Simplifying sampling for African swine fever surveillance: Assessment of antibody and pathogen detection from blood swabs

African Swine Fever Diagnosis in Africa: Challenges and Opportunities

The global spread of African swine fever (ASF) in recent decades has led to the need for technological advances in sampling and diagnostic techniques. The impetus for these has been the need to enable sampling by lay persons and to obtain at least a preliminary diagnosis in the field for early control measures to be put in place before final laboratory confirmation. In rural Africa, rapid diagnosis is hampered by challenges that include lack of infrastructure as well as human and financial resources. Lack of animal health personnel, access to affordable means to transport field samples to a laboratory, and lack of laboratories with the capacity to make the diagnosis result in severe under-reporting of ASF, especially in endemic areas. This review summarizes the challenges identified in gap analyses relevant to low- and middle-income countries, with a focus on Africa, and explore the opportunities provided by recent research to improve field diagnosis and quality of diagnostic samples used. Sampling techniques include invasive sampling techniques requiring trained personnel and non-invasive sampling requiring minimal training, sampling of decomposed carcass material, and preservation of samples in situations where cold chain maintenance cannot be guaranteed. Availability and efficacy of point-of-care (POC) tests for ASF has improved considerably in recent years and their application, as well as advantages and limitations, are discussed. The adequacy of existing laboratory diagnostic capacity is evaluated and opportunities for networking amongst reference and other laboratories offering diagnostic services are discussed. Maintaining laboratory diagnostic efficiency in the absence of samples during periods of quiescence is another issue that requires attention, and the role of improved laboratory networking is emphasized. Early diagnosis of ASF is key to managing the disease spread. Therefore, the establishment of the Africa Chapter of the Global African Swine Fever Research Alliance (GARA) increases opportunities for collaboration and networking among the veterinary diagnostic laboratories in the region.

Characterization of the monoclonal antibody and the immunodominant B-cell epitope of African swine fever virus pA104R by using mouse model

The African swine fever virus (ASFV) structural protein pA104R is the only histone-like protein encoded by eukaryotic viruses. pA104R is an essential DNA-binding protein required for DNA replication and genome packaging of ASFV, which are vital for pathogen survival and proliferation. pA104R is an important target molecule for diagnosing, treating, and immune prevention of ASFV. This study characterized monoclonal antibodies (mAbs) against pA104R and found them to recognize natural pA104R in ASFV strains with different genotypes, showing high conservation. Confirmation analyses of pA104R epitopes using mAbs indicated the presence of immunodominant B-cell epitopes, and further characterization showed the high antigenic index and surface accessibility coefficients of the identified epitope. Furthermore, the pA104R protein functions through the polar interactions between the binding amino acid sites; however, these interactions may be blocked by the recognition of generated mAbs. Characterizing the immunodominant B-cell epitope of the ASFV critical proteins, such as pA104R, may contribute to developing sensitive diagnostic tools and vaccine candidate targets.IMPORTANCEAfrican swine fever (ASF) is a highly pathogenic, lethal, and contagious viral disease affecting domestic pigs and wild boars. As no effective vaccine or other treatments have been developed, the control of African swine fever virus (ASFV) relies heavily on virus detection and diagnosis. A potential serological target is the structural protein pA104R. However, the molecular basis of pA104R antigenicity remains unclear, and a specific monoclonal antibody (mAb) against this protein is still unavailable. In this study, mAbs against pA104R were characterized and found to recognize natural pA104R in ASFV strains with different genotypes. In addition, confirmation analyses of pA104R epitopes using mAbs indicated the presence of immunodominant B-cell epitopes, and further characterization showed the high antigenic index and surface accessibility coefficients of the identified epitope. Characteristics of the immunodominant B-cell epitope of ASFV proteins, such as pA104R, may contribute to developing sensitive diagnostic tools and identifying vaccine candidate targets.

