Viremia and antibody dynamics following atypical porcine pestivirus infection: a cohort study of pigs with congenital tremor.

Atypical porcine pestivirus (APPV) induces congenital tremor (CT) in piglets. Limited understanding of the relationship between viremia, antibody responses and clinical signs has impeded the development of effective preventive measures, as it is unclear which individuals are infectious and for how long. To characterize these aspects, a cohort field study with 43 piglets from herds experiencing CT and 21 external control piglets was conducted. CT-pigs and their healthy littermates were viremic at birth, with APPV RNA detected in serum by RT-qPCR, and viremia persisted in eight of the 15 pigs from CT-litters that remained in the study until four months of age, although levels declined over time. Pigs from unaffected litters in outbreak herds became viremic after litters were mixed. All pigs had APPV-specific anti-E2 antibodies, measured by an in-house ELISA, at three days of age. Levels declined by 59 days but were elevated again at four months of age in outbreak herd pigs. Lower viral loads were associated with higher antibody levels at this age, indicating an effective immune response. Although the duration of viremia was positively associated with the duration of tremors, 22% of pigs remained viremic after clinical signs had resolved. Furthermore, neither the amount of virus in serum nor antibody levels correlated with the degree of tremors. These findings reveal that piglets born with APPV-induced CT can remain viremic until four months of age and are able to produce APPV-specific antibodies. The absence or presence of tremors cannot be used to identify which pigs are viremic and thus at risk of transmitting the virus, highlighting a challenge for disease control.