Background and history

PDNS generally affects growing and finishing pigs, although it can occasionally afflict adult pigs.

There is an association with PCV2 and PMWS. However, rare cases of PDNS have been recognised for many years. With PMWS, the incidence of the condition becomes greatly increased post-weaning.

The cause is unknown. However, current thoughts suggest it is an immune-complex-mediated disease associated with abnormal stimulation of the immune system. Lesions in affected pigs present in the blood vessel walls throughout and if viewed microscopically suggest hypersensitivity reaction to something in the bloodstream, possibly a bacterial toxin.

Skin lesions associated with PDNS

Veterinary warning

Dr Stan Done from the Central Veterinary Laboratory in the UK has warned Veterinarians to be on their guard if dealing with the acute epidemic form of PDNS.

The symptoms and post-mortem picture are very similar to Classical swine fever and include enlarged lymph glands, haemorrhages at any site, cavity, organ or tissue and the presence of fluid in any body cavity.

Such a picture should be immediately reported to the authorities for differential diagnosis

Spread

It is not known how the disease spreads between pigs or between herds or what triggers off a clinical outbreak. However sources of incoming pigs should only be purchased from herds with no history of disease.

Similar diseases

Some outbreaks, clinically and at post mortem examination, resemble Classical swine fever (CSF), (Hog cholera), or African swine fever (ASF), which in most countries are legally notifiable and, if confirmed, herds are slaughtered out. If many pigs with PDNS are subjected to detailed post mortem examination virtually all the typical gross lesions of swine fever and African swine fever are likely to be found, not in a single pig but scattered through them. This causes major problems for the farmer and the vet since the farm and such places as slaughter houses, to which pigs have been delivered, may be closed pending investigation.

Fortunately laboratory tests for CSF are rapid and accurate but tests for ASF may take several days to be sure of a negative result.

Other diseases which might be confused with PDNS include erysipelas and Actinobacillus suis. Other kidney conditions may also be confused with PDNS.

Clinical signs

• PDNS occurs mainly in growers and finishers, 12 to 14 weeks of age and sporadically in other age groups.
• The most striking sign in live clinically affected pigs is the appearance of extensive purplish red slightly raised blotches of various sizes and shapes over the chest, abdomen, thighs and forelegs.
• Over time the blotches become covered with dark crusts and then fade leaving scars.
• The pigs are depressed and may have a fever.
• They are usually reluctant to move eat, lose weight.
• Sometimes they breath heavily.
• Most pigs with skin lesions die.
• Oedema or fluid may be seen on the limbs and around the eyelids.
• Superficial lymph nodes may be enlarged.
• Diarrhoea in some pigs.
  

Within a herd, clinical signs may occur sporadically in a few pigs and the disease may then go undiagnosed. Alternatively, signs may occur in a bigger proportion of the herd and be economically damaging. Mortality in affected pigs may be around 15% with death usually occurring within a few days of onset. However, mortality can rise much higher and in one study it reached 100% in pigs aged 3 months and older. The majority of pigs that develop extensive skin blotching die. Pigs that recover are often permanently unthrifty. Affected pigs also show anorexia, depression, incoordination and stiffness a reluctance to move and a mild fever. Haemorrhages are seen in the skin, lymph glands, kidneys and spleen, hence the similarity to Swine Fever

Diagnosis

• The clinical signs are strongly suggestive but not diagnostic. Gross and microscopic post mortem examinations are needed to make a firm diagnosis.
• At gross post mortem examination lymph nodes, particularly those at the rear of the abdomen which are not usually examined, are reddened and enlarged haemorrhagic with fluid.
• There is often fluid in the abdomen.
• The most consistent lesions are in the kidneys which are swollen, pale and mottled with many small haemorrhages showing through the surface.
• Tests can be done for high urea and creatinine levels in the blood which indicates severe kidney damage. These tests may be negative if the kidneys are less severely affected. Microscopically, the lesions in the blood vessel walls are distinctive.
• Since the cause is unknown there are no specific diagnostic tests.
• Gastric ulcers and haemorrhage in the small and large intestines.

Cause

The cause is unknown. The lesions are in the blood vessel walls throughout and if viewed microscopically suggest a hypersensivity reaction to something in the blood stream, possibly a bacterial toxin. In field studies in Scotland a specific strain of Pasteurella multocida has been isolated consistently from affected pigs whereas the majority of strains isolated from unaffected pigs are different. However, cause and effect have not yet been proven.

Some herds in which the disease is occurring have been shown to be infected with porcine circovirus (PCV), which sometimes causes confusion, but PCV is widespread in pig populations and its simultaneous presence may be coincidental. However, porcine post weaning multisystemic syndrome (PMWS) sometimes occurs at the same time as PDNS or precedes it in a herd or follows it. The relationship between these two diseases is not known but each occurs in herds without the other being present. The virus porcine reproductive and respiratory syndrome (PRRS) has been detected in some pigs with PDNS and was suggested as a possible cause because it damages blood vessel walls but, again, cause and effect have not been proved and since PRRS virus is widespread in the many pig populations its presence may also be coincidental. Furthermore, not all herds with PDNS are seropositve for PRRS.

Current thoughts on the cause of PDNS suggest it is an immune complex mediated disease associated with abnormal stimulation of the immune system. This implies antibody antigen reactions. It has been postulated that the condition could be initiated by factors such as medicines, vaccines, chemicals and infectious agents. Recent tests and data have shown that Porcine circovirus Type 2 (PCV2) can be identified in a high percentage of cases the same as PMWS. Further studies are being carried out.

Prevention

• Isolate incoming pigs for 6 weeks and check the source before entry.
• Control of the disease is based on good husbandry.
• Early recognition and segregation of sick pigs is essential.
• Dissemination of the disease has been associated with the movement of pigs from affected farms.
• Disease has been associated with the mixing of pigs at weaning from multiple sources.
• Lateral movement of disease in dense pig production areas is believed to be a factor.
• Birds are thought to spread desire.
• Fungal toxins and in particular ochratoxin may affect the immune system.
• In finishing units do not buy pigs from infected sources.
• Ideally do not mix pigs less than 30kg weight.
• Keep mixing of pigs and stress to a minimum.
• Adopt all in all out procedures and disinfect buildings between batches.

Treatment

• Antibacterial medication is usually ineffective unless given preventively for a long time in advance of when the start of the disease is anticipated.
• It is a good idea to get a laboratory to isolate bacteria such as Pasteurella form the herd and to run antibacterial tests to provide a more rational choice of preventative medicine.
• There is no vaccine but if a pasteurella is isolated it would be possible to produce an autogenous vaccine.
• Immediate removal of affected pigs to a well-bedded, warm hospital pen can improve the chances of survival.