Challenge of pigs with classical swine fever viruses after C-strain vaccination reveals remarkably rapid protection and insights into early immunity

Abstract

Pre-emptive culling is becoming increasingly questioned as a means of controlling animal diseases, including classical swine fever (CSF). This has prompted discussions on the use of emergency vaccination to control future CSF outbreaks in domestic pigs. Despite a long history of safe use in endemic areas, there is a paucity of data on aspects important to emergency strategies, such as how rapidly CSFV vaccines would protect against transmission, and if this protection is equivalent for all viral genotypes, including highly divergent genotype 3 strains. To evaluate these questions, pigs were vaccinated with the Riemser® C-strain vaccine at 1, 3 and 5 days prior to challenge with genotype 2.1 and 3.3 challenge strains. The vaccine provided equivalent protection against clinical disease caused by for the two challenge strains and, as expected, protection was complete at 5 days post-vaccination. Substantial protection was achieved after 3 days, which was sufficient to prevent transmission of the 3.3 strain to animals in direct contact. Even by one day post-vaccination approximately half the animals were partially protected, and were able to control the infection, indicating that a reduction of the infectious potential is achieved very rapidly after vaccination. There was a close temporal correlation between T cell IFN-γ responses and protection. Interestingly, compared to responses of animals challenged 5 days after vaccination, challenge of animals 3 or 1 days post-vaccination resulted in impaired vaccine-induced T cell responses. This, together with the failure to detect a T cell IFN-γ response in unprotected and unvaccinated animals, indicates that virulent CSFV can inhibit the potent antiviral host defences primed by C-strain in the early period post vaccination.

Figure 1. Vaccine induced protection against leukopenia.

Leucokyte numbers remained at pre-vaccination levels throughout the experiment in animals vaccinated five days prior to challenge with A) UK2000/7.1 or B) CBR/93 (filled circles), whereas a significant (P<0.05) decrease occurred rapidly after challenge in unvaccinated animals (open circles) and animals vaccinated one day before challenge that developed clinical signs and were euthanized prior to the end of the experiment (open triangles). A smaller, non significant, decrease in leukocyte numbers was observed in animals vaccinated one day prior to challenge that did not develop clinical signs (filled triangles) and those vaccinated three days prior to challenge (filled squares). n = number of animals in groups vaccinated on day prior to challenge that either developed signs and were euthanized or remained clinically healthy. Data are mean leukocyte counts for each group of pigs. Error bars represent SEM.

Figure 2. Kinetics of vaccine induced prevention or reduction of viral RNA in blood and nasal secretions.

Viral RNA concentrations, determined by real time RT-PCR, in blood (solid lines) and nasal swab samples (dashed lines). A) High levels of viral RNA were detected in blood soon after challenge in unvaccinated animals (open circles), with nasal secretions becoming RNA positive 5–6 days later. B) No viral RNA, or very low levels in the inconclusive range of the assay, was detected in blood or nasal secretion of animals vaccinated 5 days prior to challenge (filled circles). C) High levels of viral RNA was detected in blood and nasal swab samples of one of the animals vaccinated 3 days prior to challenge with UK2000/7.1. An intermediate level of viral RNA was present in blood of one animal vaccinated 3 days prior to challenge with CBR/93 (open squares), whereas the remaining animals in these groups in both experiments (filled squares) had either a transient low level, or no, viral RNA in blood, and either no or intermediate levels of RNA in nasal secretions. D) Animals vaccinated one day prior to challenge either developed clinical disease and were euthanized by 18 dpc (open triangles) or remained healthy for the duration of the experiment (filled triangles).

Figure 3. Association of virus-specific IFN-γ and serum neutralising antibody responses with viraemia.

Pigs were vaccinated with C-strain CSFV on days -5, -3 or -1 and were then challenged, together with groups of unvaccinated pigs, with UK2000/7.1 or CBR93 CSFV isolates. According to clinical outcome following challenge, the day -1 vaccinated pigs were separated into two groups; (recov) animals that experienced mild signs followed by recovery and (euth) animals that developed severe signs necessitating euthanasia. (A) Peripheral blood leukocyte (PBL) IFN-γ responses were measured by ELISA following in vitro stimulation with C-strain CSFV or an uninfected cell preparation (Mock) and data presented as the mock corrected values. (B) Serum neutralisation titres (SNT) against the CSFV reference strain Alfort 187 were assessed in vitro. (C) Viraemia in was measured by qRT-PCR and expressed as log₁₀ viral RNA copies/µl blood. Data from the UK2000/7.1 and CBR/93 experiments were combined and results expressed as the mean data for each group of pigs and error bars represent SEM.