Prevention and control of porcine circovirus

The series of articles on porcine circovirus (PC) reaches its conclusion with the subject that has probably seen the most new breakthroughs in the last 4 years concerning this disease and its causal agent, porcine circovirus type 2 (PCV2). Logically, this statement is referring to the appearance of commercial vaccines in some European countries and in North America. If in 2005, more than seven years after the virus was associated with the disease, there still existed doubts and chiaroscuros in relation to the etiology of PC, apparently these doubts are now dissipating. However, this tendency is not due to the scientific investigations with the virus and the disease during the last 12 years or to the existence of more than 700 articles published in international journals with reviews from experts which can be found in the database of MedLine; it is due almost exclusively to the spectacular result that vaccines against PCV2 have had in scenarios of epizootic disease, especially in United States and Canada but also in Europe.

I have to admit that as a scientist I have mixed feelings here: on the one hand I feel frustration. So many hours of discussion in vain! So much incomprehension! So little appreciation for the scientific research carried out on a worldwide level over the last 12 years! But, on the other hand I feel a growing optimism, in the sense that sooner or later things are falling into place, and that time also gives a much wiser perspective of any concrete scientific investigation. However, leaving personal feelings to one side, it is important to emphasize the journey taken to get us to where we are today. At the end of the day, the story of PCV2 and PC is no more than a success story in relation to how to take on an emerging disease: after its initial description and its potential association to this new virus within a multifactorial framework, the development of control strategies was strictly necessary.

We owe the initial steps to control and prevention of PC to Dr. François Madec and his famous 20 points (Table 1). They were basically no more than a summary of how to manage a pig farm in the correct way; what the North Americans like to call “back to the basics”. Madec’s 20 points will effectively go down in the history of PC as the most consistent system which, in the absence of other possibilities, and even with no knowledge of the etiology of the process, allowed us to deal with controlling the disease. As time passed, the first studies of cases and controls indicated that there were risk factors for PC, whose control allowed an improvement in the clinical situation. The existence of concomitant diseases (especially porcine reproductive and respiratory syndrome virus, PRRSV), the use of certain vaccine adjuvants coinciding with concrete moments of the PCV2 infection, and the adoption of certain genetic lines or families of the male pig are some of these triggering factors. Some desperate proceedings were also tried, such as “serum-therapy”, although that now seems to us a long time ago. Logically, all these proceedings had their place in the ambit of a disease which had many unknown associated factors (in fact some of which are still unknown!). But all that changed after the end of 2004.

Table 1. The “20 Madec points” correspond to a series of management practices with the aim of decreasing the pressure of infection against practically any pathogen. They were (and still are) of great use in relation to the control of porcine circovirus, especially faced with the unavailability of vaccine products.

So, what happened in 2004? The answer is: various relevant phenomena. On the one hand, the launch onto the market of the first vaccine against PCV2, inactivated, applicable to sows, in France and Germany (with temporal use licenses). On the other hand, the start of the serious PC epizootic problems in North America which led to North American scientists beginning to work intensely on a disease which until then they had more or less ignored. This meant that from 2006 the North American market had practically 4 vaccines available against the virus: that which was destined for sows which we have already mentioned (exclusively in Canada) and three for use in piglets (Table 2), which demonstrated an extremely positive effect in relation to the prevention of mortality and of the number of pigs with wasting syndrome observed with PC. It would be difficult to find a pig vaccine product with a higher success level than the PCV2 vaccines. Moreover, the real benefits of these vaccines are not just based on the mortality associated to the disease, but also on other important parameters such as weight gain, litter uniformity, slaughterhouse uniformity, etc.

Table 2. Vaccines against PCV2 currently available on the world market.

Therefore, the availability of PCV2 vaccines has meant a before and after in farms infected with PC that have used them. Could we have achieved it before? Surely we could, but the existence of “blocking” patents and of extremely strict European authorities have slowed it down a great deal. There are still many countries outside Europe with serious problems with PC and where these vaccines are not yet registered. It is certain, however, that other PCV2 vaccines exist that have been developed locally in some countries, basically in South America.

Finally, we need to conclude simply by remembering that maybe we have done very little or indeed that we haven’t done anything yet. PC continues to be as multifactorial now as when we had no vaccine products available; the vaccines have, without a doubt, reinforced and confirmed the fundamental and vital role of PCV2 in the disease, but they have not resolved a single case of etio-pathogenesis of the condition. The success of vaccination does not mean that the famous “agent X” exists or not, we do not know precisely what are the concrete triggering factors, not so much at the farm level, but more at an individual level, and today so many unknown basic questions or questions with very few available data persist. What can we expect from the PCV2 vaccines from now on? Vaccination protocols will need to be refined: Who do we vaccinate? Only the sows or only the piglets, or both? What is the role of maternal immunity? Does it or does it not interfere with vaccination? What is the ideal moment for vaccination? (on this point serology, which currently receives minimal diagnostic interest, may play an important role). And so many more questions for which we have certain generic ideas but nothing concrete. And even more important…are there vaccinal failures with these products? Well, we shall see, but the first point in order to avoid these potential situations is, of course, the diagnosis. We may have some highly efficient vaccines, but we shouldn’t forget that for their application we must obtain good diagnostic information, and that in all cases we should not lose sight of good management and a good control of the concomitant diseases.