Abstract
Virus nucleic acids and antibody response to pathogens can be measured using swine oral fluids (OFs). Detection of foot-and-mouth disease virus (FMDV) genome in swine OFs has previously been demonstrated. Virus isolation and viral antigen detection are additional confirmatory assays for diagnosing FMDV, but these methods have not been evaluated using swine OF. The objectives of this study were to further validate the molecular detection of FMDV in oral fluids, evaluate antigen detection and FMDV isolation from swine OFs, and develop an assay for isotypic anti-FMDV antibody detection in OFs. Ribonucleic acid (RNA) from FMDV was detected in OFs from experimentally infected pigs by quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) from 1 day post-infection (dpi) to 21 dpi. Foot-and-mouth disease virus (FMDV) was isolated from OFs at 1 to 5 dpi. Additionally, FMDV antigens were detected in OFs from 1 to 6 dpi using a lateral flow immunochromatographic strip test (LFIST), which is a rapid pen-side test, and from 2 to 3 dpi using a double-antibody sandwich enzyme-linked immunosorbent assay (DAS ELISA). Furthermore, FMDV-specific immunoglobulin A (IgA) was detected in OFs using an isotype-specific indirect ELISA starting at dpi 14. These results further demonstrated the potential use of oral fluids for detecting FMDV genome, live virus, and viral antigens, as well as for quantifying mucosal IgA antibody response.
Résumé
Chez les porcs les acides nucléiques viraux et la production d’anticorps contre des agents pathogènes peuvent être mesurés en utilisant les fluides oraux (FO). La détection du génome du virus de la fièvre aphteuse (VFA) dans les FO de porcs a été démontrée précédemment. L’isolement viral et la détection d’antigène viral sont des épreuves de confirmation supplémentaires pour diagnostiquer la présence du VFA, mais ces méthodes n’ont pas été évaluées en utilisant des FO porcins. Les objectifs de la présente étude étaient de valider un peu plus la détection moléculaire du VFA dans les FO, d’évaluer la détection d’antigènes et l’isolement du VFA à partir de FO porcins, et de développer une épreuve pour la détection d’anticorps isotypiques anti-VFA dans les FO. L’ARN du VFA fut détecté dans les FO de porcs infectés expérimentalement par réaction quantitative en temps réel d’amplification en chaine par la polymérase utilisant la transcriptase réverse à partir du jour 1 post-infection (PI) jusqu’au jour 21 PI. Le VFA fut isolé à partir des FO aux jours 1 à 5 PI. De plus, les antigènes du VFA ont été détectés dans les FO des jours 1 à 6 PI en utilisant une épreuve sur bandelette d’immunochromatographie par flot latéral, un test rapide pouvant être réalisé à la ferme, ainsi que de 2 à 3 j PI en utilisant une épreuve immuno-enzymatique (ELISA) double-sandwich. Également, à partir du jour 14 PI des immunoglobulines A (IgA) spécifiques au VFA ont été détectées dans les FO au moyen d’une épreuve ELISA indirecte spécifique pour les isotypes. Ces résultats démontrent d’une manière additionnelle le potentiel d’utilisation des FO pour détecter le génome du VFA, du virus vivant, et des antigènes viraux, de même que pour quantifier la production d’IgA par les muqueuses.
(Traduit par Docteur Serge Messier)
Introduction
Foot-and-mouth disease (FMD) is one of the most economically important viral diseases that limits export and import of animals and animal products around the world (1). It is caused by the FMD virus (FMDV), which belongs to the family Picornaviridae (genus Aphthovirus). Foot-and-mouth disease (FMD) is characterized by: fever; vesicular lesions on the feet, tongue, snout, and teats; lameness; and depression (2,3). Although FMDV has a wide range of hosts, pigs are more severely affected than other cloven-footed animals and are known to amplify the disease by generating large amounts of aerosolized virus even before the onset of clinical signs (2,4).
Pig production has achieved high efficiency in the past few decades through large intensive-production systems. It is important to detect any infection quickly and accurately in these high-density pig farms in order to minimize the spread of a disease like FMD (5–7). Disease surveillance can ensure early detection and an easily obtainable type of sample such as oral fluids (OFs) would ease swine disease surveillance.
Oral fluids, or saliva, have been used in human medicine to diagnose infectious and metabolic diseases and hormonal disorders since 1957 (8,9). The use of OFs in veterinary medicine has become popular over the last 2 decades (10). Several recent studies in pigs have shown that OFs can be used to diagnose highly infectious viral diseases and for disease surveillance (10–15).
While the use of swine OFs for FMDV detection has previously been reported (14,15), these studies focused on detection of the FMDV genome by real-time reverse transcription-polymerase chain reaction (qRT-PCR). Other assays based on the use of swine OFs to detect FMD have not been evaluated. Therefore, the objectives of this study were to further validate the use of swine OFs for the detection of FMDV genomes and to evaluate FMDV antigen detection in OFs using a double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) and a rapid lateral flow immunochromatographic strip test (LFIST). An additional objective was to develop an ELISA for detecting anti-FMDV immunoglobulin A (IgA) in oral fluids from pigs experimentally infected with FMDV.
Materials and methods
Experimental design
The Animal Care Committee at the Canadian Science Centre for Animal and Human Health reviewed and approved the use of animals for this study under animal-use document (AUD) number 15-001. The Canadian Council for Animal Care guidelines were observed during all procedures with animals.
