In Switzerland, the last classical swine fever (CSF) outbreak in domestic pigs occurred in 1993 (Hofmann
and Bossy, 1998), and the virus was reported for the last time in wild boar in 1999 (Hofmann et al., 1999).
Re-emergence of CSF virus (CSFV) however represents a constant threat for the Swiss pig farming. Indeed,
the OIE regularly registers new cases of classical swine fever (CSF) worldwide including Europe. In Bulgaria,
Slovakia, Hungary and Russia the virus still circulates and results in clinical disease (for the current disease
situation, see
http://www.oie.int/wahis/public.php?page=home). The wild boar – the natural CSFV reservoir –
represents the major problem for disease control. Also, the extension of the European community to eastern
European countries along with reduced controls at the borders elevate the risk of re-introducing the disease
in Switzerland through import of contaminated wild boar or pork meat products. A CSFV outbreak in the domestic
pig would generate significant economic losses and restrictions in meat trade. In wild boar, the disease
often remains unapparent and is only detected through an increased mortality rate in the wild boar
population. However, in domestic pigs as well as in wild boar (if animals are monitored closely), a CSFV
isolate can manifest various degrees of virulence, from avirulent to highly virulent. Low and moderately virulent
strains result in a mild, atypical, often chronic disease, and induce a protective immunity. This contrasts
with highly virulent strains that cause acute to peracute symptoms consisting of high fever, diarrhoea, lameness
and haemorrhages. Such disease rapidly progresses within the herd with a high mortality rate, irrespective
of the age and weight of the infected pigs (van Oirschot, 1988).
Infection with low virulent strains is the most challenging situation for disease surveillance and control. The
mild symptoms resemble those of various other, partly common diseases such as porcine multisystemic
wasting syndrome (PMWS). Especially in disease-free periods as it has been the case in Switzerland for the
past decade, the awareness for CSFV among field veterinarians is low. Therefore, if CSFV is found in wild
boar or in domestic pigs, information on its virulence is crucial for guiding the surveillance measures and
supporting disease awareness:
a) The presence of low virulent strains in wild boar will require a comprehensive serological
surveillance to be implemented, due to the lack of distinct disease sign, while in domestic pigs a
thorough analysis of any animals with suspect clinical symptoms is mandatory.
b) If a highly virulent strain starts to circulate, infection of domestic pigs and wild boars is likely to be
rapidly detected, due essentially to the severity of the symptoms and the high infection and mortality
rate.
Thus, the diagnostic laboratory must rapidly provide information on the virulence of an emerging wild boar
isolate. Genetic correlates with virulence are missing, despite extensive sequence information available for
various CSFV isolates. Such knowledge would allow a rapid diagnostic procedures to be applied, providing
essential knowledge to support monitoring and molecular epidemiology of CSFV. It would improve disease
preparedness of veterinarians in the field, and allow appropriate surveillance programs and control measures
to be implemented.
