Line 49: |
Line 49: |
| | | |
| ==Treatment== | | ==Treatment== |
− | ====Control====
| + | |
− | *'''NOTIFIABLE''' disease
| + | The control policy for CSF depends on the incidence and prevalence of the infection in |
− | *'''Vaccination''' (live attenuated) in endemic countries:
| + | the domestic and wild pig populations, respectively. In countries with CSF endemic in |
− | **Parts of EU are using vaccinated bait to control spread in wild boar population
| + | domestic pigs it is common practice to vaccinate against the disease, thereby, avoiding |
− | **Vaccination does not curtail spread: marker vaccine needed to distinguish virus exposure from vaccine-induced antibody
| + | serious losses. However, the simultaneous eradication of ®eld virus is improbable because |
| + | serological methods are no longer applicable for the detection of ®eld virus infections. It |
| + | is acknowledged that ®eld virus may be hidden under a `blanket' of general vaccination. |
| + | Taking this risk into account, importing countries in general do not allow the introduction |
| + | of pigs or pig products from countries that vaccinate against CSF. The preventive |
| + | measures adopted by the EU for trade with Third Countries stipulate that live pigs and |
| + | fresh pig meat can only be imported from regions or countries where no CSF has |
| + | occurred for 12 months and no vaccination against CSF was applied during the same |
| + | period. Nevertheless a policy of consistent and systematic prophylactic vaccination in |
| + | endemic situations may ultimately lead to a favourable starting point for a non |
| + | vaccination policy and the eradication of the virus. After the cessation of general |
| + | vaccination, eventual local outbreaks of residual ®eld virus must be dealt with by strict |
| + | measures to ensure prevention of virus spread and eradication of the virus. |
| + | Based on the above mentioned disadvantages of vaccination and a cost bene®t analysis |
| + | the EU banned vaccination against CSF at the end of the 1980s. Whereas most |
| + | neighbours of the EU have also adopted a similar policy, vaccination is allowed and |
| + | mostly routinely applied by many Central and Eastern European countries (Edwards |
| + | et al., 2000). In some of the latter countries only sick or clinically suspect animals are |
| + | destroyed in case of CSF outbreaks whereas all other animals of the infected herd, herds |
| + | in the neighbouring area and contact herds are vaccinated. |
| + | In case of an outbreak of CSF, all EU Member States and the other Western European |
| + | countries execute eradication measures according to the Council Directive 80/217/EEC |
| + | (Anonymous, 1980; Edwards et al., 2000). These are based on stamping out |
| + | (depopulation) of infected pig herds and possibly infected contact or (partially) |
| + | neighbouring herds, epidemiological investigations, clinical and virological investigations, |
| + | movement restrictions for live pigs, pig meat and other vectors which can transmit |
| + | CSF within zones surrounding the infected farm and restrictions on contact farms outside |
| + | these zones (Anonymous, 1980). Especially in areas with dense pig populations very |
| + | high numbers of pigs had to be destroyed in the course of the eradication measures |
| + | dealing with the outbreaks mentioned above. Only a minority of animals were killed due |
| + | to direct involvement with the infection. Most of the pigs had to be killed because of |
| + | welfare measures. The direct and indirect costs of recent CSF outbreaks in several EU |
| + | Member States so far amount to several billion Euro, and in the course of the CSF |
| + | epidemic in the Netherlands in 1997 approximately 10 million pigs were destroyed |
| + | (Saatkamp and Horst, 2000; Stegeman et al., 2000). Whereas in areas with a low density |
| + | pig population, the present control policy works very well it may well be questioned |
| + | whether it is sustainable in areas with a high density of pigs. There is a general consensus |
| + | that a number of measures must be introduced in order to reduce the vulnerability of |
| + | regions at risk, e.g., structural changes in the pig industry including trade. However, |
| + | implementation of appropriate programs might be dif®cult. Several parties, notably some |
| + | V. Moennig / Veterinary Microbiology 73 (2000) 93±102 99 |
| + | national farmers' associations requested the reintroduction of a general or at least |
| + | regional vaccination. |
| + | In principle, emergency vaccination is in agreement with EU legislation (Anonymous, |
| + | 1980). Requirements related to emergency vaccination campaigns against CSF virus have |
| + | been de®ned in the document `Guidelines for a Classical Swine Fever Emergency |
| + | Vaccination Programme' (Anonymous, 1994). However, by using conventional vaccines |
| + | and applying the mentioned guidelines, the Scienti®c Veterinary Committee of the |
| + | Commission has calculated that vaccinated animals would be excluded from the market |
| + | for up to 600 days (Anonymous, 1997). This is economically unacceptable and so far |
| + | emergency vaccination has never been used. |
| + | With the development of a ®rst generation marker vaccine against CSF the possibility |
| + | of an amendment of the existing EU emergency vaccination regulations seems feasible. A |