Twenty-four Landrace 5- to 6-week-old grower pigs were purchased from a local source in Manitoba, Canada. Upon arrival at the National Centre for Foreign Animal Disease (NCFAD), the pigs were randomly assigned into 4 groups (A to D) of 6 pigs each and housed in 4 separate containment-level 3 animal cubicles. The pigs were allowed 7 d to acclimatize to their new surroundings before virus inoculation. Food and water were provided ad libitum and the animals were monitored daily.
The inoculum, FMDV O UKG 11/2001, was grown in lamb kidney cells, as previously described (16). Two animals per group were intradermally inoculated with 1 × 106 tissue culture infective dose (TCID50) of the virus per animal in a total volume of 200 μL in 2 sites (100 μL per site) on the heel bulb of the left hind limb under isoflurane anesthesia. The inoculated animals served as a source of virus for the remaining 4 group mates through direct-contact exposure. All animals were monitored for the development of clinical signs of FMD. Clinical scores for FMD lesions and lameness were recorded for each animal daily. The formation of vesicles or development of lesions was scored on a scale of 1 to 3, depending on severity (1 — mild, 2 — moderate, and 3 — severe). Lameness was similarly scored from 1 to 3, with a score of 3 meaning the animal could not rise or walk. A score of 3 for either one of the criteria was considered the humane endpoint and the affected animal was euthanized. The clinical scores for each animal were combined to obtain the total score, with the possible maximum score of 6 per day per animal.
Serum, nasal swabs, oral swabs, and oral fluid samples were collected daily for the first 7 days post-inoculation (dpi) and weekly thereafter until the end of the experiment at dpi 28, as previously described (17).
Quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR)
The amount of viral ribonucleic acid (RNA) in oral fluids, oral swabs, nasal swabs, and serum samples was quantified by a TaqMan qRT-PCR assay that specifically amplified an 88-bp conserved region of the FMDV 3D gene. Total RNA was extracted from the samples using the MagMAX-96 Viral RNA Isolation Kit (AMB1836-5; Life Technologies, Burlington, Ontario) and the MagMAX Express-96 Magnetic Particle Processor (Life Technologies), following the manufacturer’s protocol. One-step qRT-PCR was carried out using AgPath ID One-Step RT-PCR reagents (AM1005; Life Technologies) on the Applied Biosystems 7500 Real-Time PCR Instrument (Life Technologies) as previously described (18). To determine diagnostic specificity of the qRT-PCR for oral fluids (OFs), 300 OF samples collected from groups of clinically healthy and FMDV-free pigs in Canada and the US were tested.
Virus isolation and titration
Virus was isolated from oral fluids and oral swab samples on monolayers of a fetal porcine kidney cell line constitutively expressing αvβ6 integrin (LFBKαvβ6 cells) (19,20), using a previously described method (17).
Detection of antigen by double-antibody sandwich ELISA (DAS ELISA)
The presence of viral antigen in oral swabs, oral fluids, and sera was tested by a double-antibody sandwich (DAS) ELISA, as previously described (16,21).
Detection of FMDV antigen by the lateral flow immunochromatographic strip test
Foot-and-mouth disease virus (FMDV) serotype O antigen was detected in oral fluids (OFs) by a lateral flow immunochromatographic strip test (LFIST) described previously (22). Briefly, 50 μL of OFs was mixed with a 50-μL cocktail of capture and colloid-gold-conjugated detection monoclonal antibodies in a running buffer (RapidAssays ApS, Copenhagen, Denmark). If FMDV antigen was present, immune complexes were formed between the antigen and the monoclonal antibodies. The strips were dipped into this mixture, which then migrated along the membrane to the test and control lines. The results were interpreted as previously described (22).
Detection of FMDV serotype O-specific IgA isotype antibodies in oral fluids
A modification of a previously described FMDV serotype O solid-phase competitive ELISA was used to detect FMDV serotype O-specific IgA (16,21). Briefly, 96-well ELISA plates (Nunc Maxisorb; Thermo Fisher Scientific, Waltham, Massachusetts, USA) were coated with a rabbit anti-FMDV O antibody in carbonate buffer, pH 9.6 (50 μL/well) and incubated overnight at 4°C. The contents of each well were emptied and the plates were blocked with 10% horse serum in PBST (phosphate-buffered saline, 0.05% Tween-20, pH 7.2) for 1 h at 37°C with shaking. After washing 5 times with PBST, FMDV serotype O antigen in blocking buffer (50 μL/well) was added to the plates and incubated for 1 h at 37°C with shaking. The plates were washed 5 times and oral fluids diluted 1/2 in PBST containing 5% horse serum were added to the plates at 50 μL/well. After 1 h of incubation, plates were washed and mouse anti-pig IgA (AbD Serotec, Raleigh, North Carolina, USA) at approximately 0.17 μg/mL in PBST containing 5% horse serum was added to the plates (50 μL/well) and incubated for 1 h at 37°C with shaking. The plates were washed 5 times and tetramethylbenzidine (TMB; Thermo Scientific, Rockford, Illinois, USA) substrate was added to the plates (50 μL/well). The plates were incubated in the dark at room temperature for 10 min. After adding the stop solution [2 M sulfuric acid (H2SO4)], the optical density (OD) was read at 450 nm (OD450) using an ELISA plate reader (SpectraMax Plus 384 Microplate Reader; Molecular Devices, Sunnyvale, California, USA). To determine diagnostic specificity, 299 OF samples collected from groups of clinically healthy and FMDV-free pigs in Canada and the US were tested on the same assay. In addition, oral fluids from pigs infected with swine vesicular disease virus (SVDV) and vesicular stomatitis virus (VSV) were tested to determine analytical specificity.
