This reflects infection with PPV at different stages of foetal development. The term 'tolerant carrier' has been used in relation to PPV to describe pigs that are infected as foetuses but do not die or become mummified. There is still some doubt as to whether this occurs with PPV and what significance it may have. Spread of PPV and immunity PPV can be introduced into a clean herd by carrier animals, infected faeces or mechanically by people and vehicles. The virus can survive for long periods in the environment and is difficult to kill with disinfectants.
Infected pigs can shed the virus in their faeces for up to three to four weeks after infection.
Infected boars can also shed PPV in their semen. Levels of immunity in adult sows in infected herds can vary from as low as 40 per cent to almost 100 per cent. Housing in individual stalls limits access to faeces from other pigs and therefore reduces the chances of contact with the virus. Gilts reared in isolation from an early age may also have limited access to PPV, and therefore may be susceptible at the time of their first pregnancy. As gilts are likely to have been infected with PPV more recently than older sows, the level of this temporary immunity in their progeny is likely to be greater than that in piglets from sows.