Detection of influenza A virus nucleoprotein antibodies in oral fluid specimens from pigs infected under experimental conditions using a blocking ELISA

Swine influenza A virus infection dynamics and evolution in intensive pig production systems

Swine influenza A virus (swIAV) is one of the main viral pathogens responsible for respiratory disease in farmed pigs. While outbreaks are often epidemic in nature, increasing reports suggest that continuous, endemic infection of herds is now common. The move towards larger herd sizes and increased intensification in the commercial pig industry may promote endemic infection; however, the impact that intensification has on swIAV infection dynamics and evolution is unclear. We carried out a longitudinal surveillance study for over 18 months on two enzootically infected, intensive, indoor, and multi-site pig production flows. Frequent sampling of all production stages using individual and group sampling methods was performed, followed by virological and immunological testing and whole-genome sequencing. We identified weaned pigs between 4 and 12-weeks old as the main reservoir of swIAV in the production flows, with continuous, year-round infection. Despite the continuous nature of viral circulation, infection levels were not uniform, with increasing exposure at the herd level associated with reduced viral prevalence followed by subsequent rebound infection. A single virus subtype was maintained on each farm for the entire duration of the study. Viral evolution was characterised by long periods of stasis punctuated by periods of rapid change coinciding with increasing exposure within the herd. An accumulation of mutations in the surface glycoproteins consistent with antigenic drift was observed, in addition to amino acid substitutions in the internal gene products as well as reassortment exchange of internal gene segments from newly introduced strains. These data demonstrate that long-term, continuous infection of herds with a single subtype is possible and document the evolutionary mechanisms utilised to achieve this.

A new nanobody-enzyme fusion protein-linked immunoassay for detecting antibodies against influenza A virus in different species

Circulation of influenza A virus (IAV), especially within poultry and pigs, continues to threaten public health. A simple and universal detecting method is important for monitoring IAV infection in different species. Recently, nanobodies, which show advantages of easy gene editing and low cost of production, are a promising novel diagnostic tool for the monitoring and control of global IAVs. In the present study, five nanobodies against the nucleoprotein of H9N2 IAV were screened from the immunized Bactrian camel by phage display and modified with horseradish peroxidase (HRP) tags. Out of which, we determined that H9N2-NP-Nb5-HRP can crossreact with different subtypes of IAVs, and this reaction is also blocked by positive sera for antibodies against different IAV subtypes. Epitope mapping showed that the nanobody-HRP fusion recognized a conserved conformational epitope in all subtypes of IAVs. Subsequently, we developed a nanobody-based competitive ELISA (cELISA) for detecting anti-IAV antibodies in different species. The optimized amount of coating antigen and dilutions of the fusion and testing sera were 100 ng/well, 1:4000, and 1:10, respectively. The time for operating the cELISA was approximately 35 min. The cELISA showed high sensitivity, specificity, reproducibility, and stability. In addition, we found that the cELISA and hemagglutination inhibition test showed a consistency of 100% and 87.91% for clinical and challenged chicken sera, respectively. Furthermore, the agreement rates were 90.4% and 85.7% between the cELISA and commercial IEDXX ELISA kit. Collectively, our developed nanobody-HRP fusion-based cELISA is an ideal method for monitoring IAV infection in different species.

The epidemiology of swine influenza

Globally swine influenza is one of the most important diseases of the pig industry, with various subtypes of swine influenza virus co-circulating in the field. Swine influenza can not only cause large economic losses for the pig industry but can also lead to epidemics or pandemics in the human population. We provide an overview of the pathogenic characteristics of the disease, diagnosis, risk factors for the occurrence on pig farms, impact on pigs and humans and methods to control it. This review is designed to promote understanding of the epidemiology of swine influenza which will benefit the control of the disease in both pigs and humans.

