Design and validation of optimized tools for Classical and African swine fever surveillance and pathogenesis research
Classical and African swine fever belong to the most important contagious diseases of pigs worldwide and are notifiable to the World Organization for Animal Health (OIE). While outbreaks of classical swine fever (CSF) have a long and ongoing history in Europe, up to now African swine fever (ASF) was considered an exotic disease within EU Member States. However, very recently the disease has been introduced into the European wild boar and domestic pig population in Poland, Lithuania, Latvia, and Estland. Hence, both diseases pose a high risk to the whole European pig industry and wildlife. Regardless of very similar clinical pictures that are not discriminable without laboratory diagnosis, the causative agents differ greatly. Classical swine fever is caused by a small enveloped positive single-stranded RNA virus belonging to the genus Pestivirus within the Flaviviridae family. The causative agent of ASF is a complex DNA virus of the genus Asfivirus within the Asfarviridae family representing the only DNA arthropod-borne (ARBO) virus. Classical swine fever virus (CSFV) isolates of recent European outbreaks are characterized by their moderate virulence. The clinical picture can range from an almost inapparent infection to a lethal hemorrhagic fever like illness. High variability in disease course and outcome are a challenge for both disease surveillance and pathogenetic research. Although several studies aimed to analyze basic pathogenetic mechanisms, responsible factors have never been elucidated entirely. While on the host’s side, age and immune status are acknowleged parameters, the virulence of the isolate seems to be decisive on the agent’s side. Moreover, the influence of the genetic background of the host has been discussed. To define host responses linked to different disease courses and outcome, a first animal trial was conducted with a moderately virulent CSFV strain and different pig breeds including European wild boar. In a second trial, the impact of the age was revisited in combination with the assessment of tools for active swine fever surveillance. Dysregulation of immune responses, especially cytokine reactions, seems to play a crucial role in CSF pathogenesis. Up to now, there has been a serious lack of appropriate and reliable tools for cytokine gene expression analyses, especially in pigs. To overcome this shortcoming, a harmonized TaqMan-based RT-qPCR protocol for the detection of seven swine fever relevant cytokines was developed and fully validated. This assay is now available for future studies and could be implemented also for other swine diseases including ASF. Beyond the studies focusing on underlying mechanisms, diagnostic approaches for active and passive disease surveillance in wild boar have been targeted. Sample submission is usually the bottleneck of wildlife surveillance under field conditions, even in times of increased risk. Pragmatic approaches for sampling and transport could improve the compliance of hunters. In view of the fact that an introduction of ASF and CSF is usually accompanied by an increased mortality, animals found dead have to be sampled even if they are already in various stages of decay. To this means, a dry-/ semi-dry blood swab method was implemented enabling an easy handling under field and laboratory conditions. Moreover, a simple approach for sampling of live animals for active surveillance, i.e. a “rope-in-a-bait” method, was evaluated with respect to the frequently observed subclinical CSF forms in older animals.
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