Abstract
Pre-emptive culling is becoming increasingly questioned as a means of controlling animal diseases, including classical swine fever (CSF). This has prompted discussions on the use of emergency vaccination to control future CSF outbreaks in domestic pigs. Despite a long history of safe use in endemic areas, there is a paucity of data on aspects important to emergency strategies, such as how rapidly CSFV vaccines would protect against transmission, and if this protection is equivalent for all viral genotypes, including highly divergent genotype 3 strains. To evaluate these questions, pigs were vaccinated with the Riemser® C-strain vaccine at 1, 3 and 5 days prior to challenge with genotype 2.1 and 3.3 challenge strains. The vaccine provided equivalent protection against clinical disease caused by for the two challenge strains and, as expected, protection was complete at 5 days post-vaccination. Substantial protection was achieved after 3 days, which was sufficient to prevent transmission of the 3.3 strain to animals in direct contact. Even by one day post-vaccination approximately half the animals were partially protected, and were able to control the infection, indicating that a reduction of the infectious potential is achieved very rapidly after vaccination. There was a close temporal correlation between T cell IFN-γ responses and protection. Interestingly, compared to responses of animals challenged 5 days after vaccination, challenge of animals 3 or 1 days post-vaccination resulted in impaired vaccine-induced T cell responses. This, together with the failure to detect a T cell IFN-γ response in unprotected and unvaccinated animals, indicates that virulent CSFV can inhibit the potent antiviral host defences primed by C-strain in the early period post vaccination.
Highlights
A number of live attenuated classical swine fever (CSF) vaccines have been developed by traditional methods such as passage in tissue culture (e.g. Thiverval strain), or by serial passage in rabbits [1]
With a view to providing additional information for those formulating emergency vaccination policies and to understand the role that T cell responses may have in vaccine-induced, rapid immunity we have investigated the speed with which the C-strain vaccine affords protection of susceptible in-contact animals, after challenge with two diverse CSFV strains, as well as the kinetics and magnitude of T cell responses
In-contact animals had haematology and pathology parameters (Table 2) consistent with CSF and infection was confirmed by detection of CSFV RNA in blood
Summary
A number of live attenuated classical swine fever (CSF) vaccines have been developed by traditional methods such as passage in tissue culture (e.g. Thiverval strain), or by serial passage in rabbits (e.g. the Chinese or C-strain) [1]. Due to the inability to identify infected animals by serology within a vaccinated population, the non-emergency use of such live attenuated vaccines is banned in the EU [6] and current strategies to control outbreaks are based on slaughter of all pigs on infected holdings, establishment of protection and surveillance zones, movement restrictions and, in some instances, pre-emptive culling of neighbouring uninfected herds [3,6]. This policy can lead to large scale economic and social consequences, which were most strikingly demonstrated by the devastating outbreak that occurred in the Netherlands, France, Belgium and Spain in 1997 [7,8]. Decisions on control strategies are complex, will vary for different regions, and depend on many factors, such as; herd density, production system(s), the presence of susceptible wildlife, impact on export trade and current opinions on economic versus ethical factors [11,12]
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