The first technical report on swine oral fluid diagnostics was presented at the 2005 International PRRS Symposium when Simer et al1 reported 20 of 24 pen-based oral fluid samples (83.3%) and 17 of 24 serum samples (70.8%) were porcine reproductive and respiratory syndrome virus (PRRSV) reverse transcription polymerase chain reaction (RT-PCR) positive in finishing pigs. The purpose of this review is to provide an update on the development and implementation of oral fluid diagnostics in swine medicine subsequent to this initial report.

Collection of oral fluid samples has been described at length by Prickett et al.2 In brief, cotton ropes are hung in the pen at pig shoulder height. Pigs chew on the rope, saturating the rope with oral fluids. After 20 to 30 minutes, the ropes are placed in a single-use plastic bag, the fluid is wrung from the rope, and then decanted into a tube for submission to the diagnostic laboratory. Pigs with prior experience respond immediately to the presence of the rope. In experienced groups, a 20- to 30-minute sampling period is sufficient to allow adequate participation of pigs in the pen. In pigs without prior rope sampling experience, 60 minutes is recommended to allow pigs to learn the new “game” and achieve an adequate level of participation.3

Oral fluids are most commonly collected from pens of pigs, but oral fluid samples can also be obtained from individual pigs.3 Oral fluids can be successfully collected at all production stages, ie, growing pigs3,4 and in the breeding herd for individually- or group-housed sows and boars.5,6 Samples can be collected from suckling piglets as they approach weaning age,7,8 but family sampling, ie, placing the rope so that both sows and their litters have access, has been shown to be more successful than collecting solely from the piglets. Thus, Almeida et al8 reported an approximate 73% success rate when collecting family oral fluid samples versus 44% success when collecting only from litters. From a collection of 72 family oral fluid samples and matching sera from 718 piglets, 84.4% (27 of 32 litters) were PRRSV RT-PCR positive while 24.2% (174 of 718 piglets) of serum samples were positive for PRRSV nucleic acid.

At the present time, the detection of nucleic acid or antibodies in oral fluids has been documented for most of the major swine pathogens including: Actinobacillus pleuropneumoniae (APP),9,10 African swine fever virus,11,12 classical swine fever virus,13 foot-and-mouth disease virus,14,15 influenza A virus (IAV),16-18 Lawsonia intracellularis,19 Mycoplasma spp.,20-22 porcine circovirus type 2 (PCV2),2,23 porcine epidemic diarrhea virus (PEDV),24 PRRSV,2,6,25-27 Senecavirus A (SVA),28 and others. Field applications or research on the use of oral fluid diagnostics have been described in Australia,15 Belgium,29 Canada,30 France,31 Germany,13 Italy,32 Japan,33 Malaysia,34 Poland,35 Spain,36 the United Kingdom,37,38 the United States,2 Vietnam,39 and others. Many of the assays reported in the literature have only been described under research conditions, but it is reasonable to expect their future commercialization and adoption for routine use in diagnostic laboratories.

Oral fluid testing

In the United States, veterinary diagnostic laboratories with a major swine focus began offering oral fluid-based tests to clientele in 2010. The data provided in Figure 1 and Tables 1, 2, 3, and 4 describe the number of oral fluid tests performed at Iowa State University, South Dakota State University, and University of Minnesota. The following is a review of pathogens for which testing is commonly performed and for which peer-reviewed literature is available.

Porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus was the first virus detected by RT-PCR in oral fluid samples.2 Porcine reproductive and respiratory syndrome virus oral fluid enzyme-linked immunosorbent assays (ELISA) for antibody detection have been routinely offered since 2010. In 2016, RNA detection was performed for 116,671 of the 148,526 PRRSV tests (Tables 1 and 2).

