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Porcine Reproductive and Respiratory Syndrome (PRRS)

See also chapter 6.

Clinical signs

Acute disease
(442) When introduced first into an EP and App free growing herd there is usually a period of slight inappetence and mild coughing but in some herds there are no symptoms at all. If EP and/or virulent App are present in the herd however, clinical signs may become severe with an acute extensive consolidating pneumonia with the gradual formation of multiple abscesses. Disease becomes evident within 1-3 weeks of weaning, pigs loose condition with pale skin, mild coughing, sneezing and increased respiratory rates. Mortality during this period may reach 12-15%.

Endemic disease
Once the acute period of disease has passed through the breeding and finishing herd PRRS virus normally then only becomes of significance in the early growing period, where severe endemic pneumonia can persist with periods of inappetence and wasting of pigs. Pigs become infected as maternal antibody disappears and then remain viraemic for 3 to 4 weeks continually excreting virus. Permanently populated houses maintain the virus at high levels, particularly in the first and second stage accommodation. Clinical disease is seen in pigs from 4 to 12 weeks of age and it is characterised by a fairly predictable time of onset, inappetence, malabsorption and wasting, coughing and pneumonia. In this post-weaning period mortality can rise up to 12% or more and persist inspite of antibiotic treatments. Secondary bacterial infections become evident in pigs at a later stage from 12 to 16 weeks of age from abscesses that develop in the lungs. These infections spread to other parts of the body, particularly joints with increased lameness.

Diagnosis

This is based on the history and symptoms, post mortem examinations and the known presence of the virus in the herd or by serological examinations and isolation of the virus.

Similar diseases

Chronic respiratory disease is caused by combination of respiratory pathogens including PRRS, SI, EP, App, Hps and pasteurella bacteria, particularly as they become additive. The diagnosis is then of multiple cause.

Treatment

  • In the acute disease when PRRS first enters the farm it is important to cover the period at risk, which is usually six to eight weeks, with in-feed antibiotics or by individual injections and water medication.
  • The broad spectrum antibiotics, tetracyclines, trimethoprim/sulpha, or synthetic penicillins are the medicines of choice but if EP alone is involved tiamulin or lincomycin may be used. If App is active ceftiofur could be a medicine of choice for individual treatments.
  • In endemic disease preventive medication over the period at risk using 500 to 800g of tetracycline or trimethoprim/sulpha 400g/tonne in-feed may be used but it would be advisable to identify the major bacteria involved and determine their antibiotic sensitivities.
Management control and prevention

See also Respiratory disease and control strategies.

  • Consider using segregated early weaning off site to break the endemic cycle of disease.
  • Refer to the principles of SEW and SDC in chapter 3.
  • Section all the buildings so that they can be managed on an all-in all-out basis, and clean and disinfect between each batch. It is most important to adopt this principle at the onset of disease to prevent endemic infection becoming established.
  • Consider the directions of pig movement around the farm. Change these to reduce droplet contamination from older pigs to younger pigs.
  • Consider farrowing once every two to four weeks thus giving an age break between groups and houses. This can be highly effective.
  • Consider depopulation of the first and second stage flat decks where virus is active. Before considering this step however first check by serology that the virus is no longer circulating in the breeding and finishing herds. Elimination of virus has been successfully carried out using this technique on a number of farms. The first and second stage houses are depopulated washed with hot water and detergent and left empty for two weeks. Pigs are weaned away from the farm whilst this is being carried out and then newly weaned pigs are introduced back to the nursery.
  • Live vaccines are available that can be administered by intramuscular injection to the pig at weaning time. Their use is claimed to be beneficial and pigs become resistant to field virus challenge. There is considerable debate as to the long term effects in the herd and the movement of live virus into the breeding herd with possible side effects. Discuss aspects of this with your veterinarian.

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