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When the large outbreaks of porcine circovirus disease in Europe, Asia, North America, etc. were studied, it was found that in all the countries where these health problems were analyzed, it corresponded with a very clear change in the prevalence of genotype a towards genotype b that occurred from around 2001-2004. The frequency of serious problems with genotype a was much lower than with genotype b. And the obvious question is, why did the frequency of these genotypes change? We're talking about a period where there was no vaccine-induced immune pressure. We are currently facing another change, in which the most frequently encountered genotype has shifted from b to d. Why do these changes in prevalence occur? There is speculation that vaccination pressure may have affected this change, based on the idea that vaccine coverage was more effective against b than against d. But this is still just speculation, especially so if we remember that all commercial vaccines today are based on PCV-2a.
What has been shown is that vaccines based on PCV2-a offer protection against the rest of PCV-2 genotypes.
How many genotypes of PCV-2 are recognized up until now?
Recently, in collaboration with Dr. Giovanni Franzo, from the University of Padua (Italy), we decided to create a genotype recapitulation based on genotyping criteria that is more modern than what was originally used in 2008. We took into account genotypes that were present with a certain number of sequences. In this process we were able to establish 8 genotypes (a,b,c,d,e,f,g,h), although within the genotypes listed there, some of them are poorly represented.
In summary, PCV-2c was described retrospectively in Denmark when the change in prevalence from PCV-2a to PCV-2b was being studied, in relation to the onset of clinical problems with PMWS. PCV-2c was subsequently found in some wild pigs in Brazil and also in China. PCV-2e was found in Mexico and the United States and PCV-2f was found in China. What we can say today is that the most frequently circulating genotype is d, followed by b and a. This does not mean that the other genotypes are not circulating, but they do not have the same representation and geographical distribution as the other three.
How does the PCV-2 recombination phenomenon affect vaccine effectiveness?
It is true that a recombination phenomenon occurs in circoviruses. There are changes in sections between viruses, and changes can also occur between different genotypes. How does this affect the effectiveness of the vaccine? It should be noted that vaccines are made based on the only capsid protein that PCV-2 contains, one of the simplest known viruses. If we consider a hypothetical recombination between genotype b and d that would affect this entire section, we would not expect any problems with vaccine protection. This could be a problem if recombination affected only a portion of the genome, but recombinations do not occur randomly, they take place at specific points. For these reasons, and given that no specific studies have been done, recombinations are not currently considered to be a large enough source of variation to affect the protective capacity of current vaccines. The sporadic cases of farms that have had cases diagnosed as clinical circovirus associated disease while having vaccinated against PCV-2, have not been associated with the presence of recombinant viruses.
Could the frequency of different PCV-2 genotypes change in the future?
This is something we do not know at this time, but it could happen like it has before. There is talk of the influence of vaccination pressure, but we do not know. In fact, today we still do not know why genotype b was "more successful" leading to the change in frequency from a to b took place.
Interest and discussion about the importance of genotypes has perhaps increased as a result of these sporadic cases where clinical porcine circovirus disease has been observed on farms that were vaccinating against PCV-2. When genotyped, most of these farms had genotype PCV-2 d. Many experts questioned whether this was due to this genotype escaping the vaccine's action, or it having a higher degree of pathogenicity, but I think this finding only reflects the fact that PCV-2 d is now the genotype most frequently found on farms. In fact, studies based on clinical diagnosis of porcine circovirosis have been carried out, studying which genotypes were present, and the three most frequently isolated were found to be a, b, and d. In addition, when it was seen that genotype d was increasing in frequency, and when it was questioned whether current vaccines provided protection against this genotype, experimental infections were performed and the result was that vaccines cover this genotype in the same way they do a and b.