Data analysis
Receiver operating characteristic (ROC) analysis was conducted using Version 1.3 of easyROC (23). Results for known negative OF samples and from inoculated animals starting at dpi 14, 21, and 28, which allowed the humoral response to develop, were used for the ROC analysis. The optimal cutoff was determined by the Youden method (24).
Results
Clinical scores in pigs inoculated with foot-and-mouth disease virus
Seven of the 8 pigs inoculated on the heel bulb developed fever and lesions typical of FMD at dpi 2. One directly inoculated pig died abruptly at dpi 2 without developing fever. Furthermore, 1 direct-contact animal also died abruptly without showing any clinical signs. At postmortem, necrotic myocarditis was observed in both pigs. The rest of the direct-contact animals developed fever and clinical disease at dpi 3 and 4, 24 to 48 h later than the directly inoculated pigs (Figure 1). On or before dpi 7, most of the pigs (66.7%) were either dead or euthanized for humane reasons, due to severe lesions and/or lameness.
Figure 1.
Fever and clinical scores of pigs infected with foot-and-mouth disease virus (FMDV). Two 5- to 6-week-old pigs per group of 6 were inoculated with FMDV and placed in direct contact with the rest of the group. Rectal temperatures and clinical scores ranging from 0 for normal to 6 for severe lesions and clinical signs were recorded for each animal. Directly inoculated pigs are represented in red and direct contacts in black.
Detection of FMDV in oral fluids and nasal swabs by qRT-PCR and virus isolation
Ribonucleic acid (RNA) of foot-and-mouth disease (FMDV) was detected in oral fluids (OFs) at dpi 1 in groups A, B, and D and at dpi 2 in group C and preceded the onset of clinical signs. Ribonucleic acid (RNA) of FMDV was also detected in oral swabs and nasal swabs, starting at dpi 2 for most of the directly inoculated pigs and at dpi 3 for the direct-contact pigs. Similarly, based on FMDV RNA detection in serum, viremia began at dpi 1 in directly inoculated pigs and at dpi 2 to 3 in the direct-contact pigs (Table I). Higher levels of FMDV genome were detected in oral fluids than in oral swabs and nasal swabs at corresponding days post-infection (Figure 2). Additionally, there was prolonged detection of FMDV RNA in oral fluids (up to dpi 14 and 21), whereas only 2 samples from oral swabs were positive at dpi 14 and all nasal swabs and sera were negative after dpi 7 (Figure 2 and Table I). All 300 OF samples collected from farms in Canada and the US were negative, giving a diagnostic specificity of 100%.
Table I.
Detection of foot-and-mouth disease virus (FMDV) by real-time reverse transcription polymerase chain reaction (qRT-PCR). Five- to 6-week-old pigs were inoculated with FMDV and oral fluid (OF) samples were collected from each of 4 groups using cotton ropes; sera, oral swabs, and nasal swabs were also collected from individual animals. All samples were tested by FMDV 3D qRT-PCR. Crossing threshold (Ct) of < 35.99 was considered positive for FMDV genome. The lower the Ct value, the higher the amount of FMDV ribonucleic acid (RNA). Any sample with undetermined Ct was assigned a Ct of 40. Numbers in bold are positive for FMDV genome. The first 2 pigs in each group were directly inoculated
| dpi −1 | dpi 1 | dpi 2 | dpi 3 | dpi 4 | dpi 5 | dpi 6 | dpi 7 | dpi 14 | dpi 21 | dpi 28 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Group A | |||||||||||
