National eradication programme in Denmark

PRRSV was first introduced into Denmark in 1992. The disease spread more slowly than in the rest of Western Europe, by 1996 only about 33% of the herds were infected. In July 1996 a voluntary vaccination programme was initiated, organised by the Federation of Danish Pig Producers and Slaughterhouses. Ingelvac PRRS, the vaccine used in this programme, was licensed for use in 3-18 week old pigs.

From mid October 1996, soon after the start of the National Vaccination Programme, several vaccinating herds experienced acute PRRS-like symptoms. Symptoms included an increasing number of abortions and stillborn piglets and an increasing pre-weaning mortality. From October 1996 until May 1997, the virus was identified as the US-type vaccinal strain of PRRSV, isolated from foetuses, dead piglets, pleural fluids and/or lung tissue from 114 of the herds (6).

Furthermore it was found that vaccinated boars could transmit the vaccine virus over a period of more than 8 weeks, to non-vaccinated boars (27).

Transmission via semen from vaccinal virus-infected boars was one of the ways by which the vaccine-like PRRSV was introduced into the 461 previously uninfected, non-vaccinating, herds later identified as being infected with B.I. vaccine-like PRRSV (Sørensen, 44). They were identified serologically, and vaccinal US-type virus was isolated in 80 of the herds. Other possible routes of transmission are via infected animals as well as airborne infection. The 461 herds accounted for 14% of the PRRSV-infected herds in Denmark .

After disappointing experience, the National Vaccination Programme was stopped on December 31 1996 . Ingelvac PRRS is still registered in Denmark , but may only be used for finishing herds with no breeding stock.
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