Shaky Pig APPVirus Remains Genetically Stable Over Time , By Doug MacDougald South West Ontario Veterinary Services

Congenital tremor or “Shaky Pig” was identified in Canada in the 1950’s. The exact cause of  Congenital Tremor Type AII remained a bit of a mystery for some time but was thought to be some sort of viral infection. Recently,  Next Generation Genetic Sequencing established a link between Congenital Tremor Type AII and a newly identified atypical porcine pestivirus (APPV). Atypical porcine pestivirus (APPV) is a single-stranded RNA virus from the family Flaviviridae. It is a pestivirus and this should not have been a big surprise since classical swine fever virus (CSFV), another pestivirus, is also capable of causing a similar form of congenital tremors. Shaky Pig is most common in piglets in gilt litters and the problem very rarely occurs in subsequent litters suggesting that exposure to the virus results in immunity of the gilt that is at the same time protective to the piglets.

Researchers in the Netherlands wanted to investigate the genetic molecular evolution of APPV in an affected herd over time. Because less is known about this more recently identified virus the researchers wanted to better understand how this virus changes genetically over time. The viral samples covered 2013 to 2019.

The researchers found the following:

  • the same genotype of APPV was found during the entire study period on this farm and this suggested no introductions from outside the farm.
  • the nucleotide substitutions over time did not show substantial amino acid variation in the structural glycoprotein components of the virus.
  • the evolution of the virus showed mainly purifying selection with efficient removal of any deleterious genetic mutations such that the genome remains relatively stable over time.

Take Home Message

  • The researchers concluded that with limited pressure on the virus to change at immune-dominant regions it is possible that farms can maintain APPV for years
  • Massive testing and removal of infected animals are not likely to be sufficient to clear the virus from affected farms and therefore improving on immunity is a better approach.
  • Biofeedback of replacement gilts is an important component of exposure to endemic pathogens. Because some of these pathogens can cause reproductive failure, it is important that biofeedback and exposure is established prior to breeding and pregnancy. Biofeedback is an important aid in preventing infectious reproductive failure and this is especially true when there are no vaccine alternatives. eg SMEDI virus

Submitted by Dr. Doug MacDougald

Ref:  Alba Folgueiras-González , Robin van den Braak , Bartjan Simmelink , Martin Deijs , Lia van der Hoek , Ad de Groof    Atypical Porcine Pestivirus Circulation and Molecular Evolution within an Affected Swine Herd  Viruses . 2020 Sep 25;12(10):E1080. doi: 10.3390/v12101080.