PRRS, an Underestimated Disease

Vaccination alone seldom provides a satisfactory result for the farmer.  Experience has shown that the adoption of a ‘holistic’ approach to PRRSV control gives the most consistent and reliable long term solution to controlling PRRSv.


Article by:
Dr Allison Kirwan

Porcine Respiratory and Reproductive Syndrome virus (PRRSv) is a viral disease, affecting all stages of pig production. In sows PRRSv infection may result in reproductive failure, such as abortions, low viability and stillborn piglets, or general infertility. In nursery/grower pigs it may cause pneumonia and change the way the defence mechanisms in the respiratory tract work, predisposing the animals to other common causes of pneumonia in pigs. This may result in an increase in pig mortality, increased treatments and a reduction in performance.
PRRSv is a global problem, affecting the swine industry to a greater or lesser extent, worldwide, and, where present, is of high economic significance. In Europe, the estimated annual cost associated with PRRS ranges from around €940 million up to €1.5 billion. This translates at around €100-200 per sow and €5-10 per growing pig. At present, in the UK it is estimated that just below 60% of herds are PRRSv positive, with a wide, regional variation. In Northern Ireland, PRRS prevalence is estimated at 43% and in the Republic of Ireland, at 33%.
Control of PRRSv has proven to be difficult for several reasons: the continual changes occurring with the field viruses, a limited understanding of the way the virus enters and persists in the farm and also a limited knowledge regarding the effect of PRRSv on the immune system. In practical terms, the successful control of PRRSV is now understood to be complex and to involve multiple facets of pig production:  external and internal biosecurity, regular diagnostic surveillance and a ‘whole herd’ approach to immunization, including frequent, mass vaccination of the adult herd, along with routine piglet vaccination.
A pig may be infected either by direct contact with an infected pig that is shedding virus (from bodily excretions, such as faeces, saliva, nasal discharge) or by indirect contact, via contaminated equipment, needles, staff clothing/boots or dirty pens/troughs from the previous batch of pigs. The primary source of herd to herd transmission is usually through the introduction of infected gilts or, less commonly, infected semen.  
Following contact with a naïve pig (one that has never met the virus or been vaccinated) the PRRS virus quickly infects the major immune cells of the lungs, causing inflammation in the lungs (manifesting as ‘thumping’, coughing (depending on secondary bacteria present), lethargy and reduced appetite. In breeding animals the virus then travels to the uterus where, if the animal is pregnant, it is capable of crossing the blood/placenta barrier and infecting the unborn piglets. The outcome of transplacental infection depends on the stage of pregnancy and on the immune status of the sow and the ability of the virus to cause disease. The resulting clinical signs range from nothing observable through to abortion storms, premature farrowing, and an increase in stillborn, mummified and weak piglets. Piglets that appear healthy at birth may be infected and shedding virus. In non-pregnant sows and boars the clinical signs are usually mild or absent and are usually limited to slight fever and loss of appetite. 

Table 1: A list of potential clinical signs after infection with PRRSv.

Breeding herd ·         Abortions
·         Stillbirths
·         Premature farrowing
·         Weak born piglets
·         Mummies
·         Healthy but infected piglets
·         No visible signs
Growing herd ·         Increased mortality
·         Reduced performance
·         Increased medicine use
·         Limited response to medication
·         Increased respiratory disease

Fig 2: Aborted material after PRRSv infection of pregnant animal.

In piglets, following infection, when the immune cells of the lung have been altered, the virus will act as a ‘door opener’ for other respiratory infections, usually enhancing the effect of the other pathogen, as is the case with Swine Influenza. Virus is excreted from the body through all bodily fluids (including semen) and via airborne transmission, with the pig usually, eventually, clearing the virus. Younger infected pigs are likely to act as a continuous source of infection for all the other pigs (including sows and boars) on site.

Fig 3: Ill thriving nursery pigs infected with PRRSv

Fig 4: PRRSV infected lungs, showing the darker, diseased tissue compared to lighter, air-filled healthy tissue

The control of PRRS virus has traditionally depended solely on the vaccination of adults in the herd and, to a lesser extent, vaccination of piglets.  However, vaccination alone seldom provides a satisfactory result for the farmer.  Experience has shown that the adoption of a ‘holistic’ approach to PRRSV control: frequent, mass, vaccination of the breeding herd, routine vaccination of piglets, appropriate  gilt immunisation, alongside greater attention to all aspects of internal and external biosecurity, gives the most consistent and reliable long term solution to controlling PRRSv.
To realise these aims on every PRRSv infected pig farm, a ‘5 Step Process’ diagnostic and management tool has been developed by Boehringer Ingelheim. This is a systematic platform that enables both farmers and their vet to work logically towards a common goal of PRRS virus control or elimination.  

In summary, PRRSv can be a costly, difficult and frustrating problem to deal with. External biosecurity (stopping PRRSv coming in) is important but challenging, considering the many potential breaches that can occur and the many opportunities for virus to enter the herd. Close consideration to internal biosecurity (reducing the circulation of PRRS viruses within the herd) will help to limit the spread and impact of resident viruses (see the “10 Golden Rules”). Finally, increasing the resistance of all animals on site, using a whole herd approach (frequent, mass vaccination of the entire breeding herd and routine vaccination of the grower herd), will provide maximum protection and benefit for the whole herd, over time.