Post-weaning multisystemic wasting syndrome (PMWS), now being called in the US PCVAD (porcine circovirus associated disease), has been in Europe for nearly 10 years. We have struggled with the epidemiology, immunology, the causal agent and the lack of fully licensed vaccines to control the disease. Now, things are starting to change with the introduction of porcine circovirus type 2 (PCV-2) vaccines in N. America and we can look at PCV-2 in a completely new light.
The complexity of the immuno-destructive effect of PCV-2 and its consequences in conventional herds, which have high background levels of other respiratory and enteric pathogens made us uncertain of the actual cause of the disease. After the acute mortality problem of 20-40% in growing pigs, if there was a respiratory problem before, such as PRRS virus, enzootic pneumonia (Mycoplasma hyopneumoniae) and especially Actinobacillus pleuropneumoniae, the mortality could still remain high even though it had passed to the more chronic phase of infection in older pigs. In large units in Asia, many still had mortality rates of 15-25% in the finisher sheds, but comparatively low levels in the nurseries around 1-2%.
In Europe, we have adopted the Madec 20 principles of good hygiene and management and have largely brought our mortality down to manageable commercial levels, even so, some large producers in the UK were struggling with 9% mortality in the finisher sheds and our average mortality was 6.5% in finishers in 2005 (Fowler, 2006). This may be a UK phenomenon as many countries are reporting lower levels of mortality or even PCV-2 is now considered 'a harmless infection'. I think this is probably wishful thinking and it is definitely there in the background. The severity may vary by system and breeds used etc, but I suspect, given the right, challenging environment, it could resurface.
So what has really changed? In some countries in Europe we have had a killed vaccine for sows (Circovac® - Merial) available, under a special or provisional licence and it is starting to be imported into the UK, under the cascade system with a Special Import Certificate. The lack of fully registered vaccines in the EU has been a cause of frustration amongst both veterinarians and producers.
Extensive field trial results in France on 15 farms, involving 4800 sows (Auvigne and others, 2006) described reductions of mortality before and after vaccination in growing pigs and finishing pigs (see Graph 1).
Graph 1. Comparative mortality in growing and finishing pigs, before and after sow vaccination
There was a fall in mortality in the growing pigs from 4.4% to 2.6% (41%) and in the finishing pigs from 6.6% to 5.1% (23%), which was just short of being statistically significant. If we use 2% as a target mortality in weaner and grower pigs and also in finishers (Muirhead, 1977), before PRRS & PMWS came along, there is only a limited response in the finishing pigs but this could also be a result of concurrent infections such as PRRSv and enzootic pneumonia.
(Source: Auvigne and others, 2006)
In contrast, recent work presented in the US (Desrosiers and others, 2007) using a piglet vaccine (Ingelvac® CircoFLEX™ - Boehringer Ingelheim) in a trial where half of 3850 19-59 day old weaners and growers were vaccinated, showed a drop in finisher mortality from 9.5% to 2.4% (75%) (see Graph 2).
Graph 2. Comparison of sow vaccination and piglet vaccination on finisher mortality
The reduction in mortality in the piglet vaccinated group brought the level of disease down to almost pre-PMWS target levels for finishing pigs, which was exemplary and statistically, highly significant. The herd was both PRRS and enzootic pneumonia free.
(Source: Auvigne and others, 2006; Desrosiers and others, 2007)
These findings have revolutionized my thinking about the single and simple importance of PCV-2 in the complexity of the disease syndromes that we see on farm today. Yes, the disease comes in different forms, depending on what else is there, but I think we have made it too complicated, because we were not convinced that PCV-2 was the actual cause and other factors, such as 'factor X', might be involved.
If we look at simple depletion of naturally acquired maternal antibodies in the growing pig (Thomas and others, 2005), we can see these protective antibodies going below the cut-off level for quantification and protection (S/P ratio 0.2) at 9-10 weeks of age, possibly, as one would expect from other diseases (see Graph 3).
Graph 3. Depletion of maternally derived PCV-2 antibodies in a growing pig
This therefore allows pigs beyond that age i.e. finishers to be susceptible to PCV-2 challenge and I think explains what we are seeing in the field, in Europe, where most of the sow herds have developed their own natural immunity.
(Source: from Thomas and others, 2005)
There is probably an additional twist, which explains the difference in the level of disease between herds and countries. There has to be a certain level of viral challenge or infection in the pig for severe clinical disease to be expressed (Olvera and others, 2004), and it is probable that in clean, good hygiene herds the level of challenge is relatively low, so the disease level is low. In the UK we have a lot of straw-based solid-floor systems, which tend to increase that challenge (Scott and others, 2006) and, I suspect, precipitate disease when the maternally derived immunity has waned.
Hopefully, some of the myths and confusion over PCV-2 infections can now be resolved and with the introduction of the new vaccines we can overcome this terrible condition and restore pig production to normal, at long last.
Auvigne, V., Herin, J.B., Fily, B. and Joisel, F., (2006) Evaluation of the first field results of vaccination against PCV2 with Circovac®. Proceedings of the 19th International Pig Veterinary Society Congress, Copenhagen, Denmark, p 105
Desrosiers, R., Clark, E., Tremblay, D. and Tremblay, R. (2007) Proceedings 38th American Association of Swine Veterinarians Conference, Orlando, USA, pp 143-145
Fowler, A. (2006) 2005 Pig Cost of Production in Selected EU countries (www.BPEX.org )
Muirhead, M. R. (1977) Veterinary problems of intensive pig husbandry. Veterinary Record, 99, 288-292
Olvera, A., Sibila, M., Calsamiglia, M., Segales, J. Domingo, M. (2004) Comparison of porcine circovirus type 2 load in serum quantified by a real time PCR in postweaning multisystemic wasting syndrome and porcine dermatitis an nephropathy syndrome naturally affected pigs. Journal of Virological Methods, 117, 75-80
Scott, K., Chennells, D.J., Campbell, F.M., Hunt, B., Armstrong, D., Taylor, L., Gill, B.P. and Edwards, S.A. (2006) The welfare of finishing pigs in two contrasting housing systems: fully slatted versus straw-bedded accommodation. Livestock Science, 103, 104-115
Thomas, P., Opriessnig, T., McKeown, N, Meng, X-J, Halbur, P.G., (2005) Effect of PCV2 passive antibody levels on immunization with chimeric PCV1-2 vaccine and challenge with wild-type PCV2. Proceedings of the American Association of Swine Veterinarians Meeting, Toronto, Ontario, Canada, pp 23-25
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