Extracted from The Danish National Committee for Pig Production's Annual report for 2003 (click for report)

Occurrence in Denmark

In the past six months, 20 herds a month have been diagnosed with PMWS. Thus, as of August 4, 2003, PMWS has been diagnosed in 225 herds.

Characterisation

The first 45 herds with the diagnosis PMWS in Denmark were visited by the National Committee for Pig Production in the period summer 2001 to autumn 2002. The herds were characterised and the extent of the outbreaks studied. The herds were primarily located in Jutland: 38 of the herds were sow herds with pigs until min. 30 kg; five herds purchased their pigs at weaning; the last two herds were finisher herds. Both herds with SPF, MS and conventional health status were represented. 70% of the herds were infected with PRRS, which correspond to the presumed distribution of PRRS among all Danish pig herds. Apparently, the mortality in these herds was higher than in herds without PRRS. The mortality post-weaning until 30 kg varied from 2% to 30% with an average of 11%. The study showed that batch production in the weaner unit resulted in a lower average mortality after weaning until 30 kg compared with the herds with continuous production.

Transmission of PMWS

A study made by the National Committee for Pig Production showed that PMWS could be transmitted from pigs with PMWS to pigs without PMWS. Another study showed that the risk of a pig dying after weaning was increased if the sow had high antibody titre against PCV2 before farrowing. The risk of dying after weaning was in this study not related to the number of pigs at birth, the number of pigs at weaning, the weaning age or the parity number of the sow. The study was conducted in three herds, and the result was must prominent in one of the herds.

Time of infection

In four herds with PMWS and four herds without PMWS it was studied at which age the pigs develop antibodies against PCV2. As can be seen from figures 3 and 4, the pigs in the herds with PMWS developed antibodies against PCV2 at the same time as there was an outbreak of disease in the herd. However, PMWS cannot be diagnosed by a titre increase, and blood samples cannot be used to diagnose PMWS.

Control measures

Operational efforts against PMWS were monitored in five herds. Apparently consistent sectioning and batch production resulted in a decrease in the mortality rate in two of the herds (from 23.3% to 5.9% in one herd, and from 15.1% to 4.9% in another).

As we have still not found any connection between PMWS and factors other than PCV2, it is difficult to outline specific control measures against PMWS. However, it does look as if the following procedure can reduce the mortality:

• General improvement of the management conditions – batch production, cleaning and avoidance of mixing age groups
• Limited cross-fostering in the farrowing unit
• Litter-wise weaning of pigs
• No weaning earlier than four weeks of age
• Sectioning with batch production according to the all-in all-out principle
• Emptying and disinfection of the weaner units (partial depopulation)

The control measures will also have a good effect on the management of other infections in the herd, particularly respiratory diseases.

It is important to keep in mind that even though a herd is infected with PMWS, the pigs can also be infected with the other diseases of the herd, and often these infections cause the death of the pigs.

Respiratory disorders can be controlled by vaccination or medication. The programme must be arranged so that the pigs are stressed as little as possible around weaning.

In a study of the extent of vitamin E in the blood among weaners until four weeks post-weaning, no correlation was found between the risk of dying from weaning until 30 kg and the amount of vitamin E in the blood (cf. figure 5).

Studies

The National Committee for Pig Production is still using many resources on clarifying more conditions of PMWS. In co-operation with DVI, the National Committee for Pig Production will thus initiate two very important studies in 2003. A study will be initiated to – if possible – establish the cause of PMWS, which can either be conditional upon special herd conditions, another virus than PCV2, or another, more pathogenic PCV2 type. Furthermore, it is expected to initiate a serum study in 2003.

This trial will establish whether PMWS can be controlled with serum in the herd, either a specific PCV2 antiserum or an auto serum extracted from the finishers of the herd. Besides these two projects, the National Committee is also studying other possible aspects: are some breeds more prone to PMWS than others; is there an effect of various feed additives; and does medication with a reducing effect on inflammations have a preventive or a directly curative effect. Specific measures in different herds are monitored, and the effects of complete depopulation and partial depopulation are studied.

Furthermore, it is studied whether transferring the pigs directly from the farrowing unit to other premises can reduce the losses caused by PMWS. Hopefully, the study will clarify whether an improvement is seen in the herd with a reduced stocking density in the weaner unit and whether the pigs that are transferred to other premises remain healthy.

The future

During the next year, the National Committee for Pig Production will also work intensively on clarifying various aspects of PMWS. In this connection it is important that herds with problems that resemble PMWS establish whether PMWS really is the problem. This is important in order to clarify how widespread PMWS really is, but also in order to see whether there are new problems in the herd that the herd owner needs to deal with. Furthermore, the National Committee for Pig Production has set up an Internetbased discussion group where it is possible for owners of a PMWS herd to talk with other herd owners.