Introduction

Maedi-Visna (MV), also called Ovine Progressive Pneumonia or OPP in the USA, is a common chronic viral disease of adult sheep and is caused by the retrovirus maedi visna virus (MVV).  The name is Icelandic and means laboured breathing (maedi) and wasting (visna) which is associated with neurological disease.  Here in Canada, surveys have found a very high prevalence of infection in Ontario and Quebec flocks and less so in flocks in the west and Atlantic Canada.   Infected flocks tend to have between 5 and 40% of the sheep infected with the virus, and a smaller proportion of these exhibiting signs of disease.