Don’t get complacent with evolving PCV2 virus
Vaccines for porcine circovirus type 2 (PCV2) currently do an adequate job of cross protecting against new genotypes of the virus, but swine veterinarians and producers shouldn’t let their guard down, according to Darin Madson, DVM, PhD, Iowa State University.
PCV2, which is found all over the world and may be present in more than 95% of herds, is the fastest mutating virus that infects swine, Madson said. Its circular genome and its ability to survive under many conditions, including varying temperatures, makes it incredibly resilient.
“There are very few herds that we know of that would be negative for infection,” he told Pig Health Today.
Since 2005 and 2006, when there was a spike in diseases associated with PCV2, the virus has evolved considerably. In the US, PCV2d is now the most common genotype and has been since 2012, followed by 2a, 2b and 2e.
Madson said the PCV2 virus was “much different” than most other swine viruses and mutates “on a continuous basis.” The degree of mutation hinges on the PCV2 genotypes circulating in the herd and the vaccines being used.
The other unique thing about PCV2 is that more than one viral genotype may infect the same pig. “It has the ability to recombinate with itself and make new viruses…Different genotypes can infect the same pig and then, all of a sudden, we may have a new, slightly different virus that’s circulating in that particular pig.”
The good news is that despite PCV2’s many mutations in the last dozen years, traditional PCV2 vaccines containing either the 2a genotype have provided adequate cross protection against the PCV2 genotypes circulating in US herds. The development of a new vaccine containing both the 2a and 2b antigens could provide even broader coverage as the virus evolves.
“If you can broaden antigenic diversity, this may increase protection and decrease infection in those animals,” he said.
Madson noted that research data also suggests that the more closely related the virus is the better or stronger the protection.
For example, a vaccine containing either the 2a will be more effective against 2a while still providing some cross protection against 2b, 2d and 2e. Likewise, a vaccine containing 2b will be better protect against 2b and possibly 2d, which was once known as mutant 2b. The more PCV2 genotypes pigs are exposed to, the higher the level of cross protection.
He expects the PCV2 virus to continue to change. In addition, researchers have identified what appears to be another species of the virus, PCV3. Unlike the PCV2, which is often transmitted by direct contact, the mode of transmission for PCV3 remains unknown.
The veterinarian noted that PCV3 is similar in structure to PCV2 but “about 200 base pairs larger.” it’s slightly larger,” Madson said. “But when you look at the genetic makeup…they are only about 30 percent similar,” adding that he doesn’t think PCV2 vaccines would adequately cross protect against PCV3.
“There’s a lot more diagnostic tools that need to be incorporated or developed so that we can really answer those questions,” Madson said. “Is PCV3 an actual disease or is it just a kind of a virus that’s circulating in the pig population?”