Stability of Genotube® Swabs for African Swine Fever Virus Detection Using Loop-Mediated Isothermal (LAMP) Laboratory Testing on Samples Stored without Refrigeration

African swine fever (ASF) is a transboundary viral disease which causes high mortality in pigs. In many low- and middle-income countries and in remote areas where diagnostic surveillance for ASF virus (ASFV) is undertaken, access to trained animal health technicians, sample collection, cold chain storage and transport of samples to suitably equipped laboratories can be limiting when traditional sampling and laboratory tests are used. Previously published studies have demonstrated that alternative sampling matrices such as swabs and filter papers can be tested using PCR without refrigeration for up to a week. This study used Genotube® swabs stored in temperate and tropical climates without refrigeration for four weeks after collection to demonstrate there was no change in test performance and results using loop-mediated isothermal amplification (LAMP) ASFV detection on a series of pig serum samples including serum spiked with a synthetic ASFV positive control, naturally acquired ASFV positive serum from Timor-Leste and negative ASFV serum samples. The use of Genotube® swabs for ASFV detection for surveillance purposes, coupled with testing platforms such as LAMP, can provide an alternative to traditional testing methodology where resources are limited and time from collection to testing of samples is prolonged.

Multiepitope array as the key for African Swine Fever diagnosis

African Swine Fever (ASF) is an acute hemorrhagic fever affecting suids with high mortality and morbidity rate. The causal agent of ASF, the African Swine Fever Virus (ASFV), is an icosahedral virus of 200 nm diameter, composed of an outer envelope layer of host derivation and a linear 170-190 kb long dsDNA molecule. As of today, no efficient therapeutic intervention nor prophylactic measures exist to fight ASFV diffusion, underlining the importance of the early diagnosis and the need for efficient in-field screening of ASF. Recommended guidelines for the diagnosis of ASF are unpracticable in the desirable context of the rapid in-farm screening. In this view, the design of innovative diagnostics based on a panel of multiple ASFV epitopes would amend versatility and the analytical performances of the deliverable, ensuring high quality and accuracy standards worth of implementation in rapid in-field monitoring programs. Pursuing this view, we performed epitope prediction from the major AFSV structural proteins holding the potential to be targeted in innovative rapid diagnostic tests. Selected ASFV structural protein sequences were retrieved from data repositories and their tridimensional structure was computed. Linear and 3D protein structures were subjected to the prediction of the epitope sequences, that are likely to elicit antibody production, by independent bioinformatic tools, providing a list of candidate biomarkers whose batch employment held the potential suitability for the unbiased rapid in-field diagnosis and, in turn, might be implemented in screening programs, crowing the current monitoring and control campaigns that are currently running worldwide.

Innovative Research Offers New Hope for Managing African Swine Fever Better in Resource-Limited Smallholder Farming Settings: A Timely Update

African swine fever (ASF) in domestic pigs has, since its discovery in Africa more than a century ago, been associated with subsistence pig keeping with low levels of biosecurity. Likewise, smallholder and backyard pig farming in resource-limited settings have been notably affected during the ongoing epidemic in Eastern Europe, Asia, the Pacific, and Caribbean regions. Many challenges to managing ASF in such settings have been identified in the ongoing as well as previous epidemics. Consistent implementation of biosecurity at all nodes in the value chain remains most important for controlling and preventing ASF. Recent research from Asia, Africa, and Europe has provided science-based information that can be of value in overcoming some of the hurdles faced for implementing biosecurity in resource-limited contexts. In this narrative review we examine a selection of these studies elucidating innovative solutions such as shorter boiling times for inactivating ASF virus in swill, participatory planning of interventions for risk mitigation for ASF, better understanding of smallholder pig-keeper perceptions and constraints, modified culling, and safe alternatives for disposal of carcasses of pigs that have died of ASF. The aim of the review is to increase acceptance and implementation of science-based approaches that increase the feasibility of managing, and the possibility to prevent, ASF in resource-limited settings. This could contribute to protecting hundreds of thousands of livelihoods that depend upon pigs and enable small-scale pig production to reach its full potential for poverty alleviation and food security.

A practical guide for strategic and efficient sampling in African swine fever-affected pig farms