| OF | 40.0 | 24.3 | 23.6 | 23.1 | 28.1 | 25.2 | 32.1 | 32.6 | 32.2 | 33.5 | |
| Oral swabs | |||||||||||
| 1 | 40.0 | 40.0 | 32.6 | 30.6 | 25.8 | ||||||
| 2 | 40.0 | 40.0 | 27.6 | 26.2 | 28.9 | 27.5 | 40.0 | ||||
| 3 | 40.0 | 40.0 | 38.5 | 34.3 | 23.6 | 23.7 | 38.5 | 33.5 | 39.3 | 40.0 | 40.0 |
| 4 | 38.4 | 40.0 | 40.0 | 35.6 | 28.4 | 28.1 | 30.8 | ||||
| 5 | 40.0 | 40.0 | 40.0 | 34.9 | 28.1 | 29.2 | 35.0 | 30.0 | 40.0 | 40.0 | |
| 6 | 40.0 | 40.0 | 40.0 | 32.1 | 22.7 | 36.6 | |||||
| Nasal swabs | |||||||||||
| 1 | 40.0 | 40.0 | 38.4 | 24.2 | 30.6 | ||||||
| 2 | 40.0 | 40.0 | 26.7 | 29.8 | 27.2 | 30.9 | 30.7 | ||||
| 3 | 40.0 | 40.0 | 39.2 | 36.5 | 23.1 | 31.7 | 40.0 | 40.0 | 40.0 | 40.0 | 40.0 |
| 4 | 40.0 | 40.0 | 40.0 | 35.5 | 22.0 | 22.8 | 28.3 | ||||
| 5 | 40.0 | 40.0 | 40.0 | 37.9 | 29.2 | 30.0 | 37.5 | 37.8 | 40.0 | 40.0 | |
| 6 | 40.0 | 40.0 | 38.2 | 35.3 | 22.9 | 38.4 | |||||
| Serum | |||||||||||
| 1 | 40.0 | 31.3 | 19.6 | 24.7 | 38.5 | ||||||
| 2 | 40.0 | 21.4 | 17.6 | 24.8 | 33.5 | 31.2 | 40.0 | ||||
| 3 | 40.0 | 40.0 | 40.0 | 24.1 | 17.3 | 32.9 | 36.0 | 37.4 | 40.0 | 40.0 | 40.0 |
| 4 | 40.0 | 40.0 | 35.9 | 20.8 | 17.8 | 18.9 | 29.6 | ||||
| 5 | 40.0 | 40.0 | 40.0 | 28.4 | 17.7 | 33.1 | 40.0 | 40.0 | 40.0 | 40.0 | |
| 6 | 40.0 | 40.0 | 33.0 | 18.5 | 22.8 | 36.7 | |||||
| Group B | |||||||||||
| OF | 40.0 | 28.63 | 23.28 | 19.44 | 25.74 | 26.57 | 32.31 | 27.53 | 31.84 | 33.52 | 40.00 |
| Oral swabs | |||||||||||
| 7 | 40.0 | 40.0 | 22.8 | 31.3 | 32.0 | 29.7 | 36.8 | 33.4 | |||
| 8 | 40.0 | 40.0 | 24.4 | 31.0 | |||||||
| 9 | 40.0 | 40.0 | 40.0 | 33.4 | 33.0 | ||||||
| 10 | 40.0 | 40.0 | 40.0 | 30.9 | 32.4 | 38.8 | 29.0 | 35.5 | 39.0 | 37.8 | 40.0 |
| 11 | 40.0 | 40.0 | 38.2 | 28.5 | 26.2 | 33.6 | |||||
| 12 | 40.0 | 40.0 | 40.0 | 39.2 | 33.1 | 29.2 | 40.0 | 40.0 | 40.0 | 40.0 | 40.0 |
| Nasal swabs | |||||||||||
| 7 | 40.0 | 40.0 | 32.4 | 34.1 | 35.0 | 39.9 | 40.0 | 32.5 | |||
| 8 | 40.0 | 40.0 | 24.0 | 24.7 | |||||||
| 9 | 40.0 | 40.0 | 40.0 | 34.6 | 38.1 | ||||||
| 10 | 40.0 | 40.0 | 40.0 | 34.7 | 24.7 | 31.5 | 32.8 | 32.3 | 40.0 | 40.0 | |
| 11 | 40.0 | 40.0 | 40.0 | 24.4 | 27.5 | 30.2 | |||||
| 12 | 40.0 | 40.0 | 40.0 | 40.0 | 28.1 | 28.2 | 38.1 | 28.1 | 39.6 | 40.0 | |
| Serum | |||||||||||
| 7 | 40.0 | 22.1 | 16.8 | 30.6 | 36.1 | 29.3 | 24.8 | 39.0 | |||
| 8 | 40.0 | 21.7 | 16.6 | 22.9 | |||||||
| 9 | 40.0 | 40.0 | 36.0 | 21.8 | 38.9 | ||||||
| 10 | 40.0 | 40.0 | 40.0 | 40.0 | 24.8 | 37.1 | 39.0 | 40.0 | 40.0 | 39.7 | 40.0 |
| 11 | 40.0 | 40.0 | 25.8 | 16.1 | 17.0 | 28.9 | |||||
| 12 | 40.0 | 40.0 | 40.0 | 22.6 | 36.0 | 39.4 | 39.2 | 40.0 | 40.0 | 40.0 | 40.0 |
| Group C | |||||||||||
| OF | 40.0 | 36.1 | 25.4 | 23.1 | 26.1 | 25.6 | 28.2 | 28.9 | 32.8 | 37.4 | 40.0 |
| Oral swabs | |||||||||||
| 13 | 40.0 | 35.9 | 34.4 | 36.8 | 23.0 | 20.5 | 33.5 | 34.7 | 35.4 | 40.0 | 40.0 |
| 14 | 40.0 | 40.0 | 32.6 | ||||||||
| 15 | 40.0 | 40.0 | 40.0 | 35.4 | 23.0 | ||||||
| 16 | 40.0 | 40.0 | 40.0 | 35.0 | 28.1 | 28.9 | |||||
| 17 | 40.0 | 40.0 | 40.0 | 39.3 | 26.4 | 35.3 | 34.4 | 40.0 | 32.2 | 40.0 | 40.0 |
| 18 | 40.0 | 40.0 | 40.0 | 39.5 | 33.6 | 34.6 | 39.7 | 40.0 | |||
| Nasal swabs | |||||||||||
| 13 | 40.0 | 40.0 | 28.4 | 30.1 | 21.8 | 22.3 | 27.5 | 31.6 | 40.0 | 40.0 | 40.0 |
| 14 | 40.0 | 38.4 | 30.4 | ||||||||
| 15 | 40.0 | 40.0 | 40.0 | 30.3 | 21.4 | ||||||
| 16 | 40.0 | 40.0 | 39.1 | 34.0 | 38.1 | 22.2 | |||||
| 17 | 40.0 | 40.0 | 40.0 | 40.0 | 30.1 | 28.2 | 29.3 | 37.5 | 40.0 | 40.0 | |
