PRRS Rosalía: what have we Learned in these five years?

The Rosalía strain appeared in 2020 as a particularly virulent variant of PRRS, causing significant outbreaks and posing major challenges to Spanish pig production. Five years after those first cases, Dr. Enric Mateu, veterinarian from the Universitat Autònoma de Barcelona and expert in swine health, offers us an overview of the current situation and explains how the virus has evolved, its impact, and the lessons learned over this time.

Enric Mateu, doctor in Veterinary Medicine and expert in swine health. Photo: Enric Mateu.

How has the Rosalía strain evolved over the past five years?
At this point, we can no longer speak of a single Rosalía strain. Over these five years, the virus has spread widely, undergone multiple recombinations, and experienced numerous changes in its genome. What we have now is a constellation of descendants of the original Rosalía strain, meaning a great deal of genetic diversification. However, there does not appear to be a single predominant mutation or variant. This is largely due to the virus’s own replication mechanism, which introduces mutations with every new generation of viral particles. Considering that the Rosalía strain has infected millions of animals in a very short time, and that in each of those animals hundreds of billions of viral particles have been generated, the accumulated level of genetic variation is immense. What we are seeing now is not the emergence of a new dominant variant, but rather the expression of the full potential of genetic diversity that this strain had from the start.

What impact do we see now on its virulence compared to the initial outbreaks?
In general terms, current outbreaks seem less severe than those of three or four years ago. However, this apparent reduction in virulence is probably due less to changes in the virus itself than to the progressive development of immunity in the pig population. We must take into account that in regions such as Catalonia or Aragón, the proportion of infected farms is very high. This means that many farms already have a certain degree of herd immunity, which mitigates the clinical impact of new outbreaks. Even so, we cannot generalize. Variants of Rosalía with virulence comparable to the original strain are still circulating, while others with more attenuated clinical behavior are also being detected.

What control and management strategies have proven most effective against Rosalía?
From a practical standpoint, changes in production flow have been among the most effective tools. Specifically, five-week batch management has proven to have a positive impact on health stabilization. Many farms that have implemented this system have managed to significantly reduce viral circulation, although results are not immediate and, in many cases, the process takes close to a year. However, this strategy is not viable for all farms, especially those with structural or productive limitations. That is why other measures, more accessible but equally important, have also been key—such as reinforcing internal biosecurity, applying stricter work protocols, and maintaining strict discipline in daily management. A critical aspect is vaccination. Although its effect may be limited, it provides a basic level of immunity that helps cushion the impact of new infections.

Given the usefulness of vaccination, are there commercial vaccines that provide adequate protection against this strain?
The vaccines currently available on the market offer limited protection against the Rosalía strain, although it is always preferable to vaccinate than not to. Clinically, they can help reduce the severity of symptoms and maintain parameters such as growth and weight. However, their effectiveness in curbing virus transmission is very low. The main reason for this limitation is the genetic distance between the Rosalía strain and the vaccine strains included in commercial formulations. Even so, it is worth stressing that vaccination remains a useful tool: although it does not guarantee full control of infection, it can help mitigate its effects and support health stabilization when combined with other management and biosecurity measures.

Current PRRS outbreaks appear less severe. Photo: Rotecna.

Compared to 2020, do we now see lower mortality thanks to improvements in management or infection control?
With Rosalía, certain aspects have become evident, especially in phases such as weaning and finishing, showing clearly that management plays a key role in reducing both transmission and mortality. Although figures remain high—around 7–8% mortality in weaning and 5–6% in finishing—a reduction has been achieved by applying basic principles we already knew: not mixing batches, avoiding backward animal movements, implementing all-in/all-out systems, and maintaining thorough cleaning and disinfection of facilities. However, we have also seen that the presence of concurrent infections with other pathogens greatly complicates control. Rosalía exacerbates these situations, adding an extra level of difficulty to health management.

Have more agile diagnostic tools been developed to assess immunity against Rosalía?
This remains a complex field. The main challenge is that we still do not know precisely which parameters directly correlate with effective immunity against Rosalía. We can measure antibody levels or evaluate the cellular response, and while this data provides some guidance on the animal’s immune status, it does not allow us to predict with certainty whether it will be protected against infection. In short, we have tools that provide partial information, but not a clear immunological marker that can conclusively indicate the level of protection. Therefore, in this regard, we could say we have not advanced significantly compared to the situation in 2020.

What impact has this strain had on the perception of PRRS within the sector?
Some companies and professionals have been truly concerned and have taken serious measures to tackle the virus. However, there is also a part of the sector that, without this implying a lack of interest, has not adequately valued the real impact of the disease. One factor influencing this perception has been the economic context. Prices have remained extraordinarily high, and one of the reasons has been precisely the animal shortage caused by PRRS. In 2024, the average gain per slaughter pig was around €37, providing a very wide margin. This has allowed farms to remain profitable even with high mortality, but it also creates the risk that the true extent of the health problem is not clearly perceived.

After five years, what lessons has the swine sector learned in terms of surveillance, biosecurity, and health decision-making?
In companies that have truly taken action, the way health is managed has changed significantly. Issues such as biosecurity, logistics organization, or animal movements are now approached differently, precisely because of the risk posed by this disease. The fact that many companies have chosen this path shows that they have become aware of the economic impact that a health problem of this magnitude can have, and this has helped generate greater awareness of the importance of health within the production process. It should also be noted that the Spanish swine sector has long struggled with making collective decisions and assuming shared responsibility. This lack of coordinated action has been one of the most evident weaknesses in PRRS management. Much of the virus’s initial spread was due to poor communication between operators, whether voluntary or involuntary. This is a lesson we should learn. Moreover, it would be desirable for the authorities to adopt a more active role and provide the legal or bureaucratic support needed so that the sector itself can implement more effective control measures.