Noninvasive strategies for surveillance of swine viral diseases: a review

In view of the intensive development of the swine industry, monitoring and surveillance of infectious diseases require low-cost, effective, and representative population sampling methods. We present herein the state of knowledge, to date, in the use of alternative strategies in the monitoring of swine health. Blood sampling, the most commonly used method in veterinary medicine to obtain samples for monitoring swine health, is labor-intensive and expensive, which has resulted in a search for alternative sampling strategies. Oral fluid (OF) is a good alternative to serum for pooled sample analysis, especially for low-prevalence pathogens. Detection of viral nucleic acids or antiviral antibodies in OF is used to detect numerous viruses in the swine population. Meat juice is used as an alternative to serum in serologic testing. Processing fluid obtained during processing of piglets (castration and tail-docking) may also be used to detect viruses. These matrices are simple, safe, cost-effective, and allow testing of many individuals at the same time. The latest methods, such as snout swabs and udder skin wipes, are also promising. These alternative samples are easy to acquire, and do not affect animal welfare negatively.

Guidelines for oral fluid-based surveillance of viral pathogens in swine

Recent decades have seen both rapid growth and extensive consolidation in swine production. As a collateral effect, these changes have exacerbated the circulation of viruses and challenged our ability to prevent, control, and/or eliminate impactful swine diseases. Recent pandemic events in human and animal health, e.g., SARS-CoV-2 and African swine fever virus, highlight the fact that clinical observations are too slow and inaccurate to form the basis for effective health management decisions: systematic processes that provide timely, reliable data are required. Oral fluid-based surveillance reflects the adaptation of conventional testing methods to an alternative diagnostic specimen. The routine use of oral fluids in commercial farms for PRRSV and PCV2 surveillance was first proposed in 2008 as an efficient and practical improvement on individual pig sampling. Subsequent research expanded on this initial report to include the detection of ≥23 swine viral pathogens and the implementation of oral fluid-based surveillance in large swine populations (> 12,000 pigs). Herein we compile the current information regarding oral fluid collection methods, testing, and surveillance applications in swine production.

Correlation of Anti-Salmonella Antibodies Between Serum and Saliva Samples Collected From Finisher Pigs

Saliva samples obtained by using absorptive devices, can provide an alternative diagnostic matrix to serum for monitoring disease status in pigs. The aim of this study was to investigate the correlation of anti-Salmonella antibodies between serum and saliva samples collected from pigs. Twenty individual paired serum and saliva samples were collected from a single farm. Anti-Salmonella IgG was detected in individual serum samples using a commercial Salmonella ELISA kit, validated for sera. The same kit was used with a protocol modified by extending incubation time and increasing temperature to test individual saliva samples. Anti-Salmonella IgG antibodies in pig saliva were always detected at a lower level than in the matching serum samples. A correlation (rho = 0.66; p = 0.002) and a moderate agreement (K > 0.62 p = 0.003) was found between individual Salmonella IgG in serum and saliva samples. Both correlation and the agreement levels are moderate. The size of this investigation was small, and further studies are necessary to further confirm these findings. The results of this work provide some evidence that saliva samples have the potential to be used for the diagnosis of Salmonella infection in pig farms.

