David Burch qualified from the Royal Veterinary College in 1972. After two years as houseman and five years in practice, he developed an interest in pig and poultry production and medicine, and joined the pharmaceutical industry. He worked on the development of the antibiotic tiamulin for pigs and poultry during the 1980s, and valnemulin, the first EU-approved medicated feed premix, during the 1990s. He now runs a consultancy company, Octagon Services, and is involved mainly in antimicrobial development and registration. He is currently junior vicepresident of the Pig Veterinary Society and produces its biannual publication The Pig Journal.

Skin lesions i.e. purple plaques may also be seen but these are more frequently associated with another related condition PDNS (Porcine dermatitis nephropathy syndrome) and Pasteurella multocida. Lymph nodes are enlarged, which is often the case with infection but there is depletion of the normal lymphoid cells, which produce antibodies to fight disease and replaced with macrophages in the nodes and Peyers patches in the intestine suggesting, the body is attacking its own immune system, or something inside it.

Currently, the primary cause of the syndrome is thought to be porcine circovirus type 2 (PCV-2), a virus that has been identified in pigs for many years but suddenly is associated with this new disastrous disease. It suggests that the virus has changed or mutated in some way to alter its disease-causing ability or pathogenicity. It appears to be spreading like an infection with East Anglia being the worst hit, accounting for 88% of the cases in early MAFF surveys (see table 1) but now it is reported to be spreading into the North East and causing similar productive and economic difficulties.

Due to the multitude of clinical signs, a number of possible causes have been proposed and a number of microbial agents, or changes in production system blamed for the condition. Increasingly I think this can be explained simply by the pigs that are affected have their immune system destroyed so that they cannot fight any infection. They are on a tight rope. Any stress or infectious challenge, however minor, can push them off, as they have no functioning immune system to fight it. This includes other viruses such as PRRS (Porcine reproductive and respiratory syndrome) virus and PPV Porcine parvovirus) both of which have been associated with the severe forms of the condition and are immunosuppressive in their own right. Not all pigs are badly affected, the immunity derived from maternal antibodies are likely to be critical to fight the challenge of the PCV-2 until the pigs are able to cope with it. Some vets are attempting serum treatment derived from the blood of pigs that have recovered from the infection. In a recent DEFRA survey, large farms buying in high numbers of replacement gilts were at a higher risk presumably because the maternal herd immunity was not allowed to stabilise. The incidence of salmonella was also higher.

Post-mortem examination surveys of affected pigs show a wide range of findings. Pneumonia was seen in 68% of pigs and further examination under the microscope showed 77% of pigs had a type of pneumonia, which filled the lungs with excessive fluid. An ideal breeding ground for any secondary infection especially bacteria. Similarly in 53% of cases the large intestines were filled with diarrhoeic contents and were dilated and inflamed. Pleurisy (where the lung was stuck to the chest wall) was found in18% of cases and pericarditis in 12%, indicative of bacterial infections caused by Actinobacillus pleuropneumoniae and Haemophilus parasuis. A whole range of bacteria has been isolated and this is the reason some preventive medication programs have worked on one farm and not on another. The main secondary infectious agent will vary and the major organ affected either respiratory or gut may vary from farm to farm and pig to pig.

Table 1: Survey results on the incidence of PMWS and PDNS

No single medication will cover all infections unfortunately (see table 2) and a combination approach may be more beneficial to protect against a broad infectious challenge. Certainly medication will not affect virus infections but sows can now be vaccinated against PRRSV and PPV. Mycoplasma and Haemophilus vaccines have also shown promise in some cases but others have shown that it may be a trigger factor pushing the pig off the tight rope. It possibly depends on the piglet's immunity/infectious status at the time of vaccination. It is important to seek veterinary diagnostic advice to see which bacteria are most prevalent on your farm. A combination of tiamulin and chlortetracycline gives the most broad-spectrum cover especially for respiratory infections. Other products may be needed for E.coli and salmonella control.

Table 2: Activity analysis of various in-feed medications and combinations

Key - 0 = no activity; + = 1-25%; ++ = 26-50%; +++ = 51-75%; ++++ = 76-100%; (+) = depends on ref.
*These products are MFS substances - consult your veterinary advisor for further information
**Laboratory submitted strains - may have been exposed to medication before submission

The late Mike Muirhead, on his website www.thepigsite.com, had tried to simplify the strategy to control the severity of the disease using 3 'Golden Rules'

Limit pig-to-pig contact. – the virus is spread directly from pig to pig and indirectly by a needle, surgical instrument, muck or people.

‘Stress’ is a killer – try to reduce it. Stressed animals are more likely to become diseased. Reduce exposure to microorganisms, medications may help. Consider if a procedure could be done in a less stressful manner.

Good hygiene – Don’t spread the disease around; use good cleaning, disinfection and biosecurity procedures.

Until a suitable vaccine is approved, which will control this disease effectively, we in the UK are going to have to live with PMWS and try to reduce its tremendously damaging effects on production by adjusting our management, medication and hygiene procedures, to try to stop pigs falling off the tight rope.

Reference:

Burch, D.G.S. (2001) The role of mycotoxins in PMWS - fact or fiction? Pig Journal 48, 142-147

For addition information, please visit our PMWS Technical Zone

Source: David Burch, Octagon Services - January 2005.
Review prepared for Novartis Animal Health UK, published in "Pig World".