At last, the porcine circovirus type 2 (PCV2) vaccines are starting to be available in the European Union. Following their introduction in North America, where the disease struck in 2004 and has been sweeping across the US and Canada ever since, they have gathered the most field experience to protect their herds from post-weaning multisystemic wasting syndrome (PMWS) or porcine circovirus disease (PCVD), (Loula, 2007).
Following the results of a variety of field trials to different protocols, some controlled and some 'before and after' studies with sow and piglet vaccines the following mortality results were reported (see Graph 1).
Graph 1. Comparative mortality before and after vaccination in growing pigs in N. America
In Europe we are in a different phase of the disease as generally it has been around for nearly 10 years. Some excellent Danish work (Hjulsager and others, 2007) showed the pathogenesis of PCV2 infections in pigs with PMWS (see Graph 2).
Graph 2. Pathogenesis of the virus infection in pigs developing PMWS
There appears to be a very early transmission of PCV2 and colonization of the nose, presumably from the sow. This highlights the difference between surface and humoral immunity. Antibodies circulating in the piglet's blood, derived from colostrum intake (maternally-derived antibodies - MDAs), inhibit systemic infection until between 6-9 weeks when the virus starts to multiply up in the blood (viraemic phase), spreads to the organs and lymph nodes in the body and shedding occurs in the faeces. Effectively, this is how the sow vaccines work by increasing the production of antibodies in the sow to give enhanced passive immunity in the piglets. It is licensed as giving protection to piglets for up to 5 weeks of age, but it would appear naturally to be a little longer.
Piglets that do not develop PMWS go through a similar pattern of infection except they appear to control the viraemia and presumably the damage to the immune system and organs is less, and the infection does not become overwhelming (see Graph 3).
Graph 3. Comparative viraemia profiles in PMWS affected and unaffected pigs
This is typically what has happened naturally when sow herds have been infected with the virus and the subsequent damage and mortality in the piglets is reduced from the initial, high mortality, acute phase to the chronic phase of the disease. However, is sow vaccination enough to stop the mortality and wasting subsequently, when the viraemia takes off after the MDAs have declined, this is where there is currently an uncertainty?
The sow vaccine (Circovac® - Merial) has now been approved in Europe and is being introduced. In addition, the piglet vaccines are also starting to trickle into the UK, under the Special Treatment Certificate (STC) scheme permitted under the 'cascade guidelines' for farms where the disease signs occur later, after 10 weeks of age, but it is too early to directly compare the efficacy of the two approaches.
Clinical field trials have been carried out in Europe with Ingelvac® Circoflex (Boehringer Ingelheim), a piglet vaccine, and in a trial in the UK, which I visited and reviewed, (von Richthofen and others, 2007) there was a dramatic reduction in mortality associated with PMWS from 14.3% to 4.6%, in pigs that had been vaccinated at 3 weeks of age and the trial lasted for a further 20 weeks, almost to slaughter (see Graph 4). The surviving pigs also put on an extra 6.6 kg liveweight in comparison with the unvaccinated controls (see Photo 1). The farm did not have porcine reproductive and respiratory syndrome virus (PRRSV) or enzootic pneumonia but did have some streptococcal meningitis and pleuropneumonia problems. The trial was a fully controlled study carried out according to good clinical practice (GCPv) and the pigs were kept in the same pens but identified individually, so were exposed to the same environmental conditions and virus challenge as the unvaccinated controls. The viraemia started at 6 weeks of age and peaked at 9 weeks. It can be considered a very severe natural challenge.
Graph 4. Mortality in vaccinated and controls before and after the onset of viraemia
There are advantages to both sow vaccination and piglet vaccination programmes (see Graph 5).
Graph 5. Comparison of protection from MDAs and piglet vaccination
In discussion with colleagues in the UK, some have had good responses to sow vaccination, whereas others have not wanted to invest in a full sow programme, when they would have to wait for four months for the piglets to come through and a further six months before they finish and can assess the results. Where they have got disease occurring later in the grower or in the finisher (>10 weeks of age), which is common here, they seem to prefer to take the option to vaccinate the younger pig at 3 weeks of age so that it has protection throughout the growing and finishing stages.
I think this will be the way forward here.
Hjulsager, C.K., Kristensen, C.S., Bille-Hausen, V., Baekbo, P., Dupont, K., Vigre, H., Enoe, C. and Larsen, L.E. (2007) PCV2 Dynamics in PMWS Positive Herds. Fifth Symposium on Emerging and Re-emerging Pig Diseases Krakow, Poland, p 46.
Loula, T.J. (2007) Circiviral disease and porcine circovirus type 2 (PCV2) vaccines in the USA: the US and Swine Veterinary Centre's experiences with circovirus vaccination. Pig Journal 60, 80-81.
Von Richthofen, I., Woolfenden, N., Lischewski, A. And Strachan, W.D. (2007) Field Efficacy study of a PCV2 vaccine in three week old piglets in the United Kingdom. Fifth Symposium on Emerging and Re-emerging Pig Diseases Krakow, Poland, p 122.
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