This disease has during the past year or so become of significance and considerable concern in many countries particularly Canada, the US and Europe. It is manifest as the name implies by wasting in pigs from 6 weeks of age onwards.
The disease is associated in part with a porcine circovirus (PCV), so called because its DNA is in the form of a ring. It is extremely small and hardy. There are two serotypes, Type 1 causes no known disease. Type 2 can be found in the lesions and can be isolated in pure culture. There are several different strains (biotypes and genotypes). Antibodies to circovirus type 2 have been detected in pig sera collected in Belgium in 1985 but the clinical disease was not described until 1991 in Western Canada. It has since become widespread in North America and Europe.
Young experimentally inoculated colostrum deprived pigs given Type 2 alone sometimes, develop typical lesions. However, they are more likely to develop lesions if another virus, such as porcine parvovirus (PPV) or porcine reproductive and respiratory syndrome virus (PRRSV), is inoculated at the same time. Naturally occurring clinical cases in the field seem always to have a dual infection with PCV Type 2 and some other virus but most pigs which are infected with PCV and PRRS do not develop clinical PMWS.
Serum surveys in Europe and North America have shown that infection has spread widely through the pig population but only a small proportion of seropositve herds have a history of clinical disease. It seems that most infections are sub-clinical. It is not known why some infections result in disease. Piglets may become infected before weaning.
Symptoms
Weaners & Growers
- PMWS tends to be a slow and progressive disease with a high fatality rate in affected pigs.
- Starting usually at about 6 – 8 weeks of age, weaned pigs lose weight and gradually become emaciated. Their hair becomes rough, their skins become pale and sometimes jaundiced.
- Sudden death.
- Enlarged peripheral lymph nodes.
- May show diarrhoea.
- May show respiratory distress caused by interstitial pneumonia.
- Incoordination.
- Post weaning mortality is likely to rise to 6 – 10% but is sometimes much higher.
- Clinical cases may keep occurring in a herd over many months. They usually reach a peak after 6 – 12 months and then gradually decline.
Sows & Piglets
- N/A
Causes / Contributing factors
- Infected faeces.
- Mechanical means via clothing, equipment, trucks etc.
- Possibly birds and rodents.
- Circovirus has also been detected in semen from apparently healthy boars.
- It is not known what other ways the virus spreads between pigs or between herds.
- Mixing and stress.
- Continual production.
- High stocking densities.
Diagnosis
Since most herds have antibodies to PCV, blood testing a herd usually does not help. The clinical signs are not specific and to make a diagnosis it is often necessary to post mortem several pigs. Diagnosis is based upon the presence of PCV type 2 histological lesions in lung, tonsil, spleen, liver and kidney tissues. Immunohistochemistry is used to demonstrate PCV in tissues. Probably many small mild outbreaks go undiagnosed. The gross post mortem lesions are variable. The carcass is emaciated and may be jaundiced. The spleen and many lymph nodes are usually very enlarged and the kidneys may be swollen with white spots visible from the surface. The lungs may be rubbery and mottled. Microscopically these lesions are characteristic and diagnostic particularly if the circovirus is demonstrated in them. If affected pigs are suspended by their back legs the inguinal lymph nodes appear enlarged often the size of large grapes.
Similar diseases
Many conditions, such as starvation, malnutrition, lack of water, gastric ulcers, enzootic pneumonia, coliform enteritis, swine dysentery, PRRS and other diseases, can cause similar signs. These all have to be eliminated if a specific diagnosis of PMWS is to be made. One disease deserves special mention here, porcine dermatitis and nephrosis syndrome (PDNS) because it sometimes occurs at the same time as PMWS or precedes it in a herd or follows it. The relationship between these two diseases is not known but each can occur in herds without the other being present.