| + | restricted application of a marker vaccine would require extensive serological testing in |
| + | the vaccinated population in order to detect hidden ®eld virus infections. At present no |
| + | marker vaccine has been licensed within the EU and EU Member States demand welldocumented |
| + | data on the safety and ef®cacy of the vaccine before its potential use in |
| + | emergency situations. It is understood that the criteria for the use of the marker vaccine |
| + | will be very stringent. Provided that all safety requirements are met, the period of |
| + | exclusion from the market could be considerably shortened at least for pig products after |
| + | a CSF outbreak (Anonymous, 1997). As soon as marker vaccines are suf®ciently |
| + | investigated and licensed, the `Guidelines for a Classical Swine Fever Emergency |
| + | Vaccination Programme' (Anonymous, 1994) are to be amended. The possible use of an |
| + | emergency vaccination with marker vaccines is expected to avoid the ethically |
| + | questionable and expensive large scale pre-emptive slaughter of pigs. Thereby, the |
| + | public acceptance of the eradication policy will increase and costs will decrease. Under |
| + | these circumstances the use of emergency vaccination using marker vaccines could be a |
| + | useful tool of the non-vaccination policy. |
| + | A still unresolved problem is the control of CSF in wild boar (Laddomada, 2000). |
| + | Both the prolonged persistence of virus in wildlife populations and the constant threat |
| + | of domestic pig holdings in the respective areas require an ef®cient control strategy. |
| + | Comprehensive information about the current situation in wild boar populations |
| + | is essential and new strategies have to be devised. They have to take into account |
| + | current knowledge about factors in¯uencing CSF epidemiology, e.g., wild boar |
| + | behaviour; population dynamics; in¯uence of hunting strategies; in¯uence of geographic |
| + | pro®les. |
| + | An ef®cient surveillance system must be an integral part of the control strategy. The |
| + | EU Commission has held a workshop dedicated to this topic (Anonymous, 1998) and a |
| + | working group of the Scienti®c Committee on Animal Health and Animal Welfare will |
| + | prepare a recommendation. |
| + | ===Vaccination=== |
| + | From the beginning of the century attempts have been made to develop vaccines |
| + | against CSF. However, the safety and ef®cacy of the ®rst generations of vaccines were |
| + | poor. In the 1940s ®rst experiments were made to attenuate CSFV by adapting it to |
| + | rabbits (Baker, 1946; Koprowski et al., 1946). After initial setbacks, this development |
| + | ultimately led to a very ef®cient and safe generation of live vaccines. Most attenuated |
| + | vaccines are based on the China-strain (C-strain) of lapinized CSF virus. C-strain |
| + | vaccines were and are still being used world-wide for the control of CSF in domestic pigs. |
| + | It is also used at least on an experimental basis for the oral immunisation to control CSF |
| + | in wild boar (Kaden et al., 2000). C-strain vaccines induce high titres of neutralising |
| + | antibodies and they are safe when used on pregnant animals. Their ef®cacy is |
| + | demonstrated by the observation that vaccinated pigs are protected against infections with |
| + | virulent CSF virus as early as ®ve days after vaccination. The animals are immune |
| + | throughout their economic life. However, with respect to today's global trade policy there |
| + | is a severe disadvantage in using live attenuated vaccines against CSF: Vaccinated and |
| + | ®eld-virus-infected animals cannot be distinguished because the antibody pattern induced |
| + | by the vaccine virus resembles that of reconvalescent animals. |
| + | A way out of this dilemma may be the development and use of so-called marker |
| + | vaccines, e.g., subunit vaccines consisting of single viral surface proteins, which are suf- |
| + | ®cient for the induction of protective immunity. At present two subunit vaccines containing |
| + | the viral glycoprotein E2 are under scrutiny. The respective gene is expressed in baculoviruses |
| + | grown in insect cells (Van Rijn et al., 1996). Since these cells are able to glycosylate |
| + | proteins, the resulting viral glycoprotein is expressed in a `natural'way.CSF subunit vaccines |
| + | are safe and so far their protective potency is promising, though inferior to live vaccines. |
| + | Vaccinated animals may be distinguished from infected pigs using an ELISA based on a |
| + | different viral protein as diagnostic antigen, e.g., the surface glycoprotein Erns or the |
| + | nonstructural protein NS2-3. However, not all criteria for the emergency use of marker |
| + | vaccines are well de®ned yet, and the technical merits of these vaccines have not yet been |
| + | established. More data are expected to be available during the year 1999. |
| + | Technically there is the potential for further improving CSF marker vaccines by |
| + | developing, e.g., viral vector vaccines (RuÈmenapf et al., 1991; Van Zijl et al., 1991; Hooft |
| + | van Iddekinge et al., 1996), DNA vaccines and molecularly altered infectious cDNA |
| + | clones of CSF virus (Meyers et al., 1996, Moormann et al., 1996, Ruggli et al., 1996). |
| | | |
| ==Prognosis== | | ==Prognosis== |