Herd Health: What does the future hold for PCV2?

Friday, May 30, 2008

Will a new mutated virus come along and, if so, will present vaccines work against it? Can we stop using the vaccine or do we have to go on forever? These are just some of the questions now surfacing in the wake of the recent epidemic

by ERNEST SANFORD

Now that circovirus vaccines have been such a resounding success, where do we go with circovirus (PCV2) from now on? What else does it have in store for us?

For instance, there is talk that another new mutated PCV2 virus may come along and that the current vaccines might no longer provide protection against this "new" strain. This, of course, is purely speculative.

Part of the speculation is being driven by features of the PCV2 virus currently circulating in North America which has caused the big PCVAD epidemic that we have just gone through and, in some areas, are still experiencing. This PCV2 virus is different from the one present in our pigs for decades and which caused only sporadic problems and no epidemic of the kind we have been seeing over the last three to four years. Given that background, it is possible that yet another new PCV2 virus could emerge which is totally different from the current ones we have been dealing with.

It could happen, certainly, but to counter this possibility we have the knowledge that our present vaccines have provided exceptional efficacy. These vaccines have done the job.  If a "new" variant does come along – and that's a big "if" – who's to say that the same vaccines would not do exactly what they have done with this current "new" variant causing our present problems?

Can we stop vaccinating?
Now that adequate supplies of circovirus vaccines are available and the acute phase of the Circo epidemic has been stopped cold by vaccination, the next step revolves around the decision of how long will circovirus vaccination be necessary. Will vaccination of pigs have to continue forever or can it be stopped some time in the future?

One possible way to answer that question is to examine what has happened in countries where the Circo epidemic had been raging for years before it descended on us here. Most of the European Union (EU) countries fit exactly into that category.

This is what we can glean from their experiences. The epidemic does not last forever. It seems to peak in about four or five years, then gradually wanes with more and more herds returning to more acceptable levels of mortality and production. From EU data, however, we can see that although mortality figures have improved as the epidemic phase has shifted into a more chronic, endemic form, the mortality figures have not returned to pre-epidemic levels. They remain consistently two to three per cent higher than they were before the epidemic.

Furthermore, in the EU countries, the epidemic tended to hit the pigs more in the late nursery/early grower phase of production. Now that it has moved into this endemic phase, the disease seems to have also shifted over to a later stage of production. It actually mirrors more closely the early finisher stage that we have experienced here during our epidemic. Thus, a bigger more expensive pig is affected.

There is another way to see how long pigs will have to be vaccinated for Circo. That's simply just to stop vaccinating. A bit of a scary thought, I know, but it could provide an answer. That is exactly what has been done in a few cases that have been reported back to me.

The experience has not been favourable so far. In the cases reported back to me, Circo very quickly reared its ugly head almost immediately as the unvaccinated pigs entered the stage of production that had been the focus of previous PCV2 outbreaks.

Now, there are probably cases where stopping PCV2 vaccination "cold turkey" might have had a more pleasant outcome but have not been reported to me. The old saying that "misery likes company" tends to result in the "bad news" being quickly reported while the "good news" is much slower to get out.

A more moderate approach is also possible, whereby vaccination could be stopped in a portion of the population rather than stopping all vaccination completely. So, instead of no vaccination of any pigs from a certain starting date, maybe half a barn of pigs could be left unvaccinated. Or maybe just two or three weeks' worth of weaners, or some such combination, can be used.

There is one caution. Having vaccinated and unvaccinated pigs in the same airspace might result in a reduced challenge to the unvaccinated pigs simply because of the presence of the vaccinates in the same airspace. This could delay or drag out the time when an outbreak might restart, until such time as when fewer vaccinated pigs are in the same airspace. It might appear that pigs are no longer breaking with Circo at first, but all that's happened is that the break is delayed and does not show up until later.

Vaccinating the breeding herd.
This is another area that has become a focus for queries about PCV2 vaccination – vaccinating the breeding herd (sows and gilts) for improvements in reproductive performance and protection of the breeding herd.

PCV2-associated reproductive disease is well documented, though still thought to be a relatively infrequent phenomenon. Documented cases have been primarily and almost exclusively in start-up units with gilt populations. Abortions, stillbirths, mummies and weakborn neonatal pigs are the typical presentation of PCV2-associated reproductive outbreaks. The stillborn and weakborn pigs have a classical, viral-type myocarditis.

These PCV2-associated reproductive failure cases are assumed to be occurring in PCV2-naïve gilts which manage to get through their nursery, finisher and gilt grow-out periods without being exposed to PCV2. They then enter a breeding herd, most often a gilt start-up unit, whereupon they eventually do get exposed to PCV2 (virtually all pigs worldwide eventually do). If this exposure occurs sometime in middle or late gestation, they are subject to the reproductive failure events I just described above. 

There are also scattered reports in the literature of similar occurrences linked to PCV2 in mature sows in mature breeding herds. From this background comes the query about protection against reproductive failures or possible enhancement of reproductive by vaccinating the breeding herd.

We have no controlled trials to guide us with an answer to this question. Field results from the EU countries (France, Germany and Denmark) after using the sow vaccine provide a glimpse of improvements with increases in the number of liveborn pigs per litter, decreased stillbirths, decreased postweaning mortality, reduced weaning-to-service interval and increased farrowing rate.

Anecdotal reports from the field in Canada and the United States from those who have used the two pig vaccines suggest similar findings with improved, more robust farrowing room pigs and improved viability, which carries on into the nursery and beyond.

How young can I vaccinate pigs?
This is a question that has come up more and more frequently in recent months. The labels on each of the vaccines are very clear about the ages when pigs can be vaccinated.  These ages are different for each PCV2 vaccine.

There are several attempts being made in the field to vaccinate at earlier than label ages, but the results are still inconclusive. We know from published results in trials done here in Canada with one of the two pig vaccines that pigs vaccinated at 19 days of age had some of the best results of all pigs in the trial. Furthermore, the EU label for that vaccine states that pigs can be vaccinated from 14 days of age or older.

In the field, vaccinations are being done at earlier than these ages, most notably at processing time, anywhere from three to five days of age. So far we have only anecdotal information on the outcomes of these very early age vaccinations. We have to wait for more detailed results before venturing too deeply into this area of uncertainty.

Can less than full dose of vaccine be used?
Finally, there is the question of partial dosing, usually half dose, with the pig vaccines. Partial dosing was adopted to "stretch" vaccine usage when the vaccines were in limited supply. Further-more, with the current state of our swine industry, every effort is being made to reduce input costs.

Initial reports from the field indicated that results on mortality and morbidity were similar in most cases, whether the full label or a half dose was being used. In recent months, however, reports of clinical Circo breaks have been mounting both in Canada and the United States in herds where partial dosing regimens were being used.

It is also not yet clear if the production performances with partial dosing are equal to those where a full dose is used. We do know now that even in herds which have never had any clinical signs of Circo, there are significant production and economic benefits to be gained by vaccinating pigs with PCV2 vaccine. This is another situation that will be decided over the course of time. BP

S. Ernest Sanford, DVM, Dip. Path., Diplomate ACVP, is a swine specialist with Boehringer Ingelheim Vetmedica (Canada) in Burlington. Email: esanford@bur.boehringer-ingelheim.com