| 18 | 40.0 | 40.0 | 40.0 | 40.0 | 27.2 | 35.5 | 25.2 | 32.3 | |||
| Serum | |||||||||||
| 13 | 40.0 | 23.4 | 17.2 | 28.8 | 23.3 | 26.7 | 31.6 | 40.0 | 40.0 | 40.0 | 40.0 |
| 14 | 40.0 | 20.0 | 16.7 | ||||||||
| 15 | 40.0 | 40.0 | 36.5 | 23.5 | 18.8 | ||||||
| 16 | 40.0 | 40.0 | 36.9 | 21.4 | 24.1 | 20.1 | |||||
| 17 | 40.0 | 40.0 | 39.2 | 36.5 | 18.3 | 26.1 | 34.1 | 33.8 | 40.0 | 40.0 | 40.0 |
| 18 | 40.0 | 40.0 | 37.8 | 22.0 | 36.2 | 40.0 | 38.9 | 40.0 | |||
| Group D | |||||||||||
| OF | 40.0 | 24.4 | 24.1 | 22.8 | 29.9 | 24.6 | 40.0 | 36.1 | 33.5 | 40.0 | 40.0 |
| Oral swabs | |||||||||||
| 19 | 40.0 | 40.0 | 35.4 | 37.0 | 34.6 | 32.3 | 40.0 | 40.0 | 38.4 | 40.0 | 40.0 |
| 20 | 40.0 | 36.7 | 34.0 | 37.9 | 25.1 | 31.4 | 40.0 | 37.8 | 40.0 | 40.0 | 40.0 |
| 21 | 40.0 | 39.8 | 40.0 | 30.6 | 24.0 | 30.7 | 31.5 | 29.5 | |||
| 22 | 40.0 | 40.0 | 40.0 | 40.0 | 22.4 | 24.5 | 35.9 | 40.0 | |||
| 23 | 40.0 | 40.0 | 40.0 | 36.9 | 32.3 | 24.0 | 29.8 | ||||
| 24 | 36.7 | 40.0 | 40.0 | 37.0 | 35.4 | 33.6 | 40.0 | 40.0 | |||
| Nasal swabs | |||||||||||
| 19 | 40.0 | 37.9 | 28.9 | 27.5 | 30.8 | 30.5 | 32.4 | 35.2 | 40.0 | 40.0 | |
| 20 | 40.0 | 40.0 | 31.0 | 26.5 | 26.6 | 34.1 | 27.6 | 30.4 | 40.0 | 40.0 | |
| 21 | 40.0 | 40.0 | 40.0 | 24.4 | 27.1 | 27.0 | 27.7 | 34.8 | |||
| 22 | 40.0 | 40.0 | 40.0 | 40.0 | 40.0 | 26.7 | 32.2 | 36.1 | |||
| 23 | 40.0 | 38.3 | 40.0 | 35.0 | 40.0 | 24.1 | 25.6 | ||||
| 24 | 40.0 | 40.0 | 40.0 | 37.5 | 36.1 | 32.5 | 37.7 | 40.0 | |||
| Serum | |||||||||||
| 19 | 40.0 | 24.2 | 17.6 | 31.0 | 35.4 | 39.7 | 40.0 | 40.0 | 40.0 | 40.0 | 40.0 |
| 20 | 40.0 | 23.4 | 17.6 | 26.9 | 21.9 | 33.6 | 36.8 | 40.0 | 40.0 | 40.0 | 40.0 |
| 21 | 40.0 | 39.8 | 23.7 | 17.2 | 18.3 | 24.9 | 38.4 | 37.8 | |||
| 22 | 40.0 | 40.0 | 40.0 | 23.7 | 19.3 | 34.3 | 40.0 | 40.0 | |||
| 23 | 40.0 | 40.0 | 36.4 | 19.5 | 20.9 | 27.3 | 40.0 | ||||
| 24 | 40.0 | 40.0 | 40.0 | 27.8 | 35.3 | 37.0 | 40.0 | 40.0 | |||
dpi — Days post-inoculation.
Figure 2.
Detection of foot-and-mouth disease virus (FMDV) by real-time reverse transcription polymerase chain reaction (qRT-PCR). Five- to 6-week-old pigs were inoculated with FMDV and oral fluids (OF) were collected from each of 4 groups using cotton ropes; sera (Bld), oral swabs (OS), and nasal swabs (NS) were also collected from individual animals. All samples were tested by FMDV 3D qRT-PCR. Error bars represent standard deviation of mean.
Additionally, live FMDV was isolated from OF samples at dpi 1 to 5 in all the groups except group A, in which virus isolation started at dpi 2 (Figure 3). Virus isolation from oral swabs was delayed, starting at dpi 4 for most animals. Virus titers were higher in oral fluids than in oral swabs (Figure 3).
Figure 3.
Isolation of foot-and-mouth disease virus (FMDV) from oral fluids (OF) and oral swabs (OS). Five- to 6-week-old pigs were inoculated with FMDV and oral fluids were collected from each of 4 groups using cotton ropes; oral swabs (OS) were also collected from individual animals. Error bars represent standard deviation of mean. *Time points when pigs were less interested in ropes, which reduced amount of OF.
Detection of antigen in oral fluids
Foot-and-mouth disease virus (FMDV) antigen was detected in OF samples from groups A and B at dpi 2 and at dpi 3 in all 4 groups by the DAS ELISA (Figure 4). On the other hand, 1 sample from oral swabs on dpi 2, 3, and 5 and 5 oral swabs at dpi 4 were positive for FMDV antigen (data not shown).
Figure 4.