Detection of antibodies to Toxoplasma gondii in oral fluid from pigs

Toxoplasma gondii-infected pigs play a major role as a source of infection for humans and detection of high-risk herds is essential to implement control measures at the farm level. The aim of this study was to determine whether oral fluid (OF) could be used as a matrix to detect antibodies against T. gondii in infected pigs by immunoblot (IB). For this, OF from experimentally inoculated sows (n = 8) (serial samples) and naturally exposed group-housed fatteners (n = 42 groups, one sample/group) were analysed for IgG and IgA against T. gondii-SAG1 antigen by IB. Simultaneously, each animal was serologically tested for anti-T. gondii IgG by ELISA. Specific IgG was detected in the sera of all inoculated sows from 2 to 3 weeks post inoculation (pi) and in 3.4 to 92% of the pigs in 13 out of 42 groups. Experimentally inoculated sows showed positive OF-IB results for IgA (100%) and IgG (87.5%) at 1.5 weeks pi and continued yielding positive results for IgA (87.5-75%) and IgG (50%) until 4 weeks pi; however, from 8 weeks pi the frequency of detection of both isotypes was lower, despite constantly positive IgG values in serum-ELISA. Interestingly, consecutive daily samplings for 4 days at 13 and 30 weeks pi showed inconsistent results for some sows, showing that the antibody concentration in OF is prone to timely variations. Pooled OF from groups with 91 and 92% of seropositive pigs yielded positive IB results for IgG and IgA. Fattener groups with ≤13% of seropositive pigs gave negative IB results to both isotypes. Our results showed that antibodies to T. gondii can be detected in OF from infected pigs, and that IgA seems to be a more adequate target than IgG. Although OF does not seem to be a robust matrix to assess the serological status for T. gondii in individual animals, this diagnostic approach represents an interesting non-invasive, low-cost and animal welfare friendly option as a screening method at the farm level to determine high exposure to T. gondii in the herd.

Delivery of a thermo-enzymatically treated influenza vaccine using pulmonary surfactant in pigs

Swine influenza A virus (IAV-S) infections are a major cause of economic losses for the swine industry. The vast genetic and antigenic diversity often results in mismatch between the vaccine and field strains, necessitating frequent updates of vaccines. Inactivated IAV-S vaccines are of questionable efficacy. Intra-nasally administered live vaccines are more effective but are associated with safety concerns. The objective of this study was to develop a first-generation vaccine which combines the safety and efficacy advantages of inactivated and attenuated vaccines respectively. The approach targeted fragmentation of viral nucleic acids while preserving structure. Hence, cultures of influenza A/CA/04/09 H1N1 were exposed to 44 °C for 10 min. to reversibly denature the capsid, followed by RNase treatment to digest the genomic RNA and then refolded at lower temperatures. As targeted, treated virions retained an intact structure and were not detected in the first passage in infected cells. To improve intra-nasal delivery of the vaccine antigen, the vaccine antigen was delivered in porcine lung surfactant. Both the treated vaccine alone or vaccine in combination with the surfactant elicited strong anti-HA and virus neutralizing antibodies, protection against viral shedding and lung lesions in 3-week-old piglets. There were no significant differences between the groups. Vaccine viral replication was not detected in the vaccinated pigs. The described approach can advance current immunization practices against swine influenza viruses due to the relative simplicity, high efficacy and safety and ease of adaptation to newly emerging field strains.

Assessment of abattoir based monitoring of PRRSV using oral fluids

Various porcine reproductive and respiratory syndrome virus (PRRSV) regional elimination projects have been implemented in the U.S., but none have yet succeeded. In part, this reflects the need for efficient methods to monitor over time the progress of PRRSV status of participating herds. This study assessed the feasibility of monitoring PRRSV using oral fluids collected at the abattoir. A total of 36 pig lots were included in the study. On-farm oral fluid (n = 10) and serum (n = 10) collected within two days of shipment to the abattoir were used to establish the reference PRRSV status of the population. Oral fluids (n = 3 per lot) were successfully collected from 32 lots (89%) at the lairage. Three veterinary diagnostic laboratories (VDLs) tested the sera (VDL1 and VDL3: n = 316, VDL2: n = 315) and oral fluids (VDL1 and VDL3: n = 319, VDL2: n = 320) for PRRSV antibodies (ELISA) and RNA (rRT-PCR). Environmental samples (n = 64, 32 before and 32 after pigs were placed in lairage) were tested for PRRSV RNA at one VDL. All oral fluids (farm and abattoir) tested positive for PRRSV antibody at all VDLs. PRRSV positivity frequency on serum ranged from 92.4% to 94.6% among VDLs, with an overall agreement of 97.6%. RNA was detected on 1.3% to 1.9%, 8.1% to 17.7%, and 8.3% to 17.7% of sera, on-farm and abattoir oral fluids, respectively. Between-VDLs rRT-PCR agreement on sera and oral fluids (farm and abattoir) ranged from 97.8% to 99.0%, and 79.0% to 81.2%, respectively. Between-locations agreement of oral fluids varied from 31.3% to 50% depending on the VDL. This study reported the application of swine oral fluids collected at the abattoir for monitoring PRRSV, and describes the between-VDL agreement for PRRS testing of serum and oral fluid field samples.

