Multiple PRRS strains are present simultaneously on 90 percent of pig farms. This has been shown by field studies in which the complete genetic information of the PRRS virus has been exposed. This can cost 1.8 piglets per sow. According to researcher Daniel Linhares of the American Iowa State University, this requires a robust approach to reduce losses from PRRS.
Researchers Daniel Linhares, Giovani Trevisan and Gustavo Silva of Iowa State University found that several types of PRRS virus coexist. Not only did they find multiple PRRS strains on 90 percent of the farms, it could even go up to four or more strains at the same time.
This ‘cloud’ of PRRS strains is constantly circulating and changing. ‘There is no question of ‘the’ virus, being a homologous virus that is circulating. We assume that every company affected by PRRS is dealing with a family of PRRS strains, sometimes closely related and sometimes not at all’, explains Linhares.
Time to PRRS stability varies
It can also vary how long it takes for a pig herd to stabilize after a PRRS outbreak. Research by Iowa State University reveals several factors that play a role in this.
• Temporarily stopping the supply of breeding gilts – herd closure pig herd – shortens the time to stability.
• Companies that are committed to quickly supplying negative piglets again achieve this twice as quickly as companies that do not take any measures.
• Farms with three or more PRRS strains take 12 weeks more to stabilize compared to farms with two strains or one strain.
• If no recombination between PRRS strains has been established, a farm with 1,000 sows produces 1,827 more piglets than farms with three strains or recombinations found.
Blood samples
It is important for sow farmers to know about the ‘PRRS family’ on their farm. Linhares recommends that the complete genome of PRRS be examined. This can be done by examining blood samples, especially blood samples with a low CT score. That means that a lot of virus has been found in it.
Another good option is to study tongue tips of stillborn piglets. This can show whether the infection comes from the sows or not. According to Linhares, the use of tissue fluid provides insight into PRRS activity in the entire pig herd. Saliva samples from sows and piglets just before weaning show the state of PRRS activity at that time.
Depop-repop
On farms where PRRS is rampant, depop-repop seems to be the best method to tackle PRRS. According to Linhares, it can also be done with ‘load-close-homogenize’. So first ‘herd-closure’, supplying many rearing sows at once and then not for months, and stabilizing the pig herd with PRRS vaccination. However, with this method it is not always certain how long the period of non-feeding of rearing sows may last. If new rearing sows come in too early, it may have been for nothing.
Depop-repop, i.e. the removal of the entire sow herd, intensive cleaning and disinfection, vacancy and cleaning and disinfection again before new sows arrive, is an expensive method, but it offers the most certainty if several PRRS strains are circulating on the farm.
According to Linhares, tackling PRRS is necessary, not only because of the direct losses in piglet production, but also because PRRS is known as the pioneer for other pig diseases.