Unit 1: Importance of PRRS and PMWS to the European pig industries
In recent years pig production has become more concentrated within Europe. Four countries, Denmark, Germany, Spain and France are responsible for more than half of Europe's pig output. Average herd size and pig density have increased remarkably in these countries. For example, Danish herds now have more than 400 pigs on more than 90% of the pig farms. In contrast other European countries still have a more traditional pig production, such as Poland where only 2 out of every 10 farms have more than 400 pigs. Pig density has increased considerably in Germany Denmark and the Netherlands where production is concentrated in specific geographical areas. Pig movement has also increased across Europe, where there has been some specialization in certain countries, for example Denmark producing mainly breeders and Spain mainly fatteners. These differences have had an impact on the potential risks and importance of PMWS and PRRS in the different countries.
In most of Europe, both diseases are among the most important diseases in the pig industry because of their high economic impact and they are also compromising animal welfare because of the sickness associated with the conditions. The pattern of PRRS and PMWS has changed from sudden outbreaks that spread rapidly through the pig population (i.e. epidemic stage), to frequent and constant occurrences in production pigs (i.e. endemic stage).
The main economic losses of PMWS are due to increased post-weaning mortality, reduced growth rate, increased feed conversion ratio and secondary respiratory infections resulting in increased management and treatment costs. During the early years of the epidemic the cost of the disease was estimated to be between €562 and 900 million per year across the European Union countries. However, the severity with which farms were affected differs both within and between countries. Differences in mortality rates associated with PMWS can be due to either differences in the pig production systems (all indoor vs. all outdoor vs. part indoor / part outdoor) or presence of other diseases. The proportion of pigs affected within the farm can be up to 60% (6 out of every 10 pigs) during an outbreak and varies between 1 and 30% in endemic situations. Post weaning mortality resulting from PMWS can be as high as 1 in every 4 deaths, and a sudden increase in mortality combined with wasting is one of the main reasons for suspecting an outbreak of PMWS.
The main economic losses in PRRS are through respiratory and reproductive related issues. PRRS causes respiratory malfunction in growing and fattening pigs. Infected herds are prone to getting secondary respiratory infections and show reduced growth rate, and increased production costs due to veterinary treatment and management expenses. PRRS also causes a decrease in reproductive performance: sows take longer to get pregnant, there is an increase in abortions (normally during the third trimester of pregnancy), and premature parturition and/or more stillbirths and neonatal deaths are observed. As with PMWS, the severity of PRRS infection varies widely across farms. The PRRS virus, however, keeps mutating even on the same farm; and this is more likely to happen when more than one strain is present. This makes it more challenging to control. The degree of clinical PRRS on a farm might be related to the strain of the virus that is present and its ability to replicate (this is explained in the next unit: 'PRRS - causes and predisposing factors').