In the case of African swine fever (ASF) outbreaks in pig farms, EU legislation requires a thorough epidemiological investigation to determine, among other tasks, the extent of infection in the affected farm. The main aim of this study was to implement a reliable sampling strategy to quickly obtain an overview of the extent of ASF virus spread in an affected pig farm. We developed and tested a three-step approach: (i) identification of sub-units within the affected farm, (ii) categorization of sub-units, and (iii) targeted selection of animals for testing. We used commercially available lateral flow devices (LFDs) to detect ASF antigen and antibodies under field conditions and compared them with routinely performed laboratory tests (qPCR, ELISA, IPT). The study was conducted in three commercial farms in Latvia that were affected by ASF in July 2020. One of the affected farms was relatively small with only 31 pigs, whereas the other two were large with 1800 and 9800 animals, respectively. The approach proved to be helpful and practical for efficient and reliably assess the ASF situation on the farm and to identify sub-units within a farm where infected animals are present and sub-units which might (still) be free of infection. This important epidemiological information helps to better estimate the high-risk period and to track the potential spread of infection outside the farm. It allows also to prioritize culling and, if appropriate, to pursue a partial culling strategy taking into account the absence of clinical signs, implemented biosecurity measures, quarantine and negative test results, among others. This might be of interest for large commercial farms where the infection was identified very early and has not yet spread widely. Due to its limited sensitivity, the antigen LFD test is useful for testing animals showing signs of disease.

HRP-conjugated-nanobody-based cELISA for rapid and sensitive clinical detection of ASFV antibodies

Abstract

African swine fever (ASF), which is caused by the ASF virus (ASFV), is a highly contagious hemorrhagic disease that causes high mortality to domestic porcine and wild boars and brings huge economic losses to world swine industry. Due to the lack of an effective vaccine, the control of ASF must depend on early, efficient, and cost-effective detection and strict control and elimination strategies. Traditional serological testing methods are generally associated with high testing costs, complex operations, and high technical requirements. As a promising alternative diagnostic tool to traditional antibodies, nanobodies (Nb) have the advantages of simpler and faster generation, good stability and solubility, and high affinity and specificity, although the system is dependent on the immunization of Bactrian camels to obtain the specific VHH library of the target protein. The application of Nbs in the detection of ASFV antibodies has not yet been reported yet. Using a phage display technology, one Nb against the ASFV p54 protein that exhibited high specificity and affinity, Nb8, was successfully screened. A HEK293T cell line stably expressing Nb8-horseradish peroxidase (HRP) fusion protein was established using the lentiviral expression system. Following the optimization of the reaction conditions, the Nb8-HRP fusion protein was successfully used to establish a competitive enzyme-linked immunosorbent assay (cELISA) to detect ASFV-specific antibodies in pig serum, for the first time. There was no cross-reaction with healthy pig serum, porcine pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), porcine epidemic diarrhea virus (PEDV), and classical swine fever virus (CSFV) positive sera. The optimal cut-off value for the cELISA by ROC analysis was 52.5%. A total of 209 serum samples were tested using the developed cELISA and a commercial ELISA kit. The results showed that the relative specificity of the cELISA was 98.97%, and the relative sensitivity of the cELISA was 93.3%, with the percent agreement between the two ELISA methods being 98.56%. In conclusion, a specific, sensitive, and repeatable cELISA was successfully developed based on the Nb8 as a probe, providing a promising method for the detection of anti-ASFV antibodies in clinical pig serum.

Key points

• We successfully screened a specific, high affinity nanobody against ASFV p54 protein.

• We establish a method for continuous and stable expression of Nb-HRP fusion protein using a lentiviral packaging system.

• We establish a nanobody cELISA detection method that can monitor an ASF infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11981-4.

Rapid and ultra-sensitive detection of African swine fever virus antibody on site using QDM based-ASFV immunosensor (QAIS)

African swine fever (ASF) is a swine viral disease that could cause highly contagious and extremely high mortality, causing huge economic losses to the pig industry. As there is currently no vaccine and effective treatment methods. Therefore, early monitoring is one of the most important solutions to prevent and control ASF. In this study, the dual QDM recombinant virus protein 30 and 54 (P30 and P54) probes and pre-incubation in vitro were proposed for the first time as QDM based-ASFV immunosensor (QAIS) for the ultra-sensitive quantitative detection of ASFV antibodies in serum. In the range from serum dilution of 1:1000 to 1:64000, it showed a good linear relationship (R² = 0.9947), and its detection sensitivity was 1:64000 dilution. Compared with commercial enzyme-linked immunosorbent assay (ELISA) and colloidal gold immunochromatographic strip (CGICS), its detection sensitivity was improved by at least one order of magnitude and four orders of magnitude respectively. In addition, the whole ASFV antibody screening test can be completed in 25 min with simple operation. The performance and practicability of the established QAIS sensor have been verified by ASF-ELISA kit, and its coincidence rate was as high as 98.7% in 151 clinical samples. We firmly believe that the proposed QAIS sensor could potentially be applied to point-of-care testing (POCT) for quantitative ASFV antibody in pig farms.

