For a very long time the “viral” cause of Congenital Tremor or Shaky Pig was unknown. In 2015 researchers in the United States used next generation sequencing to identify Atypical porcine pestivirus (APPV), genus Pestivirus, in serum. The disease was then characterized as congenital tremor type A-II and is the most common cause of Congenital Tremor. Although the virus can spread horizontally (pig to pig), the clinical disease is normally observed only after in utero infection of a naïve female during pregnancy. With very rare exceptions the affected females will normally have only one litter affected by the disease and after that they are protected by long term neutralizing immunity. The only treatment for the condition is supportive care. Piglets that continue to be able to nurse on the sow will eventually grow out of the tremor. Most control focuses on biofeedback to replacement gilts that aims at getting the gilts infected prior to their first pregnancy. While venereal transmission has been proposed as a potential route, the pathogenesis and transmission dynamics of APPV remain incompletely understood. These Norwegian researchers wanted to investigate the presence of APPV in semen from Norwegian breeding boars, its temporal shedding patterns, and the potential impact of APPV on semen producing organs.
The researchers found the following:
- Of the 110 breeding boars that were initially screened there were 50.9% (56/110) that were APPV-positive in semen based on reverse transcription quantitative PCR (RT-qPCR). This was a much more significant detection rate than detected in previous studies.
- The virus load in semen was highly variable, with concentrations up to 4.0 × 10^6 genomic copies (GC)/mL.
- Extended sampling of a smaller subset of APPV-positive boars (n = 19) revealed that virus shedding in semen typically persisted for less than 3 months but one boar had intermittent shedding for up to 7 months. Figure 1 APPV Shedding Pattern In Boars
- At slaughter, the virus was detected in reproductive organs and accessory glands. The virus was identified even in boars with negative APPV semen and serum samples. There was no microscopic evidence of damage of the reproductive tissues.
Take Home Messages:
- APPV-positive semen from breeding boars appears to represent a potentially important source of APPV transmission.
- The researchers suggest that their next step is to set up a study that will assess the viability of APPV in AI semen and clarify its role in transmission. Virus isolation for infectivity studies and experimental infections in naïve sows need to be done.
- The researchers suggest that most of these infections in boars appear to be as a result of transient “horizontal” (animal to animal) infections rather than congenital pestivirus persistent infections that are associated with in utero infection.
- Rapid boar turnover facilitating genetic improvement has the potential to continue to add “fresh kindling to the fire” and maintain boar semen as a persistent source of viral infection in some boar studs. APPV is very difficult to keep out of a sow herd so the best protection is getting gilts infected before their first pregnancy.
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