Semen can play a crucial role in PRRS virus transmission. This is due to the special features of the virus shedding by this route and the extensive use of artificial insemination.
Though viraemia usually is very short in boars, from few a days to a maximum of two-to-three weeks, they can shed the virus in semen for several weeks after infection. In addition, amount of virus necessary to infect a susceptible sow by sexual exposure tends to be much lower that the amount of virus usually present in semen from an infected boar. Nevertheless, transmission by semen appears more likely to occur at the early stages of infection due to the high titres of virus in the semen. It is also important to note that a high variability in virus shedding among boars is observed, and that boars are intermittent shedders. Consequently, a negative isolated PCR result in serum or semen does not rule out the possibility of shedding by this route; more than one sample from a given boar should be analysed at different time periods.
Pigs are extremely susceptible to infection by parenteral exposure; very few PRRS virus particles are sufficient to infect pigs by this route. Therefore, any event, practice or contaminated material that could result in breaks in the skin barrier can potentially facilitate the transmission of the virus, such as: teeth clipping, ear notching, tail docking, inoculations with medications (needles)… In relation to the latter point, it has been demonstrated that infection can occur following the use of the same needle used to vaccinate infected pigs. In addition, because the virus is present in oral fluids, normal pig social behaviour and aggressive interactions (bites, scrapes, abrasions…) can also result in parenteral infection.