recent years. In this study a simple, fast, relatively costeffective multi-primer RT-PCR assay to differentiate genetic lineages of type Asia1 viruses was developed. Efforts were made in the design of novel lineage-specific primers and in optimization of the multi-primer assay protocol in conjunction with the use of the serotype specific primer for confirmation of serotype Asia1 virus. This assay promises to be an effective tool in molecular epidemiological investigation of FMD in the country.

Microarray-based detection and typing of foot-and-mouth disease virus

Foot-and-mouth disease virus (FMDV) is the most economically important veterinary pathogen because of its highly infectious nature and the devastating effects the virus has on the livestock industry. Rapid diagnostic methods are needed for detection and typing of FMDV serotypes and differentiation from other viruses causing vesicular diseases. We developed a microarray-based test that uses a FMD DNA chip containing 155 oligonucleotide probes, 35-45 base pair (bp) long, virus-common and serotype-specific, designed from the VP3-VP1-2A region of the genome. A set of two forward primers and one reverse primer were also designed to allow amplification of approximately 1100 bp of target sequences from this region. The amplified target was labelled with Alexa-Fluor 546 dye and applied to the FMD DNA chip. A total of 23 different FMDV strains representing all seven serotypes were detected and typed by the FMD DNA chip. Microarray technology offers a unique capability to identify multiple pathogens in a single chip.

Validation of a LightCycler-based reverse transcription polymerase chain reaction for the detection of foot-and-mouth disease virus

A specific reverse transcription polymerase chain reaction (RT-PCR) for the detection of the polymerase gene (3D) of foot-and-mouth disease virus (FMDV) was developed and validated with an analytical sensitivity of equal to, to 1,000 times higher than that of a single passage virus isolation. The performance of the RT-PCR was determined in 180 runs. After implementation, 5.3% of the tests had to be rejected due to invalid controls (e.g. cross-contamination of negative controls). The diagnostic sensitivity, determined using 124 samples from experimentally infected animals, was 91.9% for RT-PCR and 84.7% for virus isolation. Diagnostic specificity, determined by testing 258 samples from uninfected animals, was 100% by both tests. Of the 627 samples tested by RT-PCR and virus isolation, 85 reacted positively in both tests (13.5%) and 447 negatively in both tests (71.3%). One sample was positive by virus isolation and negative by RT-PCR (0.2%), 94 samples were positive by RT-PCR and negative by virus isolation (15%). The majority (84 of 94) of the 15% RT-PCR positive and virus isolation negative samples were among other samples from farms that reacted positively by both tests. The new RT-PCR is a robust, reliable and sensitive test, provided that adequate measures are taken to prevent cross-contamination. A possible preventive measure is to exclude ELISA positive samples from the RT-PCR testing.

Detection of carrier cattle and sheep persistently infected with foot-and-mouth disease virus by a rapid real-time RT-PCR assay

The detection of foot-and-mouth disease virus (FMDV) in persistently infected carriers among exposed ruminants is of great importance in disease control. For this purpose, a real time, fluorogenic reverse transcription polymerase chain reaction (real-time RT-PCR) assay was evaluated for the identification of FMDV carrier animals. The results indicate that this real time RT-PCR assay may be suitable for detection of FMDV carrier animals.

The pathogenesis and diagnosis of foot-and-mouth disease

The pathogenesis of foot-and-mouth disease (FMD) is reviewed, taking account of knowledge gained from field and experimental studies and embracing investigations at the level of the virus, the cell, the organ, the whole animal and the herd or flock. The review also addresses the immune response and the carrier state in FMD. Progress made in understanding the pathogenesis of the disease is highlighted in relation to developments in diagnosis and methods of control.

Evaluation of automated RT-PCR to accelerate the laboratory diagnosis of foot-and-mouth disease virus

Automated fluorogenic (5' nuclease probe-based) reverse transcription polymerase chain reaction (RT-PCR) procedures were evaluated for the diagnosis of foot-and-mouth disease (FMD) using suspensions of vesicular epithelium, heparinised or clotted blood, milk and oesophageal-pharyngeal fluid ('probang') samples from the United Kingdom (UK) 2001 epidemic and on sera from animals experimentally infected with the outbreak serotype O FMD virus strain. A MagNA Pure LC was initially programmed to automate the nucleic acid extraction and RT procedures with the PCR amplification carried out manually by fluorogenic assay in a GeneAmp 5700 Sequence Detection System. This allowed 32 samples to be tested by one person in a typical working day or 64 samples by two people within 10-12 h. The PCR amplification was later automated and a protocol developed for one person to complete a single test incorporating 96 RT-PCR results within 2 working days or for two people to do the same thing in around 12 h. The RT-PCR results were directly compared with those obtained by the routine diagnostic tests of ELISA and virus isolation in cell culture. The results on blood, probang and milk samples were in broad agreement between the three procedures but specific RT-PCR protocols for such material have to be fully optimised as perhaps have the positive-negative acceptance criteria. However, the automated RT-PCR achieved definitive diagnostic results (positive or negative) on supernatant fluids from first passage inoculated cell cultures and its sensitivity was greater than ELISA on suspensions of vesicular epithelium (ES) and at least equivalent to that of virus isolation in cell culture. The combined tests of ELISA, virus isolation in cell culture and RT-PCR might, therefore, only be required for confirmation of a first outbreak of FMD in a previously FMD-free country. Should a prolonged outbreak subsequently occur, then either ELISA plus RT-PCR or else RT-PCR alone could be used as the laboratory diagnostic tool(s). Either approach would eliminate the requirement for sample passage in cell culture and considerably advance the issue of laboratory diagnostic test results.

