Monitoring for classical swine fever virus persistence and seropositivity in vaccinated pig farms using on-farm sentinel pigs during the pre-elimination phase toward a CSF-free status

Abstract

Classical swine fever (CSF) is a transboundary viral disease that causes high mortality and systemic hemorrhages in domestic pigs and wild boars. In Taiwan, long-term implementation of compulsory vaccination has effectively prevented CSF, with no confirmed cases reported since 2006. On the path toward CSF elimination, a national surveillance program was initiated in 2021 for a two-year period. This program included monitoring unvaccinated on-farm sentinel pigs (with maternally derived antibody but without further vaccination with age) raised on 300 vaccinated farms annually, as well as dead pigs from rendering plants and culled sows from slaughterhouses. Sentinel pigs were sampled twice, at 3–12 weeks and 18–24 weeks of age, and tested for CSFV RNA and neutralizing antibodies (NA). CSFV vaccine strain RNA was detected in eight sentinel pigs from three farms, while all others were CSFV negative. The seropositive rates for anti-CSFV NAs in the first inspection were 68.0 % in 2021 and 70.8 % in 2022, and in the second inspection, they declined significantly to 10.8 % and 9.5 %, respectively. Seropositivity declined with age, indicating MDA waning without new infections. Among 1277 dead pigs and 1199 culled sows tested, tests for wild-type CSFV RNA were negative, while vaccine strain RNA was detected in 1.6 and 2.8 % of dead pigs in 2021 and 2022, respectively. These results suggest that wild-type CSFV persistence is minimal under the current vaccination strategy. Overall, these results demonstrate the significance of the surveillance program, and Taiwan is well-positioned toward CSF elimination.