Postvaccination serosurveillance of foot-and-mouth disease through virus-neutralizing and nonstructural protein antibody tests on pig farms in Taiwan: 2009-2020.

The use of virus-neutralizing (VN) and nonstructural protein (NSP) antibody tests in a serosurveillance program for foot-and-mouth disease (FMD) can identify pig herds that are adequately vaccinated, with a high percentage of pigs with VN positive antibody titers; these tests can also help identify pigs with NSP-positivity that have previously been or are currently infected even in vaccinated herds. To identify infected herds and manage infection, the combination of VN and NSP antibody tests was used in Taiwan's serosurveillance program implemented simultaneously with the compulsory FMD vaccination program. The result was the eradication of FMD: Taiwan was recognized by the World Organization for Animal Health as an FMD-free country without vaccination in 2020. Evaluation of the compulsory vaccination program incorporated in the FMD control program in Taiwan revealed that the vaccine quality was satisfactory and the vaccination program was effective during the period of compulsory vaccination (2010-2017). Sound immunological coverage was achieved, with 89.1% of pigs having VN antibody titers exceeding 1:16 in 2016. This level of immunological coverage would be expected to substantially reduce or prevent FMD transmission, which was borne out by the results of the NSP tests. We identified farms having positive NSP reactors (very low annual prevalence) before the cessation of FMD vaccination in July 2018; however, detailed serological and clinical investigations of pigs of all ages in suspect herds demonstrated that no farms were harboring infected animals after the second half of 2013. Thus, the results revealed no evidence of FMD circulation in the field, and Taiwan regained FMD-free status.