Detection of foot-and-mouth disease virus (FMDV) antigen in oral fluids (OF) by the double-antibody sandwich (DAS) ELISA. Five- to 6-week-old pigs were inoculated with FMDV and oral fluids were collected from each of 4 groups using cotton ropes and tested for FMDV antigen by a DAS ELISA. Samples with an optical density (OD) of > 0.1 were considered positive for FMDV antigen. Grp = group.
Similarly, FMDV antigen was detected by LFIST at dpi 2 to 5 in OF samples from all 4 groups. The OF samples became negative for FMDV antigen at dpi 6 with the exception of group C that remained weakly positive.
Detection of IgA antibody in oral fluids
An optimal positive cutoff of OD ≥ 0.75 for anti-FMDV IgA in oral fluids (OFs) was determined by the Youden method (24). The distribution (scatter grams) of all the negative and positive samples is shown in Figure 5A. Diagnostic sensitivity and specificity were both 100%. The OF sample became positive for anti-FMDV IgA by DPI 14 (Figure 5B). There was no cross-reactivity with OF samples positive for antibodies to swine vesicular disease virus (SVDV) and vesicular stomatitis virus (VSV).
Figure 5.
Detection of anti-foot-and-mouth disease virus (FMDV) IgA antibody in oral fluids (OF) by isotype-specific ELISA. Five- to 6-week-old pigs were inoculated with FMDV and oral fluids were collected from each of 4 groups using cotton ropes and tested for FMDV antigen by an IgA-specific ELISA. A — Distribution of FMDV antibody negative and positive OF samples and the cutoff optical density (OD) (broken horizontal line). B — Temporal IgA response in OF from experimentally inoculated pigs. Negative oral fluids (n = 299) were collected from farms and experimental animals before inoculation.
Discussion
Collection of oral fluids (OFs) is cost-effective, stress-free for the animals, and has been proposed as an appropriate type of sample for disease surveillance (10,12,15). Testing oral fluids for the early detection of reportable diseases like FMD could therefore be highly beneficial to the pork industry around the world. The objective of this study was to evaluate the potential use of OFs as an alternate type of sample for detecting FMDV in experimentally infected pigs. Similar work has been reported by other groups (14,15). However, in addition to virus genome detection in these studies, we evaluated virus isolation and viral antigen detection by a double-antibody sandwich enzyme-linked immunosorbent assay (DAS ELISA) and a lateral flow immunochromatographic strip test (LFIST), which is a rapid pen-side test. Furthermore, we demonstrated the suitability of OF samples for detecting anti-FMDV antibodies (IgA) post-infection.
In this study, directly inoculated pigs developed viremia by dpi 1 and clinical signs by dpi 2, but these were delayed by 24 to 48 h in direct contacts, which is consistent with previous findings in pigs (24–26). The detection of FMDV genome in OFs from groups of FMDV-infected pigs preceded the onset of clinical signs, which is similar to previous findings (14,15). This is significant in that the presence of FMDV in an apparently healthy herd can be detected using swine OFs. In the event of an outbreak, OFs preemptively sampled from apparently healthy pigs within a radius of the outbreak could be tested by qRT-PCR and rapid control measures implemented before the appearance of clinical signs.
We reported the detection of SVDV in oral fluids from groups of pigs with subclinical disease (17). Similar to our previous report for SVDV (17), oral fluids are better than oral swabs and nasal swabs for detecting FMDV. This could be attributed to prolonged chewing on ropes (20 to 30 min), which enhanced the contents of oral fluids. In a previous study (15), the quantification cycles (Cq) for detecting FMDV RNA were lower for oral fluids (OFs) than for oral swabs, which indicates that levels of FMDV genome are higher in OFs than in oral swabs. In addition, Vosloo et al (14) reported the detection of FMDV genome in OFs but not in oral swabs in a group of pigs challenged with FMDV serotype A. Nevertheless, they also reported contrary results for a specific group of pigs challenged with FMDV serotype O (14) in which oral swabs (OSs) were positive, while samples from oral fluids (OFs) were negative for FMDV serotype O genome at some sampling points. The possibility was raised that proteolytic enzymes might have affected virus recovery in OFs due to the 30-minute collection time at room temperature. This does not appear to be the case, however, since the oral fluids (OFs) in our study were collected in a similar manner to that in another study (15).
Virus isolation is considered a confirmatory test for FMDV. Isolating FMDV from OFs as early as dpi 1 therefore adds value to the use of this type of sample for diagnosing foot-and-mouth disease (FMD). Detmer et al (28), however, isolated virus from nasal swabs and not from oral fluids (OFs) from pigs infected with influenza A. This could be explained by the fact that, during sampling of oral fluids, the ropes were left in the pen for 6 or 12 h rather than for the standard 20 to 30 min (28). Another possibility is that the virus on the ropes was inactivated since influenza A virus becomes inactivated on highly porous surfaces in less than 12 h (29).
Some laboratories, especially in FMD-endemic countries, are not equipped for qRT-PCR testing. On the other hand, ELISA can be carried out in most laboratories worldwide. Thus, the detection of FMDV antigen in OFs by DAS ELISA means that most laboratories can use this type of sample to diagnose FMD. Although the DAS ELISA is considered relatively less sensitive than qRT-PCR, it is one of the routine diagnostic tests discussed in the OIE Manual of Diagnostic Tests and Vaccines for Terrestrial Animals (30).
