By Eduardo Fano, DVM, MSc, PhD, Boehringer Ingelheim

Despite the fact that DNA viruses have a lower mutation rate than RNA viruses, porcine circovirus type 2 mutation should be expected, as viruses evolve in an effort to survive.

In 2010, a novel mutation reported in China was designated mPCV2b. Two years later, Iowa State University reported the presence of this novel strain in the United States, and it was later reclassified as PCV2d. Since 2012, PCV2d is the only novel mutation that has been reported.^1,2 

Given the potential costs of a PCV2 outbreak, it’s natural for veterinarians to wonder: Can existing PCV2a-based vaccines provide cross-protection against PCV2d, or are homologous PCV2d vaccines needed?

Previous studies have shown that PCV2a vaccines do, in fact, provide cross-protection against PCV2b.^3,4 In other studies, pigs vaccinated with PCV2a vaccines and challenged with PCV2d showed reduced viremia, increased antibody titers and increased average daily weight gain when compared to non-vaccinates.^5,6 

To further this evidence, Boehringer Ingelheim performed an additional study to evaluate the efficacy of a PCV2a-based vaccine (Ingelvac CircoFLEX®) and an experimental PCV2d-based vaccine against a virulent PCV2d challenge.⁷ 

Putting vaccines to the test

In this randomized, blinded challenge study, 100 colostrum-deprived 3-week-old pigs were divided into four groups of 25. On day 0, one group received a PCV2a vaccine (INGELVAC CIRCOFLEX), another received an experimental PCV2d vaccine and two control groups were administered an adjuvant only.

On day 28, all pigs were challenged with a PCV2d strain isolated at the Iowa State University Veterinary Diagnostic Laboratory. At this time, it was determined that the pigs had also been naturally exposed to PCV2a.

Figure 1: Lymphoid tissue lesion scores by group, mean and median, represented by the diamond and horizontal line, respectively.

A mixed PCV2a/PCV2d challenge

The results? None of the pigs that were administered the PCV2a or PCV2d vaccines died following the challenge, while the two control groups experienced 48% and 33% mortality.⁷ Similarly, no vaccinated pigs were clinically affected, while 22% and 50% of pigs in the respective control groups showed evidence of respiratory distress, poor body condition and/or recumbency.⁷

During histopathology, all pigs were assigned a lymphoid tissue lesion score, based on lymphoid depletion and immunohistochemistry. Both vaccinated groups were spared from lymphoid tissue lesions, while the control groups were severely affected (Figure 1).⁷ 

Compared to the control groups, vaccinated pigs also experienced significantly reduced viremia and significant increases in ADWG.⁷

PCV2a-based vaccine demonstrated cross-protection
These results suggest that both PCV2a and PCV2d vaccines can provide similar protection against a severe, mixed PCV2a/PCV2d infection.⁷ Even more important, the study provided an answer to the original question by demonstrating that INGELVAC CIRCOFLEX, a PCV2a vaccine, can provide effective cross-protection against a virulent PCV2d challenge.⁷ 

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References

INGELVAC CIRCOFLEX® is a registered trademark of Boehringer Ingelheim Vetmedica GmbH, used under license. ©2019 Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All Rights Reserved. POR-1350-CFX0519