Advances in oral fluid antibody-based surveillance for porcine reproductive and respiratory syndrome virus (PRRSV)

Abstract

Recent decades saw rapid growth and extensive consolidation in swine production systems. An unintended consequence of this development has been an exacerbation in the circulation of viruses and a reduction in our ability to prevent, control, and/or eliminate impactful them. The work described in this dissertation is focused on one of these viral pathogens: porcine reproductive and respiratory syndrome virus (PRRSV). As we seek new approaches to PRRSV prevention and control, it has become apparent that clinical observations cannot provide the basis for effective health management decisions. Rather, good swine health management must rely on diagnostic methods and sampling approaches that systematically provide timely and reliable data at the lowest cost. One solution to this problem could be oral fluids, an aggregate specimen collected from one pig or group of pigs by the use of an adsorptive device (cotton rope). The use of oral fluids as an efficient and practical method of sampling for the surveillance of swine viral diseases in commercial farms was first proposed in 2008 (Prickett et al., 2008). Today, the use of oral fluids has been rapidly adopted by swine producers and veterinarians and direct and/or indirect detection of at least 22 swine viral pathogens have been reported in this specimen (Bjustrom-kraft et al., 2018; Trevisan et al., 2019). This dissertation compiles the current knowledge on oral fluid-based surveillance and further advances the development of oral fluid diagnostic technology. Specifically, Chapter 2 provides an overview of oral fluid-based surveillance of viral pathogens in swine and provides guidelines for their use. Further, Chapter 2 explains the diagnostic properties of swine oral fluids vis-à-vis the detection of viral nucleic acids and virus-specific antibody and contrasts oral fluids with other diagnostic specimens used in veterinary medicine, i.e., serum, nasal swabs, buccal swabs. The basics of oral fluids collection, sample handling, and a brief troubleshooting is also included. Chapter 3 reviews the general issues in persistent virus infections and the biological issues that affect test performance. Using PRRSV as an example, Chapter 3 discusses the relationship between "disease transition stages" and "diagnostic transition stages" and uses a meta-analyses of PRRSV diagnostic data (n = 4307 results) from 19 refereed publications (1995-2018) to construct three prediction equations for PRRSV detection by RT-PCR, bioassay, and ELISA. Chapter 4 evaluated the diagnostic performance of three commercial PRRSV oral fluid antibody ELISAs by testing 564 samples collected longitudinally from twelve 14-week-old pigs vaccinated with a PRRSV modified live vaccine. Serum samples (n = 132) from the same animals were likewise tested on a PRRSV serum ELISA to provide a baseline antibody response. The positivity rates for each oral fluid ELISA were calculated and compared to the serum ELISA and, finally, pairwise comparisons among the oral fluid ELISAs were performed. As an aid for veterinarians and diagnosticians, the estimated diagnostic sensitivity and specificity of each ELISA was provided over a range of cutoffs. Chapter 5 focused on the adaptation of the PRRSV oral fluid ELISA to surveillance. Specifically, this chapter focused on improving the diagnostic specificity of the test by estimating test performance using experimental and field oral fluid samples of known PRRSV status and oral fluid samples from commercial production sites of unknown PRRSV status. Based on a total of 3350 oral fluid specimens, estimates of test performance were provided over a range of cut-offs (S/P ≥ 0.4, 0.6, 0.8, 1.0, 1.2, 1.4) in terms of diagnostic sensitivity and specificity and positivity rates. Based on the aggregate analyses, a cut-off S/P ≥ 1.0 was determined as optimal for field PRRSV surveillance.