Although porcine reproductive and respiratory syndrome virus (PRRSV) vaccines have been available in North America for almost 30 years, many vaccines face a significant hurdle: they must provide cross-protection against the highly diverse PRRSV strains. This cross-protection, or heterologous vaccine efficacy, relies greatly on the vaccine’s ability to induce a strong immune response against various strains—heterologous immunogenicity. Thus, this study investigated vaccine efficacy and immunogenicity of a modified live virus (MLV) against four heterologous type 2 PRRSV (PRRSV-2) strains. In this study, 60 pigs were divided into 10 groups. Half were MOCK-vaccinated, and the other half vaccinated with the Prevacent® PRRS MLV vaccine. Four weeks after vaccination, groups were challenged with either MOCK, or four PRRSV-2 strains from three different lineages—NC174 or NADC30 (both lineage 1), VR2332 (lineage 5), or NADC20 (lineage 8). Pre-and post-challenge, lung pathology, viral loads in both nasal swabs and sera, anti-PRRSV IgA/G, neutralizing antibodies, and the PRRSV-2 strain-specific T-cell response were evaluated. At necropsy, the lung samples were collected to assess viral loads, macroscopical and histopathological findings, and IgA levels in bronchoalveolar lavage. Lung lesions were only induced by NC174, NADC20, and NADC30; within these, vaccination resulted in lower gross and microscopic lung lesion scores of the NADC20 and NADC30 strains. All pigs became viremic and vaccinated pigs had decreased viremia upon challenge with NADC20, NADC30, and VR2332. Regarding vaccine immunogenicity, vaccination induced a strong systemic IgG response and boosted the post-challenge serum IgG levels for all strains. Furthermore, vaccination increased the number of animals with neutralizing antibodies against three of the four challenge strains—NADC20, NADC30, and VR2332. The heterologous T-cell response was also improved by vaccination: Not only did vaccination increase the induction of heterologous effector/memory CD4 T cells, but it also improved the heterologous CD4 and CD8 proliferative and/or IFN-γ response against all strains. Importantly, correlation analyses revealed that the (non-PRRSV strain-specific) serum IgG levels and the PRRSV strain-specific CD4 T-cell response were the best immune correlates of protection. Overall, the Prevacent elicited various degrees of efficacy and immunogenicity against four heterologous and phylogenetically distant strains of PRRSV-2.

As genetic analysis becomes less expensive, more comprehensive diagnostics such as whole genome sequencing (WGS) will become available to the veterinary practitioner. The WGS elucidates more about porcine reproductive and respiratory syndrome virus (PRRSV) beyond the traditional analysis of open reading frame (ORF) 5 Sanger sequencing. The veterinary practitioner will require a more complete understanding of the mechanics and consequences of PRRSV genetic variability to interpret the WGS results. More recently, PRRSV recombination events have been described in the literature. The objective of this review is to provide a comprehensive outlook for swine practitioners that PRRSV mutates and recombines naturally causing genetic variability, review the diagnostic cadence when suspecting recombination has occurred, and present theory on how, why, and where industry accepted management practices may influence recombination. As practitioners, it is imperative to remember that PRRS viral recombination is occurring continuously in swine populations. Finding a recombinant by diagnostic analysis does not ultimately declare its significance. The error prone replication, mutation, and recombination of PRRSV means exact clones may exist; but a quasispecies swarm of variable strains also exist adding to the genetic diversity. PRRSV nonstructural proteins (nsps) are translated from ORF1a and ORF1b. The arterivirus nsps modulate the hosts’ immune response and are involved in viral pathogenesis. The strains that contribute the PRRSV replicase and transcription complex is driving replication and possibly recombination in the quasispecies swarm. Furthermore, mutations favoring the virus to evade the immune system may result in the emergence of a more fit virus. More fit viruses tend to become the dominant strains in the quasispecies swarm. In theory, the swine management practices that may exacerbate or mitigate recombination include immunization strategies, swine movements, regional swine density, and topography. Controlling PRRSV equates to managing the quasispecies swarm and its interaction with the host. Further research is warranted on the frequency of recombination and the genome characteristics impacting the recombination rate. With a well-defined understanding of these characteristics, the clinical implications from recombination can be detected and potentially reduced; thus, minimizing recombination and perhaps the emergence of epidemic strains.

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The objective of this study was to evaluate the effects of two dietary feeding periods of tiamulin in combination with chlortetracycline for the control and treatment of swine respiratory and enteric disease and subsequent growth performance. The study used 1,151 commercial crossbred barrows and gilts in a randomized complete block design. Pigs were housed in single-sex groups of 25 at a floor space of 0.69 m2/pig. There were two dietary treatments: 1) nonmedicated controls and 2) 39 mg/kg tiamulin + 441 mg/kg chlortetracycline (TIACTC) fed from days 7 to 20 and again days 49 to 62. There were 23 pens per treatment group. Daily observations were made throughout the study, including the number of pigs in each pen coughing, with diarrhea, or showing signs of lameness as well as the number of pigs in each pen requiring individual therapy treatment for each symptom. Pigs were weighed as a group on days 0 (for allocation purposes), 7, 21, 49, 61, 89 (start of marketing), and at time of slaughter. Within pen, animals were selected by visual appraisal and sent for slaughter over 4 wk to a commercial slaughter facility where HCW was collected and used to calculate carcass yield. There was no difference (P > 0.05) between treatments for the incidence of morbidity or mortality. Pigs fed TIACTC tended to have less coughing observations (P = 0.10) and less diarrhea observations (P = 0.08) during the study period, and had less observations of lameness (P < 0.001) and required less treatments than nonmedicated controls (P < 0.001). For the overall study period, pigs fed TIACTC had greater (P < 0.05) total BW gain (43.3 kg greater/pen) and greater (P < 0.05) ADG and ADFI than controls. There was no effect (P > 0.05) of treatment on G:F. Overall, pigs fed TIACTC weighed 1.3 kg heavier (P < 0.05) at the start of marketing and completed the study with an overall BW advantage of 1.6 kg (P < 0.05) compared to controls. The difference between treatments for live BW increased with marketing group (1.0 kg in marketing group 1 and 3.3 kg in marketing group 4). Pigs fed TIACTC had greater (P < 0.05) HCW (1.0 kg) than controls; however, there was no difference (P > 0.05) between treatments for carcass yield. The results of this study suggest that feeding TIACTC was successful at controlling respiratory and enteric disease and, consequently, improved growth performance and carcass weight of grow-finish pigs.