Global Dominance of PCV2d

In just over two decades, the landscape of porcine circovirus type 2 (PCV2) has changed dramatically. In the late 1990s, the PCV2a genotype was the predominant one in most pig-producing countries. Over time, it was displaced by PCV2b, which quickly became the main driver of the outbreaks that triggered clinical signs in the mid-2000s. However, for more than a decade now, a single predominant genotype has been identified: PCV2d, which today is the most frequently detected genotype in commercial farms worldwide.

This phenomenon, known as a genotype shift, is not a random change. Field sequences show that PCV2d carries specific genetic variations in the capsid protein, the main target of the immune system. These changes can alter how the virus is recognized by antibodies generated against previous genotypes, giving PCV2d an advantage in populations with high immunity to PCV2a or PCV2b.

The trend is not limited to a specific region. Studies in Asia, Europe and the Americas consistently show that, over the past ten years, the relative frequency of PCV2d has increased steadily, surpassing other genotypes, even in farms with well-established vaccination programs. This genetic replacement is documented through prevalence curves showing how PCV2d, initially a minority, has reached detection rates above 70–80% in some countries.

Furthermore, the circulation of PCV2d is not restricted to domestic pigs. Cases have been reported in wild boar populations, suggesting the existence of wildlife reservoirs that may contribute to the persistence of the virus and its genetic exchange with strains from intensive production. This interaction between populations reinforces the need for continuous monitoring of circulating variants.

From a practical standpoint, understanding this genetic evolution is essential. It is not enough to simply control the infection; it is necessary to monitor which genotypes are circulating to anticipate potential changes in the effectiveness of control measures. In practice, this means incorporating molecular diagnostic tools that not only detect the virus but also identify its genotype.

The dominance of PCV2d is the result of an evolutionary process that combines immune pressure, genetic adaptations, and broad transmission opportunities. Understanding this process is not just an academic exercise, it is a direct investment in herd health and productivity.

Summary
• PCV2d, now the dominant genotype, has displaced PCV2a and PCV2b due to its immunological advantage, which allows it to evade immunity generated against previous genotypes.
• Globally documented presence in intensive production farms and wildlife, with prevalence exceeding 70–80% in some countries.
• Control challenges: its ability to evade the immune response requires vaccines and control programs adapted to current genetic diversity.

References

• Franzo G., et al. (2016). Porcine circovirus type 2 (PCV2) evolution before and after the vaccination introduction: a large scale epidemiological study. Sci. Rep.
• Huang L., et al. (2021). Epidemiological and genetic variation analysis of emerging porcine circovirus type 2d. BMC Vet. Res.
• Phuong T., et al. (2018). Genetic characterization of porcine circovirus type 2 (PCV2) in Vietnam. Arch. Virol.
• D’Annunzio R., et al. (2023). PCV2d in Latin American swine populations. Transbound. Emerg. Dis.