Swine veterinarian Steve Henry outlines the growing relevance of the three federally licensed porcine circovirus vaccines.
As one of the leaders of the Kansas State University (KSU) team assembled to help reduce the devastating impact of porcine circovirus type 2 (PCV2) in the state's hog operations, Steve Henry, DVM, Abilene, KS, is starting to admire the way the vaccines are performing.
The multidisciplinary team conducted a series of vaccine field trials from 2005-2007, when the disease first wreaked havoc on some northern Kansas herds. All three products were shown to significantly reduce mortality rates.
But the major improvement in growth rates was a pleasant surprise, he says.
In 2006, an initial study of a 300-sow herd negative for porcine reproductive and respiratory syndrome (PRRS), suffered from greatly elevated finishing mortalities due to PCV2.
Four finisher groups were tested, each housed as commingled vaccinates and control pigs in hoop barns. Pigs were vaccinated with two doses of circovirus vaccine (Intervet) at 3 and 6 weeks of age. Mortality was markedly reduced, from 17% in controls to 6% in vaccinates.
“Most striking and unexpected was the magnitude of the growth improvement in vaccinates vs. controls,” Henry remarked in a talk during the American Association of Swine Veterinarians annual meeting in March in San Diego, CA. “Vaccinates weighed 20 lb. more at market, a 6.8% advantage over controls. Finishing weights were 297 lb. for the vaccinates vs. 278 lb. for the controls.” (See Figure 1.)
But most importantly, weight distribution within treatment groups was much more equalized, including minimizing lightweight pigs. “Even the best pigs, those we thought were growing normally, were improved,” he says.
“In 35 years of practicing swine veterinary medicine, I have never seen any product inserted in pigs that would give you that kind of response and such a phenomenal shift in population distribution,” he declares.
From these results, it became apparent that vaccination impacted virus load and antibody response. “Even in the face of constant viral challenge in the commingled barns, vaccination resulted in a 10-fold reduction in virus load compared to controls,” Henry reports. “This vaccine-induced rise in antibody titer level was sustained throughout the study period.”
Second Vaccine Trial
A second field trial compared Fort Dodge Laboratories' single dose circovirus vaccine given at 4 weeks of age with Intervet's two-dose product at 4 and 6 weeks of age and a control group.
Henry suggests both vaccines equally reduced mortality, with a slight weight gain advantage for the two-dose vaccinated pigs.
As with the previous study, a shift in the distribution of weights occurred at 160 days of age, “clearly demonstrating the positive influence of porcine circovirus immunization on the entire group. Again, this improvement was noted in all pigs in the population, including the fast-growing pigs without any evidence of clinical porcine circovirus-associated disease (PCVAD),” he points out.
The across-the-board improvement in growth response from circovirus vaccination strongly suggests that performance losses due to the infection have been grossly understated, Henry states.
Genetic Differences
In the spring of 2006, a genetic multiplier contracted PCVAD, producing unthrifty finishing pigs. The herd was negative for PRRS and Mycoplasmal pneumonia.
The herd contained a Duroc-based line and a synthetic line comprised of Duroc, Pietrain and Large White genetics. Genetic line production and crosses were evaluated. Pigs were vaccinated with two doses at 3 and 5 weeks of age (Intervet), and commingled with control group pigs.
All of the lines showed significant improvement in growth rates. However, the rate of improvement was dramatically different between the genetic lines, Henry says. At 150 days of age, the Duroc-line vaccinated group showed a 10% increase in growth to 221 lb., compared to 201 lb. for the Duroc-line control pigs. However, vaccinated pigs were only 5 lb. heavier than unvaccinated controls for the synthetic line. The crosses were intermediate, with an 8-lb. difference in weight between vaccinated and unvaccinated pigs.
“This study supports the clinical impression that there are host genetic response differences in pigs immunized against porcine circovirus type 2 (PCV2), and it also lends credibility to the oft-mentioned belief from the field that some genetic lines seem to be more affected during PCVAD outbreaks,” he comments.
Passive Immunity
The KSU PCV2 team conducted a number of trials to assess the role passive maternal protection might play in limiting effective immunization.
In one trial, full or half doses of a two-dose regimen vaccine (Intervet) were given to 7- and 21-day-old pigs or to 21- and 35-day-old pigs.
Results showed the full dose of vaccine at 21 and 35 days of age protected 93% of vaccinates. Half doses of vaccine at 21 and 35 days of age protected 76% of vaccinates.
In younger pigs vaccinated at 7 and 21 days of age, the full and half dose vaccine protection was 70% and 65%, respectively. Passive antibody titers appeared to interfere with a post-vaccination antibody response, and many of these pigs became infected later in the growing period, Henry says.
Another trial compared Intervet, Fort Dodge and Boehringer Ingelheim circovirus vaccines in a farm severely affected with the disease. Vaccines again apparently failed to induce an antibody response in the face of high passive titers. The two-dose product afforded greater antibody production than the one-dose products, he says.
“Maternal passive antibody levels are important in the effort to predictably and effectively immunize as many animals as possible,” Henry states.
Virus Elimination Study
Studies suggest that immunization greatly decreases viral load. A collaborative study at KSU and Michigan State University immunized pig flows on the respective research farms.
Both are closed, batch production, high-health systems with excellent sanitation programs, providing a good framework to examine the potential for virus elimination, according to Henry.
The ability to derive PCV2-negative pigs from positive breeding herds was demonstrated prior to the availability of vaccines through selection based on maternal antibody, followed by isolation.
Future Work
The KSU team, Iowa State University and South Dakota State University have joined forces utilizing funding from the National Pork Board to develop antibody tests to objectively assess immunization efficiency.
“These results will help pave the way for next-generation vaccine development,” asserts Henry.