Nucleic acid detection

Prickett et al2 first reported the detection of PRRSV by quantitative RT-PCR (qRT-PCR) in oral fluids collected in the field from 8-week-old pigs. Oral fluid qRT-PCR-positive results were coincident with RT-PCR-positive serum samples, ie, showed 77% agreement. Under experimental conditions, Prickett et al25 reported that PRRSV RNA was detected in oral fluid samples from 3 to 35 days post inoculation (DPI), with sporadic positives thereafter. Similar results were obtained from individual boars inoculated with modified-live virus, type 1 PRRSV, or type 2 PRRSV.6 On the first DPI, virus was detected in 10% of the boars sampled (7 of 69); by 3 DPI, virus was detected in 100% of boars sampled (67 of 67).6 Cumulatively, the literature indicates that PRRSV RNA can be detected for at least 36 DPI in oral fluid samples.5,25,26,33,35,40-44

Sequencing

Successful PRRSV sequencing from oral fluids has been described.35,45,46 Kittawornrat et al45 obtained PRRSV open reading frame-5 sequences from 2 of 6 RT-PCR-positive oral fluid samples from pre-weaned pigs. Zhang et al46 reported successful full-genome sequencing from oral fluid samples with RT-PCR cycle threshold (Ct) values between 18.7 and 20.6, whereas no full-length sequences were obtained from oral fluids with Ct values between 22.9 and 35.4.

Antibody detection

Porcine reproductive and respiratory syndrome virus IgG antibody is detected in oral fluids by ELISA between 7 and 14 days after inoculation or vaccination.5,25-27,40,47 Kittawornrat et al,27 working with oral fluid samples from individually housed boars and a serum ELISA adapted to oral fluids, reported that IgM was detectable at 3 DPI, IgA at 7 DPI, and IgG at 8 DPI. Antibody responses in oral fluids mirrored antibody responses in serum. Maternal PRRSV IgG is readily detected in pigs from PRRSV-positive sow herds and may be detected for up to 60 days post-weaning; however, a PRRSV IgM-IgA (dual isotype) ELISA was shown to detect pig-specific IgM and IgA, even in the presence of maternal IgG.48 Porcine reproductive and respiratory syndrome virus antibody ELISA testing has been well documented in the literature and may provide a cost-effective approach to PRRSV monitoring and surveillance.

Influenza A virus

As shown in Tables 1, 2, 3, and 4, IAV oral fluid testing has been offered for routine testing since 2010. Nucleic acid detection was performed for 42,261 of the 47,454 IAV tests in 2016 (Table 1 and 2).

Nucleic acid detection

Detmer et al49 first reported the detection of IAV nucleic acid in oral fluid samples from both experimentally and naturally infected pigs. Under experimental conditions, IAV RNA was detected in oral fluids from 3 to 21 DPI; whereas, no IAV RT-PCR-positive nasal swabs were detected past 7 DPI.50 Ramirez et al43 reported highly variable detection patterns for IAV infection in 10 wean-to-finish barns. Cumulatively, the literature indicates that IAV RNA can be detected in oral fluids, but widely variable detection patterns have been noted in the literature.35,37,50-54

Sequencing

Influenza A virus sequencing has been described in the literature.49,51,53 Detmer et al49 obtained hemagglutinin (HA) sequences from 2 of 4 positive oral fluid samples submitted for analysis. Panyasing et al53 reported unsuccessful attempts to sequence HA and neuraminidase genes, but successfully sequenced M genes for all 18 IAV qRT-PCR-positive oral fluid samples collected from neonatal pigs. In oral fluid field samples submitted for routine analysis, HA sequences were obtained from 34 of 61 (55.7%) samples with Ct values < 25; 5 of 34 (14.7%) samples with Ct values between 25 and 29.9; and 0 of 39 (0%) samples with Ct values > 30 (Jianqiang Zhang, Personal Communication).

Virus isolation

Isolation of IAV from oral fluids in pigs is difficult and reports of both success and failure may be found in the literature. Detmer et al49 and Allerson et al51 were not able to isolate and sequence IAV from oral fluid samples. However, Romagosa et al54 reported 51.4% (19 of 37) of RT-PCR-positive oral fluid samples were also positive by virus isolation. Similarly, Goodell et al16 reported successful IAV virus isolation, but isolation was significantly less likely in oral fluids when compared to nasal swabs, particularly in vaccinated animals. Virus isolation was successful in 82 of 180 (45.6%) oral fluid samples with Ct values < 25; 62 of 346 (17.9%) samples with Ct values between 25 and 29.9; and 21 of 407 (5.2%) samples with Ct values between 30 and 35 (Jianqiang Zhang, Personal Communication). Additional research is needed to determine the best time to collect samples and the optimum laboratory protocol for successful IAV virus isolation.16,49 As reviewed by Baron et al55 in the context of human immunodeficiency virus, the extreme hypotonicity of oral fluids (one-seventh the tonicity of interstitial fluid) severely reduces virus infectivity. This is a factor that should be considered for future research because, like humans, swine oral fluids are hypotonic and may have an impact on the isolation of IAV and other viral agents from porcine oral fluids.

