Post-weaning multisystemic wasting syndrome (PMWS) has been in Asia for nearly a decade and like in other parts of the world has had a ravaging effect on pig production as it hit each country. Mortality levels of 30% were not uncommon and left many units with chronic respiratory disease problems, especially in the finishing units. With the outstanding, reported efficacy of the new PMWS vaccines recently described in North America, the prospects are looking good for Asian pork production.
The disease PMWS is now known to be as a result of Porcine Circovirus type 2 (PCV2) infections and is increasingly being called PCVD or Porcine Circovirus Disease. The virus damages the immune system of the pig, which protects it from a number of other infections and thereby allows them, such as PRRS virus (Porcine Reproductive and Respiratory Syndrome virus), enzootic (mycoplasmal) pneumonia and especially Actinobacillus pleuropneumoniae in Asia, to have an increased lethal effect. It is thought that some of these infections may encourage the multiplication of circovirus and hence exacerbate the disease.
I saw the first cases of PMWS, as early as 1998, in Malaysia when I was regularly travelling to the region. We were shown small, emaciated, jaundiced growers following an outbreak of a skin haemorrhage problem thought to be associated with a swine fever outbreak, but in hindsight, it was probably PDNS (Porcine Dermatitis Nephropathy Syndrome), which is also associated with Circovirus and is possibly a forerunner of PMWS.
Following on from this time, in several large units across the Far East, I could often see the effects of outbreaks of PMWS as well as the more usual infections we had grown to live with and control either by vaccination and sometimes the extensive use of antimicrobials. Some farms tried to change their systems to reduce stress (reducing mixing pigs), as well as following more all-in/all-out procedures and 3 site production, instead of multi-age, single-site production, which was common at that time. By reducing the stress and the infectious challenge, it helped to reduce overall mortality (see Graph 1) especially in the nursery (3-10 weeks of age) and to a large extent in the finishing sheds, although it would still be considered higher than expected targets.
Graph 1. Mortality rates before and after stress reduction, all-in/all-out procedures and 3-site production
Management has played a major role of controlling the disease in Europe also, especially in those countries, which have followed the 'Madec' recommendations. In the last two years PMWS has taken off in N. America and they face the same problems we have seen several years ago. Fortunately, they have been able to introduce a number of new PCV2 vaccines for piglets, which have proven surprisingly protective and effective in the control of PMWS (see Graph 2) as well as a sow vaccine in Canada.
Graph 2. Mortality result differences from a variety of trials in N. America with PCV2 vaccines
(Presented at the AASV conference March 2007)
In many countries, their national herds have developed immunity to the virus and we have seen a changing pattern to the disease and tend to see it later on in the finishing pigs rather than in the younger nursery stage. This is primarily due to the production of maternal antibodies in the breeding herds following virus exposure. This can offer variable protection to the piglet from 0-10 weeks of age, as it is dependent on having high antibodies circulating in the sow and getting a good intake of colostrum into the piglet. This can be boosted by sow vaccination, but the duration of protection is still thought to be limited. The piglet vaccines however have been shown to give protection throughout the growing and finishing period, and trials suggest that the younger the pig is vaccinated (currently 3 weeks of age) the better the overall protection, as they reduce the virus multiplication and the destruction of the piglet's immune system (see Graph 3) in the early stages of life. It also seems to reduce the problems associated with the secondary infections such as PRRS.
Graph 3. Piglet passive immunity and piglet active immunity after vaccination
PCV2 vaccination really has been an exciting breakthrough in controlling PMWS. It is hoped that when the vaccines start to roll out around the world the extraordinary benefits seen in N. America will be repeated in other continents, not just for the circovirus-associated diseases but also in the general health of herds. The perspectives of at last controlling this disease are looking good, not just for N. America but for Asia as well.
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