Rinderpest virus lineage differentiation using RT-PCR and SNAP-ELISA

An RT-PCR/ELISA system has been developed that detects and differentiates Rinderpest virus (RPV) from the other closely related morbillivirus of ruminants, Peste des petits Ruminants virus (PPRV). In addition, using lineage specific probes, it is possible to determine whether the virus sample is wild-type or vaccine, and the likely origin of the outbreak if it is wild-type. It involves carrying out a RT-PCR with one digoxygenin (Dig)-labelled primer followed by a hybridisation step with a virus-specific, biotin-labelled, probe. The hybridisation step is carried out in an ELISA format on a streptavidin-coated plate. The DIG-labelled products are detected using a specific anti-DIG monoclonal antibody and an anti-mouse horseradish peroxidase conjugate. The hybridisation step replaces nucleotide sequencing or nested PCR for confirmation of the identity of DNA product. The assay is fast and easy to carry out and can give semi-quantitative estimates of the virus content of samples.

Diagnosis and screening of foot-and-mouth disease

Foot-and-mouth disease (FMD) diagnostic methods are reviewed. As the presence of clinical signs alone is inconclusive, laboratory diagnosis should always be carried out. The presence of FMD virus can be demonstrated by cell culture isolation, complement fixation test, ELISA or the more recent polymerase chain reaction (PCR) method. Serological diagnosis is also a valuable tool. The virus neutralization test has been replaced by ELISA and the antibody response to some viral non-structural proteins allows to discriminate between vaccinated and infected animals on a herd basis. More rapid and accurate tests as well as an earlier detection system in preclinical state are still needed.

Detection of all seven serotypes of foot-and-mouth disease virus by real-time, fluorogenic reverse transcription polymerase chain reaction assay

A fluorogenic RT-PCR (5'-nuclease probe-based) assay using a primer/probe set designed from the internal ribosomal entry site region of the virus genome was developed for the specific detection of all seven serotypes of foot-and-mouth disease (FMD) virus in epithelial suspensions and cell culture virus preparations. The reverse transcription polymerase chain reaction (RT-PCR) specifically detected FMD virus in sample submissions from the UK 2001 FMD outbreak with greater sensitivity than our conventional RT-PCR procedure and our routine diagnostic procedures of ELISA and virus isolation in cell culture. The fluorogenic RT-PCR provides relatively fast results, enables a quantitative assessment to be made of virus amounts and can handle more samples and/or replicates of samples in a single assay than the conventional RT-PCR procedure. Therefore it is seen as a valuable tool to complement the routine diagnostic procedures for FMD virus diagnosis.

Aspects of the persistence of foot-and-mouth disease virus in animals--the carrier problem

Foot-and-mouth disease virus (FMDV) is a member of the Aphthovirus genus in the Picornaviridae family. Seven distinct serotypes, each including a wide range of variants, have been defined. FMD, affects wild and domesticated ruminants and pigs, is difficult to control and is the major constraint to international trade in livestock and animal products. After the acute stage of infection, FMDV may cause a prolonged, asymptomatic but persistent infection in ruminants. Also, vaccinated or naturally immune animals subsequently exposed to live virus may become persistently infected (the so-called carriers), a situation which can result in export embargoes if vaccination is included in a country's control policy.

The early pathogenesis of foot-and-mouth disease in pigs infected by contact: a quantitative time-course study using TaqMan RT-PCR

Foot-and-mouth disease (FMD) is a highly contagious, economically important virus disease of cloven-hoofed animals. The objective of the present study was to examine the early pathogenesis of FMD in pigs by a quantitative time-course study. Under experimental conditions, recipient pigs were infected by contact with donor pigs affected by FMD. Every 24 h from day 1 to day 4 after exposure, two recipient pigs were selected randomly, killed and necropsied. A range of tissues were analysed by a quantitative TaqMan RT-PCR method and by titration of FMD virus on primary bovine thyroid cells. The titres of virus determined by assay in cell culture and calculated from the quantitative TaqMan data correlated strongly (r>0.9), thereby establishing the validity of the TaqMan calculations. The data indicated that the replication of virus in the lungs contributes only in small part to airborne virus excretion. Sites in the pharynx, trachea and nasal mucosa are probably more important in that regard. The sites of earliest virus infection and possibly replication in recipient pigs appeared to be in the pharynx (soft palate, tonsil and floor of pharynx). The data indicated that FMD virus replication in pigs is rapid and that the majority of virus amplification occurs in the skin. A model for the progression of infection is proposed, indicating initial spread from the pharyngeal region, through regional lymph nodes and via the blood to epithelial cells, resulting in several cycles of virus amplification and spread.