Abstract
Foot-and-mouth disease virus (FMDV) poses significant economic threats to livestock industries worldwide, with its serotype O being the most prevalent globally and responsible for all outbreaks in Europe in the last 25 years. This retrospective multi-centre study evaluated the dose-response relationship after intra-oropharyngeal (IOP) inoculation of pigs with FMDV O, combining independently obtained results from three research institutes. Four virus isolates were evaluated: a recombinant O/FRA/2001 virus containing the P1-coding region of O/BUL/2011, O/TAW/97, O/UKG/34/2001, and O/Manisa/TUR/69, at doses up to 8 × 10^6 TCID(50) using IOP inoculation. In all experiments, intradermal heel bulb (IDHB) inoculation served as a positive control. Overall, results of IOP were variable and strain-dependent, without a dose-response relationship, while IDHB reliably caused infection even at low doses. No single factor responsible for the observed variation was identified. These findings indicate that, unlike IDHB, the outcome of IOP inoculation does not consistently lead to infection with FMDV.