Detection of Aujeszky's disease virus DNA and antibody in swine oral fluid specimens

Aujeszky's disease virus (ADV) continues to circulate in commercial swine populations in many regions and in feral swine populations in most parts of the world, that is, ADV continues to present a risk to pork producers everywhere. Current DIVA vaccines and assays are highly effective in the control and/or eradication of ADV, but detection of wild-type ADV infection relies on testing individual pig specimens, for example, serum or muscle exudate ("meat juice"). Oral fluid specimens have been shown to be highly effective for the surveillance of a variety of swine pathogens and could offer the means to improve the efficiency of ADV surveillance in the field. In this study, the temporal patterns of ADV DNA and antibody detection in oral fluid and serum specimens were established in ADV-inoculated pigs (n = 14) using gB and gE PCRs, virus neutralization (VN) and three commercial serum antibody ELISAs (gB bELISA, gI bELISA and ADV iELISA). ADV DNA was detected in oral fluid samples (20% to 100%) from 3 to 21 days postinoculation (DPI), but not in serum. ADV antibody was detected in oral fluid specimens at DPI ≥ 10 with the gB bELISA (36% to 79%) and ADV iELISA (29% to 100%), but not the gI bELISA. These results suggest that oral fluid could be used as an alternative to individual pig sampling for ADV surveillance using PCR- and/or antibody-based assays.

Herd-level infectious disease surveillance of livestock populations using aggregate samples

All sectors of livestock production are in the process of shifting from small populations on many farms to large populations on fewer farms. A concurrent shift has occurred in the number of livestock moved across political boundaries. The unintended consequence of these changes has been the appearance of multifactorial diseases that are resistant to traditional methods of prevention and control. The need to understand complex animal health conditions mandates a shift toward the collection of longitudinal animal health data. Historically, collection of such data has frustrated and challenged animal health specialists. A promising trend in the evolution toward more efficient and effective livestock disease surveillance is the increased use of aggregate samples, e.g. bulk tank milk and oral fluid specimens. These sample types provide the means to monitor disease, estimate herd prevalence, and evaluate spatiotemporal trends in disease distribution. Thus, this article provides an overview of the use of bulk tank milk and pen-based oral fluids in the surveillance of livestock populations for infectious diseases.

Comparison of the efficacy of a commercial inactivated influenza A/H1N1/pdm09 virus (pH1N1) vaccine and two experimental M2e-based vaccines against pH1N1 challenge in the growing pig model

