Research update

Terminology:  use of the term "epidemic" rather than "epizootic"
Until recently we used the term "epizootic" for animal epidemics, but recently international veterinary epidemiologists rejected these terms (which would make them "epizootiologists" rather than "epidemiologists" - and even more awkward words than that if they deal with bird or fish epidemics!) so back to the old words, which have the advantage that lots of other people know what we are talking about!
see international epidemiology terms

Warning: PDNS confusion with swine fever

Both diseases can produce symptoms of:

• pyrexia (fever)
• >10% mortality in growing or finishing pigs
• deaths in adult pigs
• lameness or neurological signs

Both diseases can produce lesions of:

• enlarged haemorrhagic lymph nodes
• haemorrhages in tissues, organs or body cavities
• fluid in body cavities (pericardial, peritoneal, pleural)



haemorrhagic swollen lymph node - medial retropharyngeal
(courtesy of Dr. Stan Done, Veterinary Laboratories Agency UK)

Classical Swine Fever     African Swine fever

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Post-weaning Multisystemic Wasting Syndrome (PMWS)

this emerging health problem of pigs has been recognised in the USA, Spain, France, Netherlands, Germany, Italy, Ireland and now the UK

Reasons for concern

PMWS outbreaks persist for some months and there is no particularly effective cure for the disease.

Clinical signs

Starts at about 6 - 16 weeks of age, especially between 8 - 12 weeks. Pigs show chronic wasting, pale skin color, and enlarged lymph nodes. They usually develop jaundice and a decreased growth rate. In some less specific cases, there are some respiratory and digestive (diarrhoea, gastric ulcers) signs. The proportion of weaners affected varies but the mortality in those that are affected is high.

Post-mortem findings

The gross lesions are characteristic. The carcase is emaciated and jaundiced. The spleen and lymph nodes throughout the body are very enlarged, the kidneys sometimes are swollen with white spots visible and the lungs are rubbery and mottled. Microscopically the lesions are also characteristic.

History

PMWS has been recognised in the USA, Spain, France, Netherlands, Germany, Italy, Ireland. It was first diagnosed in the United Kingdon in 1999.

Cause

Porcine circovirus type 2 (PCV2) has been demonstrated in the lesions of PMWS, sometimes in association with other viruses such as PRRS and parvovirus.
However, PCV2 is also found in herds which do not have problems of PMWS.

Diagnosis

The diagnosis of PMWS is based on the age affected, typical wasting, possibly with diarrhea, and the autopsy lesions. Differential diagnostics should include all other possible causes of wasting in pigs.

Control

There are no vaccines available. Biosecurity precautions (isolating incoming pigs and having farm perimeter security) seems to be important in preventing herd outbreaks of PMWS.

Porcine Dermatitis and Nephropathy Syndrome (PDNS)

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Porcine dermatitis and nephropathy syndrome (PDNS) is a relatively new and economically damaging disease occurring mainly in growers and finishers and occasionally in adult swine.

Reasons for concern

PDNS is extrmely distressing to affected pigs and can occur in expensive outbreaks. There is no clear knowledge as yet of how to prevent or treat, although antibiotics seem to help some cases.

PDNS is a serious concern because the clinical signs and lesions closely resemble those of Classical Swine Fever and African Swine fever If reported to the government veterinary service the herd or slaughterhouse will be closed until laboratory tests have eliminated CSF. This may take a week or more.

Clinical signs

The most striking clinical sign in severely affected grower or finisher pigs is an extensive dermatitis mainly over the chest, abdomen, thighs and forelegs appearing as purplish red bumps of varying sizes and shapes. The pigs are depressed and may have a fever and be reluctant to move or eat. They may breathe heavily. Mortality is often around 15% but can be much higher. Pigs that recover may be permanently unthrifty.