Identification of one novel epitope targeting p54 protein of African swine fever virus using monoclonal antibody and development of a capable ELISA

African swine fever (ASF) is one of the most lethal viral diseases affecting both domestic pigs and wild boars. The acute infection of the ASF disease in domestic pigs leads to a 100% mortality rate with symptoms including high fever, vascular changes, cyanosis of the skin. Until now, there are no commercial vaccines and antiviral drugs available for ASF control. Therefore, the spread of ASF poses great economic losses to the pig industry and the ecosystems in the affected countries. A rapid and capable method was urgently needed to monitor ASFV-specific antibodies for controlling the spread of ASFV. In this study, we obtained one strain of monoclonal antibody (mAb) against the p54 protein of ASFV, and the target epitope of the mAb was determined to be ¹⁷⁵YTHKDLENSL¹⁸⁴. The experimental results demonstrated that the monoclonal antibody could successfully recognize the exogenously expressed p54 protein and the chimeric virus constructed in our laboratory. The mAb could be used as a detection tool for the development of ASF vaccine strains. In addition, the ELISA established by using the obtained synthetic epitope peptide as the antigen had high sensitivity, good specificity and showed the great potential for ASF epidemic monitoring and control.

African Swine Fever in Wild Boar in Europe-A Review

The introduction of genotype II African swine fever (ASF) virus, presumably from Africa into Georgia in 2007, and its continuous spread through Europe and Asia as a panzootic disease of suids, continues to have a huge socio-economic impact. ASF is characterized by hemorrhagic fever leading to a high case/fatality ratio in pigs. In Europe, wild boar are especially affected. This review summarizes the currently available knowledge on ASF in wild boar in Europe. The current ASF panzootic is characterized by self-sustaining cycles of infection in the wild boar population. Spill-over and spill-back events occur from wild boar to domestic pigs and vice versa. The social structure of wild boar populations and the spatial behavior of the animals, a variety of ASF virus (ASFV) transmission mechanisms and persistence in the environment complicate the modeling of the disease. Control measures focus on the detection and removal of wild boar carcasses, in which ASFV can remain infectious for months. Further measures include the reduction in wild boar density and the limitation of wild boar movements through fences. Using these measures, the Czech Republic and Belgium succeeded in eliminating ASF in their territories, while the disease spread in others. So far, no vaccine is available to protect wild boar or domestic pigs reliably against ASF.

A CRISPR/Cas9 eraser strategy for contamination-free PCR end-point detection

Polymerase chain reaction (PCR), a central technology for molecular diagnostics, is highly sensitive but susceptible to the risk of false positives caused by aerosol contamination, especially when an end-point detection mode is applied. Here, we proposed a solution by designing a clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 eraser strategy for eliminating potential contamination amplification. The CRISPR/Cas9 engineered eraser is firstly adopted into artpcr reverse-transcription PCR (RT-PCR) system to achieve contamination-free RNA detection. Subsequently, we extended this CRISPR/Cas9 eraser to the PCR system. We engineered conventional PCR primers to enable the amplified products to contain an implanted NGG (protospacer adjacent motif, PAM) site, which is used as a code for specific CRISPR/Cas9 recognition. Pre-incubation of Cas9/sgRNA with PCR mix leads to a selective cleavage of contamination amplicons, thus only the template DNA is amplified. The developed CRISPR/Cas9 eraser, adopted by both RT-PCR and PCR systems, showed high-fidelity detection of SARS-CoV-2 and African swine fever virus with a convenient strip test.

African Swine Fever Laboratory Diagnosis-Lessons Learned from Recent Animal Trials

African swine fever virus (ASFV) causes a hemorrhagic disease in pigs with high socio-economic consequences. To lower the impact of disease incursions, early detection is crucial. In the context of experimental animal trials, we evaluated diagnostic workflows for a high sample throughput in active surveillance, alternative sample matrices for passive surveillance, and lateral flow devices (LFD) for rapid testing. We could demonstrate that EDTA blood is significantly better suited for early ASFV detection than serum. Tissues recommended by the respective diagnostic manuals were in general comparable in their performance, with spleen samples giving best results. Superficial lymph nodes, ear punches, and different blood swabs were also evaluated as potential alternatives. In summary, all matrices yielded positive results at the peak of clinical signs and could be fit for purpose in passive surveillance. However, weaknesses were discovered for some matrices when it comes to the early phase of infection or recovery. The antigen LFD showed variable results with best performance in the clinical phase. The antibody LFD was quite comparable with ELISA systems. Concluding, alternative approaches are feasible but have to be embedded in control strategies selecting test methods and sample materials following a “fit-for-purpose” approach.