Swine influenza A viruses (IAV-S) found in North American pigs are diverse and the lack of cross-protection among heterologous strains is a concern. The objective of this study was to compare a commercial inactivated A/H1N1/pdm09 (pH1N1) vaccine and two novel subunit vaccines, using IAV M2 ectodomain (M2e) epitopes as antigens, in a growing pig model. Thirty-nine 2-week-old IAV negative pigs were randomly assigned to five groups and rooms. At 3 weeks of age and again at 5 weeks of age, pigs were vaccinated intranasally with an experimental subunit particle vaccine (NvParticle/M2e) or a subunit complex-based vaccine (NvComplex/M2e) or intramuscularly with a commercial inactivated vaccine (Inact/pH1N1). At 7 weeks of age, the pigs were challenged with pH1N1 virus or sham-inoculated. Necropsy was conducted 5 days post pH1N1 challenge (dpc). At the time of challenge one of the Inact/pH1N1 pigs had seroconverted based on IAV nucleoprotein-based ELISA, Inact/pH1N1 pigs had significantly higher pdm09H1N1 hemagglutination inhibition (HI) titers compared to all other groups, and M2e-specific IgG responses were detected in the NvParticle/M2e and the NvComplex/M2e pigs with significantly higher group means in the NvComplex/M2e group compared to SHAMVAC-NEG pigs. After challenge, nasal IAV RNA shedding was significantly reduced in Inact/pH1N1 pigs compared to all other pH1N1 infected groups and this group also had reduced IAV RNA in oral fluids. The macroscopic lung lesions were characterized by mild-to-severe, multifocal-to-diffuse, cranioventral dark purple consolidated areas typical of IAV infection and were similar for NvParticle/M2e, NvComplex/M2e and SHAMVAC-IAV pigs. Lesions were significantly less severe in the SHAMVAC-NEG and the Inact/pH1N1pigs. Under the conditions of this study, a commercial Inact/pH1N1 specific vaccine effectively protected pigs against homologous challenge as evidenced by reduced clinical signs, virus shedding in nasal secretions and oral fluids and reduced macroscopic and microscopic lesions whereas intranasal vaccination with experimental M2e epitope-based subunit vaccines did not. The results further highlight the importance using IAV-S type specific vaccines in pigs.

Using oral fluids samples for indirect influenza A virus surveillance in farmed UK pigs

Influenza A virus (IAV) is economically important in pig production and has broad public health implications. In Europe, active IAV surveillance includes demonstration of antigen in nasal swabs and/or demonstration of antibodies in serum (SER) samples; however, collecting appropriate numbers of individual pig samples can be costly and labour‐intensive. The objective of this study was to compare the probability of detecting IAV antibody positive populations using SER versus oral fluid (OF) samples. Paired pen samples, one OF and 5–14 SER samples, were collected cross‐sectional or longitudinally. A commercial nucleoprotein (NP)‐based blocking ELISA was used to test 244 OF and 1004 SER samples from 123 pens each containing 20–540 pigs located in 27 UK herds. Overall, the IAV antibody detection rate was higher in SER samples compared to OFs under the study conditions. Pig age had a significant effect on the probability of detecting positive pens. For 3–9‐week‐old pigs the probability of detecting IAV antibody positive samples in a pen with 95% confidence intervals was 40% (23–60) for OF and 61% (0.37–0.80) for SER (P = 0.04), for 10–14‐week‐old pigs it was 19% (8–40) for OF and 93% (0.71–0.99) for SER (P < 0.01), and for 18–20‐week‐old pigs it was 67% (41–85) for OF and 81% (0.63–0.91) for SER (P = 0.05). Collecting more than one OF sample in pens with more than 25 less than 18‐week‐old pigs should be further investigated in the future to elucidate the suitability of OF for IAV surveillance in herds with large pen sizes.

Field-Deployable Reverse Transcription-Insulated Isothermal PCR (RT-iiPCR) Assay for Rapid and Sensitive Detection of Foot-and-Mouth Disease Virus

Foot‐and‐mouth disease (FMD) is a highly contagious viral disease of cloven‐hoofed animals, which can decimate the livestock industry and economy of countries previously free of this disease. Rapid detection of foot‐and‐mouth disease virus (FMDV) is critical to containing an FMD outbreak. Availability of a rapid, highly sensitive and specific, yet simple and field‐deployable assay would support local decision‐making during an FMDV outbreak. Here we report validation of a novel reverse transcription‐insulated isothermal PCR (RT‐iiPCR) assay that can be performed on a commercially available, compact and portable POCKIT^™ analyser that automatically analyses data and displays ‘+’ or ‘−’ results. The FMDV RT‐iiPCR assay targets the 3D region of the FMDV genome and was capable of detecting 9 copies of in vitro‐transcribed RNA standard with 95% confidence. It accurately identified 63 FMDV strains belonging to all seven serotypes and showed no cross‐reactivity with viruses causing similar clinical diseases in cloven‐hoofed animals. The assay was able to identify FMDV RNA in multiple sample types including oral, nasal and lesion swabs, epithelial tissue suspensions, vesicular and oral fluid samples, even before the appearance of clinical signs. Clinical sensitivity of the assay was comparable or slightly higher than the laboratory‐based real‐time RT‐PCR assay in use. The assay was able to detect FMDV RNA in vesicular fluid samples without nucleic acid extraction. For RNA extraction from more complex sample types, a commercially available taco^™ mini transportable magnetic bead‐based, automated extraction system was used. This assay provides a potentially useful field‐deployable diagnostic tool for rapid detection of FMDV in an outbreak in FMD‐free countries or for routine diagnostics in endemic countries with less structured laboratory systems.

