Nearly completing their second year of use, commercial circovirus vaccines continue to be intensely tested by the research team at Kansas State University (KSU). Vaccines have excelled in virtually all trials.
Researcher Steve Dritz, DVM, summarizes some of the group's major findings of trials conducted at research farms and in commercial herds:
The vaccines continue to clearly reduce mortality in infected pigs.
The vaccines improve growth rates in approximately 85% of pigs in the population that don't even show clinical signs of porcine circovirus type 2 (PCV2).
“We found that growth rate was a much more sensitive indicator of the impact of PCV2 than was clinical signs,” Dritz says.
Diagnostic testing can spell out what is happening inside a pig due to PCV2. “But it's very hard to tell when growth rates are being affected when the pigs look clinically normal,” he emphasizes.
Steve Henry, DVM, Abilene (KS) Animal Hospital, has begun to study the weight distribution of PCV2-affected pigs and marketing patterns as a means of determining when the population starts shifting backwards. This may serve as an early indicator that there are circovirus problems on the farm, Dritz reports.
The vaccines improve growth rates of the entire population, essentially eliminating the tail-enders.
“When you start impacting the growth rate on all of the pigs in the population, it has a huge impact on the economics,” he stresses. Return-on-investment for circovirus vaccine has never been lower than 2:1 and often reaches 4:1 to 7:1. “In infected herds, we have not had a trial where there wasn't a good response,” he says.
“We think we're getting to the point where we are reducing virus loads in herds, meaning there isn't as much circulating virus, and with the lower amounts of circulating virus, the response to the vaccine may not be as great,” Dritz comments.
But Henry has warned that the circovirus vaccine is one that veterinarians and producers shouldn't be too hasty to pull out of their herd health toolbox.
“We did leave unvaccinated pigs in a commercial operation recently to see if they would become viremic, and sure enough, they did,” reports KSU graduate student Megan Potter, DVM. In that trial, 85 pigs on Farm #1 were left unvaccinated, while 150 pigs on Farm #2 were vaccinated at 5 and 8 weeks of age (Intervet's Circumvent PCV).
Impact of Genetics
The vaccinated pigs on Farm #2 showed no signs of PCV2 in pooled serum sets submitted for polymerase chain reaction (PCR) testing. Farm #1, however, showed signs of infection starting at 6 weeks of age (6%), at 9 weeks (38%) and also at 12 weeks of age (75%). These are percentages of the pools tested.
“This seems to answer a big question about how long the virus persists after a lengthy period of vaccination — and it appears the virus is still there,” adds Potter.
Also in that trial on Farm #1, where pigs were left unvaccinated, 3-week-old weaned pigs tested negative for PCV2. “That is telling us that we are weaning negative pigs, so they are not getting circovirus from the sow, meaning if they are moved to a clean environment, producers should be able to keep them negative,” Dritz explains.
Moreover, Farm #1 that was left unvaccinated is normally a vaccinated herd that occasionally experiences clinical signs. The fact that circovirus remains after regular periods of vaccination shows that vaccination is not the only answer to this problem, he notes.
“The circovirus vaccines are excellent. There is no question about it,” Dritz declares. “But they are not totally the answer. Producers still need to pay close attention to environmental sanitation.”
In another KSU/Abilene (KS) Animal Hospital study, researchers evaluated the impact of genetic backgrounds and PCV2 vaccination on circovirus infection in a high-health herd free of porcine reproductive and respiratory syndrome (PRRS) and Mycoplasmal pneumonia.
Potter says the study was done in response to reports that genetic background affects the severity of porcine circovirus-associated disease (PCVAD).
The 130-day study compared the response to PCV2 vaccine across genetic combinations by using increased growth rate as a response measure.
Studied was a 1,700-sow, boar multiplier herd in Kansas that was diagnosed with PCV2 infection in early 2006. There were four genetic combinations in the operation. The Duroc-based sire combination (A×A) and the Pietrain-based dam combination (B×B) were both synthetic breed combinations. In the A×B combination, the sire was Duroc-based and the mother was Pietrain-based. In the B×A synthetic combination, the sire was Pietrain-based and the dam was Duroc-based, Dritz explains.
Three factors were evaluated in the trial — vaccine status, genetics and gender (boar or gilt). “We commingled the controls and the vaccinated pigs in the same pens to challenge the vaccine as much as possible,” he adds. A total of 454 pigs were placed on test when weaned at 21 days of age. Pigs were vaccinated at weaning, with a second dose administered two weeks later (Intervet's Circumvent PCV).
In the study, pigs were weighed at Day 0 (weaning), and again at Days 40, 70, 105 and 130, with blood samples collected at Days 0, 40 and 130.
“There were some reports that Durocs were more susceptible (to PCVAD) and that Pietrains were more resistant,” Dritz says.
Those suggestions were borne out.
“Though the trial was not focused on determining mortality, mortality among vaccinates in the synthetic combination A×A pigs was 8.3% and the combination A×B mortality was 9.7%. In the synthetic combination B×B pigs, mortality was 5.3%, and the combination B×A pigs had 0% mortality,” Potter explains. The percentages represent the results for the vaccinated pigs (Figure 1 ).
Figure 2  illustrates the off-test market weights of the four treatment groups tested on two genetic combinations. Of the vaccinates, the synthetic Duroc combination (A×A) recorded the lowest final weight of 220 lb., the synthetic Duroc combination (A×B) a weight of 232.9 lb., beaten out by the Pietrain synthetic combination (B×A) at 236.9 lb., but not by the synthetic Pietrain combination (B×B), with a market weight of 225.7 lb.
The study showed a four-fold increase in the impact of circovirus infection vaccination on the synthetic Duroc line (A×A) vs. the three other genetic combinations, according to the researchers.
Even the fastest-growing pigs in the genetics study appeared to benefit from vaccination. And vaccination greatly lowered viremia (viral presence in the blood) at both Day 40 and Day 130, and overall compared to unvaccinated control pigs.
Authors note that based on growth performance, the different genetic backgrounds responded differently to the circovirus vaccine even though they were commingled in the same pen.
“Even though those pigs were commingled when they were vaccinated, we weren't able to detect virulent virus in the majority of the pigs' serum because the vaccine was providing protection from infection,” Dritz points out.
“This study clearly showed that vaccination will increase growth rate even in a high-health herd of both clinically and non-clinically affected pigs,” he concludes.
Dick Hesse is a virologist with the Kansas State Veterinary Diagnostic Laboratory who is integrally involved with the use of herd profiling and developing next-generation diagnostics for circovirus.
He says despite the effectiveness of the circovirus vaccines, many questions remain to be answered, including:
What is the role of maternal immunity in blocking vaccination?
Will vaccine efficacy decay over time as a result of genetic mutation?
Can a serological test be developed to differentiate vaccination from field infection?
Are circovirus-associated co-factors circulating in the herd?
Hesse says the poultry industry has used flock profiling to monitor a host of avian pathogens for decades. Routine flock profiling effectively monitors health and optimizes production.
He says the easiest profile to generate for PCV2 is the herd “snapshot.” Collect serum samples from the sow herd and from pigs at 3, 6, 9, 12, 15 and 18 weeks of age. Because of cost, samples are usually pooled in sets of five to detect for presence of the virus and genotype.
“If the herd has a history of PCVAD, it is important to test for the presence and onset of infectious co-factors such as PRRS, swine influenza virus and mycoplasma,” Hesse says.
Serum samples should be submitted to an approved diagnostic laboratory for analysis.
By using the information learned, producers can make cost-effective health management decisions, Hesse says.