What Causes Maedi Visna?

Maedi visna is caused by a virus, specifically a small ruminant lentivirus (SRLV) which belongs to the Retroviridae family and is called maedi visna virus (MVV). Caprine arthritis encephalitis (CAE), a disease of goats, is caused by a similar virus often grouped with MVV and termed SRLV's. However there are considerable strain differences within both MVV and CAEV and not all sub-types are equally pathogenic (disease causing) for both species.   

Genes Important in Classifying the Virus

There are a few genes which are very important in understanding the classification of these viruses.  These genes encode the structural proteins of the virion (virus): 

The capsid p25 protein is the most abundant protein and elicits a strong antibody response during infection.

Lentivirus Groups

Using various well-conserved sequences on these genes, SRLV’s have been divided into groups A, B, C and D.  Group A viruses tend to be those responsible for MV and Group B for CAE.  Groups C and D don’t appear important in North America.  Within Group A there are 7 subtypes (A1-A7) with A2 being associated with American isolates, and in group B there are 2 subtypes (B1 and B2) with B1 being isolated from goats world-wide.    In recent research in Ontario and Quebec sheep flocks it appears that sheep isolates were mostly A2 and goat isolates are B1 with no evidence of strains that more commonly infect both species.  More work is ongoing to further investigate the strains present in Ontario goats. Additionally both slow and rapid replicating strains of MVV have been identified which correlates with different levels of pathology in the animal.  

How the virus infects the host's cell and replicates new virus

Visit these websites for more information on retroviruses:
  • An excellent diagram of the virus and how it replicates here
  • A short video of how a retrovirus replicates here
  • A very detailed tutorial of how a retrovirus replicates here
  • A very detailed 3D animation of how a retrovirus replicates here