Combining the ease of collecting oral fluids (OFs) with a rapid pen-side test would simplify detection of FMD in pigs. The lateral flow immunochromatographic strip test (LFIST) is rapid, easy to perform, and has been used to diagnose FMD (22). We have shown in this study that LFIST detects FMDV antigen in oral fluids (OFs), meaning a pig farmer or attendant can easily collect OFs, test by LFIST, and make a preliminary diagnosis of FMD. This is significant since effective control of outbreaks and eradication of FMD depend on rapid detection. Furthermore, we had previously shown that the FMDV genome could be detected in OFs from infected pigs with a rapid field-deployable reverse-transcription insulated isothermal (RT-iiPCR) assay (31).
A previous study also explored mucosal antibody response in FMDV-infected pigs using saliva and reported the detection of FMDV-specific IgA, but not IgG and IgM (27). Therefore, in this study we focused on the detection of IgA in OFs, corroborating previous observations on saliva (26). The detection of anti-FMDV IgA in oral fluids (OFs) confirms the usefulness of this type of sample for serological diagnosis of FMD in pigs by the IgA ELISA.
In conclusion, foot-and-mouth disease virus (FMDV) and antibodies to FMDV can be detected in oral fluids (OFs) from FMDV-infected pigs. This confirms the relevance of oral fluids from swine for routine virological surveillance as well as the potential for using OFs for serological surveillance of swine herds for FMDV.
Acknowledgments
The authors thank the animal care staff at the National Centre for Foreign Animal Disease (NCFAD) for their support with animal experiments. We are also grateful to Amber Papineau, Melissa Goolia, Tim Salo, and Kate Hole for technical support.
This study was funded by the US National Pork Board (NPB# 14-286) and the Canadian Food Inspection Agency (CFIA).
References
- 1.Paarlberg PL, Lee JG, Seitzinger AH. Potential revenue impact of an outbreak of foot-and-mouth disease in the United States. J Am Vet Med Assoc. 2002;220:988–992. doi: 10.2460/javma.2002.220.988. [DOI] [PubMed] [Google Scholar]
- 2.Alexandersen S, Mowat N. Foot-and-mouth disease: Host range and pathogenesis. Curr Top Microbiol Immunol. 2005;288:9–42. doi: 10.1007/3-540-27109-0_2. [DOI] [PubMed] [Google Scholar]
- 3.Grubman MJ, Baxt B. Foot-and-mouth disease. Clin Microbiol Rev. 2004;17:465–493. doi: 10.1128/CMR.17.2.465-493.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Arzt J, Baxt B, Grubman MJ, et al. The pathogenesis of foot-and-mouth disease II: Viral pathways in swine, small ruminants, and wildlife; myotropism, chronic syndromes, and molecular virus-host interactions. Transbound Emerg Dis. 2011;58:305–326. doi: 10.1111/j.1865-1682.2011.01236.x. [DOI] [PubMed] [Google Scholar]
- 5.Alexandersen S, Donaldson AI. Further studies to quantify the dose of natural aerosols of foot-and-mouth disease virus for pigs. Epidemiol Infect. 2002;128:313–323. doi: 10.1017/s0950268801006501. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Kitching RP, Alexandersen S. Clinical variation in foot and mouth disease: Pigs. Rev Sci Tech. 2002;21:513–518. doi: 10.20506/rst.21.3.1367. [DOI] [PubMed] [Google Scholar]
- 7.Murphy C, Bashiruddin JB, Quan M, Zhang Z, Alexandersen S. Foot-and-mouth disease viral loads in pigs in the early, acute stage of disease. Vet Rec. 2010;166:10–14. doi: 10.1136/vr.b5583. [DOI] [PubMed] [Google Scholar]
- 8.Galloway JW, Keijser BJ, Williams DM. Saliva in studies of epidemiology of human disease: The UK biobank project. Periodontol 2000. 2016;70:184–195. doi: 10.1111/prd.12108. [DOI] [PubMed] [Google Scholar]
- 9.Wormwood KL, Aslebagh R, Channaveerappa D, et al. Salivary proteomics and biomarkers in neurology and psychiatry. Proteomics Clin Appl. 2015;9:899–906. doi: 10.1002/prca.201400153. [DOI] [PubMed] [Google Scholar]
- 10.Prickett JR, Zimmerman JJ. The development of oral fluid-based diagnostics and applications in veterinary medicine. Anim Health Res Rev. 2010;11:207–216. doi: 10.1017/S1466252310000010. [DOI] [PubMed] [Google Scholar]
- 11.Prickett JR, Kim W, Simer R, Yoon KY, Zimmermman J. Oral-fluid samples for surveillance of commercial growing pigs for porcine reproductive and respiratory syndrome virus and porcine circo-virus type 2 infections. J Swine Health Prod. 2008;16:86–91. [Google Scholar]
- 12.Kittawornrat A, Wang C, Anderson G, et al. Ring test evaluation of the repeatability and reproducibility of a porcine reproductive and respiratory syndrome virus oral fluid antibody enzyme-linked immunosorbent assay. J Vet Diagn Invest. 2012;24:1057–1063. doi: 10.1177/1040638712457929. [DOI] [PubMed] [Google Scholar]
