Tale of two viruses: PCV3 impact unknown while PCV2 continues to evolve

Genetic sequencing demonstrates that porcine circovirus type 3 (PCV3) is significantly different from PCV2, but otherwise little is known about the virus or its impact on swine health, Clayton Johnson, DVM, director of health at Carthage Veterinary Service, Carthage, Illinois, told Pig Health Today.

“We can sequence both of the viruses and then compare their sequences to each other. And when we do, we see that PCV3 is a relative of PCV2 but a dramatically different relative…” Johnson said.

It’s clear that PCV3 is widespread among pigs, but unknown is whether it causes disease. “That’s a big question and [it’s] hotly debated,” said Johnson, who last year moderated a panel discussion featuring academicians, scientists, diagnosticians and practitioners experienced with PCV disease.

Mummification linked to PCV3

Mummification issues in sow herds positive for PCV3 in the absence of other pathogens have been reported by swine veterinarians. In some cases, they believe PCV3 has been a driver of those dead fetuses, he said.

Others feel strongly that evidence is lacking as to whether PCV3 causes disease.

“Trying to understand the difference between infection and disease is extremely difficult and it takes more time than a disease, say, that’s an epidemic pathogen,” Johnson said. With porcine reproductive and respiratory syndrome virus or porcine epidemic diarrhea virus, there’s obviously a disease and diagnostic results, and it quickly becomes clear there’s a pathogen resulting in disease.

No cross-protection expected

There appears to be agreement that if PCV3 turns out to be a cause of disease, it’s unlikely PCV2 vaccines would provide any protection due to the genetic differences between the two viruses, he said.

Not enough is known about why a pathogen that has historically lived within a population in harmony turns into a disease-causing pathogen. It’s impossible to predict, he continued. For now, PCV3 needs to be carefully monitored and, when pigs get sick, diagnostic samples obtained.

“We do the normal battery of tests for the horses, if you will, that we expect in the disease investigation. If we don’t see any horses, but we hear the hoof beats, we look for zebras…And we’ve got a tremendous ability to look for zebras these days. Next-generation sequencing gives us a whole host of technology capabilities we haven’t had before,” Johnson said.

PCV2 concerns

He considers vaccination for PCV2 a “success story.” In his career, he’s seen the lack of vaccines for control and the resulting devastation. That’s in contrast to now, when the development of vaccines has completely changed the situation to a point where “we feel generally we have control over it.”

Although vaccination has helped control acute PCV2 disease, the pathogen also needs a lot of monitoring because it continues to evolve. Like many other practitioners, Johnson is finding the most common genotype of PCV2 circulating in herds now is PCV2d, whereas PCV2a and PCV2b were once prevalent.

Besides the expected continued evolution of PCV2, another concern about the virus is its role in subclinical disease, which Johnson said can “absolutely” be a problem that may have an impact on performance that’s not obvious. That raises the question of just how well the vaccines are working.

Determining vaccine efficacy

“As astute as we try to be with making observations, our eyes are not always the best measurement of disease impact. Fortunately, with subclinical disease, most of the disease impacts can be seen, if nothing else, through average daily gain,” he said, but research is needed to determine vaccine efficacy.

“How do we know that protection is optimal and that it could or couldn’t be better? Well, we’ve got the ability to do good research,” comparing the performance of vaccinated and unvaccinated pigs. Growth, reproductive performance and a host of other metrics can be evaluated to determine if vaccinated pigs perform better than non-vaccinates, Johnson said.

He cautioned, however, against assuming that any drops in performance are due to PCV. “I would tell you that each situation is unique. I wouldn’t go straight to PCV. I think that’s circumventing several steps in the diagnostic pathway.”

First, he’d encourage producers to trust their own judgment if they work with the pigs daily, and if not, they should talk to caregivers to try and find any relevant changes.

“What do they see that’s different? What are they treating a little more for in terms of individual pig treatments? What are the other parameters in the barn that are changing? Is water intake changing? Have we changed anything in how we’re managing the pigs?” he said.

“You’ve got to peel back that onion one layer at a time. And I would start at the most common challenges we see, and I’d go from there,” Johnson said.