Scientific Opinion on the assessment of the control measures of the category A diseases of Animal Health Law: African Swine Fever

EFSA received a mandate from the European Commission to assess the effectiveness of some of the control measures against diseases included in the Category A list according to Regulation (EU) 2016/429 on transmissible animal diseases ('Animal Health Law'). This opinion belongs to a series of opinions where these control measures will be assessed, with this opinion covering the assessment of control measures for African Swine Fever (ASF). In this opinion, EFSA and the AHAW Panel of experts reviewed the effectiveness of: (i) clinical and laboratory sampling procedures, (ii) monitoring period and (iii) the minimum radius of the protection and surveillance zone, and the minimum length of time the measures should be applied in these zones. The general methodology used for this series of opinions has been published elsewhere; nonetheless, specific details of the model used for the assessment of the laboratory sampling procedures for ASF are presented here. Here, also, the transmission kernels used for the assessment of the minimum radius of the protection and surveillance zones are shown. Several scenarios for which these control measures had to be assessed were designed and agreed prior to the start of the assessment. In summary, several sampling procedures as described in the diagnostic manual for ASF were considered ineffective and a suggestion to exclude, or to substitute with more effective procedures was made. The monitoring period was assessed as non-effective for several scenarios and a longer monitoring period was suggested to ensure detection of potentially infected herds. It was demonstrated that the surveillance zone comprises 95% of the infections from an affected establishment, and therefore is considered effective. Recommendations provided for each of the scenarios assessed aim to support the European Commission in the drafting of further pieces of legislation, as well as for plausible ad hoc requests in relation to ASF.

Joining the club: First detection of African swine fever in wild boar in Germany

African swine fever (ASF) has spread across many countries in Europe since the introduction into Georgia in 2007. We report here on the first cases of ASF in wild boar detected in Germany close to the border with Poland. In addition to the constant risk of ASF virus (ASFV) spread through human activities, movements of infected wild boar also represent a route of introduction. Since ASF emerged in Western Poland in November 2019, surveillance efforts, in particular examination of wild boar found dead, were intensified in the regions of Germany bordering with Poland. The first case of ASF in wild boar in Germany was therefore detected by passive surveillance and confirmed on 10 September 2020. By 24 September 2020, 32 cases were recorded. Testing of samples from tissues of carcasses in different stages of decomposition yielded cycle threshold values from 18 to 36 in the OIE-recommended PCR, which were comparable between the regional and national reference laboratory. Blood swabs yielded reliable results, indicating that the method is suitable also under outbreak conditions. Phylogenetic analysis of the ASFV whole-genome sequence generated from material of the first carcass detected in Germany, revealed that it groups with ASFV genotype II including all sequences from Eastern Europe, Asia and Belgium. However, some genetic markers including a 14 bp tandem repeat duplication in the O174L gene were confirmed that have so far been detected only in sequences from Poland (including Western Poland). Epidemiological investigations that include estimated postmortem intervals of wild boar carcasses of infected animals suggest that ASFV had been introduced into Germany in the first half of July 2020 or even earlier.

Multiplex and visual detection of African Swine Fever Virus (ASFV) based on Hive-Chip and direct loop-mediated isothermal amplification

African swine fever is caused by African swine fever virus (ASFV), and has a mortality rate approaching 100%. It has already caused tremendous economy lost around the world. Without effective vaccine, rapid and accurate on-site detection plays an indispensable role in controlling outbreaks. Herein, by combining Hive-Chip and direct loop-mediated isothermal amplification (LAMP), we establish a multiplex and visual detection platform. LAMP primers targeting five ASFV genes (B646L, B962L, C717R, D1133L, and G1340L) were designed and pre-fixed in Hive-Chip. On-chip LAMP showed the limits of detection (LOD) of ASFV synthetic DNAs and mock samples are 30 and 50 copies per microliter, respectively, and there is no cross-reaction among the target genes. The overall performance of our platform is comparable to that of the commercial kits. From sample preparation to results readout, the entire process takes less than 70 min. Multiplex detection of real samples of ASFV and other swine viruses further demonstrates the high sensitivity and specificity of Hive-Chip. Overall, our platform provides a promising option for on-site, fast and accurate detection of ASFV.