Kinetics of influenza A virus nucleoprotein antibody (IgM, IgA, and IgG) in serum and oral fluid specimens from pigs infected under experimental conditions

Indirect influenza A virus (IAV) nucleoprotein (NP) antibody ELISAs were used to compare the kinetics of the NP IgM, IgA, and IgG responses in serum and pen-based oral fluid samples collected from 82 pigs followed for 42 days post inoculation (DPI). Treatment categories included vaccination (0, 1) and inoculation (0, 1) with contemporary H1N1 or H3N2 isolates. Antibody ontogeny was markedly affected by vaccination status, but no significant differences were detected between H1N1 and H3N2 inoculated groups of the same vaccination status (0, 1) in IgM, IgA, or IgG responses. Therefore, these data were combined in subsequent analyses. The correlation between serum and oral fluid responses was evaluated using the pen-based oral fluid sample-to-positive (S/P) ratios versus the mean serum S/P ratios of pigs within the pen. IgM responses in serum and oral fluid were highly correlated in unvaccinated groups (r=0.810), as were serum and oral fluid IgG responses in both unvaccinated (r=0.839) and vaccinated (r=0.856) groups. In contrast, IgM responses were not correlated in vaccinated groups and the correlation between serum and oral fluid IgA was weak (r∼0.3), regardless of vaccination status. In general, vaccinated animals exhibited a suppressed IgM response and accelerated IgG response. The results from this study demonstrated that NP-specific IgM, IgA, and IgG antibody were detectable in serum and oral fluid and their ontogeny was influenced by vaccination status, the time course of the infection, and specimen type.

Kinetics of the porcine reproductive and respiratory syndrome virus (PRRSV) humoral immune response in swine serum and oral fluids collected from individual boars

BACKGROUND

The object of this study was to describe and contrast the kinetics of the humoral response in serum and oral fluid specimens during acute porcine reproductive and respiratory syndrome virus (PRRSV) infection. The study involved three trials of 24 boars each. Boars were intramuscularly inoculated with a commercial modified live virus (MLV) vaccine (Trial 1), a Type 1 PRRSV field isolated (Trial 2), or a Type 2 PRRSV field isolate (Trial 3). Oral fluid samples were collected from individual boars on day post inoculation (DPI) -7 and 0 to 21. Serum samples were collected from all boars on DPI -7, 0, 7, 14, 21 and from 4 randomly selected boars on DPI 3, 5, 10, and 17. Thereafter, serum and oral fluid were assayed for PRRSV antibody using antibody isotype-specific ELISAs (IgM, IgA, IgG) adapted to serum or oral fluid.

RESULTS

Statistically significant differences in viral replication and antibody responses were observed among the three trials in both serum and oral fluid specimens. PRRSV serum IgM, IgA, and IgG were first detected in samples collected on DPI 7, 10, and 10, respectively. Oral fluid IgM, IgA, and IgG were detected in samples collected between DPI 3 to 10, 7 to 10, and 8 to 14, respectively.

CONCLUSIONS

This study enhanced our knowledge of the PRRSV humoral immune response and provided a broader foundation for the development and application of oral fluid antibody-based diagnostics.