Review
Clinical Signs and Epidemiology of Classical Swine Fever: A Review of New Knowledge

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Abstract

Although classical swine fever (CSF) has been well known for decades and epidemics still occur, clinical diagnosis continues to cause problems for veterinary practitioners. This is due to the extensive differential diagnosis, further complicated by the emergence of new diseases such as porcine reproductive and respiratory syndrome (PRRS) and porcine dermatitis and nephropathy syndrome (PDNS). In addition, acute, chronic and prenatal courses of CSF have to be distinguished. As a cause of considerable economical losses within the EU, control of CSF requires knowledge of the primary outbreaks and spread of the disease.

Genetic typing of CSF virus isolates has proved to be a potent method of supporting epidemiological investigations. Phylogenetic analysis of CSF virus strains and isolates originating from different continents has allowed three genetic groups and several subgroups within these groups to be distinguished. Whereas isolates belonging to group 3 seem to occur solely in Asia, all CSF virus isolates of the 1990s isolated in the EU belonged to one of the subgroups within group 2 (2.1, 2.2, or 2.3) and were clearly distinct from former CSF reference viruses, which belong to group 1.

Within the EU, different strategies are followed for the eradication of CSF in domestic pigs and in wild boar. While a strict non-vaccination policy is followed for domestic pigs, eradication of the disease in wild boar is more complex, and oral immunisation together with special hunting strategies have been applied. Recently, marker vaccines with a companion discriminatory test designed to allow differentiation between vaccinated animals and animals having recovered from field virus infection have been developed. Preliminary studies indicated that the discriminatory tests had a reduced sensitivity and specificity. Further improvements are therefore necessary before marker vaccines can be considered for emergency use in EU Member States. Prevention of CSF remains the main objective within the EU.

Introduction

Classical swine fever (CSF) is an OIE List A disease causing major damage in the pig populations of the EU Member States and in other countries worldwide. The causative agent of CSF is a small enveloped RNA virus which belongs to the family Flaviviridae, genus Pestivirus, together with bovine viral diarrhoea (BVD) virus and border disease (BD) virus of sheep. The virions are particles with a hexagonally shaped, electron-dense inner core structure of about 30 nm, surrounded by a spherical envelope with diameters ranging between 40 and 60 nm (Wengler et al., 1995). Pestiviruses are antigenically closely related, and as polyclonal sera are highly cross-reactive between all pestiviruses (Darbyshire, 1960), problems can arise in serological diagnosis. There is no serological relationship to the members of the other genera of the Flaviviridae. In cell culture, most CSF virus (CSFV) strains and isolates are non-cytopathic (ncp), and cytopathogenic effects of some isolates have been attributed to the existence of defective interfering particles (Mittelholzer et al., 1997). In vivo, it has been found that CSFV infection can induce apoptosis (Sato et al., 2000).

As several reviews covering different aspects of pestiviruses and CSF have been published recently (Meyers & Thiel, 1996; Moennig & Plagemann, 1992; Thiel et al., 1996), this review will focus on new facts mainly concerning the epidemiology of the disease in Europe, the genetic typing of new virus isolates, and progress in vaccine development. In addition, as the diagnosis of primary CSF outbreaks continues to cause difficulties to veterinary practitioners and government veterinary officers, the clinical signs of the disease will be surveyed.

Section snippets

Clinical signs of classical swine fever

The main route of infection in field cases is oronasal by direct or indirect contact with infected pigs or by feed which is contaminated with virus, e.g., swill. In areas with a high density of pigs, virus spread easily occurs between neighbouring pig holdings (Fritzemeierj et al., 2000). Disease transmission via the semen of infected boars may also occur (Floegel et al., 2000). The incubation period in individual animals is about one week to 10 days. Under field conditions, symptoms may only

Chronic course of classical swine fever virus infection

The chronic form of CSF is always fatal. It develops when pigs are not able to mount an effective immune response against the infection. Initial signs are similar to die acute infection. Later, predominantly non-specific signs are observed, e.g. intermittent fever, chronic enteritis and wasting. Animals may survive for 2–3 months before they die. CSF virus is shed from the onset of clinical signs constantly until death. Antibodies may be temporarily detected in serum samples, as the immune

Prenatal course of infection and late onset of disease

Although the course of infection in the sow is often subclinical, CSF virus is able to cross the placenta of pregnant animals, thereby infecting fetuses during all stages of pregnancy. The outcome of transplacental infection of fetuses mainly depends on the time of gestation and viral virulence, respectively. Infection during early pregnancy may result in abortions and stillbirths, mummification and malformations. All of this will lead to a reduction in the fertility index in the holding.

Antigenic and genetic typing

Extensive studies have been performed using monoclonal antibodies (mabs) against viral proteins for differentiating between pestiviruses (Greiser-Wilke et al., 1990; Paton et al., 1995; Wensvoort et al., 1989), thus leading to the creation of a reference panel of mabs (Edwards et al., 1991). In addition, mabs have been used for typing CSF virus isolates and other pestiviruses (Kosmidou et al., 1995; Paton et al., 1995). Two panels of mabs, 12 directed against the E2 and 11 against the ERNS

Epidemiology in Europe

At the end of the 1980s, CSF was considered an infection at the brink of eradication in the EU. With the implementation of a common market within the EU in 1990, countries which still used live CSF vaccines were required to stop vaccination in order to achieve a common status of ‘CSF free’ in all member states. Instead of vaccination strict control measures were applied, e.g. killing of infected, suspect and contact animals and movement restrictions. However, in the following years several EU

Eradication

EU Council Directive 80/217/EEC (Anon., 1980) is currently under extensive revision, and the revised version, Council Directive 2001/89/EC of 23 October 2001 (Anon., 2001a), takes into account experiences gained during the past decade under the non-vaccination policy. It will deal in particular with high-density pig areas, eradication of CSF in wild boar and strategies for the use of marker vaccines. In addition, a ‘Diagnostic Manual’ (Anon., 2001b) will be available with the new Directive to

Use of marker vaccines

Live attenuated vaccines, such as the ‘lapinised Chinese strain’ (C-strain) of CSF, are safe and powerful tools for the prophylactic protection of domestic pigs against CSF. In the EU, vaccination was banned in 1990. However, in many central and eastern European countries, as well as overseas, this vaccine is still being used. Where a severe emergency exists, provisions have been made in EU Directive 80/217 (Anon., 1980) for limited vaccination of domestic pigs. However, since animals

Recommendations

From a purely scientific point of view several recommendations can be made.

  • 1.

    As it is not possible to control the heat treatment of swill before feeding it to pigs, a total ban on swill feeding would be appropriate. Swill feeding is still the major risk for primary CSF outbreaks in domestic pigs and wild boar. Public awareness should be heightend with regard to the dangers of swill feeding of wild boar and domestic pigs in outdoor holdings.

  • 2.

    The control of the influx of meat and meat products from

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