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Hive-Chip firstly realized simultaneous detection of multiple genes of ASFV, largely avoiding false-negative results.

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Without nucleic acid extraction, direct LAMP was firstly incorporated into the Hive-Chip for visual detection.

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Because very little operation and no complicate instrument is required, on-site detection is possible for this platform.

Epitope mapping of African swine fever virus (ASFV) structural protein, p54

In the absence of a vaccine for African swine fever virus (ASFV), diagnostic tools are critical for early detection and implementation of control measures. Along with other immunogenic proteins, p54 is a good serological target for conducting ASF detection and surveillance. In this study, a panel of 12 mouse monoclonal antibodies (mAbs) was prepared against a baculovirus-expressed p54(60-178) polypeptide. Further screening showed that five mAbs were positive for reactivity against ASFV-infected cells and recombinant p54 proteins. Mapping studies using five polypeptides and 12 oligopeptides, showed that mAb #154-1 recognized a conserved polypeptide sequence, p54(65-75), and was placed into Group 1. Mabs #143-1 and #7 recognized a region covered by p54(93-113) and were placed into Group 2. Group 3 consisted of mAbs #101 and #117, which recognized p54(118-127). Sera from pigs infected with the low virulent OURT 88/3 strain recognized the same p54 region covered by the Group 3 mAbs. When tested in a neutralization format, only mAb #143-1 showed neutralization activity above background. Together, the results identify important antigenic and immunogenic regions located on p54, which provide new tools for improving ASFV diagnostics.

Rift Valley Fever Virus, Japanese Encephalitis Virus, and African Swine Fever Virus: Three Transboundary, Vector-Borne, Veterinary Biothreats With Diverse Surveillance, and Response Capacity Needs

Early detection of emerging foreign animal diseases is critical to pathogen surveillance and control programs. Rift valley fever virus (RVFV), Japanese encephalitis virus (JEV), and African swine fever virus (ASFV) represent three taxonomically and ecologically diverse vector-borne viruses with the potential to be introduced to the United States. To promote preparedness for such an event, we reviewed the current surveillance strategies and diagnostic tools in practice around the world for these emerging viruses, and summarized key points pertaining to the availability of existing guidelines and strategic approaches for early detection, surveillance, and disease management activities. We compare and contrast the surveillance and management approaches of these three diverse agents of disease as case studies to emphasize the importance of the ecological context and biology of vectors and vertebrate hosts. The information presented in this review will inform stakeholders of the current state of surveillance approaches against these transboundary foreign animal disease which threaten the United States.

Porcine reproductive and respiratory syndrome virus RNA detection in different matrices under typical storage conditions in the UK

In the UK, approximately 40 per cent of the pig breeding herds are outdoors. To monitor their porcine reproductive and respiratory syndrome virus (PRRSV) status, blood is collected commonly from piglets around weaning. Sample collection in British outdoor pigs often occurs during the early morning hours when the piglets tend to accumulate inside sheltered areas. For practical reasons, dry cotton swabs are occasionally used for blood collection and stored at room temperature until arrival in the laboratory. Detection of PRRSV RNA is a function of viral concentration, sample type and storage condition. To evaluate a possible impact of the sampling protocol on PRRSV1 detection, experimentally spiked blood samples using three dilutions of a representative PRRSV1 strain were prepared. In addition, blood samples from pigs naturally infected with PRRSV were obtained from a PRRSV-positive British herd. Spiked blood and blood from infected pigs were used to obtain sera, dry or wet (immersed in saline) polyester or cotton swabs and FTA cards. The different samples were stored for 24 hours, 48 hours or 7 days at 4°C or 20°C and tested by a real-time reverse transcriptase PRRSV PCR assay. Under the study conditions, the best matrix was serum (96.7 per cent), followed by wet swabs (78 per cent), dry swabs (61.3 per cent) and FTA cards (51 per cent). Polyester swabs (76 per cent) showed a better performance than cotton swabs (63.3 per cent). The reduction in sensitivity obtained for swabs and FTA cards was particularly high at low viral concentrations. The results indicate that wet polyester swabs should be used whenever possible.