Antibody detection

Panyasing et al18 first reported detection of IAV-specific antibodies in oral fluid samples using a blocking ELISA based on the viral nucleoprotein (NP). Using a NP indirect ELISA, IAV IgM antibody responses peaked at 8 DPI and declined quickly thereafter while IgA and IgG were detected around 6 DPI and lasted through the conclusion of the study (42 DPI).17 In this same study, Panyasing et al17 showed a rapid anamnestic oral fluid antibody response in vaccinated animals. Cumulatively, the literature agrees that IAV antibodies can be detected in oral fluids as early as 6 DPI.17,39,52,56,57

Porcine coronaviruses

The majority of research on the porcine coronaviruses has focused on PEDV. Cumulatively, the research strongly supports the use of oral fluids for PEDV detection. Similar assumptions have been made for other porcine coronaviruses, ie, transmissible gastroenteritis virus and porcine deltacoronavirus (PDCoV), on the strength of the PEDV research.

Porcine epidemic diarrhea virus

Porcine epidemic diarrhea virus RT-PCR testing for oral fluids was implemented in 2013 and was used extensively thereafter, as reflected in the test numbers reported in Tables 1, 2, 3, and 4. Oral fluid antibody testing for PEDV became available in 2016 (Table 3). Reverse transcription PCR testing was performed for 69,324 of the 73,494 PEDV tests conducted in 2016 (Tables 1 and 2).

Nucleic acid detection. Bjustrom-Kraft et al24 published the first peer-reviewed study on the detection of PEDV in oral fluid samples by RT-PCR. Detectable levels of PEDV were found in fecal swabs, oral fluids, and pen fecal samples collected in the field following a natural planned exposure to PEDV. Significant differences were detected between individual fecal swabs and pen-based oral fluid; oral fluids had lower Ct values indicating higher virus concentrations. PEDV was detected in oral fluids for approximately 69 days post exposure (DPE). Under experimental conditions, Bower et al58 reported detection of PEDV by RT-PCR in fecal swabs and oral fluids from 1 to 35 DPI in both sample types.

Antibody detection. Bjustrom-Kraft et al24 reported the detection of PEDV IgG and IgA in oral fluid samples collected 13 days after natural planned exposure. Porcine epidemic diarrhea virus IgA sample to positive ratio (S/P) responses in oral fluid increased until 97 DPE whereas oral fluid IgG responses peaked at 13 DPE and declined thereafter.

Porcine deltacoronavirus

Under experimental conditions, Zhang59 reported detection of PDCoV in oral fluids from 3-week-old pigs. Individual fecal swabs, pen-based feces, and oral fluids were collected and PDCoV RNA was detected from 7 to 28 DPI, 7 to 14 DPI, and 7 to 35 DPI, respectively. Homwong et al60 evaluated PDCoV RT-PCR testing results from routine submissions (n = 602) to the University of Minnesota Veterinary Diagnostic Laboratory and found that oral fluid samples were more likely to test positive for PDCoV than feces.

Less commonly used oral fluid tests in the United States

Tests are offered at the diagnostic laboratories for several pathogens for which little peer-reviewed literature is available.

Porcine circovirus type 2

As shown in Tables 1, 2, and 4, routine PCV2 oral fluid testing began in 2010. Relatively few tests have been performed in recent years, which suggests that the current PCV2 vaccines are effective.61 Porcine circovirus type 2 was detected in oral fluids from each of the three sites with at least 1 to 2 positive samples in oral fluids by quantitative polymerase chain reaction (qPCR) in 2008.2 Similar results were reported in PCV2-inoculated 11-week-old pigs where PCV2 was detected by qPCR from 2 DPI until the conclusion of the study (98 DPI).23 Ramirez et al43 reported 508 of 600 (84.7%) oral fluid samples were PCV2 positive by qPCR in 10 wean-to-finish barns. Van Cuong et al39 reported a slightly lower PCV2 detection rate (61%) in 68 farms throughout Vietnam. Under experimental conditions, PCV2 antibody (IgG, IgA, and IgM) was first reported in 2011.23 All PCV2-inoculated pigs seroconverted between 14 and 21 DPI (IgG, IgA, and IgM), and antibody responses remained detectable through the conclusion of the study (98 DPI).