- 13.Giménez-Lirola LG, Mur L, Rivera B, et al. Detection of African swine fever virus antibody in serum and oral fluid specimens using a recombinant protein 30 (p30) dual matrix indirect ELISA. PLoS ONE. 2016;11:e0161230. doi: 10.1371/journal.pone.0161230. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Vosloo W, Morris J, Davis A, et al. Collection of oral fluids using cotton ropes as a sampling method to detect foot-and-mouth disease virus infection in pigs. Transbound Emerg Dis. 2015;62:e71–75. doi: 10.1111/tbed.12196. [DOI] [PubMed] [Google Scholar]
- 15.Grau FR, Schroeder ME, Mulhern EL, McIntosh MT, Bounpheng MA. Detection of African swine fever, classical swine fever, and foot-and-mouth disease viruses in swine oral fluids by multiplex reverse transcription real-time polymerase chain reaction. J Vet Diagn Invest. 2015;27:140–149. doi: 10.1177/1040638715574768. [DOI] [PubMed] [Google Scholar]
- 16.Moniwa M, Embury-Hyatt C, Zhang Z, et al. Experimental foot-and-mouth disease virus infection in white tailed deer. J Comp Pathol. 2012;147:330–342. doi: 10.1016/j.jcpa.2012.01.010. [DOI] [PubMed] [Google Scholar]
- 17.Senthilkumaran C, Bittner H, Ambagala A, et al. Use of oral fluids for detection of virus and antibodies in pigs infected with swine vesicular disease virus. Transbound Emerg Dis. 2016 doi: 10.1111/tbed.12563. [DOI] [PubMed] [Google Scholar]
- 18.Moniwa M, Clavijo A, Li M, Collignon B, Kitching PR. Performance of a foot-and-mouth disease virus reverse transcription-polymerase chain reaction with amplification controls between three real-time instruments. J Vet Diagn Invest. 2007;19:9–20. doi: 10.1177/104063870701900103. [DOI] [PubMed] [Google Scholar]
- 19.LaRocco M, Krug PW, Kramer E, et al. Correction for LaRocco et al. A continuous bovine kidney cell line constitutively expressing bovine αVβ6 integrin has increased susceptibility to foot-and-mouth disease virus. J Clin Microbiol. 2015;53:755. doi: 10.1128/JCM.03220-14. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Fukai K, Morioka K, Yamada M, et al. Comparative performance of fetal goat tongue cell line ZZ-R 127 and fetal porcine kidney cell line LFBK-αVβ6 for foot-and-mouth disease virus isolation. J Vet Diagn Invest. 2015;27:516–521. doi: 10.1177/1040638715584156. [DOI] [PubMed] [Google Scholar]
- 21.Kittelberger R, Nfon C, Swekla K, et al. Foot-and-mouth disease in red deer — Experimental infection and test methods performance. Transbound Emerg Dis. 2017;64:213–225. doi: 10.1111/tbed.12363. [DOI] [PubMed] [Google Scholar]
- 22.Yang M, Caterer NR, Xu W, Goolia M. Development of a multiplex lateral flow strip test for foot-and-mouth disease virus detection using monoclonal antibodies. J Virol Methods. 2015;221:119–126. doi: 10.1016/j.jviromet.2015.05.001. [DOI] [PubMed] [Google Scholar]
- 23.Biosoft Project. easyROC: A Web-tool for ROC Curve Analysis (Version 1.3) [Last accessed January 22, 2017]. Available from: http://www.biosoft.hacettepe.edu.tr/easyROC/
- 24.Youden WJ. Index for rating diagnostic tests. Cancer. 1950;3:32–35. doi: 10.1002/1097-0142(1950)3:1<32::aid-cncr2820030106>3.0.co;2-3. [DOI] [PubMed] [Google Scholar]
- 25.Alexandersen S, Quan M, Murphy C, Knight J, Zhang Z. Studies of quantitative parameters of virus excretion and transmission in pigs and cattle experimentally infected with foot-and-mouth disease virus. J Comp Pathol. 2003;129:268–282. doi: 10.1016/s0021-9975(03)00045-8. [DOI] [PubMed] [Google Scholar]
- 26.Alexandersen S, Oleksiewicz MB, Donaldson AI. The early pathogenesis of foot-and-mouth disease in pigs infected by contact: A quantitative time-course study using TaqMan RT-PCR. J Gen Virol. 2001;82:747–755. doi: 10.1099/0022-1317-82-4-747. [DOI] [PubMed] [Google Scholar]
- 27.Pacheco JM, Butler JE, Jew J, Ferman GS, Zhu J, Golde WT. IgA antibody response of swine to foot-and-mouth disease virus infection and vaccination. Clin Vaccine Immunol. 2010;17:550–558. doi: 10.1128/CVI.00429-09. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Detmer SE, Patnayak DP, Jiang Y, Gramer MR, Goyal SM. Detection of influenza A virus in porcine oral fluid samples. J Vet Diagn Invest. 2011;23:241–247. doi: 10.1177/104063871102300207. [DOI] [PubMed] [Google Scholar]
- 29.Bean B, Moore BM, Sterner B, et al. Survival of influenza viruses on environmental surfaces. J Infect Dis. 1982;146:47–51. doi: 10.1093/infdis/146.1.47. [DOI] [PubMed] [Google Scholar]
- 30.World Organisation for Animal Health (OIE) Manual of Diagnostic Tests and Vaccines for Terrestrial Animals. [Last accessed January 23, 2017]. Available from: http://www.oie.int/fileadmin/Home/eng/Health_standards/tahm/2.01.05_FMD.pdf.
- 31.Ambagala A, Fisher M, Goolia M, et al. Field-deployable reverse transcription-insulated isothermal PCR (RT-iiPCR) assay for rapid and sensitive detection of foot-and-mouth disease virus. Transbound Emerg Dis. 2016 doi: 10.1111/tbed.12554. [Epub ahead of print] [DOI] [PMC free article] [PubMed] [Google Scholar]