Classical Swine Fever

Description

Classical swine fever is a highly contagious, haemorrhagic disease of swine which is caused by a Togavirus. Presentation may be actue, sub-acute, chronic or persistent, and the disease is indistinguishable in the field from African Swine Fever. Acutely, classical swine fever is characterised by severe depression, high fever and superficial and internal haemorrhages, with many cases resulting in death. Depression, anorexia and pyrexia are seen in chronic classical swine fever. Transplacental infection is also possible and results in persistently infected piglets.

Aetiology

The causative agent of classical swine fever is a small, enveloped virus of around 40nm diameter. The genome is comprised of single stranded RNA, which is positive sense and contains about 12,300 bases. The sequence of the genome is known, and codes four structural and seven non-structural proteins. The classical swine fever virus is relatively stable in excretions and in fresh meat products including ham, salami and other similar sausages. It is, however, easily inactivated by detergents, common disinfectants and heat.

Classical swine fever virus is a Togavirus within the Pestivirus genus of the Flaviviridae. As such, it is closely related to the bovine viral diarrhoea (BVD) virus of cattle, and the border disease virus of sheep.

Signalment

Domestic pigs and other swine of any age may become infected with classical swine fever.

Transmission and Pathogenesis

In field cases of CSFV, transmission is mainly oronasal by direct or indirect contact with infected pigs. Infected feed or pork products may also cause spread of disease, and transmission in semen can occur. Once the virus gains entry to the host an incubation period of around 7 days ensues, but this may vary from 4-10 days. Initially, virus infects the epithelial cells of the tonsillar crypts before spreading via the lymphatics to regional lymph nodes. From here, classical swine fever virus enters the blood stream and then replicates in the spleen, bone marrow and lymph nodes before spreading to further tissues. Replication in the endothelial cells of blood vessels leads to apoptosis, causing superficial and internal haemorrhages. CSFV also causes a thrombocytopenia which contributes to haemorrhage by impairing primary haemostasis. In acute CSF this angiopathy causes pig death in association with shock and the febrile response. Surviving swine go on to develop a chronic form of the disease where joint and enteric lesions are seen resulting from tissue infarction.

The outcome of transplacental infection of foetuses depends largely on the point of gestation and may result in abortions, stillbirths, mummifications, malformations or the birth of weak or persistently viraemic piglets. Although persistently infected piglets may be clinically normal at birth, they grow poorly, excrete virus over long periods and invariably die from CSF.

Diagnosis

Clinical Signs

Although the incubation period for classical swine fever is generally less than ten days, in the field it may take up to four weeks for clinical signs to become apparent in a population. Disease severity varies with virulence, immune status and the age of the animal: this means that although acute, chronic and congenital forms of the disease can be appreciated, there is no "classic" disease presentation.

In the acute form, animals are almost always pyrexic. In piglets the fever may exceed 40°, but in adults temperatures may be no higher than 39.5°. Lethargy, conjunctivitis, lymphomegaly may be seen, as well as respiratory signs and diarrhoea. Neurological signs such as gait abnormalities, incoordination and convulsions are also common. The most telling sign of classical swine fever is haemorrhage of the skin. These arise in the second or third week post-infection on the ear, tail, abdomen and medial aspect of the limbs and persist until death. CSF virus also causes severe leukopenia and immunosuppression, which can to secondary enteric or respiratory infections which may cause confusion by masking or overlapping the more typical signs of CSF. With increasing age of infected animals, the clinical signs of acute CSF become less specific and diagnosis more difficult. Alao, acute classical swine fever is indistinguishable from African swine fever and so care must be taken when formulating a diagnosis. Other differential diagnoses for acute CSF are erysipelas, PRRS, purpura haemorragica, PWMS, PDNS, Salmonellosis and Pasteurellosis. Classical swine fever should also be considered in any pyrexic enteric or respiratory disease case that is not responsive to antibiotics.

The chronic form of classical swine fever develops when pigs fail to mount an effective immune response to viral infection. Initially, the signs are similar to the acute form of the disease, but symptoms become less specific as the course progresses. For example, pigs may display chronic enteritis, loss of condition, lameness or intermittent pyrexia. In a herd, mortality may be increased or there may be large numbers of runty pigs. Although animals may survive some months after contracting chronic CSF, the disease is always eventually fatal and animals continue to shed virus until death.

The course of infection in older, breeding-age animals is often subclinical; however, CSFV is able to cross the placenta at any stage of pregnancy. The outcome of transplacental infection is highly dependent on the stage of gestation, and also virulence. During early pregnancy, transplacental CSFV infection may cause abortions, mummifications, congenital malformations or stillbirths. Infection occuring after 50-70 days gestation can lead to the birth of persistently viraemic piglets. These may appear clinically normal at birth, but grow poorly and occasionally show congenital tremor. Persistently infected piglets also shed virus until their inevitable death, acting as a reservoir for virus and making major contributions to the maintenance of infection in the population. It is therefore important to consider classical swine fever as a differential diagnosis of reduced fertility in addition to parvovirus, PRRS, leptospirosis and Aujeszky's disease.

Pathology

In acute classical swine fever, the major pathological change is multiple haemorrhages. This is seen as many purple blotches in the skin, and as sub-capsular bleeding in association with swelling and oedema in all lymph nodes. A "turkey egg" appearance to the kidneys is displayed, with haemorrhage varying from petechiae to ecchymoses. and petechiae can also be observed in the larynx, bladder and heart as well as on serosal surfaced. A non-purulent encephalitis is sometimes seen.

Chronic: 

Pathological changes are less typical, especially the lack of haemorrhages on organs and serosae. In animals displaying chronic diarrhoea, necrotic and ulcerative lesions on the ileum, the ileocaecal valve and the rectum are common. Since clinical signs of chronic CSF are rather non-specific, a broad range of other diseases must be considered as part of any differential diagnosis.

Treatment

Control

• NOTIFIABLE disease
• Vaccination (live attenuated) in endemic countries:
  • Parts of EU are using vaccinated bait to control spread in wild boar population
  • Vaccination does not curtail spread: marker vaccine needed to distinguish virus exposure from vaccine-induced antibody

Links

• Defra - Classical Swine Fever

References

1. Moennig, V (2000) Introduction to classical swine fever: virus, disease and control policy, Veterinary Microbiology, 73, 93-102.
2. Moennig, V et al (2003) Clinical Signs and Epidemiology of Classical Swine Fever: A Review of New Knowledge, The Veterinary Journal, 165, 11-20.
3. Paton, DJ and Greiser-Wilke, I (2003) Classical swine fever – an update, Research in Veterinary Science, 75, 169-178.