FlagT4G Vaccine Prevents Transplacental Transmission of Highly Virulent Classical Swine Fever Virus after Single Vaccination in Pregnant Sows

Abstract

The transplacental transmission of CSFV and the resulting persistent congenital infection in newborn piglets have been abundantly discussed in pregnant sows suffering from virus infection. Importantly, the availability of safe commercial vaccines with proven efficacy to prevent the generation of congenital and postnatal persistent infections in pregnant sows are critical tools for controlling the disease in CSF endemic areas. Here, we demonstrate the high efficacy of a single dose of the recombinant FlagT4G vaccine to provide solid protection in pregnant sows against transplacental transmission of a highly virulent CSFV. Pregnant sows vaccinated with FlagT4G at 44 days of gestation elicited a strong CSFV-specific antibody response, with neutralizing antibody levels above those required for protection against CSFV. Importantly, after the challenge with a highly virulent CSFV, all foetuses from FlagT4G-vaccinated sows lacked CSF macroscopic lesions and showed a complete absence of the challenge virus in their internal organs at day 79 of gestation. Therefore, pregnant sows safely vaccinated with FlagT4G without affecting reproductive efficacy are efficaciously protected, along with their foetuses, against the infection and disease caused by a CSFV virulent field strain.

Keywords: FlagT4G vaccine; classical swine fever virus; immune response; neutralizing antibodies; protection; transplacental transmission; vaccine efficacy.

Figure 1

Clinical signs evaluated in pregnant sows after FlagT4G vaccination and CSFV challenge. The clinical signs were monitored daily during the trial. Different colours represent the severity of the clinical signs according to the legend. Sow number 3 was euthanized at 9 dpv, following animal welfare law, despite not showing clinical signs of CSF. Sows 5 and 6 were non-vaccinated, being the CSFV challenge control group.

Figure 2

Antibody response generated in pregnant sows after FlagT4G vaccination and CSFV challenge. Sera samples from FlagT4G-vaccinated sows (represented in green symbols) and non-vaccinated sows (represented in orange symbols) were evaluated for the presence of antibodies against E2 using a commercial ELISA. The results are expressed in % blocking, and values above 40% (grey-shaded area) are considered positive. * Sample taken at 12 dpc.

Figure 3

IFN-α response in sera from pregnant sows after FlagT4G vaccination and CSFV challenge. Sera samples from FlagT4G-vaccinated sows (represented in red symbols) and non-vaccinated sows (represented in blue symbols) were tested to analyze the response of the cytokine IFN-α. The results are expressed in units/mL.

Figure 4

CSFV RNA detection in pregnant sows at different time points after vaccination and CSFV challenge in samples and tissues. The sample were analyzed by (a) the specific CSFV Margarita strain RT-qPCR test, represented in red, and by (b) the generic RT-qPCR test, represented in blue. The RNA load, according to the Ct value, is represented as low, moderate, or high according to the intensity of the red and blue colours in the scale for (a) and (b), respectively. Ct values above 40 were considered negative (represented by X). * The samples from sow 5 were taken at 12 dpc. MLN (mesenteric lymph node).

Figure 5

CSFV RNA detection by the RT-qPCR test in foetuses from vaccinated and non-vaccinated sows after CSFV challenge. Samples from foetuses were evaluated by the specific CSFV Margarita strain and the generic CSFV RT-qPCR assays. Results are shown in a colourimetric scale representing the range of values for the detection of both CSFVs: low for Ct values of 29 or above, moderate for Ct between 28 and 23, and high for values under 22. The results obtained are represented in red and blue for the specific CSFV Margarita and generic CSFV tests, respectively. The symbol “−” represents a negative result. In litter number 5, only one serum sample could be evaluated since the other foetuses in the litter were mummified after CSFV infection.

Figure 6

IFN-α level in foetuses from vaccinated and non-vaccinated sows after CSFV challenge. Sera samples of foetuses from vaccinated sows (represented by red symbols) and non-vaccinated sows (represented by blue symbols) were tested to analyze the response of the cytokine IFN-α. The results are expressed in units/mL. In litter number 5, only one serum sample could be evaluated since the other foetuses in the litter were mummified after CSFV infection.