Experimental infection of pigs with a classical swine fever virus isolated in Japan for the first time in 26 years

Abstract

Following an outbreak of classical swine fever (CSF) in Japan, 2018, CSFV JPN/1/2018 was isolated from an infected pig sample. In this study, we carried out a comparative experimental infection in pigs using this strain and the highly virulent ALD strain and compared outcomes, including clinical manifestation, virus shedding patterns and antibody responses. Although pigs inoculated orally or intramuscularly with JPN/1/2018 developed hyperthermia and had decreased leucocyte numbers, they survived for the whole experimental period and showed less severe clinical signs than those infected with the ALD strain. We confirmed the presence of characteristic multifocal infarction of the margin of the spleen that arises following infection with JPN/1/2018, albeit that this finding was not observed in all infected pigs. Both viruses efficiently spread to contact pigs in a similar manner, suggesting in transmissibility between the two strains. Viral RNAs were detected in all clinical samples, especially whole blood samples, before the pigs developed hyperthermia until at least approximately 2 weeks after inoculation. Our findings will be valuable for the investigations into epidemic events occurring in Japan and for establishing diagnostic strategies and control measures against CSF.

Keywords: Japan; classical swine fever virus; experimental infection.

Fig. 1.

Clinical findings of pigs of Groups 1–3. (a) Diarrhea in pig No. 10 at 3 dpi. (b) Severe neurological symptoms of abnormal swimming behavior in pigs Nos. 9 and 10 at 5 dpi. (c) Reddened skin in the hypogastrium in pig No. 6 at 15 dpi. (d) Congestion in the palpebral conjunctiva in pig No. 5 at 8 dpi.

Fig. 2.

Body temperature and leucocyte number in pigs of Groups 1–3. Body temperature (solid line) and leucocyte counts (dotted line) from Groups 1–3 were shown. Pig Nos. 9 and 10 were euthanized at 5 dpi. (#) Pig number.

Fig. 3.

Fluorescent antibody assay. Pig No. 3. Specific antigen was is clearly detected in the crypt epithelial cells in the tonsil at 14 dpi.

Fig. 4.

Pathological findings in pigs of Groups 1–3. (a) Congestion and whitish clouding of the meninges in pig No. 10 at 5 dpi. (b) Formation of multiple abscesses (white arrows) in the tonsil in pigs No. 9 at 5 dpi. (c) Multifocal hemorrhagic infarction of the margin of the spleen in pig No. 2 at 14 dpi. (d) Button ulcers (black arrows) in the colon in pig No. 1 at 14 dpi.