Porcine Reproductive and Respiratory Syndrome (PRRS)

The effects in breeding herds are reproductive failure and signs of respiratory disease. Weaned and finishing pigs suffer from respiratory disorders due to secondary infection, which are worsened by the immuno-modulatory properties of the PRRS virus (PRRSV).

About PRRS

• Etiology
• Consequences
• Pathogenesis
• Clinical signs
• Interactions
• Diagnosis
• Control

For detailed information on PRRS in pigs also see the website www.porcilis-prrs.com.

Etiology
The PRRS virus is a single-strand RNA virus.

Classification:
Order: Nidovirales
Family: Arteriviridae
Genus: Arterivirus.

top

Consequences of PRRS
The economic impact of PRRS in a finishing unit is substantial. In a breeding herd it is dramatic but in general only lasting for some month’s till the breeding herd reaches a stabile situation. However the reproductive problems may reoccur if the breeding herd become destabilised due to recycling of PRRSv from the finishing herd or excreting gilts.

top

Pathogenesis of PRRSV infection
The PRRS virus is transmitted by various means. The virus enters and replicates in macrophages. Disease may be subclinical or clinical. The syndrome presented, respiratory or reproductive, is dependent on the age of the pigs affected.

top

Clinical signs
Sows and piglets
PRRSV infection can cause severe reproductive damage:

• premature farrowings
• stillborn or mummified piglets
• weak PRRSV-positive piglets (50% die soon after birth)
• delayed return to service

Stillborn and weak piglets at farrowing as a result of PRRS infection in the sow

In addition to reproductive failure, sows and gilts may show:

• anorexia
• fever
• lethargy
• pneumonia
• agalactica
• red/blue discolouration of the ears and vulva
• subcutaneous and hind limb oedema
• delayed return to oestrus after weaning
• in rare cases death.

Neonatal piglets
Neonatal piglets can display a variety of clinical signs. The most characteristic are dyspnea, tachypnea and death.

Growers and finishing pigs
PRRSV infection alone is often subclinical. It is however indirectly responsible for huge economic losses in finishing herds due to its major role in the multifactorial Porcine Respiratory Disease Complex (PRDC).

If clinical signs are present, they are usually respiratory:

• fever
• sneezing
• hyperpnea
• dyspnea
• coughing
• pneumonia
• lethargy
• periocular edema
• oculonasal discharge

Experimental challenge of SPF pigs with PRRSV alone usually produces no clinical signs.

top

Interactions between PRRSV and other pathogens
PRRSV interacts with other pathogens to cause severe respiratory symptoms in finishing pigs.

Mechanism of interactions
The mechanism by which PRRSV interacts with other pathogens is still under investigation. However, it has been demonstrated experimentally that:

• Mycoplasma enhances the pathological effects of PRRSV
• PRRSV enhances the pathological effects of Porcine Influenza virus
• PRRSV predisposes pigs to infection and disease caused by Streptococcus suis
• PRRSV can cause a loss of the bactericidal function of pulmonary intravascular macrophages.

The role of PRRSV
It is still not possible to conclude which are the most important combinations of pathogens or to define the importance of PRRSV in the severity of respiratory disease in finishing pigs. This is difficult to prove experimentally due to pathogenicity of virus strains used, timing of infections, serological status of pigs used and the lack of predisposing or stress factors due to housing and/or management. Clinical experience in the field has shown that the occurence of new respiratory pathogens result in increasingly complex respiratory problems and improved performance has been observed in PRRSV-vaccinated herds.

top

Diagnosis of PRRSV infection
Methods used to diagnose the presence of PRRSV infection are:

• Serology
• Virus isolation (VI)
• Reverse Transcriptase Polymerase Chain Reaction (RT-PCR)
• Nucleotide or amino acid sequencing

top

PRRS control in different pig herds
For both growing and finishing pigs as well as for breeding stock the following has proven effective:

• Vaccination
  A live vaccine for vaccinating the whole herd is available. See Vaccines for more information.
• Management strategies
  As always, management practices have an important part to play. Gilts entering the breeding unit need to be acclimatized, strict adherence needs to be paid both to the all-in/all-out principle and to the unidirectional flow of animals during the nursery and growing phases.

top