Transmission of Classical Swine Fever Virus in Cohabitating Piglets with Various Immune Statuses Following Attenuated Live Vaccine

SIMPLE SUMMARY: Classical swine fever is a highly dangerous pathogen affecting domestic pigs. Vaccination with the modified live vaccine is critical for preventing and controlling classical swine fever. However, many factors, such as maternally derived antibodies via colostrum, could interfere with live vaccine efficacy, leading to incomplete protection in commercial herds. In this study, we investigated the transmission of classical swine fever virus in experimental piglets with various post-vaccination immune statuses. A specific-pathogen-free piglet infected with classical swine fever virus served as the viral donor and primary invader, and was cohabited with piglets with maternally derived antibodies that had or had not undergone vaccination. According to the results, most of the piglets with maternally derived antibodies that were vaccinated were fully protected from contact transmission from the viral donor and blocked viral transmission to the third party (those piglets secondarily exposed through cohabitation). Cell-mediated immunity, represented by specific interferon-γ-secreting cells, served as the key to the viral clearance and recovery. Contrarily, the unvaccinated piglets with low levels of maternally derived antibodies had accelerated classical swine fever virus infection following viral invasion. In conclusion, vaccination still induces solid immunity in commercial herds under maternally derived antibody interference and can block viral transmission in herds. ABSTRACT: Classical swine fever (CSF) is a systemic hemorrhagic disease affecting domestic pigs and wild boars. The modified live vaccine (MLV) induces quick and solid protection against CSF virus (CSFV) infection. Maternally derived antibodies (MDAs) via colostrum could interfere with the MLV’s efficacy, leading to incomplete protection against CSFV infection for pigs. This study investigated CSFV transmission among experimental piglets with various post-MLV immune statuses. Nineteen piglets, 18 with MDAs and 1 specific-pathogen-free piglet infected with CSFV that served as the CSFV donor, were cohabited with piglets that had or had not been administered the MLV. Five-sixths of the piglets with MDAs that had been administered one dose of MLV were fully protected from contact transmission from the CSFV donor and did not transmit CSFV to the piglets secondarily exposed through cohabitation. Cell-mediated immunity, represented by the anti-CSFV-specific interferon-γ-secreting cells, was key to viral clearance and recovery. After cohabitation with a CSFV donor, the unvaccinated piglets with low MDA levels exhibited CSFV infection and spread CSFV to other piglets through contact; those with high MDA levels recovered but acted as asymptomatic carriers. In conclusion, MLV still induces solid immunity in commercial herds under MDA interference and blocks CSFV transmission within these herds.