Post-mortem findings

At autopsy examination, lymph nodes, particularly at the rear of the abdomen, may be red and enlarged and there may be fluid in the abdomen. The most consistent lesions are in the kidneys, which appear mottled with many small haemorrhages. Multiple skin spots (papules) may be seen, particularly on the hindlimbs.

Histopathology

Photos of the histological changes have been published by Rosell et al (see Literature). A hyper-immune reaction in the blood vessel walls is the primary lesion of porcine dermatitis and nephropathy syndrome. This may progress to systemic necrotising vasculitis. When renal lesions are present, they take the form of severe diffuse fibrinous glomerulitis in acute cases, or interstitial fibrosis and glomerular sclerosis in chronic cases. Skin papules have been linked with dermal necrotising vasculitis.

Lymphocyte depletion is common in lymphoid tissues. Multinucleated syncitia may be present in trabecular or subcapsular sinuses, or in the follicular centres of lymph nodes. Occasionally, granulomas with epithelioid and syncitial cells have been observed.

History

PDNS was first reported in the UK in Scotland in 1993 but it has since been diagnosed in more than 120 herds in England and become particularly widespread in East Anglia in the autumn and winter of 1999. It has been reported in many other pig producing countries including Ireland.

Cause

The cause is unknown but may be involve a toxin or other antigenic substance produced by bacteria or viruses. Early investigations in the 1990s carried out by Jill Thomson in the SAC veterinary investigation centre near Edinburgh consistently revealed a Pasteurella multocida. All isolates from PDNS cases were the same but differ from most other Pasteurella isolates. More recent studies suggest that Porcine circovirus type 2 is usually involved in PDNS, though possibly not alone.

PDNS outbreaks sometimes, but not always, occur in association with PMWS, but the relationship between the two diseases is as yet unclear.

Diagnosis

Diagnosis is based on the typical clinical signs, gross and microscopic lesions and the elimination of other possible causes, particularly CSF.

Circovirus

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The name "circovirus" was given because this family of viruses have their DNA is in the form of a ring.

They are the smallest known viruses and survive well in the environment.

There are two types of Circovirus that can be found in swine, Type I and Type II, sometimes called PCV1 and PCV2.

Porcine circoviruses appear to be host specific for the pig. Other circoviruses, distinct from porcine circoviruses, cause disease in birds, plants and humans.

Circovirus Type I

Type 1 PCV was first reported in 1974 and is widespread throughout western pig populations. It was once reported to be associated with congenital tremor of piglets but this has never been confirmed and is probably wrong.

PCV 1 is currently believed to be harmless (non-pathogenic) to infected pigs.

Type I has in the past been known to contaminate porcine kidney cell lines used for virus research and vaccine production.

Circovirus Type 2

Type II porcine circovirus has recently been identified and is associated with PWMS (Post weaning Multisystemic Wasting Syndrome) in various countries: France, Spain, United Kingdom, USA.

Type 2 PCV was first reported in 1991 in Western Canada associated with the then newly described disease PMWS. Circovirus type 2 is thought to be widespread in pig herds in different countries.

Porcine circo virus type 2 has been shown to be shed in faeces and nasal secretions of recently infected pigs and presumably this is how it spreads from pig to pig. It is not known how it spreads from herd to herd but, like other viruses, movement of pigs between herds is probably a major factor. Since it is a relatively hardy virus, it is also likely to spread on clothes and equipment.

Until recently, tests for detecting and identifying circovirus was only available for research purposes but a very sensitive PCR (polymerase chain reaction) technique is now available for routine use. The test can distinguish between the two types of Circovirus.

There are several blood (serological) tests for circovirus antibodies, but their interpretation is proving to be difficult. In a study done in France on over 200 farms, 80% of the farms with no clinical signs of PMWS were serologically positive for circovirus. Also in the UK, a farm that tested serologically positive for circovirus type 2 has not shown any clinical signs of PMWS.

Despite rumours, we do not know of any vaccine or cell line contamination involving Type II Circo virus.