Senecavirus A

For SVA, 3,598 oral fluid-based tests have been conducted (Tables 1, 2, and 3). Senecavirus A detection in oral fluids has been documented under field conditions.28 Although no clinical signs were observed, SVA was detected by RT-PCR in oral fluid samples at day zero in one of the sites. The fact that 9 of 10 serum samples were SVA positive on the same farm supported the validity of the oral fluid results. Little peer-reviewed research is available on SVA, but this initial report suggests oral fluids may be a useful sample for monitoring and surveillance of SVA.

Bacterial pathogens

Little research has been done on the detection of bacterial pathogens in oral fluid samples. Regardless, peer-reviewed publications reporting detection by polymerase chain reaction under experimental or field conditions have included the following bacterial agents: APP,9,62 Brachyspira spp.,63 Erysipelothrix rhusiopathiae,64 Haemophilus parasuis,62 L intracellularis,19 Mycoplasma spp.,20,21,62 Pasteurella multocida,62 Salmonella,19 and Streptococcus suis.62

Bacterial pathogens for which antibodies are reportedly detected in oral fluids include: APP,9,10 E rhusiopathiae,64 and Mycoplasma spp.22

General conclusions

Pig production changed dramatically over the last several decades from smaller single-site farms to larger multisite production systems.65 These changes made it possible for producers and veterinarians to achieve higher production efficiencies, but also facilitated the appearance of production diseases, ie, multifactorial diseases and the appearance of new, high-impact pathogens, such as PRRSV and PEDV.66-69

Diagnostic medicine needs to respond to new disease challenges with new methods capable of providing timely, accurate, informative results. Individual pig samples, such as serum or swabs, have historically served this purpose, but individual animal sampling is not compatible with efficient surveillance in contemporary swine production systems. As an alternative to individual animal samples, Prickett et al2 described the use of pen-based oral fluid samples (rope testing) for the detection of PRRSV and PCV2 in growing pigs. Since this initial report, oral fluid-adapted nucleic acid and antibody tests have been reported for many of the major swine pathogens and oral fluid-based surveillance has been widely adopted by swine veterinarians and swine producers. This process will continue as more and better tests are adapted to the oral fluid matrix.

Nevertheless, there are good reasons to exercise caution. In particular, the peer-reviewed literature has shown that nucleic acid and antibody assays can be adapted to oral fluids, but the literature has also consistently shown that the procedures need to be carefully modified for optimum performance with the oral fluid matrix.70,71 Chittick et al70 and Gibert et al36 working with PRRSV and Goodell et al71 working with IAV found significant differences in test performance among RT-PCR protocols offered in veterinary diagnostic laboratories. Once optimum protocols are identified, they should be broadly implemented to achieve reproducibility among diagnostic laboratories. Overall, the development of oral fluid-based testing has changed the way we monitor disease in swine populations. However, careful work on the part of researchers and critical thinking on the part of producers and veterinarians will be needed to fully develop a reliable and robust oral fluid diagnostics system capable of meeting the current and future needs of the swine industry.

Acknowledgements

The authors would like to acknowledge and thank Dr Stephanie Rossow and Michele Leiferman from the University of Minnesota, Travis Clement from South Dakota State University, and Dr Luis Giménez-Lirola and Wendy Stensland from Iowa State University for their significant contributions to oral fluid test development and to this manuscript.

Conflict of interest

None reported.

Disclaimer

Scientific manuscripts published in the Journal of Swine Health and Production are peer reviewed. However, information on medications, feed, and management techniques may be specific to the research or commercial situation presented in the manuscript. It is the responsibility of the reader to use information responsibly and in accordance with the rules and regulations governing research or the practice of veterinary medicine in their country or region.

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