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