Despite the appreciable gains that circovirus vaccines have made in controlling porcine circovirus-associated disease (PCVAD), 10 major questions need answers to improve the understanding of this organism and the disease it spawns, says Robert Desrosiers, DVM, Boehringer Ingelheim Vetmedica (BIVI), Inc. of Canada.
Desrosiers spoke at the 2008 symposium on circovirus in San Diego, CA, sponsored by BIVI. Following are his comments:
Where are we with agents XYZ? For various reasons, some researchers cling to the idea that an organism referred to as Agent X may be involved with PCVAD. Three disease agents, porcine parvovirus, porcine reproductive and respiratory syndrome and Mycoplasmal pneumonia, in conjunction with porcine circovirus type 2 (PCV2), have been shown to exacerbate PCVAD problems beyond an infection of PCV2 alone.
There is no reason to believe, however, that those known swine pathogens are the extent of PCVAD co-infections. In fact, recent evidence suggests that the Torque Teno virus may possibly play a role in PCVAD. Activation of the immune system by certain vaccines or adjuvants has also been shown in certain situations to trigger PCVAD, suggesting agents XYZ could play a role.
While the possibility remains of new agents being associated with PCVAD, extensive research shows that quite simply, when PCV2 is controlled, the disease and related losses are also eliminated.
Can virulence vary between PCV2 isolates? The evidence appears quite contradictory around the world, and regardless, it does not seem to fully explain the explosion of PCVAD problems in the last decade.
Is it possible to have PCV2-associated losses without having clinical signs? This is an important question since nearly all swine herds are infected with PCV2, yet not all herds appear to suffer losses from this pathogen. Field trials suggest there are situations where the PCV2 virus may have contributed to performance losses, but without being recognized as such. In one case, pigs vaccinated with a one-dose vaccine, Ingelvac CircoFLEX, were about 6 lb. heavier at the end of the study and had 5% less mortality than control pigs.
Can PCV2 be transmitted by boar semen? Studies of hundreds of cases in foreign countries discount the role of semen in the spread of PCVAD. Infectious PCV2 can be found in semen; in commercial boar studs, it is common to detect PCV2 in semen samples by polymerase chain reaction (PCR). However, in one study where naïve gilts were inseminated with semen containing infectious PCV2, they didn't get infected.
Still, more work is probably needed before semen can be removed from the list of potentially relevant factors in the epidemiology of PCV2.
Is PCV2 the cause of porcine dermatitis and nephropathy syndrome (PDNS)? While there has been a multitude of experimental infections with this organism around the world, none has provided conclusive evidence that it could cause PDNS.
However, there appears to be some links based on reports on PCV2 and PDNS in a number of countries. And in many farms where PDNS was present, it virtually disappeared following the use of PCV2 vaccines in piglets.
How important is it for PCV2 vaccines to produce antibodies? Unlike some vaccines, it appears that there is no correlation between antibody production and protection for PCV2 vaccines. In a Quebec study using Ingelvac CircoFLEX, pigs vaccinated between 19 and 59 days of age had almost four times lower mortality than control group pigs. Yet in serological tests, none of the pigs vaccinated at 26 days of age had seroconverted or shown antibody titers to PCV2 by Day 30. Similar results have been found in U.S. studies with circovirus vaccines.
The end result is that some PCV2 vaccines can induce excellent protection in field situations, even when they do not produce a strong antibody response as measured by serological tests following vaccination.
Can very young pigs be effectively vaccinated in the presence of maternal immunity? Because baby pigs are handled at processing time, many producers and veterinarians express an interest in vaccinating against PCV2 at an early age. Two key questions arise: Can vaccination overcome maternal immunity, and will immunity last until pigs are marketed?
While it appears to be both convenient and successful in some situations, it may not always produce satisfactory results. Accordingly, caution is needed before making changes in vaccination timing, particularly when the current program has already been successful.
How do current vaccines compare in terms of efficacy and safety? Three PCV2 vaccines have been available in Canada: a vaccine licensed for use in gilts and sows (Merial), a two-dose vaccine for use in pigs (Intervet) and a one-dose vaccine for pigs (Ingelvac CircoFLEX).
Overall results showed pigs born from vaccinated sows had mortality results similar to pigs born from non-vaccinated sows. Also, the two-dose and one-dose pig vaccines produced superior results. In safety trials conducted at Iowa State University, Ingelvac CircoFLEX produced significantly fewer lesions 14 days after vaccination than the two other vaccines.
Should we vaccinate gilts and sows? The gilt/sow vaccine (Merial) was the first PCV2 vaccine available in Canada, and many herds used it. The consensus is that in many cases, it did not produce the results that were expected. When problems were occurring in the nursery, these problems were often delayed and transferred to the finishing unit. Inconsistent results led to most producers not relying solely on gilt/sow vaccination to protect their herds. PCV2 appears to rarely affect reproductive performance.
Will vaccination for PCV2 continue forever? Stopping vaccination now would seem risky. Those interested in trying should start small and vaccinate only one half of one barn. That way, if losses occur in non-vaccinates, they will be limited to a relatively small number of animals.
European countries have seen a decline in PCVAD, despite the fact that vaccine wasn't available until years after the outbreaks began. In North America, it is believed that herds suffered losses due to PCV2 before the epizootic of PCVAD.
In some cases, PCV2 vaccines have raised the performance to levels that are better than they were before the PCVAD outbreaks. And in other herds, with no apparent clinical signs of PCVAD, vaccination still seems to produce a benefit. So, for now, vaccination remains a vital cog in herd health programs on many farms.