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PCV2 evolution: looking for the best match of the virus to its target cell
PCV2 is a virus in constant evolution, and it has gone extremely fast. PCV2 evolution happened before the mass vaccination with mostly PCV2a based vaccines. Even when you vaccinate, PCV2 is still causing infections in herds. The combined vaccination/infection immunity has been one of the driving forces behind the evolution of PCV2 in the direction of a dominating PCV2d strain. Another driving force is its better replication in lymphoblasts compared to older strains.
After years of fast evolution, it looks like PCV2d as the most predominant strain is a little bit more stable, but I am convinced this will not be the last evolution of the virus. In this context, it is extremely important to understand why PCV2d has become the dominant strain nowadays.
PCV2d: the fittest of them all... so far
The answer to the question above is that it seems that PCV2d is much better in binding and getting into the lymphoblasts and therefore replicating to higher levels compared to the old strains. We have studied how PCV2d changed its surface to have a better binding to its receptors on the lymphoblasts and this small change is extremely important. So, working in vitro, if you take lymphoblasts and you incubate them with different strains of PCV2, you can see important differences. With PCV2a or PCV2b there is limited replication, but if you put PCV2d the replication will be much higher. PCV2a and PCV2b are still present but PCV2d is the “winner”. One theory is that due to this very good “match” of PCV2d and the lymphoblast receptors the only evolution that could be successful is if a new strain becomes even better in interacting with receptors. This will create a new dominant strain.
If a new dominant strain will arise, it is important to find out the mechanism that the virus used to outperform PCV2d.
The importance of pig genetics: an opportunity and a threat
It has been shown that different pig genetic lines have a different response to PCV2, both in live animals but also through studies in vitro. Those in vitro studies are trying to identify the factors that could explain why the genetic background of the pig plays a role in allowing the virus to have a different impact. The study of those genetic differences could provide very valuable information about the mechanisms behind this resilience in some animals that would help developping new control strategies, e.g. by selection.
At present, there is a strong trend in the livestock industry to narrow its genetic basis. This reduction in biodiversity will limit the possibilities for selection. It is very important to conserve a gene pool in pigs. As a believer in the benefits of biodiversity, and the importance of genetics in the answer to infection, loss of resilience/resistance genes could be very dangerous when next new aggressive variants arrive; it will limit the possibilities to select for more resilient/resistant animals.
The need for research
I was involved with PCV2 research from very early on, when the impact due to the PCV2 associated diseases were creating huge economic losses to the swine industry globally. Back then, as it usually happens when a viral crisis arises, we had financial resources to investigate the virus and plenty of valuable information came from these working groups. Nowadays, the resources for research are very limited.
It is important not to forget that it is extremely relevant to keep understanding those viruses that maybe are not causing an immediate threat, to find out how they evolve, and why they suddenly become a threat to animal and/or human health. COVID-19 opened the eyes of many people that not only hot topics should receive funding, but memory can be very fragile.
Regarding PCV2, it is important to understand the evolution and the changes of the virus, and if at any point the vaccine is less effective, we should be able to react in a proper way.
It is important to be prepared and be able to have control systems in place for whatever virus that may emerge.
