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Porcine Reproductive and Respiratory Syndrome Virus (PRRS)

See also chapter 6.

Clinical signs

(341) When first introduced into a herd PRRS virus infection has a marked affect on all lactating sows over a 6-8 week period. They show varying degrees of inappetence and mild illness but the most striking sign is agalactia, not necessarily with oedema of the udder or mastitis, but just a very poor milk flow. In part this is associated with inappetence and reluctance of the sick sow to drink. The temperature may be normal or elevated. Mastitis or urinary infections may occur as secondary effects. Typical signs in the sucking piglets are also evident (these are given later in this chapter under problems in disease in piglets). Stress at farrowing may activate latent infections and this is a common experience 6 to 12 months after the initial outbreak of PRRS.


When disease first appears in the herd the most striking picture is the high mortality in piglets that are weak at birth and the poor milking and agalactia in sows. Antibodies become evident in serum 10-14 days after exposure and virus isolation can be carried out during the clinical phases.


  • Medication in acute outbreaks should include long-acting injections of OTC 2 to 3 days prior to farrowing.
  • Alternatively top dress the sows feed for 4 days before and 10 days after farrowing or during the total period of lactation with either CTC, OTC or trimethoprim/sulpha. Use 15-20g per day of 10% premixes (or pro-rata).
  • Medication during an acute outbreak should be maintained for a period of up to 6 to 8 weeks.
  • Once a herd immunity has developed it is uncommon to see disease again at a herd level however if the virus continues to circulate in the breeding herd in individual animals and in particular the incoming pregnant gilts, top dress the feed with CTC, OTC, or TMS for 4 days pre and 3 days post-farrowing. Also assess acclimatisation procedures for replacement gilts in the breeding herd as a whole.
Management control and prevention.
  • Consider vaccination and/or acclimatisation of incoming gilts.

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