Transmission of classical swine fever virus depends on the clinical course of infection which is associated with high and low levels of virus excretion.

Infection with moderately virulent strains of classical swine fever virus (CSFV) can lead to different courses of disease: either (sub)acute, resulting in death or recovery, or chronic disease. The virus excretion dynamics between these courses are quite dissimilar, but it is not known if this also results in differences in virus transmission. In this study, the excretion and transmission dynamics of the moderately virulent Paderborn strain were studied in 15 one-to-one experiments. In these experiments, a single inoculated pig was housed with a single susceptible contact pig from day 1 post-inoculation (p.i.). Each contact pig that became infected was removed and replaced by a new contact pig at day 17 p.i. and day 26 p.i. Infection of contact pigs was monitored by reverse transcription quantitative real-time PCR on oropharyngeal fluid samples. Five of the inoculated pigs developed the chronic form or died during the acute phase (high excreting pigs), while 10 pigs recovered from the infection (low excreting pigs). In the first contact period, there was no significant difference in virus excretion between the high and low excreting pigs, while in the second and third contact period, high excreting pigs excreted significantly higher quantities of virus. Over the entire study period, the reproduction ratio differed significantly between the high (143 [56.3-373]) and low excreting pigs (23.1 [11.5-45.0]). This indicates the importance of high excreting pigs in transmission of CSFV. Furthermore, this study showed the rate of CSFV infections from a contaminated environment.