Q & A on PRRS
What is the current thinking about the porcine reproductive and respiratory syndrome?
PIG INTERNATIONAL: Professor Martelli, where would you place PRRS today in terms of its economic importance to the world's pig industries?
MARTELLI: PRRS has become one of the most important disease entities affecting pigs worldwide, with effects costing the equivalent of many hundreds of millions of US dollars every year. Most producers think of it first as causing reproductive failure in sows, but we now know that approximately 85% of its total cost relates to the effects of the infection in the growing and finishing stages of production.
PIGI: Has the virus changed over the past 5-10 years?
M: It can mutate, even in the same herd, so changes have been inevitable. Distinct North American and European genotypes were recognised from the early days and at first it was thought that the American one was less stable so more likely to change genetically, but the last 5 years have shown that the European type is equally capable of changing.
PIGI: Were such changes behind the recent reports of a high-fever version in Asia (China and Vietnam) and the outbreak in Sweden after years of the country staying virus-free?
M: Quite honestly we do not know. Probably not in the Swedish case, they had done well to stay free of the virus for so long. The Asian episodes may have involved PRRS in a mix of pathogens. What is clear is that the process of mutation tends to continue at the same pace over years rather than by sudden jumps. Also, we see all the time that genetically different isolates of PRRS virus differ in virulence in terms of their ability to cause disease as well as in their antigenic make-up. However, this does not mean the American genotype is necessarily more virulent than the European one or the other way round. The variations in virulence occur within genotypes, not from one genotype to another.
PIGI: Has our knowledge of the virus improved significantly?
M: Certainly, the last 10 years have brought plenty of research with good results. But there is still the frustration that we do not know enough to control the disease properly. For example, knowledge is lacking about exactly what happens in the pig in terms of its immune response when it becomes infected. Nor is it clear how the virus can transmit among herds. A key problem in practice is that although we know enough to be able to eradicate the virus from a herd quite easily, there are gaps in the know-how on keeping a virus-free status. Eradication is very costly and you need to balance the risk of re-infection carefully before you start. All we can say is that the risk is less where the unit practises strict biosecurity and is in an area of relatively low pig density.
PIGI: What can we achieve by vaccination?
M: The vaccines available today cannot be completely protective against the disease and viraemia, with positive effects on persistence and the viral loading of the tissues, unless the field and vaccine viruses are homologous. In the real world of on-farm conditions we are dealing with heterologous strains of virus. Infection will not be prevented, so in that sense the protection is less than 100%. But the pigs will still be 70-90% protected through reductions in the magnitude and duration of the viraemia together with fewer clinical signs and less viral loading. In my opinion this is a really good result, bearing in mind the severe economic impact of PRRS. Remember I said at the start that 85% of the cost of the disease occurred in the grow-finish stages. You are talking about a lot of money that can be saved by effective vaccination.
PIGI: Can we expect even better vaccines?
M: Yes, but it may be 5 years or more before they arrive. One focus of research today is to develop a new generation of PRRS vaccines that can protect completely even in field conditions of heterologous strains. Moreover, we want to be able to differentiate between the antibodies resulting from natural infection and those from vaccination. That means marker vaccines, along the lines of those already available against Aujeszky's disease/pseudorabies. For the moment it is a dream, but that dream will become a reality one day!