It’s been reported that over 18 million pigs have been affected by the Rosalia strain in northern Spain.

The new strain is highly pathogenic, with a particular effect on breeding stock and high rates of mortality in sows, abortion storms and stillborn piglets.

PRRS is a single-strand RNA virus and, like COVID-19 in humans and influenza in pigs, it multiplies as it passes through its host, changing or mutating very slightly.

If this occurs when other similar virus strains are also circulating, it can recombine, creating a new strain. Rosalia is made up of at least four different PRRS fragments found across Europe.

“Although it’s unlikely that this particular strain will reach Ireland any time soon, pig farmers shouldn’t be complacent about PRRS,” cautions technical manager at MSD Animal Health Maureen Prendergast.

“The industry needs to be vigilant to the risks and alert to the potential emergence of new strains. Farmers and vets should report any unusual features of disease in their pigs.”

Controlling PRRS in Ireland

Herd-to-herd transmission of PRRS can occur following the introduction of infected animals or the use of infected semen.

The virus can also be spread via contaminated equipment and there are reports that the virus can spread on the wind for up to 3km.

The proximity of other farms and the movement of lorries are believed to have been important factors in the transmission of the Rosalia strain in Spain.

“Acquired immunity continues to be the best protection against PRRS,” explains Dr Prendergast. “Rather like we see with COVID-19, vaccination protects against the effects of the virus and works best when all animals in the herd are vaccinated.

"For example, Porcilis PRRS reduces morbidity caused by the disease and improves daily growth and feed conversion in growing pigs. Breeding pigs see an improvement in reproductive performance and reduction of transplacental virus transmission when vaccinated with Porcilis PRRS.”

Gilt management is key to stability of the breeding herd. Gilts may have been exposed to disease and still be potentially infected with field strain of PRRS, so need to be immunised prior to joining the rest of the breeding herd.

Early selection is recommended to allow the full course of vaccine to be administered and allow a suitable interval prior to service to allow immunity development.

Your vet will advise the recommended procedure for your farm.

Sows should receive regular PRRS vaccination, either before every service on a ‘6-and-60’ programme, or at regular intervals as part of a blanket immunisation programme.

Gilts or sows continue to shed virus for a short time post-vaccination as their immunity develops. This time period will depend on the individual vaccines, but ideally use a stable, low-shedding vaccine.

Ensuring effective PRRS vaccination

There is a range of commercial live vaccines against PRRS available to farmers in Ireland.

The European Medicines Agency recently recommended general guidelines to ensure the safe use of these vaccines, to mitigate the risk and the frequency of recombination between PRRS viruses including PRRS vaccine strains.

All pig farmers are now advised:

  • To only vaccinate healthy animals.
  • To only use one vaccine throughout the herd, avoiding the use of different companies’ vaccines for breeders and rearing piglets.
  • To select a vaccine with a reduced and self-limiting shedding period of the vaccine virus (information available in the SPC).
  • To respect a transition time before introducing a new vaccine and, if needed, to undertake diagnostics to confirm the absence of any other vaccine or field virus strain circulating on farm.
  • To speak to their vet about the new EMA guidelines before making any changes, in order to identify the best vaccination programme for their farm.
  • At a glance: Porcilis PRRS

  • Sow, gilt and piglet vaccine in one bottle.
  • Intramuscular or IDAL administration.
  • Individual sow or ‘blanket’ vaccination possible.
  • Suitable for piglets from two weeks of age.
  • Established in Ireland for 20 years.
  • Providing up to 24 weeks’ immunity against PRRS.
  • Can be administered alongside Porcilis vaccines for PCV, M. hyo and Lawsonia
  • For more information

    Farmers should discuss their vaccination regime with their vet, who can also advise on diagnostics, improving sow fertility and on-farm biosecurity.

    Further information is available from the vaccine SPC, your veterinary practitioner or MSD Animal Health, Red Oak North, South County Business Park, Leopardstown, Dublin 18, Ireland. MSD Animal Health can be contacted at 01-297 0220 or emailed at